All about intermittent fasting


Intermittent fasting (IF) involves relatively brief, intermittent or random periods of not eating. IF may have some benefits for health, longevity, and fat loss, but should be done judiciously.

Note to readers: We wrote this article in 2009. Since then, based on the latest scientific evidence and lots of additional experience, we’ve updated and revised our position on intermittent fasting. We’ve even done some cool experiments on ourselves. To read more about our experiences, and to see some updated recommendations, check out our free e-book on Intermittent Fasting.

What is intermittent fasting?

Fasting is a pretty simple concept: it’s a period of time during which you don’t eat. During some types of fasts, you don’t drink anything either.

For most medical purposes, a person is assumed to be fasting if they’ve gone without food for 8-12 hours.

Intermittent fasting (IF) is a period of food abstinence alternated with a period of eating.  It could include alternate day fasting, or fasting once or twice per week. Common types of IF include:

  • Full-day fasting. For example, “Eat Stop Eat” recommends two days of fasting each week, from one dinner to the next.  It’s a 23+ hour fast.
  • Many research studies have examined alternate day fasting, or ADF, in which research subjects (human or animal) eat normally one day, then don’t eat anything the next. During the eating day, subjects usually eat ad libitum, or as much as they want. This typically results in a caloric surplus on the eating day.
  • Partial-day fasting with an evening “eating window”. An example is the “Fast 5”, in which eaters consume their required calories for the day in 5 hours, fasting the other 19; or the “Warrior Diet“, in which eaters undereat or fast for 20 hours, then deliberately overeat during a 4-hour period in the evening.
  • Partial-day fasting with an “eating window” that is timed around training if possible. The Leangains approach, for instance, is aimed at people with a relatively higher level of activity, and suggests an “eating window” of 8 hours (for men) and up to 10 (for women).

IF vs CR

IF should not be confused with another popular form of modified fasting known as caloric restriction (CR).

This is a diet in which total calorie intake is reduced to around 60-80% of maintenance without a reduction in nutritional requirements. Unlike IF, which as its name implies is done intermittently, CR is done consistently, with reduced calories every day rather than every other day, once or twice a week, or randomly.

To understand the differences between the two approaches, compare three types of caloric intake over the course of a week for a hypothetical person whose maintenance intake is normally 2000 calories per day.

  • With CR (blue bar), they might eat 1500 calories daily, every day, for an average daily intake of 1500 calories over the course of a week. This is 75% of their maintenance intake.
  • With an IF approach, and the goal of caloric restriction (green bar), they might vary their intake from 2000-2300 calories on eating days, and then twice a week (Tuesday and Friday), take in no calories at all. As with the CR approach, this gives them an average of about 1500 calories per day over the course of the week. However, the daily intake varies widely. A recent study suggests that IF-style caloric restriction might work better for people looking to lose weight. It’s easier to reduce calories on alternating days than to do it all the time, every day.
  • If the eater wished to eat at maintenance, they might also vary their calories using an IF-type approach, but eat a significant surplus on the non-fasting days (between 2550-2700) calories a day to make up for the fasting day deficit. This gives them an average daily intake of 2000 calories over the course of the week.

Why is intermittent fasting so important?

Much of the information about fasting is related to spiritual well-being.  Those with a strong spiritual sense claim that forgoing food for several hours allows them to focus on religious pursuits.  Followers of the Bible will recall that Moses and Jesus engaged in fasting for 40 days to replenish the spirit.

One of the fundamental rituals of Islam is fasting during the month of Ramadan, a 4-week period of food and fluid abstinence between the hours of sunrise to sunset.  The length of the fast may vary from 10 to 19 hours each day.  Many studies have observed individuals following Ramadan fasts.

Outside of the spiritual realm, fasting for health and weight management interests the masses, with nearly 14% of U.S. adults reporting the use of fasting to control body weight.

Others are interested in fasting’s potential to manage chronic diseases and increase longevity. For instance, a recent study in the American Journal of Cardiology (Horne et al 2008), which examined a study population of Mormons, suggested that in these groups who regularly fast for religious purposes, fasting might also lower the risk of coronary artery disease and diabetes.

New York Times graphic comparing normal and calorically restricted monkeys.
New York Times graphic comparing monkeys on normal and calorically restricted diets. Click to enlarge.

