Fasting is a practice that involves completely abstaining from eating or avoiding certain foods for a period of time. It has been practiced 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, some of which are described below. Generally, it involves very low or no calorie intake for 1 to 4 days per week, then eating normally on non-fasting days. Supporters claim that this style of eating is more sustainable than traditional diets.

Fun facts on intermittent fasting

  • There is debate among researchers about the best method of intermittent fasting.
  • Intermittent fasting may be more effective for fat loss than muscle building.
  • Fasting may influence cancer progression, as well as a response to cancer treatment.
  • Studies have shown that people following an intermittent fasting plan can also adhere to a moderate-intensity exercise program.

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Intermittent fasting involves allotted periods where little to no food is consumed.

Plans vary in the frequency and length of fasts. Dr. John Berardi, an expert on exercise physiology and nutrient biochemistry, experimented with various intermittent fasting plans. He published his results in Experiments with Intermittent Fasting. Below is a summary of some fasting plans.

Alternate day fasting

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

Two days per week fasting

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

Daily intermittent fasting

Daily intermittent fasting limits eating to a certain number of hours each day. The 16:8 diet is a common method. It involves fasting for 16 hours per day, leaving an eight-hour window for eating. The Leangains method is a plan that utilizes a 16:8 fasting approach, in addition to other recommendations. Daily intermittent fasting is also commonly referred to as time-restricted eating.

Advocates of intermittent fasting say the following benefits can be achieved:

Longevity

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Some studies have linked intermittent fasting to a longer, more healthful life.

Some researchers report that years of animal studies have shown a link between restriction of calories, fewer diseases, and longer life. Scientists have studied the mechanisms behind those benefits and their translation to humans.

Insulin-like growth factor-1 (IGF-1) is a hormone linked to certain diseases that affect lifespan, such as cancer and type 2 diabetes. Some experts report that eating increases IGF-1 production. Fasting may be a way to decrease IGF-1 levels, which could potentially lower risk of chronic diseases and extend lifespan.

Cancer

Studies have reported that restricting calories decreases IGF-1 levels, which results in slower tumor development. A very small study in people with cancer found that fasting reduced some of the side effects of chemotherapy, including fatigue, nausea, vomiting, and diarrhea.

Due to potential negative effects, long-term calorie restriction is not recommended for people with cancer. Short-term calorie restriction, such as intermittent fasting, may be an option for this group of people.

Neurological diseases

Intermittent fasting may also impact cognition. In a study of mice with genes for Alzheimer's disease, intermittent fasting improved performance on measures of cognitive decline associated with aging.

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.

Intermittent fasting is not appropriate for everyone. According to Dr. Michael Mosley, intermittent fasting is not recommended for people with the following conditions:

  • being underweight
  • eating disorders
  • type 1 diabetes
  • type 2 diabetes that is controlled by medication
  • pregnancy (or women who are breast-feeding)
  • recent surgery
  • mental health conditions
  • fever or illness
  • conditions for which Warfarin is prescribed

Drawbacks of fasting discussed in Experiments with Intermittent Fasting include:

  • changes in mood
  • extreme hunger
  • low energy
  • obsessive thoughts about food
  • binge eating behavior

However, most people report these feelings and behaviors in the first few weeks of intermittent fasting.

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Intermittent fasting has shown a similar effect on weight loss to traditional diets.

A study in male veterans compared the effects of a 5:2 diet (eating normally 5 days, fasting 2 days) versus a traditional diet on weight loss and laboratory values.

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

A review of studies comparing intermittent fasting with traditional diets found similar results. Researchers reported that both types of diets led to similar amounts of weight loss.

Most weight loss plans result in some loss of lean body mass. A 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 dieters.

A study in men following a resistance-training program found that intermittent fasting resulted in a significant loss of body fat. Men following a normal diet did not have a significant change in their body fat. Both groups maintained their lean body mass.

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 estimated calorie needs on feast days. However, they do not eat enough to make up the deficit from fast days. Other researchers report, however, that people unintentionally eat less on non-fasting 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 consumed on fasting days from 500 to 1,000. Other plans recommend decreasing the number of fasting days each week.

People interested in trying intermittent fasting should consider whether or not it will work with their lifestyle. Fasting stresses the body, so it may not be beneficial for people already dealing with significant stressors.

Special occasions and social gatherings commonly revolve around food. 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 when competition or practice falls on fasting days.