What is Intermittent Fasting and Is It for Everyone?

Reviewed by Dr. Jonathan Bonnet, MD, MPH, Judy Singer, RD and Heather Hodson, RD

Over the years there have been several trending wellness strategies aimed at improving health and longevity. One of these strategies that continues to grow in popularity is intermittent fasting.

Among the many purported benefits of intermittent fasting are increased longevity, improved stress resilience, weight loss, and a decreased risk for diseases, including some cancers. But does the evidence really back up these claims? Will everyone benefit from some form of calorically-restricted or time-restricted eating?

Here, we explain how intermittent fasting works, how it may impact your body, and whether or not it can help to optimize your health.

What is intermittent fasting?

There are several forms of intermittent fasting (IF), but the central focus involves giving your body timed and periodic breaks from eating.

Animal and human studies have shown that during periods of fasting, triglycerides (a type of fat found in your blood) are broken down to fatty acids and glycerol, which are then used for energy. The liver also converts fatty acids to ketone bodies into a major source of energy for many tissues in the body, especially the brain.

Essentially, a metabolic switch takes place that replaces the use of glucose (or sugar) as a source of energy for the body with the use of fatty acids and ketone bodies as a source of fuel. These same ketone bodies are generated when following a ketogenic diet and may have health benefits, however, fasting elicits additional adaptations that are thought to be beneficial for health and longevity too.

What are the different types of intermittent fasting protocols?

Some of the most common IF protocols include:

  • Periodic prolonged fasting: Also known as 5:2 intermittent fast, this protocol involves fasting for up to 24 hours once or twice a week and eating regularly for the rest of the week.

  • Daily time-restricted feeding: Restricting eating periods to some predetermined feeding window. The most popular form limits the eating window to 8 hours a day, every day. The exact timing of the feeding window can be flexible; for example, you can choose to eat between the hours of 8am and 5pm one day and 12pm and 8pm the next day. In this way, you can adjust the window to accommodate for social gatherings and events as needed. It is felt that earlier eating periods are likely more beneficial for health than later ones. This method of intermittent fasting may have fewer adverse effects than other protocols.

  • Complete alternate day fasting: Though the exact parameters of this strategy can vary, the basic premise involves alternating eating and fasting days where fasting days involve zero caloric intake.

  • Fasting mimicking diets: A form of a low-calorie, low-sugar, low-protein diet that attempts to recreate the metabolic effects of fasting while still allowing the individual to have some nutrition. These diets generally consist of vegetables, soups, bars, broths, juices, herbal teas, supplements, etc. that provide a very low level of caloric intake.

  • Water only fasting: the most restrictive version of fasting where the only thing consumed is water, often with some minerals during the duration of the fast. Medical supervision is recommended, particularly for those with underlying medical conditions. Do not embark on a water only fast for longer than 24 hours without medical supervision.

Why do people practice intermittent fasting?

Hundreds of studies in animals and several conducted in humans have shown that there may be several benefits of intermittent fasting, including:

  • Extended life-span (though this benefit may vary depending on sex, diet, and genetic factors)

  • Reduced free-radical production

  • Weight loss

  • Improved glucose regulation

  • Increased stress resistance

  • Suppressed inflammation

  • Supports cellular repair and removal of damaged molecules

  • Slows or reverses cellular aging

  • Reduces risk of some diseases (including cancer)

Each person’s goal with incorporating IF into their routine may differ. For example, some at risk for type 2 diabetes or cardiovascular disease may be advised to try IF to try and improve their metabolic or heart health markers. However, metabolic and weight changes induced through intermittent fasting may not be lasting. Some human studies have shown that when a “normal diet” is resumed, health benefits dissipate. In particular, weight loss tends to be short lived. Additional research in humans is needed to fully understand the short and long term benefits and potential risks.

Is intermittent fasting safe?

Research largely suggests that short term IF is safe for most people, regardless of whether benefits are experienced.

Still, there are some groups for which IF may not be safe or recommended, including those with:

  • History of anorexia or disordered eating

  • Uncontrolled hyperthyroidism (overactive thyroid)

  • Advanced cerebrovascular insufficiency or dementia

  • Advanced liver or kidney insufficiency

  • Preconception, pregnancy, or nursing

  • Certain medical conditions, such as Type 1 diabetes

Much of the data we have on the benefits of IF have been collected from studies conducted on animals, not humans. Researchers continue to measure how these results compare with the long-term benefits and effects reported in humans.

What’s more, of the existing clinical research conducted on humans, most subjects have been overweight, middle-aged adults. More research on younger, older, and healthy-weight subjects is needed to determine efficacy and safety for these groups.

If you have any medical condition or take daily medicines for any reason, it is recommended that you speak to your physician first before attempting any type of fast or stopping any medication.

Note: Fasting for periods of more than a handful of days may require purposeful re-introduction of food to avoid refeeding syndrome (marked electrolyte abnormalities leading to heart, lung, and neurological consequences from the large influx of fluid and nutrients). If you are going to consider longer fasts, be sure to speak with a medical provider so that you can do it safely.

Will I benefit from intermittent fasting?

Whether or not you will benefit from IF is an excellent question to ask your provider when discussing your health goals and health history.

Many people will experience some short term health benefits from intermittent fasting. However, sustainability is also key. When choosing the right diet or form of caloric restriction for your needs, finding a strategy that is easy to follow will help to ensure that you continue to reap the rewards.

If you or your provider have any concerns about the impacts of IF on your long-term health, consider creating a customized plan that incorporates multiple checkpoints where you and your provider can discuss the advantages and drawbacks of the restricted eating on your physical and mental health.

Specifically, quality research on the psychological impacts of fasting have yet to be done. If you’re concerned about fasting’s impacts on your relationship with food and eating, reach out to your provider for guidance.

When it comes to finding the right strategies for safely improving your health and relationship with health, Forfend can help. Our whole person exams can help you optimize your approach to whole body wellness.


Effects of Intermittent Fasting on Health, Aging, and Disease. (2019).

Fasting Therapy - an Expert Panel Update of the 2002 Consensus Guidelines. (2013).

Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease. (2019).

Intermittent Fasting: Is the Wait Worth the Weight? (2019).

Refeeding Syndrome. (2021).

Research on intermittent fasting shows health benefits. (2020).

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