By Joshua N. Farr, Ph.D., and Scott B. Going, Ph.D.
Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.
Joshua N. Farr, Ph.D., is Assistant Professor of Medicine in the Division of Endocrinology,Diabetes, Metabolism and Nutrition, Department of Endocrinology, College of Medicine, MayoClinic, Rochester, Minnesota. His research focus includes clinical-investigative studies in osteoporosis as well as in bone biology using mouse and cell models.
Scott B. Going, Ph.D., an ACSM member, is Professor and Head of the Department of Nutritional Sciences in the College of Agriculture and Life Sciences at The University of Arizona, Tucson. His research includes studies on the effects of exercise and diet on soft tissue composition and bone in children and adults.
The following commentary reflects Dr. Farr’s and Dr. Going’s views relating to the research article that they and their colleagues authored and which appeared in the December 2013 issue of Medicine and Science in Sports and Exercise® (MSSE).
The incidence of osteoporosis is projected to triple by year 2040, reflecting increased longevity and sedentary lifestyles. Despite considerable effort, our understanding of the fundamental mechanisms that drive age-related bone loss remains incomplete. However, there is now nearly universal consensus that early-life experiences are important for disease risk. Indeed, osteoporosis is widely considered a pediatric disorder, waiting to manifest itself later in life. Thus, the peri-pubertal years are recognized as an opportune period to modify bone density, size, and shape – traits that tend to track throughout life. In girls, over 25% of adult bone mineral is laid down during just two years surrounding peak linear growth – this is as much bone as a woman will lose from age 50 to 80 years. But can building a stronger skeletal during growth counteract the inevitable loss of bone with aging?
While the answer to this question is not known, the time has come to understand how modifiable factors can foster optimal skeletal health during development because, if maintained, changes would be clinically significant. Examples of such modifiable factors include the mechanical loading environment, i.e., exercise. Mechanical loading has been known for over a century to induce robust skeletal modeling adaptions, as well as skeletal muscle forces and muscle-derived osteogenic factors that mediate crosstalk with bone cells.
Given that the peri-pubertal years represent a unique period when bone is most responsive to exercise, it is easy to hypothesize that physical activity and skeletal muscle quality are important determinants of bone strength during growth.
Perhaps surprisingly, however, relatively little attention has been given to the effects of physical activity and muscle quality on bone mineral density (BMD) and bone structural strength in children and adolescents, particularly in prospective studies. Using dual energy X-ray absorptiometry (DXA) to investigate BMD adaptations has been a problem because DXA only measures bone in a two-dimensional image and can’t capture changes in the thickness of bone that naturally occurs with growth. In addition, DXA cannot assess bone structure, a significant limitation, as geometric adaptations such as increases in bone size may confer gains in bone strength that track into later life. While there are plausible reasons to believe this is true, longitudinal studies in children in different stages of maturation using appropriate imaging methods that can accurately assess bone morphology have been lacking.
Recently, we tested the hypothesis that physical activity and muscle quality are significant determinants of changes in volumetric BMD and bone structural strength (assessed by peripheral quantitative computed tomography, or pQCT), in a 2-year longitudinal study of 248 healthy girls, aged 9 to 12 years. Our findings suggest that physical activity is associated with more optimal gains in bone density and strength, and that poor muscle quality may put girls at risk for suboptimal bone development. Clearly, our study points to the need for more work to define the optimal type and amount (“dose”) of exercise required to induce optimal skeletal adaptations during growth and to determine whether changes are maintained and ultimately reduce fracture risk.