Each year, more than 170 million people visit national forests for recreation. Experts estimate that the physical activity associated with these visits burns 290 billion food calories. Jeff Kline, from the US Forest Service's Pacific Northwest Research Station (Oregon, USA), and colleagues examined the extent that national forests might provide public health benefits by estimating the net energy expended for a range of outdoor activities engaged in by visitors to national forest lands.   The team found that hiking, walking, downhill skiing, fishing, relaxing, camping, relaxing, and driving for pleasure are among the primary activities – accounting for 68% of all visits to the national forests.  Annual energy expenditures in national forest recreation represent 6.8 million adults and almost 317,000 children meeting the Centers of Disease Control and Prevention guidelines regarding regular aerobic physical activity for a year.