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.1 It’s a secret because this potentially powerful habit had until recently in many ways been virtually forgotten especially regarding our health.2

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.3

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.4 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.

Disclaimer: While intermittent fasting has many proven benefits, it’s still controversial. A potential danger regards medications, especially for diabetes, where doses often need to be adapted. Discuss any changes in medication and relevant lifestyle changes with your doctor. Full disclaimer

This guide is written for adults with health issues, including obesity, that could benefit from intermittent fasting.

People who should NOT fast include those who are underweight or have eating disorders like anorexia, women who are pregnant or breastfeeding, and people under the age of 18.

 

 

What is intermittent fasting?

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.5 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.6 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.

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.7 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.


336,876 viewsDo you want to keep watching? Part 2: How to maximize fat burning

 

 


 

Intermittent fasting for weight loss

Balancing eating and fasting

At its very core, intermittent fasting simply allows the body to use its stored energy. For example, by burning off excess body fat.8

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.9 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.

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”).10

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).

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.11

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.12

To restore balance or to lose weight, we may simply need to increase the amount of time spent burning food energy.13 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.14 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.15 However, there are a many potential benefits beyond this, some of which have been known since ancient times.16

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:17

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

Intermittent fasting results

  • Keto and intermittent fasting: "I am completely blown away by the changes"
  • How Gino reversed his type 2 diabetes by doing the opposite
  • Intermittent fasting: down 42 pounds in 14 months

 
More intermittent fasting results
 


 

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.29 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.30

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.31

Learn more about longer fasts

 

 

Intermittent fasting FAQ

Questions and Answers About Fasting

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.32 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.33 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.34 Instead, your body can burn stored energy (like body fat) for energy.

However, for long duration aerobic exercise, eating before exercise may increase performance.35 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.36 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.37
  • 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.38 Taking some extra salt often helps mitigate such headaches.39
  • 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.40 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.41 Instead, hunger comes in a wave. If you simply ignore it and drink a cup of tea or coffee, it will often pass.42

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 3-4.43 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.44

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.45

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.46 Learn more

What are your top tips for intermittent fasting?

Here are the nine top tips, briefly:

  • Drink water
  • Stay busy47
  • Drink coffee or tea48
  • 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.49 It may also increase the effect on weight loss and type 2 diabetes reversal, etc.50
  • 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.51

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.52 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.53 Learn more:

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.54

Other than that, there is no special reason why women should not fast.55 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.56 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”.57 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.58 Learn more

Will I lose weight?

Most likely.59 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.60

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.61 Learn more

More Q&A

Many more questions and answers about intermittent fasting
 


 

How to get started


Now that you know more about intermittent fasting, how do you get started? I suggest following these steps:

  • Decide what type of fast you want to do
  • Decide upon the length of time you want to fast
  • Start fasting. If you do not feel well, or if you have any concerns, then stop
  • Continue all your usual activities outside of eating. Stay busy and live normally. Imagine you’re “eating” a full meal of your own fat
  • Break the fast gently
  • Repeat
Yes. It can be that simple.

Extra video preparation

If you feel that you want more preparation first, then watch our intermittent fasting video course.

If you want to, you can jump straight to the get-started sections of the course:

  • The left one below is about how to do a 24-hour fast (it’s surprisingly simple).
  • The right one is a real challenge, a 7-day fast (are you up for it?).
How to Get Started with Fasting – Dr. Jason Fung
The Power of a 7–Day Fast – Dr. Jason Fung
 

 
 
 

Resources to learn more

 

The intermittent fasting video course

 

Interviews

 

Presentations

Books

Untitled-7Untitled-8diabetes-code-darker-background1

Dr. Fung’s books The Obesity Code, The Complete Guide to Fasting and The Diabetes Code are available on Amazon.62

Website

Dr. Fung’s website: IDMprogram.com

 
  1. Journal of the Academy of Nutrition and Dietetics 2015: Intermittent fasting and human metabolic health [overview article]

  2. Regarding calling fasting a “secret” or “virtually forgotten”: this is debatable as many people do fast regularly, not least for religious or spiritual purposes. But intermittent fasting for health or weight loss has had very limited attention from nutritional authorities in the last few decades:

    The ancient secret of weight loss

    More background:

    Cell Metabolism 2014: Fasting: molecular mechanisms and clinical applications [overview article]

  3. Between January 2010 and January 2019, Google searches for intermittent fasting increased by about 100X or 10,000 percent:

    Google Trends: Intermittent fasting

  4. Intermittent fasting can help people lose weight. Note that many of the studies on this investigate restricting calories to very low levels for a limited time, like one or more days per week (e.g. 500 calories per day). Actual fasting means restricting calories to close to zero for a time, so it may or may not be even more effective.

