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Title: DIETARY BORON: EVIDENCE FOR ESSENTIALITY AND HOMEOSTATIC CONTROL IN HUMANS AND ANIMALS

Author
item HUNT, CURTISS

Submitted to: Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/1/2005
Publication Date: 8/1/2006
Citation: Hunt, C. 2006. Dietary boron: evidence for essentiality and homeostatic control in humans and animals. In: Fangsen Xu, Heiner E. Goldback, Patrick H. Brown, Richard W. Bell, Toru Fujiwars, Curtiss D. Hunt, Sabine Goldberg, Lei Shi, editors. Advances in Plant and Animal Boron Nutrition. Boron 2005, September 10-13, 2005, Wuhan, China. p. 230-246.

Interpretive Summary:

Technical Abstract: Boron is a bioactive element and a number of advances have been made in satisfying the five criteria for essentiality in humans and higher animals. 1) The element is present in tissues of different animals at comparable concentrations. Plasma concentrations (ug/mL) of boron are comparable in humans (0.034-0.095), rats (0.055), and chicks (0.077-0.152). In the same species, boron concentrations are also comparable in liver, brain, and bone. New evidence for a specific mammalian borate transporter supports the earlier finding that the intracellular accumulation of boron occurs against a concentration gradient. In postmenopausal women, boron retention did not increase when low dietary intakes of (0.36 mg B/d) were increased nearly ten-fold (to 3.23 mg B/d). Also, lactating mothers who consumed self-selected diets had stable concentrations of boron in milk (~0.030 ug/mL) during the first 3 months of lactation. Together, these findings support the concept of boron homeostasis. 2) An essential mineral has a range of safe exposures that maintain optimal tissue concentrations and functions. Also, every trace element has a toxic range when its safe exposure is exceeded. Boron dose response experiments to determine embryo-larval malformations in the frog X. laevis have demonstrated the pattern of areas of survival, deficiency, optimization, toxicity, and lethality that are characteristic of an essential element. Mature American women (51-70 y) consume on average 1.34 mg of boron daily (1st percentile, 0.31; 99th percentile, 3.34 mg/d). When expressed on a molar basis, the average dietary intake of boron for this age-sex group (0.12 mmol/d) is substantially greater than that of manganese (0.04 mmol [2.42 mg]/d), copper (0.01 mmol [0.76 mg]/d), or molybdenum (0.0008 mmol [0.076 mg]/d). Even so, boron has a low order of toxicity; the DRI upper limit for boron is 20 mg/d for adults. 3) The organism can neither grow nor complete its life cycle without the element in question. In experiments with lower animals (frogs and fish), embryological development did not proceed normally in the absence of extracellular boron. In rat dams fed a low-boron diet (0.04 ug/g), the number of implantation sites was reduced significantly compared to dams fed a boron-adequate diet (2.00 ug/g). 4) Withdrawal of the element produces similar physiological or structural abnormalities regardless of species. Its presence reverses or prevents these abnormalities. Because boron is essential for all vascular plants, it is not surprising that boron deprivation appears to perturb physiological processes in frogs, zebrafish, chicks, rats, pigs, and humans. For example, in one or more of these species, boron deprivation induced signs of insulin resistance, impaired reproduction, altered mineral and steroid metabolism, defective bone structure, or an abnormal inflammatory response to antigens. 5) The element should have a direct influence on the organism and be involved in its metabolism. Boron is integral to the structure and function of several specific biomolecules including three antibiotics, a quorum-sensing signal produced by bacteria, and dimers of rhamnogalacturan-II (a plant cell wall compound). Full confirmation of boron essentiality in humans and animals awaits identification of a specific boron-requiring biomolecule that is utilized by humans or animals. Several biomolecules synthesized by humans have high boron-binding affinities and include S-adenosylmethionine and the diadenosine polyphosphates [ApnA] Ap3A, Ap4A, Ap5A, and Ap6A. Further characterization of these and other biomolecules that form complexes with boron under physiologic conditions should provide insights into the specific biochemical function(s) of boron in humans and animals.