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

Increased dietary protein consumption is thought to cause calciuresis, a negative 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 increasing crude protein (CP) content (114, 142, 171 and 190 g/kg DM) but approximately on an iso-nutrient basis with regard to metabolisable energy, calcium and phosphorus. Increased protein consumption modestly (NS) enhanced calciuresis and resulted in significant (P < 0.01) limb skewness. This could not, however, be ascribed to osteopaenic bones, and compared with animals consuming lower protein rations, the bone mineral density (BMD) and vertebral trabecular bone volume of animals fed high protein diets were significantly increased: theBMDof thoracic vertebrae was positively related to the CP intake (r=0.62; P < 0.001). In animals consuming higher protein diets, skeletal radiology and quantitative bone histology revealed no evidence of increased bone turnover as would be expected in animals that are in negative calcium balance. No relationship existed between limb skewness and the growth rate of lambs. However, the ratio 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 skeletal deformities induced by increased protein consumption in sheep.

The effects of oxalate in tropical grasses on calcium, phosphorus and magnesium availability to cattle

SUMMARYCattle were fed four hays in mineral balance experiments. Two of the hays (Cenchrus ciliaris and Setaria sphacelala var. sericea) contained 1·8 and 1·3% total oxalates and provided above- and below-maintenance intakes of calcium respectively. These grasses contain calcium oxalate crystals. The other two hays (Aristida spp –Bothriochloa spp. mixture and Triticum aestivum) contained 0·1% oxalates, and also provided above- and below-maintenance intakes of calcium. The absorptions of calcium from the hays providing above-maintenance intakes were 51% for the high and 57% for the low oxalate hay. At below-maintenance intakes, the cattle were in negative calcium balance and calcium absorptions were 52% for the high and 64% for the low oxalate hay. While in negative calcium balance the cattle were given single doses of calcium oxalate, followed by single doses of either limestone or rock phosphate. The cattle fed S. sphacelata hay absorbed sufficient calcium from calcium oxalate to achieve positive calcium balance, although the amount absorbed was only 52% of that absorbed from limestone. The cattle fed T. aestivum hay also absorbed calcium from calcium oxalate, but in insufficient amount to result in a positive calcium balance. The amount absorbed was 47% of that absorbed from rock phosphate. The results demonstrate that in tropical grasses containing calcium oxalate crystals, the availability of calcium is about 20% lower than it is in grasses containing little oxalate. Adaptation to oxalate may improve the ability of the rumen to utilize calcium oxalate. It is suggested that the availability of calcium to cattle grazing tropical grasses should be considered to be a maximum of 50%. Neither magnesium nor phosphorus absorptions from tropical grasses were affected by oxalate.

The effects of oxalate in some tropical grasses on the availability to horses of calcium, phosphorus and magnesium

SUMMARYHorses fed various species of tropical grass hays containing more than 0·5% total oxalate were in negative calcium balance of up to 41 mg/kg live weight/day in mineral balance experiments. In most cases, a concurrent negative phosphorus balance of up to 22 mg/kg live weight/day was measured. These results were in contrast to those obtained when grass hays low in oxalate were fed. Magnesium balance was little affected by oxalate. The degree of negative calcium balance could not be reconciled with total calcium and total oxalate concentrations, but could be explained if a major fraction of calcium in the grasses was in the form of calcium oxalate crystals and if calcium in this form was unavailable for absorption. It is suggested that soluble oxalates exert a variable but smaller effect on absorption of the remaining calcium fraction of the grasses. In general, nutritional secondary hyperparathyroidism may occur in horses grazed solely on pastures in which calcium to oxalate ratios are less than 0·5, such ratios usually being found in grasses containing more than 0·5% total oxalate.

The inability of horses to absorb calcium from calcium oxalate

SUMMARYSix horses were fed Setaria sphacelata var. sericea cv. Narok hay in a mineral balance experiment and were in negative calcium balance. This should have ensured maximum efficiency of calcium absorption. Only 6·3% of calcium from a single dose of 1·32 kg calcium oxalate was retained by the horses, which supports the suggestion that horses cannot utilize calcium in tropical grasses if it is present as calcium oxalate crystals.

The Influence of Glucose and Crude Fibre (Wheat Bran) on the Rate of Intestinal 47Ca Absorption The Influence of Glucose and Wheat Bran on Calcium Absorption

With evidence of induction of increased urinary excretion of calcium by the ingestion of glucose and sucrose there is the theoretical possibility in these circumstances of at least a transient negative calcium balance. In this study the ingestion of glucose or glucose equivalent was found to stimulate 47Ca absorption from the intestine both in normal subjects and in idiopathic calcium stone formers. This induced increase in the rate of 47Ca absorption by glucose can be negated by the addition of crude fibre in the form of wheat bran.

Some Biokinetic Aspects of Calcium Metabolism in Dairy Cows

A study was made to determine how cows which have been prefed low Ca:high P diets are better able to maintain normal serum Ca levels than are cows prefed high Ca:low P diets. Radioactive calcium (Ca45) was employed as a tracer. A procedure is presented by which the reservoirs of mobilizable skeletal Ca may be estimated. It appears that these reservoirs of skeletal Ca are larger and are turning over more slowly in cows which were prefed low Ca:high P diets. Since we found earlier that the low Ca: high P cows were in negative calcium balance, the larger stores of mobilizable Ca must accrue at the expense of stable bone Ca. A comparison of bone Ca specific activity to serum Ca specific activity indicates that nearly all the trabecular bone Ca and from 7 to 17% of the cortical bone Ca (depending upon dietary Ca/P) may be included in the ‘mobilizable Ca pool.’