Fasting means deliberately going without food, drink, or both. It can mean:
Going without any food.
Going without certain types of food.
There are many kinds of fasting:
Medical fasting: You may be asked to fast for a certain amount of time before a medical test or surgery, often 8 hours or more.
Religious fasting: Many people fast as part of their religion. Religious fasting may involve eating nothing on certain days, eating nothing from sunrise to sunset for a month, or not eating meat, dairy, and eggs for several weeks at a time.
Fasting to detoxify the body: Some people use fasting as a way to rid the body of toxins. There is no evidence that going without food for a period of time does this, though.
Is fasting a good way to lose weight?
No, it isn't. Here's the problem with fasting (eating nothing) for days at a time: Because your body isn't getting fuel (food), it goes into survival mode and slows your metabolism. Your body doesn't burn calories as fast as it did. You may lose weight, but it's mostly water and muscle, not fat.
Then when you return to normal eating, the water weight you lost during fasting comes back. You may even gain extra weight because your slower metabolism doesn't burn calories as fast as it used to.
Some people use alternate-day fasting as a way to lose weight. They eat nothing every other day. This only helps you lose weight if you can control your hunger on your "eating" days and don't overeat. That's because no matter when you eat or don't eat, you only lose weight when you eat fewer calories than your body needs.
Can fasting be good for your health?
There is much debate over the health benefits of long-term fasting (eating only broth, juice, or water for days at a time). There isn't enough evidence to support the health claims.
That type of fasting can actually be dangerous, especially for people who have other medical problems.
Bloomer KG, et al. (2010). Effect of a 21-day Daniel fast on metabolic and cardiovascular disease risk factors in men and women. Lipids in Health and Disease, 9: 94. Also available online: http://www.lipidworld.com/content/9/1/94.
Whitney E, Rolfes SR (2011). Metabolism: Transformations and interactions. In Understanding Nutrition, 12th ed., pp. 205–229. Belmont, CA: Wadsworth.
How this information was developed to help you make better health decisions.