Maternal Obesity, 25-Hydroxy Vitamin D Concentration, and Bone Density in Breastfeeding Dyads

Objective To examine the association between maternal body mass index (BMI) and serum 25-hydroxy vitamin D [25(OH) D] concentration and bone density in mother-infant pairs. Study design The study was a secondary analysis of 234 exclusively breastfeeding dyads who were recruited in the first postpartum month for a randomized controlled trial of maternal vs infant vitamin D supplementation. Mean 25(OH) D concentrations and bone mineral density (BMD) were compared by BMI group. The adjusted association between maternal BMI and 25(OH) D and bone density was examined at 1, 4, and 7 months postpartum. Results Obese breastfeeding women had lower 25(OH) D concentrations and higher BMD than lean women at all 3 time points (P < .01). Higher maternal BMI was associated with lower maternal serum levels of 25(OH) D at 1, 4, and 7 months postpartum (adjusted beta = -0.45 ng/ml per kg/m(2), 95% CI -.076, -0.14, at 1 month) and higher BMD at the same time points (beta = 0.006 BMD z score; 95% CI 0.003, 0.01 at 1 month). Seventy-six percent of infants were vitamin D deficient at 1 month of age. Infants born to overweight and obese mothers had lower 25(OH) D concentrations than infants of lean mothers (P < .01). For infants in the maternal supplementation group, higher maternal BMI was associated with lower 25(OH) D concentrations at 4 months (beta = -0.68; 95% CI -1.17, -0.20) and lower bone density at 7 months (beta = -0.001; 95% CI -0.002, -0.0001). Conclusions In exclusively breastfeeding dyads, maternal obesity is associated with lower maternal and infant serum 25(OH) D concentrations, which may impact infant bone density.