How much this rise in obesity has been driven by excess calorie intake and how much by decreased physical activity has been a topic of debate for years but has been difficult to pin down, Swinburn said. He and his colleagues estimated those proportions by devising a series of equations that took into account energy intake, energy expenditure, and body size in 963 children and 1,399 adults. They also analyzed the US Dept. of Agriculture (USDA) food-supply data to determine how much food had actually been delivered to the US population between 1970 and 2002.

Using that information, they then predicted the increases in weight in children and adults between 1971 and 1976 and between 1999 and 2002. If the predicted weight equaled the observed weight, increased food consumption was presumed to be the cause of the weight increase. If the predicted weight was higher or lower than the weight that was actually observed, it was assumed that changes in physical-activity levels were the cause.

As shown in National Health and Nutrition Examination Survey (NHANES) data, the mean weight gain in children was as the researchers had predicted--4.0 kg--and therefore attributed to increased calorie consumption alone. In adults, the predicted increase in mean weight--10.8 kg--was higher than the observed mean weight gain--8.6 kg--and this implied that increased physical activity might have moderated the effect to some extent.

"Americans have been eating more; the USDA data clearly show this. But US epidemiological data shows that physical-activity levels haven't really changed all that much. So I think we have to be much more focused on the energy-intake side of the energy-balance equation in understanding what the drivers of obesity are and also in working out what the solutions are," Swinburn commented. "We still need to continue to promote increases in physical activity, because exercise has a lot of positive physiologic benefits, but our level of expectation about the impact of physical activity on weight gain has to be a bit more tempered."

Promoting physical activity has been the favored approach to solving the problem of obesity by politicians and the food industry, said Swinburn. "It's relatively uncontroversial, there are no commercial competitors, it's a positive thing to do, so politicians, egged on by the food industry, heavily promote the physical-activity side of the equation."

Swinburn said that the food industry has been "extraordinarily successful" in promoting excessive intake of calories. "They've worked their marketing out to the nth degree. They've got the products that we like to eat, they've got the price right--in fact the price of junk food has been coming down for years and is getting cheaper and cheaper. The food industry has also mastered promotion, especially to the most vulnerable and impressionable members of society--children. "Over the past 30 years they have become very sophisticated in marketing and advertising that is particularly iniquitous in relation to kids. They are adept in the way they turn kids into liking, preferring, demanding, and pestering for the foods that they advertise."

"The main cause of the obesity epidemic in this country is the wide availability of high-caloric foods and the fact that we are eating way too many calories in the course of a day. Exercise has much less impact."

Sorrentino said that about 90% of weight loss is achieved by cutting calories; only about 10% of weight loss is achieved by significantly increasing physical activity.

There needs to be a population-based approach to teach people how to count and cut calories, choose whole foods instead of packaged foods, and increase their awareness of just how fattening going out for dinner can be.

"Studies have shown that when you go out to eat, most individuals will eat on average 500 more calories per meal than they would eat at the same meal at home. There are now huge varieties of fast food, in packages and in fast-food restaurants, and they are usually calorically dense, full of carbohydrates, and sweetened, so they taste good and you want more. Years ago, you had to prepare food; now they're all prepared for you."

Presented at the 2009 European Congress on Obesity.