What you should know

The fasting process

As the body uses up the nutrients from the last meal, it begins to enter the fasted state. Eventually, once it exhausts its immediate fuel stores, it turns to stored fuel for energy.

Since the body has a limited ability to store carbohydrate, it increasingly draws energy from protein (for example, via the breakdown of muscle tissue for amino acids), which results in high levels of ammonia that must be excreted. As fasting time is lengthened, body fat and storage protein becomes a major fuel source.  A high amount of fat breakdown can result in elevated ketones.

Fasting & insulin

With no carbohydrate coming into the body during a fast, insulin secretion is also depressed. Lowered insulin may be one mechanism by which the beneficial effects of fasting occur.

Stages of fuel use during fasting

Fasting outcomes

Physiological highlights of fasting include:

  • Decreased fasting insulin
  • Reduced glucose uptake by muscle
  • Reduced protein synthesis
  • Increased glucagon & catecholamines
  • Increased liver glycogen breakdown
  • Formation of glucose from amino acids
  • Fat breakdown and ketone production
  • Decreased growth hormone
  • Decreased metabolic rate

Fasting & aging

Eating is a form of stress on the body.  It takes energy to digest/absorb food. Metabolizing nutrients creates free radicals and other potentially harmful by-products.

This may, in part, stimulate the processes responsible for aging. The rate at which aging occurs is species-specific, suggesting a genetic influence, so it’s not solely based on food/calories.  Nonetheless, caloric intake seems to affect the rate of aging -– a lowered food intake does appear to slow the aging process.

Intermittent vs. chronic

However, when examining the potential advantages of fasting, it’s important to distinguish between chronic caloric deprivation (as in prolonged starvation studies, discussed further below) and intermittent, brief periods of fasting (e.g. 12-36 hours).

The research suggests that brief, relatively infrequent periods of fasting may be beneficial. Chronic, prolonged fasting, however, may be detrimental. More than 2 days of fasting results in protein loss, decreased visceral proteins (blood cells, internal organs, serum proteins), impaired immune responses and impaired wound healing.

Thus: Although eating is a form of stress, so is long-duration nutrient deprivation.

For example, some of the most comprehensive data on responses to a lengthy period of semi-starvation was collected by Ancel Keys.

During the 1940s, Keys experimented with healthy men at the University of Minnesota.  The study included a 168-day period with the men consuming only 40% of their usual dietary intake. The men began bingeing, became excessively preoccupied with food and developed an otherwise disordered eating pattern.

Voluntary food restriction (e.g. fasting) and extreme dieting can lead to compulsive eating, binges and gorging.  This has also been noted in prisoners of war.

Fasting vs. “-rexia”

It is also important to distinguish fasting as a deliberate, informed religious and/or health choice from disordered eating behaviour such as anorexia nervosa, which is defined as self-starvation for the purpose of attaining an extremely low bodyweight. Health consequences of anorexia include a reduced rate of growth, constipation, stomach rupture, anemia, kidney problems, heart problems, osteoporosis, and death.

Sometimes, the lines get blurry, as in the case of orthorexia. If you’re considering CR or IF for health purposes, it’s worth examining your motivations first.

Impact and effects of fasting

Body composition

Initial body composition plays a major role in determining the length of survival when fasting.  Much of the information on fasting and survival has been obtained from prisoners of war, those on hunger strikes and victims of famine. Observational studies including these population subsets don’t provide the most accurate portrayal of fasting due to confounding health factors such as fear of death, illness and injury.

However, a recent study that looked at the effects of caloric restriction in lean and fat mice found that the two types of mice responded differently. The fat mice benefited from caloric restriction, while the lean mice did not. Such data may bolster at least part of the evidence gathered from these fasting populations.

There have been documented hunger strikes indicating that individuals die after losing a substantial percentage of body weight.

In normal weight mammals, death usually occurs with a loss of ~40% of body weight.  A normal weight individual might be able to survive ~60 days when fasting, while an obese individual might be able to survive ~200 days. Data on the longest fast indicates it was 382 days long and done by an obese man in the United Kingdom who lost 75% of body weight.

Food intake and body weight

Mice placed on alternate-day fasting schedules showed no decrease in overall food intake or body weight.  The animals made up for lost time on the days they were allowed to eat.  However, the mice did improve glucose and insulin levels as well as insulin sensitivity.  Less “brain stress” was also noted.

