'Agreement is growing that the source of the obesity epidemic lies in an environment that produces an energy gap, where energy intake exceeds energy expenditure even by as little as 100 excess calories per day,' wrote Clarke and co-authors Patrick O'Malley, Lloyd Johnston, John Schulenberg and Paula Lantz, all researchers at ISR.

The finding that young women consistently exercised less than young men, suggests that differences in energy expenditure could play a role in gender disparities in obesity and overweight.

The study will be published in the October issue of the American Journal of Public Health.