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Nutrition for Athletic Performance: A Focus on Hydration

By Marie Spano, MS, RD, CSCS, CSSD

A new position paper from the Academy of Nutrition and Dietetics (the Academy), the American College of Sports Medicine (ACSM), and the Dietitians of Canada, focusing on the various nutrition factors that influence athletic performance, was published in the March issues of the Journal of the Academy of Nutrition and Dietetics and Medicine & Science in Sports & Exercise

An evidence-based analysis that involved more than eight years of nutrition research, the position paper offers new perspectives on calorie, nutrient, and fluid intake; nutrition strategies for exercise recovery; the use of performance-enhancing supplements; and the importance of individualized strategies to meet each athlete’s nutrient needs, performance goals, and demands while considering food preferences, training schedule, and medical history.1

This article will discuss the latest guidelines on achieving and maintaining proper hydration for optimal athletic performance.

Fluid Losses
Before learning how to achieve and maintain hydration, it’s good to know the consequences of fluid losses and how the body inherently protects itself. During exercise, muscular contractions generate heat, which must be dissipated to avoid an excessive rise in core body temperature, a dangerous drop in blood volume and subsequent cardiovascular strain, and altered metabolic and central nervous system functioning. Sweat helps the body cool down, although excessive sweat and electrolyte losses can impair athletic performance and result in health complications.1 Sweat fluid losses >2% of body weight, particularly in hot weather, can impair cognitive functioning and aerobic performance. When sweat fluid losses rise to 3% to 5% of body weight in cooler environments, performance declines in high-intensity sports, sports requiring technical skills, and aerobic exercise. Cardiac output, sweat production, and blood flow to skin and muscle are impaired when sweat fluid losses rise to 6% to 10% of body weight,1,2 so it’s imperative that athletes are well hydrated before, during, and after exercise.

Assessing Hydration Status
Acute changes in body weight often reflect fluid fluctuations in athletes who are calorically balanced. Athletes can assess day-to-day hydration changes by keeping track of body weight each morning after voiding and before eating and drinking. In addition, dietitians can assess hydration status by measuring urine specific gravity (USG) and urine osmolality (UOsmol) to determine the concentration of solutes in urine. A USG reading <1.020 is indicative of euhydration. (RDs should give athletes a sealed urine cup so they can collect their first urine in the morning.) USG and UOsmol should be measured first thing in the morning, because values can be misleading if urine is assessed during periods of rehydration. If a dehydrated athlete consumes large volumes of fluid, they will urinate large volumes of fluid for a period of time before becoming euhydrated. However, during this period, USG and UOsmol are misleading, as both values will indicate that the athlete has achieved euhydration when in fact they’re still dehydrated.

Hydration Recommendations
Attaining and maintaining proper hydration will be different for every athlete because fluid needs are based on individual training demands, environmental conditions, and sweat losses. The position paper provides the following general guidelines that dietitians can tailor to individual clients.

Before Exercise
“Athletes who begin exercise in a hypohydrated state will have a difficult time achieving euhydration during training,” says Laura J. Kruskall, PhD, RDN, CSSD, LD, FACSM, FAND, director of nutrition sciences at the University of Nevada Las Vegas. General recommendations suggest consuming 5 to 10 mL/kg bodyweight over a two- to four-hour period before exercise to achieve a pale urine color and allow for sufficient time to void excess fluid. However, the recommendation to assess urine color is controversial. The 2007 ACSM position paper “Exercise and Fluid Replacement” notes that “Urine color and urine volume are often subjective and might be confounded.” Also, urine will be pale in color during the immediate rehydration period after dehydration although the athlete is still dehydrated. To achieve proper hydration, athletes may want to add sodium to their sports drinks or preworkout meals to help retain the fluid they’re consuming. The amount of sodium they add to drinks and meals depends on their total dietary sodium intake and sodium losses through sweat while training in a particular environment. An experienced sports dietitian can help athletes determine whether they need more sodium before exercise and how much they should consume.

During Exercise
Sweat is a chief factor that influences hydration during and after exercise. The amount and the rate at which athletes sweat varies tremendously, generally ranging from 0.3 to 2.4 L/h. Athletes can estimate sweat losses by weighing themselves before and after exercise in minimal clothing after voiding while accounting for total fluid consumed during the workout. Given individual differences in sweat rates, dietitians can help athletes develop a hydration plan that helps them limit fluid losses to <2% body weight while preventing overhydration. Overhydration, particularly with water or other low-sodium beverages, can dilute blood sodium, resulting in dangerously low blood sodium levels, or hyponatremia, which occurs when plasma sodium is <135 mmol/L. In general, most athletes can maintain an adequately hydrated state by consuming 0.4 to 0.8 L/h of fluid.

Sodium is the primary electrolyte lost through sweat and those losses may increase an athlete’s risk of cramping, particularly when combined with substantial fluid losses through sweat. Sodium losses generally range between 230 and 1,610 mg per liter of sweat depending on heat acclimatization, diet, sweat rate, and genetics. During exercise, athletes should consume a beverage that contains sodium if they lose a considerable amount of sodium through sweat. Athletes with high sweat rates (>1.2 L/h), salty sweat, or those exercising for more than two hours are more likely to have considerable sodium losses.

After Exercise
After a workout, the goal of rehydration is to improve fluid balance and restore sodium losses. Many athletes finish exercising with a fluid deficit. They may continue sweating after they’re done and lose additional fluid through urine. A greater fluid volume is necessary to replace fluid losses to make up for 125% to 150% of the fluid deficit (1.25 to 1.5 L/fluid for each pound of bodyweight lost). To restore sodium, athletes shouldn’t limit sodium intake postexercise. Consuming small amounts of table salt may be helpful to restore sodium lost through sweat. Since sweat sodium losses vary among athletes as does habitual dietary sodium intake, a sports dietitian must take both variables into account when making postexercise sodium recommendations.

Hypohydration and electrolyte losses can impair cognitive thinking, athletic performance, and health. “An experienced sports dietitian has the knowledge and skillset to assess an athlete and develop an individualized hydration plan that can be tailored for environmental changes,” says Roberta Anding, MS, RD, LD, CSSD, CDE, FAND, sports dietitian in the department of orthopedic surgery at Baylor College of Medicine in Houston. The Academy has created standards of practice that help dietitians develop the skills and level of experience required to guide athletes on the timing, amounts, and types of foods, fluids, and supplements they need for peak athletic performance, career longevity, and health, Anding says.

— Marie Spano, MS, RD, CSCS, CSSD, is the sports dietitian for the Atlanta Hawks of the National Basketball Association and works with professional, college, and recreational athletes in various sports. Spano has appeared on NBC, ABC, Fox, and CBS affiliates, and authored hundreds of magazine and trade publication articles. She’s coeditor of the NSCA’s Guide to Sport and Exercise Nutrition.

1. Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: nutrition and athletic performance. J Acad Nutr Diet. 2016;116(3):501-528.

2. American College of Sports Medicine, Sawka MN, Burke LM, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377-390.