Dr Jason Fung on Intermittent Fasting  medical review by Dr. Andreas Eenfeldt, MD

https://www.dietdoctor.com/intermittent-fasting  Updated October 15, 2019

Intermittent fasting is a way to cycle between periods of fasting and eating. It’s currently a very popular method to lose weight and improve health.

Intermittent fasting might actually be an ancient secret of health. It is ancient because it has been practiced throughout all of human history.

It’s a secret because this potentially powerful habit had until recently in many ways been virtually forgotten especially regarding our health.

However, many people are now re-discovering this dietary intervention. Since 2010, the number of online searches for “intermittent fasting” has increased by about 10,000 percent, with most of the increase happening in the last few years.

Intermittent fasting might provide huge benefits if it is done right, including loss of excess weight, reversal of type 2 diabetes and many other things.

 Plus, it can save you time and money.

The goal of this beginner’s guide is to provide everything you need to know about intermittent fasting, in order to get started.

Intermittent fasting – isn’t that starvation?

No. Fasting differs from starvation in one crucial way: control. Starvation is the involuntary absence of food for a long time, leading to severe suffering or even death.

It is neither deliberate nor controlled.

Fasting, on the other hand, is the voluntary withholding of food for spiritual, health, or other reasons. It’s done by someone who is not underweight and thus has enough stored body fat to live off. Intermittent fasting done right should not cause suffering, and certainly never death.

Food is easily available, but you choose not to eat it. This can be for any period of time, from a few hours up to a few days or – with medical supervision – even a week or more. You may begin a fast at any time of your choosing, and you may end a fast at will, too. You can start or stop a fast for any reason or no reason at all.

Fasting has no standard duration, as it is merely the absence of eating.

Anytime that you are not eating, you are intermittently fasting. For example, you may fast between dinner and breakfast the next day, a period of approximately 12-14 hours. In that sense, intermittent fasting should be considered a part of everyday life.

It is perhaps the oldest and most powerful dietary intervention imaginable.

Consider the term “break fast”. This refers to the meal that breaks your fast – which is done daily. Rather than being some sort of cruel and unusual punishment, the English language implicitly acknowledges that fasting should be performed daily, even if only for a short duration.

Intermittent fasting is not something queer and curious, but a part of everyday, normal life. It is perhaps the oldest and most powerful dietary intervention imaginable.

Yet somehow we have missed its power and overlooked its therapeutic potential.

Learning how to fast properly gives us the option of using it or not.

To get started, either watch our brief video course on intermittent fasting, or keep reading below.

It is important to realize that this is normal and humans have evolved to fast for shorter time periods – hours or days – without detrimental health consequences.

Body fat is merely food energy that has been stored away. If you don’t eat, your body will simply “eat” its own fat for energy.

Life is about balance. The good and the bad, the yin and the yang. The same applies to eating and fasting. Fasting, after all, is simply the flip side of eating. If you are not eating, you are fasting. Here’s how it works:

When we eat, more food energy is ingested than can immediately be used. Some of this energy must be stored away for later use. Insulin is the key hormone involved in the storage of food energy.

    Eat Food —–à Increase Insulin ——àStore Sugar in Liver, Produce Fat in Liver or Muscle

Insulin rises when we eat, helping to store the excess energy in two separate ways. Carbohydrates are broken down into individual glucose (sugar) units, which can be linked into long chains to form glycogen, which is then stored in the liver or muscle.

There is, however, very limited storage space for carbohydrates; and once that is reached, the liver starts to turn the excess glucose into fat. This process is called de-novo lipogenesis (meaning literally “making new fat”).

Some of this newly created fat is stored in the liver, but most of it is exported to other fat deposits in the body. While this is a more complicated process, there is almost no limit to the amount of fat that can be created.

So, two complementary food energy storage systems exist in our bodies. One is easily accessible but with limited storage space (glycogen), and the other is more difficult to access but has almost unlimited storage space (body fat).

        Burn Stored Sugar or Fat ß——Decrease Insulin ß—– No Food or Fasting

The process goes in reverse when we do not eat (intermittent fasting). Insulin levels fall, signaling the body to start burning stored energy as no more is coming through food. Blood glucose falls, so the body must now pull glucose out of storage to burn for energy.

Glycogen is the most easily accessible energy source. It is broken down into glucose molecules to provide energy for the body’s other cells. This can provide enough energy to power much of the body’s needs for 24-36 hours. After that, the body will primarily be breaking down fat for energy.

So the body only really exists in two states – the fed (insulin high) state and the fasted (insulin low) state. Either we are storing food energy (increasing stores), or we are burning stored energy (decreasing stores). It’s one or the other. If eating and fasting are balanced, then there should be no net weight change.

If we start eating the minute we roll out of bed, and do not stop until we go to sleep, we spend almost all our time in the fed state. Over time, we may gain weight, because we have not allowed our body any time to burn stored food energy.

To restore balance or to lose weight, we may simply need to increase the amount of time spent burning food energy.

That’s intermittent fasting.

In essence, intermittent fasting allows the body to use its stored energy. After all, that’s what it is there for. The important thing to understand is that there is nothing wrong with that. That is how our bodies are designed. That’s what dogs, cats, lions and bears do. That’s what humans do.

If you are eating every third hour, as is often recommended, then your body will constantly use the incoming food energy. It may not need to burn much body fat, if any. You may just be storing fat.

Your body may be saving it for a time when there is nothing to eat.

If this happens, you lack balance. You lack intermittent fasting.

Intermittent fasting benefits

Intermittent fasting’s most obvious benefit is weight loss.

However, there are a many potential benefits beyond this, some of which have been known since ancient times.

The fasting periods were often called ‘cleanses’, ‘detoxifications’, or ‘purifications’, but the idea is similar – e.g. to abstain from eating food for a certain period of time, often for health reasons. People imagined that this period of abstinence from food would clear their bodies’ systems of toxins and rejuvenate them. They may have been more correct than they knew.

Some of the purported health benefits of intermittent fasting include:

• Weight and body fat loss
• Increased fat burning
• Lowered blood insulin and sugar levels 
• Possibly reversal of type 2 diabetes
• Possibly improved mental clarity and concentration
• Possibly increased energy
• Possibly increased growth hormone, at least in the short term
• Possibly an improved blood cholesterol profile
• Possibly longer life
• Possibly activation of cellular cleansing by stimulating autophagy
• Possibly reduction of inflammation

Fasting offers many important unique advantages that are not available in typical diets.

Where diets can complicate life, intermittent fasting may simplify it. Where diets can be expensive, intermittent fasting can be free. Where diets can take time, fasting saves time. Where diets may be limited in their availability, fasting is available anywhere. And as discussed earlier, fasting is a potentially powerful method for lowering insulin and decreasing body weight.

Here are even more reasons to try it, along with more details: The 7 practical benefits of intermittent fasting

How to intermittent fast

Shorter fasts (<24hrs)

Intermittent fasting offers plenty of flexibility. You can fast for as long or short as you like, but fasts longer than a few days may require medical supervision. Here are some popular regimens. Generally, shorter fasts are done more frequently.

16:8

This way of doing intermittent fasting involves daily fasting for 16 hours. Sometimes this is also referred to as an 8-hour eating ‘window’. You eat all your meals within an 8-hour time period and fast for the remaining 16 hours. Generally, this is done daily or almost daily.

