Hydration and Exercise, Part 1
Until recently relatively small losses of body weight due to dehydration during exercise were thought to be detrimental to athletic performance. The threshold for drops in performance related to dehydration has long been considered to be around 2% of body weight . So a 150-pound man who lost 3 pounds or a 125-pound woman who dropped 2.5 pounds during exercise were expected to slow down as a result. In 1996 the American College of Sports Medicine (ACSM) went a step further suggesting that the loss of any body weight due to dehydration could potentially result in poor performance or even health issues . Their position stand was that an athlete should drink enough to replace all water lost through sweating and that this would likely be in the range of 18 to 36 ounces (600-1200mL) of fluid per hour. Last year the ACSM officially changed their stand by adopting the more common 2% of body weight as the upper end of what an athlete should lose due to dehydration . This time they stopped short of making recommendations on how much to consume.
Two-percent is still too low according to Tim Noakes, PhD, a South African exercise physiologist who has led the fight to get the ACSM and other sports organizations to relax their standards on drinking during exercise . Why would Noakes want us to drink less during racing and training? The reason is something called “hyponatremia” – the dilution of the body’s stores of sodium. Athletes who were encouraged to replace all fluids lost while exercising as per the 1996 ACSM position stand put themselves at risk for this condition which is worse than the dehydration they were trying to prevent, according to Noakes . And he believes that 2% still encourages excessive drinking and encourages risk.
By over-hydrating while exercising, sodium concentrations drop and put you at risk for hyponatremia. Some early signs of hyponatremia are bloating around elastic bands in socks and the waistbands of shorts, disorientation, headache, nausea, muscle cramps, lethargy, confusion, reduced coordination and tunnel vision. In recent years there have been deaths in back-of-the-pack runners related to hyponatremia. Drinking an excessive amount of water for several hours during exercise (4 hours is considered the danger threshold) increases your risk. One study of Ironman-distance triathletes going way back to 1987 found that as few as 8% and as many as 30% of the race finishers experienced mild to severe hyponatremia .
It’s generally accepted that elite athletes have a lower risk of hyponatremia but a higher risk of dehydration since they are focused on performance and may take in less fluid than demanded by the body. Slower athletes simply have more time to drink and may well believe it is in their best interests to consume a lot of water. The mistaken notion that we should finish a workout or race with no loss of body weight further increases the risk, especially in long events.
So what does all of this mean for your training and racing? How much should you drink during a race or workout? What should you drink? How much weight loss due to dehydration is acceptable and can be well-tolerated by the body? Should you drink extra amounts in the days and hours leading up to a race in hot conditions so that you’re hyperhydrated? How do you gauge your level of dehydration following exercise and then go about replacing lost fluid? Does age have any implications for hydration? I’ll address these questions and what the research and my experience suggest are the answers in my next blog.
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