First, the mitochondria (cellular furnaces where fat and other nutrients are burned) increase in size and total number inside muscle fibers. Second, muscle fibers build up more aerobic enzymes - special chemicals involved in fat-burning. Third, aerobic exercise appears to increase levels of myoglobin, a muscle compound that accelerates the transfer of oxygen from the bloodstream into the muscle fibers. Larger mitochondria and more of them, greater levels of aerobic enzymes, and increased blood flow - these factors all boost the fat-burning capability of muscle fibers. The more aerobically fit you become, the more your body learns to burn fat for energy. So you can see why aerobic exercise is so important for leaning out. Endurance athletes have known these things all along.

The key, however, is eating the right kinds of carbs, in the right amounts. Glucagon opposes the effect of insulin. When blood sugar is low, glucagon is released, and this typically occurs several hours after a meal is eaten. Glucagon then activates the conversion of glycogen to glucose in the liver in response to low blood sugar levels. It also signals the body to start burning fat for energy, because the body is running low on carbohydrates, its preferred fuel source. The ratio of insulin to glycogen in your body largely determines whether you will gain fat or lose it. You can control this ratio naturally by adjusting the protein and carbohydrate proportions in your diet and combining foods in the proper manner. Here’s how you can partition your food more effectively, so it can be used to burn fat (and keep it off), plus build fat-burning muscle tissue: