Obesity is a medical condition in which excess body fat has accumulated to the degree that it may have a negative effect on health, leading to reduced life expectancy. Obesity is associated with many diseases: heart disease, type 2 diabetes, breathing difficulties during sleep, certain types of cancer, and osteoarthritis. Obesity is most commonly caused by a combination of excessive dietary calories, lack of physical activity, and genetic susceptibility. It is defined by body mass index (BMI), and further evaluated in terms of fat distribution via the waist-hip ratio and total cardiovascular risk factors.

Obesity is a leading preventable cause of death worldwide. The primary treatment for obesity (persons with a BMI between 25 and 35) is a combination of dieting and physical exercise. Anti-obesity drugs may also be taken to reduce appetite and inhibit fat absorption.

Surgery is considered the most effective treatment for morbid obesity—persons with a BMI greater than 40, as well as those with a BMI between 35 and 40 with attendant weight related illnesses, who have tried and failed conservative weight loss therapies.