Dr. Stacy Sims, an exercise physiologist at Stanford and the co-founder of Osmo Nutrition, soaked up all she could from her experiences in the endurance world, from high school cross-country to crewing at Purdue University to cycling to triathlon. This sort of field knowledge has propelled her development into one of the world's most recognized researchers on the subject of hydration—her list of consulting clients includes the likes of Team Garmin and the USA Olympic Cycling Team. As a matter of consequence, it was in the time-bending haunts of the Queen K, racing the Hawaii Ironman, that Sims' PhD topic popped into her head. At New Zealand's University of Otago she would begin chasing the various intricacies of thermoregulation and hydration.
Although the Ironman has been around for nearly 40 years, hydration continues to be one of the most vexing problems for the racer to get a handle on. We use our training sessions and early season races to find that sweet spot that enables a good performance and steers us clear of the danger-zone of over-hydrating into a state of stomach-sloshing hyponatremia.
While you can get away in a shorter race, even on a muggy day, with not worrying about how much your drink (or drink at all, for example, in a 5K run). But we know that when the race is clocked not just in minutes but in hours, drinking water and/or sports drinks becomes necessary to go the distance. Sims says the research is clear on this point: A 2-percent drop in your optimal water body level can cost you up to an 11 percent drop in max VO2 power.
"Basically, when you're below your optimal hydration level, you have less blood available for circulation," Sims says. When your body has to ration out the water supply, there's a priority list involved, and the organs, for example, are going to come before the working muscles. "You lose out on optimal delivery of oxygen and nutrition to the working muscles." Sims adds that complicating this standard is that clear urine—"peeing clear"— is not an accurate way to judge whether you're optimally hydrated or not.
There are numerous complexities involved in hydrating properly during an Ironman—including, Sims says, that men and women are not created equal. "Women are not small men" is a catchphrase Osmo Nutrition puts forward in their marketing (they even paste it to a water bottle they sell). Most of the hydration research performed in endurance athletics uses men as their test subjects, Sims says, an aberration that omits the hormone-induced water losses at work in the menstruation cycle.