    JBI Database of Systematic Reviews and Implementation Reports 2018: Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis [strong evidence]

    Obesity Reviews 2017: Short‐term intermittent energy restriction interventions for weight management: a systematic review and meta‐analysis [strong evidence]

    Obesity (Silver Spring) 2016: A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity [moderate evidence]

    The American Journal of Clinical Nutrition 2015: Health effects of intermittent fasting: hormesis or harm? A systematic review [moderate evidence]

    Intermittent fasting and type 2 diabetes:

    JAMA Network Open 2018: Effect of intermittent compared with continuous energy restricted diet on glycemic control in patients with type 2 diabetes: a randomized noninferiority trial [moderate evidence]

    Diabetes Research and Clinical Practice 2016: The effects of intermittent compared to continuous energy restriction on glycaemic control in type 2 diabetes; a pragmatic pilot trial [moderate evidence]

    BMJ Case Reports 2018: Therapeutic use of intermittent fasting for people with type 2 diabetes as an alternative to insulin [case reports, very weak evidence]

  5. Cambridge Dictionary: Starvation is “the state of having no food for a long period, often causing death”.

    Collins Dictionary: “Starvation is extreme suffering or death, caused by lack of food.”

  6. At Diet Doctor, we define intermittent fasting as the absence of eating or drinking significant amounts of calories.

    Another commonly used definition is a reduction in food intake i.e. eating restricted, small amounts. However, when we use the word “fasting”, we’re referring to the absence of eating.

  7. There could be more powerful dietary interventions, compared to not eating at all for a significant period. However, we’re not aware of any, at least not for the treatment of conditions such as obesity or type 2 diabetes.

    No comparative randomized trials have been done, so this statement is based on theory and clinical experience. [weak evidence at best]

  8. International Journal of Obesity 2011: The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomised trial in young overweight women [moderate evidence]

    Cureus 2018: Intermittent fasting: the choice for a healthier lifestyle [moderate evidence]

  9. This statement presumes that the person fasting is not underweight, under the age of 18, pregnant or breastfeeding.

    The American Journal of Clinical Nutrition 2015: Health effects of intermittent fasting: hormesis or harm? A systematic review [moderate evidence]

  10. Nutrients 2018: Regulation and metabolic significance of de novo lipogenesis in adipose tissues [overview article]

  11. Obesity (Silver Spring) 2017: Flipping the metabolic switch: understanding and applying health benefits of fasting [overview article]

  12. There’s no good evidence that eating regularly and often (or snacking regularly) has any benefits. It may be bad for weight loss or metabolic issues:

    British Medical Journal 2019: Effect of breakfast on weight and energy intake: systematic review and meta-analysis of randomised controlled trials [strong evidence]

    Diabetologia 2014: Eating two larger meals a day (breakfast and lunch) is more effective than six smaller meals in a reduced-energy regimen for patients with type 2 diabetes: a randomised crossover study [moderate evidence]

    British Journal of Nutrition 2010: Increased meal frequency does not promote greater weight loss in subjects who were prescribed an 8-week equi-energetic energy-restricted diet [moderate evidence]

    Hepatology 2014: Hypercaloric diets with increased meal frequency, but not meal size, increase intrahepatic triglycerides: a randomized controlled trial [moderate evidence]

    PLOS One 2012: Effects of meal frequency on metabolic profiles and substrate partitioning in lean healthy males [moderate evidence]

    Obesity (Silver Spring) 2012: Effects of manipulating eating frequency during a behavioral weight loss intervention: a pilot randomized controlled trial [moderate evidence]

    British Journal of Nutrition 1997: Meal frequency and energy balance [overview article]

  13. One way to increase the time spent burning food energy is to skip or postpone breakfast. It has been proven that this can result in weight loss:

    British Medical Journal 2019: Effect of breakfast on weight and energy intake: systematic review and meta-analysis of randomised controlled trials [strong evidence]

    Another, more extreme, version is to completely fast every second day. This seems to potentially result in significant weight loss:

    The American Journal of Clinical Nutrition 2005: Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism [weak evidence]

  14. Researchers have found that when people follow calorie-restricted diets, they lose more weight when eating two meals within a narrow time frame than when they eat the same food spread out over six meals.

