Obesity natural history, complications and prognosis

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).^[1][2] 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.^[2]

Excessive body weight is associated with various diseases, particularly cardiovascular diseases, diabetes mellitus type 2, obstructive sleep apnea, certain types of cancer, and osteoarthritis.^[1] As a result, obesity has been found to reduce life expectancy.^[1]

Obesity increases the risk of many physical and mental conditions. These comorbidities are reflected predominantly in metabolic syndrome.^[1] Metabolic syndrome being a combination of medical disorders, which includes diabetes mellitus type 2, high blood pressure, high blood cholesterol, and high triglyceride levels.^[23]

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.^[24]

Obesity is one of the leading preventable causes of death worldwide.^{[25] [26][27]} 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/m^2[28] in non smokers and at a BMI of 24–27 kg/m² in current smokers and increases with changes in either direction.^[29][30] Obesity increases the risk of death in current and former smokers as well as in those who have never smoked.^[30] A BMI of over 32 has been associated with a doubled mortality rate among women over a 16-year period^[31] and obesity is estimated to cause an excess 111,909 to 365,000 death per year in the United States.^[27][1] Obesity on average reduces life expectancy by six to seven years.^[32][1] A BMI of 30–35 reduces life expectancy by two to four years^[28] while severe obesity (BMI > 40) reduces life expectancy by 20 years for men and five years for women.

Although the negative health consequences of obesity in the general population are well supported by the available evidence, health outcomes in certain subgroups seem to be improved at an increased BMI, a phenomenon known as the obesity survival paradox.^[33] The paradox was first described in 1999 in overweight and obese people undergoing hemodialysis,^[33] and has subsequently been found in those with heart failure, and peripheral artery disease (PAD).^[34]

In people with heart failure, those with a BMI between 30.0–34.9 had lower mortality then those with a normal weight. This has been attributed to the fact that people often lose weight as they become progressively more ill.^[35] Similar findings have been made in other types of heart disease. People with class I obesity and heart disease do not have greater rates of further heart problems than people of normal weight who also have heart disease. In people with greater degrees of obesity, however, risk of further events is increased.^[36][37] Even after cardiac bypass surgery, no increase in mortality is seen in the overweight and obese.^[38] One study found that the improved survival could be explained by the more aggressive treatment obese people receive after a cardiac event.^[39] Another found that if one takes into account chronic obstructive pulmonary disease (COPD) in those with PAD the benefit of obesity no longer exsists.^[34]

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