A sedentary lifestyle may contribute more to obesity rates in the United States than the number of calories taken in. A new study out of the Stanford University School of Medicine has found that over the last two decades, the amount of exercise has declined drastically while little change has occurred in the number of calories ingested.

The study, published in the American Journal of Medicine on Jul. 7, found that the percent of people who reported participating in no physical activity during their down time had risen drastically between 1988 and 2010. For women, 19 percent reported no exercise in 1988 but that number rose to 52 percent in 2010. Men’s numbers rose from 11 to 43 percent in the same time period.

However, researchers noted that none of the data collected indicated a significant rise in calorie intake during the 20 years in question, though researchers are quick to point out that the types of calories ingested may not be “optimal.”

The rate of obesity has risen during this time period as well. In women the rate has increased from 25 percent to 35 percent and for men the increase has been from 20 to 35 percent. According to the study, abdominal obesity rates, which were studied independently of other types of obesity, have been on the rise as well. Each year has seen a 0.27 percent increase in abdominal measurements for men and a 0.37 increase for women.

The most notable increase in obesity was recorded among women between the ages of 19 and 35 years. It is thought that part of the reason why this has occurred is because socioeconomic hardships have left single mothers trying to feed and clothe their children too exhausted to seek out physical activity in their rare leisure time.

Researchers conducting the study also note that the distinct correlation between the increasing lack of exercise and obesity rates do not prove causation. It is a noteworthy trend that could provide another insight into the complex picture of obesity rates in the United States.