Individuals who are extremely obese have higher rates of mortality

Class III obesity (BMI greater than 40 kg/m2) is linked to higher rates of mortality, according to a study published in this week's PLOS Medicine. Cari Kitahara and colleagues from National Cancer Institute, US, found that mortality rates for a wide range of diseases, particularly heart disease, cancer, and diabetes, were higher in individuals with class III obesity compared to those in the normal weight range.

The researchers reached these conclusions by pooling data from 20 prospective (mainly US) cohort studies from the National Cancer Institute Cohort Consortium. After excluding individuals who had ever smoked and people with a history of chronic disease, the analysis included 9,564 adults who were classified as class III obese based on self-reported height and weight at baseline and 304,011 normal-weight adults. Among the participants with class III obesity, mortality rates (deaths per 100,000 persons per year) during the 30-year study period were 856.0 and 663.0 for men and women, respectively, whereas the mortality rates among normal-weight men and women were 346.7 and 280.5, respectively. Heart disease was the major contributor to the higher mortality rate among class III obese individuals, followed by cancer and diabetes. Furthermore, the risk of all-cause death and death due to heart disease, cancer, diabetes, and several other diseases increased with increasing BMI. Compared with having a normal weight, having a BMI between 40 and 59 kg/m2 resulted in an estimated loss of 6.5 to 13.7 years of life.

The accuracy of these findings is limited by the use of mostly self-reported height and weight measurements to calculate BMI and by the use of BMI as the sole measure of obesity. These findings may not be generalizable to all populations. Nevertheless, these findings indicate that class III obesity is associated with a substantially increased rate of death and highlight the need to develop more effective interventions to reduce class III obesity.

The authors say: "Class III obesity is associated with excess rates of total mortality and mortality due to a wide range of causes, particularly heart disease, cancer, and diabetes, and that the risk of death overall and from these specific causes continues to rise with increasing values of BMI."

They continue: "We found that the reduction in life expectancy associated with class III obesity was similar to (and, for BMI values above 50 kg/m2, even greater than) that observed for current smoking."

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