In general, adopting a fasting protocol is safe for most people. However, there are some individuals who should avoid fasting and instead concentrate on maintaining a balanced diet and avoiding overeating or eating too often.

WHO SHOULD AVOID INTERMITTENT FASTING?

Consult a qualified healthcare professional before changing your diet or exercise routine, including adopting an intermittent fasting routine.

If you happen to be pregnant, breastfeeding, over the age of 65, have a history of disordered eating, have a medical condition, or are currently taking medication, it's important to consult with your healthcare provider before considering intermittent fasting as an option.

Longevity

During fasting, cells undergo an adaptive stress response that may account for its many beneficial effects.

Alternate-day fasting improves markers of oxidative stress, a measure of longevity.

Intermittent fasting has been shown to protect against many age-related diseases, including diabetes and cardiovascular disease.

Early time-restricted feeding (in which the last meal of the day is eaten before late afternoon) can lead to an increase in sirtuin (an essential protein family associated with longevity and aging) and autophagy expression (the clearing of unhealthy cells).

Routine long-term intermittent fasting may result in greater longevity.

gut microbiome

The effects of intermittent fasting on the microbiome of the digestive tract have received little attention.

When you consider how quickly changes in nutrition may impact the microbiome, it is easy to see how fasting could have a comparable impact.

You can start with the sleep-wake cycle because it is the most familiar form of fasting. Most people eat multiple meals throughout the day, typically beginning with breakfast and finishing with supper or a late-night snack.

It is recommended that they sleep for eight hours, during which time they will have unintentionally fasted. In this state, the gut microbiota can recover from its hard work of digesting food and creating vital molecules.

Evidence suggests that our gut microbiomes and circadian rhythms are interconnected. The bacteria in our bodies affect our regular sleep schedule and vice versa. Consequently, sleep deprivation affects the microorganisms in the digestive tract.

The population and variety of the gut microbiome rely on the brief fasting phase known as sleep. This knowledge will help lay the groundwork for a future argument that prolonged fasting may be more helpful than regular fasting while we sleep.

Inflammation

Research suggests intermittent fasting can be beneficial for inflammation. A study conducted on 110 volunteers celebrating Ramadan—a holy month for Muslims when no food or drink is ingested from dawn to dusk for 30 days in a row—found that prolonged intermittent fasting positively affects the inflammatory response of the body.

HEART HEALTH

A study conducted on 101 overweight and obese adults with pre-diabetes found that three weeks of either time-restricted fasting (16/8) or alternate-day fasting reduced triglyceride and glucose levels. A separate review of alternate-day fasting trials from 3-12 weeks long found that body weight, body fat, total cholesterol, and triglycerides were lowered in normal-weight, overweight, and obese humans.

Blood Glucose & Weight-loss

Body weight is impacted by numerous factors, from your hormones to your blood sugar. In human studies, intermittent fasting has been shown to help prevent insulin resistance and leptin resistance, which may assist with weight management.

Furthermore, a review of intermittent fasting programs determined that alternate-day and whole-day fasting trials effectively reduced body weight, body fat, and total triglycerides.

Blood glucose and insulin go up when you eat and down when you don’t eat (barring type 1 or 2 diabetes)

What you eat has a differential impact on the rise in blood glucose (e.g., simple carbs and sugars will raise it the most, grains will raise it more than fibrous carbs like lettuce, etc.)

High blood glucose is correlated with mortality

Almost everyone has an increased resting blood glucose level as we age

Hormone Health

In endurance athletes, time-restricted eating led to a significant decrease in free testosterone (not to the extent that it should be avoided unless you have low testosterone at baseline)

Studies in athletes have shown significant reductions in cortisol due to time-restricted eating.

Shortening the feeding window to less than 8 hours can increase stress hormones and pro-inflammatory hormones and decrease some sex hormones.

Effects on fertility: reducing food intake or food calories too much can have a negative impact on the ovulation cycle in women and sperm count in men because signals in the brain are looking for good reproductive conditions without stress

Alternate-day fasting (eating one day, fasting the other) can lead to weight loss and positively affect glucose control – but it’s less likely to sustain over time.

Time-restricted eating + caloric restriction creates the best hormonal environment for fat loss.

