Birth weight, birth length, and bone density in prepubertal children: evidence for an association that may be mediated by genetic factors.

There is an incomplete understanding of the contribution of early growth to bone accrual in childhood. The aims of this longitudinal study were to examine the association between growth variables at birth, 1 month, and 8 years and bone density in prepubertal children. Weight and length at both birth and 1 month of age were measured in 1988 as part of a prospective study for sudden infant death syndrome. A total of 330 children (47%) and 278 of their mothers were then contacted in 1996 for measurement of anthropometrics and bone density. Birth weight, birth length, and length gain (but not weight gain) in the first month all made significant contributions to areal bone density (BMD, g/cm(2)) at all sites at age 8 even after taking into account subsequent weight and height gain (model R(2) 14-39% depending on variable and site). Adjustment for potential environmental confounders did not alter these findings, however, adjustment for maternal BMD markedly reduced the early life associations (particularly for birth weight). Though early life factors were weakly associated with bone mineral apparent density (BMAD, g/cm(3)) in correlation analysis, subsequent height and weight gain were the only significant independent contributors to BMAD. In conclusion, early life anthropometrics make little contribution to BMAD (other than through their correlation with later growth) but make significant independent contributions to BMD suggesting that the growth trajectory of bone is determined very early in life. In addition, the contribution of body size at birth to bone growth in early life appears to be mediated by genetic factors although it is possible that it may be mediated by poorly measured or as yet unidentified determinants of body size at birth.