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It takes time to digest your food and turn it into glycogen, which is the primary fuel your muscle cells actually use during exercise. Glycogen is made out of glucose (which comes from carbohydrates) and is stored in both your muscle cells and liver. As long as you're eating enough nutrients to meet your activity needs, your body can store enough glycogen to handle about 2000 calories worth of high-intensity activity or 4000 calories worth of lower-intensity activity—even if you haven’t eaten in a while.
If you’re highly active, you should plan your meals and snacks so that you don’t run out of muscle glycogen at the wrong time (like in the middle of an exercise session). A marathon runner can deplete her glycogen stores before the end of a single race (called “bonking” or “hitting the wall”). A more casual exerciser can run out of glycogen after a few days of not eating enough carbohydrates and total calories to replace it.
Action Step: Eat enough total calories to support your activity level. The combination of a low-calorie diet and a high level of exercise will force your body to breakdown muscle tissue to meet your immediate energy needs. Total calorie deficits of more than 500-1000 calories per day will actually inhibit exercise performance—even for moderate exercisers.
The most important window for replenishing glycogen is the four to five hours immediately after a vigorous exercise session. During this time, the enzymes responsible for this process are more active and effective.
Most healthy people don’t really need to eat immediately before exercise. But eating a small snack or meal 20-60 minutes before an exercise session will trigger an insulin response that helps glucose enter your muscle cells, making it easier to exercise without discomforts like dizziness, faintness, or a general lack of energy. But if you or your stomach prefers not to eat before a workout, you should have plenty of muscle glycogen to fuel one to two hours of moderate to vigorous exercise before eating. To learn more about exercising in the morning or on an empty stomach, read this Ask the Expert Q & A.
Action Step: Eat a good post-exercise meal. Sports nutritionists recommend that active people eat about 250-300 calories (with a 4:1 ratio of carbs to protein) within 90 minutes after exercising. This breaks down to about 50-60 grams of carbs and 12-15 grams of protein.
When you put these facts together into one big picture, you can see that keeping your glycogen tanks topped off is the key factor to maintaining your ability to perform at a high level while losing weight; the Action Steps outlined above will help you do just that.