Though health experts often cite reduced physical activity levels among people in developed nations as one of the main contributors to the rising problem of obesity, it has been unclear just how sedentary people have become.

Dr. Steven Heymsfield, deputy director of the Obesity Research Center at St. Luke's-Roosevelt Hospital in New York compiled data from many previous studies that assessed the energy expenditure of people as well as a variety of wild mammals, including orangutans, wolves, wildcats, bats, jack rabbits, squirrels and mice.

He wanted to get an idea of what physical activity levels were like before we were 'domesticated.'" He figured that animals living in the wild have physical activity levels similar to primitive humans.

The data suggest that modern-day humans in developed nations are considerably less active than generations past who had to scrape and scrounge for food and fight off predators, just as wild animals still do.

In most wild mammals, for every calorie they burn at rest during the day, they burn another calorie or two in physical activity. In people who are typical city dwellers, for every calorie they burn at rest, they burn only half a calorie in physical activity. That's half or less than half of the calories that mammals burn in the wild.

However, the study also found that some groups of people -- athletes and army recruits -- burn almost as much energy a day as animals, a level that is probably consistent with early humans.

American Society for Clinical Nutrition San Diego, CA February 24, 2002