Water: Recommendations and how dehydration affects performance

The main mediator of nutrients and toxins in the body is water, which accounts for up to 60% of the human body weight (1). Insufficient fluid intake slows down most physiological processes during exercise, including the removal of lactate from muscles, oxygen delivery, and lower body temperature, which in turn reduces performance. Dehydration lowers blood volume and blood pressure, leading to cardiovascular exhaustion, which can have irreversible effects on health.

Dehydration of just 1-2% of body weight causes an increase in heart rate, body temperature, glycogen utilization, and a decrease in cardiac output, cognitive function, anaerobic power, and time to exhaustion (2, 3). Gonzalez-Alonso et al showed that 2 hours of moderate-intensity cycling without hydration resulted in an 18% decrease in cardiac output, a 13% increase in heart rate, and a 5% increase in blood pressure (4).

The main fluid loss during exercise occurs through sweating and respiration, with 1 to 2 liters/hour lost. The rate of fluid loss is dependent on genetics, environment, and exercise type. When a person exercises in extreme conditions, such as high air temperature and low barometric pressure, sweating increases. Therefore, daily water intake should vary depending on these factors.

Recommendations:

Indicators of dehydration include dry mouth and dark yellow urine. There are strategies to maintain fluid levels during exercise. Current recommendations include:

1. Replenish fluids before exercise (5). The athlete should slowly drink approximately ∼5–7 ml/kg body weight at least 4 hours before exercise. If the athlete has dark or very concentrated urine, they should slowly drink an additional ∼3–5 ml/kg approximately 2 hours before exercise (6).

2. Avoid excessive water intake during exercise. Consume 1 to 2 liters per hour.

3. Replace any remaining losses after exercise (5).

Water plays a key role in athletic performance, but there are other factors such as sodium and carbohydrates. An imbalance in just one of these affects the function of water in the body and also reduces performance.

Further reading:

Belval, L. N. et al (2019) Practical Hydration Solutions for Sports, Nutrients, 11(7), pp. 1550.