    Diabetologia 2014: Eating two larger meals a day (breakfast and lunch) is more effective than six smaller meals in a reduced-energy regimen for patients with type 2 diabetes: a randomised crossover study [moderate evidence]

  15. See references below.

  16. Nutrition 2017: Unraveling the metabolic health benefits of fasting related to religious beliefs: a narrative review [overview article]

  17. The evidence for most of these benefits, beyond weight loss, is not conclusive. This is due to a lack of studies.

    Existing high-quality (RCT) studies of intermittent fasting almost always assume a limited amount of calories (e.g. 500 kcal) a day for fast days. True fasting with no eating or calories for a similar specified time may or may not be more effective – there are no good studies to prove this.

    The common intermittent fasting method of 16:8 (fasting for 16 hours per day) has not been tested in high-quality studies. However, advice to skip breakfast is similar, and has been tested with resulting weight loss.

    British Medical Journal 2019: Effect of breakfast on weight and energy intake: systematic review and meta-analysis of randomised controlled trials [strong evidence]

    The evidence for the effects of longer full fasting periods is limited to theories, anecdotes and fairly extensive clinical experience. [weak evidence]

  18. Intermittent fasting can help people lose weight. One example of a slightly longer daily fasting period is to just skip breakfast. This has been shown to result in weight loss:

    British Medical Journal 2019: Effect of breakfast on weight and energy intake: systematic review and meta-analysis of randomised controlled trials [strong evidence]

    Most longer studies on fasting investigate restricting calories to very low levels for a limited time, like one or more days per week (e.g. 400 calories per day). Actual fasting means restricting calories to close to zero for a time, so it may or may not be even more effective.

    Obesity Reviews 2011: Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss? [strong evidence]

    International Journal of Obesity 2011: The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomised trial in young overweight women [moderate evidence]

    Obesity (Silver Spring) 2016: A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity [moderate evidence]

    Cureus 2018: Intermittent fasting: the choice for a healthier lifestyle [moderate evidence]

    Diabetologia 2014: Eating two larger meals a day (breakfast and lunch) is more effective than six smaller meals in a reduced-energy regimen for patients with type 2 diabetes: a randomised crossover study [moderate evidence]

    Journal of Translational Medicine 2016: Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males [moderate evidence]

  19. Journal of Translational Medicine 2016: Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males [moderate evidence]

  20. Nutrition Reviews 2014: Time-restricted feeding and risk of metabolic disease: a review of human and animal studies [moderate evidence]

    The British Journal of Nutrition 2008: Effect of Ramadan fasting on some indices of insulin resistance and components of the metabolic syndrome in healthy male adults [weak evidence]

    PloS One 2012: Effects of meal frequency on metabolic profiles and substrate partitioning in lean healthy males [weak evidence]

  21. To our knowledge, there are still no randomized trials exploring the effect of true (zero calorie) intermittent fasting. Here are the best existing studies:

    BMJ Case Reports 2018: Therapeutic use of intermittent fasting for people with type 2 diabetes as an alternative to insulin [very weak evidence]

    JAMA Network Open 2018: Effect of intermittent compared with continuous energy restricted diet on glycemic control in patients with type 2 diabetes: a randomized noninferiority trial [moderate evidence]

    Diabetes Research and Clinical Practice 2016: The effects of intermittent compared to continuous energy restriction on glycaemic control in type 2 diabetes; a pragmatic pilot trial [moderate evidence]

  22. Nature Reviews Neuroscience 2018: Intermittent metabolic switching, neuroplasticity and brain health [overview article]

  23. In this study, intermittent fasting combined with calorie restriction improved mood and vigor in aging men:

    The Journal of Nutrition Health and Aging 2013: Efficacy of fasting and calorie restriction (FCR) on mood and depression among ageing men [moderate evidence]

