Sports Science Update: Drinking By Thirst Works

Runners drink enough when they drink by feel.

Written by: Matt Fitzgerald

In the first half of the 20^th century, drinking during training runs was considered soft, and drinking during races was actually classified as cheating. Then medical experts figured out that dehydration had some rather nasty consequences for thermoregulation during exercise, and attitudes and standards changed.

In fact, with the advent of sports drinks in the 1960s, the pendulum swung completely in the other direction. Generations of athletes were taught to fear any amount of dehydration like the plague and to “drink as much as possible” during exercise to prevent it. But this led to a new set of problems. Since the human gastrointestinal system was not really designed to absorb fluid and nutrients during vigorous activity, only relatively modest volumes of fluid and nutrient consumption can be tolerated. Thus, a common consequence of trying to “drink as much as possible”, especially during running, with its constant jostling of the GI system, is GI distress.

Worse, in extreme cases, drinking as much as possible can cause a condition called hyponatremia, or water intoxication, which is defined as a dangerous lowering of the blood sodium concentration. Hyponatremia became a widespread problem in the last decade, as a flood of slower marathoners came into the sport. Slower marathoners are more susceptible to hyponatremia because their slower pace and longer exercise duration enable them to drink more.

In response to this epidemic, the International Marathon Medical Directors Association in 2002 released new guidelines for hydration during marathons, advising runners to simply drink according to their thirst. The scientific literature squarely supports this recommendation. Studies have repeatedly shown that drinking to completely offset body fluid loss during exercise offers no advantage for thermoregulation or performance compared to drinking “ad libitum”, while increasing the risk of GI distress.

The “obey your thirst” doctrine receives further validation from a new study conducted by researchers at McMaster University. The study involved eight trained high school runners. On three separate occasions, following a period of heat acclimatization, these runners ran for 75 minutes at a moderate intensity on treadmills in a hot environment and immediately thereafter ran at high intensity until exhaustion. The runners were given a different drink in each trial: water, flavored water, and a sports drink. In all three trials they were instructed to drink according to their thirst. The researchers found that, in all three trials, the runners voluntarily drank amounts matching their body fluid losses through sweating. Time to exhaustion in the high-intensity run at the end of the trial was equal regardless of the type of beverage consumed.

The authors of the study concluded, “Under exercise conditions more closely reflecting real-life situations, heat-acclimatized adolescent male runners can appropriately gauge fluid intake regardless of the type of beverage made available, resulting in consistency in exercise performance.”[sgi:MattFitzgerald]