Exercise More Critical Than Calcium For Adolescent Bones

"Although calcium intake is often cited as the most important factor for healthy bones, our study suggests that exercise is really the predominant lifestyle determinant of bone strength in young women," said Tom Lloyd, Ph.D., professor of health evaluation sciences, Penn State College of Medicine, Penn State Milton S. Hershey Medical Center. "There was only a small positive relationship between calcium intake and bone variables, but a significant association between sports exercise score and young adult bone mass and strength."

The study, titled "Lifestyle Factors and the Development of Bone Mass and Bone Strength in Young Women," was published in the June issue of Journal of Pediatrics.

The study's goal was to obtain a comprehensive view of how calcium intake, oral contraceptive use and exercise were related to the development of peak bone mass and to the development of young adult hip bone bending strength. Bone bending and torsion strength offers a better representation of bone strength than bone mineral density.

Studies have shown that most of a female's bone mass is built between the ages of 13 and 15 and then slowly lost in the last four decades of her life. Therefore, attaining optimal bone mass and bone strength in adolescence may offer the best protection possible against osteoporosis – bone loss in post-menopausal women and the elderly.

"That is why understanding how modifiable lifestyle factors affect bone accrual and development of bone strength is of critical clinical importance," Lloyd said. "If we know how to optimize bone building and strength in youth, we may be able to develop a preventive strategy to reduce the more than 1.3 million fractures a year, which cost $14 billion a year."

The study of 80 women is part of the Penn State Young Women's Health Study, an ongoing observational study begun in 1990 with the enrollment of 112 healthy female adolescents who are representative of Caucasian females attending public schools in Pennsylvania. The Penn State researchers followed the natural progression of cardiovascular, reproductive and bone health in these women.

For 10 years, researchers tracked a number of factors. Body composition and hip bone measurements were taken yearly with a dual X-ray absorptiometry (DXA) bone densitometer. Although bone mineral density has been widely used as the primary means of determining bone fracture risk, it is not a direct measurement of bone strength, which depends on both the material and structural properties of bone. So, the research team used recently-developed software to calculate specific measures of bone strength and geometric properties from the DXA data.

The team also collected calcium intake information by recording three-day diet records every six months for the first four years of the study and then yearly thereafter. Total average calcium intake in the group was between 618 mg/day and 1498 mg/day.

Oral contraceptive use was recorded, and physical activity was determined using a sports exercise questionnaire that listed 28 activities including school-based activities like soccer and marching band, outside of school activities like dance and aerobic classes, and individual activities like walking and running. A cumulative exercise score was calculated from the 10 years of data.

Study results showed that no significant relationship exists between average daily calcium intake and total bone gain from ages 12 to 22 or in young adult hip bone mineral density. A comparison of the oral contraceptive users and nonusers showed no difference in total body and hip bone mineral density suggesting oral contraceptives have no effected on bone density.

"Our statistical analysis of sport-exercise in adolescence showed that exercise is responsible for between 16 percent and 22 percent of the variation in hip bone mineral density and bending strength," Lloyd said. "Overall, data from the Penn State Young Women's Health Study indicates that daily calcium intake greater than 500 mg/day during adolescence does not result in clinically appreciable increased body bone accrual or increased adult bone mass."

Lloyd said additional, similar longitudinal studies of other ethnic and racial groups are needed to more fully evaluate the importance of the results for American women.

The study team included: Moira Petit, Ph.D., and Hung-Mo Lin, Sc.D., Department of Health Evaluation Science, Penn State College of Medicine, and Thomas J. Beck, Sc.D., Department of Radiology, Johns Hopkins University. The study was funded by the U.S. Department of Health and Human Services and the General Clinical Research Center at Penn State Milton S. Hershey Medical Center.