For example, you may eat all your meals within the time period of 11:00 am and 7:00 pm. Generally, this means skipping breakfast, but some people prefer to skip dinner instead. You typically eat two or three meals within this 8-hour period.

20:4

This involves a 4-hour eating window and a 20-hour fast. For example, you might eat between 2:00 pm and 6:00 pm every day and fast for the other 20 hours. Generally, this would involve eating either one meal or two smaller meals within this period.

Learn more about shorter fasts 

Longer fasts (>24 hours)

24-hour fasts

This way of doing intermittent fasting involves fasting from dinner to dinner (or lunch to lunch). If you eat dinner on day 1, you would skip the next day’s breakfast and lunch and eat dinner again on day 2. This means that you are still eating daily, but only once during that day. This would generally be done two to three times per week. Learn more 

5:2 fast

This is the version of intermittent fasting that has the most scientific support, as most studies on intermittent fasting has featured similar advice.

Dr. Michael Mosley popularized this variation of intermittent fasting in his book ‘The Fast Diet’.

5:2 involves five regular eating days and two fasting days. However, on these two fasting days, you are allowed to eat 500 calories on each day. These calories can be consumed at any time during the day – either spread throughout the day or as a single meal. Learn more 

Alternate-day fasting

Another related approach to 5:2 is to have “fasting” days with 500 calories not just twice a week, but every other day. Learn more 

36-hour fasts

This involves fasting for the entire day. For example, if you eat dinner on day 1, you would fast for all of day 2 and not eat again until breakfast on day 3. This is generally 36 hours of fasting. This might provide a more powerful weight loss benefit. The other great benefit is that it avoids the temptation to overeat dinner on day 2. Learn more 

Extended fasting

The first rule of extended fasts is to always check with your health care provider to ensure you are not at risk for fasting complications.
Generally for fasts greater than 48 hours, I recommend a general multivitamin to avoid micronutrient deficiency. The world record for fasting is 382 days (although we don’t recommend this!), and going 7-14 days may be possible for some people.

I discourage people from fasting for more than 14 days due to high risk of refeeding syndrome, a dangerous shift in fluids and minerals that can occur when food is re-introduced after a long fast.

Learn more about longer fasts 

Intermittent fasting FAQ

Who should NOT fast?

You should not do intermittent fasting if you are:

• Underweight (BMI < 18.5) or have an eating disorder like anorexia.
• Pregnant – you need extra nutrients for your child.
• Breastfeeding – you need extra nutrients for your child.
• A child under 18 – you need extra nutrients to grow.

You can probably fast, but may need medical supervision, under these conditions:

• If you have diabetes mellitus – type 1 or type 2.
• If you take prescription medication.
• If you have gout or high uric acid.
• If you have any serious medical conditions, such as liver disease, kidney disease, or heart disease.

Won’t intermittent fasting put me into starvation mode?

Not likely. This is the most common myth about intermittent fasting, and generally it’s not true.

 In fact, some studies indicate that intermittent fasting may even increase the basal metabolic rate (at least initially) and might potentially improve overall body composition.

 Learn more 

Can I exercise during fasting?

Yes. You can continue all your usual activities, including exercise, while fasting. You do not need to eat before exercising to provide energy.

Instead, your body can burn stored energy (like body fat) for energy.

However, for long duration aerobic exercise, eating before exercise may increase performance.

This is good to know if you’re competing.

Keep in mind that it may be important to drink fluids and replenish sodium (salt) around exercise when fasting. Learn more 

What are the possible side effects?

There can be a number of possible nuisance side effects of intermittent fasting.

Here’s what to do if you encounter them:

• Hunger is the most common side effect of intermittent fasting. This may be less of an issue if you’re already on a keto or low-carb, higher-fat diet.

Learn more.

• Constipation is common. Less going in means less going out. You don’t need medications unless you experience discomfort. Standard laxatives can be used to help.
• Headaches are common and tend to disappear after the first few times on fasts.

 Taking some extra salt often helps mitigate such headaches.

• Mineral water may help if your stomach tends to gurgle.
• Other possible side effects include dizziness, heartburn and muscle cramps. Learn more 

A more serious side effect is the refeeding syndrome. Fortunately, this is rare and generally only happens with extended fasts (5-10 days or more) when one is undernourished. Learn more 

Why does my blood sugar go up during fasting?

While this does not happen with everyone, it can occur due to hormonal changes that occur during intermittent fasting. Your body is producing sugar in order to provide energy for your system. This is a variation of the Dawn Phenomenon and in general is not a concern as long as blood sugars are not elevated the rest of the day.

Learn more 

How do I manage hunger?

The most important thing to realize is that hunger usually passes like a wave. Many people worry that hunger during intermittent fasting will continue to build until it is intolerable, but this does not normally happen.

Instead, hunger comes in a wave. If you simply ignore it and drink a cup of tea or coffee, it will often pass.

During extended fasts, hunger will often increase into the second day. After that, it gradually recedes; and many people report a complete loss of hunger sensation by day 3-4.

 Your body is now being powered by fat. In essence, your body is ‘eating’ its own fat for breakfast, lunch and dinner and therefore is no longer hungry. Learn more 

Won’t intermittent fasting burn muscle?

That depends on the person and the duration of the fast. During fasting, the body first breaks down glycogen into glucose for energy. After that, the body increases fat breakdown to provide energy. Excess amino acids (the building blocks of proteins) are also used for energy, but the body does not burn its own muscle for fuel unless it has to.

Some studies, however, suggest that leaner individuals are at higher risk of lean body mass loss, and even reduced metabolic rate. Yet it appears this is less of a concern with obese subjects.

In my experience with over 1,000 patients on various intermittent fasting regimens, I have not yet seen a single case of significant muscle loss.

 Learn more 

What are your top tips for intermittent fasting?

Here are the nine top tips, briefly:

• Drink water
• Stay busy
• Drink coffee or tea
• Ride out the hunger waves
• Give yourself one month to see if intermittent fasting (such as 16:8) is a good fit for you
• Follow a low-carb diet between fasting periods. This reduces hunger and makes intermittent fasting easier. It may also increase the effect on weight loss and type 2 diabetes reversal, etc.
• Don’t binge after fasting

Learn more practical intermittent fasting tips 

How do I break a fast?

Gently. The longer the fast, the more gentle you might have to be.

Eating too large a meal after fasting (a mistake that we have ALL done, myself included) can give you a stomach ache. While this is hardly serious, people usually learn quickly to eat as normally as possible after a fast.

Isn’t it important to have breakfast every morning?

Not necessarily. This appears to be an old misconception, based on speculation and statistics, and it does not hold up when it’s tested.

 Skipping your morning meal gives your body more time to burn fat for energy. Since hunger is lowest in the morning, it may be easiest to skip it and break your fast later in the day.

 Learn more:

• Skipping breakfast does not lead to eating more
• NYT: Sorry, there’s nothing magical about breakfast

Can women fast?

Yes, but there are exceptions. Women who are underweight, pregnant or breastfeeding should not fast.

Furthermore, for women trying to conceive, be aware that – perhaps especially for athletic women with low body fat percentage – intermittent fasting might increase the risk of irregular menses, and lower the chance of conception.

Other than that, there is no special reason why women should not fast.

Women can have problems during intermittent fasting, but so can men. Sometimes women do not get the results they want, but that happens to men, too.

