Gravity is a harsh mistress, along with her cohorts, Metabolism and Aging. They are largely responsible for the phenomenon known as "middle age spread," the tendency of many people to gain weight as they enter their forties and fifties. There are a number of factors which conspire to turn 20 year old athletes into 50 year old couch potatoes with expanding waistlines, some of which are out of anyone's control, such as genetics and the natural effects of the aging process.
One reason people tend to gain weight as they age is a change in metabolism. As we age, our digestive systems tend to work less efficiently, meaning less food energy is burned off as calories and more of it is stored as fat deposits. A child may be able to burn off excessive calories through play sessions or sports, but a sedentary middle age office worker who consumes the same meal may find more of it collecting around his abdomen as fat. This slowing of metabolism causes older people to gain weight which ordinarily would have been consumed during the digestive process.
Another reason why people gain weight as they get older is a lack of dietary restraint. Adults are no longer under the control of parents or school dietitians who made sure they received nutritious meals and controlled portions. There were certain foods such as pizza and marbled steaks which were considered problematic for growing children. Some people are capable of maintaining these early dietary habits, but others discover the pleasure of eating fatty or sugary junk foods high in carbohydrates and empty calories. Many workers also discover that a quick meal from a fast-food restaurant is often more convenient that preparing a healthier meal from home. Years of poor eating habits almost inevitably cause adults to get heavier as they age.
Many people begin their adult working lives in physically or mentally demanding entry-level jobs, which may help them stave off their tendency to gain weight. Younger adults may also continue to participate in organized sports or take up physically challenging hobbies such as jogging or yoga. As people age, however, they may find themselves in more sedentary positions at work and with busier family schedules at home. Relaxation and de-stressing become more of a priority than weight loss or exercise. An increasingly sedentary lifestyle with less motivation for unnecessary exertion can cause many people to gain weight as they enter middle age.
There are some studies which suggest a genetic reason why certain people gain weight as they age. The tendency to become obese or thin may be predetermined by a person's genetic family history. Every person's body handles the storage of excessive fat in different ways, and there may very well be a medical reason why certain people get heavier as they age, such as an imbalance in the thyroid or a disease affecting the digestive process. Some proponents of holistic medicine believe that adults gain weight whenever their digestive tracts or liver have been compromised by an accumulation of toxins. Once these toxins are safely removed through special cleansing programs, the liver and digestive tract should work more efficiently.