To complicate the problem, many older people are less active than when they were younger, yet continue to eat the same amount of food. People who quit smoking often gain some weight because nicotine is no longer artificially stimulating their metabolism and because they tend to eat instead of smoke, says G. Michael Steelman, M.D., vice president of the American Society of Bariatric Physicians, who is in private practice in Oklahoma City. And strict dieting can also make you gain weight. When you restrict calories too much, your body thinks it's starving and slows down your metabolism. When you begin eating again, it doesn't shift completely back into normal, and you put on the pounds more easily, explains Dr. Steelman.
Some medical problems can trigger weight gain.
An underactive thyroid, although not a cause of significant weight gain, slows the metabolism, allowing calories to be stored more easily as fat. People with diabetes who begin taking insulin may notice an increase in weight. And, in rare instances, hormonal disturbances that cause an overproduction of insulin or cortisol in the body also cause weight gain. As a side effect, some medications can stimulate the appetite, slow the metabolism or allow calories to be stored more easily as fat. Women taking birth control pills or estrogen replacement therapy probably will notice a weight gain of about five pounds or so, says Dr. Robertson.