With countless medical advancements and efforts to curb smoking, one might assume that life expectancy in the United States would be improving. Yet recent studies report that the rate of improvement in American mortality has been slowing down during the last three decades. According to a new Penn study, a rise in obesity is to blame.

The research was performed by Samuel Preston, a professor of sociology in the School of Arts and Sciences; Yana Vierboom, a graduate student in demography at Penn; and Andrew Stokes, an assistant professor of global health at Boston University. They used data from successive cohorts of the National Health and Nutrition Examination Survey (NHANES) from 1988 to 1994 and 1999 to 2010, as well as from the NHANES linked mortality files, which include follow-up statistics into death records through December 2011. The final sample consisted of 25,269 adults ages 40 to 79.

Rather than use a typical measure of obesity—body mass index, or BMI, recorded at baseline—the researchers calculated each person’s lifetime maximum BMI. They found this measure to be more successful at predicting mortality because it is less susceptible to weight loss associated with illness, which biases estimates of the link between BMI and mortality. It also conveys important elements of weight history that may have enduring effects on health.

“We established a time trend of mortality within this data set by dating every observation as it moved forward,” Preston says. “We estimated that the impact of rising obesity was about twice as important for mortality trends as the impact of declining smoking. Smoking is such an important variable in mortality analysis, and U.S. mortality is improving faster than it otherwise would because of reductions in smoking, but it’s not improving fast enough to offset the effect of obesity.”

The researchers estimated that the mortality decline in the U.S. would have been about a half-percentage point faster than it actually was if obesity hadn’t risen. The study found that rising BMI reduced gains in life expectancy at age 40 by 0.9 years during this period.

“These results underscore the importance of the obesity epidemic for American health and mortality,” Preston says. “When it’s having this large an impact on the national level of vital statistics, it puts the spotlight on the importance of stopping and reversing the rise in obesity.”