Carrying extra weight increases a person’s risk of developing diabetes, heart disease, and other serious illnesses. Studies have shown, however, that being overweight or at the low end of obese on the body mass index (BMI) may not necessarily shorten lifespans in the United States, resulting in the obesity paradox.

New research by Andrew Stokes, a Ph.D. student in demography and sociology in Penn Arts & Sciences, suggests that these studies substantially underestimate the mortality risks associated with excess weight in the United States. His study, “Using Maximum Weight to Redefine Body Mass Index Categories in Studies of The Mortality Risks of Obesity,” was published in the March issue of the journal Population Health Metrics.

“The scholarly community is divided over a large meta-analysis that found that overweight is the optimal BMI category and that there are no increased risks associated with obese class 1,” Stokes says.

Normal weight is indicated by a BMI of 18.5-24.9, overweight is indicated by a BMI of 25.0-29.9, obese class 1 is a BMI of 30.0-34.9, and obese class 2 is a BMI of 35.0 and above.

Skeptics of the meta-analysis and other studies that have found weak associations between excess weight and mortality argue that the findings are likely driven by biases, especially by illness-induced weight loss.  

Stokes agrees: “Using BMI at the time of the survey to assess the mortality risks of overweight and obesity is problematic―especially in older populations―because slimness can be a marker of illness.”

Researchers have attempted to address this bias by eliminating people who are ill from their samples. However, according to Stokes, such measures are inadequate because information on illness is ascertained by self-reporting and not everyone with an illness has been diagnosed.

Stokes uses individuals’ highest BMI in life to predict mortality rates. He says that in the previous literature, the normal weight category combines data from low-risk, stable-weight individuals with high-risk individuals who have experienced weight loss. Use of weight histories makes it possible to separate the two groups and redefine the reference category as people who were consistently normal weight throughout their lives.

Stokes conducted the analyses using data from the National Health and Nutrition Examination Surveys from 1988-1994 and 1999-2004 linked to the National Death Index through 2006 on U.S. adults ages 50-84 who never smoked.   

The project was supported by a grant from the National Institute on Aging.