Fasting is a practice that involves completely abstaining from eating or avoiding certain foods for a fixed period. People have practiced fasting for centuries, primarily for religious purposes.

In recent years, intermittent fasting has become increasingly popular with people looking to lose weight or improve their health.

There are various methods of intermittent fasting. Typically, it involves consuming few or no calories for 1–4 days per week, then eating a more regular diet on nonfasting days.

Some supporters claim that this style of eating is more sustainable than traditional diets.

In this article, we look at the main methods of intermittent fasting, their possible benefits, and the potential risks.

a woman eating lunch as part of her intermittent fasting diet planShare on Pinterest
A person wanting to try intermittent fasting can choose from a range of plans.

Different diet plans vary in the length of time people should fast and how regularly they fast.

No method is better overall than any other. However, some people might find they can sustain a particular eating pattern more successfully or see better results personally.

The options mean that those looking to try intermittent fasting can find a plan that suits them.

Alternate day fasting

Dr. Krista Varady, an associate professor of nutrition at the University of Illinois, Chicago, created the every-other-day diet, which she based on her research findings.

As its name implies, this plan involves alternating "fast" and "feast" days.

Fasting days consist of a single 500-calorie meal at lunchtime. People do not have to restrict what, when, and how much they eat on feasting days.

Other alternate day fasting plans involve completely abstaining from food every other day.

2 days per week fasting

Developed by Dr. Michael Mosley, the fast diet involves fasting 2 days per week. On fasting days, females eat 500 calories, and males eat 600 calories. People maintain their usual eating routines for the remaining 5 days.

Daily intermittent fasting

Daily intermittent fasting restricts eating to a certain number of hours each day. The 16:8 diet is a common method that means fasting for 16 hours per day, leaving an 8-hour window for eating.

The Leangains method is a plan that uses a 16:8 fasting approach alongside other recommendations. People also refer to daily intermittent fasting as time restricted eating.

Several studies, although moderate in size, have demonstrated the weight loss benefits of intermittent fasting, according to this 2018 review of studies. The study's authors recommend further research before doctors recommend the approach for weight loss.

Advocates of intermittent fasting say the following benefits are possible alongside weight loss.

Longevity

A 2016 review found that years of animal studies showed a link between restriction of calories, fewer diseases, and longer life. Scientists have studied the mechanisms behind those benefits and how they translate to humans.

A 2011 study linked the hormone insulin-like growth factor-1 (IGF-1) to certain diseases that affect lifespan, such as cancer and type 2 diabetes.

Authors of a 2014 study found that eating proteins increases IGF-1 production. Fasting to restrict calories may be a way to decrease IGF-1 levels. This could potentially lower a person's risk of chronic diseases and extend their lifespan.

Cancer

A 2014 minireview reported that restricting calories decreases IGF-1 levels, which may result in slower tumor development.

A 2019 review in people with cancer found that fasting reduced some of the side effects of chemotherapy and increased its effectiveness. The study authors suggested that fasting may deprive cancer cells of nutrients, making them more susceptible to the toxins in chemotherapy.

Doctors do not recommend long-term calorie restriction for people with cancer. Calorie restriction can also be challenging to sustain.

Short-term calorie restriction plans, such as intermittent fasting, may be an option for those who have cancer. However, people with cancer must talk to their doctor and dietitian before starting a fasting diet.

Appetite suppression can be a side effect of cancer treatment, and excessive weight loss can pose a health risk for people receiving treatment.

Neurological diseases

Intermittent fasting may also impact cognition.

According to a 2018 review, animal studies show that restricting calories through intermittent fasting can slow signs of cognitive and motor decline.

The same review suggests that these fasting methods may also help the nerves protect themselves against injury.

While many of these studies have not been in humans, intermittent fasting shows promise as a remedy for some effects of aging. More extensive research is now awaited in human subjects.

Blood sugar

Some studies have shown that intermittent fasting improves insulin sensitivity more than traditional diets, but other studies have not shown the same advantage.

Researchers have also reported that intermittent fasting and traditional diets lead to comparable decreases in hemoglobin A1c.

More research is necessary to confirm the long-term risks and benefits of intermittent fasting. However, it shows a great deal of promise on a short-term basis.

