Vitamin D mediated Calcium and Phosphorus metabolism in cattle

Abstract

Vitamin D metabolites and the major mammalian minerals phosphorus (P) and calcium (Ca) have a complex and detailed relationship. The relationship, especially for Ca, is delicately controlled by a range of hormones and is largely dependent on the animal's physiological state. Studies have identified that Vitamin D metabolites given in supraphysiological doses to cattle can manipulate Ca and P metabolism. Furthermore, large doses of the metabolite 25-hydroxy vitamin D (25-vitD) can replace the actions of active vitamin D, 1,25-dihydroxy vitamin D (1,25-vitD) and initiate increased absorption of both Ca and P from the digestive tract in mice. These findings led to the hypothesis that 25-vitD could be used to favorably manipulate the metabolism of both Ca and P in cattle. The studies undertaken within this thesis are focused on the use of 25-vitD in both the beef and dairy industries. A group of beef breed steers were recruited and trained to halter, stand in metabolism crates and spend extended periods of time in individual pens. For studies concerning Ca, urine was the most important measurement. An increase in urinary Ca excretion demonstrated that there has been an increase in available Ca, either from the diet or skeletal reserves. With P studies faecal P excretion and plasma P were important guides to changes in P metabolism. The results of the metabolism studies undertaken in the thesis identify that Ca and P homeostasis is manipulated by 25-vitD and the majority of the increase in availability of both Ca and P originates from the diet. This is further supported by the absence of bone degradation. Thus, the inclusion of 25-vitD in a typical anionic transition diet will increase Ca absorption prior to parturition, enabling labile bone Ca stores to remain intact and available for immediate use at parturition whilst increasing the amount of Ca available to the animal from both bone and diet. Furthermore, the combination of 25-vitD and anionic salts has physiological implications that allow sufficient generation of extracellular Ca at parturition. An increase in plasma 25-vitD concentrations, to approximately 375 ng/ml, facilitated an increase in the concentration of plasma P and a reduction in faecal P, which indicates that dietary P absorption was increased.