Although the World Health Organization (WHO) sees obesity as a global epidemic, the US still heads the list, with obesity affecting over a third of American adults.
The study is potentially important for the food and beverage industry, which has taken a dual approach to tackling obesity. It has focused its efforts not only on product reformulation to reduce trans fats, saturated fats and sugar, but also on encouraging increased physical activity.
But the study’s leader, Professor Boyd Swinburn, chair of population health and director of the WHO Collaborating Centre for Obesity Prevention at Deakin University in Australia, said: “There have been a lot of assumptions that both reduced physical activity and increased energy intake have been major drivers of the obesity epidemic…This study demonstrates that the weight gain in the American population seems to be virtually all explained by eating more calories. It appears that changes in physical activity played a minimal role.”
The researchers took a sample of 1,399 adults and 963 children and tested how many calories they burned in free-living conditions. They were then able to establish what their calorie intake would need to be in order to maintain a stable weight or, for the children, to maintain a normal growth curve. They then used national survey data of US weights in the 1970s and early 2000s and compared actual weight gain to expected weight gain if food were the only factor.
More food, more exercise
The results suggested that children are just as active as they were in the 70s, and increased energy intake alone accounts for higher average weights today, the researchers said.
For adults, on the other hand, Swinburn said that exercise had actually increased over the past 30 years, leading to an average weight gain of 18.9lbs (8.6kg). If adults had not increased levels of physical activity, extra energy intake would have led to a gain of 23.8lbs (10.8kg), he said.
“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,” Swinburn said.
Going back to the 70s
He added that in order to return to the average weights of the 70s, children would have to reduce their daily energy intake by about 350 calories, equivalent to a can of soda and a small portion of French fries, and adults would have to consume about 500 calories less, or about one large hamburger.
Despite the study’s conclusions, Swinburn said that the role of physical activity should not be ignored because it carries additional health benefits, but added that public health policy needed to be shifted so it does not over-emphasize exercise’s potential effects for weight loss.
“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 said.