Less Exercise Behind Teen Girls' Weight Gain

The growing time teenage girls spent on the phone or in front of the TV rather than on the playing field or at the gym may add up to an extra 20 pounds by the end of their adolescence, according a new study.

Researchers found a steep decline in physical activity during the teenage years was largely responsible for weight gain commonly seen among adolescent girls.

They say the results suggest that a drop in physical activity rather than a sudden increase in calories may be fueling weight gain among teenaged girls and contribute to childhood obesity.

Obesity is caused by an increase in calorie intake (eating more), a reduction in energy expenditure (moving less), or both. But researchers say that pinpointing the cause for the current rise in childhood obesity has been difficult.

Read WebMD's "Better Late Than Never for Exercise"

Less Exercise Leads to Weight Gain

In the study, researchers followed a group of nearly 2,200 black and white girls in three different U.S. cities from ages 9-10 to 18-19. Throughout the study, researchers measured the girls' body mass index (BMI, a measure of weight in relation to height used to indicate obesity) and body fat, as well as monitored their physical activity level and calorie intake.

The results showed the girls' physical activity levels significantly declined during adolescence while the number of overweight and obese girls doubled -- without a significant increase in their calorie intake.

Only about a third of white girls remained physically active throughout their teenage years compared with 11 percent of black girls.

Changes in physical activity scores significantly affected changes in BMI. A drop in physical activity was associated with an increase in BMI.

They say the differences in BMI between physically active and inactive girls tripled between ages 9-10 and 18-19 years.

Inactive white girls experienced an average weight gain of 9 to 13 pounds, and inactive black girls gained an average of to 13 to 20 pounds.

In addition, the results showed that black girls were significantly heavier and fatter throughout adolescence than white girls. Their average calorie intake increased by an average of 120 calories per day by the end of the study while white girls' calorie intake was unchanged.

Read WebMD's "Dancing Your Way to Better Health

Staying Active May Fight Childhood Obesity

Researchers say their results suggest that programs to prevent the decline in physical activity experienced during adolescence may be effective tools in preventing childhood and adult obesity.

For example, they estimate that increasing the girls' physical activity by the equivalent 2.5 hours of brisk walking per week could prevent the weight gain experienced by the inactive girls.

In an editorial that accompanies the study in today's online edition of The Lancet, John J. Reilly of the University of Glasgow says that increased physical activity during adolescence would not only combat the epidemic of childhood obesity but it would also provide benefits in improving children's bone and heart health.

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCE: Kimm, S. The Lancet, July 14, 2005, online edition.