Exercise builds strong bones, but teens miss out

Physical activity in childhood leads to stronger bones later on, but inactive teenagers, including most girls, may lose out on the gains, a study shows.

The study also found that very active children might have strong bones as young adults, even if they don’t get much exercise as teenagers.

Researchers from the University of Iowa studied 530 participants in the Iowa Bone Development Study, an ongoing examination of how genetics, diet, and physical activity affect bone strength from childhood to young adulthood. They measured physical activity at ages 5, 8, 11, 13, 15, and 17 years by having subjects wear an accelerometer for 3 to 5 consecutive days, including a weekend day. At age 17 years, participants underwent radiography and tomography scans to measure bone density, strength, and brittleness and assess the geometric properties of the bones, an important determinant of strength.

Girls and boys with the most moderate-and-vigorous physical activity (MVPA) throughout childhood had greater bone mass and better geometry at 17 years than less active young people. Very few participants, and fewer than 6% of girls, achieved high childhood levels of MVPA, however. By late adolescence almost all the girls were inactive. Boys also got less exercise as time went on but were more active than girls.

In early childhood, girls were active for an average of 46 to 48 minutes a day, but the amount of exercise dropped to 24 minutes a day by the time they were aged 17 years. Boys had average MVPA levels of 60 to 65 minutes a day at young ages, which decreased to 36 minutes a day by age 17 years.

The benefits of high preadolescent MVPA levels carry over into adulthood, even among youth who are less active in the teenaged years, the study suggests.

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