Should You Really Be Drinking All That Water?
WHEN IT COMES to hydration during exercise, the mainstream message of the day is "drink early and often." But that's exactly what got a 39-year-old athletic, healthy woman into dire trouble back in 2007.
According to the case report, the woman was brought to the emergency room at Baylor University Medical Center suffering from a splitting headache, nausea, vomiting, and severe lethargy. She was disoriented and borderline unresponsive, but managed to mutter out the events and conditions leading up to her sudden illness. She had played tennis when it was 100 degrees out, then weightlifted afterwards. How long? Oh, about two hours. How much water did you drink? I kept myself very hydrated; I have a one-liter water bottle and drank about four liters.
Laboratory tests would soon confirm the doctors' suspicions. On a mission to stay properly hydrated, the woman had almost inadvertently drunk herself into a coma.
"STAY HYDRATED." Repeated position statements from the prestigious the American College of Sports Medicine (ACSM) dating back to 1975 have drilled this "conventional wisdom" into athletes' collective psyche. The ACSM's latest recommendations urge drinking fluids before and during exercise in order to "prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance." The ACSM further encourages athletes to imbibe afterwards, up to 1.5 liters of fluid for every kilogram of body weight lost.
The advice seems fairly commonsense, but as a result, exercisers in the developed world seem to be growing increasingly waterlogged. Of runners surveyed at the London Marathon in 2010, only a quarter planned to drink according to their thirst. Furthermore, 13% of 488 runners sampled at the Boston Marathon in 2002 were found to have hyponatremia, the state that put the aforementioned woman in danger, in which salt levels in the blood fall below acceptable levels. Other studies have reported incidence rates as high as 29%. The likely cause is overhydration.
Salts are regularly vilified in modern health, but we can't live without them. Sodium salts are particularly vital, necessary for the regulation of blood and body fluids, transmission of nerve impulses, and heart activity. When we drink excessive amounts of water, salt concentrations fall, sometimes to dangerously low levels.
Death from exercise-induced hyponatremia is quite rare, but noticeably on the rise, prompting many scientists to re-examine the touted wisdom on water intake during physical activity.
WRITING IN THE journal Extreme Physiology and Medicine, exercise physiologist James David Cotter says the "just keep drinking" mentality seems to stem from a lack of trust in the human body.
"Drinking to limit changes in body mass is commonly advocated, rather than relying on behavioral cues (mainly thirst) because the latter has been deemed too insensitive."
But when Cotter reviewed the scientific literature comparing the ACSM's drinking guidelines to drinking "ad libitum" (when thirsty), he found the former to offer limited to no tangible benefits as far as reducing heat illness and increasing cognitive and physical performance.
When exercise scientist Dr. Timothy Noakes conducted a stress test of sorts, he came to the same conclusion. Noakes had 18 athletes run 25 kilometers in 112-degree heat, telling them to drink water at their pleasure, and monitoring them throughout the entire route. Even under these extreme conditions, their bodies functioned quite well. Noakes noted that "humans are the mammals with the greatest capacity for exercising in extreme heat."
As a chemical compound, it's hard to dispute water's importance. As Cotter states, it is:
The medium in which metabolism occurs; a reactant and a product; the basis by which the volume of cells, tissues and organs is maintained; a shock absorber (e.g. for the brain); the medium for the mass-flow transport of gases, substrates, heat, hormones etc.; a thermal reservoir with a uniquely high specific heat capacity, hereby being capable of accepting or releasing large amounts of thermal energy with little change in tissue temperature, and; the substrate for evaporative cooling via sweating, which helps give humans an unparalleled versatility for moving in hot environments.
But more is not necessarily better. In fact, as stated earlier, it might actually be dangerous. Cotter asserts that drinking based on one's thirst is appropriate in the vast majority of environmental and exercise settings, and far less risky.
Source: Cotter et al. "Are we being drowned in hydration advice? Thirsty for more?" Extreme Physiology & Medicine 2014, 3:18