Proper Fluid Intake in Sports

During sport, various mechanisms balance rising body temperature. The most important are increased skin blood flow and sweating. Sweating is the most effective way to lose heat during exercise, a protective bodily response. However, we lose water and electrolytes along with heat. Unless these are replaced during and after exercise, performance cannot improve and, at greater levels, health may suffer.

To replace sweat losses, we first need to know their size. Too little or too much water and electrolytes can harm the body. Sweating varies with age, sex, height, weight, genetic differences, clothing, air temperature and exercise intensity. Because many variables are involved, the simplest estimate uses the difference in weight before and after exerciseto determine an approximate personal value.

- Can Sweating Remove Toxins from the Body?

Dehydration (water loss equal to %2 of body weight, or for a 70 kg person 1,4 lt of fluid) and high body temperature have been observed to substantially reduce mental and cognitive performance, including perception and comprehension (depending on individual tolerance). Although dehydration does not directly affect aerobic performance, physiological increases in temperature, cardiovascular load and glycogen breakdown, alongside nervous system effects, reduce performance. Electrolyte loss may also cause muscle cramps.

Hyperhydration (excess fluid in the body) results from excessive drinking. Normally, excess fluid is excreted by the kidneys as urine and normal hydration returns within hours. During exercise, however, reduced kidney blood flow decreases urine output. This may cause hyponatremia (low plasma sodium concentration). Although dehydration is more common, hyperhydration-induced hyponatremia has more dangerous consequences.

One factor affecting sweating is sex: women sweat less and lose fewer electrolytes than men, mainly because of smaller body size and slower metabolism. Mouse studies found sodium transporters (Na/K ATPase) more active in male than female brains. This indicates greater hyponatremia-related edema risk in women. The difference was attributed to female sex hormones.

Another factor is age : people aged 65 and older generally have normal fluid levels because fluid balance adjusts slowly and plasma electrolytes are high, but they are also more sensitive to water loss. Increased plasma electrolytes also raise blood pressure. This age group should be particularly cautious about hyponatremia from excess fluid intake.

Eatingalso increases fluid intake and retention. We generally obtain electrolytes lost in sweat, sodium and potassium, from food. At rest, little is lost in urine and intake meets requirements. Exercise produces extra electrolyte losses that must be replaced.

Before exercise, fluid intakeshould aim to restore normal body fluid levels. Approximately 3-4 hours before exercise, drinking 3-5 ml/kg (for a 70 kg person, 210-350 ml) is recommended. Urine indicates hydration: producing urine after drinking suggests normal levels, while dark or absent urine suggests deficiency. Excess drinking offers no performance advantage; it causes discomfort during exercise and harms performance.

During exercise, because dehydration and electrolyte balance affect performance, adequate fluids should be consumed where possible. As noted, needs depend on many factors. Studies found sweat fluid losses varying between 0.4-1.8 lt. Researchers also measured approximate sweat losses in different people and conditions:

1. At 18 degrees Celsius, running at 8,5 km per hour, a 50 kg athlete sweats 0.43 lt hourly, a 70 kg athlete 0.65lt, and a 90 kg athlete loses 0.86lt through sweat.
2. At 28 degrees Celsius, running at 8,5 km per hour, a 50 kg athlete loses 0.52 lt hourly, a 70 kg athlete 0.75 lt, and a 90 kg athlete 0.97 lt through sweat.

Also, exercise duration, activity intensity and harsh weather create electrolyte and carbohydrate needs alongside water needs. Sports drinks containing 20-30 meq/l sodium, 2-5 meq/l potassium and 30-60 g carbohydrate are recommended under these conditions. Carbohydrate supplementation benefits blood glucose and glucose stores for activities lasting more than 1 hour.

After exercise, replace lost water and electrolytes. If there is sufficient time (12 hours), normal meals and water immediately after exercise can meet needs. With less than 12 hours available, 1,5 lt of fluid per kilogram lost is recommended.

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