The influence of dietary crude protein intake on bone and mineral metabolism in sheep

Increased dietary protein consumption is thought to cause calciuresis, a ne gative calcium balance and increased bone loss that may result in skeletal deformities and fracture. To explore this hypothesis, 40 approximately 100- day-old meat-type Merino ram lambs were fed, for 6 months, diets with an in creasing crude protein (CP) content (114, 142, 171 and 190 g/kg DM) but app roximately on an iso-nutrient basis with regard to metabolisable energy,cal cium and phosphorus. Increased protein consumption modestly (NS) enhanced c alciuresis and resulted in significant (P less than or equal to 0.01) limb skewness. This could not, however, be ascribed to osteopaenic bones, and co mpared with animals consuming lower protein rations, the bone mineral densi ty (BMD) and vertebral trabecular bone Volume of animals fed high protein d iets were significantly increased: the BMD of thoracic vertebrae was positi vely related to the CP intake (r = 0.62; P less than or equal to 0.001). In animals consuming higher protein diets, skeletal radiology and quantitativ e bone histology revealed no evidence of increased bone turnover as would b e expected in animals that are in negative calcium balance. No relationship existed between limb skewness and the growth rate of lambs. However, the r atio of Ca:P in the forelimb (r = -0.98), vertebrae (r = -0.72) and rib (r = -0.42) was found to be inversely correlated with increased protein intake and resulted from an increase in the phosphorus content of bone, while the amount of bone calcium was unaffected. We conclude that qualitative micro- architectural abnormalities, and not mere bone loss, may underlie the skele tal deformities induced by increased protein consumption in sheep.