Vitamin D status and skeletal changes during reproduction- A longitudinal study from late pregnancy through lactation

Abstract

Low vitamin D status has been associated with sub-optimal bone health. During both pregnancy and postpartum, it has been speculated that vitamin D status may affect maternal bone health, due to its importance in maintaining the calcium homeostasis in the body. The overall aim of this thesis was to evaluate vitamin D status and bone changes during pregnancy and postpartum in women living in the vicinity of Gothenburg, Sweden. Ninety-five fair-skinned pregnant women and 21 non-pregnant and non-lactating controls were recruited. Blood samples, anthropometric data, information about sun exposure and lactation habits and four-day food diaries were collected in the third trimester of pregnancy and two weeks (baseline), four, 12 and 18 months postpartum. Serum concentrations of 25-hydroxyvitamin D (25OHD) were analyzed. Bone status was assessed postpartum with dual-energy X-ray absorptiometry (DXA) and high-resolution peripheral quantitative computed tomography (HR-pQCT). In the third trimester, mean 25OHD concentration was 47±18 (mean±SD) nmol/L. During the first year postpartum, no change in mean 25OHD concentration was found and no association between duration of lactation and changes in 25OHD concentrations was observed. Estimates of sun exposure and use of vitamin D supplements were found to be major determinants both for 25OHD concentrations during pregnancy and for the variation in changes in 25OHD concentrations postpartum. During the first four months postpartum, bone decreases were observed at several skeletal sites in women lactating four months or longer. At 18 months postpartum, cortical volumetric bone mineral density and trabecular thickness at the ultradistal tibia were still significantly lower than baseline in women lactating nine months or longer. Calcium intake and 25OHD concentrations appear to have different influences on the cortical and trabecular bone changes postpartum. In conclusion, a majority of the women were vitamin D insufficient in the third trimester of pregnancy. No change in mean 25OHD concentration was observed during the first year postpartum. Longer follow-up than 18 months is needed to confirm whether women with long lactation fully recover their bone minerals after weaning or whether the postpartum bone changes could potentially lead to an increased fracture risk in later life.