A recent Chinese study published in The Journal of Clinical Endocrinology & Metabolism seems to show that body mass index and waistline measurements do not tell the whole story when determining cardiometabolic risk factors in some ethnic groups.
According to the Hormone Health Network, as of January 2020, more than 2 billion adults have obesity — a global pandemic. No longer misconstrued as stemming from a simple “lack of self-control,” we now know that the pathophysiology of obesity is multifactorial, ranging from hormonal imbalance to genetics to environmental, cultural, and psychosocial components. As we continue to delve into these complexities, it starts to feel like the proverbial safe to which there is a combination, but the combination is locked up in the safe.
- In a cross-sectional study of two large nationwide surveys in China and the U.S., the associations between waist circumference and BMI and cardiometabolic risk factors were compared among multiple ethnic groups.
- Residual waist circumference (the proportion of waist circumference not related to BMI) was significantly associated with all cardiometabolic risk factors in Chinese subjects, some of those factors in non-Hispanic Whites and Blacks, and none of them in Mexican Americans and non-Hispanic Asians.
- When using BMI and waist circumference to screen for obesity-related health risks, clinicians should consider ethnicity-specific susceptibility.
Nevertheless, researchers in China have unlocked one more clue. A team from the Shanghai Institute of Endocrine and Metabolic Diseases and the Shanghai National Center for Translational Medicine, Ruijin Hospital, of the Shanghai Jiaotong University School of Medicine, Shanghai, China, have found an association between Chinese ethnicity and greater risk of serious complications from obesity.
Setting the Record Straight
In “Chinese Adults are More Susceptible to Effects of Overall Obesity and Fat Distribution on Cardiometabolic Risk Factors,” published in The Journal of Clinical Endocrinology & Metabolism (JCEM) in February, the team used 2010 data from the Noncommunicable Disease Surveillance Point System, a nationally representative sample of Chinese adults, and data collected in the U.S. from 2005 to 2016 in the National Health and Nutrition Evaluation Survey (NHANES) to compare the effects of waist circumference and body mass index (BMI) on risk for developing diabetes, high blood pressure, and heart disease among various ethnicities.
Waist circumference and BMI are the two most commonly used diagnostic indicators of obesity, yet they do not reveal the full picture, as they do not reflect the same level of fat mass and abdominal obesity or whether ethnic differences exist. So, Weiqing Wang, MD, PhD, and team (Ruizhi Zheng, Mian Li, Min Xu, Jieli Lu, Tiange Wang, Meng Dai, Di Zhang, Yuhong Chen, Zhiyun Zhao, Shuangyuan Wang, Hong Lin, Yufang Bi, Yu Xu, and Guang Ning) set out to determine whether overall obesity and fat distribution have ethnicity-specific effects on five different ethnic populations.
“Racial and ethnic differences in susceptibility of obesity-related health problems should be noticed while screening for high-risk individuals using BMI and waist circumference.” – Weiqing Wang, MD, PhD, Shanghai Institute of Endocrine and Metabolic Diseases; Shanghai National Center for Translational Medicine, Ruijin Hospital, of the Shanghai Jiaotong University School of Medicine, Shanghai, China
Piggybacking on prior studies that have demonstrated links between obesity metrics (in particular, waist circumference and BMI) and cardiometabolic risk factors, these researchers took another look at the data and came up with different findings. Whereas previous studies showed that a larger waist circumference was the main culprit in degree of risk, Wang and team saw flaws in how that finding was arrived at, given the knowledge that an association exists between waist circumference and BMI. This so-called multicollinearity of the study variables is an acknowledged pitfall of some regression analyses that causes grave errors and therefore untrustworthy results. This team wanted to set the record straight.
They got around the multicollinearity problem by isolating the variables so their impacts could be independently assessed in two separate linear regression models: waist circumference was the dependent variable and BMI was the independent variable. The key here is the new metric they developed out of their statistical analysis method: residual waist circumference. By removing the variation of BMI from waist circumference measures, they obtained the proportion of waist circumference not explained by BMI, in other words, the measurement of central obesity adjusted for overall adiposity.
From the two study populations, a total of 126,284 adult subjects were included: 98,532 Chinese, 5,316 Mexican Americans, 13,487 non-Hispanic Whites, 6,980 non-Hispanic Blacks, and 1,969 non-Hispanic Asians. Not surprisingly to the team, significant differences among the ethnic groupings emerged for systolic and diastolic blood pressure, hemoglobin A1c, triglycerides, and high- (HDL) and low-density lipoprotein (LDL) cholesterol. These differences included that Chinese and non-Hispanic Asians had lower BMI than the other three groups, non-Hispanic Blacks had the lowest triglyceride level, and Chinese had the lowest cholesterol levels. Chinese had the highest blood pressure levels but were lowest in terms of taking anti-hypertensive medicine. Also as expected, both obesity metrics were positively correlated with blood pressure, hemoglobin A1c, triglyceride, and LDL cholesterol and negatively correlated with HDL-cholesterol level among all ethnic groups.
Perhaps the biggest surprise was that although waist circumference was highly correlated with BMI in all ethnicities, residual waist circumference was uncorrelated with BMI in all groups. Yet, residual waist circumference was associated with all cardiometabolic risk factors in the Chinese group alone (although it was associated with some risk factors in non-Hispanic Whites and Blacks). “Our study demonstrated that Chinese adults were more susceptible to the effects of overall obesity and abdominal fat accumulation on blood pressure and triglycerides than those in other racial and ethnic populations,” Wang stated in an Endocrine Society press release. The Chinese group tended to have more abdominal fat deposition, which can lead to dyslipidemia and cardiovascular disease.
Waist circumference and BMI are the two most commonly used diagnostic indicators of obesity, yet they do not reveal the full picture, as they do not reflect the same level of fat mass and abdominal obesity or whether ethnic differences exist.
Noting Racial and Ethnic Differences
The upshot is, BMI and waist circumference do not take ethnicity into account, which could mean that risk for cardiovascular disease gets missed for certain ethnicities, like Chinese adults who may have a comparatively low BMI and waist circumference, yet a proportion of abdominal fat deposition high enough to significantly raise their risk. “These racial and ethnic differences in susceptibility of obesity-related health problems should be noticed while screening for high-risk individuals using BMI and waist circumference,” said Wang in the press release.
Another takeaway from their study is a caution to researchers to account for multicollinearity when designing studies and interpreting results. Ignoring it can lead to important inaccuracies such as obfuscation of some results and unwarranted predominance of others. They also caution against overinterpreting their study results, calling for additional studies to continue investigating ethnicity-specific differences in obesity-related health complications.
Horvath is a freelance writer based in Baltimore, Md. She wrote about the ENDO 2021 Presidential Plenary sessions in the January and March issues.
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