New research: Americans are obese because they eat too much

While American scientists disagree about the causes of obesity, from genes and the environment to chemicals, TV and video games, Australian scientists have proposed just one cause: overeating. And they have clear numbers to support their theory.

Professor Boyd Swinburn of Deakin University in Australia studied how much food an adult needs to maintain their weight and a child to grow properly. From US food consumption figures for the 1970s and the 2000s, his team worked out how much the average American eats, predicted the expected change in weight, and then compared it with the real change in weight from health agency data.

According to the researchers, the weight gain predicted from eating more matched the real weight gain in children, meaning no other factor was needed to explain why American children have gained weight. Adults gained less than predicted (18.9 pounds instead of 22.3), which may mean today's adults exercise more than those of the 1970s.

Many theories have linked weight gain with eating more, but no study had put numbers on the relationship. This one shows that overeating plays the main role, while physical activity had a limited effect.

"To get back to 1970s weights, children would have to cut about 350 calories a day, about a small portion of fries, and adults about 500 calories a day, about a large hamburger," Swinburn said. "Or children would have to walk an extra two and a half hours and adults an extra two hours every day. That may be beyond most people, so the only practical solution is to eat less." He added: "This study doesn't play down the importance of physical activity at all, but it suggests priorities, and the priority is to eat less."

More than two thirds of Americans are now overweight or obese, and different groups trade blame for the causes of obesity and its diseases, while the government has neither proved any of the accusations nor acted seriously on them.