Effect of number of suckling pups and dietary calcium on bone mineral content and mechanical properties of femurs of lactating rats.

The bone loss that occurs during lactation in rats was aggravated by increasing the number of suckling pups and was further accentuated by feeding the rats a calcium (Ca)-deficient diet. The bone loss was evenly distributed along the whole length of the femur in severe cases. In less severe cases, the metaphyses were affected more than the midshaft. Mechanical properties of femurs, i.e., bone 'strength', 'stiffness', 'toughness' and 'ductility', were all affected in lactating rats suckling seven to eleven pups and fed the 0.1% Ca diet. The positive correlation between bone 'strength' and ash weight is consistent with the concept that when there is loss of bone, bone becomes more susceptible to fracture. The fall in serum ionized Ca and the rise in serum iPTH were closely related to the intensity of lactation and were profoundly affected by litter size and Ca concentration of the diet. We conclude that the change in Ca homeostasis in lactating rats is due to the large loss of Ca required for milk production and that the loss of Ca is associated with hyperfunction of the parathyroid gland. Furthermore, a Ca-deficient diet severely reduced bone mineral content and affected the mechanical properties of femurs of lactating rats adversely, especially those suckling large litters of pups.