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Exercise in the Heat and Adequate Hydration

Bushman, Barbara A. Ph.D., FACSM

ACSM'S Health & Fitness Journal: July/August 2013 - Volume 17 - Issue 4 - p 4–7
doi: 10.1249/FIT.0b013e318296bdda
DEPARTMENTS: Wouldn't You Like to Know?

Although I enjoy exercising outdoors during the summer months, I do fi nd the heat to be a challenge. I try to stay hydrated when doing longer endurance sessions, but I am not sure I’m getting in the right amount of fl uids. I’m confused about how much I should be drinking as well as what to drink (i.e., water, sports drinks)

Disclosure: The author declares no conflict of interest and does not have any financial disclosures. Barbara A. Bushman, Ph.D., FACSM, is a professor at Missouri State University. She holds four ACSM certifications: Program Director, Clinical Exercise Specialist, Health Fitness Specialist, and Personal Trainer. Dr. Bushman has authored papers related to menopause, factors influencing exercise participation, and deep water run training; she authored ACSM’s Action Plan for Menopause (Human Kinetics, 2005), edited ACSM’s Complete Guide to Fitness & Health (Human Kinetics, 2011) and promotes health/fitness at


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A:Summer is a wonderful time to get outside and be active. However, exercising in the heat, typical in many locations during the summer months, can increase the risk of heat cramps (Box 1. Heat Cramps) and heat illnesses (Box 2. General Non-Fluid-Related Recommendations for Preventing Heat Illness) (3). Many factors can affect exhaustion experienced by active individuals, including (3):

* duration and intensity of exercise

* the individual’s aerobic capacity (V˙O2max)

* physical conditioning

* acclimatization of exercise-heat stress

* personal factors (e.g., medications, supplements, sleep)

* environmental conditions

* hydration status (Box 3. Hydration Status: Urine Color)

When considering the potential interrelationships and interactions among and between factors, it is apparent that no one universal recommendation related to fluid consumption during exercise in the heat can be applied. An individualized approach is best (7). There are concerns related to consuming too little fluid as well as for consuming a volume that is greater than what is needed to replace sweat losses (9).

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Sweating is an important mechanism for heat loss in the body (Box 4. Dissipating Heat). With prolonged exercise in hot environments, individuals can lose more sweat than is replaced by taking in fluids (3) (Box 5. What about Sweat?). Sweat production and skin blood flow will start to decline with fluid deficits of 3% to 5% (3). Water deficits greater than 2% to 3% of body weight may result in decreases in performance; adequate fluid intake is recommended before exercise as well as during and after exercise (2). Before exercise, planning ahead will allow for optimal hydration status and excretion of any excess fluid through urination (2). During exercise, the target should be to drink enough to cover the losses caused by sweating — NOT in excess of sweat rate (2, 4). The 2007 American College of Sports Medicine Position Stand, “Exercise and Fluid Replacement,” provides guidance on fluid replacement to sustain appropriate hydration during exercise (9).

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Fluid Intake Before Exercise

The goal is to start exercise in a euhydrated or normal level of hydration. To this end, consuming 5 to 7 mL of water or sport beverage per kg body weight (2 to 3 mL/lb) is recommended at least 4 hours before exercise (9). Additional fluid might be needed if this level of consumption does not result in urine production or if the urine is dark or concentrated. If so, drinking some additional fluid a couple hours before the event will still allow time for urine output to return to normal (avoiding the need to void during the competition or exercise bout) (9).

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During Exercise

During exercise, the goal is to avoid excessive dehydration and excessive changes in electrolyte balance. The Position Stand does not include specific recommendations on a fluid/electrolyte replacement schedule because of the many factors that impact sweat rate and the electrolyte concentration of sweat, including clothing and equipment, metabolic requirements, duration of the session, weather conditions, training status, heat acclimatization, and genetic factors (9). Instead, the Position Stand recommends monitoring body weight changes, with a goal to prevent excessive dehydration (i.e., postexercise body weight no more than 2% below preexercise body weight) (9). As an example of the variability of requirements, the Position Stand highlights fluid intake between 0.4 to 0.8 L/hour for marathon runners; higher rates for faster heavier runners in warm environments and lower rates for slower lighter runners in cooler environments (9).

If the exercise session or competition is longer (45 to 50 minutes or more) or involves high-intensity intermittent exercise, drinking a carbohydrate-electrolyte beverage seems to offer benefits when compared with water only (4). In these instances, beverages containing 6% to 8% carbohydrates are recommended (2). Sports drinks typically contain carbohydrates in this range as well as electrolytes (e.g., sodium, potassium) to help replace losses in the sweat. Sodium in sports beverages stimulates thirst, which helps encourage fluid consumption (9).

