Science Of Sport: Rehydration And Fluid Balance

Are Athletes In Danger Of Going Too Far?

Concerns about the potential risks of over-hydration and consequential hyponatraemia (low blood sodium levels) for endurance athletes have been raised in several quarters following the death of a female runner during the 2002 Boston marathon.

The athlete, who died of hyponatraemic encephalopathy (swelling of the brain caused by severely reduced blood sodium), had reportedly ingested excessive amounts of fluids before and during the race.

Meanwhile, recent reports in the medical literature seem to confirm a trend that marathon medical experts have observed: hyponatraemia has become increasingly common with the increasing participation of recreational athletes in endurance events.

In a recent editorial in the British Medical Journal, the world-famous South African exercise physiologist Professor Tim Noakes made the provocative claim that current guidelines on hydration, which have been in existence for the past 30 years, have been biased by sports drink sponsorship, do not take account of athletes’ individual needs and have led to the over-consumption of fluids during exercise (1).

The implication of these guidelines is that dehydration poses the greatest threat to health and wellbeing during exercise in the heat. Thirst is said to be an unreliable indicator of fluid needs, and athletes are advised to start replacing lost fluid before they feel thirsty. As recently as 1996, the American College of Sports Medicine advised athletes to consume the maximum amount that could be tolerated – up to 600-1200ml per hour.

Noakes believes that the notion that athletes must remain fully hydrated before, during and after exercise to maintain a high level of performance has led to a dangerous situation in which some athletes are bound to over-hydrate.

His views have been quoted in recent newspaper articles published on both sides of the Atlantic. But while he may have some truth on his side, there is a risk that such scaremongering headlines as ‘Water: can you overdose?’ and ‘Drinking too many fluids while exercising may be dangerous’ will lead to a situation in which athletes fail to hydrate sufficiently and thus limit their performance potential.

New hydration guidelines have now appeared. Noakes himself has produced a set for the Hydration Advisory Board of the International Marathon Medical Directors’ Association. And the sporting body USA Track and Field has striven for a middle ground in its own new guidelines.

A topic once discussed only in sports medicine circles has become a source of debate among athletes, coaches and the lay press. And the upshot is that many athletes simply don’t have a clue what rehydration strategy to employ.

But what is over-hydration (also known as hyperhydration) and how can it lead to such deadly consequences? Noakes blames it on what he calls the ‘dehydration myth’: that collapse during or after prolonged exercise is caused by heat exhaustion due to dehydration and that both can be prevented by inordinate fluid ingestion.

Cases of ‘water intoxication’ in marathon runners were reported as far back as 1988. And there are numerous publications describing the life-threatening consequences of fluid overload in runners, triathletes, army personnel and hikers, all of whom tend to engage in low-intensity activity over long periods of time in hot conditions.

The rise in marathon participation by recreational runners has led to an increase in competitors running for four hours or more – and it is people in this type of situation who are most likely to consume too much fluid.

Hyponatraemia results from an abnormally low concentration of sodium in blood plasma – the normal range being 136-142 mmol per litre(2). A sustained decrease in plasma sodium concentration disrupts the osmotic balance across the blood-brain barrier, leading to a rapid influx of water into the brain. This, in turn, leads to swelling of the brain, which can progress to confusion, seizure, coma and even death. The severity of the illness depends on the severity of the sodium reduction: a decrease in plasma sodium concentration to 125-135 mmol/litre will tend to cause mild symptoms, such as bloating or mild nausea, but concentrations below 125mmol/litre produce more severe symptoms, while a drop below 120mmol/litre can be life threatening.

Various factors can lead to a fall in sodium concentration, including loss of sodium in sweat and decreased sodium intake. But the rapid intake of large quantities of fluids, which effectively dilutes the blood, appears to be the major cause of dangerous reductions.

The symptoms of hyponatraemia overlap with those of heat injury, but the former is uniquely defined by low plasma sodium levels. When hyponatraemia occurs in athletes, it is most commonly characterised by severe dilution of the blood plasma caused by the dramatic influx of water.

As Tim Noakes points out, confusion between hyponatraemia and heat illness can be very dangerous at the site of collapse, since the latter condition tends to be treated by administration of intravenous (IV) fluids, which are bound to exacerbate an already life-threatening situation(3).

