Submitted to: Biomedical Research on Trace Elements
Publication Type: Review Article
Publication Acceptance Date: 8/11/2008
Publication Date: 12/1/2008
Citation: Hunt, C. 2008. Dietary boron: possible roles in human and animal physiology. Biomedical Research on Trace Elements. 19(3):243-253.
Technical Abstract: Boron is a bioactive element of low molecular weight. Since discovery of the first boron biomolecule, boromycin, in 1967, several other similar biomolecules are now well-characterized. Most recently described was a bacterial cell-to-cell communication signal that requires boron, autoinducer-II. Boron is a natural constituent of the diet and human consumption is not trivial: ~1.34 mg/d for males aged 51 to 70 y. Gastrointestinal absorption of boron approaches 100% but, even so, boron has a low order of toxicity. For adults, the Tolerable Upper Intake Level (UL) for boron is 20 mg/d, i.e., 20-fold typical intakes. The Institute of Medicine has not set an Estimated Average Requirement (EAR), Recommended Dietary Allowance (RDA), or Adequate Intake (AI) for boron because the collective body of evidence has yet to establish a clear biological function for boron in humans. The evidence to date suggests that humans and higher animals (frog, zebrafish, chick, rat, and pig) may use boron to support normal biological functions. There are several lines of evidence that suggest boron depletion prevents growth and completion of the life cycle in animal models. For example, in rats, a low-boron diet reduced the number of implantation sites compared to a diet supplemented with boron. Numerous findings from independent laboratories indicate that boron is beneficial for bone health. For example, boron deprivation reduced bone strength in pigs and rats and induced abnormal limb development in frogs. Observations in human studies suggest that an increase in boron intake (0.36 to 3.23 mg/d) by postmenopausal women resulted in a 5% increase in urinary calcium excretion. Because increases in dietary calcium often result in increased urinary calcium excretion, this finding may reflect an increase in intestinal calcium absorption. Dietary boron decreased plasma insulin concentrations while maintaining glucose concentrations in the rat. It is possible that dietary boron may reduce the amount of insulin needed to maintain glucose levels, thus limiting '-cell deterioration. Research in several animal species indicates that boron has a role in immune processes. Dietary boron decreased localized inflammatory responses in a pig model. In a rat model, dietary boron reduced paw swelling in rats following intradermal injection of M. butyricum to induce arthritis. Boron may be under homeostatic control in humans and other mammals through regulatory mechanisms that remain undefined. For example, human blood boron concentrations are insensitive to changes in dietary boron intake and the concentration of boron in milk from mothers of full term healthy infants is highly conserved across time despite the fact that dietary intake of boron typically varies widely with food intake patterns and drinking water sources. Most likely, apparent concentration of boron against a gradient indicates the existence of boron specific transporters. This line of evidence for the homeostatic control is enhanced further by the discovery of a specific mammalian borate transporter, NaBC1, expressed in the basolateral membranes of epithelial cells in tissues with excretory functions including kidney. Further experimentation with the boron transporter is a promising avenue of research to characterize the mechanisms of action responsible for the beneficial effects of dietary boron.