Water overload in sport: who is at risk?

Is it possible to drink too much water during endurance exercise? The answer is definitely ‘yes’. Water overload (hyponatremia) is a potentially life-threatening condition. Read on to learn who is at risk and what you can do to prevent it.

Hyponatremia occurs when the sodium concentration in the blood drops to a dangerously low level. Sodium serves an important role in the body by acting as an electrolyte and helping with muscle contraction. If more water is drunk than the kidneys are able to deal with then this can dilute sodium in the body just the same as adding fresh water to salt water makes it less salty.

Before you get too concerned every time you have a glass of water, take heart knowing that water overload 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 of hyponatremia occurring in people with mental illnesses who consumed massive quantities of water in a short period of time.

Hyponatremia has also been described in otherwise well-hydrated people who have drunk around 3 litres of fluid over an hour in an attempt to foil drug tests. In some cases however, hyponatremia 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 water overload. A ground-breaking 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 hyponatremia and 0.6 per cent had such a low level ohypof blood sodium that it was considered critical.

The researchers surveyed the runners about their fluid consumption during the race. Hyponatremia was found to be most often associated with the following characteristics of runners:

• 

  Drinking more than 3 litres of fluid during the race

• More than 1 kg of weight gain from the start to the finish
• 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 hyponatremia in marathon runners equally apply to any activity that involves continuous strenuous activity for more than 4 hours (bushwalking or even military manoeuvres for example) when combined with excess fluid consumption.

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 water overload

Symptoms of hyponatremia 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 hyponatremia mirror those of dehydration making the condition difficult to self-diagnose. If hyponatremia is misdiagnosed as simply dehydration then consuming more water will only make the condition worse.

Mild cases of hyponatremia can be managed by taking extra salt combined with water restriction until most of the fluid is urinated out. Standard medical treatment of hyponatremia involves intravenous administration of a concentrated salt solution as well as using diuretics to speed water loss.

While a firm diagnosis of hyponatremia 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 include swelling of the hands and feet or a watch or ring that has become much tighter.

How can you protect yourself from water overload

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 hyponatremia occurs in endurance athletes is thought to be due to muscle damage causing a 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 hyponatremia. Importantly though, before an event it is thought that hyponatremia 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.

So how does a person 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.

What it all means

Hyponatremia is a potentially dangerous condition caused by excessive drinking combined with abnormal water retention during exercise. People should be aware of the symptoms of water overload 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.