A British study, involving 5,500 children and published in the latest issue of PLoS Medicine, used accurate methods to measure the fat mass of the children and the amount of physical activity they were taking. The researchers, based at the University of Bristol, concluded that low levels of activity, particularly moderate and vigorous activity, play an important role in the development of obesity.

While it would appear to be commonsense that anyone who is overweight or obese has been doing less physical activity than other people, body fat and energy expenditure are both hard to measure accurately. It has not therefore been clear which factor eating too much or doing too little is most responsible for the rising prevalence of obesity. The researchers employed advanced techniques an Actigraph accelerometer for activity and an X-ray emission absorptiometry scanner for fat mass. They considered that the top 10% of the children in terms of fat mass were obese.

The results show consistently that the lower the level of childrens activity the higher their fat mass. The effect was stronger in boys than in girls. The statistical association was found to be greater for moderate and vigorous activity than for total activity. While the researchers acknowledge their work to have some limitations, they urge that encouraging greater activity, especially moderate and vigorous activity, should be a key part of policies to address the obesity epidemic.

The research was part of the long-term Avon Longitudinal Study of Parents and Children, which is looking at many different aspects of health.