Commentary: role of hydration in health and exercise

Why we feel thirsty during exercise

Sweat is a relatively dilute plasma secretion containing more water than electrolytes. As a result sweating increases blood sodium concentrations and osmolality. These increases are sensed by receptors in the hypothalamus, which respond by producing hormonal and behavioural changes designed to maintain the osmolality within the homeostatically regulated range.

Hormonal changes increase renal sodium and water conservation; reflex stimulation of the anterior cingulate gyrus produces the conscious sensation of thirst that drives water seeking behaviour. As the sensation of thirst rises, exercise performance becomes progressively impaired—a useful control since it reduces the exercise intensity and hence the possibility of continuing large water loss through sweating.

The sensation of thirst ceases when sufficient fluid and solute (electrolyte) has been ingested to correct the blood osmolality. This control ensures that humans always drink just enough but not too much. There is no intrinsic biological drive that will cause overdrinking. Voluntary overdrinking causes blood osmolality to fall, which should inhibit thirst and reduce the pituitary release of antidiuretic hormone (ADH). ADH regulates water reabsorption in the distal renal tubules and is one of the most potent human hormones. Paradoxically, athletes who develop exercise associated hyponatraemic encephalopathy report persistent thirst and retain fluid, even though their blood osmolality is falling; this suggests the presence of the syndrome of inappropriate ADH secretion.1 2

Dehydration in a sports setting

Dehydration is not a medical illness.1 Correctly used, the term refers to a reduction in total body water content. Thus dehydration’s only symptom is thirst—the effect of which is to induce drinking.3 There is barely any risk that dehydration can occur in healthy athletes competing in a modern endurance event in which ample fluid is available.1 Only when the total body water is reduced by about 15%—as occurs in those lost in the desert without water for more than 48 hours—is voluntary motor activity completely inhibited, resulting in paralysis.1

Confusion arose when the erroneous belief that all athletes who collapse after exercise are suffering from a dehydration induced heat illness was promoted as part of the false “science of hydration.”1 This dictated that people collapsing needed to drink more fluids during exercise and immediate resuscitation with large volumes of intravenous fluids.

However, athletes who collapse are neither hotter nor more dehydrated than control runners who complete the same races without collapsing.1 4 Sporadic cases of heatstroke are also not caused by dehydration.1 5 Hydration influences regulation of body temperature in competitive athletes only indirectly. The key determinant of the body temperature during exercise is the exercise intensity or metabolic rate; the greater the intensity, the higher the temperature.1

Humans can raise their body temperatures slightly more (adaptive heterothermy) when forced to exercise without any or optimum fluid replacement. The higher body temperature increases the gradient for heat loss by convection, thus reducing the need to increase sweat losses to maintain safe body temperatures and conserving water.

Treatment of collapsed athletes

Clinicians often assume that athletes collapse after exercise because they are hypotensive (and hyperthermic) as a result of dehydration. In fact, most have exercise associated postural hypotension, a form of vaso-vagal fainting that occurs in susceptible individuals within seconds or minutes after exercise terminates.1 4 6The treatment is recovery in the head down Trendelenberg position.7

Clinical signs of dehydration are unreliable in detecting substantial fluid loss in athletes completing endurance events.8 Athletes who finish exercise with thirst are mildly dehydrated and need to eat and drink in order to replace their solute and fluid deficits.1 Athletes who complete exercise without thirst do not need any specific treatment. Any presenting complaints in athletes who do not report that they are also thirsty cannot be caused by dehydration.

By contrast, the symptoms and signs of overhydration are unmistakable. The athlete shows marked changes in cerebral function, from mild withdrawal, to confusion, seizures, and coma. The diagnosis is confirmed by measuring blood sodium concentration; the lower the value, the more severe the fluid overload.2 9

Overhydration is treated by absolute fluid restriction and bolus 3-5% hypertonic saline infusions for those with confusion or coma.10 This rapidly reverses the mental confusion in people with mild hyponatraemia11 and produces rapid diuresis in those with exercise associated hyponatraemic encephalopathy. If brain swelling is so advanced that there is a high risk of respiratory arrest or cerebellar coning, diuretics and intravenous mannitol infusions may be needed.12