Essentials of Nutrition for Sports

These studies are often touted by coaches, sport scientists, and
companies that market hydration products.

It is not that simple; and not all studies come to this conclusion. For example, four other studies showed that:

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More than 20% of both boys and girls lost more than 2% of body weight during a triathlon cons

isting of an 800-meter swim, 30-

kilometer bike, and 8-kilometer run. Performance was better in the more dehydrated athletes.

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Neither 2% nor 4% dehydration a

dversely affected performance.

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A 3.5% reduction in body weight from dehydration was not associated with decreased muscular strength or endurance compared with a 1.3% reduction in body weight.

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One-hour cycle time-trial performance was not improved by hydration during the event.

Bottom Line

In the real world, modest dehydration probably does not affect

performance, and for events less than one hour, hydration has not been proven as important as it is for longer events.

Although laboratory performance has been shown to worsen

with modest dehydration, in real world competition many 10-K runners and 40-K bicycling time trialists do not drink, even in desert events. During races of less than one hour at maximum effort the disruption of rhythm, the time cost of drinking and the aerodynamic drag of water bottles, may justify not drinking.

The longer the event, the more important it is to pay attention to

hydration. Prehydrate and pay attention from the get go—because once behind, it is often difficult to catch up.

Thirst Sensation

Your body responds to dehydration with a sense of thirst. If you

are thirsty, dehydration has already happened. The older we get, the slower our body’s thirst response becomes. Recent Studies

Athletes are often unaware of and underestimate their fluid

losses and replacement.

In one study, experienced runners

lost almost 2-quarts (liters) in

a 10-mile run. They estimated their sweat loss to be 1.1 quarts (liters).

In Hawaii Ironman subjects, dehydration was 20 times more

common than overhydration.

Estimating Sweat Rate

Athletes can roughly estimate sweat rate by weighing themselves

before and after exercise and accounting for urine losses and fluid ingestion.

One quart (liter) of water

weighs about 2 pounds.

However, a scale can be misleading. Glycogen binds, on average, three times its weight in water.

Since glycogen depletion is common in events longer than one hour, some weight loss reflects decreased glycogen stores rather than hydration status per se. Glycogen stores normally average about 1 pound (500 grams). If glycogen is co

mpletely exhausted, 4 pounds (2

kilograms) of weight loss can be attributed to glycogen exhaustion—1 pound of glycogen and 3 pounds

of associated bound water.

Prehydration

If you are training or racing in hot weather, or repeatedly

exercising within a short span of time, particular attention must be given to adequate prehydration in the days before racing.

Nutrition for Sports, Essentials of 28