The rise in obesity in the United States since the 1970s is virtually all due to increased food intake, a public health expert at Deakin University in Melbourne has revealed. Professor Boyd Swinburn worked with researchers from Pennington Biomedical Research Center in the US to determine how much of the obesity epidemic was caused by excess calorie intake and how much by reductions in physical activity.

"There have been a lot of assumptions that both reduced physical activity and increased energy intake have been major drivers of the obesity epidemic," Swinburn says. "Until now, nobody has proposed how to quantify their relative contributions to the rise in obesity since the 1970s. This study demonstrates that the weight gain in the American population seems to be virtually all explained by eating more calories - it appears that reductions in physical activity played a minimal role."

While the study used US data, Swinburn believes a similar situation is likely to apply in Australia. But for developing countries undergoing rapid economic transition and urbanisation, it is likely reduced physical activity is playing a larger role in their obesity epidemic.

The research is the first to examine the question of the proportional contributions to obesity by combining metabolic relationships, the laws of thermodynamics, epidemiological data and agricultural data. By analysing a range of data, the researchers predicted how much weight they would expect Americans to have gained over the 30-year period studied if food intake was the only influence and compared that to actual weight gain over that time.

They found that in children, the predicted and actual weight increase matched exactly, indicating the increases in energy intake alone over the 30 years studied could explain the weight increase.

"For children, the predicted weight gain of 4.0 kg matched the actual weight gain and for adults, the predicted weight gain of 10.8 kg was a little more than the actual gain of 8.6 kg," Swinburn says.

"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 soft drink and a small portion of French fries) and almost 500 calories a day for adults (about one large hamburger).

"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."

Swinburn emphasises that physical activity should not be ignored as a contributor to reducing obesity and should continue to be promoted because of its many other benefits. But that expectations regarding what can be achieved with exercise need to be lowered and public health policy shifted more toward reducing excess energy intake.

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