Obesity is measured using body mass index, or BMI. BMI is determined by dividing a person's weight in kilograms by a person's height in meters squared. A person whose BMI is 25.0 to 29.9 kg/m2 is considered overweight. A BMI of 30 to 39.9 kg/m2 indicates obesity. A BMI of 40 kg/m2 or more indicates extreme obesity.

What is going on in the body?

Food is made up of calories, or units of energy. Physical activity and normal body metabolism burn calories. When a person takes in more calories than the body uses, the extra calories are stored as fat. These fat cells enlarge or decrease in size depending on the balance of energy in the body. When fat cells build up or accumulate, it causes obesity.

What are the causes and risks of the condition?

Obesity is a complex condition. It is caused by an interaction among a number of factors. These varied factors fit into these categories: