Seasons and Fluid Needs

Your clients need good advice on fluid replacement in every season.

MOST OF YOUR clients are probably aware that increasing physical activity levels demand an increased fluid intake. Water loss during exercise in the heat is obvious: Clients get hot, sweaty and thirsty. But fluid needs also increase when exercising in a cold environment. Your clients need good advice on fluid replacement in every season.

Fluids R Us

A lean, adult body is about 60 percent water by weight. All physiological processes, including energy metabolism, muscular contraction and temperature regulation, work best with adequate hydration. A water loss of even 1 or 2 percent can cause a decrease in physical performance.

We lose water every day through urination, bowel movements, sweating and breathing. Any factor that increases any of these losses increases fluid requirements. For example, exercise increases water loss through extra sweating and breathing. Blood loss, vomiting and diarrhea cause extra fluid losses, as well. People who use diuretics (water pills) to lose weight increase their risk of dehydration.

Dehydration hurts performance

Understanding the physiological effects of dehydration shows clients why adequate hydration is important. When water loss exceeds replenishment, water is lost from all areas of the body. Water levels decline in both the intracellular and extracellar fluid, which can interfere with electrolyte concentrations and optimal performance.

Blood volume also declines as dehydration occurs. The decrease in blood volume hurts physical performance, since it decreases the amount of blood the heart can pump per beat. One of the early signs of dehydration is an increased heart rate (beyond a person's normal heart rate at rest or for a given amount of work). If blood volume continues to decline, cardiac output may decline, as well. Muscles need oxygen and fuel to work, so performance decreases as cardiac output is reduced.

Since there is not enough blood for every system, the body tries to maintain blood volume to the muscles by constricting vessels in the skin. Blood flowing near the skin allows heat to escape from the body, which is helpful during vigorous exercise. A reduction in skin blood flow means less heat is lost and body temperature rises. To make matters worse, the body decreases sweat production when not enough water is available, so the body has even more difficulty getting rid of heat. You can see why dehydration can lead to heat illness.

Exercise in the cold increases fluid needs, too

While dehydration is more likely when exercising in hot weather, it is a concern in cold weather, as well. Cold air contains less moisture. When this cold air leaves the lungs, it is fully humidified and warmed. Vigorous exercise for a prolonged period of time is, therefore, accompanied by significant fluid loss via respiration.

In a cold environment (water or air), the body attempts to conserve its heat by reducing circulation to the skin. An increased core blood volume stimulates the kidneys to produce more urine to reduce blood volume. But as exercise progresses and clients warm up, their bodies now open circulation to the skin to get rid of heat. A higher blood volume is needed, so they must replenish those lost fluids.

Water is also lost as sweat when clients exercise in the cold, although not usually to the same extent as in the heat. Wearing too much clothing can increase heat and sweating, so cold weather exercisers must learn to shed layers as they warm up so they don't get too hot.


Clark, N. Nancy Clark's Sports Nutrition Guidebook. Human Kinetics: Champaign, Ill., 2003.

McArdle, W.D., F.I. Katch and V.L. Katch. Sports & Exercise Nutrition. Lippincott, Williams and Wilkins: Philadelphia, Pa., 2005.

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