wid Rep acement Strategies for Exercise in Hot Weather

Scott J. Montain, PhD1, Ronald J. Maughan, PhD2, and Michael N. Sawka, PhD1 'U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts Medicine, University of Aberdeen, Scotland

A thletes should drink fluids thirst and body needs, some perature is further increased. during training and during com- "voluntary" dehydration will still The adverse effects of dehydra- petition to prevent the detri- occur. One must make a conscious tion on body temperature are mental effects of dehydration on effort to drink, even when not sufficient to negate the thermal physical performance. This is es- thirsty. Flavoring and cooling advantages gained by being physi- pecially important during hot fluids will increase their palatabil- cally fit and heat acclimated. In weather because sweating rates ity and help minimize voluntary other words, the time and effort rise as the temperature increases. dehydration. spent becoming heat acclimated During hot weather, seden- The purpose of this article is will be wasted if athletes do not tary activities will produce sweat- to discuss fluid replacement strat- drink enough to prevent dehy- ing rates of 0.3 to 1.2 liters per egies to enable optimal athletic dration. hour. During physical activity, performance. More detailed infor- There is a common belief sweating rates of 1.0 to 2.5 L/hr mation on exercise and fluid re- that training and heat acclima- are likely. These high sweating placement can be found in a tion will reduce the need to drink rates will increase daily fluid needs recent position statement by the fluids during exercise. This is not from 2 to 4 L/day (range from American College of Sports Medi- true. Heat acclimation usually in- sedentary persons to active ath- cine (1995), as well as in other creases the need to drink fluids, as letes) in temperate climates to published research (Marriot, acclimation increases sweating 4 to 10 L/day in hot weather. 1994; Maughan, 1991; Sawka & rate. The goal of fluid replacement Pandolf, 1990). To avoid dehydration, heat is to match fluid intake to sweat acclimated athletes must either loss in order to avoid dehydration. Detrimental Effects drink larger volumes of fluid or Thirst itself is not an accurate in- of Dehydration drink more often to offset the in- dicator of fluid needs, as sensa- creased sweat rate. Likewise, if tions of thirst are not usually Moderate levels of dehydration (2 they fail to drink enough fluids to perceived until the individual has to 5% body weight) have been meet their needs, heat acclimated a water deficit of approximately shown to reduce athletic perfor- persons will become dehydrated 2%of body weight. Studies report mance as much as 50%; the effects more rapidly than their nonaccli- that even ad libitum water intake are most detrimental when the mated counterparts. results in incomplete water re- weather is hot and exercise inten- placement ("voluntary" dehydra- sity is high. What to Drink tion) during exercise and/or heat Dehydration hinders physical exposure. Because of these factors, performance, at least in part, by The goals of drinking are to re- athletes will often become dehy- increasing the thermal strain of place fluid and electrolyte losses drated by 2 to 6% of body weight exercise. A water deficit of 1 % of (from sweat) and supplement the during physical activity. body weight is enough to elevate body's limited energy stores. The Although heat acclimation body temperature, and as the wa- balance between the demands for improves the relationship between ter deficit increases, body tem- water, electrolyte, and carbohy-

O 1996 Human Kinetics 24 Athletic Therapy Today July 1996 drate depends on the type and Athletes in weight category Prior to exercise, the best duration of the event, the climatic or weight sensitive sports should fluid to drink is probably a sports conditions, and the individual's choose their drinks &th caution. drink such as , , own characteristics. Sports drinks usually contain ap- Sport, , etc. If the athlete consumes a nor- proximately 5-8% . This is because these sports drinks ma1 diet and the event is less than Drinking- 5 liters of contain small amounts of sodium, 45 min in duration, tap water is can provide up to 1,600 calories- which allows more of the fluid to sufficient. Even if the event lasts this may be more than half the be retained than when drinking longer than 45 min but less than athlete's total daily energy needs. plain water. However, if food is 3 hrs, electrolyte replacement is Soft drinks generally- contain consumed together with plain not necessary.For longer events, even more carbohydrate (10- water, the advantage of sports however, and when food will not 12%). Persons drinking 5 L of soft drinks is minimized. be consumed prior to competi- drinks a day will get over 2,000 tion, carbohy- calories, not Drinking drate and elec- counting what During Exercise trolyte replace- they consume ment should be in solid food. Ensure that adequate fluids are considered. Another disad- available at training and competi- The most ef- vantage of soft tion sites. For all competition, ath- fective drinks for drinks is car- letes should bring drinks they are fluid and carbo- bonation, which comfortable drinking. Make every hydrate replacement contain 2 gives the false impression of a attempt to cool the beverages, as to 8% carbohydrate and 20-30 full stomach and thus tends to people generally will drink more mmol/L (0.5-0.7 g/L) sodium. reduce fluid consumption. of a cool beverage than a warm These formulations enable fast one. The athlete should be en- fluid delivery and the addition of Drinking couraged to drink whenever he or sodium enhances fluid retention. Before Exercise she gets the opportunity. Although more energy can be de- During training, athletes livered with higher concen- Athletes should begin each train- should experiment with different trations (15-20%, or 150-200 ing session fully hydrated. It is a drinking schedules to determine g/L, or even more), this will slow good habit to drink 400 to 600 ml their individual fluid needs and the stomach emptying rate and approximately 1 hour before be- how best to replace fluid losses. limit fluid delivery. ginning exercise; this will offset Fluid loss can be determined by The major difference be- any preexisting water deficit. If weighing oneself before and after tween the commercial sports this volume does not induce uri- exercise. This should become drinks is taste. Athletes should ex- nation prior to exercise, this prob- part of the normal training rou- periment with different sports ably means the athlete is still tine. Most of the weight loss is drinks and the of each dehydrated from the previous water, and 1 kg weight loss = 1 L brand. Since taste preferences workout. fluid. Check the amount of fluid change during physical exertion, Preexisting dehydration can consumed by counting the num- individuals should determine be a problem during early morn- ber of drinking bottles used each their drink preferences both at ing training sessions because day. rest and during exercise. water loss occurs during sleep. En- The rate at which fluids can In addition, since the athlete courage athletes to keep water at be replaced by mouth is limited by will drink 4 to 10 L of fluid a day, their bedside so they can take how fast the fluid is emptied from several different drinks (water, drinks if they wake up during the the stomach and how fast it can sports drinks, other fluids) or night. In addition, they should be absorbed from the small intes- flavors of the same drink should drink immediately after awaken- tine. Since gastric emptying and be made available to provide ing in the morning to offset the fluid absorption rates vary from variety. overnight fluid losses. one person to another, each ath-

JU~Y1996 Athletic Therapy Today