The major reason for the obesity epidemic that has gripped the United States in the past three decades is increased food intake, not reduced physical activity, according to a study released Friday at the European Congress on Obesity in Amsterdam.

The study is the first to quantify the relative contributions of food and exercise habits to the growing number of Americans with bulging waistlines.

“In the U.S., over the last 30 years, it seems that the food side of the equation has changed much more than the physical activity side,” Professor Boyd A. Swinburn, director of the World Health Organization Collaborating Center for Obesity Prevention at Deakin University, Melbourne, Australia, noted in a telephone interview with Reuters Health.

Weight gain in the American population seems to be virtually all due to the consumption of more calories, with declines in physical activity playing only a minor role, Swinburn explained.

“We absolutely need to continue to promote increased physical activity and a healthy diet because they are both obviously beneficial factors in terms of obesity,” he emphasized. “But when it comes to placing priorities, I think it needs to be on reducing energy intake. It’s particularly important for policymakers to focus on the energy intake side of the equation.”

In the study, Swinburn and his colleagues calculated how much adults need to eat in order to maintain a stable weight and how much children need to eat in order to maintain a normal growth curve.

They then figured out how much Americans were actually eating, using national food supply data from the 1970s and the early 2000s. This information allowed them to predict how much weight Americans would be expected to gain over the 30-year study period if food intake were the only influence.

Next, the investigators determined the actual weight gained over the study period using data from a nationally representative survey that recorded the weight of Americans in the 1970s and early 2000s.

In children, according to Swinburn and colleagues, the predicted and actual weight increase matched exactly, which indicates that the increases in energy intake alone over the 30 years studied could explain the added pounds, they say.

In adults, the data predicted that they would be 10.8 kg (23.8 pounds) heavier, but in fact they were only 8.6 kg (18.9 pounds) heavier. This finding, Swinburn noted, “suggests that excess food intake still explains the weight gain, but that there may have been increases in physical activity over the 30 years that have blunted what would otherwise have been a higher weight gain.”

“To return to the average weights of the 1970s, we would need to reverse the increased food intake of about 350 calories a day for children (about one can of fizzy drink and a small portion of French fries) and 500 calories a day for adults (about one large hamburger),” Swinburn noted in a statement from the meeting.

“Alternatively, we could achieve similar results by increasing physical activity by about 150 minutes a day of extra walking for children and 110 minutes for adults, but realistically, although a combination of both is needed, the focus would have to be on reducing calorie intake,” he added.

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By Megan Rauscher, Reuters