Body Mass Index (BMI), though often used to define obesity, does not tell us about body shape or body fat distribution. Central obesity, aka “belly fat,” has been associated with higher risk of mortality, but is often only measured among those who are overweight or obese and not among people classified as normal weight.
Researchers from the University of Iowa used BMI and waist circumference data from the Women’s Health Initiative, a U.S. cohort study of over 156,624 postmenopausal women recruited from 1993-1998 and followed through early 2017. The authors then linked the data to study-obtained death records and the National Death Index. They also measured age, education level, income, physical activity, smoking, and other confounding variables.
Of the 156,624 women, 43,838 (28%) died. Compared to women with normal BMI and normal waist circumference, women with high waist measurements (>35 inches) were more likely to have died regardless of their BMI. For example, those with normal BMI and high waist circumference were 31% more likely to have died (hazard ratio 1.31, 95% confidence interval 1.20 to 1.42). Similarly, women classified as obese with high waist circumference were 30% more likely to have died (hazard ratio 1.30, 95% confidence interval 1.27 to 1.34).
The authors acknowledged several limitations, including the lack of imaging data to more accurately measure visceral fat (fat surrounding internal organs). They also acknowledged that measurements were taken at a single point in time, so they don’t know the effects of changing BMI and waist circumference over time.
In a related article, the authors suggested that BMI alone “…leaves many people in the dark about the risks from (belly fat)”.
Sun, Y., Liu, B., Snetselaar, L. G., Wallace, R. B., Caan, B. J., Rohan, T. E., … & Bao, W. (2019). Association of Normal-Weight Central Obesity With All-Cause and Cause-Specific Mortality Among Postmenopausal Women. JAMA network open, 2(7), e197337-e197337.
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