HOW FOOD FUELS YOU:

Although your body burns some stored fat when you exercise, its main fuel is carbohydrate that's been stored in the muscles and liver in the form of glycogen. When your glycogen stores are depleted, your body will indeed tap more of its fat reserves, but at what price? Without readily available fuel, you're not likely to feel too energetic. "And you won't burn more of anything if you can't muster the enthusiasm to master your toughest sets. On the other hand, if you eat before exercise, whether it's a large meal several hours in advance or a small snack only minutes ahead of time, you'll have the extra oomph you need for an energetic and effective workout.

Here's the reason: Before carbohydrate is tucked away in your muscles and liver as glycogen, it enters your bloodstream in the form of glucose (also called blood sugar), a readily available source of energy that helps perk you up when you're feeling hungry and fatigued. If the glycogen stored in your muscles and liver is low, your body can rely on glucose for fuel; if you already have a fair amount of stored glycogen, your body will use the glucose as a secondary source of energy and spare the glycogen. "It means that you have two sources of fuel as opposed to one, so you can last a lot longer," says Jackie Berning, RD, a sports nutritionist and assistant professor in the department of biology at the University of Colorado at Colorado Springs.
　
Something else you should consider is that the muscles and liver can only store so much glycogen. It's important to "top up" your reserves fairly often, even if you haven't been doing much: During a long night's sleep, the body depletes as much as 80 percent of the glycogen stored in the liver. "That's why eating a little something before you exercise in the morning can really help," says Nocton. Moreover, it doesn't take long to deplete stored glycogen during exercise, and it gets used up even faster when the weather is warm. "If you're playing an intense tennis match without having eaten and it's warm outside, it may take only 30 to 40 minutes before you deplete your glycogen," says Berning. Eating before a match will not only help you last a lot longer, it will also help settle the gastric juices that make your stomach growl and ward off the feelings of lightheadness and fatigue that can make it difficult to perform well.
　
　
　
　
WHAT TO EAT WHEN

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-fibre 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.