December 24th, 2010
10:20 AM ET

Can drinking lots of water help you lose weight?

Every weekday, a CNNHealth expert doctor answers a viewer question. On Friday, it's Dr. Melina Jampolis, a physician nutrition specialist.

Question asked by Tony of Shelton, Connecticut:

Is it true if you drink lots of water, it will help you to lose weight? Is it bad for your body if you consume too much of it?

Expert answer:

Hi, Tony. Research does suggest that drinking plenty of water may help you lose weight. An abstract by Dr. Brenda Davy, associate professor of human nutrition, foods and exercise at Virginia Tech, presented at last year's obesity conference in Phoenix, Arizona, showed that people who drank two glasses of water 20 to 30 minutes before every meal lost weight more quickly initially and lost significantly more weight than those who didn't.

In another study by Davy and her group, published last year in the Journal of the American Dietetic Association, she found that people who drank water before meals ate an average of 75 fewer calories at that meal. This may not seem like much, but if you ate 75 fewer calories at lunch and dinner for the next year, you could lose about 14½ pounds! In addition, being even 1 percent dehydrated can cause a significant drop in metabolism, which can also interfere with weight loss.

Finally, it is very difficult for the body to differentiate hunger from thirst. If you don't drink enough water throughout the day, you may mistake thirst for hunger and eat more than you really need, which can also impair weight loss. So staying well hydrated is important, particularly if you are trying to lose weight. And don't forget to eat lots of water-based foods like soups, vegetables and low-fat dairy, which are equally important for weight loss, as they lower the calorie density of meals. That can help you reduce calories without reducing portions.

To answer the second part of your question, yes, drinking massive amounts of water (gallons and gallons) can cause a dangerous condition known as hyponatremia (low sodium levels in the blood), which can cause confusion, irritability and seizures and may even lead to a coma.

This condition is very rare in healthy people but can sometimes be seen in the elderly or in endurance athletes who sweat significantly and drink water only to replace lost fluids. Most people should be far more concerned with not drinking enough water versus drinking too much.

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