Effects on health
Excessive body weight is associated with various diseases, particularly cardiovascular diseases, diabetes mellitus type 2, obstructive sleep apnea, certain types of cancer, and osteoarthritis. As a result, obesity has been found to reduce life expectancy.
Mortality
Obesity is one of the leading preventable causes of death worldwide. Large scale American and European studies have found that mortality risk varies with BMI; the lowest risk is found at a BMI of 22.5–25 kg/m2 in non smokers and at a BMI of 24–27 kg/m2 in current smokers and increases with changes in either direction. Obesity increases the risk of death in current and former smokers as well as in those who have never smoked. A BMI of over 32 has been associated with a doubled mortality rate among women over a 16-year period and obesity is estimated to cause an excess 111,909 to 365,000 death per year in the United States. Obesity on average reduces life expectancy by six to seven years. A BMI of 30–35 reduces life expectancy by two to four years while severe obesity (BMI > 40) reduces life expectancy by 20 years for men and five years for women.
Morbidity
Obesity increases the risk of many physical and mental conditions. These comorbidities are reflected predominantly in metabolic syndrome. Metabolic syndrome being a combination of medical disorders, which includes diabetes mellitus type 2, high blood pressure, high blood cholesterol, and high triglyceride levels.
Complications are either directly caused by obesity or indirectly related through mechanisms sharing a common cause such as a poor diet or a sedentary lifestyle. The strength of the link between obesity and specific conditions varies. One of the strongest is the link with type 2 diabetes. Excess body fat underlies 64% of cases of diabetes in men and 77% of cases in women.
Health consequences can be categorized by the effects of increased fat mass (osteoarthritis, obstructive sleep apnea, social stigmatization) or by the increased number of fat cells (diabetes, cancer, cardiovascular disease, non-alcoholic fatty liver disease). Increases in body fat alter the body's response to insulin, potentially leading to insulin resistance. Increased fat also creates a proinflammatory state, increasing the risk of thrombosis.