Studies show that the average weight loss for women and men who fast is similar.

Learn more about women and intermittent fasting 

Isn’t fasting the same as reducing calories?

No, not necessarily. Fasting can reduce the time you spend eating and primarily addresses the question of “when to eat”.

Calorie reduction addresses the question of “what and how much to eat”. They are separate issues and should not be confused with each other.

Fasting may reduce calories but its benefits extend far beyond that.

Learn more 

Will I lose weight?

Most likely.

It is almost inconceivable that you will not lose weight if you do not eat.

In theory it’s of course possible to eat more after fasting, cancelling out the weight lost. But studies generally show that people tend to end up eating significantly less overall.

I call intermittent fasting “the ancient secret of weight loss” because it might be one of the most powerful dietary interventions for weight loss, yet it has been mostly ignored by doctors and dieticians for a long time.

 Learn more 

Intermittent fasting – questions & answers

By Dr. Andreas Eenfeldt, MD, q&a by Dr. Jason Fung, MD – Updated September 10, 2019

Here you can find common questions about intermittent fasting with answers from our top expert, Dr. Jason Fung.

Choose a topic below or scroll down for every question and answer.

1.                      Who can use intermittent fasting (IF)?

2.                      Different variants of IF

3.                      Things to consider during fasting

4.                      Blood sugar and other tests during fasting

5.                      Dietary advice between fasting periods

6.                      Weight loss problems and IF

7.                      Type 2 diabetes and IF

8.                      Positive effects of IF beyond weight loss and type 2 diabetes

9.                      Potential negative effects of IF and LCHF

10.                   IF and ketosis

11.                   Recommendations on blood tests

Do you have other questions about fasting for Dr. Jason Fung? Watch our in-depth interview with him or ask him directly on our membership site (free trial).

1. Who can use intermittent fasting?

Is fasting an option for children who need to lose weight?

Fasting is not an option for children. My advice is to severely restrict added sugars and snacking. Reducing down to 2 meals per day is also possible, but not longer duration fasting.

My daughter who is 31 and a healthy weight exercises (rowing) four times a week. She wants to know if she can fast or is this not recommended for people who exercise?

Not only is it safe, but training in the fasted state has several theoretical benefits that many elite level athletes are using. So, yes, it is highly recommended.

Can women fast during pregnancy and after birth during breast feeding?

I don’t advise fasting during pregnancy or breast feeding. Short term (<24 hrs) fasts may be OK for some, but definitely not longer term fasts. There is a concern of nutrient deficiency which I think far outweighs any potential benefit.

How should intermittent fasting be used in conjunction with resistance training to maximize muscle growth and fat burning?

Should there be any differences between intermittent fasting on training days and non-training days? And during the fasting periods – or days – is it advisable to take supplements such as BCAA’s to prevent muscle loss?

There are many different schedules. Most people fast for 24 hrs and then do their training – this is called ‘training in the fasted state’. Since growth hormone is high, you theoretically will recover and build muscle faster in this state.

There is minimal muscle loss during short term fasting, so BCAA is not needed, but often used by bodybuilders. It’s efficacy is unknown, with mostly anecdotal evidence. Many athletes follow a schedule of 24 hr fast, then exercise, then breaking the fast with a high protein meal.

How appropriate is intermittent fasting for teenagers?

Not appropriate. Certainly occasional short fasts, (less than 24 hours) are fine but not longer ones. Even most religions do not make children fast because their bodies require more nutrients to grow.

Is fasting still a good idea if you are trying to conceive? Should Intermittent fasting be used instead, or none at all?

You can certainly try although we don’t have good data one way or the other. Fasting should not be used during pregnancy however.

2. Intermittent fasting – different variants

Are there significant differences in benefits from 24-hour fasting vs. multi-day fasting vs. 16:8 fasting?

The main difference, as you may suspect, is that shorter fasting periods are less effective and are usually done more frequently. So a 16:8 fast is often done daily, whereas a 24 hr fasting period is done 2-3 times per week. For more severe insulin resistance, I tend to prescribe longer fasting periods, whereas for maintenance I tend to prescribe shorter ones.

The fasting protocol that fits my life style best is a fasting all day with a 4 to 5 hour eating window in the evening. I feel that I could do this daily during the work week. Is this recommended? How many days a week of IF is healthy?

Periods of fasting less than 24 hours (20 hours fasting, 4 hr eating) or ‘Warrior’ style fasting can be done daily. The term ‘healthy’ always depends upon what your goals are. If you are simply trying to lose weight, then fasting can be done as needed for that. There are no known negative health consequences to eating only during 4 hours of the day.

I have done 18hr, 24hr and 3 days without any real difficulties and have switched it up during the week, depending how I feel and whether I have social plans. Is it a good idea to change the fasting routine regularly, or would I be better off sticking to a 24hr regime for consistency?

I am not aware of any good data on this, but personally, I believe that it is much better to switch things up so that the body does not have a chance to adapt. However, sometimes this inconsistency leads to people not fasting at all, which is also bad.

So it all depends upon your ‘style’. If a regular routine works better for you for compliance reasons, then do so. However, physiologically, I think changing things up all the time works better.

My hubby is very overweight, he weighs 164kg and is 179 cm tall, his fasting glucose is normal between 4.8 and 5.6 mmol/L his insulin is very high, when he was first tested it was 32.2 uU/mL… I know that keeping his insulin low will be the key to his weight loss, but what would be better? A daily 16:8 IF or a 24 hour fast 2 times a week? What would you recommend?

Both are acceptable fasting regimens. It only depends upon which one you prefer and which one gives you the best results. We use both regimens. My own feeling is that the 24 hr fast 2-3 times per week is stronger than the 16:8 regimen, but you must decide which is best for you.

I would like to ask about people who work night shifts? Can you just suggest what pattern of daily fasting might be good ? I think maybe from 14:00-22:00 eating window ( 2-3 meals and than fast during the night ) of course it doesn’t have to fit everyone.

I am also interested about feeding window, isn’t it the same when I eat from 10:00-18:00 as if I eat from 13:00 – to 21:00? when I go to bed 22:00-23:00 ? I mean isn’t it better to eat sooner the day so the energy will be used better or is it just another myth that evening eating leads to weight gain?

People working night shifts often have problems with sleep deprivation and disturbed circadian rhythms. This may result in elevated cortisol that makes it difficult to lose weight. This pathway is separate from the insulin pathway that is typical in most people.

The time of day you eat probably matters, but only a little. Some advise eating in the morning, others at night, so there is little consensus. I think it is more important to try both and see which works best for you, both from your lifestyle perspective and weight loss.

When people use longer fasting regimes, what is the trigger to cease fasting? I have been on an LCHF diet for over six months and have been intermittent fasting (with some food) with very slow results. I finally bit the bullet last weekend and started a fast that has lasted 4 days, each evening telling myself I can eat scrambled eggs for breakfast. I wake up not hungry, so the fast continues (broth, water, coffee with cream and coconut oil). I have lost about 12kgs since last October and have about 20 – 25kgs to go. I’m feeling good (though taking the chicken off the bones after soup making this morning was very tempting) and wonder how long to continue. Until I’ve lost 10kgs? 20kgs? It is a daily decision for me to continue the fast, so I don’t want to ‘commit’ to “until I’ve lost 10kgs more” but to have an idea of what it is possible to pursue would be really interesting. I know you’ve had patients who’ve gone on for quite a while!

