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Fasting - Topic Overview

What is fasting?

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.

    This information is produced and provided by the National Cancer Institute (NCI). The information in this topic may have changed since it was written. For the most current information, contact the National Cancer Institute via the Internet web site at http:// cancer .gov or call 1-800-4-CANCER.

    WebMD Medical Reference from Healthwise

    Last Updated: January 25, 2013
    This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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