Causes of obesity.

The prevalence of obesity has been rising steadily over the last several decades and is currently at unprecedented levels: more than 68% of US adults are considered overweight, and 35% are obese (Flegal et al., JAMA 303:235-241, 2010). This increase has occurred across every age, sex, race, and smoking status, and data indicate that segments of individuals in the highest weight categories (i.e., BMI > 40 kg/m(2)) have increased proportionately more than those in lower BMI categories (BMI < 35 kg/m(2)). The dramatic rise in obesity has also occurred in many other countries, and the causes of this increase are not fully understood (Hill and Melanson, Med Sci Sports Exerc 31:S515-S521, 1999).