As a Wellness Metric, Body-Mass Index Might Fall Short

By Stephen Miller Sep 23, 2009

A recent report suggests that body-mass index (BMI)—a metric commonly used in wellness programs—might not be the best indicator of health care costs.

"BMI is a single value calculated as a function of a person's height and weight, and it is often used to categorize relative obesity," says Jon Shreve, co-author of the report, Impact of Height, Weight and Smoking on Medical Claims Costs, released by actuarial and consulting firm Milliman. In a growing number of health and wellness programs, achievement of an improved BMI is tied to individual goals that serve as a basis for incentive rewards, such as a rebate on health care premiums.

"According to the World Health Organization, a person with a BMI of 30 or more is considered obese and thereby considered high risk for chronic diseases, including diabetes and cardiovascular diseases," says Shreve, a principal at Milliman. "Our research shows that BMI is an inconsistent measure of build for men and women of different heights, making it a poor predictor of expected medical claim costs."

Adds study co-author Mary van der Heijde, a consulting actuary at Milliman, "When calculating BMI, weight is divided by height squared, but our research determined that weight divided by height cubed provides a better prediction of medical claim costs associated with height and weight. Research suggests that other metrics, such as the Rohrer's Index (RI), better coincide with medical claim costs."

A Better Metric?

RI is a modified form of the BMI, dividing weight by height cubed (rather than squared). This variation, the authors found, improves the linear correlation between overall health care costs and the incidence of select conditions (such as cancer and diabetes) dependent on a person's build. Furthermore, the cubed form produces tighter maximum and minimum levels for standard combinations of height and weight, they found. When RI is coupled with medical information, it supplies more consistent predictions of overall health risk, according to the research.

Among the specific findings in the report, a "healthy" BMI of 22 for females between 40 and 49 years old was associated with a range of average health care costs, as was a less healthy BMI of 27. "However, the actual-to-expected cost ratio is much more consistent if we use the Rohrer's Index," Shreve says. "This consistency means that we may be able to better associate a particular RI value with a reasonable range of costs, which is not possible with BMIs."

Stephen Milleris an online editor/manager for SHRM.​

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