A little light reading about human water needs.
Kathleen Zelman, MPH, RD/LD
WebMD
Weight Loss Clinic
Reviewed By Charlotte Mathis, MD
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.
"If you follow your thirst, you won't go wrong, says David Perlow, MD, an Atlanta urologist. 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."
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?
Approximately 80% of our water intake comes from drinking water and other beverages, and the other 20% comes from food. 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:
Food Percentage Water
Lettuce (½ cup) 95%
Watermelon (½ cup) 92%
Broccoli (½ cup) 91%
Grapefruit (½ cup) 91%
Milk (1 cup) 89%
Orange juice (¾ 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. 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?
"Water intoxication can 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.
"Trust your thirst instinct to make sure you get enough fluids and, of equal importance, void frequently," suggests Perlow.
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."
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.
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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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MYTHBUSTERS, Discovery Channel:
"While it's a good idea to drink plenty of water, there is no scientific or medical basis for the specific amount of eight glasses per day. Water intake needs vary from person to person, depending on age, size and amount of physical activity. We can also get water from food and other beverages, so we don't necessarily need to get all of our fluids in the form of drinking water."
This is from ClassBrain.com:
"The "8 by 8" rule comes from a report that was created in 1945, what they didn't tell us all these years is that the water that we consume in our food is included in the total, which means that the average person only needs to drink about four glasses of water a day. You can rest easy that you're actually drinking the correct amount of water for a healthy lifestyle."
From RealAge.com:
"And, contrary to popular belief, there is no evidence to suggest that drinking eight 8-ounce glasses of water each day will make you healthier. How much water you need depends on many factors, including your
body size, how active you are, your age, and your
diet.
FURTHER READING:
http://calorielab.com/news/2006/05/28/8-
glasses-of-water-a-
diet-urban-legend
http://dms.dartmouth.edu/news/2002_h2/08
aug2002_water.shtml
http://www.snopes.com/medical/myths/8gla
sses.asp
http://www.eurekalert.org/pub
_releases/2002-08/dms-al080802.php
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