It's not a lack of exercise and an abundance of food making Americans fat. It's just all that food.

Experts agree that both eating less and exercising more are important for combating obesity, but a study presented Friday at the European Congress on Obesity in Amsterdam found that nearly all the extra pounds Americans packed on in the last 30 years come from what we stuff in our mouths, and not our sedentary lifestyle.

Scientists at Deakin University in Melbourne, Australia tested a large sample of adults and children. They took into account biological, epidemiological and food supply evidence to determine the causes of weight gain in the U.S.. They found that for kids the problem was food alone. For adults, it was predominantly food, but there may have been increases in physical activity that slowed weight gain to some degree.

Wouldn't it be great if we could return to the average weights of the '70s, which were nine pounds less for children and 17 pounds less for adults? It's possible. All it would take is a decrease in food intake by about 350 calories a day for kids and by 500 calories for adults. "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," says study leader Boyd Swinburn in a press release, "but realistically, although a combination of both is needed, the focus would have to be on reducing calorie intake."