Studies during Ramadan fasting have shown some weight loss from body fat stores.  Initial weight loss is likely due to body water changes and decreased intestinal bulk.

Other studies using Ramadan fasts show no weight loss.  This may be due to food overcompensation after sunset.  Studies have even shown an increase in overall calorie intake.  Resting metabolism may decrease during a fast in response to energy conservation.  However, fewer meals each day doesn’t always translate into a lower metabolic rate.

Other studies using a very low calorie diet observe weight loss.  But they also notice lower leptin levels, increased cortisol levels, and increased hunger/appetite.


Physical activity patterns don’t seem to significantly change during Ramadan fasts.

Aerobic fitness might be slightly diminished.  There are potential negative effects of fasting on muscle protein synthesis, but these effects might be dissipated with adequate essential amino acid consumption.

Blood chemistry

Favorable changes in good cholesterol (HDL) are noticed during Ramadan fasting.  This might be due to infrequent, large meals.  However, these changes return to baseline levels within one month of a normal eating schedule.  Other favorable changes include a lower homocysteine level and tendency for blood clotting.

An unfavorable change that might occur is impaired morning glucose tolerance.  Yet, in monkeys who undergo CR, glucose regulation seems to improve.

Other forms of modified fasting can decrease total cholesterol (during fasts).  This decrease was due to both a decrease in bad and good cholesterol levels.  Again, when regular eating patterns resumed, cholesterol numbers trended back to baseline.  One study revealed no significant differences in the occurrence of heart attacks or chest pain during times of fasting.


A study including 636 individuals with colon cancer and over 1,000 controls found that among men, those who ate the fewest meals had half the risk of colon cancer compared with the “middle” eating frequency group.  No associations were detected in women.  Fasting also seems to increase cancer resistance in mice.

Aging and the brain

Studies on monkeys introduced CR to the public.  These studies showed that long-term CR disrupted the thyroid and growth hormone axis in monkeys.  While this may sound disturbing, it might actually be why CR works to lengthen life span.  Reproductive cycles can be altered and puberty can also be delayed with restricted nutrition.

Fasting may also protect the brain from age-related neurodegenerative diseases such as Alzheimer’s and Parkinson’s. A recent study that compared caloric restriction to a higher intake of fatty acids (also known to enhance brain function) in older people showed that cutting calories by 30% for three months improved memory significantly. The researchers suggested that caloric restriction’s brain benefits may come from lowered insulin and levels of inflammatory C-reactive protein.

In mice, long-term intermittent fasting may enhance learning.  So, if you have any mice friends taking trigonometry next semester, pass along this article.

What happens afterwards?

As food is re-introduced after a fast, several effects may be noticed.  Electrolytes can become abnormal, some vitamins can be depleted, ketone bodies can increase, edema may occur, and liver function can become abnormal.  Appetite-regulating hormones tend to be low or unchanged.  Many dangers are associated with re-feeding after a fast.

We know that after periods of food restriction, disordered eating patterns may result.  Does this also occur with regular Ramadan fasting?  One study says no. On the other hand, some people may emerge from Ramadan with residual weight gain from the abundant and rich foods consumed during the permitted eating times from dusk to dawn.

Other interesting fasting factoids

Since the days of Hippocrates, fasting was recommended to improve health.  Do you think your nutritionist is a hard-ass?  Check out this quote from my man Hippocrates:

“Obese people and those desiring to lose weight should perform hard work before food.  Meals should be taken after exertion and while still panting from fatigue.  They should, moreover, only eat once per day and take no baths and sleep on a hard bed and walk naked as long as possible.”

In 1557, English poet Thomas Tusser remarked in his work A Hundreth Good Pointes of Husbandrie:

“Make hunger thy sauce, as a medicine for health.”

Acetaminophen (Tylenol) induced liver toxicity is more common in people who fast.

Alternate day fasting may improve asthma in obese individuals.  It may also delay other chronic health conditions.

Some of the physiological and psychological effects of CR that occur in animals may impact human life differently.

Summary and recommendations

Dr. Berardi’s insight:

The idea that calorie restriction can enhance longevity isn’t a new one.  Many animal models have demonstrated that reducing calorie intake by 30-50% can dramatically increase lifespan.

But before dropping your calorie intake by 30-50%, consider the following.

