Burning fat generates about nine food calories per gram, which means that a pound of pure fat is worth about 4,000 calories (though because fat isn't really pure, the true number is closer to 3,500 calories). My daily energy needs amount to about 2,800 calories, of which fewer than 1,000 will be used to power physical activity (the majority of my calorie needs are devoted to infrastructure—servicing my body's background metabolic requirements). Absolute starvation—not a good idea, by the way—would, in theory, burn away about 13 ounces of fat in a 24-hour period. Activity plays a role in weight loss, and, as I don't need to tell anyone who has tried to lose or gain weight, other factors do as well. Appetite, which is partially controlled by hormone effects we are just beginning to understand, is obviously important. So is the caloric density of the food we choose to eat (fat contains more than twice as many calories as the same weight of carbohydrate or protein; many vegetables are full of water and cellulose, both of which contribute little or no calories to our intake). Additional factors (like differences in fat absorption) affect the efficiency with which food is converted into energy or body mass.