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Hydration in the heat for young athletes

Young athletes, their parents and coaches frequently ask about safe ways to hydrate in the heat. They want to know how much to drink and what to drink in training or competition. Confusion can arise from conflicting hydration guidelines and roiling debate. Let me try to clarify.

By E. Randy Eichner, M.D., F.A.C.S.M

Fundamentals

Vigorous exercise in the heat prompts sweating for thermoregulation via evaporative cooling. But sweat rates vary considerably among individual athletes, even in identical environmental settings. In some young athletes exercising hard in the heat, sweat rates can exceed two liters per hour. Heavy sweating can lead to dehydration.

There are three reasons why dehydration matters in sports and sports medicine. Dehydration influences: (1) Exercise performance; (2) the risk of exertional heat stroke (EHS); and (3) the risk of heat or sweat cramping.

“If they exercise longer, especially in the heat, young athletes should drink regularly to offset ongoing sweat loss.”

Also, in some settings, athletes may wrongly perceive they are dehydrated, and overdrink in response. This can increase the chance they will suffer exertional hyponatremia (EHN), which can be fatal.

This column keys on dehydration and athletic performance in the heat. Details of fluid and salt balance as they relate to EHS, cramping and EHN are better left for future columns.

Athletic performance in the heat

The effect of dehydration on athletic performance is varied and debated. Dehydration of 2-3% (measured by weight loss) can impair aspects of performance in, for example, rowing, cycling, sprinting, soccer and basketball. Prior dehydration of 2% can also impair middle-distance track running, but not strength or power performance. The prevailing view, based on 50 years of research, is that dehydration degrades aerobic exercise performance.

Studies also show that athletes, especially young men, often begin training or competition on the dry side. For example, by urine specific gravity, half or more of boys in summer football, NCAA athletes, NBA basketball players and recreational exercisers in fitness centers begin their exercise at least slightly dehydrated. And studies show that during prolonged exercise in the heat, many athletes drink less than their sweat losses, thus enhancing their dehydration.

To maintain daily electrolyte balance, a normal diet will typically suffice, even if only water is consumed during exercise.”

So the prevailing view is that dehydration is detrimental, yet many athletes begin dehydrated and get more dehydrated when competing in the heat. What to make of this? How much dehydration is detrimental, and how best to prevent it? This is where a challenging view comes in. Let me explain.

The debate

The prevailing view is that dehydration (loss of greater than 2% of body weight) degrades aerobic exercise performance in both warm-hot and temperate climates. Likely mechanisms include augmented hyperthermia, increased cardiovascular strain and increased perception of effort.

Consensus holds that, during prolonged exercise in the heat, it is better to drink water than nothing, and a sports drink with carbohydrate and electrolytes may improve performance more than water alone. By this view, athletes should drink regularly, aiming to offset sweat losses to avoid dehydrating by greater than 2-3% of body weight.

The challenging view is that thirst, not “dehydration,” impairs exercise performance, so an athlete can optimize performance by drinking to thirst. Drinking “beyond thirst” to prevent an arbitrary level of dehydration will gain the athlete nothing and may lead to fatal EHN. Proponents argue that, absent thirst, dehydration much greater than just 2-3% is trivial and may be beneficial, enabling the “lighter” athlete to go faster. Despite roiling debate, “drink to thirst” is the minority view. Yet it has merit in its clarion call to avoid overdrinking.

A lively and informative debate on these two views, by Sawka and Noakes, is recommended reading. (See Sawka and Noakes 2007.) I also reference four other articles from which I draw practical lessons.

Practical Lessons

For brief exercise, especially in mild conditions and when starting well-hydrated, young athletes may need to drink no fluids. But water should be readily available.

If they exercise longer, especially in the heat, young athletes should drink regularly to offset the ongoing sweat loss, say three to eight ounces every 20 minutes for young teens and somewhere between 16 to 32 ounces every hour for young adults. The aim is to avoid losing more than 2-3% of body weight. Those who sweat heavily may need more fluid than this. It is key to individualize drinking rates.

To estimate how much to drink in a distance race, for example, athletes can weigh themselves without their clothes on before and after running one hour at race pace and in race conditions, drinking fluids as they normally would. To remain adequately hydrated during a race, they may drink the amount of fluid consumed during the one-hour test run plus one pint per hour for each pound lost during the test run. Athletes should never drink more than they sweat. A weight gain means they drank too much. Caution in using this rule of thumb is key in a very long race, where drinking to match even a small overestimate of sweat loss could result in fluid overload and EHN.

“It is vital that parents, coaches and health care providers monitor the athletes, heed what they say and watch for warning signs.”

To maintain daily electrolyte balance, a normal diet will typically suffice, even if only water is consumed during exercise. A light-colored first-void morning urine suggests good hydration. Some athletes lose so much salt in sweat that they need more dietary sodium and extra salt in their sports drink. Look for more detail on this in a future column on cramping.

Repeat same-day exercise bouts—as in tournaments and football two-a-days—are a challenge. Between bouts, athletes should eat healthy snacks and rehydrate prudently—but not overdrink—while resting in air-conditioning. At the end of the day’s competition, they should catch up with fluid and salt needs, along with good nutrition.

Should it be water or a sports drink? For most young people in most athletic settings, when they can eat meals before and after exercising, water is fine. But the longer and harder they go in the arena, the hotter it is, the heavier they sweat, the sooner they have to compete again and the fewer their chances for regular snacks, the greater the role for sports drinks with needed carbohydrate and electrolytes.

Finally, it is vital that parents, coaches and health care providers monitor the athletes, heed what they say and watch for warning signs, such as undue fatigue, muscle cramping, vomiting, heavy breathing, confusion or other signs of heat exhaustion. Better safe than sorry.

By E. Randy Eichner, M.D., F.A.C.S.M.

Dr. Eichner is professor emeritus of medicine at University of Oklahoma Medical Center, and served for 14 years as team internist for the Oklahoma Sooners football team. In 2011, he received the National Athletic Trainers Association President’s Challenge Award for lifetime achievement benefitting the health of athletes. Dr. Eichner is a columnist for Current Sports Medicine Reports, a fellow and past trustee of the American College of Sports Medicine and a founder of the American Medical Society for Sports Medicine. He received his medical degree from Johns Hopkins University School of Medicine.

 

References

Bergeron MF. Hydration in the pediatric athlete – how to guide your patients. Curr. Sports Med. Rep. 2015; 14:288-93.

Cheuvront SN, Carter R III, Sawka MN. Fluid balance and endurance exercise performance. Curr. Sports Med. Rep. 2003; 2:202-8.

Maughan RJ, Shirreffs SM. Dehydration and rehydration in competitive sport. Scan. J. Med. Sci. Sports 2010; 20 (Suppl 3):40-7.

Sawka MN, Burke LM, Eichner ER, et al. Exercise and fluid replacement. ACSM Position Stand. Med. Sci. Sports Exerc. 2007; 39:377-90.

Sawka MN, Noakes TD. Does dehydration impair exercise performance? Med. Sci. Sports Exerc. 2007; 39:1209-17.