Hydration is a major factor in athletic performance because water helps move heat, nutrients, oxygen, and waste through the body. During exercise, muscles produce heat and the body loses water and salts through sweat. If fluid loss becomes too large, the heart works harder, body temperature rises, and coordination can decline.
Good hydration supports endurance, strength, focus, and safer recovery.
Key Facts
- Percent body mass loss = (mass before exercise - mass after exercise) / mass before exercise × 100%
- 1 kg of body mass lost during exercise is approximately 1 L of fluid lost, if food and urine changes are small.
- Sweat rate = (fluid lost + fluid consumed - urine produced) / exercise time.
- A body mass loss of about 2% can begin to reduce endurance, power output, and mental focus for many athletes.
- Electrolytes such as sodium help maintain fluid balance, nerve signals, and muscle contraction.
- Heart rate often rises during dehydration because lower plasma volume reduces stroke volume, so the heart beats faster to maintain cardiac output.
Vocabulary
- Hydration
- Hydration is the state of having enough water in the body to support normal body functions and performance.
- Dehydration
- Dehydration occurs when the body loses more fluid than it takes in, reducing the water available for cooling, circulation, and cell function.
- Electrolyte
- An electrolyte is a dissolved ion, such as sodium or potassium, that helps control fluid balance and electrical signals in nerves and muscles.
- Sweat rate
- Sweat rate is the amount of fluid an athlete loses through sweating per unit of time.
- Thermoregulation
- Thermoregulation is the body's process of keeping internal temperature within a safe range.
Common Mistakes to Avoid
- Ignoring body mass changes before and after exercise, because this is one of the easiest ways to estimate fluid loss and plan replacement.
- Drinking only when performance already feels poor, because thirst can lag behind fluid loss during intense or hot conditions.
- Replacing sweat loss with plain water only during long events, because heavy sweating can also remove sodium and increase the risk of electrolyte imbalance.
- Assuming the same hydration plan works for every athlete, because sweat rate depends on body size, fitness, clothing, heat, humidity, intensity, and genetics.
Practice Questions
- 1 An athlete has a mass of 70.0 kg before practice and 68.6 kg after practice. What percent of body mass did the athlete lose?
- 2 During a 2 hour training session, a runner loses 1.8 kg of body mass, drinks 0.6 L of water, and produces no urine. Estimate the runner's sweat rate in L per hour.
- 3 Two athletes lose the same amount of sweat during a game, but one plays in humid air and the other in dry air. Explain why the athlete in humid air may have more trouble cooling down.