Bicycle Network: Health Matters
H20 to go?
If drinking some water is good, then drinking more must be better, right? Not always, as Gavin Wright explains
Ultra Cycling, the website of the American Ultra Marathon Cycling Association, has listed 10 mistakes endurance athletes make. The number-one mistake might surprise you. It’s over-hydration. Also, last year Cape Town hosted the 1st International Exercise-Associated Hyponatremia (EAH) Consensus Development Conference, which resulted in a warning that too much water can actually kill. But what is EAH and why is it so dangerous?
Bodily fluids
Normally the body does a good job shifting sodium around the body. With heavy exercise, though, the sodium concentration can rise as water is lost through sweating. More water is needed to restore fluid levels and lower this sodium concentration. The hypothalamus (the ‘super-gland’ at the base of the brain) controls this process by initiating thirst.
When the body’s fluid intake is greater than its fluid loss, EAH, also called ‘water intoxication’ and known to marathon cyclists as simply ‘too-much-water’ can occur. In EAH, the sodium concentration in your circulatory system is too low. If the sodium levels drop over a period of a few days, the brain has defences to deal with the problem: the human body can be very clever with shifting sodium to where it’s needed. But if sodium levels drop quickly as a result of heavy exercise, these defences don’t kick in. The extra water passes from the circulatory system into the cells and makes them swell. This is a particular problem inside the skull, which doesn’t readily expand. Brain swelling can cause brainstem herniation, which is often fatal.
What is EAH?
EAH was not reported before 1985, and its occurrence has increased with advice not to rely on thirst as an early indication of dehydration, and with the well-meaning over-provision of drinks at races. Since last year’s EAH conference, though, marathon and triathlon athletes are relying more on thirst as an indicator. Also, drinks on these races are now being rationed.
EAH is not a common condition among cyclists, apart from in intense cycling marathons. However, anyone cycling any distance in heat or heavily for over four hours would do well to be aware of the dangers. Risk factors associated with EAH are low body mass, exercising for more than four hours, inexperience, high availability of drinking fluids and extreme heat or cold, but an almost sure indicator is weight gain during performance. Conversely, weight loss during training is a sign of dehydration.
How much fluid is too much?
Educating athletes and limiting the availability of fluid at races are effective ways to reduce EAH. In an Ironman-distance triathlon, drink stations every 20km for cycling and 2.5km for running, and every 5km in a standard marathon, have been shown to eliminate EAH. In a ride such as Around the Bay in a Day, EAH is unlikely as drink stations are quite far apart.
Dr Andrew Garnham of Alphington Sports Medicine Clinic notes that athletes will overcompensate for unexpected conditions, such as high heat, on the day of an event. “For optimum performance it is better to rehydrate before you feel thirsty,” says Graham, “but bear in mind your body is well adapted to losing a kilo or two during heavy exercise. Don’t do anything you wouldn’t normally do in training.”
Replacing minerals
Garnham also has a helpful suggestion for athletes to restore their mineral nutrients. “When we start to lose sodium, we develop a taste for salt. People find themselves craving chips. Soup is probably the most palatable way of rehydrating – try putting salt in Gatorade, it’s horrible.”
Our bodies and their workings are all different, but most stomachs are incapable of absorbing more than a litre of fluid per hour. For commuter and recreational cyclists, underhydration is still more of a risk than overhydration, but if you’re drinking more than this you may be drinking too much.
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