T D Noakes, Discovery Health chair of exercise and sports science (2012) BMJ 2012;345:e4171
Water is the major constituent of the human body and the total body water content is tightly regulated. The goal is to ensure that the water content of the cells and hence their size remains within a homeostatically regulated range.
Humans evolved as long distance persistence hunters on the arid savannahs of south and east Africa. We inherited the capacity to regulate our body temperatures during prolonged exercise in dry heat despite quite large reductions in total body water—no other mammal has the equivalent capacity.
Humans do not regulate fluid balance on a moment to moment basis. Because of our evolutionary history, we are delayed drinkers and correct the fluid deficits generated by exercise at, for example, the next meal, when the electrolyte (principally sodium but also potassium) deficits are also corrected.1 As a result, there is no need to completely replace any fluid deficit as it develops either at rest or during exercise. Instead people optimise their hydration status by drinking according to the dictates of thirst.
Over the past 40 years humans have been misled—mainly by the marketing departments of companies selling sports drinks—to believe that they need to drink to stay “ahead of thirst” to be optimally hydrated. In fact, relatively small increases in total body water can be fatal. A 2% increase in total body water produces generalised oedema that can impair athletic and mental performance; greater levels of overhydration result in hyponatraemic encephalopathy— severe cerebral oedema that produces confusion, seizures, coma, and ultimately death from respiratory arrest.