选词填空[A] positive [B] additional [C] duration [D] effective [E] shed[F] physical [G] food [H] functions [I] participated [J] rely[K] cut [L] repeatedly [M] uses [N] little [O] obviousPerhaps like most Americans you have some extra pounds to 41 . You may even have tried a fad diet or two, but found yourself right back where you started. The key to weight loss is regular 42 activity. And surprisingly, you don't have to give up eating or make the gym your second home to see long-term, 43 effects.You body needs a certain amount of energy to maintain basic 44 such as breathing, blood circulation and digestion. The energy required to keep your organs functioning is referred to as the resting or basal metabolic rate.Any time you are active, 45 energy is required. It is obtained from glycogen and fat stored in the blood, liver, and muscles. The key to losing weight is to draw on the fat rather than on the carbohydrate reserves.Which of the two energy sources you use depends on the intensity and 46 of your activity. The higher the intensity, the more your body will pull from the stored carbohydrates. The lower the intensity, the more your body will 47 on fat as its fuel.Aerobic exercise is most 48 for weight loss. When you perform aerobic activities you 49 contract large muscle groups such as your legs and arms. Walking, running, rollerblading, swimming, dancing, and jumping jacks are all forms of aerobic activity.Surprisingly, if your aerobic activity is low to moderately intense and of long duration, you will burn more fat than if you had 50 in a short burst of high-intensity exercise. In short, a brisk 30-minute walk will burn fat while a 100-yard sprint will burn glycogen.