How much you really need - and how it can help you lose weight.
By Kathleen Zelman, MPH, RD/LD
WebMD Weight Loss Clinic - Feature
Reviewed By Michael W. Smith, MD
Water is one of the most basic elements of life but figuring out how much we ought to drink hasn't always been so simple.
Most of us grew up thinking we needed to drink eight glasses of water each day, in addition to any other drinks we might choose. But the latest recommendations say that we no longer need to worry about drinking specific amounts of water. Instead, we can simply satisfy our thirst with any beverage.
Of course, water -- clean, refreshing, and calorie-free -- is the ideal beverage of choice. And some folks swear by its weight loss powers, including Mireille Guiliano, author of the best-selling book French Women Don't Get Fat.
To help make the facts about water crystal clear, WebMD asked experts for the skinny on just how much water we need, and whether drinking water can really help keep those extra calories at bay.
The New Fluid Guidelines
A 2002 study published in the American Journal of Physiology questioned the old recommendation of 8 ounces of water, eight times a day. After a thorough review, researcher Heinz Valtin concluded there was inadequate evidence that healthy adults -- living in temperate climates and not engaged in rigorous activities -- need large amounts of water.
For normal, healthy adults, Valtin recommended simply drinking when thirsty. And he reported that even caffeinated drinks can count toward satisfying our fluid requirements.
In February 2004, the Institute of Medicine (IOM) issued new recommendations that agree with Valtin's findings. The new guidelines remove the eight-glasses-a-day recommendation, and say healthy adults may use thirst to determine their fluid needs. Exceptions to this rule include anyone with a medical condition requiring fluid control; athletes; and people taking part in prolonged physical activities or whose living conditions are extreme.
How Much Is Enough?
The IOM report did not specify requirements for water but made general fluid intake recommendations of 91 ounces (that's 11-plus cups a day) for women and 125 ounces (15-plus cups a day) for men. Remember, these guidelines are for total fluid intake, including fluid from all food and beverages.
Approximately 80% of our water intake comes from drinking water and other beverages, and the other 20% comes from food. Assuming these percentages are accurate for most of us, the recommended amount of beverages, including water, would be approximately 9 cups for women and 12.5 cups for men.
While 20% may seem like a lot of fluid to get from food, many common food items are mostly water. Here are some foods with high water content, according to the American Dietetic Association:
|Lettuce (½ cup)||95%|
|Watermelon (½ cup)||92%|
|Broccoli (½ cup)||91%|
|Grapefruit (½ cup)||91%|
|Milk (1 cup)||89%|
|Orange juice (3/4 cup)||88%|
|Carrot (½ cup)||87%|
|Yogurt (1 cup)||85%|
|Apple (one medium)||84%|
When You Need More
Physical activity, heat, and humidity can increase our fluid needs. In these situations, keep water bottles close at hand and drink frequently to avoid dehydration. If you're going to be physically active for long periods, consider sports drinks that hydrate and provide easily usable sugar and electrolytes.
Illnesses accompanied by increased body temperature, excessive perspiration, vomiting, frequent urination, or diarrhea can also increase our fluid needs. Be sure to drink plenty of liquids if you have one of these conditions, and see a doctor if your fluid losses are excessive or prolonged.
How Much Is Too Much?
Scientists on the IOM panel did not set an upper limit for water.
"Water intoxication is very rare, although it has been seen in fraternity pranks. That can be very serious and result in death" says David Perlow, MD, an Atlanta-based urologist.
One recent study of Boston Marathon runners showed that one in three marathon runners was drinking too much water during a race -- probably because they were following recent advice to drink as much as tolerated.
If you follow your thirst, you won't go wrong, Perlow says. He notes that pre-modern man never ran around sipping on a water bottle. A dry mouth indicated it was time to run to the stream for a drink.
"Trust your thirst instinct to make sure you get enough fluids and, of equal importance, void frequently," suggests Perlow.
Perlow says the bladder is like a balloon. When you make infrequent trips to the bathroom, it can become overstretched -- which can result in problems with incomplete emptying, he explains.
He recommends 7-12 trips to the toilet daily for most healthy people.
Water and Weight Control
For years, drinking water has been recommended for weight loss -- despite the fact that it satisfies thirst and not hunger. Barbara Rolls, PhD, an expert on thirst and satiety, points out that thirst and hunger are regulated by entirely different mechanisms.
A recent study by Rolls and colleagues at Penn State University looked at whether people who drank water with lunch took in fewer calories than those who drank other low-calorie beverages. They found that drinking water had little effect on total calorie consumption at the meal.
"In all of our research, we have never been able to show that water can cause weight loss," says Rolls. The only way drinking water can help you lose weight is if you substitute it for higher-calorie beverages or foods, she explains.
However, eating foods with high water content can help dieters, by increasing the fullness factor.
"When you add water to a bowl of vegetables as in soup, the soup has greater satiety than when the vegetables are eaten alone with a glass of water," explains Rolls, author of The Volumetrics Eating Plan and The Volumetrics Weight Control Plan. "When water is incorporated into food or shakes, satiety is increased and subjects ultimately eat less food."
The weight loss benefits of water stem from several facts:
- Foods that incorporate water tend to look larger.
- The higher volume of these foods provides greater oral stimulation.
- Most important, when water is bound to food, it slows down absorption and lasts longer in the belly.
A soon-to-be-published study by Rolls and colleagues shows that a high-volume eating plan resulted in more weight loss than a low-fat eating plan.
If you want to lose weight, Rolls recommends an eating plan that includes plenty of high-volume foods such as fruits, vegetables, broth-based soups, and oatmeal, along with adequate fluids to satisfy your thirst.
The experts agree: Drinking water -- either sparkling or flat and perhaps with a twist of citrus -- is a great, noncaloric way to satisfy your thirst.
But if you struggle with drinking water, you're off the hook as long as you:
- Enjoy plenty of high-volume foods, including fruits and vegetables.
- Satisfy your thirst with a variety of healthy, low-calorie beverages such as 100% fruit juice, skim or low-fat milk, tea, and, of course, water.
- Pay attention to signs of dehydration, such as dry mouth and concentrated urine, which indicate a need for more liquids.
Originally Published May 5, 2005.
Medically updated April 10, 2008.
SOURCES: American Journal of Physiology, Aug. 8, 2002. Appetite, April 2005. News release, Institute of Medicine, Feb.11, 2004. American Dietetic Association web site. David Perlow, MD, urologist, Atlanta. Barbara Rolls, PhD, author, The Volumetrics Weight-Control Plan and The Volumetrics Eating Plan; and professor, nutritional sciences, Pennsylvania State University, State College, Pa. WebMD news article: "Marathon Runners Drink Too Much."
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