Ultra-endurance athletes need to avoid overdrinking during triathlons if they want to avoid a serious episode of hyponatraemia (an abnormally low blood concentration of sodium). That's the key message from a major review of exercise-associated hyponatraemia by experts from South Africa and New Zealand.

Since it was first described in 1985 by sports scientist Tim Noakes, one of the authors of the current paper, hyponatraemia has increasingly been recognised as a common and potentially serious complication of ultra-endurance exercise, particularly for women. With a reported incidence of 18% in the New Zealand Ironman triathlon and 29% in the Hawaiian event, it has become the commonest medical complication of ultra-distance exercise, with a wide variation in severity from asymptomatic to life-threatening.

Despite the prevalence and risks of this condition, there is no consensus on its origin, with two conflicting theories in circulation. The first of these proposes that hyponatraemia is caused by large water and salt losses in the sweat, leading to dehydration as well as sodium depletion; the second suggests that athletes with hyponatraemia are overhydrated without a substantial sodium deficit.

In this paper, Noakes and his colleagues come down firmly in favour of the latter theory, mostly because of the evidence that hyponatraemia is associated with minimal weight change or even weight gain, which is more suggestive of fluid overload than dehydration.

Why the fluid overload? A series of 25 published case reports show a mean fluid intake which is higher than that generally recommended for ultra-distance exercise (500-1,000 mL/h). Interestingly, of nine athletes who developed hyponatraemia with intakes lower than 1L/h, eight were women, suggesting that standard recommendations for fluid intake may be too high for female athletes, who have a smaller body weight and may exercise at lower absolute work rates, with lower sweating rates. However, the authors point out that most asymptomatic cases of hyponatraemia occur in athletes are modestly dehydrated during exercise, indicating that other factors are also involved.

Fluid overload can result from failure to absorb fluid from the intestine or retention of fluid in the plasma - the liquid portion of the blood. Noakes inclines to the latter theory, which implies an inability of the kidneys to excrete fluid at high rates during prolonged exercise when rates of fluid ingestion are 750 mL/h or greater.

To prevent hyponatraemia, the researchers conclude, athletes have to avoid over-drinking. In a previous study, Noakes had suggested that a fluid intake of 500 mL/h is appropriate for most non-competitive athletes during prolonged exercise, while another expert has recommended intakes of 500-1,000 mL/h in ultra-distance events - both lower than the American College of Sports Medicine's recommendation of 600-1,200 mL/h.

One study involving informing triathletes of appropriate fluid intake rates and reducing the availability of fluids at support stations reduced the percentage treated for hyponatraemia in the medical tent from 24% to 4%.

'The advice to prevent overhydration by curtailing fluid intake has been criticised,' acknowledge the researchers, 'but it needs to be emphasised that appropriate rates of fluid intake for ultra-distance exercise have not been established and the standard recommendations for fluid intake, including those of the American College of Sports Medicine, are based on data from shorter distance events.

'As already noted, some athletes still develop hyponatraemia at modest fluid intakes and for slower athletes, or for those who are small or female, fluid intakes should be at the lower end of the recommended range. Nor is there any evidence that the mild deyhydration found in athletes who follow these guidelines is detrimental to their health.'

The role of salt supplementation in the prevention of hyponatraemia has yet to be determined. But the authors advise athletes to choose a sodium-rich sports drink as their replacement fluid during ultra-distance exercise, rather than water or a sodium-free soft drink.

Emerg Med 2001 Mar 13(1), pp 17-27

Isabel Walker