Measurement of rectal temperature and plasma sodium concentration should allow for accurate diagnosis. And Noakes argues that if rectal temperature does not exceed 40oC at the time of collapse, heat illness is not to blame and IV treatment should not be offered. He also points out that, following a recent 226k Ironman Triathlon, beginning at midday in sweltering temperatures of 30oC, not one of the 390 participants required IV treatment, suggesting that the prevalence of heat illness has been exaggerated.

In favour of adequate hydration

Noakes argues that, while hyperhydration can be life-threatening, dehydration caused by exercise in the heat will only reduce performance and temporarily affect wellbeing. There is no evidence, he insists, that modest levels of dehydration in endurance athletes – body mass losses of 2-8% – have any health consequences during exercise.

But, while it may be true that endurance athletes participating at low exercise intensities are at a low risk of ill effects, those exercising at higher intensities and in extremely hot conditions may suffer from heat illness caused by rising body temperature. This can, at worst, result in circulatory collapse due to failure of the body’s normal thermoregulatory system and an uncontrolled rise in core body temperature.

Dehydration of 8% is equivalent to a loss of 4kg through sweat for a 50kg runner, or 5.6kg for a 70kg runner – losses which are not insignificant while racing in the heat. Exercise-induced increases in rectal temperatures up to 41oC, induced by rapid running in the heat over a short duration, may be tolerated, but if they are accompanied by dehydration, core temperatures will tend to rise uncontrollably because of impaired sweat loss.

Tim Noakes has argued at length that heat illness caused by dehydration is a rare occurrence during endurance events such as marathons, and that many reported incidents are down to misdiagnosis (1,2,5). He also argues that the advice to drink copious amounts of fluids during prolonged exercise stems from laboratory research that does not take into account real life factors, such as the convective cooling effect of outside air.

Additionally, events in very warm, humid climates are often held early in the morning, before temperatures soar, while many city marathons are held in the spring or autumn to avoid the summer heat. Such factors obviate the need to drink ‘as much as possible’, but many athletes still obey this dictum.

Nevertheless, however exaggerated the links between dehydration and heat illness, there can be little doubt about the detrimental effects of dehydration on performance. Studies carried out over the past 50 years have shown that severe fluid losses decrease the capacity to perform submaximal endurance exercise. As little as 1-2% dehydration can increase core temperature and cardiovascular strain, thereby increasing the perception of workrate. And water loss as low as 1.6% can increase running time over 1,500m, 5,000m and 10,000m and reduce run time to exhaustion (4).

Greater losses of 4% can reduce high-intensity exercise capacity (eg maximal cycling) by up to 50% due to a reduced VO2max, while reducing skeletal muscle performance by 15% (5).

So, while dehydrated athletes are not necessarily at risk of heat illness, there is no doubt that their performance can be adversely affected. And fears about hyponatraemia should not be allowed to stop them understanding and meeting their fluid needs when performing endurance exercise in the heat.

So what advice should athletes take? Douglas J Casa, the assistant professor who wrote the USA Track and Field’s new hydration guidelines, says athletes should be educated about their own individual fluid needs(6). He believes that athletes’ fluid intakes need to be governed by their individual sweat rates, which are influenced by body weight as well as exercise intensity and environment.

Tim Noakes advises athletes to heed their thirst and employ the same individualised hydration strategies in competition as they do in training. Elite runners can achieve adequate hydration by ingesting as little as 200ml or as much as 800ml per hour, depending on individual requirements. But runners further back in the pack are moving at a slower pace, with heat accumulation less likely. A reasonable approach for these athletes is to drink no more than required, as indicated by thirst, rather than as much as possible(7). All athletes should also ensure adequate salt intake in the fluids they take on board during and after exercise to counteract salt losses in sweat.

Clare Whitehead

References
British Medical Journal (2003) 327;113-114 
Sports Science Exchange 88 (2003) 16(1). GSSI 
The Physician and Sportsmedicine (2000) 28(9); 71-76 
Medicine and Science in Sports and Exercise (1985): 17(4); 456-61 
Journal of Applied Physiology (2001) 90; 1057-1064 
The Physician and Sportsmedicine (2003) 31(7) 
IMMDA-AIMS Advisory Statement by IAAF (2002) 17(1);15-24 

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