You’re right about the football players. A 2011 study found that 75 percent of NFL teams hyperhydrate (the medical term for over-hydrating) an average of five to seven players on their teams, most often at the players’ request. Why? In theory, it’s to stave off the performance-impeding effects of dehydration, including poor thermoregulation, cardiac output, and metabolic functioning, as well as fatigue. In reality, hyperhydration can indeed guard against dehydration—but only if you’re drinking the right thing.
Hyperhydrating with water, studies have shown, is not nearly as effective as hyperhydrating with a drink that contains glycerol, a naturally occurring chemical that can be dissolved in water or a sports drink. Water passes through the body relatively quickly, but glycerol increases fluid retention, making it much better at keeping dehydration at bay. In fact, studies have shown that downing a glycerol solution before a long workout or race can increase endurance time to exhaustion by up to 24 percent, and reduce core temperature.
Simon Van Rosendal, a leading researcher on glycerol hyperhydration, recommends in a study taking in 1.2 grams of glycerol per kilogram of body weight in 26 milliliters per kilogram of body weight of fluid over a period of 60 minutes, 30 minutes prior to exercise. That’s about 82 grams of glycerol in 60 ounces of fluid for a 150-pound person. However, Van Rosendal only recommends hyperhydrating if “the exercise is likely to induce a reduction in body weight of greater than 2 percent because dehydration by less than 2 percent bodyweight is very unlikely to cause a decrease in performance,” he writes in an email. In fact, he points out, even a bodyweight decrease of four to six percent is unlikely to decrease performance in many athletes.
If it’s going to be hot at your race, Van Rosendal says glycerol hyperhydration may also be more beneficial than if you were racing in a temperate or cold environment. “The potential benefit," he writes, "Is that an athlete will start exercise with extra fluid in their body so that they can lose more fluid through sweating before they start to enter an overall negative fluid balance.”
Physically, glycerol hyperhydration before a big bout of endurance exercise like a marathon can be beneficial. Resarchers have found that the increase in body mass following hyperhydration does not affect running economy, and side effects, including nausea, gastrointestinal discomfort, and light-headedness, are rare.
Legally, however, glycerol hyperhydration is a minefield. The World Anti-Doping Association put glycerol on its list of prohibited substances in 2010—not because glycerol is considered an unfair performance enhancer, but because it can help mask the use of other banned substances by diluting those drugs in an athlete’s blood. Researchers, however, are currently contesting the WADA decision, writing that glycerol doesn’t significantly alter the results of doping blood tests. So perhaps it’s best to restrict experimentation with glycerol hydration to long training runs.
THE BOTTOM LINE: Hyperhydrating with a glycerol solution has been shown to attenuate the performance impairing effects of dehydration. Glycerol, however, is currently on the list of WADA banned substances. So if you think you may be drug tested, you’re better off sticking with a more traditional hydration plan, like starting your race hydrated, and drinking according to thirst as you run.