He is less active than a normal person. He is using less energy. But it is more likely that his relative inactivity is an inherited tendency. Studies have shown that inherited traits probably play an important part in whether or not we are likely to become overweight. Probably the most significant inherited problem is that of reduced metabolism.
Metabolism is the sum total of all the chemical processes that occur in the body.
These processes use up energy. Even in a state of complete rest and starvation metabolism is taking place and calories are being used. For a long time it was thought that overweight people had a low B.M.R. compared with normal people. Experiments have shown that this is not usually so. The basal metabolism of most obese people is the same as that of normal people.