    Nutrition 2017: Unraveling the metabolic health benefits of fasting related to religious beliefs: a narrative review [overview article]

  24. This study only lasted for a few days and it’s unclear how long the effect would last:

    Nutrition, Metabolism and Cardiovascular Diseases 2013: Randomized cross-over trial of short-term water-only fasting: metabolic and cardiovascular consequences [moderate evidence]

  25. Cureus 2018: Intermittent fasting: the choice for a healthier lifestyle [systematic review of randomized trials; strong evidence]

    Scientific Reports 2015: Effects of weight loss via high fat vs. low fat alternate day fasting diets on free fatty acid profiles [moderate evidence]

  26. This effect remains to be proven in humans:

    Aging Cell 2019: Programmed longevity, youthspan, and juventology [overview article]

  27. This is a discovery that was awarded the 2016 Nobel Prize in medicine:

    Aging Research Reviews 2016: Impact of intermittent fasting on health and disease processes [overview article]

    Cell Metabolism 2014: Fasting: molecular mechanisms and clinical applications [overview article]

  28. Free Radical Biology and Medicine 2007: Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma [weak evidence]

  29. JBI Database of Systematic Reviews and Implementation Reports 2018: Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis [strong evidence]

    Obesity Reviews 2017: Short‐term intermittent energy restriction interventions for weight management: a systematic review and meta‐analysis [strong evidence]

  30. Longer fasts require significant reserves of excess stored body fat, and should probably not be done by lean people. Learn more

    Postgraduate Medical Journal 1973: Features of a successful therapeutic fast of 382 days’ duration

  31. European Journal of Endocrinology 2007: Metabolic and hormonal changes during the refeeding period of prolonged fasting [weak evidence]

  32. However, results from studies are not entirely consistent. One reason might be that many “intermittent fasting” studies actually test intermittent severe caloric restriction, i.e. they allow a limited amount of food during fasting. Depending on the food that is eaten, this might have less positive effects on metabolic rate.

  33. In a 2016 randomized, controlled study, obese adults were assigned to either fast every other day or eat a calorie-restricted diet every day. Those in the intermittent fasting group showed less slowing in metabolic rate during the 8-week study and greater improvement in body composition after 32 weeks of follow up compared to people in the calorie-restricted group:

    Obesity (Silver Spring) 2016: A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity [moderate evidence]

    American Journal of Clinical Nutrition 2007: A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults [moderate evidence]

    American Journal of Clinical Nutrition 2000: Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine [weak evidence]

    The American Journal of Clinical Nutrition 2005: Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism [weak evidence]

  34. Scandinavian Journal of Medicine & Science in Sports 2018: Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis

  35. Scandinavian Journal of Medicine & Science in Sports 2018: Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis [meta-analysis of randomized and non-randomized studies; moderate evidence]

  36. BMC Complementary and Alternative Medicine 2018: Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting [overview article]

  37. Obesity Reviews 2014: Do ketogenic diets really suppress appetite? A systematic review and meta-analysis [strong evidence]

    Obesity 2007: The effects of a low-carbohydrate ketogenic diet and a low-fat diet on mood, hunger, and other self-reported symptoms [moderate evidence]

  38. This is based on clinical experience. [weak evidence]

  39. Intermittent fasting results in sodium loss:

    The American Journal of Medicine 1971: Fasting—a review with emphasis on the electrolytes [overview article]

  40. Endocrine Practice 2005: The dawn phenomenon revisited: implications for diabetes therapy [overview article]

  41. International Journal of Obesity 1990: Hunger/craving responses and reactivity to food stimuli during fasting and dieting [weak evidence]

  42. This is primarily based on clinical experience. [weak evidence]

  43. In a recent study of more than 1,400 people who fasted for anywhere from 4 to 21 days, 93% of participants reported that they were not hungry during the study period:

    PloS One 2019: Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects [weak evidence]

  44. Nutrition Journal 2013: Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial [moderate evidence]

    Research suggests that when obese people fast, hormonal changes occur that lead to a decrease in total body protein breakdown, which includes muscle:

    American Journal of Public Health and the Nation’s Health 1968: Influence of fasting and refeeding on body composition [weak evidence]

