According to the National Institutes of Health, metabolism is the name for the process your body uses to convert the food you eat into energy. The energy your body does not use is stored as fat. Many people believe that this process slows as you age, causing weight gain. However, this is a misconception, since your metabolism is more directly related to your level of activity throughout your life.
While your metabolism is your body's process of converting food into energy, it is connected to your body's expenditure of energy. You burn energy when you are active; however, even when you are not active, your body still uses energy for processes like breathing and healing. This is called your basal metabolic rate--or BMR--which is determined by how much weight and muscle your body has. Muscle burns more calories than fat, so if you are overweight you burn fewer calories even when resting, causing your metabolism to seemingly slow.
Metabolism and Age
Thermogenesis, the food processing part of your metabolism, actually stays fairly steady throughout your adult life. The illusion that your metabolism slows as you age actually occurs because as you get older, your muscle mass decreases and your amount of fat tends to increase, decreasing your BMR and the amount of calories your body burns. This in turn decreases the amount of calories you need to take in through your diet.
Metabolism and Weight Gain
One key reason that older people sometimes have lower muscle mass and higher amounts of fat is their level of physical activity. When you are less active, you lose muscle mass and gain fat, which burns calories more slowly than muscle. This causes your BMR to decrease, decreasing the amount of calories you need. However, many people do not reduce their calorie intake appropriately, leading to more weight gain and an even lower BMR. To prevent this, you need to stay active throughout your life, especially as you age.
When you get older, you need to stay active to maintain muscle mass and keep your body fat levels low. This will help your BMR stay higher, which means your body burns more calories, and will help you feel like your metabolism is higher. However, according to the American Institute for Cancer Research, you should also stay active and integrate weight and strength training because as you strengthen your body, you may prevent injuries, ease arthritis pain and even reduce cancer risk.