12.3: We Are Mainly Water

Whereas many of us can survive months without food, none of us—fat or thin—can survive more than a few days without water. A lack of water is felt immediately. In extreme circumstances, as in a summer desert, the absence of water can mean death in as few as 12 hours.

Body water is often categorized as either intracellular water (water within cells, representing about 60% of total body water) or extracellular water (water outside of cells, representing about 40% of total body water) (see Fig. 12-2). But this categorization can be misleading when envisioned as two separate and distinct compartments of water. Rather, there is a constant exchange of extracellular and intracellular water. This exchange is the means by which nutrients are carried into our cells and the cells’ products are carried out.

The sugar glucose, for example, needs to be dissolved in the water of digestive fluids (extracellular fluid) before it enters the intestinal cell to become a part of its intracellular fluid. This glucose coming from food is then passed by way of blood plasma (an extracellular fluid) to the intracellular fluid of cells throughout the body.

Figure 12-2: Body’s fluid compartments. About two-thirds of the adult body is water (60% intracellular, 40% extracellular).

We see an example outside of our bodies by looking at the one-celled animals of ponds and tide pools. This same flow of nutrients between extracellular (the pond) and intracellular fluid takes place. These one-celled creatures are separate from, yet a continuous part of, the pond in which they live. Our own cells are much the same.

Although the percentage of our body weight that’s water varies from person to person and diminishes from birth to death, water is always the major component of the body. Newborn infants have the most water, averaging about 75% of total body weight. Older adults may be only slightly more than 50% water.

In between these extremes of age, the average adult body is about 60 to 70% water. This percentage is mainly determined by the proportion of body fat. Recall that fat doesn’t hold water. It follows that the fatter you are, the smaller the proportion of water in your body.

Indeed, the fact that body water has little to do with fat suggests why it’s futile to tamper with the water balance of the body when trying to lose body fat. There’s no value in trying to restrict water intake or to promote water loss. Similarly, there’s no point in devices and exercises which are intended mainly to cause water loss through sweating. While such water losses temporarily lower body weight, they don’t reduce the true problem of fatness.

The Water Game in Quick Weight-Loss Schemes

There really is a pill that will make you lose weight. Just as quick-loss diet programs advertise, you really can see your weight fall two, four, six pounds, even more, within a couple of days. It’s a gratifying experience for the frustrated dieter.

Unfortunately, this “magic pill” is a diuretic. It works by stimulating the kidneys to take water out of the body and excrete it as urine. So the diuretic pill has shed pounds all right, but it has nothing to do with fat.

Our body water does have its ups and downs. For example, body water can vary by several pounds during a woman’s menstrual cycle. Water retention generally peaks as the menstrual period begins, and can make the woman feel fat.

The amount of sodium in a day’s food can raise and lower the body’s water content, by a matter of pounds. We tend to be the lightest before breakfast—our stomachs are empty; we have lost some water through evaporation during sleep; and we usually urinate upon getting out of bed.

The ups and downs of body water create the impression that fat can come and go with incredible speed, and can easily deceive the reducer who relies only on the scale.

Take Sue, who says, “I sure wish I could control my appetite. I played grueling tennis last Sunday for 3 hours in that sun. Then, I skipped lunch, and sat in the sauna instead. Sure enough, I’d lost 5 pounds. But then, you know what? At Ed’s dinner party that night, I went crazy over the prime rib and had some daiquiris with a lot of appetizers. When I got on the scale before I went to bed, I couldn’t believe it. All that work to lose 5 pounds, and then at that one dinner I ate it all back!”

Such things happen all the time. The weight loss and gain are real. But it’s an illusion that the food and drink really made the person 5 pounds fatter. Let’s see what the body is actually doing.

For Sue to have eaten her way to a 5-pound gain in body fat would mean an extra 15,000 calories or so. Ten pounds of prime rib, 10 daiquiris, and 2 trays of appetizers? Probably not.

Sue may have spent 800 calories in 3 hours of vigorous tennis. (Though actually, she didn’t spend the whole 3 hours playing tennis. She rested 10 minutes between each set and spent some 20 minutes changing courts and arguing scores.) And the sauna didn’t use up much beyond basal metabolic need. But even at that, an 800 calorie expenditure represents less than a quarter pound of body fat.

But the sweat loss was considerable, and Sue neglected to drink water during the match. Between the tennis, the hot sun, and the sauna, 5 pints of perspiration were lost, actually representing the 5 pounds (“a pound a pint”).

Weak after the sauna, Sue drank long at the water fountain. At home, while changing for dinner, she drank a couple of cans of diet soda. Famished, she had big helpings at the dinner party. Still thirsty, she had more water, a couple of daiquiris, and a few more cans of diet soda poured over a lot of ice. Her body water came back up.

Calorie-wise, her tennis, skipping lunch, and all that extra food at dinner balanced out to about zero. As usual, need and appetite stayed pretty close. But the dip and then rise of body water had made her experience seem much more dramatic.