AMSTERDAM, THE NETHERLANDS - No, you don't have an unusual metabolism. No, you're not big-boned. You can't even blame your sedentary lifestyle. Sorry - and we really hate to tell you this - but you're just eating too many donuts.
A study being presented today at the European Congress on Obesity claims to be the first to examine the relative contribution of food and exercise habits to America's obesity epidemic.
There has never been a clear consensus on whether a wobbly tum is caused by too much food or too little exercise, making it difficult for the do-gooders to know where best to focus public health initiatives.
Aiming to rectify this, researchers combined metabolic relationships, the laws of thermodynamics, epidemiological data and agricultural data, and concluded that the rise in obesity in the United States since the 1970s is almost entirely due to increased energy intake.
"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," said the study's leader, Professor Boyd Swinburn, chair of population health and director of the World Health Organization Collaborating Centre for Obesity Prevention at Deakin University in Australia.
The scientists started by testing 1,399 adults and 963 children to determine how many calories their bodies burned in total under free-living conditions. The test is the most accurate measure of total calorie burning in real-life situations.
Once they had determined each person's calorie-burning rate, Swinburn and his colleagues were able to calculate how much adults needed to eat in order to maintain a stable weight and how much children needed to eat in order to maintain a normal growth curve.
They then worked out how much Americans were actually eating, using national food supply data (the amount of food produced and imported, minus the amount exported, thrown away and used for animals or other non-human uses) from the 1970s and the early 2000s.
The researchers used their findings to predict how much weight they would expect Americans to have gained over the 30-year period if food intake were the only influence. They used data from a nationally representative survey (NHANES) that recorded the weight of Americans in the 1970s and early 2000s to determine the actual weight gain over that period.
"If the actual weight increase was the same as what we predicted, that meant that food intake was virtually entirely responsible. If it wasn't, that meant changes in physical activity also played a role," Swinburn said. "If the actual weight gain was higher than predicted, that would suggest that a decrease in physical activity played a role."
The researchers found that in children, the predicted and actual weight increase matched exactly, indicating that the increases in energy intake alone over the 30 years studied could explain the weight increase.
"For adults, we predicted that they would be 10.8kg heavier, but in fact they were 8.6kg heavier. That suggests that 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.
Getting back to our parents' size would mean cutting back by about 500 calories per day for an adult, and about 350 calories a day for children - so just snatching a can of fizzy drink and a small portion of fries from your kids each day should do it.
Swinburn emphasized that physical activity should still be promoted because of its many other benefits. But he said people should be a bit more realistic about what ten minutes' jogging is likely to achieve, and should concentrate on eating less if they really want to lose weight.