Exercise hydration

What and how much should you drink when exercising?
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02.Too much water?

While for most, constantly topping up with water regardless of thirst may be unnecessary, for some endurance athletes it has proven deadly. A condition called hyponatraemia, meaning low (hypo) sodium (natrium), has caused the deaths of marathon runners and other endurance athletes, hikers and military personnel during training.

About hyponatraemia

Hyponatraemia occurs when sodium levels in the fluid between cells are reduced relative to water, causing water to move from there into tissue cells, making them swell. Hypnotraemic encephalopathy refers specifically to brain swelling, and results in dizziness, nausea, confusion and muscle disorders, and can lead to unconsciousness, coma or death. It is increasingly occurring in exercise situations due to overhydration. Indeed, experts in the field have noted that while there are no documented cases of anyone dying of dehydration during endurance sport events, people have died of hyponatraemia caused by overhydration. And it's not just sport: the Medical Journal of Australia reported that several hikers walking the Kokoda track have suffered from exercise-associated hyponatraemia, while a hiker’s death in Tasmania in 2011 was attributed to hyponatraemia caused by overhydration.

Hyponatraemia in athletes

Researchers investigating this relatively recent trend have observed that so entrenched is the mantra to “stay ahead of your thirst”, long-distance runners and other athletes tend to drink at drink stations regardless of how thirsty they feel. Consequently, athletes are finishing events heavier than they started due to drinking more fluids than they lose through sweat and other processes. A study of a sample of runners in the 2002 Boston Marathon found 35% of runners gained weight during the race, 13% had mild hyponatraemia and 0.6% had critical hyponatraemia. One woman (not a study participant) died from hyponatraemia after the race. An Australian study on adolescent elite athletes at a one-day sports camp found that they drank more than enough, and were mildly hyponatraemic.

Some people seem to be more at risk of hyponatraemia than others. Risk factors include length of time exercising (longer race times create greater risk) and amount of liquid consumed. Some studies have found women are more at risk, though others put this down to smaller body size and slower race times (therefore larger consumption of fluid) than gender. It may also have a genetic basis.

Can sports drinks help prevent it?

If drinking too much water can cause hyponatraemia, does drinking sports drinks solve the problem? Sports drinks contain sugar and salts, with the sugars providing an energy source and assisting absorption of water, and the salts replacing those lost through sweat. In addition, the salts and sugar make you a little thirsty and encourage you to drink more, therefore providing adequate – though some would argue excessive – hydration.

When it comes to hyponatraemia, however, the concentration of salts in sports drinks is lower than the saline concentration in blood. When you sweat, you lose about 900-1400mg of salt per litre, and while some drinks contain almost this much sodium (for example, Gatorade Perform Endurance contains 840mg of sodium per litre), most sports drinks typically contain 230-510mg/L. This means the levels of salts in the drinks don’t quite replace the amount of salt lost through sweat, and it’s the dilution of sodium in the body that causes hyponatraemia.

So while sports drinks don’t necessarily prevent hyponatraemia - and people drinking sports drinks have suffered hyponatraemia - it makes sense when heavy salt losses through sweat are likely, such as during endurance sports and long hikes in hot weather, to drink sports drinks instead of plain water, because you’re replacing at least some of the salts.

Official recommendations for preventing exercise-related overhydration and dehydration

The advice for marathon and other long distance runners from the International Marathon Medical Directors Association (IMMDA) is to “drink to thirst”: don’t drink if you’re not thirsty, don’t feel compelled to drink at every station and be mindful it’s normal to lose a little weight during a race.

While it doesn’t seem very scientific, in fact it’s based on the latest scientific evidence. If you need better guidance than this, they recommend you work out your sweat rate – how much sweat you lose over a given time and different conditions – by weighing yourself before and after a training run. Some say adults should expect to lose one to two per cent of body weight in sweat, and most agree that more than two per cent affects physical and mental performance.

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