One, people become less physically active as we age, so the number of calories we burn off via activity declines. Loss of muscle contributes to body fat gain because one's resting metabolic rate is linked to how much lean body mass one has. Thirdly, our bodies ability to use fat as a fuel declines as we age, so more of the dietary fat we eat is stored, rather than used for energy. However, in order to continue to stave off creeping obesity successfully, some research suggests we may have to slowly increase our physical activity as we age, rather than decrease it, which is usually what happens. In order to lose weight and keep it off, one must devise a way to construct a calorie deficit, where the total number of calories consumed is less than the number of calories used, thereby requiring the body to make up this calorie difference by taking fat out of storage and using it for energy.
Exercise can create such a calorie deficit in two ways.
Aerobic Exercise significantly increases the total number of calories burned per day due to activity. Aerobic exercise also increases the body's ability to burn fat as a fuel, and to do so at higher exercise intensities than would otherwise be the case. However, the 30 minutes of exercise can be broken up into smaller sessions spaced throughout the day if desired. Although any form of aerobic exercise is good, high intensity exercise is preferable to low intensity exercise for weight control. Strength Training can also be an effective way to alter body composition, in this case, however, the changes are somewhat more passive.