Dehydration during training

Author: Frédéric Delavier and Michel Gundill: Chapter from the book "Sports Supplements for Athletes".
2011-10-18

The harder and longer the muscles work, the more heat our body releases. Sweating is the most effective way for our body to get rid of excess heat. An athlete can easily produce up to one liter of sweat per hour. However, in extreme conditions, this figure can rise to three liters.

Dehydration in an athlete's body immediately affects not only performance but also health. Although our body is made up of 60% water, it has virtually no water reserves that we can expend without consequences. Loss of water due to excessive sweating results in immediate water deficit. This is fundamentally different from the energy reserves our body stores abundantly in the form of fat tissue and glycogen.

Fluid loss leads to a decrease in blood plasma volume (the liquid part of the blood). With each heartbeat, the heart pumps less blood, thus losing strength. It has to beat faster to adequately nourish all tissues. But this is only a partial compensation, as blood flow to the muscles eventually decreases, leading to insufficient oxygen delivery to the tissues and slowed removal of metabolic waste products generated during repeated muscle contractions. The muscles start to "suffocate" and perform worse in aerobic mode. Additionally, due to the difficulties associated with metabolism, body temperature may rise. All these physiological deviations manifest very quickly, causing a feeling of fatigue. The first signs of fatigue appear when water loss reaches about 2% of body weight.

Elite athletes have blood plasma volume circulating at 20%-30% higher than average, which provides additional protection against dehydration. Laursen (2006) demonstrated that high-level triathletes can handle fluid loss of up to 3% of their body weight. Therefore, amateur athletes must clearly understand that they should not try to keep up with professionals who can better handle dehydration than many others.

Theoretically, athletes should replenish water losses right before competitions. But in practice, this rarely happens. While it is indeed easy to lose a liter of water in an hour, athletes rarely consume more than half a liter of fluids per hour during competitions. It has been found that the amount of water they typically consume is around 200-300 milliliters per hour. This amount seems to correspond to the maximum absorption capacity of fluids by our digestive system per hour. If more than 800 milliliters are consumed per hour, there is a risk of digestive problems.