Calcium restriction during lactation has minimal effects on post-weaning mineral metabolism and bone recovery

Dietary calcium (Ca) restriction during lactation in the rat, which induces intra-cortical and endocortical remodeling, has been proposed as a model to study bone matrix maturation in the adult skeleton. The purpose of this study was to assess the effects of dietary Ca restriction during lactation on post-weaning mineral metabolism and bone formation. Mated female Sprague-Dawley rats were randomized into groups receiving either 0.6% Ca (lactation/normal Ca) or 0.01% Ca (lactation/low Ca) diets during lactation. Virgin animals fed normal Ca were used as controls (virgin/normal Ca). At the time of weaning, animals on the low Ca diet were returned to normal Ca and cohorts of all three groups were sacrificed at days 0, 1, 2, 7, and 14 post-weaning. Lactation caused bone loss, particularly at the endocortical surface, but the amount was not affected by dietary Ca. Rats in the lactation/low Ca group had increased cortical porosity compared to the other groups, particularly within the size range of secondary osteons. Dietary Ca restriction during lactation did not affect post-weaning bone formation kinetics or serum Ca and phosphate levels. In both lactation groups, there was a transient increase in phosphate and fibroblast growth factor 23 (FGF23) post-weaning, which trended toward virgin/normal Ca levels over time. Thus, the additional challenge of low dietary Ca during lactation to induce intra-cortical remodeling in the rat has minimal effects on bone formation kinetics and mineral metabolism during the post-weaning period, providing further justification for this model to study matrix maturation in the adult skeleton.