First, there are a lot of questions left unanswered. For example:

Does this information translate to humans?
Are there other ways to live longer besides eating 30-50% less?
How exactly does CR work and can we mimic those other ways?
Are there any risks associated with eating this much less?

Second, the current comparisons between a “typical” diet and a calorie restricted one are limited.

For example, those eating a “healthy”, Precision Nutrition-style diet would be less likely to suffer the same decline and disease that those eating a “typical” diet would face. (Compare, for instance the suggested human-equivalent content of the monkey diets in the image above — not only are calories reduced, but nutrition quality is improved. Thus in that case, it’s not apples to fewer apples; it’s more like apples to fried eggs.)

So yes, if you’re going to keep up your typical North American diet and refuse to make improvements, calorie restriction might be for you. But that might not be a choice you have to make.

After all, chronic calorie restriction isn’t easy. As researcher Dr. Jay Phelan, an evolutionary biologist at the University of California, Los Angeles, and a co-author of a paper that predicts that the maximum life span gain from calorie restriction for humans would be just 7 percent (and more likely 2 percent) suggests:

“Calorie restriction is doomed to fail, and will make people miserable in the process of attempting it.”

(Full article in International Herald Tribune)

In the end, eating well, supplementing intelligently, and subscribing to a life-long exercise program may prove even more powerful than calorie restriction.  And you won’t have to suffer the restriction, deprivation, and, well, scrawniness associated with this approach.

A low body weight presents its own risks: studies show that those with low body weight and low muscle mass are at a higher risk for a loss of independence into their elder years due to the natural losses in bone and muscle that occur.

And here’s another interesting piece of data: studies are showing that the nutritional supplement resveratrol may work through the same pathways as calorie restriction to enhance longevity.

So don’t stop eating just yet. Although calorie restriction works in animals, it’s not yet proven in humans.

And if it does turn out to work in humans, we don’t know how powerful the effects will be.  But more to the point, who cares when calorie restriction is so difficult and carries its own risks, including exchanging quality of life for longevity?

Why not learn the good lifestyle habits associated with smart eating, proper supplementation, and lifelong exercise? That way you can eat, exercise, and be merry while living long and living well.

In my opinion, here’s the best health advice (not coincidentally, this advice also leads to better body composition for most people):

  1. Eat a varied diet.
  2. Eat from a wide selection of natural, unprocessed foods in moderation.
  3. Exercise at least 5 hours per week with a varied training program.
  4. Eat enough calories to support your exercise.
  5. Eat enough calories to produce a slight gain in muscle mass or at least the prevention of muscle loss.
  6. Don’t eat so many calories so that you’re gaining body fat or maintaining fat above 12% for men and 21% for women.
  7. Sleep between 7-9 hours a night.
  8. Have stress management strategies like a hobby, prayer, meditation, yoga, journaling, etc.
  9. If you choose to experiment with IF, read our experiences first, and proceed with caution and informed curiosity.

Ryan’s insight:

We already have one of the largest ongoing food restriction experiments taking place in North America: the cycle of yo-yo dieting.  This pattern results in food preoccupation, overeating, poor health and a focus on calories rather than healthy food.

Intermittent fasting and CR to improve health is unnecessary for most people.  Simply eating the appropriate amount of food to maintain a healthy weight and energy level would be enough for the masses.

Much of the fasting and CR data talks strictly about calories.  But what about nutrients and food?

If North America participated in CR, their version would probably result in scurvy and rickets because the daily menu would consist of 100 calorie packs and low sugar candy.

In the clinical nutrition community, fasting regimens are currently not recommended as a treatment for overweight clients.  This is because of the potential for ventricular fibrillation, lactic acidosis and vitamin/electrolyte deficiencies.  Sudden death syndrome has also occurred during fasting or re-feeding.  Much of the weight loss noticed with a short-term fast is from glycogen, water and does not seem to substantially impact fat mass.

In the end, if you fast for spiritual purposes, that’s great.  If you’re hoping that fasting will improve body composition and health long-term, I wouldn’t jump on board just yet.  Without even thinking about it, most healthy people have a substantial gap between dinner and breakfast.

In essence, someone with healthy meal habits will likely be fasting for about 40-50% of every 24 hour period.  If you usually eat your last meal at 6pm and then your first meal the next day at 6am, that means 50% of your life is already spent fasting.


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