Health and Medicine

Association between Class III Obesity (BMI of 40–59 kg/m2) and Mortality: A Pooled Analysis of 20 Prospective Studies

Cari M. Kitahara, Alan J. Flint, Amy Berrington de Gonzalez, Leslie Bernstein, Michelle Brotzman, Robert J. MacInnis, Steven C. Moore, Kim Robien, Philip S. Rosenberg, Pramil N. Singh, Elisabete Weiderpass, Hans Olov Adami, Hoda Anton-Culver, Rachel Ballard-Barbash, Julie E. Buring, D. Michal Freedman, Gary E. Fraser, Laura E. Beane Freeman, Susan M. Gapstur, John Michael Gaziano, Graham G. Giles, Niclas Håkansson, Jane A. Hoppin, Frank B. Hu, Karen Koenig, Martha S. Linet, Yikyung Park, Alpa V. Patel, Mark P. Purdue, Catherine Schairer, Howard D. Sesso, Kala Visvanathan, Emily White,Alicja Wolk, Anne Zeleniuch-Jacquotte, Patricia Hartge
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Summary

Background The prevalence of class III obesity (body mass index [BMI]≥40 kg/m2) has increased dramatically in several countries and currently affects 6% of adults in the US, with uncertain impact on the risks of illness and death. Using data from a large pooled study, we evaluated the risk of death, overall and due to a wide range of causes, and years of life expectancy lost associated with class III obesity. Methods and Findings In a pooled analysis of 20 prospective studies from the United States, Sweden, and Australia, we estimated sex- and age-adjusted total and cause-specific mortality rates (deaths per 100,000 persons per year) and multivariable-adjusted hazard ratios for adults, aged 19–83 y at baseline, classified as obese class III (BMI 40.0–59.9 kg/m2) compared with those classified as normal weight (BMI 18.5–24.9 kg/m2). Participants reporting ever smoking cigarettes or a history of chronic disease (heart disease, cancer, stroke, or emphysema) on baseline questionnaires were excluded. Among 9,564 class III obesity participants, mortality rates 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 contributed largely to the excess rates in the class III obesity 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 (rate differences = 51.2 and 29.2 in men and women, respectively). Within the class III obesity range, multivariable-adjusted hazard ratios for total deaths and deaths due to heart disease, cancer, diabetes, nephritis/nephrotic syndrome/nephrosis, chronic lower respiratory disease, and influenza/pneumonia increased with increasing BMI. Compared with normal-weight BMI, a BMI of 40–44.9, 45–49.9, 50–54.9, and 55–59.9 kg/m2 was associated with an estimated 6.5 (95% 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) y of life lost. A limitation was that BMI was mainly ascertained by self-report. Conclusions Class III obesity is associated with substantially elevated rates of total mortality, with most of the excess deaths due to heart disease, cancer, and diabetes, and major reductions in life expectancy compared with normal weight.

Key Points

  • 1Class three obesity leads to significantly higher mortality rates compared to normal weight
  • 2Major causes of excess mortality include heart disease, cancer, and diabetes
  • 3The study combines data from multiple countries to provide a comprehensive analysis
  • 4Individuals with higher BMI within the class three category face increased years of life lost
  • 5The results underscore the urgent public health implications of rising obesity rates.

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