There is no set limit. It depends upon how you are feeling and your goals. We have had people go as long as a month, but this is under direct medical supervision. We have many people doing 10 day to 2 week fasts as well, but again, under supervision. There are many people who do a 1 week fast themselves or as part of a ‘cleanse’.

However, I cannot provide personalized advice and you would need to make your own decision with your physician.

I’m very afraid of fasting because every time I tried, I get the common cold. How should I begin the fasting process? Should I start with a shorter fast and then progressing with more and more hours?

I don’t think there is any link. You can certainly try skipping breakfast 2-3 times per week and working upwards from there. Some people prefer to work themselves in slowly, and others to jump in with both feet. Kind of like a swimming pool. Some wade in, others cannonball right in. Your choice.

As a Muslim person I am required to do fasting for 30 days from Thursday, 18th June and I have done so far for three days, fasting between 18 to 20 hours a day. Any tips to maximise the benefits of the fasting for this period. In fact even after Ramadan, fasting period, it is recommended adult still fast 2 days a week, Monday and Thursday. This is almost reiteration of what you said in your interview with Andreas.

Some people tend to overeat and indulge in foods before sunrise and after sundown. I would recommend to not eat before sunrise and eat as normally as possible after sundown. Some studies of Ramadan have shown weight gain during this period for that reason.

I would like to fast for a small amount of weight loss (10 lbs or so) and overall health. I have tried the 24-hour fast twice on different weeks and I was so hungry for dinner I overstuffed myself both times and then proceeded to eat more than usual the next day. Is this overstuffing desire normal? Or is this something that will normalize over time, and if so, how long will it take? Or is my body telling me the 24-hour fast isn’t for me?

You should try to eat as normal a dinner as possible. You will eat slightly more than usual, but as you get used to the fasting, it should get better. This is quite common. Many people will find that the overeating will cause some stomach upset.

3. Intermittent fasting – things to consider during fasting

Can you drink bone broth while you are fasting?

Yes – Bone broth is highly recommended. It contains numerous minerals and vitamins and is quite ‘filling’ in terms of reducing hunger pangs. The other benefit is that you can add a good amount of sea salt to it. The other fluids taken during a fast – water, tea, coffee – don’t have sodium and you can become dehydrated. Mild dehydration, for example may lead to cramps and headaches during longer fasting.

So, yes, bone broth is highly recommended (recipe). Also very ‘Paleo’ in the sense that this is a very traditional food with a long tradition.

While fasting is it okay to drink coffee with milk? I can’t do black coffee. I have a little milk with my coffee.

Technically, the milk does not fall within the guidelines of a true fast, but the small amount of milk/ cream added to coffee improves compliance tremendously for some people. So in our program, we absolutely allow milk/ cream in coffee, but no sweeteners or sugar.

I am sorry for this stupid question but how much is little amount of cream ?:-)))) I want to hear it from you because otherwise I am starting thinking that 250 ml are ok, which I don’t think so:-) I am on my day 3 of fasting and today I had whipped heavy cream ( 33% percent ) 2x 40g in my coffee. Is this too much?

Yeah, I was more thinking along the lines of 1-2 teaspoons, not the whole carton. Because cream still contains milk protein, it will stimulate insulin which defeats the point of fasting. Pure fat, such as coconut oil, will be less.

Do you suggest the use of any liquids other than water during a 16 to 18 hour fast? Is regular black coffee or tea with caffeine ok or does caffeine raise blood sugar?

Tea (all kinds), coffee, and bone broth are all acceptable. I allow a small amount of cream or coconut oil in coffee for compliance sake, although technically, it would not be allowed. I don’t worry about the caffeine content.

What is your opinion on the use of Xylitol for sweetening coffee and tea in the fasting state? Would insulin levels still rise?

There is little information on the sugar alcohols including xylitol. So I don’t really know if they are acceptable or not. However, when in doubt, my position is that these are not traditional, real foods, so best avoided.

I already fasted a few times. My longest run was 14 days some time ago. I’m now fasting again, day 3 right now, feeling terrific by the way. I’m doing all my traveling on the bicycle, so I have some problems with cramps in my legs. Magnesium and 1 tablespoon salt in water really helps but salty water is getting ugly on my tongue… so I found this cup-soup: 0.6g fat, 1g carbs, 0.2g protein, 2.7g salt per portion… am I too paranoid because of the 1g carbs? One in the morning & one in the evening is okay?

If you feel better taking the soup, then, yes, I wouldn’t be too concerned about the small amount of carbs.

What do you recommend for constipation, bloating, heavy stomach? I found adding fibre makes the bloating worse.

Fiber is often our first approach, but if that fails, we often use laxatives such as milk of magnesia or senna tea. Constipation is common and often bothersome. Stool softeners often help with the problem.

When an intermittent fast is complete (say after 16 to 20 hours) , does one then consume that day’s entire allocated macros for dinner? As an example, after a 20 hour intermittent fast would a person then consume a dinner of 1200 calories in the “standard” LCHF ratios of ~5% carbs, ~20% protein, and ~75% fats? Or should the gross number of dinner calories be ratcheted down? I’ve tried the 1200 calorie “break fast” method, and it’s very difficult! But an alternate “break fast” dinner of, say 600 calories at the LCHF macro ratio of 5/20/75 means that one is getting only about 25g protein daily, which becomes questionable if one is doing 20/4 fasting every day.

I don’t recommend counting calories. After the fast, I would try to eat as normally as possible. That would be your usual dinner but perhaps a slightly larger portion of it.

Remember that protein intake on a fasting day will be much lower than normal. On your eating day, you can simply make up for it by taking higher dose, although most people eat at least 3-4x as much protein than what is needed for normal health.

Is drinking two tablespoons of apple cider vinegar (in water) during the fast acceptable, or should I move that to a meal?

Dr. Jason Fung: Yes, both are acceptable. Drinking 2 tbsp at night lowers morning sugars

How about high blood pressure patients, how do you substitute salt for them during fast? Is salted bone broth still good option? Should they be careful about the amount?

In uncomplicated hypertension, I generally don’t advise a low salt diet – just a regular salt diet. With bone broth, the amount of sodium is still going to be far less than what is taken on a regular eating day, so I would not worry.

4. Blood sugar and other tests during fasting

When I fast on just chicken broth my blood glucose drops to 65 and then on the 5th day to 55 mg/dl.
Have you you observed a drop like this in your fasting patients?

We often see this drop during fasting.

Should i worry if my blood sugar while fasting goes up to in the 140’s [Around 8]?

This is quite common, and due to breakdown of glycogen or production of new glucose in response to some of the hormonal changes of fasting. It is neither good nor bad.

Think of it this way. Your body is simply moving sugar from its stores (glycogen and fat) and pushing it into the blood. Here your body has a chance to burn it off.

I purchased a glucose and ketone meter last week. Measured my glucose on Friday after fasting for about 21 hours and the reading was 3.6 mmol with the ketones at 1.9 mmol. It worried me a bit that the glucose was this low, should I be worried? On Saturday I went for a 6 km trail run at a high tempo and my glucose reading afterwards were 2,4 mmol. I would like to stay in ketosis, but my fluctuating glucose readings are making me nervous. I do not experience any of the low glucose symptoms, but I tend to become paranoid when the numbers don’t look right!

