EFFECTS OF DIETARY ACIDITY ON CALCIUM BALANCE AND MOBILIZATION, BONE MORPHOLOGY AND 1,25-DIHYDROXYVITAMIN-D IN PREPARTAL DAIRY-COWS

The effect of an acid or alkali diet was investigated in 14 mature dai ry cows during the last 28 days of pregnancy. The acid diet reduced th e incidence of parturient hypocalcaemia compared with the alkali diet and was associated with higher blood ionised calcium and plasma chlori de concentrations and lower blood pH and acid-base excess before partu rition. Plasma 1,25-dihydroxyvitamin D concentrations before parturiti on were increased by the acid diet but the concentrations of parathyro id hormone and intact 1-86 parathyroid-hormone-related peptide were un affected. The estimated fractional calcium absorption and calcium mobi lisation rate during an ethylenediamine tetraacetic acid infusion were increased by the acid diet 14 days before parturition. Cortical bone remodelling occurred in all the animals during late pregnancy but was particularly evident in the cows given the acid diet. The data suggest that an acid diet ameliorates parturient hypocalcaemia by enhancing c alcium mobilisation before parturition by increasing calcium absorptio n and bone resorption, these increases possibly being mediated by incr eases in the plasma concentration of 1,25-dihydroxyvitamin D.