How Food Fuels Your Body

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. Understand that you won't burn more of anything if you can't muster the enthusiasm to master your toughest sets of exercises. 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 blood stream 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. This means that you have two sources of fuel as opposed to one, so you can last a lot longer during your daily workouts.

Something else you should consider is that the muscles and liver can only store so much glycogen. It's important to "top off" 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. Moreover, it doesn't take long to deplete stored glycogen during exercise, and it gets used up even faster when the weather is warm. We’re heading into Spring, so take heed.