The Truth About Hydration in the Heat

Most articles about exercising in the heat are all about hydration. But did you know that drink fluids during exercise in hot weather actually does very little to prevent the body's core temperature from rising?  It's true, and the studies prove it.

For example, a 2007 study from the University of Exeter, England, found that fluid consumption did not prevent a rise in body temperature or improve performance in a half-marathon running event. This was the first study to monitor internal body temperature continuously throughout a real race, using high-tech sensors that runners actually ingested the night before the race, which took place in hot and humid conditions.

More: Measure Your Sweat Loss for Optimal Hydration

Runners consumed as much or as little fluid as they wished during the race, and there was a high degree of variability in drinking rates. Runners replaced between 6 and 73 percent of body fluid losses over the course of the run. Researchers found no correlation between the amount of fluid runners consumed and their body temperature or performance. Thus, they concluded that drinking fluid had no effect on body temperature or performance in this context.

However, there is another way to interpret these results. Evidence from other recent studies suggests that the nervous system regulates body temperature and performance during exercise in the heat through a mechanism called regulatory anticipation. Essentially, the brain allows the body to work hard enough—and only hard enough—to reach his highest safe core body temperature, which is more or less the same in all humans.

More: 15 Hydration Facts for Athletes

Therefore, as long as they are working at maximum capacity—as one does during a race—runners competing in the heat will reach the same core body temperature whether drinking has a cooling effect or not, because inasmuch as it does have a cooling effect, the runner's brain will simply allow him to run a little harder so that he still reaches the same body temperature.

But, if this is so, wouldn't the authors of this study at least have observed a performance benefit to hydration? The answer is that they probably would have observed a performance benefit if they had looked for one within individual runners (by having each of them run the race twice—once without fluid consumption and once at the runner's natural rate of fluid consumption), but instead they looked for a general correlation between drinking rate and performance in the general study population. Presumably, however, each runner instinctively consumed fluid at the proper rate to maximize his individual performance.

More: Hydration and Electrolyte Tips for Race Day

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