Of course, growing children require more calories. The average weight gain in young adults averages 0.2 to 0.8 kg per year. Single gene defects that produce a defined disease marked by obesity, such as the Prader-Willi syndrome, are extremely rare. Food intake is regulated via neural circuits located in the hypothalamus. Leptin inhibits food intake and increases energy expenditure via an interaction with specific leptin receptors located in the hypothalamus.
Social factors play a major role in weight gain.
Persons who emigrate to a more urbanized culture tend to gain weight. Behavioral or environmental changes in life, such as smoking cessation, are associated with weight gain. Weight gained during holiday periods and festivals is more than at other times of the year and tends not to be lost. Using a definition of obesity as a BMI of 30 or more, half the U.S. population qualifies as obese. However, the distribution of fat has importance in determination of risk.