12.6: Water Balance

Water balance (amount taken in = amount lost) is carefully regulated by the kidneys and the thirst center in the brain. When water intake is less than we need, we become thirsty and our urine becomes more concentrated. Conversely, when our water intake is more than we need, we don’t feel thirsty, and our urine becomes more dilute. Under normal, everyday circumstances, the body automatically regulates our water balance, and we don’t have to be concerned with it.

But the thirst mechanism doesn’t work as well in older adults,^1,2 so they need to make a point of drinking water, even when they aren’t thirsty. Also, when extraordinarily large amounts of water are lost—as in severe diarrhea, vomiting, fever, or heavy sweating—the body’s control mechanisms may not work well enough or fast enough to prevent dehydration (e.g., thirst isn’t a reliable gauge of needed water replacement for the active athlete).

Because so many bodily functions depend on water, the balance between loss and intake must be maintained within reasonable limits. There’s only so much forgiveness (see Fig. 12-3).

As dehydration develops, the body has its priorities. As we’ve seen before, the body tends to protect first those organs and functions which are most necessary to survival. Foremost, the body needs to maintain its blood volume for adequate circulation. For this reason, the body will permit only so much water to be used for perspiration when we’re hot and dry.

Figure 12-3: Effects of Dehydration.

But this adjustment can work only so long. It’s a kind of bodily stalling tactic, which has some strict time limits. As water ordinarily expended in sweat is held back when we’re hot, the body temperature begins to rise, and it can’t rise more than 6 or 7 degrees before collapse becomes imminent.

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The first President Bush, at age 82, was hospitalized briefly after becoming dehydrated and dizzy while golfing in Palm Springs, California.

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With just a 5% drop in body water, a person may begin to show early signs of heat exhaustion, such as weakness, headache, and a weak and fast pulse. When body water drops 10%, a person is at risk of heat stroke, which can be fatal if not treated promptly.

Even if heat isn’t a problem, continued dehydration will cause a fall in blood volume. This can cause a fall in blood pressure and a weak pulse— and a fast pulse, as the heart beats faster in trying to maintain circulation. The fall in blood volume and circulation can lead to its own set of potentially fatal complications, such as an inability to produce urine.

Lest one think that the problem of water intake is always one of not enough, it’s worth mentioning that there’s a psychological disturbance called compulsive water drinking—a compulsion to drink enormous amounts of water to the point where the kidneys can’t get rid of the overload fast enough. It results in a potentially lethal condition called water intoxication. Tissues swell, making the brain particularly vulnerable because it’s encased in the skull.

Early symptoms include headache and vomiting. (Recall that headache can also be a symptom of dehydration. Headaches often serve as a subtle—or not so subtle—indication that something is out of kilter.) Tragically, fraternity pledges have died from water intoxication from being forced to drink huge amounts of water during hazing.

Alcohol and Dehydration

Alcohol increases urination and can thus be dehydrating, and dehydration causes some of the misery of a hangover. So drinking plenty of water during or after cocktail parties can spare one from some of the discomforts of the morning after.

In hot weather, the body may have less water available to deal with alcohol. So a beer, with its higher water-to-alcohol ratio, is a better choice in hot weather than a cocktail.

Combine the hot weather with heavy exercise or a high altitude (where the drier air and reduced atmospheric pressure hastens evaporation from the skin and lungs), and alcohol can carry a lot more kick. For one thing, the effect of alcohol increases as it becomes a larger percentage of the blood.

When we’re low on water, the volume of blood circulating in our bodies is less. So every ounce of alcohol we drink is a bigger percentage of the blood; it takes less alcohol to make us a lot more fuzzy.

So after a tennis match in the sun, have a drink of water before (or, better yet, instead of) one with alcohol. And on a passenger jet, you might want to take the same precautions. Dehydration can make a substantial contribution to jet lag. Despite pressurization, the air is thin and dry—like that of high altitude on land. Limit alcohol intake, and drink plenty of non-alcoholic beverages.

The Variables of Water Need

Considering the variations in weather, individual activity, alcohol intake, etc., it’s clear that the need for water is quite variable. At a minimum, it’s estimated that adults need about 2 quarts of water each day to replace normal losses. About a quart is lost as urine, and the rest is lost in perspiration, expired air, and feces.

