When the body lacks water

Physical performance begins to decline when there is just a 2% loss of body fluid

Decline in physical performance due to dehydration

Source: Yoshida et al. Eur J Appl Physiol, 2002, 87: 529-534

When the body is hot, blood vessels in the skin dilate to increase blood flow and allow perspiration in order to regulate the temperature. However, when we sweat a lot, we lose body fluid. When the level of body fluid falls, body temperature rises, and we face the risk of overheating.

When you lose body fluid equivalent to just 2% of your body weight, you feel thirsty and your body's ability to perform starts to fall. When you lose 3%, you get very thirsty, and experience symptoms such as feeling faint and suffering a loss of appetite. When you lose 4% to 5% of your body weight, you experience dehydration symptoms such as fatigue, headaches and dizziness. Finally, if you lose 10% or more of your body weight, there is a risk of death.

Dehydration and signs or symptoms

Moisture reduction rate
(% of body weight)
The main symptoms of dehydration
2% Thirst
3% Burning thirst, absent-mindedness, loss of appetite
4% Flushing of the skin, irritability, rise in body temperature, exhaustion, decrease of urine volume but increase in its concentration
5% Headache, feeling feverish
8-10% Swaying, convulsions
over 20% Anuria, death

The following reference was provided by Takeshi Morimoto:
Nakano, S. Sports medicine science, Chapter 2, Function of body fluids and blood, (1999), p. 40. Partially modified and with a reference to Adolph EF & Associates, Physiology of man in the desert. Hafner Publishing Company, New York,(1947), p. 191.

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