Americans have heard time and again that eating right and getting exercise will help prevent or reverse obesity. While some argue that obesity statistics have begun to plateau in America, the condition can still be considered an epidemic. According to the Centers for Disease Control and Prevention, over one third of adults in the U.S. can be considered obese. Also, Americans spent around $147 billion on obesity-related medical care in 2008. The condition, which is considered to be completely preventable, increases the risk of diabetes, stroke, heart disease and even some types of cancer.

But what is more important – eating a healthy diet or engaging in physical activity? According to a new study published in the American Journal of Medicine, lack of exercise is the main reason we’re getting fat.

No Change in Overall Caloric Intake

Perhaps the most surprising aspect of the study, conducted by researchers at Stanford University’s School of Medicine, is the finding that Americans’ overall caloric intake actually didn’t change much from 1988 to 2010. This conclusion challenges the common assumption that obesity in the U.S. is mostly due to people simply eating more food.

During the same time period, however, researchers found that the number of people living a sedentary lifestyle increased dramatically. The percentage of men in the U.S. that reported to participate in zero physical activity during their free time went from around 11 percent to 43 percent from 1988 to 2010. Even worse, women saw an increase from 19 percent to 52 percent.

Trend Has Hit Women Hardest

Researchers also found that body mass index increased by around 0.37 percent each year for both women and men. However, the age group that saw the most dramatic rises were women aged 18 to 39. When abdominal obesity was addressed, it was found that men’s average waistlines increased by 0.27 percent each year while women averaged an increase of 0.37 percent annually. A waist circumference of 40.16 inches or larger is considered obese for men, while the cutoff point for women is 34.65 inches.

Also, despite caloric intake staying the same in general, it was found that women who reported to engage in no physical activity did see an increase in calories consumed over the years studied.

Sedentary Lifestyle: The Bottom Line

The study seems to suggest that getting regular physical activity – not necessarily eating less – is the number one factor in preventing or counteracting obesity. However, researchers maintain that the relationship between a sedentary lifestyle and obesity is correlative, not causative. In other words, researchers did not find that a lack of physical activity caused the obesity trend, although common sense would deem it an important factor.

Additionally, as has already been pointed out by some skeptics, the study is perhaps presuming that the current average for caloric intake is acceptable. Instead, it could be argued that calories consumed must be cut in order to correspond with our increasingly sedentary lifestyles. So, while the study does reinforce the need for physical activity among the U.S. population, it certainly doesn’t mean that we can start eating whatever we want so long as we get some exercise.

The full study is available here.