A common rule of thumb is that adults need about a quart of water for each 1,000 calories consumed. Infants need more water—about a quart and a half per 1,000 calories. (As will be discussed soon, we meet our water need not only through drink, but also through the substantial amount of water in “solid” food.)

This amount doesn’t take into account such stresses as a hot or dry climate or strenuous activity. Because the loss of water by evaporation from the skin and lungs is a primary way of cooling the body, a combination of heat and exertion can markedly increase water usage. Soldiers working in desert heat can lose up to a quart of water an hour.

Water and the Athlete

As with other nutrients, there are misconceptions about water and athletic performance. Even today, some parent coaches tell their young players to limit the amount of water they drink even during a vigorous soccer game on a hot day. In reality, unless one gulps enormous amounts of water, there’s little or no risk of “cramps” or other problems.

To the contrary, the risk is one of substandard performance—or a serious health problem—if play continues when the players need water. Thirst isn’t a discomfort to be endured. It’s a warning sign of bodily need in almost all cases. (But an absence of thirst isn’t necessarily a signal that the body doesn’t need water.)

Strenuous athletic events in hot weather offer particular dehydration problems. As stated earlier, water loss may be so fast that athletes can’t depend on thirst, and often can’t even drink and absorb water fast enough to keep pace with their water need. For this reason, athletes should drink a lot of water before, during, and after athletic events that last more than a half-hour, even if they aren’t thirsty. Cold water may be preferable because it’s absorbed a little faster, and is itself cooling. But unchilled water will do quite well.

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Activity is partially taken into account by the tie-in of water need to calorie need—more physical activity requires more calories. All activity is heat-generating, but strenuous activity is all the more so.

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Athletes can monitor their dehydration by weighing themselves before and after an athletic event or work-out. The aim is to minimize weight loss by drinking plenty of water, and to drink enough water afterwards. For every pound of weight loss, two 8-oz. glasses of water are needed for rehydration (“a pint a pound”).

In most circumstances, “sports drinks” don’t provide any performance advantage over plain water (except perhaps the psychological advantage of believing performance will be enhanced). But in endurance events lasting more than 90 minutes, consuming drinks containing about 6 to 8% carbohydrate (glucose, sucrose, or glucose polymer) beginning about 30 minutes before fatigue sets in, does seem to enhance performance. (Most sports drinks are 5-8% carbohydrate.)

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One-third cup of table sugar (sucrose) per quart of water makes a “sports drink” of 7% carbohydrate. Add a bit of unsweetened Kool-Aid for flavor and color. Adding a bit of sodium also can help by speeding the absorption of the sugar and water. Adding 1/8 teaspoon of table salt (sodium chloride) per quart, provides about 70 mg sodium per cup.

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Taking sodium or other electrolytes lost in sweat isn’t generally necessary, and doesn’t enhance performance. The one exception is the need for sodium in the drinking water during endurance events lasting more than 4 hours. In such situations, sweat losses are extraordinarily large and prolonged.

Other nutrients, such as amino acids and vitamins, taken immediately before or during an event can be detrimental to performance if taken in large amounts. Such excesses mean more waste products to excrete through the urine, and increased urine production hastens dehydration.

To sum up, it’s very important for an athlete to drink plenty of water to keep from becoming dehydrated. For athletic endeavors lasting less than 90 minutes, plain water is usually all that’s needed for optimal performance. For endurance events of more than 90 minutes, carbohydrate-containing drinks can enhance performance when taken during the event, starting about 30 minutes before the expected time of fatigue. For endurance events of 4 hours or more, sodium-containing drinks should be taken during the event.

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For events lasting less than 90 minutes, plain water is a good “sports drink.”

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For all the emphasis placed on preventing dehydration, there’s a caveat—don’t over-do it. In many marathons, runners load up on water before the race, and then are offered water and sports drinks at every mile along the route.

A study of several hundred runners in the 2002 Boston Marathon found excessive fluid consumption in 22% of the women and 8% of the men, as measured by abnormally low levels of sodium in the blood (from water overload), regardless of whether they drank water or sports drinks.3 Most of those with excessive fluid intake gained weight during the race—an indication of water overload (“a pint a pound”), just as weight lost during a race is a measure of dehydration. A 28-year-old woman who ran that race (but wasn’t in the study) died soon afterwards from severe water overload.