Obesity is a medical condition in which excess body fat has accumulated to an extent that it may have a negative effect on health. People are generally considered obese when their body mass index (BMI), a measurement obtained by dividing a person's weight by the square of the person's height—despite known allometric inaccuracies[a]—is over 30 kg/m2; the range 25–30 kg/m2 is defined as overweight. Some East Asian countries use lower values. Obesity is correlated with various diseases and conditions, particularly cardiovascular diseases, type 2 diabetes, obstructive sleep apnea, certain types of cancer, and osteoarthritis. High BMI is a marker of risk, but not proven to be a direct cause, for diseases caused by diet, physical activity, and environmental factors. A reciprocal link has been found between obesity and depression, with obesity increasing the risk of clinical depression and also depression leading to a higher chance of developing obesity.
Obesity has individual, socioeconomic, and environmental causes, including diet, physical activity, automation, urbanization, genetic susceptibility, medications, mental disorders, economic policies, endocrine disorders, and exposure to endocrine-disrupting chemicals. While a majority of obese individuals at any given time are attempting to lose weight and often successful, research shows that maintaining that weight loss over the long term proves to be rare. The reasons for weight cycling are not fully understood but may include decreased energy expenditure combined with increased biological urge to eat during and after caloric restriction. More studies are needed to determine if weight cycling and yo-yo dieting contribute to inflammation and disease risk in obese individuals.
Obesity prevention requires a complex approach, including interventions at community, family, and individual levels. Changes to diet and exercising are the main treatments recommended by health professionals. Diet quality can be improved by reducing the consumption of energy-dense foods, such as those high in fat or sugars, and by increasing the intake of dietary fiber. However, large-scale analyses have found an inverse relationship between energy density and energy cost of foods in developed nations. Low-income populations are more likely to live in neighborhoods that are considered "food deserts" or "food swamps" where nutritional groceries are less available. Medications can be used, along with a suitable diet, to reduce appetite or decrease fat absorption. If diet, exercise, and medication are not effective, a gastric balloon or surgery may be performed to reduce stomach volume or length of the intestines, leading to feeling full earlier or a reduced ability to absorb nutrients from food.