THE BIOCHEMICAL EFFECTS OF PHYSIOLOGICAL AMOUNTS OF DIETARY BORON IN ANIMAL NUTRITION MODELS

This review summarizes evidence that supports working hypotheses for the roles of boron in animal model systems. It is well established that vascular plants, diatoms, and some species of marine algal flagellates have acquired an absolute requirement for boron. although the primary role of boron in plants remains unknown. Recent research findings suggest that physiologic amounts of supplemental dietary boron (PSB) affect a wide range of metabolic parameters in the chick and rat model systems. Much of the current interest in boron animal nutrition began with the initial finding that PSB stimulates growth in cholecalciferol (vitamin D-3)-deficient chicks, but does not markedly affect growth in chicks receiving adequate vitamin D-3 nutriture. The finding suggests that boron affects some aspect of vitamin D-3 metabolism or is synergistic with Vitamin D-3 in influencing growth. Vitamin D-3 regulates energy substrate utilization, and current research findings indicate that dietary boron modifies that regulatory function. The concentration of circulating glucose, the most thoroughly investigated metabolite to date, responds to PSB, especially during concomitant vitamin D-3 deficiency. In chicks, PSB substantially alleviated or corrected vitamin D-3 deficiency-induced elevations in plasma glucose concentrations. The influence of vitamin D-3 on cartilage and bone mineralization is mediated in part through its role as a regulator of energy substrate utilization; calcification is an energy-intensive process. There is considerable evidence that dietary boron alleviates perturbations in mineral metabolism that are characteristic of Vitamin D-3 deficiency. In rachitic chicks, PSB alleviated distortion of the marrow sprouts of the proximal tibial epiphysial plate, a distortion characteristic of vitamin D-3 deficiency. in ovo injections of boron or 1,25-(OH)(2)-vitamin D-3 reduced the abnormal height of the growth plate of 1-day-old chicks hatched from vitamin D-3-deficient eggs. Also, in vitamin D-deficient rats, PSB improved the apparent absorption and retention of calcium and phosphorus, and increased femur magnesium concentrations. Current findings lend support to the hypothesis that boron alleviates the symptoms of vitamin D-3-deficiency by enhancing utilization or sparing minimal supplies of an active vitamin D-3 metabolite. Also, boron and vitamin D-3 have the same overall effect on the local utilization of energy substrates. A corollary of the hypothesis is that some of the effects of dietary boron will be overshadowed by the effects of adequate amounts of dietary vitamin D-3.