Before we get into exercise-specific hydration, it’s important to pay attention to your daily needs outside of physical activity. Each day you lose water through breathing, sweating, and using the bathroom, and it’s important to replace these losses. Properly addressing everyday hydration also helps prepare you for training and races – if you show up to the starting line dehydrated, it’s much more likely that you’ll suffer a dehydration-related drop in performance during the event.
How much fluid a day? There’s no one perfect answer, and it likely depends on your body composition, but the Institute of Medicine has made the following recommendations:
- Men: 3 liters (about 13 cups) of fluid from beverages
- Women: 2.2 liters (about 9 cups) of fluid from beverages
Hydration for Exercise
Your most important goal with hydration during endurance exercise is to prevent both dehydration and hyponatremia. Dehydration occurs when you lose too much fluid and aren’t replacing it, and can lead to declining performance and increased risk of heat injury. Hyponatremia occurs when you take in too much fluid and dilute your blood’s sodium levels. Hyponatremia can cause serious side effects, including nausea, vomiting, and disorientation in mild forms, and life-threatening complications like respiratory arrest, coma, and death in severe cases.
Dehydration is much more common among runners than hyponatremia, but hyponatremia generally presents more serious concerns and complications.
The goal here is balance. Ideally, you want to replace enough fluid so that you only lose about 2% of your body weight. You want to avoid losing much more than that, or you start to risk performance impairment. You also don’t want to gain weight, as this increases your risk of hyponatremia. For example, let’s say you are a 170 pound athlete. A 2 percent loss in body weight would be the equivalent of losing about 3.5 pounds. During exercise, we would want you to avoid losing much more than 3.5 pounds (risk of dehydration impairing performance) as well as avoid gaining any weight (risk of hyponatremia).
Because of this careful balance, there’s no one “right” amount to drink during exercise. It depends on many factors, including your weight, training level, speed, sweat rate, sweat composition, and more.
The best way to estimate your hydration needs is using a combination of two methods:
First, conduct a sweat test during a training session, ideally under the conditions that you expect for race day (so if you expect the race weather to be hot, get out there on a hot day). Weigh yourself before and after training, and be sure you note how much you were drinking during your training session. If you’ve gained weight, you’re likely drinking too much. If you’ve lost more than 2-3% of your body weight, you probably want to drink a bit more. If you’ve lost around 2% of your body weight, you’re likely right on track.
Second, pay attention to your body’s thirst cues. Drinking to thirst is a very successful strategy for many athletes, however I’d disagree with those who say that it is the only strategy anyone ever needs to use. Why? Some people don’t pay attention to their thirst; the aid stations may be spaced at a distance that don’t allow you to drink to your thirst; and older athletes have a decreased thirst mechanism.
For these reasons, I recommend conducting the sweat test to get an estimate of your hydration needs and develop a loose plan for hydrating during exercise. This may indicate that you need to carry a fuel belt, or may guide you towards drinking at certain aid stations. Then, during the event, you have your plan in place and ready to execute – but can (and should) defer to your body’s physiologic cues when in question of whether to drink or not. If you feel dry mouth or thirsty, drink. If you feel nauseous or sloshing, slow down the fluid intake.
Share with us: Do you get concerned about hydration during events? How do you handle exercise hydration?