Q&A: The importance of using waist-to-hip ratio to measure obesity

Key takeaways:

• People with BMI-defined severe obesity but a healthy waist-to-hip ratio did not have increased mortality risk.
• When treating Hispanic and Latino adults, PCPs should use the ratio rather than BMI, experts said.

PCPs should use the waist-to-hip ratio rather than BMI to evaluate obesity among Hispanic and Latino adults, according to experts.

Yanbo Zhang, PhD, an assistant professor of pathology at the Albert Einstein College of Medicine, and colleagues recently published a study in JAMA Network Open that evaluated general or central obesity and mortality among Hispanic and Latino adults in the United States and how effective certain metrics are at predicting outcomes.

Healio spoke with Zhang to learn more about their findings, health equity and what PCPs should take away from their research.

Healio: Why did you decide to conduct this study?

Zhang: The Hispanic and Latino population is the second largest and one of the fastest growing racial or ethnic groups in the U.S., and the prevalence of obesity is high in this population. Although evidence regarding the association between obesity and mortality is well-established among non-Hispanic white and Asian populations, there is limited evidence among U.S. Hispanic and Latino adults. Considering the unique physiologic, sociocultural and lifestyle features of the U.S. Hispanic and Latino population, the evidence among other populations might not be directly extrapolated into this population.

Healio: Why is obesity an important area of research, especially related to health equity?

Zhang: Obesity has been linked to increased risks of major noncommunicable diseases, including but not limited to cardiometabolic diseases, dementia, depression and cancers. It contributed to more than 5 million deaths and huge disease burdens in 2019 globally. Besides, obesity is more prevalent among Hispanic/Latino adults compared with non-Hispanic white adults in the U.S. A lack of evidence on the obesity-mortality association among Hispanic/Latino populations also reflects the lack of representation of racial and ethnic minority groups in obesity research.

Healio: Will you briefly describe your study’s findings? Why were these findings important?

Zhang: In this study, we found that a greater waist-to-hip ratio (ie, the ratio of waist circumference to hip circumference) — which is a measure of central obesity (ie, waist-to-hip ratio equal to or greater than 0.9 and 0.85 for men and women, respectively) — was associated with higher mortality risk. If we use BMI to define obesity and adopt the obesity definition from non-Hispanic white adults, severe obesity (BMI equal to or greater than 35 kg/m²) was associated only with higher mortality risk among adults with unhealthy waist-to-hip ratio (waist-to-hip ratio equal to or greater than 0.9 and 0.85 for men and women, respectively). This is different from non-Hispanic white adults, in whom overweight (defined as BMI that is equal to or greater than 25 and less than 30 kg/m²), moderate obesity (BMI that is equal to or greater than 30 but less than 35 kg/m²) and severe obesity (BMI equal to or greater than 35 kg/m²) were all associated with higher mortality risk compared with normal weight (BMI is equal to or greater than 18.5 but less than 25 kg/m²).

This finding is very important. First, it suggests that we should prioritize using the waist-to-hip ratio to evaluate obesity among U.S. Hispanic and Latino adults, and that using BMI alone could lead to wrong risk stratification. Second, we should reconsider whether it is reasonable to use the BMI cutoffs for non-Hispanic white adults among U.S. Hispanic/Latino adults. Specific cutoffs among U.S. Hispanic/Latino adults might be necessary, although more data are needed. We also found that people with severe obesity (defined by BMI) but a healthy waist-to-hip ratio did not have an increased mortality risk. This interesting finding needs future studies to validate and explain the mechanisms.

Healio: What are the clinical implications of your findings?

Zhang: For U.S. Hispanic and Latino individuals, they should pay more attention to their waist-to-hip ratio to prevent long-term adverse health outcomes. For health professionals, it is necessary to incorporate waist-to-hip ratio into risk stratification among U.S. Hispanic and Latino adults, and that evaluating BMI or waist circumference alone could lead to a wrong decision. For government and professional associations, it is necessary to reevaluate the associations of BMI and waist circumference with risks of cardiometabolic diseases and mortality among U.S. Hispanic and Latino adults, and clinical cutoffs for obesity should be established in this population.

Healio: Will you discuss the importance of PCPs using other measures alongside BMI?

Zhang: Obesity could be divided into general obesity and central obesity. General obesity is abnormal or excessive fat accumulation in the whole body, and it doesn't consider the region where fat accumulates. We used BMI to evaluate general obesity. BMI is defined as the body weight divided by the square of the body height, and it is an inaccurate measure of body fat content, as it does not account for muscle mass or bone density. Central obesity is abnormal or excessive abdominal fat accumulation, and waist circumference is a convenient measure to evaluate central obesity. We also considered the waist-to-hip ratio, and since it considered both waist and hip circumference, this measure could reflect both subcutaneous and abdominal adipose accumulation.

It is important to consider other measures alongside BMI when evaluating people's body fat. As indicated above, BMI cannot distinguish fat mass from fat-free body mass (eg, bones and muscles), and it also cannot evaluate regional body fat distribution. Particularly in U.S. Hispanic and Latino adults, a greater waist-to-hip ratio is associated with higher mortality risk regardless of individuals' comorbidities or BMI, which should be prioritized in clinical evaluation and epidemiological surveillance.

Healio: Is there anything else you would like to add?

Zhang: We would like to highlight a group of adults with severe obesity but a healthy waist-to-hip ratio. This group had a similar mortality risk as those without obesity and with a healthy waist-to-hip ratio, but they had a lower mortality risk compared with those with an unhealthy waist-to-hip ratio regardless of BMI. Although we don't have sufficient evidence to explain this phenomenon, and this group had a small number of participants and death events which could make the result unstable, we still believe it deserves future studies to validate and explain it. In our study, we found this group of adults had the greatest hip circumference, as well as favorable blood glycemic and lipid levels, which could be explained by the fact that adipose tissue in the buttocks and thighs is associated with lower metabolic risk. But still, this phenomenon needs future validation and more explanation.

We are not confident enough to entirely dismiss overweight and moderate obesity in a clinical setting because we investigated only the mortality risks and did not investigate other health outcomes including but not limited to diabetes, CVD, cancer, etc. Also, it’s worth noticing that ‘overweight’ and ‘moderate obesity’ here are defined according to the criteria for white adults, and there is a possibility that the definitions of overweight and moderate obesity could be different and need to be set in U.S. Hispanic and Latino adults.

References:

• High body-mass index — level 2 risk. https://www.healthdata.org/results/gbd_summaries/2019/high-body-mass-index-level-2-risk. Accessed Feb. 9, 2024.
• U.S. Department of Health and Human Services Office of Minority Health: Obesity and Hispanic Americans. https://minorityhealth.hhs.gov/obesity-and-hispanic-americans. Accessed Feb. 9, 2024.
• Zhang Y, et al. JAMA Netw Open. 2024;doi:10.1001/jamanetworkopen.2023.51070.