Listen to your body. If you feel unwell, then stop. Otherwise, if you are worried, reduce the fasting duration.

5. Dietary advice between fasting periods

Dear Dr. Fung, I have been following your blog and protocols since last July with much success, and have the Insulin Index paper from Marty Kendall. I look forward to your every post.

What do you suggest about using Whey Protein Powders and what is their impact on insulin? Mark Sisson (who sells it) wrote whey can lower insulin yet often heard is that as a concentrated Dairy, whey protein raises insulin.

I advise against using whey powder for the main reason that it is a highly processed chemical, not a real food. Whey stimulates insulin highly, but also is very satiating, so effect on weight likely closer to neutral.

Is it necessary to take a multi vitamin on the HFLC or Ketogenic diet?

If you eat a diet predominantly of whole, unprocessed, real foods, then no, it is not necessary.

Seems the rage today is taking care of the gut flora from both probiotics and prebiotics. Are there foods you recommend that help the gut flora. Some diets like “Perfect Health Diet” recommend “safe starches” such as potatoes and rice to feed the good bacteria in the gut. Do you personally eat rice or potatoes? Can we get sick from not getting adequate amounts of these “safe starches” and can you comment on proper care of the gut?

I know that I will ruffle some feathers, but I am quite skeptical about the gut micro biome theory with regards to obesity. So, there are no food I recommend for that reason.

With regards to rice and potatoes – these are not necessary in any diet. Personally, I eat both, simply because I enjoy them occasionally. However, I am not trying to lose weight. If I were, yes I would restrict rice and potatoes.

It is not carbs per se that cause obesity, but insulin. While starches raise insulin, there are ways to bring it back down. You can add protective factors such as fibre and vinegar. You can also alter meal timing (intermittent fasting) and adding natural fats.

[Editorial note: While fiber, fat and even vinegar can slow down the absorption of some carbohydrates and delay or blunt the resulting glucose rise, this does not mean it completely prevents the glucose and insulin spikes. Furthermore, even though it may delay the spike in blood sugar, it can extend the overall time blood sugar remains elevated. Therefore, we do not recommend this technique as a “safe” way of eating more carbs. If you are absolutely craving that potato and are going to have it one way or the other, then by all means take these steps to make it somewhat “safer”. But to get the benefits of a low-carb diet you are still better off not having it to begin with.]

I use high fat Mayonnaise as extra fat to my meals. It has zero crabs,Protein and 11g of Saturated Fat. However it says in that serving there are 70mg of Sodium. Does this causes any problem?

Except in certain medical circumstances, sodium restriction is not necessary. This was recently shown by Dr. Salim Youssef in the New England Journal of Medicine.

There is ongoing debate about the need for sodium restriction, but for otherwise healthy people, I do not worry about restricting sodium.

Two questions about alcohol and Ketosis/Fasting…
1) I already heard Alcohol “stops” Ketosis. Does it mean that if you drink your body burns the alcohol to get rid of it and then (after the alcohol is gone) immediately continues ketosis?

2) I once made the dumb decision to drink a bottle of red wine during a longer fast on day 6 or 7 (or something like that). I really felt horrible that night but in the morning… no problem anymore. Can drinking alcohol during a longer fast be dangerous? And: does the body ONLY burn alcohol while being drunk? (I remember feeling cold and extremely hungry that night…)

I generally advise patients to stop alcohol during the period that you are trying to lose weight/ reverse diabetes for the simple reason that the body can burn the alcohol for fuel, and we are trying to get the body to burn stored sugar. I’ve also heard it ‘stops ketosis’ for a similar reason.

Its interesting that a lot of low-carb experts talk just about carbs and blood sugar, Its so uncommon knowledge that proteins does stimulate insulin too? And that its not about blood glucose (only) but also about how often you eat whatewer you eat…. (if its not pure coconut fat or lard 🙂 …

Yes, where most people have a carb-centric view of obesity, I have an insulin-centric view of obesity. That is, carbs are a large part of insulin, but not the whole picture.

What do you think about opinions on fibre in the diet being dangerous and able to “tear holes in the bowel wall”? Is too much fiber bad? Also what’s your thoughts on the diets effect on pH acidity/alkalinity of our body?

I don’t think fibre is inherently toxic. Actually, I think that it may help protect against insulin spikes. I don’t think fibre is bad for the gut. I also don’t agree with the acid/alkaline theory.

I am confused by some position on carbs vs net carbs and that fiber should count… what is the bottom line on net or not net carbs?

I am of the opinion that the main issue is high insulin levels. Therefore, fibre, which tends to reduce insulin, should not be counted and think that the net carbs (carbs minus fibre) concept makes good sense.

What if any limitations are there for an individual to follow the LCHF lifestyle if they have had their gallbladder removed.

None that I am aware of.

I keep on experimenting different combinations of fasting and LCHF diet. I have easy to overeat especially after fasting. Is it dangerous to break the fast with a large meal immediately. I am so hungry when I start eating so it easily becomes too much food in the stomach. Get hurt and diarrhea. Can it be unhealthy to the stomach in the long term to go between en empty and a full stomach ?

I don’t believe it dangerous, but it sure is uncomfortable. That is generally why it is easier to plan a small ‘break fast’ meal. A handful of nuts, a small salad and then give yourself at least half and hour to digest.

6. Weight loss problems and intermittent fasting

I’ve been following a strict LCHF diet for 6 months and have lost 20 lbs. I’m now 20-30 pounds from a healthy weight for my female frame (5’1), but my weightloss has stalled. I recently watched your video on fasting and thought I would give it a try since nothing else is working. I’m coming up on 72 hours of fasting and have only lost .7 of a pound! Talk about discouraging! I’m not diabetic, I don’t have high fasting blood sugar, and I don’t have high blood pressure. After day 1 of my fast my morning ketones were at 1.2 mm and after 48 hours of fasting they measured 1.5mm. I just don’t know what to do at this point, I’m beyond frustrated. I know you can’t give specific medical advise but any broad based suggestions or recommendations you can give would be greatly appreciated. Also please know, I’m not delusional about my “healthy” weight goal. I’m not talking about vanity pounds here…I weigh 157 and am only 5’1! My circumference (belly button measurement not sucking it in) is 40!!! Please help!

The initial rapid weight loss seen in fasting is often water. Since you have followed LCHF, you are not losing much of the water weight. After that, it averages 1/2 pound of weight loss per day of fasting. So, your weight loss is a little bit lower than average, but not unheard of. If a pound of fat is 3500 calories and you normally eat 1800 calories, then 2 full days of fasting will produce 1 pound of weight loss. It is possible that your metabolism runs a little slow and therefore you only get 0.2-0.3 pounds of weight loss.

If you are not getting the results that you desire, you can try to change your regimen. Do the fasting on a regular basis. Try longer fasts. Try shorter more frequent fasts. There is no ‘best’ regimen. The key is to try it and find the one that suits you and gives you results.

Since incorporating Intermittent Fasting, my latest A1C came back at 5.6! But, while I have managed to gain complete control over my Diabetes, I am still struggling to lose and maintain any weight loss (I am 5’3″ and 175lbs). In addition to the T2 Diabetes, I am also 50 yrs old and I think both of these factors contribute a negative impact on my ability to lose weight.
Because my blood sugar control is well maintained, I have turned my attention to insulin control to see if this might be my problem. After two weeks, however, I am still only managing to maintain my weight and not lose, which is frustrating.

