"Normal" BMI May Mask Risk

Frank is a 43-year-old man with high blood pressure, high cholesterol, and a clinically normal BMI. At his last check-up, Frank asked you if he really needs to worry about his BP and cholesterol, since he is of normal weight.

How do you advise your patient?
(Answer and discussion on next page)

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Answer: Everyone should be aware that being of clinically normal BMI does not mean that they are healthy.

Much of the talk about the obesity epidemic centers around body mass index (BMI). The trouble with BMI, of course, is that its usefulness is limited for those who are extremely tall or short or who are particularly muscular. Further, while being overweight or obese is linked to greater risk of cardiometabolic abnormalities like high blood pressure, poor cholesterol scores, or insulin resistance/diabetes, that does not mean that all persons with a higher BMI are necessarily in poor metabolic health. Conversely, being of clinically normal weight usually means a lower risk of those same cardiometabolic abnormalities—but again, that risk is not nonexistent.

The Research

A team of researchers funded by grants from the National Institutes of Health took a closer look at data gathered in 2 large-scale studies that included over 7600 adults of 5 different racial/ethnic groups (white, Chinese American, African American, Hispanic, and South Asian) living in the United States.
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The participants responded to detailed culture-specific dietary questionnaires and were subjected to a battery of tests including blood pressure, glucose and cholesterol scores, and even abdominal CT scans to measure visceral fat. The authors could then classify each participant by their BMI according to the World Health Organization's guidelines for their ethnic group and focus their research on those who had clinically normal BMIs but also had at least 2 abnormal scores in the following tests: blood pressure, HDL cholesterol, triglyceride scores, or glucose scores. These people were termed MAN (Metabolic Abnormalities Normal weight).

We make much of the fact that some 30% of Americans are obese and so forth, but this study found that regardless of ethnic group, 29.1% of the participants with a clinically normal BMI were MAN—that is, they had at least 2 abnormal clinical results that we normally associate with being overweight or obese. By comparison, 35.8% of those with BMI in the obese range were metabolically normal.

The Results

The authors used data regarding the likelihood of those 4 metabolic abnormalities occurring in whites with a BMI of 25 to estimate what BMI score would yield the same number of metabolic abnormalities in the various ethnic groups. After taking into account age, sex, and the interaction between race and BMI, they found that to experience the same health risks as whites with a BMI of 25, African Americans would have a BMI of 22.9, Hispanics would have a BMI of 21.5, Chinese Americans would have a BMI of 20.9, and South Asians would have a BMI of 19.6.

What’s the “Take-Home”?

It's tempting for a patient to look at their BMI and think that they are perfectly healthy and have no reason to be concerned about their diet. We know, however, that biologically speaking, ethnicity does matter. Everyone, but especially those of non-white ethnicity, should be aware that being of clinically normal BMI does not mean that they are healthy. Help your patients see the bigger picture with their metabolic scores along with the adjusted BMI for their ethnicity.

Reference:

Gujral UP, Vittinghoff E, Mongraw-Chaffin M, et al. Cardiometabolic abnormalities among normal-weight persons from five racial/ethnic groups in the United States: a cross-sectional analysis of two cohort studies. Ann Int Med doi:10.7326/M16-1895.