Humans have been fasting throughout our existence, sometimes because food was not available, or as a part of major religions, including Islam, Christianity, Judaism and Buddhism. As hunter-gatherers humans went for stretches when food was not available, our bodies evolved to be able to function without food for extended periods of time.

Intermittent fasting is a term for an eating pattern that cycles between periods of fasting and eating. It is not a “diet” in the conventional sense. It is more accurately described as an “eating pattern.” Intermittent fasting methods may involve daily 16 hour fasts or fasting for 24 hours twice a week.

When you fast, several things happen in your body on the cellular and molecular level, for example, your body changes hormone levels to make stored body fat more accessible.

Other changes that occur in your body when you fast include:

  • The levels of human growth hormone (HGH) skyrocket, increasing as much as 5-fold.  HGH plays a role in decreasing fat and increasing muscle mass.
  • Insulin sensitivity improves, and levels of insulin drop dramatically, making stored body fat more accessible.
  • When you have fasted, your cells initiate cellular repair processes, like autophagy, where cells digest and remove old and dysfunctional proteins that build up inside cells.

Studies on the benefits of fasting have been promising, although not a lot of research has been conducted with humans.  Fasting has been shown to improve biomarkers of disease, reduce oxidative stress and preserve learning and memory functioning. One theory about why fasting provides physiological benefits comes from Mark Mattson, a researcher at the National Institute on Aging.  He theorizes that during the fasting period, cells are under a mild stress.  The cells respond to the stress adaptively by enhancing their ability to cope with stress and, maybe, to resist disease.

Studies show that intermittent fasting can be a very powerful weight loss tool. In one study from 2014, fasting caused weight loss of 3-8% over periods of 3-24 weeks.

According to this study, people also lost 4-7% of their waist circumference, meaning that they lost a significant amount of the harmful belly fat.  Intermittent fasting may also reduce the amount of muscle mass lost compared to continuous caloric restriction.

The bottom line? The main reason this works, is that it helps you eat fewer calories overall. If you binge and eat massive amounts during the eating periods, then you may not lose any weight at all.  This method of weight loss is not a good choice for everyone.  Those who have eating disorders, diabetes, are pregnant or breast feeding, and children should not consider fasting. This is just another tool to put in your health box, not an end-all be-all solution.