The physiological effects of dietary boron

Boron may be an essential nutrient for animals and humans. Dietary boron influences the activity of many metabolic enzymes, as well as the metabolism of steroid hormones and several micronutrients, including calcium, magnesium, and vitamin D. Boron supplementation in rats and chicks has been shown to increase bone strength. Boron may also play a role in improving arthritis, plasma lipid profiles, and brain function. Additional research is necessary to further clarify boron's influence in human and animal physiology, as well as determine a dietary requirement for humans.

Boron in plants: deficiency and toxicity

Boron (B) is an essential nutrient for normal growth of higher plants, and B availability in soil and irrigation water is an important determinant of agricultural production. To date, a primordial function of B is undoubtedly its structural role in the cell wall; however, there is increasing evidence for a possible role of B in other processes such as the maintenance of plasma membrane function and several metabolic pathways. In recent years, the knowledge of the molecular basis of B deficiency and toxicity responses in plants has advanced greatly. The aim of this review is to provide an update on recent findings related to these topics, which can contribute to a better understanding of the role of B in plants.

Bone formation controlled by biologically relevant inorganic ions: role and controlled delivery from phosphate-based glasses

The role of metal ions in the body and particularly in the formation, regulation and maintenance of bone is only just starting to be unravelled. The role of some ions, such as zinc, is more clearly understood due to its central importance in proteins. However, a whole spectrum of other ions is known to affect bone formation but the exact mechanism is unclear as the effects can be complex, multifactorial and also subtle. Furthermore, a significant number of studies utilise single doses in cell culture medium, whereas the continual, sustained release of an ion may initiate and mediate a completely different response. We have reviewed the role of the most significant ions that are known to play a role in bone formation, namely calcium, zinc, strontium, magnesium, boron, titanium and also phosphate anions as well as copper and its role in angiogenesis, an important process interlinked with osteogenesis. This review will also examine how delivery systems may offer an alternative way of providing sustained release of these ions which may effect and potentiate a more appropriate and rapid tissue response.

Formation of rhamnogalacturonan II-borate dimer in pectin determines cell wall thickness of pumpkin tissue

Boron (B) deficiency results in inhibition of pumpkin (Cucurbia moschata Duchesne) growth that is accompanied by swelling of the cell walls. Monomeric rhamnogalacturonan II (mRG-II) accounted for 80% to 90% of the total RG-II in B-deficient walls, whereas the borate ester cross-linked RG-II dimer (dRG-II-B) accounted for more than 80% of the RG-II in control plants. The results of glycosyl residue and glycosyl linkage composition analyses of the RG-II from control and B-deficient plants were similar. Thus, B deficiency does not alter the primary structure of RG-II. The addition of (10)B-enriched boric acid to B-deficient plants resulted within 5 h in the conversion of mRG-II to dRG-II-(10)B. The wall thickness of the (10)B-treated plants and control plants was similar. The formation and possible functions of a borate ester cross-linked RG-II in the cell walls are discussed.

Regulation of enzymatic activity: one possible role of dietary boron in higher animals and humans

It is well established that vascular plants, diatoms, and some species of marine algal flagellates have acquired an absolute requirement for boron (B), although the primary role remains unknown. Discovery of naturally occurring organoboron compounds, all ionophoric macrodiolide antibiotics with a single B atom critical for activity, established at least one biochemical role of B. The unusual nature of B chemistry suggests the possibility of a variety of biological roles for B. At physiological concentrations and pH, B may react with one N group or one to four hydroxyl groups on specific biological ligands with suitable configuration and charge to form dissociable organoboron compounds or complexes. Suitable ligands include pyridine (e.g., NAD+ or NADP) or flavin (e.g., FAD) nucleotides and serine proteases (SP). B reacts with the cis adjacent hydroxyls on the ribosyl moiety of the nucleotides or, in the serine proteases, the N on the imidazole group of histidine or the hydroxyl group on the serine moiety. Reversible inhibition by B of activity of SP or oxidoreductases that require pyridine or flavin nucleotides is well known. Therefore, a proposed essential role for B is as a regulator of relevant pathways, including respiratory burst, that utilize these enzymes.

