Bone Abstracts

Breastfeeding women resorb trabecular bone to supply much of the calcium content of milk. Few studies have examined the speed and extent of BMD recovery after weaning, or the factors that predict a greater post-lactation increase in BMD. We hypothesize that weight-bearing, nutrition, hormones, and other factors facilitate bone formation after lactation.

The aims of the Factors Affecting Bone Formation After Breastfeeding Pilot Study (FABB-Pilot) were to determine the magnitude of increase in BMD at 6 months after weaning, and provide preliminary data to enable pre-specification of predictive variables to be tested in a future larger-scale FABB study.

We recruited women who had breastfed exclusively (no formula) or near-exclusively (1–2 formula/solid feeds per day) for 4–6 months, and by so doing should have experienced a significant decline in BMD. At time of planned weaning and 6 months later, we measured BMD, whole body fat and lean mass, and hip structural analysis by DXA; blood calcium, ionized calcium, PTH, PTHrP, estradiol, 25OHD, calcitriol, P1NP, CTX; and urine Ca/Cr. Questionnaires administered at both time points assessed nutrition, weight-bearing activities, and other factors that may influence bone recovery.

31 women (31.6±3.5 years, 97% Caucasian) enrolled at 26±2.0 weeks post-partum and completed the baseline measurements. Approximately 80% had breastfed exclusively for 6 months. 30 women completed the follow-up assessments. Mean thoracic spine BMD increased 5.1% (0.713–0.749 g/cm², P<0.01); lumbar spine increased 4.0% (0.971–1.01 g/cm², P<0.03), while cortical sites (hip, total body) remained unchanged. Estradiol increased (115–198 pmol/l, P<0.01), PTH increased (60–69 ng/ml, P<0.04), and 25OHD declined (80 vs 64 nmol/l, P<0.01), while calcium and ionized calcium did not change.

In conclusion, trabecular (spine) BMD increases significantly in the first 6 months post-weaning accompanied by increased estradiol and PTH. The factors that promote this post-lactation increase in BMD remain to be identified.