I have two questions:

1) Will adding a high fat source to a protein meal reduce the insulinogenic effect of the protein meal? For example, if I was eating a 6 oz. steak for dinner, would the insulin response be reduced if I included a high fat item such as avocado or sour cream as opposed to just eating the steak?

2) I know you have stated that the best fasting regimen for each person is the one they can maintain, but if a person is suffering from extreme insulin resistance, would a longer fasting period be more helpful? And, if so, do you believe in that situation a longer fast of at least a week or more would have a significantly better effect than say a 3 day fast every now and then added to regular intermittent fasting?

1. I would replace some of the steak, which is high in protein with the avocado or other natural fat. Protein can still stimulate insulin, as opposed to fat, which does so to a much lesser degree.

2. Longer duration fasts are much more effective than shorter one. We routinely start longer duration fasts for people on higher doses of medications because of their higher insulin resistance.

7. Intermittent fasting and type 2 diabetes

I am a type 2 diabetic off all meds and have been on a LCHF diet for 8 months. When i try fasting for multiple days my blood sugars scores are in the low 60’s and have been taught that this range is to low and need to have some food. Is this normal when fasting and short term affect. Is it normal for sugar levels to be low during fasting? Also ketone levels average per day is 2.5.

If you are taking blood sugar medications, then you must be very careful to avoid low sugars (hypoglycemia). Otherwise, your body should be able to manufacture new glucose during fasting, although in rare cases, some people still develop hypoglycemia.

Sugars should normally be low during fasting and ketones increased. This is the normal situation as ketones are produced as energy for the brain.

Does the presence of ketones in the urine indicate improved (lower) insulin in someone with T2 diabetes when the ketones are the result of ketogenic diet?

Ketones are produced by the body when insulin levels are low for the brain to use as fuel. Some people measure it to make sure that their diet is sufficiently low in carbohydrates

What exactly does eGFR of 80ml/min mean in a type 2 Diabetic and what can the patient do to prevent further kidney disease?

An eGFR of 80 is pretty close to normal. The only way to prevent diabetic kidney disease is to get rid of the diabetes. Anything else is just rearranging chairs on the Titanic

I heard you on one of your talks talking about the new diabetes drug Farxiga, SGLT2. How will that work with fasting with only one meal a day?

The SGLT2 inhibitors get rid of sugar in the urine, which works well with the fasting. With fasting, we encourage the body to burn off the sugar. The drugs make you urinate out the sugar.

There are, however, side effects, as with any drug, especially urine infections, but in general, I think it is a good drug.

I am a type 2 diabetic and am on a LCHF diet and am now fasting in the day, my normal day is I have a litre off water on the way to work with unsalted nuts, approx 50g and drink 2 or 3 black coffees in day and when I get home have dinner, either meat or fish with veg. If I’m am still hungry I will have some blueberries and strawberries with double cream. I have been doing this for a week and a half and have lost 10 lbs and have a blood sugar now of between 6 and 7 and this is reducing each day. Is this approach ok ? What changes to my eating do you recommend?

If you are seeing good results, then clearly this approach is working for you and I wouldn’t change it. In general, I advise following an LCHF diet with intermittent fasting. Your approach clearly qualifies.

Dr Fung. I have really recently discovered you and you’ve been an absolute revelation. My life was full of clouds and you’ve made the sun shine. Thank you so much for what you do.
I’m waiting for an appointment with my doctor (UK – 2 weeks wait) to enquire about any bad effects on my varied medications for heart and kidney problems but I’ve already done one 39 hour fast last Tuesday and am currently on another as I felt so good. I took my “normal” 32u of Lantus but obviously not my usual Humalog. I also, in the absence of any advice, took my other meds. My blood sugar following the fast was 3.6 so did I use too much insulin? What would your advice be? And thank you again in advance.

I cannot legally give you advice for your personal situation, but in our clinic, we try to keep blood sugars in to 8-10 range. This is higher than normal because it is far more dangerous to become hypoglycemic during fasting.

For this reason, we carefully monitor sugars during fasting and we always advise that it is better to take less insulin than more. If you take too little, you can always take more.

If you take too much, you will need to take sugar to reverse the hypoglycemia, which defeats the whole point of fasting in the first place.

What is best fasting regime to help diabetic who had high HbA1c above 10 ?

There is no ‘best’ fasting regimen. More intensive regimens produce faster results, but are not ‘better’. Some prefer to go slow and others fast. Some will find shorter duration fasts better and others find longer is better. Please see my ‘Fasting’ series of blogs on my website for more information.

Hi Dr Fung, I just watched your talk on the key to obesity. Fabulous! My question is about how Metformin works. My understanding is that it stops the liver from making glucose. So, from your talk, it seems that it would be a good thing for the liver to convert its fat into glucose, then wouldn’t the Metformin get in the way of fixing a fatty liver?

Metformin stops the process of gluconeogenesis. So, yes, it does nothing to prevent fatty liver.

I’m on Metformin. I absolutely can’t take this med without food as it will cause me serious gastro issues. The last fast that I did, I simply discontinued the med for the 4 days and then resumed when I started eating solids again. Is that a wise thing to do, or am I potentially causing a problem?

I can only say that we often hold metformin during fasting in our IDM clinic due to GI upset. This is OK as long as blood sugars remain in a good range. I cannot comment on whether you should take them yourself.

I’m a 48 Y0 female, obese, T2, 2000 mg Metformin/day, no insulin or any other meds, in good health otherwise. I have noted that I am unable to produce any appreciable blood ketones UNLESS I fast completely (water, broth, clear tea only) for a minimum of 3 days. Even the 90% fat “Aktin’s fat fast” will not produce blood ketones (I don’t bother with urine dip sticks). I am presuming that this is due to the IR.

1. Is it common for a very IR individual to have to go to this extreme measure to begin producing ketones?
2. If extended fasts (4-ish days) are performed between 2 and 4 times per month, and a LCHF diet is followed when not fasting… in your experience, how long might it take to resolve or at least dramatically improve this level of IR?

1. Ketones are produced by the body to feed the brain instead of using glucose. If you have enough glucose coming out of storage, there is no reason for your body to make ketones. So, yes, that is normal, and likely due to your IR.

2. There is no normal and depends upon the individual. Some people reverse their IR within weeks and others take more than a year.

I have had type 2 diabetes for 17 years, I am now 67. In my studies concerning reversing diabetes, I have come across a couple of statements saying in effect that for those of us who have had diabetes for a long period of time it is more difficult to reverse diabetes? Is this accurate information?

That is certainly our experience. If you are recently diagnosed with T2D, it generally reverses quickly. If you have had it for 17 years, it will take more than a few months at least.

I am type 2 diabetic, and usually my fasting blood sugar is the highest of the day. It will go up 40+ overnight with no food eaten, even a fasting day. I have tried eating something, a boiled egg perhaps, and it does keep the sugar a bit lower in the morning. Is that enough to break my fast, or do you have another suggestion? I have talked to other diabetics who have the same problem, and no helpful advise from their doctors. Vinegar maybe? Any suggestions appreciated.

