With countless advances in medical research, you would expect human life expectancy to only get better. But last year, life expectancy in the US dropped for the second year in a row – and, according to a new study, the obesity epidemic might have played a part.
The data suggests rising obesity rates have reduced gains in life expectancy at age 40 by 0.9 years.
"These results underscore the importance of the obesity epidemic for American health and mortality," said co-author Samuel Preston, a sociology professor in the School of Arts and Sciences.
"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."
The study compared data from the National Health and Nutrition Examination Survey to mortality rates, between 1988 to 1994 and 1999 to 2010.
To measure the rates of obesity across this period of time, the researchers calculated the lifetime maximum BMI of each of the 25,260 adults – aged 40 to 79 - in the final sample.
They chose to focus on the lifetime maximum BMI because it is better at predicting mortality than just a simple body mass index, which is more susceptible to incorporating weight loss associated with illness. The new measure is also superior because it takes into account weight history, which may have long-lasting effects on health.
The data reveals if BMI is not included in death rates (1.81 percent per year), life expectancy at age 40 would have risen from 37.6 years in 1988 to 41.4 years in 2011.
On the other hand, if BMI is incorporated into the death rate (2.35 percent per year), life expectancy at age 40 would have risen to 42.3 years in 2011.
In other words, the decline in mortality would have been about 0.5 percent faster if obesity hadn't risen.
While this doesn't seem like much, the study suggests rising obesity is offsetting other medical advances.
"We estimated that the impact of rising obesity was about twice as important for mortality trends as the impact of declining smoking," said Preston.
"Smoking is such an important variable in mortality analysis, and US 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 study was published in Proceedings of the National Academy of Sciences.