Social Position Affects Bone Mass in Childhood Through Opposing Actions on Height and Weight

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Abstract
We studied relationships between social position of the mother in pregnancy and bone mass of the child at age 9.9 years. The tendency for social position to increase bone area and bone mass through a positive influence on height was opposed by a negative effect of social position on weight and fat mass. Evidence that social factors influence skeletal growth raises the possibility that bone mass acquisition in childhood is socially determined. To clarify the role of social factors in bone mass acquisition in childhood, we studied relationships between these variables in the Avon Longitudinal Study of Parents and Children (ALSPAC). Measures of the mother's social position during pregnancy were linked to DXA results obtained at age 9.9 years in 6,702 children. Linear regression analyses were carried out after adjusting for age and gender. Because social position may affect height and weight of the child, analyses were repeated after adjusting for these additional variables. Measures of social position in pregnancy were unrelated to total body BMC in analyses adjusted for age and gender alone. However, after adjusting for height, which was positively related to social position, a strong negative association was observed between BMC and housing tenure (p < 0.001), maternal education (p < 0.001), paternal education (p < 0.001), and social class (p < 0.001). Similar results were obtained for bone area. After adjusting for weight as well as height, an association between social position and BMC and bone area was no longer observed. Hence, social position seems to exert opposing height- and weight-dependent effects on BMC and bone area in childhood. In further analyses, we found that adjusting for fat mass of the child led to similar results to those obtained with weight. Social position in childhood seems to be positively related to bone mass acquisition in childhood as a consequence of enhanced gain in height (i.e., longitudinal growth). However, this influence is counteracted by the tendency for increased fat deposition in those from a lower social position to increase bone area, presumably reflecting the stimulation of appositional bone growth.