Study: Inadequate physical activity worsens as teenagers become adults

While promoting physical activity and encouraging people to limit the time they spend watching television are important throughout life, those efforts are critical before adolescence, a new University of North Carolina at Chapel Hill investigation concludes.

That’s because the physical activity picture worsens rather than improves as teens make the transition into young adulthood, UNC researchers found in the largest national study of changes in exercise patterns over time.

Especially needed are efforts to get Hispanic and black girls to become more active, those scientists say. “In a sample originally representing more than 20 million school-aged youth, we found that only 36 percent achieved five or more sessions of moderate to vigorous physical activity weekly,” said Dr. Penny Gordon-Larsen, assistant professor of nutrition at the UNC schools of public health and medicine. “However, of those 36 percent, only a staggering 4.4 percent maintained this level of activity into adulthood.

“Also, across the teenage and adult years, close to half of the respondents spent more time watching TV or playing computer games than recommended.”

A report on the research appears in the latest issue of the American Journal of Preventive Medicine. Besides Gordon-Larsen, who led the study, authors are nutrition graduate student Melissa C. Nelson, and Dr. Barry M. Popkin, professor of nutrition.

“Poor diet and physical inactivity are responsible for roughly 400,000 deaths in the United States each year and may soon become the leading causes of death for Americans,” Gordon-Larsen said. “Previous studies at UNC and elsewhere have shown that all major racial and ethnic groups and both sexes get too little physical activity, and black and Hispanic girls face the greatest risk of being significantly overweight or obese and inactive.”

In the new research, she and colleagues wanted to concentrate on possible changes in physical activity between adolescence and young adulthood since few scientists have studied what happened to activity levels over that part of the life cycle.

The work involved analyzing extensive data gathered from 13,030 teens in 1994-95 and again in 2001 through Waves I and III of the National Longitudinal Study of Adolescent Health. Begun at UNC, that larger Add Health project entailed extensive, confidential interviews with thousands of adolescents across the United States to learn their attitudes and behaviors related to family, school and religion, alcohol and drug use, sexual activity and other health-related subjects.

“Very few teens were active when young, and even fewer remained active into adulthood,” Gordon-Larsen said. “We saw similar differences, although of less magnitude, for TV and video viewing and computer and video game use. Just under half of teens engaged in more than 14 hours of screen time per week. Of those who had the recommended 14 or fewer hours, 17 percent increased their screen time as adults.”

Other findings, based on estimates, were that:

• An average of more than 12 million adolescents did not achieve five or more sessions of moderate to vigorous physical activity a week.
• That number approached an average of 16 million in Hispanic and black females.
• About 6 million teens who achieved five or more sessions of significant physical activity per week failed to do so as adults.
• Conversely, fewer than 1 million teens exercised enough and continued to do so as adults.
• Blacks who were active as teens were more likely than whites or Hispanics to remain so into young adulthood.

“By the time individuals reach adolescence, most are already not engaging in enough physical activity and spend too much time sitting in front of video screens of one kind or another to meet national recommendations,” Gordon-Larsen said. “Thus, interventions must begin before adolescence, and this is particularly true for Hispanic and black girls.”

The National Institutes of Health supported the research. The National Institute of Child Health and Human Development supported the Add Health study with cooperative funding from 17 other agencies.