Naturally, the fact that you shouldn't exercise on an empty stomach doesn't mean that you should eat a three-course meal 10 minutes before hitting the gym. In general, the closer you get to your workout start time, the fewer calories you should eat. The nutrients that make up those calories should also shift. Because it takes the body four to six hours to digest fat, about three hours to digest protein and about two hours to digest carbohydrates, it's important to winnow down the protein and fat content of your meal or snack as you get closer to exercise. "You're not going to want to eat a plate of french fries two hours before working out, because the blood is going to rush to your stomach to digest that while it's also trying to rush to your exercising muscles," says Berning. "In the end, it doesn't do a very good job of either one."
So, here are a few rules of thumb to follow: If your workout is four hours away, eat a regular meal that combines protein, fat and carbohydrates, then have a small carbohydrate-rich snack closer to your exercise session to tide you over. Three hours before working out, make it a smaller meal and lighten up a bit on the protein and fat. Thirty to 90 minutes before exercise, have a snack of easily digested carbohydrates (see below). If you only have the 15 minutes between, say, leaving your office and hitting the gym to grab something, go for a sports drink or a few Saltines. Also keep in mind that while eating high-fiber foods is important for good health, they're best eaten after or long before exercise, since they can cause bloating and other annoyances that will make you feel uncomfortable when working out.
