Dehydration

Dehydration is a reduction in total body water that occurs when fluid losses through sweating, urine, breathing or other routes exceed fluid intake. It is often expressed as a percentage of body mass loss, with deficits greater than about 2% commonly impairing performance.

Causes and consequences of fluid deficits

During exercise, body water is continually lost via sweat and ventilation; in warm environments or during high-intensity sessions, sweat rate can reach 0.5–2.0 litres per hour. If these losses are not replaced, blood volume decreases and plasma becomes more concentrated, triggering thirst and hormonal responses to conserve water. Early signs of dehydration include thirst, a dry mouth and lips, reduced urine output and darker urine. As water deficit grows, people may notice fatigue, headache, dizziness, decreased coordination and an elevated heart rate. At about a 2% loss of body mass, endurance declines, core temperature rises and cognitive performance worsens. More severe dehydration can progress to heat exhaustion or heat stroke, a medical emergency. Risk factors include inadequate drinking, heavy sweat rates, hot or humid weather, restrictive clothing and illnesses that cause vomiting or diarrhoea.

Monitoring and preventing dehydration

Simple self-checks can help detect dehydration: monitoring urine colour, tracking body weight changes before and after exercise and noticing thirst cues. To minimise fluid deficits, athletes should start sessions well hydrated by drinking around 500 ml of fluid two to three hours before exercise and continue to sip roughly 200 ml every 15–20 minutes during activity. Replacing the weight lost after exercise with water or a beverage containing electrolytes supports recovery; for longer or intense sessions, drinks with sodium and a small amount of carbohydrate can aid fluid retention and energy supply. Avoiding excessive caffeine and alcohol, which increase urine output, and paying attention to environmental conditions and personal sweat rate are also important. Treating mild dehydration involves drinking water or a sports drink promptly; more serious cases may require oral rehydration solutions or medical attention.

By recognising the signs of dehydration and matching fluid intake to sweat losses, athletes can sustain physical and cognitive performance and reduce the risk of heat illness. Thoughtful hydration planning — before, during and after exercise — helps keep the body functioning at its best.

Related Terms: Hydration, Hypohydration, Hyperhydration, Prehydration, Rehydration