Dehydration stresses the body: Your body temperature rises, your heart beats faster, you burn more glycogen, you have trouble concentrating, and exercise feels harder. Some athletes are more tolerant of dehydration than are others, but for the most part, the more dehydrated you are, the greater the strain you will experience.
Whereas fitness exercisers (who work out for 30 to 60 minutes at a moderate pace three or four times a week) can easily maintain water balance by eating and drinking normally, athletes who exercise hard day after day can become chronically dehydrated if they fail to fully rehydrate daily. Football players in full uniform might lose far more fluids than they would think to consume. Having sweat-loss data eliminates the guesswork.
Most athletes who lose more than 2 percent of their body weight through sweat, lose both their mental edge and their physical ability to perform well, especially in hot weather. Yet during cold weather, you are less likely to experience reduced performance even at 3 percent dehydration. That is, a runner feels less impact of dehydration on her performance during a cold winter run than she does during the same run in the summer heat.
Dehydration of 3 to 5 percent negatively affects muscle strength and short intense bursts of anaerobic performance, such as weightlifting. A sweat loss of 9 to 12 percent of body weight can lead to death. Warning signs of heat illness are muscle cramps, nausea, vomiting, headache, dizziness, confusion, disorientation, weakness, reduced performance, an inability to concentrate, and irrational behaviour.
Fluids Before Exercise
The goal of drinking before you exercise is to start exercising when your body is in water balance, not in deficit from the previous exercise session. You might need 8 to 12 hours to rehydrate. The goal is to drink 5 to 10 ml per kilogram of body weight at least four hours before the exercise task (ACSM 2016). For a 68 kg athlete, this equates to 300 to 600 millilitres of fluid. By hydrating several hours pre-exercise, you have time to eliminate the excess before starting the exercise event.
If you drink a beverage containing sodium (110 to 275 ml of sodium per 240 ml) or eat a few salty snacks or sodium-containing meals, the sodium will stimulate your thirst so that you drink more; the sodium also helps retain the fluid so it doesn’t go in one end and out the other. If you sweat heavily, consuming 300 to 700 milligrams of sodium in the two to three hours before exercise can help maintain your sodium balance. There’s no need to try to hyperhydrate. The body can absorb just so much fluid—and you will end up needing to urinate during the event. Overhydrating can also dilute your blood sodium; if you then continue to aggressively drink fluids during exercise, you can increase your risk of becoming waterlogged and developing hyponatremia, a potentially fatal condition related to diluted blood, causing an abnormally low sodium level.
Fluids During Exercise
The goal of drinking during exercise is to prevent excessive dehydration, as defined as a loss of more than 2 percent of body weight as a result of a water deficit. If you are exercising hard enough to risk becoming dehydrated, you should drink periodically during exercise. If you will be exercising for more than two or three hours, you should know your sweat rate to prevent the performance decline associated with small cumulative mismatches between how much fluid you need and how much fluid you are losing via sweat. Because few athletes actually make the effort to learn their sweat rates, a starting point is to drink as desired, according to thirst.
What should you drink during exercise? The recommended fluid replacer contains a little sodium to stimulate thirst and enhance carbohydrate absorption, and a little carbohydrate (sugar) to provide energy. You can consume these nutrients through standard foods such as pretzels and bagels as well as commercial sports foods, which can be more convenient for runners, triathletes, and other endurance athletes.
When you are exercising hard for more than an hour (or doing less intense, longer exercise), consuming 120 to 240 calories of carbohydrate (30 to 60 g) per hour along with water can improve your performance. If you’ll be out for more than two and a half hours, bump that intake up to 60 to 90 grams of carbohydrate per hour (ACSM 2016). Carbohydrate helps maintain normal blood glucose levels so you are able to enjoy sustained energy. Sports drinks are an easy way to get carbohydrate plus water. For example, 480 ml of Gatorade offers 25 grams of carbohydrate and 100 calories; 480ml of Powerade offers 35 grams of carbohydrate and 140 calories. Practice drinking large volumes of fluid during training to help you adapt to the fluid load and prevent stomach sloshing and discomfort during competition.
Fluids After Exercise
After sweaty exercise, your goal is to fully replace all lost fluids and electrolytes. How aggressively you rehydrate depends on how quickly you need to recover before your next exercise session and how big a fluid–electrolyte deficit you incurred. Most active people can recover with normal meals (that contain a little sodium) and plain water. If you are significantly dehydrated and need to exercise again within 12 hours, then you need to be more aggressive with your rehydration program and sprinkle extra salt on your food if you had high sodium losses through sweat.
Drinking 50 percent more fluid than you lost in sweat will enhance rapid and complete recovery from dehydration. (The extra fluid accounts for what gets lost via urine.) Sipping fluids over time maximises fluid retention and is preferable to drinking large amounts in one sitting. If you become dehydrated during an unusually long and strenuous bout of exercise, you should drink frequently for the next day or two. Your body may need 24 to 48 hours to replace the fluid you lost in sweat.
If you become more than 7 percent dehydrated (via sweat losses, diarrhoea, or vomiting), you will likely end up requiring intravenous fluid replacement under a doctor’s care. In most cases, there is no advantage to taking fluids by IV, unless for medical necessity. Your best bet is to stay out of the medical tent in the first place by knowing your sweat rate and drinking accordingly.
Reproduced by permission from N. Clark, Nancy Clark’s Sports Nutrition Guidebook, 6th ed. (Champaign, IL: Human Kinetics, 2020).