Body mass index-specific waist circumference thresholds may improve mortality risk stratification in women

1. In this prospective cohort study, body mass index-specific waist circumference (WC) thresholds were found to improve mortality risk stratification in a nationwide sample of women.

2. This effect was consistent even after removing potential confounders such as diet, physical activity, and sleep.

Evidence Rating Level: 2 (Good)

Study Rundown: Body mass index (BMI) is a poor standalone measure of excess adiposity, and although waist circumference (WC) can effectively measure adiposity, the recommended WC thresholds fail to complement BMI. A recent report recommended a set of WC thresholds based on BMI, which this study aimed to validate in a nationwide cohort of women. It was found that, within each BMI category, multivariable-adjusted mortality risk was higher among participants with larger WC compared with those with normal WC. Two additional validation cohorts were created based on women who had undergone a hormone therapy and/or a low-fat dietary modification trial, and based on women from the Southern United States or at centers with dual x-ray absorptiometry (DXA). Further stratifying BMI categories by WC thresholds was found to improve mortality discrimination compared with BMI alone among the first cohort but not the second. However, risk stratification improved among both cohorts, with greater effects being noted in the first cohort. Sensitivity analyses showed similar results when diet, physical activity, and sleep were removed as predictors. The generalizability of this study was limited by the use of healthy volunteers, the exclusion of men and younger women, and the lack of assessment of cause-specific mortality. Nevertheless, this study suggested that BMI-specific WC thresholds may have utility in the assessment of adiposity and can allow for more accurate risk stratification.

Click to read the study in AIM

Relevant Reading: Waist circumference as a vital sign in clinical practice: a Consensus Statement from the IAS and ICCR Working Group on Visceral Obesity

In-Depth [prospective cohort]: This study aimed to validate BMI-specific WC thresholds in a nationwide cohort of women. Participants were originally part of the Women’s Health Initiative (WHI) study, which recruited women living around 40 geographically dispersed centers within the United States between 1993 and 1998. Women were recruited to participate in a hormone therapy trial and/or a low-fat dietary modification trial, with the option to join a calcium plus vitamin D trial after 1 year. The development cohort for this study was made up of the WHI’s observational study group, which comprised women from 33 centers who were ineligible for the clinical trial because they had had recent history of invasive cancer, recent history of myocardial infarction or stroke, or were consuming less than 32% of energy from fat. Two validation cohorts were formed: Validation Cohort 1 was derived from the clinical trial participants from the same 33 centers, while Validation Cohort 2 comprised participants from the remaining 7 centers, which were either located throughout the Southern United States or had DXA. Participants were excluded if had significant unintentional weight loss, had BMI <18.5 or ≥50 kg/m², or were missing predictor variables. Validation Cohort 1 had half the prevalence of prior CVD and cancer but a higher prevalence of obesity (72.6% vs. 59.1%) compared with the development cohort. Validation Cohort 2 had nearly twice the proportion of women who identified as Black (12.7% vs. 6.4%) and nearly triple the proportion who identified as Hispanic (9.1% vs. 3.1%) compared with the development cohort. Validation Cohort 1 had a higher proportion of large WC compared with Validation Cohort 2 (21.9% vs. 18.2%). Across BMI categories, participants with a larger WC had higher mortality risk. For instance, women with overweight and larger WC (hazard ratio [HR], 1.19 [95% CI, 1.15 to 1.24]) had higher mortality risk compared with those with overweight but normal WC (HR, 0.96 [95% CI, 0.93 to 0.99]). Stratifying BMI categories by WC thresholds improved mortality discrimination over BMI alone at 10 years for Validation Cohort 1, with the c-statistic increasing from 60.7% (95% CI, 59.5% to 61.9%) to 61.3% (95% CI, 60.2% to 62.5%). This corresponded to an improvement of 0.7% (95% CI, 0.3% to 1.0%); however, there was no change in c-statistic for Validation Cohort 2. Risk stratification was improved for both Validation Cohorts 1 (continuous net reclassification improvement [NRI], 20.4% [95% CI, 17.3% to 23.6%]) and 2 (continuous NRI, 12.3% [95% CI, 8.5% to 16.0%]) at 10 years. Overall, this study suggested the effectiveness of BMI-specific WC thresholds in improving risk stratification but not in mortality discrimination.

Image: PD

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