The biochemical effects of physiologic 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 D3)-deficient chicks, but does not markedly affect growth in chicks receiving adequate vitamin D3 nutriture. The finding suggests that boron affects some aspect of vitamin D3 metabolism or is synergistic with vitamin D3 in influencing growth. Vitamin D3 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 D3 deficiency. In chicks, PSB substantially alleviated or corrected vitamin D3 deficiency-induced elevations in plasma glucose concentrations. The influence of vitamin D3 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 D3 deficiency. In rachitic chicks, PSB alleviated distortion of the marrow sprouts of the proximal tibial epiphysial plate, a distortion characteristic of vitamin D3 deficiency.(ABSTRACT TRUNCATED AT 250 WORDS)

Borate Transport and Cell Growth and Proliferation: Not Only in Plants

Boron is an abundant mineral essential for the life cycle of plants and may play a role in animal development and growth. Very little is known about boron homeostasis in plant and animal cells and the physiological roles of boron in animals. The recent identification of boron transporters, BOR1 in plants and NaBC1 in mammals, and that NaBC1 functions as an electrogenic Na+-coupled borate transporter essential for cell growth and proliferation open the way to probe the roles of boron in cellular function and physiology.

The importance of boron nutrition for brain and psychological function

Boron (B) nutriture has been related to bone, mineral and lipid metabolism, energy utilization, and immune function. As evidence accumulates that B is essential for humans, it is important to consider possible relationships between B nutriture and brain and psychological function. Five studies conducted in our laboratory are reviewed. Assessments of brain electrical activity in both animals and humans found that B deprivation results in decreased brain electrical activity similar to that observed in nonspecific malnutrition. Assessments of cognitive and psychomotor function in humans found that B deprivation results in poorer performance on tasks of motor speed and dexterity, attention, and short-term memory. However, little support was found for anecdotal reports that supplementation with physiologic amounts of B helps alleviate the somatic and psychological symptoms of menopause. Parallels between nutritional and toxicological effects of B on brain and psychological function are presented, and possible biological mechanisms for dietary effects are reviewed. Findings support the hypothesis that B nutriture is important for brain and psychological function in humans.

In vivo and in vitro effects of boron and boronated compounds

Boron is ubiquitously present in soils and water. Associated with pectin it is essential for vascular plants as a component of cell walls, and it stabilizes cell membranes. It is required for the growth of pollen tubes and is involved in membrane transport, stimulating H(+)-pumping ATPase activity and K+ uptake. However, a high boron concentration in the soils is toxic to plants and some boronated derivatives are used as herbicides. An absolute requirement for boron has not been definitively demonstrated in animals and humans. However, experiments with boron supplementation or deprivation show that boron is involved in calcium and bone metabolism, and its effects are more marked when other nutrients (cholecalciferol, magnesium) are deficient. Boron supplementation increases the serum concentration of 17 beta-estradiol and testosterone but boron excess has toxic effects on reproductive function. Boron may be involved in cerebral function via its effects on the transport across membranes. It affects the synthesis of the extracellular matrix and is beneficial in wound healing. Usual dietary boron consumption in humans is 1-2 mg/day for adults. As boron has been shown to have biological activity, research into the chemistry of boronated compounds has increased. Boronated compounds have been shown to be potent anti-osteoporotic, anti-inflammatory, hypolipemic, anti-coagulant and anti-neoplastic agents both in vitro and in vivo in animals.

The justification for providing dietary guidance for the nutritional intake of boron

Because a biochemical function has not been defined for boron (B), its nutritional essentiality has not been firmly established. Nonetheless, dietary guidance should be formulated for B, because it has demonstrated beneficial, if not essential, effects in both animals and humans. Intakes of B commonly found with diets abundant in fruits, vegetables, legumes, pulses, and nuts have effects construed to be beneficial in macromineral, energy, nitrogen, and reactive oxygen metabolism, in addition to enhancing the response to estrogen therapy and improving psychomotor skills and cognitive processes of attention and memory. Perhaps the best-documented beneficial effect of B is on calcium (Ca) metabolism or utilization, and thus, bone calcification and maintenance. The paradigm emerging for the provision of dietary guidance that includes consideration of the total health effects of a nutrient, not just the prevention of a deficiency disease, has resulted in dietary guidance for chromium (Cr) and fluoride; both of these elements have beneficial effects in humans, but neither has a defined biochemical function. Knowledge of B nutritional effects in humans equals or is superior to that of Cr and fluoride; thus, establishing a dietary reference intake for B is justified. An analysis of both human and animal data suggests that an acceptable safe range of population mean intakes of B for adults could well be 1-13 mg/d. Recent findings indicate that a significant number of people do not consistently consume more than 1 mg B/d; this suggests that B could be a practical nutritional or clinical concern.

