Even though medical science has advanced, benefiting people across a large cross-section of the United States, and smoking is on a downward trend too, positive effects from these developments — as seen in a decrease of overall mortality rates — are somewhat blunted by a corresponding increase in obesity in the country, a study found.

Researchers from University of Pennsylvania (UPenn) and Boston University looked at data from the National Health and Nutrition Examination Survey (NHANES) for years between 1988 and 1994, and from 1999 to 2010. They also looked at mortality files linked to the NHANES data, which were followed up till December 2011. The total sample size for the study was 25,269 adults, aged between 40 and 79 years.

Since the study was looking at potential links between mortality and obesity, the researchers looked at the body mass index (BMI) of each of the individuals in the study. BMI, a measure of body fat, is calculated by dividing your weight (in kilograms) by the square of your height (in meters). A BMI of within 18.5 and 25 is considered normal, below 18.5 is underweight, over 25 and up to 30 is overweight, while upward of 30 qualifies as obese.

The Centers for Disease Control and Prevention further breaks down obese BMI into three classes — between 30 and 35, 35 and 40, and over 40. The third category is referred to as extreme or severe obesity.

For the purposes of their study, the researchers calculated each participant’s BMI using the maximum weight each of the individuals had in their lifetimes, to arrive at the lifetime maximum BMI. This calculation was to avoid any biases on account of weight loss from illnesses, and it also accounts for weight history which may have long-term effects on people’s health.

Analyzing the data with the lifetime maximum BMI showed a distinct effect of obesity over mortality trends. The researchers found that life expectancy for a 40-year-old would grow from 37.6 years in 1988 to 41.4 years in 2011, keeping in mind the growth in obesity during that period. If the increasing obesity was left out of the equation, however, a 40-year-old’s life expectancy would grow to 42.3 years in 2011, or 0.9 years more than the obesity-affected scenario.

"We established a time trend of mortality within this data set by dating every observation as it moved forward. 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," Samuel Preston from UPenn summarized in a statement Monday.

Obesity affects over one in every four Americans, and associated health costs run close to $200 billion a year. Some of the diseases related to obesity include cardiovascular diseases, strokes, type 2 diabetes and some types of cancer.

"These results underscore the importance of the obesity epidemic for American health and mortality. 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," Preston said.

The study is scheduled to be published in the journal Proceedings of the National Academy of Sciences.