While smoking rates have been constantly declining in the U.S. in the last few decades, obesity rates have been on the rise. A new study suggests that if these trends continue over the next 15 years, the negative effects of obesity on life expectancy will outweigh the positive effects gained from declining smoking rates.

The study, published in the current issue of the New England Journal of Medicine, forecasted life expectancy (in terms of actual years and quality-adjusted years) for a typical 18-year old based on past rates in smoking and Body Mass Index (BMI).

Life expectancy is the number of years that an average person can expect to live within a certain country. Quality-adjusted years consider the quality of life that a person has, given the burden of disease. For example, if a person is healthy, their quality-adjusted score for one year is 1. However, if that same person suffers from a major disease, their score is somewhere lower than 1.

BMI is a measure of body fat based on height and weight. Adults with BMI between 25 and 30 are considered overweight, while those with BMI over 30 are considered obese.

The main conclusion from the study was that the negative effects of increasing BMI overwhelmed the positive effects of declines in smoking in different scenarios. For a typical 18-year old, increases in life expectancy were reduced by 0.71 years and 0.91 quality-adjusted years between 2005 and 2020.

One of the most interesting findings is that if all American adults quit smoking and became of normal weight by 2020, life expectancy of an 18-year old would increase by 3.76 years and 5.16 quality-adjusted years.

As there are more large people than smokers in our population, obesity will continue to win the race. That is, unless we start taking care of our bodies, exercising more and making better food choices.