The Importance of Hydration
The body has a remarkable ability to regulate daily body water and electrolyte balance, however exercise and environmental stress (i.e., temperature, humidity) can significantly challenge this ability. Water is essential in athletics and affects performance more than any other nutrient.
During physical activity the body creates heat, which must be dissipated in order to continue the activity. The primary physiologic mechanism for cooling the body is sweat production; the body cools as the sweat evaporates from the skin. The inability to produce sufficient sweat, which occurs when blood volume is reduced due to decreased fluid intake, causes the body to overheat. Therefore, for normal thermoregulation it’s critical that athletes consume ample fluids to maintain a proper sweat rate.
The consequences of even minimal hydration can be severe – a loss of just 1% of total body weight can lead to elevated core body temperature, 3-5% loss results in cardiac strain and inability to dissipate heat. At 7% loss, collapse is likely. These rates of dehydration are not uncommon. Athletes who train or compete in the heat can easily dehydrate 2%-6%, and research has shown that most replace only about 50% of the sweat lost during exercise with fluid consumption.
Depending on exercise intensity and sweat rate, athletes may require an additional 3-4 gal/day above the 2-3 qt/day required under normal circumstances. Sweat rates for cyclists have been reported to be 0.29-1.25 liters/hour1, but can vary widely depending upon temperature, humidity, exercise intensity, type of clothing worn, level of fitness, and the athlete’s genetic predisposition for sweating. In addition, research has shown that wearing a commercial cycling helmet does not increase the rider’s level of heat strain, or perceived heat sensation of the head or body2.
With the high sweat rates typical of training and competition, dehydration and hyperthermia are inevitable unless fluid is ingested to match the volume being lost through sweat.
Fluid Needs Before and During Exercise
The ACSM (American College of Sports Medicine) recommends that athletes consume approximately 16 oz. of fluid 2 hours before exercise to ensure adequate hydration. On warm days, an additional 8 to 16 oz are recommended 30-60 minutes before exercise. ACSM guidelines also recommend that fluids be cool and flavored to enhance palatability and stimulate consumption. During exercise, athletes should consume liquids containing both carbohydrate and sodium chloride to provide energy and promote rehydration. Thirst should not be used as an indicator of hydration status. It is an emergency sensation by the body that marks the onset of performance-reducing problems. Athletes should drink even if they don’t feel thirsty. A general guideline for most athletes is
6 to 8-oz every 15 minutes.
Fluid Needs After Exercise
Following exercise, the general recommendation is to consume 20-oz of liquid for every pound of body weight lost. To ensure rapid and complete hydration athletes need to consume both water and sodium chloride, which should occur through consumption of foods and liquids over the course of 12 to 24 hours.
Factors to Consider When Choosing a Sports Beverage
An important factor in choosing a sports beverage is the type and concentration of carbohydrate it contains. Research has shown that both of these factors directly impact the rate at which fluid empties from the stomach. Smaller carbohydrate molecules (i.e., glucose, sucrose, fructose, maltodextrin) are preferred because they empty faster from the gut. A well-formulated sports drink usually contains electrolytes and 14-16gms of carbohydrate per 8-oz serving, providing a 6%-7% carbohydrate concentration. Formulations with greater concentrations have been shown to significantly slow gastric emptying, resulting in two complications: slower delivery of fluids to tissues, compounded by dehydration, and higher risk of GI distress.
Being mindful of these few facts about proper hydration will ensure a healthy cycling season all year, not just during the summer months.
1. Rehrer, N, et al. Sweat losses during various sports. Aust J Nutr Diet. 1996; 53:S13-S16.
2. Rasch W, et al. Selective brain cooling is affected by wearing headgear during exrcise. J Appl Physiol 1993;74:1229.
