A new study suggests that waist-to-hip ratio (WHR) has a more linear correlation with all-cause mortality than either body mass index (BMI) or fat mass index (FMI) and might replace BMI as a metric of choice .
Is BMI good enough?
Comparing total mass to height, the body mass index (BMI) metric has become the most popular measure of adiposity, despite its obvious limitations. BMI fails to account for many important things, such as large muscle mass in athletes or low muscle mass in people with sarcopenia (muscle loss), yet it remains central to discussions about obesity and its health outcomes.
Recently, several attempts have been made to find a replacement for BMI that would be just as convenient but more reflective of the true relationship between obesity and disease . Among the suggested metrics are fat mass, fat mass index (FMI), waist circumference, and waist-to-hip ratio (WHR). This study highlights WHR as a good candidate.
No J-curve for WHR
The study uses data from UK Biobank, a repository of health information on half a million British citizens. Its sheer size and the vast array of metrics it contains make it a gold mine for researchers. BMI, FMI (the ratio between fat mass and height), and WHR were all measured at baseline, and multi-year follow-up provided information on mortality. The researchers adjusted their model for several mortality-related parameters: age, sex, smoking status, diabetes status, alcohol consumption, cholesterol profile, blood pressure, and obesity-associated genetic variants.
In all models, both BMI and FMI showed a J-shaped association with all-cause mortality, with the hazard ratio lowest at 25.5 BMI (borderline overweight). This is a familiar picture, although the causes and the clinical relevance of the seeming increase in mortality for lower BMIs are hotly debated. Contrarily, WHR showed a monotonic relationship, with the lowest hazard ratio associated with the lowest WHR. Not surprisingly, among all mortality causes, all three measurements were most strongly correlated with cardiovascular mortality.
Developing a connection
The researchers added another level of analysis via mendelian randomization (MR). This technique looks at traits, in this case BMI, FMI, and WHR, that are associated with outcomes. MR essentially mimics the randomization found in clinical trials and allows researchers to see through the environmental factors, such as lifestyle.
MR showed that among the three metrics, WHR had the strongest association with all-cause mortality, although the difference between it and FMI was not statistically significant. However, considerable sex differences emerged. According to MR, the association between WHR and mortality was much stronger in males, while in females, FMI took the crown. Interestingly, in the observational models, sex-related differences were small.
Still, according to the researchers, their MR findings “support a possible causal relationship between WHR and mortality”, which suggests that WHR can be used not only as a clinical marker but as an intervention target as well. They cite another MR study that found WHR was more strongly associated with blood pressure than BMI .
WHR as a primary metric
Interpreting their results in the context of the J-shaped relationship between mortality and BMI/FMI, the researchers suggest that even with low BMI, increased abdominal fat mass is still unhealthy. As previous research shows, lower BMI can be due to disease-related cachexia  or malnutrition, even with abdominal fat still present, especially in older people.
The researchers suggest using WHR alongside BMI to improve risk stratification for patients and maybe even incorporating WHR as a primary outcome for future clinical interventions. Their MR results, the authors say, might mean that targeting WHR specifically in males can bring considerable benefits.
As usual, the study was not without limitations. First, UK Biobank’s population is relatively genetically homogeneous, which makes generalizing results to other populations harder. Second, adiposity measures were only assessed at baseline. Still, these results are an important addition to the growing body of knowledge threatening to dethrone BMI as the main measurement of adiposity in diagnostics and research.
In this cohort study, compared with BMI, WHR had the strongest, most robust, and consistent association with all-cause mortality and was the only measurement unaffected by BMI. Current WHO recommendations for optimal BMI range are inaccurate across individuals with various body compositions and therefore suboptimal for clinical guidelines. Future research is needed to explore whether using WHR as the primary clinical measure of adiposity would help to improve long-term health outcomes in distinct patient populations compared with BMI. Our results provide further support to shift public health focus from measures of general adiposity, such as BMI, to adiposity distribution using WHR.
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