Recent Studies Lead to Changes in The American College of Sports Medicine's Guidelines on Fluid Intake

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The American College of Sports Medicine (ACSM) recently updated its guidelines on fluid intake for runners during a race, citing new research on hyponatremia from the University of South Africa in Capetown

Dr. Martin Schwellnus observed runners at the 56-kilometer Two Oceans Marathon in South Africa, separating “cramping and non-cramping runners by body mass and finishing time.” Schwellnus found that runners with cramps were less dehydrated and had normal levels of electrolytes, such as sodium, magnesium, and zinc, than runners who did not suffer from cramps. A separate study by Schwellnus that measured post-race hydration and electrolyte levels in Ironman competitors showed similar results.

The researchers, led by Dr. Timothy Noakes, performed several studies. One study by

According to exercise scientists Ross Tucker and Jonathan Dugas, who recently published the book Runner’s World: The Runner’s Body with Matt Fitzgerald, the conclusions reached by Schwellnus “suggest that overhydration, not dehydration, characterizes people with cramps.”

However, recent studies have shown that the excess drinking that runners abide by can actually cause fatal overhydration, or hyponatremia. This conclusion led the ACSM to change its guidelines, instructing runners to drink fluids to prevent dehydration, defined as “more than a 2% weight loss.”

Tucker and Dugas said that the new guidelines reflect the results of recent research showing that “drinking more fluids lowers, not raises, blood sodium levels.” The authors performed sodium-level tests on runners right after the completion of a marathon, which showed that “a hypothetical 154-pound runner who runs for two hours at a moderate pace will lose 1 liter of sweat per hour. The runner’s starting sodium concentration would be 140 millimoles per liter.” However, if the runner drank two liters of water over the two hours, the sodium level fell to 128 millimoles per liter, which, according to the researchers, is nearing the hyponatremia level. Though a runner who drank two liters of a sports drink like Gatorade during the same period of time would have a slightly higher sodium concentration (130 millimoles per liter), he or she would still be considered hyponatremic.