Myth: Protein is the most important nutrient for athletes.

Reality: "It is true that athletes need more protein than sedentary people. They just don't need as much as they think. And they probably don't need it from supplements; they're probably getting plenty in their food," Rosenbloom says.

But timing matters. Rosenbloom recommends that after weight training, athletes consume a little bit of protein -- about 8 grams, the amount in a small carton of low-fat chocolate milk -- to help their muscles rebuild.

"That's probably all you need," she says. "You don't need four scoops of whey powder to get that amount of protein."