Yes this is a common problem and is to do with the Dawn Phenomenon. Your body has a surge of ‘counter regulatory’ hormones just before waking that pumps sugar into the blood. This is why your fasting sugar is high despite not eating.

This reflects your body’s insulin resistance. In essence, your body is moving too much sugar from storage into the blood. It just means that your body has too much sugar overall.

You could either continue your fasting schedule and give it more time, or you could increase duration of fasting if you want to speed it up.

In your opinion when to say type 2 dietetes is “reversed”, at what blood glucose numbers?

I consider diabetes to be reversed when you are taking no medication and able to maintain an A1C of less than 6%.

8. Positive effects of IF beyond weight loss and type 2 diabetes

Lots of people get relief from indigestion when they start a LCHF diet. Why is that? I need to convince my nephew that LCHF is his best approach and at 23 years old, nutrition isn’t really his thing so I can’t convince him to read your blog! He gets terrible heartburn and I’m sure it is due to his high carb, high soda diet! (He’s also overweight!)

My guess is that there are two main effects. One, the focus on real food rather than highly processed ones is very important to maintain normal function of the GI system. Two, losing weight often improves symptoms of reflux.

Dr. Fung – I am 70 and overweight, but have lost 30 lbs on the LCHF diet over about 6 months. Initially, my BP also dropped and my doctor took me off my BP meds because of the lightheadedness I experienced with the BP drop. But, now my BP is up again, even with the addition of ramipril, and my weight loss has stalled. I am still at a BMI of about 30. Do you think the 16:8 IF might help both theses issues?

The link between BP and diet is not quite as tight as with blood sugars. I think trying a 16:8 IF sure couldn’t hurt.

In your practice, do you see any reversal of kidney damage for patients who diligently follow IF? If so, is it in certain stages of kidney disease or at certain ages of patients? Or it when they also follow a ketogenic diet when non-fasting? I just wonder if there is any hope of improving CKD once you rid yourself of diabetes T2. Recently I mentioned to my endocrinologist about the mice study that showed full reversal of kidney disease on a ketogenic diet, and he said “But we’re not mice!” Your thoughts, please?

It is quite rare for renal function to improve. It has happened perhaps to one person in 300. While the diabetes (type 2) can be reversed, the kidney function cannot. Even in studies of pancreas transplant of type 1 diabetes, the kidney does not improve for 10 years after normalization.

LCHF diet did not budge my weight and only modestly reduced my Type II Diabetes until I started the Intermittent fasting. I saw my first numbers ever in the 80’s and 90’s. Now my Thyroid has gone bonkers. I’ve had hypothyroidism for maybe 20 years and take synthetic hormones to balance it. The dosage has remained constant over this time. I’ve suddenly become hyperthyroid with my TSH dropping to zero. The doctor reduced the medication to almost nothing. However, my fasting blood glucose has risen back into the 130 – 140 range. I eat no sugar and only low glycemic vegetables and a lot of fat and oil. My liver must be dumping all the glucose into my blood. Have you had any experience with this scenario and do you have any suggestions?

There is not much literature about fasting and change in thyroid dosing, so I can’t help you there, but I have heard several anecdotal cases of the same thing happening where thyroid function suddenly improves. With the high blood sugars, the only thing to do is to continue with your plan. It certainly seems that you are now mobilizing some of that stored sugar.

I am just wondering if it is the same treatment with diabetes 1 and type 2? Low carb high fat?

Yes. Both types of diabetes would improve on LCHF compared to a standard 50% carbohydrate diet. The reasons are entirely different in the two types, though.

9. Potential negative effects of intermittent fasting and LCHF

Is it normal that during fast I am quite cold? like hands and feet … especially feet at night thanks. Hope Its not the sign my REE [Resting Energy Expenditure] is going down.

No, it is not normal to be cold – it means your REE is going down. Try to eat sufficiently on eating days.

I have started intermittent fasting by skipping breakfast. Today i saw this on medXp: In study, skipping meals is linked to abdominal weight gain

I am sure the study was not done with low carb diets so there is that. What do you think?

This was an animal study and I would ignore it. Most studies ‘proving’ that you must eat all the time prove nothing of the kind. Eating all the time to lose weight? I don’t think so.

My health care practitioner is completely on board with LCHF – in fact, advocates it for all patients. I have had limited success in using LCHF in fat loss. Although gaining muscle, my weight loss is minimal after 4 months of solid, no-cheating LCHF plan (mild to moderate ketosis).
I am currently trialling 16:8. My HCP is completely AGAINST fasting, as he claims that there is definitely protein status loss. He didn’t mention starvation mode or anything like that, just a major concern that the level of protein in the body is always affected by IF
What is your take on this? Does protein get used in the same way that fat stores do? If so, can you compensate by eating more protein in the 8 hour window?

There is minimal protein loss is a 16:8 regimen or even after 4 days of fasting. It is only a myth. I cover intermittent fasting in depth on my site starting with part I.

Why would your body store energy as fat if it starts burning protein (muscle) for energy? The answer is that it does not. It is only a myth.

Have there been any adverse effects with the gallbladder and fasting in your experience? I had done a 14 day, 500cal/day, no fat diet and when I went back to low carb diet I had exteme pain in the gall bladder region, when I ate fats again. I am concerened about having this pain reoccur with fasting, as the gall bladder would not be working, (no need to produce bile), as I would not be eating anything. What are your thoughts?

We have not yet had any problems with gallstones or biliary colic during fasting. I have not yet found any discussion of gallbladder problems with fasting, and literally millions of people fast regularly

I read about Problems with intermittent or longer lasting fasting and thyroid problems, especially hypothyroidism (or hashimoto). Can you say something about that?

Fasting does not cause hypothyroidism unless you become severely iodine deficient. Hashimoto’s disease is an autoimmune disease and is not much related to diet, good or bad.

There are many problems that occur with fasting, though. The most common are constipation, headaches, and hunger pangs. There are strategies to deal with these. Many medications can also cause problems on an empty stomach and should be discussed with a physician.

Anything more serious than that is not part of normal fasting and should be investigated.

I have been LCHF/IF for a while…I eat approx. 1200 cal with less than 5% carbs…every third day. Comfortable for me…but can one end up with too many ketones…and what might their effect be?

Unless you have type 1 diabetes, the ketones are normal and are produced by the body for the brain to use instead of glucose.

Does fasting cause loss of lean muscle due to gluconeogensis? Some weight loss studies give patients some protein to prevent lean muscle breakdown. Does this make sense?

No, it does not. I address Fasting physiology in a multi-part series here.

In the first 24 hours after eating, you will use amino acids (from dietary protein) as substrate for gluconeogenesis. Afterward, glycerol from the breakdown of triglycerides provides the substrate. Protein (muscle etc.) is conserved during fasting.

Ask yourself this. Do you believe that Mother Nature designed us to store excess energy as fat, but as soon as we need it, we start burning protein instead of fat? No, protein is conserved and fat is burned for fuel during fasting.

I am not looking for medical advice, just your thoughts on a general situation, what do you think would happen in general, in a hypothetic situation like this. You have somebody who are severly overweight, have been diagnosed with diabetes type-2 and have been on insulin for several years. The next day you remove all their medications and let them fast for 3 days. Off all the medications would such a person experience dangerously low sugars? I know that in un-diabetic people, bloodsugar never goes too low when they fast, even with weeks of fasting. I thought it only was dangerous for diabetics in general to fast if they used drugs, that ramped up their insulin production and they failed to meet this with adequate carbohydrate intake, during a fast.

