How much water is too much during exercise? Recent health reports have highlighted a potentially dangerous condition that can be experienced by endurance athletes who over-consume water.
Being well hydrated is an important part of ensuring a sportsperson can perform at their maximum potential. Recent medical reports however have raised serious health concerns about drinking too much water during endurance sports because of the risk of a condition known as hyponatraemia. Hyponatraemia is where the sodium concentration in the blood drops to a dangerously low level. Sodium serves an important function in the body by acting as an electrolyte and helping with muscle contraction and nerve impulse transmission so the body needs to keep the concentration of this electrolyte in a fairly tight range. One condition where hyponatraemia can occur is when more water is drunk than the kidneys are able to deal with – resulting in more water being retained by the body than normal and therefore dilution of the sodium.
Hyponatraemia in healthy people is incredibly rare. Most cases occur because of medical problems where water cannot be excreted efficiently, such as in people with impaired kidney function. Some isolated case reports have also described hyponatraemia occurring in people with mental illnesses who consumed massive quantities of water in a short period of time. Hyponatraemia has also been described in people who have drunk around 3 litres of fluid over an hour in an attempt to foil drug tests. In some cases however, hyponatraemia can occur in otherwise healthy people who drink too much water during long-duration endurance exercise.
Who is at risk?
People who compete in endurance events over many hours such as marathons and ironman triathlons are especially at risk of hyponatraemia. A recent study published in 2005 in the New England Journal of Medicine measured the sodium levels of 766 runners at the finish line of the 2002 Boston Marathon. Thirteen per cent of the runners were found to have hyponatraemia and 0.6 per cent had such a low level of blood sodium that it was considered critical. The researchers also surveyed the runners about their fluid consumption during the race. Hyponatraemia was found to be associated with the following characteristics of runners:
• Drinking more than 3 litres of fluid during the race
• More than 1 kg weight gain
• Female sex
• Body mass index less than 20 kg/m2
• Drinking fluid every mile during the race
• A race time of greater than 4 hours
The risk factors for hyponatraemia in marathon runners equally apply to any activity that involve continuous strenuous activity for more than 4 hours (bushwalking or even military manoeuvres for example) when combined with excess fluid consumption. Athletes who rely heavily on water during an event and who also take on little salt to replace sodium losses through sweat are thought to be also be at risk. Athletes with a small body size (typical of many endurance sportspeople) are thought to be at even greater risk especially if they sweat a lot and are ‘over zealous’ in their drinking habits during an event.
Signs of hyponatraemia
Symptoms of hyponatraemia present as throbbing headaches, nausea, cramps, dizziness, confusion and lethargy. If the condition progresses and sodium levels fall even further, swelling of the brain can occur ultimately resulting in coma and possibly even death. Unfortunately, the initial symptoms of hyponatraemia mirror those of dehydration making the condition difficult to self-diagnose. If hyponatraemia is misdiagnosed as simply dehydration then consuming more water will only make the condition worse. Mild cases of hyponatraemia can be managed by taking extra salt combined with water restriction until most of fluid is urinated out. Standard medical treatment of hyponatraemia involves intravenous administration of a concentrated salt solution as well as using diuretics to speed water loss. While a firm diagnosis of hyponatraemia requires a blood test, something that would not be readily available at the finish line of an endurance event, some possible signs that a person may be fluid overloaded and at risk of hyponatraemia include swelling of the hands and feet or a watch or ring that has become much tighter.
How can you protect yourself?
Normally, our bodies are perfectly capable of maintaining water and sodium balance even if we drink several litres of water in a short period of time. Why hyponatraemia occurs in endurance athletes is thought to be due to muscle damage and the subsequent release of stress hormones. This metabolic response can result in reduced water excretion by the kidneys causing water to accumulate in the body and thus lowering the sodium concentration. Once the body’s water balance mechanism is impaired, drinking even a small amount of extra water can cause symptoms of hyponatraemia including swelling of the brain. Importantly though, before an event it is thought that hyponatraemia cannot occur because the body can excrete the extra water drunk so all athletes are encouraged to stay well-hydrated during their training and daily activities. Pre-event, drinking about 500 mL of fluid two hours before race time will allow the body plenty of time to remove the extra water while still ensuring the athlete is well hydrated.
It should be taken in context that death from hyponatraemia in healthy, exercising people is an extremely rare occurrence. Normally, sports people are focussed on staying well hydrated during an event as losing more than 2 per cent of body weight in fluid can impair performance. So how does an athlete walk the fine line between staying well hydrated during an endurance event while at the same time not risking becoming fluid overloaded? For events of several hours in duration, self weighing may be a good option. If a person is well hydrated at the start of an event then ‘weight gain’ during the event may mean a person is carrying too much extra fluid and should ease up on the drinks. Events such as the Boston Marathon now have scales at all drink stations in the second half of the race. As scales may not be readily available at the start and finish lines of events, athletes should consider weighing themselves before and after their more intense training sessions that mimic competition conditions. Knowing how your own body responds to activity and fluid hydration strategies will enable you to be better prepared for competition and keep your hydration status in the optimal zone.
Hyponatraemia is a potentially dangerous condition caused by excessive drinking combined with abnormal water retention during exercise. Athletes should be aware of the symptoms of hyponatraemia and aim for adequate, but not excessive, fluid intake during endurance-type sports. During the event, it is recommended that a person aims to consume about 300 to 500 mL of fluid (preferably a sports drink that contains salt) every 30 minutes. For slower, smaller athletes who are exercising in mild conditions, less fluid may be required while for well-trained athletes competing at high intensity in warm conditions more fluid may be needed.
Almond CSD et al. Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine 2005;352:1550-1556
Noakes TD. Overconsumption of fluids by athletes. British Medical Journal 2003;327:113-114
Gatorade Sports Science Center <www.gssiweb.com>
Edited by Astera, 22 September 2010 - 03:25 PM.