Other Well-Researched Benefits Of TRE

If the main goal is to lose weight (i.e., weight loss, not health), while the quality of your food does matter, fat loss will occur as long as the calories burned to exceed the calories consumed.

Many factors impact calories burned (e.g., resting metabolic rate, hormones, exercise, non-exercise activity thermogenesis, etc.)

Time-restricted eating anchors the gene systems of the body and provides a more stable circadian rhythm.

Benefits you can’t see: regulation of liver health, inflammation, decreased blood pressure, reduced oxidative stress, enhanced healthy microbiota, clarity of mind

Fasting during sleep is critical for longevity: during sleep, we’re undergoing cellular processes related to reducing inflammation and clearing out debris from the brain that clears dementia.

Defining Various fasting protocols

The overnight fast

The standard fast- (12 hours minimum) that almost by definition everybody does between dinner and breakfast. Since about two-thirds of the “fasting” period would be spent sleeping, this plan is often appealing to beginners. Example: Finish dinner at 7 pm, eat breakfast at 7 am the following day = 12-hour overnight fast

Time-restricted feeding (TRF or 18/8)

All calories are consumed during an 8-hour window, followed by 16 hours of fasting. This fasting schedule may appeal to those not hungry early in the day.

The idea is to increase that window from an overnight fast into a slightly longer one.

To put the overnight fast example used above into time-restricted feeding terminology, you would call it a 12/12 (meaning 12 hours of fasting followed by a 12-hour window)

You can extend that and say a 14/10 window, a 16/8 window, or an 18/6 window.

The first number refers to the period of time that has no nutrient exposure and then followed by the window in time in which one would be able to consume food (typically ad libitum, aka without restriction)

Alternate day fasting (ADF)

You fast every other day and usually eat on non-fasting days.

Generally speaking, ADF means that every other day you do something other than eating ad libitum

Most strict ADF = Consume nothing everything other days (this would be challenging to maintain over the long haul)

More common versions of ADF = every other day you have a hypo-caloric day

For example, 1,000 calories on the “fasting” day and then eat ad libitum (or their average calories amount) on the non-fasting day

We could also refer to this as “alternate day calorie restriction” instead of ADF

Intermittent fasting (IF)

This term has become popular with the public

But many people mistake IF for TRF… in other words, they say intermittent fasting, but they mean time-restricted feeding.

Peter describes IF as undergoing a fast (at least 24 hours) at some frequency.

For example, once every month (or every quarter, or every six months, etc.), you would go an extended period (>24 hours) without consuming calories.

Fasting Mimicking Diet (FMD)

The FMD is a trademarked protocol coined by a company called L-Nutra which sells a prepackaged five-day “fast,” and it’s based on the work of Valter Longo

This product is ~900 to 1000 calories on the first day and ~700 to 750 calories for days two through five

Prolonged fasting/multiple-day fasting

This one is for more experienced fasters.

You can do prolonged “fasting” via a reduced-calorie “fast” (e.g., the five days Fast-Mimicking Diet)… anywhere from three to seven days, typically at about 500 to 700 calories a day (with a customized macronutrient profile)

–An actual fast is water only (plus minerals):

A “true” prolonged fast would be nothing but water (but also includes tea and some minerals with zero calories) for a period of time (typically three to seven days at a time)

Mineral (sparkling) water is also okay; while there is some debate, I find that Black coffee and tea are OK.

However, if you are pretty caffeine dependent, I recommend NOT giving up your coffee because fasting can be hard enough without dealing with caffeine withdrawals.

Eliminating coffee during a fast might improve the “gut rest” you get from an actual fast.

But some observers have suggested that coffee actually enhances autophagy late enough in the fast (though Peter is skeptical about that)

WEEKLY 24-HOUR FAST

This approach involves fasting for more extended periods once a week.

WARRIOR DIET

Like OMAD (one meal a day), the Warrior Diet involves fasting for 20 hours daily and consuming a large meal in the evenings. Small amounts of certain foods (including fruits, vegetables, and broth) are permitted during intermittent fasting hours.

I encourage you to experiment with a protocol that works for you! Not only are the health benefits numerous, but working in a regular fasting protocol also builds discipline, which translates to all areas of your life.