This is an excerpt from NSCA's Guide to Sport and Exercise Nutrition-2nd Edition by NSCA -National Strength & Conditioning Association & Bill I. Campbell.
By Jennifer Bunn
Maintaining fluid and electrolyte balance is crucial for people who engage in aerobic endurance exercise. In fact, a fluid loss of 2% to 3% of body weight has been shown to reduce exercise performance in both hot and temperate environments (31, 42). Aerobic athletes should aim to be euhydrated before the start of the activity or performance, expect to have a mild net loss of body fluid during the exercise, and then replenish the loss of fluid, carbohydrate, and electrolytes after completion of the bout.
People should begin exercise euhydrated and with normal electrolyte levels. Good hydration practices during the day, focusing on the consumption of fluids and foods with high water content such as fruits and vegetables, should be the main goal. If at least 8 to 12 hours have passed since the last exercise session and fluid consumption is sufficient, the person should be close to a euhydrated state. On the other hand, for someone who has lost a significant amount of fluid and has not replenished with fluids and electrolytes in the amounts needed to establish euhydration, an aggressive preexercise hydration protocol is in order (2).
Hydration should begin several hours before exercise to allow absorption and urine output. At least 4 hours before exercise, athletes should consume approximately 0.08 to 0.1 fluid ounces per pound (5-7 ml/kg) of body weight. They should consume more fluid at a slow rate—0.05 to 0.08 fluid ounces per pound (3 to 5 ml/kg) of body weight in addition to the aforementioned fluid recommendations—2 hours before exercise if the person is not urinating or if the urine is dark (2). Consuming sodium-rich foods at this time can help stimulate thirst and retain fluids. If sodium is consumed in a beverage, the recommended amount is 20 to 50 mEq/L (460-1,150 mg/L) of fluid (2).
A common practice for athletes before an event is to attempt to hyperhydrate with water. This practice is not advised because it increases the risk of urination during the event and could dilute the sodium levels in the body, thus increasing the risk of hyponatremia (40). For promotion of a euhydrated state before training or competition, fluid palatability is of utmost importance. Palatability or the lack of it will contribute to or detract from preexercise hydration strategies. The fluids should typically be lightly sweetened, should contain sodium, and should be cool in temperature. Beverages with carbohydrate and sodium have been shown to promote fluid retention up to 4 hours after ingestion while in a euhydrated state (43).
The goal of drinking during exercise is to prevent excessive dehydration (greater than 3% of body weight reduction from water loss) and excessive changes in electrolyte balance (2). Athletes may also choose to drink ad libitum, or according to their thirst preferences. This method of hydrating has been shown to result in 2% to 3% loss of body weight with no negative effect on performance (23). Other fluid guidelines suggest that athletes may follow an individualized fluid plan but should aim for 3 to 8 fluid ounces (90-240 ml) of a 6% to 8% carbohydrate–electrolyte beverage every 15 to 20 minutes during exercise lasting longer than 60 to 90 minutes (2, 69). Note, however, that a recent meta-analysis showed that drinking ad libitum during aerobic exercise caused greater weight loss of about 2% of body weight compared with programmed drinking in which athletes lost only 1% of their body weight during exercise (23). Further, ad libitum drinking improved performance over programmed drinking by approximately 1% (23). Evidence does not seem to support drinking as much as possible during exercise to attenuate all fluid losses, and research also suggests not to exceed ingestion of more than 27 fluid ounces per hour (800 ml/h) during exercise (48).
Consuming carbohydrate during exercise maintains blood glucose levels and reduces fatigue. A sport drink typically contains the following (31, 32):
- 20 to 30 mEq of sodium (460-690 mg) per liter
- 2 to 5 mEq of potassium (78-195 mg) per liter
- About 5% to 10% carbohydrate concentration
Energy bars, gels, and other foods, depending on a person’s needs and preferences, can also supply this combination (10). Consuming beverages with sodium (20-30 mEq/L fluid) or snacks containing sodium will help stimulate thirst and retain water (53). Like sodium, a sport beverage with protein may increase fluid retention, but take caution because this item could result in some gastrointestinal distress (64). Athletes should train first with the beverage they intend to consume for any performance or race days.
After exercise, the goal is to fully replenish any fluid and electrolyte deficit from the exercise bout (2). Athletes should consume 150% of the lost weight to achieve normal hydration within 6 hours after exercise (42). Therefore, practically speaking, the recommendation is to ingest 24 fluid ounces (720 ml) of fluids for every pound (0.45 kg) of body weight lost during training. Although plain water is effective for rehydration, athletes should consider a sport drink or consume their water with foods that contain electrolytes such as sodium and chloride to replace electrolyte losses (14).
As a whole, alcoholic and caffeinated beverages have diuretic effects, but such effects are transient and therefore those beverages do contribute to daily hydration recommendations. But if rapid rehydration is the goal postexercise, the recommendation is to avoid alcoholic and caffeinated beverages in the first few hours after activity (14). The fluid chosen in the postexercise period should promote rapid rehydration.
Depending on the amount of time before the next exercise session, consuming sodium-rich foods and beverages with water after competition or a training session should suffice. Sodium is one of the key nutrients that athletes should consume in the postexercise period to return to a euhydrated state because it will help retain ingested fluids and stimulate thirst. Although sweat sodium losses differ among individuals, which can make individual sodium prescription difficult during this period, a little extra salt added to meals or snacks may be particularly useful for those with high sweat sodium losses (2).