    Journal of Clinical Endocrinology and Metabolism 1983: Whole body protein breakdown rates and hormonal adaptation in fasted obese subjects [weak evidence]

  45. The American Journal of Clinical Nutrition: Loss of body nitrogen on fasting [non-randomized trial; weak evidence]

    International Journal of Obesity: Thyroid hormone changes in obese subjects during fasting and a very-low-calorie diet [randomized trial; moderate evidence]

  46. We consider consistent clinical experience from experienced practitioners weak evidence.

  47. This recommendation is based on experience from experienced clinicians. [weak evidence]

  48. Journal of the American College of Nutrition 2012: Coffee, hunger, and peptide YY [weak evidence]

    The Journal of Nutritional Biochemistry 2011: Antiobesity effects of green tea catechins: a mechanistic review [overview article]

  49. Low-carb diets have been shown to reduce hunger:

    Obesity Reviews 2014: Do ketogenic diets really suppress appetite? A systematic review and meta-analysis [strong evidence]

  50. PloS One 2015: Dietary intervention for overweight and obese adults: comparison of low-carbohydrate and low-fat diets: a meta-analysis [strong evidence]

    BMJ Open Diabetes Research & Care 2017: Systematic review and meta-analysis of dietary carbohydrate restriction in patients with type 2 diabetes [strong evidence]

  51. This recommendation is based on experience from experienced clinicians. [weak evidence]

  52. The old idea that breakfast is important for health or weight control is mainly based on observational studies, a notoriously weak form of evidence.

    When tested this idea does not appear to hold up, at least not for weight loss. A recent meta-analysis of randomized clinical trials found that people assigned to skip breakfast ate less overall and lost more weight than those assigned to eat breakfast daily:

    British Medical Journal 2019: Effect of breakfast on weight and energy intake: systematic review and meta-analysis of randomised controlled trials [strong evidence]

    Even the observational data is inconsistent, for example with findings like in the study below: “compared to breakfast eating, skipping breakfast was significantly associated with better health-related quality of life and lower perceived stress.”

    International Journal of Environmental Research and Public Health 2018: Eat or skip breakfast? The important role of breakfast quality for health-related quality of life, stress and depression in Spanish adolescents [weak evidence]

  53. Obesity (Silver Spring) 2013: The internal circadian clock increases hunger and appetite in the evening independent of food intake and other behaviors

  54. The Journal of Clinical Endocrinology & Metabolism: Effects of fasting on neuroendocrine function and follicle development in lean women [randomized trial; moderate evidence]

  55. This study is sometimes used as evidence that women may have a different response than men to intermittent fasting when it comes to the glucose response to a meal. We’re unconvinced. The study was very small with few participants and this difference may have been random.

    Obesity 2005: Glucose tolerance and skeletal muscle gene expression in response to alternate day fasting [weak evidence]

  56. International Journal of Obesity 2011: The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomised trial in young overweight women [moderate evidence]

    Cureus 2018: Intermittent fasting: the choice for a healthier lifestyle [moderate evidence]

    American Journal of Clinical Nutrition 2007: A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults [moderate evidence]

  57. Note, however, that many studies of intermittent fasting actually test an intermittent severe caloric restriction. So depending on the definition of the term “intermittent fasting” there may be some connection. We primarily recommend true intermittent fasting, i.e. not eating at all during fasting periods, as this may be more effective.

  58. This study found that average daily calorie intake were significantly lower in alternate day fasting group (24h fasts) compared to calorie reduction group:

    Obesity (Silver Spring) 2016: A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity [moderate evidence]

    Benefits of intermittent fasting

  59. Obesity (Silver Spring) 2016: A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity [moderate evidence]

    Journal of Translational Medicine 2016: Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males [moderate evidence]

  60. Here’s an example:

    British Medical Journal 2019: Effect of breakfast on weight and energy intake: systematic review and meta-analysis of randomised controlled trials [strong evidence]

  61. Obesity (Silver Spring) 2016: A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity [moderate evidence]

  62. Diet Doctor will not benefit from your purchases. We do not show ads, use any affiliate links, sell products or take money from industry. Instead we’re funded by the people, via our optional membership. Learn more