The Australian National Children's Nutrition and Physical Activity Survey tracked 4,400 youngsters ages 2 to 16. The survey was sponsored by several government departments and a food industry group, the Sydney Morning Herald reported.

Researchers found that teens ages 14 to 16 eat diets much higher in fat and sugar, and containing fewer fresh fruits and vegetables, than younger children. They also spend more time on their computers or watching television and less time in physical exercise.

Lynne Cobiacs of Flinders University said the findings on teenage girls are especially worrying since bad habits can increase their risk of osteoporosis in later life. She said 80 percent do not have adequate intake of calcium.

"Girls this age also reported doing the least amount of physical activity," she said. "These two factors combined put them at risk of developing weak bones as they grow older."