Growth hormone is a potent fat fighter, which is one of the reasons why the hormone is so popular with athletes and adults trying to control body composition. Overeating decreases growth hormone levels, which can cause weight gain. This is one reason why it is so easy to gain body fat with even short-term changes in lifestyle, such as overeating during a one- or two-week vacation.
University of Michigan researchers, led by Andrea Cornford, found that a two-week period of forced overeating (4,000 calories per day) decreased 24-hour growth hormone secretion by 80 percent. After two weeks, the average person gained 5 pounds, increased body fat by 10 percent, and increased resting insulin levels by 400 percent. Overeating for several weeks triggers large metabolic changes that make the body more susceptible to fat gain. (Journal of Clinical Endocrinology Metabolism, published online January 5, 2011)