When he's not in the lab, Gareth Wallis can be found on the rugby field. The Integrative Biology Department postdoctoral fellow has learned from experience that top conditioning is essential for surviving the fast tempo and brutal hits of Bay Area club rugby. Off the field, Wallis researches exercise metabolism—his latest findings could change the way female athletes eat, and strike a blow at the low-carb faction.

Men metabolize carbohydrate for immediate use while exercising, but women, with 8 percent to 10 percent more body fat than men, use more of their fat stores. This may be because estrogen stimulates fat metabolism, or because women have more slow-twitch muscle fibers, which promote fat metabolism.

Using thick, sugary syrup (glucose powder mixed with water) as a form of pure carbohydrate, Wallis tested the metabolic threshold of low, medium, and high concentrations of carbohydrate on men and women during an intense cardiovascular workout. Although both metabolized carbohydrate at about the same rate—60 grams per hour, or the equivalent of a half a cup of maple syrup, or a liter of Gatorade—the women suffered from "severe gastrointestinal discomfort," including nausea and vomiting, as soon as they consumed more than a gram of carbohydrate per minute. The strain was less marked in men: the result, Wallis speculates, not of metabolic differences but of men's longer intestinal system, spreading the burden of wasted carbs.

So although carbohydrates are important for sports performance, it seems women need to take care. Wallis warns that his research applies only to prolonged exercise: "For the average gym-goer, such high concentrations aren't necessary." Someone hopping on a treadmill after work, for example, doesn't need a liter of Gatorade.

What should the average gym rat consume before a workout to maximize performance? According to Wallis, "50 to 60 percent of your energy should come from carbohydrates, with 15 to 20 percent from protein, and the rest fats." He explains, "Without carbohydrates we lose an immediate energy source. You'll become lethargic and your workouts will fail."

Wallis's own pre-rugby snack is deliciously simple. "A large bowl of pasta with tomato sauce," he says. "With some cheese for taste. And water, of course."