People with diabetes who take insulin or medication should consult with their doctor before starting a diet, as they must maintain their blood sugar levels.

Failure to maintain correct blood sugar can result in low blood sugar. This may lead to a coma or even death.

A 2017 review found that although intermittent fasting produced favorable results in people who had overweight or obesity, it could lead to less desirable effects in those who had a lower weight. These unwanted effects include:

  • changes in mood
  • extreme hunger
  • low energy
  • obsessive thoughts about food
  • overeating on days without restricted calories
  • binge eating behavior
  • tension
  • depression
  • anger
  • fatigue
  • confusion

Most people report these feelings and behaviors in the first few weeks of intermittent fasting.

The same review also highlights that restricting calories in this way may interfere with the female menstrual cycle.

Dr. Mosley does not recommend intermittent fasting for people with the following:

  • underweight
  • eating disorders
  • type 1 diabetes
  • medication controlled type 2 diabetes
  • pregnancy or breastfeeding
  • recent surgery
  • mental health conditions
  • fever or illness
  • conditions for which they take warfarin

Is it good for weight loss?

A 2018 study in male veterans compared the effects of a 5:2 diet to a traditional eating plan, taking into account weight loss and laboratory values. A 5:2 diet means eating a regular diet for 5 days and fasting for 2 days.

Both diets resulted in a similar amount of significant weight loss.

Also, the 2017 review of studies compared intermittent fasting with traditional diets and found similar results. Again, researchers reported that both types of diets led to similar levels of weight loss.

Is it good for muscle building?

Most weight loss plans result in some loss of lean body mass. The same 2017 review of studies found that intermittent fasting and traditional diets result in similar amounts of muscle loss. Exercise and adequate protein intake may help preserve lean body mass in people following this diet plan.

A 2016 study on males following a resistance training program found that intermittent fasting resulted in a significant loss of body fat.

Males following a regular diet did not see a significant change in their body fat levels. Both groups maintained their lean body mass.

Before starting an intermittent fasting diet plan, some people may have questions.

Can I still exercise?

In her interview with The Atlantic, Dr. Varady talks about exercise in people following the every-other-day-diet.

After the first 10 days, their activity levels were similar to people following a traditional diet or an unrestricted eating plan. It may be most beneficial for exercise sessions to end 1 hour before mealtime.

Won't I eat too much on feast days?

According to Dr. Varady, people do eat more than their calorie needs on feast days.

However, they do not eat enough to make up the deficit from fast days. Other researchers report that people unintentionally eat less on nonfasting days as well.

Will I be hungry on fasting days?

Dr. Varady reports that the first 10 days on the every-other-day diet are the most challenging.

Calorie-free beverages, such as unsweetened tea, may help offset hunger.

Do I still fast once I'm ready to maintain my weight?

Some plans, such as the every-other-day diet, also include a weight maintenance phase, which involves increasing the number of calories on fasting days from 500 to 1,000.

Other plans recommend decreasing the number of fasting days each week.

If a person has specific conditions or health requirements, it may benefit them to seek consultation with a doctor or dietitian before starting a restrictive diet.

People interested in trying intermittent fasting should consider whether or not it fits their lifestyle. Fasting stresses the body, so it may not be beneficial for people who are already dealing with significant stressors, such as illness.

Special occasions and social gatherings commonly revolve around food and drink. Intermittent fasting could impact participation in those activities.

Those who train for endurance events or engage in other types of intense exercise should also be aware that intermittent fasting might affect their performance if competition or practice falls on fasting days.

Q:

I’ve tried this and keep eating far too much on the nonrestricted days. How can I reduce my intake of unhealthful foods when I am not fasting?

A:

Slow it down. Take time to eat your meal and be aware of what you’re eating. Are you eating because you’re hungry or are you eating more because you fear that you’ll be hungry in the future?

Gradually increase your intake on “feast” days, rather than hitting up the pantry right after you wake up. Drink a glass of water, have a smoothie, or small meal before stuffing yourself.

Also, make sure to eat a balanced meal on “feast” days. Have nutritious meals and snacks available at home when you are ravenous after fasting.

Miho Hatanaka, RDN, L.D. Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.