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

This review summarizes evidence that supports working hypotheses for t he roles of boron in animal model systems. It is well established that vascular plants, diatoms, and some species of marine algal flagellate s have acquired an absolute requirement for boron. although the primar y role of boron in plants remains unknown. Recent research findings su ggest that physiologic amounts of supplemental dietary boron (PSB) aff ect a wide range of metabolic parameters in the chick and rat model sy stems. Much of the current interest in boron animal nutrition began wi th 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 synergi stic with Vitamin D-3 in influencing growth. Vitamin D-3 regulates ene rgy 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 d ate, responds to PSB, especially during concomitant vitamin D-3 defici ency. In chicks, PSB substantially alleviated or corrected vitamin D-3 deficiency-induced elevations in plasma glucose concentrations. The i nfluence of vitamin D-3 on cartilage and bone mineralization is mediat ed in part through its role as a regulator of energy substrate utiliza tion; calcification is an energy-intensive process. There is considera ble evidence that dietary boron alleviates perturbations in mineral me tabolism that are characteristic of Vitamin D-3 deficiency. In rachiti c chicks, PSB alleviated distortion of the marrow sprouts of the proxi mal tibial epiphysial plate, a distortion characteristic of vitamin D- 3 deficiency. in ovo injections of boron or 1,25-(OH)(2)-vitamin D-3 r educed the abnormal height of the growth plate of 1-day-old chicks hat ched from vitamin D-3-deficient eggs. Also, in vitamin D-deficient rat s, PSB improved the apparent absorption and retention of calcium and p hosphorus, and increased femur magnesium concentrations. Current findi ngs 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 su bstrates. 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.