And then there are those who can't take in enough. Women are susceptible because they typically have a smaller appetite than men, says Parker-Simmons, who helps the U.S. ski and snowboarding team with nutrition. And it doesn't take long for a caloric deficit to result in amenorrhea—a cessation of the menstrual cycle. (Losing your period is not necessarily a function of body fat, says Meyer, but can happen simply from eating 500 or so fewer calories than you need for five days in a row.) For athletes struggling to keep weight on, Meyer says she'll stick with healthful, good-quality foods but just prescribe more of them to boost calories.
Altitude. Traveling to high altitudes to compete or train can wreak its own havoc. The body dehydrates and also needs more energy, since the heart is working harder to pump blood, says Parker-Simmons. Her solution: sports drink, which addresses both issues. (Liquid calories should not be in the form of alcohol, which will only increase dehydration.) Some athletes, particularly women, also need to take in more iron while they're at high altitudes, but this varies a lot from person to person. (Self-medication is therefore a bad idea; see a nutritionist or physician and get your iron status tested before you use supplements.)
Cold. The outside temperature isn't so important if you're warm on the inside—dressed appropriately, and working out at a level that keeps your core temperature steady. But it can be a factor in some events, like downhill skiing, ski jumping, and snowboarding, where there are blocks of time with no activity—weather delays, chairlift trips, long waits before the start at the top of the mountain. "When you really start shivering, you burn carbs more and your glycogen is depleted," says Meyer. Since that's the last thing you want to do before a big event, it's time to reach for a warm sports drink or hot chocolate.