Yes, in the diabetic situation it is possible to get both high and low blood sugars. This should always be undertaken with physician supervision. As you point out, in the non diabetic, this hypoglycemia does not happen, but in diabetics, with their high baseline insulin levels, it is a strong possibility.

We have seen this before, where diabetics who were taking a very high dose of insulin were able to get off their medications, but became hypoglycemic off all medications. It typically passes in a few weeks time, but in the meantime, clients can become symptomatic.

I just saw that it is possible for a T2-diabetic to get too low blodsugar when fasting even when not taking medicine, which worries me as I live by myself.
I´m 60 and have T2 since 17 years (insulin since 15 years as I had bad side effect from Metformin). Before I started moderate LCHF 2 years ago I had 100 i.u. insulin/day.
After a year and half on moderate LCHF I took 29 units/day. And after having seen your videos I got inspired to quit insulin a month ago, so I had to eat more strict LCHF, eat 2 times a day and I have done a few 24 hour fasting, and the last month I only took 1-3 units a few times a week (I try to keep it under 10mmol).
Bloodsugar during this month has been 7-10mmol/l with hardly no insuline.
I have now been fasting for one day on only water and feel good and I wonder if there is a risk to get to low bloodsugar during sleep.
Is there really a risk of to low bloodsugar when I have taken it so slow and let my body get used to less carbohydrates?
Wouldn´t people with pre-diabetes get to low sugar too then?
So how do a t2 diabetic fast then?
Is there a minimum blood sugar level before bed or should I consume something?

The body will start producing glucose when it gets too low to prevent hypoglycaemia, except if you are taking medications. We do occasionally see hypoglycaemia on people taking no meds, but this is usually when people have just recently stopped insulin and presumably, the mechanism for gluconeogenesis is ‘rusty’. This usually improves within a few weeks although can last longer.

I recently had someone suggest to me that fasting may affect gut bacteria (it seems that everything these days is about the gut bugs…) Have you heard of this?

Everything you eat or do not eat will affect your gut bacteria. Whether it is important is a different question. I tend to think that it is of minor importance, but evidence is so far scarce.

I try to combine 24 hours of fasting with longer periods. 3 days fasts every 2 weeks or 7 day fasts every third week or so. It is very flexible. I feel good during fasts as well. I manage to work, go every day by bicycle and even do my exercise 2-3 times/weak at the gym ( not very regularly). Have no medications or problems with my health either.
Do you have any contra-indications on how often I should fast? Have read about fasts on your blog and even listened to your wonderful lectures on you tube and understands that it is individual and free up the experiment as long as you feel good. Yet, I am a little afraid (even though I am a registered nurse) because I combine short fasts periods (24 hours fasting) with the longer fasting periods quit often. Can it be too much of a good thing and dangerous for my health in a longer turm of time to do my fasts?

If you are feeling well, there is no contraindication, except in children, pregnant and breastfeeding. Also, if diabetic, medications may need to be adjusted.

What you say makes so much sense. I struggle, however, with serious nausea when I try to fast. Is there anything I can do to relieve or prevent nausea?

The most common reason is other medications especially iron and aspirin. Check those. Otherwise, you can try to build up to the longer fasting periods.

I understand that fasting may push up the blood level of norepinephrine. My husband has atrial fibrilllation. Will fasting make him more prone to arrhythmia?

It is certainly possible. If it happens, then he should stop.

I am very interested about your opinion in book It Starts With Food, from Dallas and Melissa Hartwig, Their program Whole 30 is very well known, concretely I am interested about your opinion in two things,
About fasting and about frequency of eating, they recommend to eat 3 times a day, and they suggest to eat as soon as you wake up, even if you are not hungry, which I don’t agree based on information from your blog, … they say that if you are not hungry in the morning its bad and it means your hormones are not in order…

They say that leptin is connected with our consumption and if we start to eat too late that whole process is disrupted then, and leptin and cortisol does bad values then, …..

Also about fasting they are saying that prolonged periods without food (more than 8 hours) are too stresfull and bad for your cortisol control. and that it can raise stress levels.

They say that cortisol then protects the body fat, and etc. My question is do you have some studies about cortisol and fasting? Do you measure cortisol in your clients? I suggest that If I measure blood glucose and it goes low during fasting like to 3,8 molar then I don’t have problems right? its normal not to be hungry in the morning isn’t it?… and I don’t have to stuffed myself with food my body obviously don’t want at that time.

If you are not hungry in the morning, then I think you should not eat. This is called ‘listening to your body’. It doesn’t mean your hormones are messed up, it means your body is telling you that you don’t need to eat. So you should not.

Leptin is not likely affected by skipping breakfast. This sounds like something that is made-up. It is not necessary to eat constantly. Neither does missing a meal increase cortisol. I fail to see how forcing yourself to eat when you are not hungry is going to help you lose weight.

There are actually very few studies on cortisol and fasting and they are mostly contradictory. I do not measure cortisol in our clinic patients. If your problems are cortisol (stress) related, then changing your diet is not going to be very effective. Instead, you need stress relief (meditation, yoga, massage, pain management etc.) I try to stick to diet related issues.

Cortisol is normally raised in the morning as part of the circadian rhythm. This is normal and found in everybody. This helps release some of the stored sugar in the morning to give your body some energy for the upcoming day.

Therefore, you do not ‘need’ to eat because your body has already given you energy. You have plenty of sugar available for whatever you need to do in the morning without eating. Since you are ‘tanked up’, you do not feel hunger. Most of morning hunger is trained through decades of eating as soon as we get up.

Are there any possible connections between kidney stones and ketosis? Do you feel this way of eating can affect blood/urine PH or anything else that would increase the chances of stones? And are their preventive measures that can be taken, while still following the plan?

There is no connection, unless you do not stay hydrated. It is important to consume enough water as well as salt/ electrolytes during fasting. This will help prevent kidney stones.

10. Intermittent fasting and ketosis

I’ve recently read that ketogenic diets can lead to renal tubular acidosis. How concerned should I be? Whats your opinion on this?

I would not be concerned. As a nephrologist, I am referred all cases of RTA and I have perhaps seen only 3 or 4 cases in the last 15 years – all of which were not related to diet. RTA would be amongst the very lowest of my concerns with a ketogenic diet.

11. Recommendations on blood tests

What blood tests do you recommend if I suspect Insulin Resistance/Metabolic Syndrome?

The tests I focus on are the fasting glucose and the hemoglobin A1C. I also test ALT to look for fatty liver and Triglycerides (indicates a diet very high in carbs).

Are you able to provide general information about symtoms that are commonly associated with excess cortisol? How would one find out whether cortisol was a relevant factor?

Cortisol excess can be difficult to diagnose for certain, but most people will know if they are under excessive stress (psychological, physical, chronic pain etc).

More

You’ve reached the end of the Q&A-page. Feel free to return as it’s updated every month with new questions and answers.

Do you have other questions about fasting for Dr. Jason Fung? Watch our in-depth interview with him or ask him directly on our membership site (free trial).

We also have a 45-minute presentation by Dr. Fung on “the key to obesity” – insulin resistance – and how to reverse it. This presentation is on the membership pages (free trial).