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How Much Weight Will I Lose on a Water Fast?

author image Milo Dakota
Since 2005, Milo Dakota has ghostwritten articles and book manuscripts for doctors, lawyers, psychologists, nutritionists, diet experts, fitness instructors, acupuncturists, chiropractors and others in the medical and health profession. Her work for others has appeared in the "Journal of the American Medical Society" and earned accolades in "The New York Times." She holds a Master of Art in journalism from the University of Michigan.
How Much Weight Will I Lose on a Water Fast?
Woman filling up a glass with tap water. Photo Credit Wavebreakmedia Ltd/Wavebreak Media/Getty Images

Reduce your daily caloric need by 500 calories and you’ll lose a pound a week, according to standard measurements of calories and pounds. Reduce calories by 2,000 a day, and you’ll lose 4 lbs. of fat. That’s the basic math behind the water fast, which involves consuming nothing but water. Practitioners and proponents boast weight-loss claims of up to 10 lbs. in three days. This may be true, but the weight loss is more likely from water and muscle. A water fast poses multiple heath risks and is not scientifically proven as a permanent weight loss method.

Water Fast

Any diet that promises fast results holds appeal. The water fast, which has been practiced for thousands of years by Buddhists and others, is no exception. But the water fast, as well as other less-stringent fasts, was originally intended as a method of cleansing the spirit and mind, not as a diet program. And practitioners generally engaged in quiet meditation and did not lead hectic modern lifestyles. Fasting is meant to be supervised under the guidance of a skilled mentor, says Rev. Heng Sure, director of the of the Berkeley Buddhist Monastery.

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Fasting to Lose Weight

The fast is virtually guaranteed to help you lose weight temporarily. Women, on average, need 2,000 calories a day to maintain their weight. This depends, of course, on your starting weight, your age, your level of activity and your metabolism. A 25-year-old woman who works out moderately and weighs 130 lbs. needs 2,077 calories a day to maintain her weight, according to calculations available on CalorieControl.org. A typical man needs 2,700 calories a day to maintain his weight. Using the 2,000-calorie-per-day figure and the standard measurement of a pound of fat equaling 3,500 calories, a woman who consumed nothing but water for 10 days would lose 5.7 lbs.

Losing Muscle

You may lose more than fat, particularly if you are already thin to begin with. A fast may help you shed excess water and fecal matter that will push through your system when the body is deprived of nourishment. This toxic cleansing effect is promoted as a benefit of fasting, but the American Cancer Society says there is no scientific evidence to support claims that toxins stored in the large intestine are harmful. If you are lean to begin with, you could lose muscle. According to Dr. Kevin Hall, an investigator at the National Institute of Health in Bethesda, who published a report in the “International Journal of Obesity,” it only takes about 600 calories to support a pound of muscle and people who do not have excess fat will lose lean muscle mass instead when they reduce their caloric intake.


Fasting can be dangerous. The body reacts to a fast in the same way it does to starvation. Fasting can cause immediate problems in children, the elderly and anyone with auto-immune disorders and prolonged fasts can harm your liver, lead to anemia and cause kidney stones, according to Kelly Grimes, author of "Fasting: Body Cleansing or Body Starving?" Less serious side effects include headaches, nausea and muscle aches.

Safer Choices

Some of the side effects of fasting can be offset by adding juice, which contains sugar and other nutrients to sustain energy, or small meals to the fast. Some Buddhists, for instance, eat one meal a day and successfully continue this type of fast for long periods of time. Adding water to your diet, instead of substituting it for food, can help you lose weight. The results are less dramatic but safer. Researcher Michael Boschmann and colleagues reported in a 2003 issue of "Journal of Clinical Endocrinology and Metabolism" that drinking six glasses of water a day could yield a 5-lb. weight loss in a year.

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