Extremely obese people — those with a body mass index (BMI) greater than 40 kg/m2 — have higher mortality rates, and most of these additional deaths are caused by cancer, heart disease, and diabetes, according to a study published online July 8 in PLOS Medicine.

Cari Kitahara, PhD, from the Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, Maryland, and colleagues found that the accumulating excess risk resulted in major reductions in life expectancy after age 40 years that were comparable to the risks of normal-weight smokers.

The researchers excluded anyone who had ever smoked cigarettes or who had a history of chronic disease from the study.

They estimated that BMI categories of 40 to 44.9, 45 to 49.9, 50 to 54.9, and 55 to 59.9 kg/m2 were associated with an estimated 6.5 (95% confidence interval [CI], 5.7 - 7.3), 8.9 (95% CI, 7.4 - 10.4), 9.8 (95% CI, 7.4 - 12.2), and 13.7 (95% CI, 10.5 - 16.9) years of life lost, respectively.

In a pooled analysis of 20 prospective studies from the United States, Sweden, and Australia, among 9564 extremely obese (class 3) participants, mortality rates (deaths per 100,000 per year) were 856.0 in men and 663.0 in women during the study period (1976 - 2009). Among 304,011 normal-weight participants, rates were 346.7 and 280.5 in men and women, respectively. Deaths from heart disease were especially high in the extremely obese group (rate differences, 238.9 and 132.8 in men and women, respectively), followed by deaths from cancer (rate differences, 36.7 and 62.3 in men and women, respectively) and diabetes (51.2 and 29.2 in men and women, respectively).

One limitation of this study is that researchers relied on mostly self-reported, as opposed to measured, height and weight. Another is that weight was ascertained only once at the beginning of follow-up, so researchers could not examine changes in risk if weight changed. However, the findings shed light on what is emerging as a major public health problem in several high-income countries.

6% in United States Morbidly Obese

In the United States, 6% of adults are now morbidly obese, and these individuals accounted for 20% of the total per capita healthcare expenditures in 2000, the study notes.

This large study is particularly important because extreme obesity used to be fairly uncommon, so study sizes were small and little is known about total and cause-specific mortality rates.

"If current global trends in obesity continue, we must expect to see substantially increased rates of mortality due to these major causes of death, as well as rising health-care costs. These results underscore the need to develop more effective interventions to combat this growing public health problem," the authors conclude.

This work was supported in part by the Intramural Research Program of the National Cancer Institute, National Institutes of Health. Certain data were provided by the Vital Statistics Administration, Maryland Department of Health and Mental Hygiene, Baltimore. One coauthor is a member of the Editorial Board of PLOS Medicine. The other authors have disclosed no relevant financial relationships.

PLOS Med. 2014;11:e1001673.