Influence of heredity and environment on bone density in adolescent boys: a parent-offspring study

The purpose of the present parent-offspring study was to investigate the influence of heredity and environment on bone density in young men. Another aim was to discover whether the same genetic factors influence bone mass, lean mass and muscle strength. Fifty families including a father, mother and one son were investigated. The mothers (aged 44.5 +/- 4.4 years) and fathers (aged 47.1 +/- 4.4 years) generally had a sedentary lifestyle with little physical activity. As a contrast, all but three of the sons (aged 17.0 +/- 0.4 years) were active in ice hockey training. Bone mineral density (BMD, g/cm2) of the total body, head, lumbar spine and femoral neck was measured using dual-energy X-ray absorptiometry. Muscle strength of the hamstrings and quadriceps muscles was also measured in the boys. BMD values of different sites in the fathers, mothers and sons were adjusted for weight, height, age, and any significant influence of environment. Heritability estimates were obtained as regression coefficients with the boys' adjusted BMD as dependent variable and the adjusted midparent bone density (father BMD + mother BMD/2) as independent variable. Accordingly, heritability explained 34-54% of the variation in the sons' BMD. Midparent BMD of several sites also predicted the boys' lean mass and quadriceps strength, and midparent-offspring differences in lean mass predicted midparent-offspring differences in BMD of the total body, head and spine (beta = 0.30-0.51, p < 0.05). The sons were found to have almost 30% higher femoral neck BMD than their fathers, and physical activity (hours/week) predicted BMD at several sites among the sons beta = 0.26-0.34, p < 0.05). In conclusion, heritability is a main determinant of the variance in BMD in young men. Based on the results we suggest that the same genetic factors may influence bone mass, lean mass and muscle strength by affecting body size. The present study also emphasizes the importance of physical activity for the development and maintenance of BMD in men.