Many teens do not get enough exercise and spend too much time in front of a television or computer screen. Yet, the few who are physically active do not often stay that way as they enter adulthood, according to a team of North Carolina researchers.

Less than one million of the more than 20 million school-aged youth represented by the study sample engaged in at least five sessions of moderate to vigorous physical activity per week and continued to do so as young adults.

More than 12 million, on average, did not get that much exercise as adolescents or young adults.

Consequently, efforts to prevent inactivity and too much television watching or video game playing “are critically needed before adolescence,” particularly among Hispanic and black females, writes study author Dr. Penny Gordon-Larsen, of the University of North Carolina at Chapel Hill and her colleagues.

Afterwards, the transition to young adulthood, “is an important time to promote physical activity, reduce TV and video viewing and computer and video game use, and encourage those who are already active to maintain adequate amounts of physical activity,” they add.

Their study findings are based on survey responses from more than 13,000 junior and senior high students enrolled in the National Longitudinal Study of Adolescent Health, a study group representing more than 20 million school-aged adolescents. The participants were first surveyed in 1994-1995 and again in 2001.

Overall, most of the sedentary behaviors and few of the healthy exercise behaviors reported during adolescence were carried over into adulthood, Gordon-Larsen and her team report in the American Journal of Preventive Medicine.

For example, 61 percent of the seventh through twelfth graders who did not to engage in at least five exercise sessions per week remained similarly inactive during their young adult years, while less than five percent of physically active teens continued their level of physical activity as young adults.

Black women were three times more likely than white women to remain active from adolescence to early adulthood. Still, 31 percent of physically active black, white, Hispanic and Asian adolescents seemed to become couch potatoes as young adults, neglecting to maintain the same level of activity that they reported just a few years earlier.

“I didn’t expect that we would see quite the drop (in physical activity levels) that we did,” Gordon-Larsen told Reuters Health. “I was very surprised that these kids who were active didn’t remain active,” she said.

The American Academy of Pediatrics’ Committee on Public Education’s’ recommends no more than two hours of so-called “screen time” per day, including television or video watching, and computer or video game use. More than a third (37 percent) of adolescents who reported no more than 14 hours of screen time per week reported the same habits as young adults, the report indicates.

Still, 17 percent of youths increased their screen time as adults, and nearly a quarter (23 percent) of teens who spent a lot of time in front of the box carried those high screen times into adulthood.

Despite the low rates of physical activity found among the study group, the authors speculate that the prevalence of inactivity may actually be worse.

They write that their data represent a “conservative estimate of the magnitude of failure” to meet the physical activity guidelines set by the US Centers for Disease Control and Prevention and the American College of Sports Medicine among adolescents transitioning to young adulthood.

Both organizations recommend that people get at least 30 minutes of moderate physical activity most days of the week, if not every day.

SOURCE: American Journal of Preventive Medicine, November 2004.