March 20, 2014
By Harvey B. Simon M.D.
Harvard Medical School
Water accounts for about half of a person's body weight. And in today's America, people tend to carry water with them everywhere. Those clear plastic bottles filled with water have become icons for health-conscious men.
Life on earth evolved from water, and water is essential for human health. But is it really necessary to drink the prescribed eight cups – two quarts – a day, whether or not you're thirsty? How much should you drink and what fluids are best?
The Wisdom of the Body
Your fluid balance depends on two key factors: sodium (salt) and water. They are essential to life. So your body has a sophisticated system to regulate salt and water. Special cells in the brain are in charge. If you need more water, they make you feel thirsty and tell your kidneys to retain water. If you have too much fluid, the brain cells tell your kidneys to put out (excrete) lots of water.
Normal kidneys can excrete as little as two pints of fluid per day, if the body is dry. Or as much as 20 quarts (it's true) if it's flooded. Even minor changes in water balance are enough to set the regulatory process in motion. For example, just a 1% deficiency of water or excess of sodium will produce a powerful sensation of thirst.
Under ordinary circumstances, you don't have to tally up the fluids you drink. Instead, let thirst be your guide: Drink when you're thirsty, but don't force fluids when you're not.
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On average, you need about 1 quart of water for each 1,000 calories you burn. For most men, that means about 2 quarts a day. About 60% of the body's needs are met by the water contained in food. In normal conditions, just 1 1/2 pints of fluid will make up the difference.
When You Need Extra Water
Thirst is a reliable guide to your fluid needs, but it has an important drawback: it's slow. By the time you feel thirsty, your body will be dry. You may suffer some of the consequences of dehydration, such as impaired concentration, irritability, headache and fatigue. These are temporary, but unpleasant. Fortunately, you can stay ahead of the game by anticipating special fluid needs. Think ahead and drink ahead.
When will you need extra water? Here are some factors to keep in mind:
- Exercise — When you exercise, your muscles generate heat. To get rid of that heat, you sweat. In warm, humid weather, you can lose up to 1 1/2 quarts per hour. To stay ahead, drink 6-8 ounces before you start working out, and take similar amounts at frequent intervals. After you finish exercising, spend some time at the water fountain before you shower. If you drink enough you won't feel thirsty — or, for that matter, grumpy or tired. You'll know you have enough when your urine is clear and plentiful. Or you can simply check your weight before and after exercise (minus your sweaty clothes, of course). For each pound of weight you lose, you'll need to drink a pint of fluid. And despite the popularity of sports drinks that contain sugar and potassium, plain old water is best.
- Climate — It's obvious that you'll lose water in sweat when it's hot and/or humid. But you'll also need more water when it's cold and dry. That's because you lose water through your lungs every time you exhale. In an average climate, it amounts to a pint a day, but when the air is dry you'll lose substantially more water.
- Air travel — Dry air is responsible here, too. You'll enjoy your trip more if you drink even before you're thirsty.
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How Illness Affects Thirst
Illness can increase a person's fluid requirements. For example:
- People who form kidney stones should drink at least 2 1/2 quarts a day to prevent recurrences.
- Urinary tract infections call for extra hydration. Diarrhea can cause excess fluid and mineral loss. Sports drinks are a good way to replace them.
- You'll also need more fluid every time you have a fever. That's because your body's metabolism speeds up by about 7% with each degree of fever.
Bladder cancer is a special problem for men, since three of every four cases occur in males. The Harvard-sponsored Health Professional Follow-up Study of 47,909 men found that extra water may help reduce risk.
Men who averaged more than 2 1/2 quarts of fluid a day had half the risk of bladder cancer as men who drank less than half as much. In all, each eight ounces of daily fluid appears to lower the risk of bladder cancer by 7%. All types of fluid were helpful; water was best of all by a narrow margin.
A high intake of fluids may protect the bladder by keeping the urine dilute. The Health Professionals Study also found that a high fluid intake reduces a man's risk of kidney stones, which are much more common in men than women. But when it comes to preventing kidney stones, not all beverages are created equal. Coffee, tea, wine and beer were all better than water, while apple juice and grapefruit juice actually increased the risk of kidney stones.
Men need more water than women. In a typical day, the average man drinks about 18 ounces more than his female counterpart. But he loses 24 ounces more. This fluid balance helps explain the gender gap in bladder cancer and kidney stones.
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Water Myths and Realities
- Dehydrating beverages — It is true that alcoholic and caffeinated beverages make the kidneys excrete large amounts of dilute urine. But the effect is temporary, and normal thirst mechanisms will kick in, restoring order.
- Constipation — It's true that dehydration dries out the stools, leading to constipation. A high-fiber diet with a high-fluid intake can help relieve constipation. If you take fiber supplements, take them with water.
- Weight loss — Athletes sometimes try to "make weight" by dehydrating themselves. It's a very hazardous practice that's been banned from competition. Still, some dieters try to lose weight by filling up on fluids. A stomach full of water may suppress hunger, but the effect is temporary; water can never substitute for a sound program of diet and exercise, the only way to lose weight and keep it off.
- Dry skin — It's a common problem, especially during winter when the air is dry. But putting water on your skin will only make the problem worse. And drinking won't help much either. The solution: Add oil, not water, to your skin by applying lubricating lotions.
- Respiratory infections — Extra water can help. It thins out mucus, making it easier to clear your sinuses, throat and chest. In addition to drinking, you can get the extra water you need by inhaling warm mist or steam.
- Bottled water — Although many people buy bottled water because they like its taste, others pay for water because they think it's more healthful than their local water. It's not necessarily so. Many brands of bottled water are low in calcium and magnesium but high in sodium. In many cases, municipal water is healthier, if not tastier (or trendier), than bottled water.
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Harvey B. Simon, M.D., is an Associate Professor of Medicine at Harvard Medical School and a member of the Health Sciences Technology Faculty at Massachusetts Institute of Technology. He is the founding editor of the Harvard Men's Health Watch newsletter and author of six consumer health books, including The Harvard Medical School Guide to Men's Health (Simon and Schuster, 2002) and The No Sweat Exercise Plan, Lose Weight, Get Healthy and Live Longer (McGraw-Hill, 2006). Dr. Simon practices at the Massachusetts General Hospital; he received the London Prize for Excellence in Teaching from Harvard and MIT.