Bone resorption and dietary calcium in pregnancy-a window to future maternal bone health.

Background: Pregnancy is characterized by increased bone turnover and reversible loss of bone mineral density (BMD) to meet fetal calcium demands. The long-term effect of bone turnover and maternal diet in pregnancy on maternal bone is not well established.

Objective: We aimed to determine if an association exists between [1] bone resorption, [2] dietary calcium, and [3] serum 25-hydroxyvitamin D in pregnancy with maternal BMD 5-year postpartum.

Design: This is a prospective, longitudinal study of 107 women recruited to the ROLO low glycemic index dietary intervention trial in pregnancy and followed-up at 13, 28, and 34 weeks' gestation and 5 years' postpartum. At 13 and 28 weeks' gestation, a biomarker of bone resorption, urine cross-linked N-telopeptide of type I collagen (uNTX), was measured. At the 5-year follow-up BMD was measured using dual-energy X-ray absorptiometry. Anthropometry, dietary intakes, and serum 25-hydroxyvitamin D were measured in pregnancy and at 5 years. Multiple linear regression, controlling for confounders, was used for analysis.

Results: Mean BMD at 5 years was 1.208 g/cm. In pregnancy, 24-34% reported dietary calcium intakes <800 mg/day. Vitamin D deficiency (< 30 nmol/L) was observed in 38-41% of women in pregnancy and in 29% of women at the 5-year follow-up. At 13 and 28 weeks' gestation, uNTX levels greater than the median were associated with 0.060 and 0.050 g/cm lower BMD 5 years later, respectively. Dietary calcium <800 mg/day in trimester 3 was associated with 0.072 g/cm lower BMD 5 years later. Vitamin D deficiency at 5 years, but not in pregnancy, was associated with lower BMD.

Conclusion: Higher bone resorption and low dietary calcium in pregnancy were associated with lower BMD 5 years later. These findings could enable the identification of women at risk of declining of BMD in later life, but further research is needed. Adequate dietary calcium should be advised in the antenatal setting to promote lifelong maternal bone health.