One other consideration is ensuring that the fluid consumed moves from the gut into the blood. For any fluid to provide benefit, the fluid must move from the stomach (gastric emptying) and be absorbed from the intestine into the blood (intestinal absorption). Gastric emptyingcan be optimized by increasing the amount of fluid in the stomach and by keeping carbohydrate concentration lower than 8% (2).

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After Exercise

The goal after exercise is to replace any fluid or electrolytes deficiencies that resulted from the exercise bout. Time permitting, normal meals and snacks (with sufficient sodium) consumed along with a sufficient volume of water can restore normal hydration (9). Consuming items with sodium during this time can stimulate thirst and help with fluid retention (9). The consumption of a large volume of fluid in a short period will increase urine production, which is not optimal for rehydration (4). Thus, when faced with a situation where rapid recovery from dehydration is needed (e.g., short recovery period), a replacement schedule of 1.5 L of fluid for each kilogram of body weight lost is suggested (the larger fluid amount intended to help account for the increased urine production) (9).

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As with many aspects of life, a balance is needed when looking at fluid consumption. Although the focus is often on ensuring adequate fluid intake, active individuals can overconsume fluids (8). Fluid intake should not exceed sweat losses; therefore, weight gain during exercise is evidence of excessive fluid intake (4). Fluid consumption that exceeds sweating rate seems to be a factor leading to exercise-associated hyponatremia (EAH), a situation where plasma sodium becomes too low (9). Low plasma sodium can result in symptoms of increasing severity, including headache, vomiting, swollen hands and feet, restlessness, confusion, disorientation, and wheezy breathing; in extreme cases, the end result can be seizure, coma, and even death (9). During prolonged exercise, the kidneys may not be able to compensate for imbalances in intake and loss of salt and water (8). EAH can occur in situations of prolonged heavy sweating where sodium is not replaced or with excessive water intake (2). Generally, hyponatremia seems to be caused by overdrinking (before, during, and even after) in events lasting less than 4 hours (9). However, for athletes in ultraendurance events, sodium losses can result in hyponatremia no matter if overdrinking or underdrinking; in such cases, replacing some of the sodium lost may be necessary (9). In a recent analysis of 5 years of 161-km ultramarathons (events that take 15 to 30 hours to complete), EAH was identified in both overhydrated and dehydrated athletes, with the occurrence appearing to be impacted by race day temperature; in cooler races, runners with EAH were more likely to beoverhydrated than dehydrated, but inthe hottest races, runners with EAH were more likely to be dehydrated than overhydrated (6).

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Exercising in the heat presents some unique challenges for active individuals. Fluid replacement is one factor worthy of special consideration. Ensuring adequate, but not excessive, fluid consumption is the goal. This can be monitored by checking body weight differences between preexercise and postexercise (9). In addition, observing urine color can be helpful; pale yellow urine, the color of lemonade, can be predictive of euhydration (4).

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1. American College of Sports Medicine. ACSM’s Resource Manual for Guidelines for Exercise Testing and Prescription. 6th ed. Philadelphia (PA): Lippincott Williams & Wilkins; 2010: p. 868.
2. American College of Sports Medicine, American Dietetic Association, Dietitians of Canada. Joint position stand. Nutrition and athletic performance. Med Sci Sports Exerc. 2009; 41 (3): 709–31.
3. Armstrong LE, Casa DJ, Millard-Stafford M, Moran DS, Pyne SW, Roberts WO. American College of Sports Medicine position stand. Exertional heat illness during training and competition. Med Sci Sports Exerc. 2007; 39 (3): 556–72.
4. Casa DJ, Clarkson PM, Roberts WO. American College of Sports Medicine Roundtable on hydration and physical activity: consensus statements. Curr Sports Med Rep. 2005; 4: 115–27.
5. Coelho LGM, Ferreira-Junior JB, Martini ARP. Head hair reduces sweat rate during exercise under the sun. Int J Sports Med. 2010; 31: 779–83.
6. Hoffman MD, Hew-Butler T, Stuempfle KJ. Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners. Med Sci Sports Exerc. 2013; 45 (4): 784–91.
7. Maughan RJ, Shirreffs SM. Development of individual hydration strategies for athletes. Int J Sports Nutr Exerc Metab. 2008; 18: 457–72.
8. Montain SJ, Sawka MN, Wenger CB. Hyponatremia associated with exercise: risk factors and pathogenesis. Exerc Sports Sci Rev. 2001; 29 (3): 113–7.
9. Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007; 39 (2): 377–90.
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Recommended Readings and Resources

Helpful brochure from ACSM to use with clients:
For more information on hydration and exercising in the heat, check out these ACSM Position Stands (
• Nutrition and Athletic Performance (2009)
• Exercise and Fluid Replacement (2007)
• Exertional Heat Illness During Training and Competition (2007)
© 2013 American College of Sports Medicine.