Mechanisms of organic acids and boron induced tolerance of aluminum toxicity: A review

Aluminum is a major limiting abiotic factor for plant growth and productivity on acidic soils. The primary disorder of aluminum toxicity is the rapid cessation of root elongation. The root apex is the most sensitive part of this organ. Although significant literature evidence and hypothesis exist on aluminum toxicity, the explicit mechanism through which aluminum ceases root growth is still indefinable. The mechanisms of tolerance in plants have been the focus of intense research. Some plant species growing on acidic soils have developed tolerance mechanisms to overcome and mitigate aluminum toxicity, either by avoiding entry of Al³⁺ into roots (exclusion mechanism) or by being able to counterbalance toxic Al³⁺ engrossed by the root system (internal tolerance mechanism). Genes belonging to ALMT (Aluminum-activated malate transporter) and MATE (Multidrug and toxin compounds extrusion) have been identified that are involved in the aluminum-activated secretion of organic acids from roots. However, different plant species show different gene expression pattern. On the other hand, boron (B) (indispensable micronutrient) is a promising nutrient in the tolerance to aluminum toxicity. It not only hinders the adsorption of aluminum to the cell wall but also improves plant growth. This review mainly explains the critical roles of organic acid and B-induced tolerance to aluminum by summarizing the mechanisms of ALMT, MATE, internal detoxification, molecular traits and genetic engineering of crops.

Boron: the essential element for vascular plants that never was

Although a requirement for boron is a well-established feature of vascular plants, its designation, for almost a century, as essential is challenged and, instead, the proposal is made that it has never been so as conventionally defined. This is because an alternative interpretation of published evidence negates its compliance with one of the criteria for essentiality, that its effects are direct. The alternative, here postulated, is that boron is, and always has been, potentially toxic, a feature which, for normal growth, development and reproduction, needed to be nullified. This was enabled by exploitation of boron's ability to be chemically bound to compounds with cis-hydroxyl groups. Although particular cell wall carbohydrate polymers, glycoproteins and membrane glycolipids are among candidates for this role, it is here proposed that soluble phenolic metabolites of, or related to, the components of the pathway of lignin biosynthesis, themselves potentially toxic, are primarily used by vascular plants. When metabolic circumstances allow these phenolics to accumulate endogenously in the cytoplasm, their own inherent toxicity is also alleviated, partially at least, by formation of complexes with boron. This chemical reciprocity, enhanced by physical sequestration of the complexes in vacuoles and/or apoplast, thus achieves, in a flexible but indirect manner, a minimization of the inherent toxicities of both boron and relevant phenolics. In these ways, the multifarious outcomes of impairments, natural or experimental, to this interplay are responsible for the lack of consensus to explain the diverse effects observed in the many searches for boron's primary metabolic role, here considered to be nonexistent. In particular, since a toxic element cannot have 'deficiency symptoms', those previously so-called are postulated to be largely due to the expressed toxicity of phenylpropanoids. A principal requirement for the otherwise toxic boron is to nullify, by means of its indirect chemical and physical sequestration, such expression. In these ways, it is therefore neither an essential nor a beneficial element as currently strictly defined.

Dietary boron, brain function, and cognitive performance

Although the trace element boron has yet to be recognized as an essential nutrient for humans, recent data from animal and human studies suggest that boron may be important for mineral metabolism and membrane function. To investigate further the functional role of boron, brain electrophysiology and cognitive performance were assessed in response to dietary manipulation of boron (approximately 0.25 versus approximately 3.25 mg boron/2000 kcal/day) in three studies with healthy older men and women. Within-subject designs were used to assess functional responses in all studies. Spectral analysis of electroencephalographic data showed effects of dietary boron in two of the three studies. When the low boron intake was compared to the high intake, there was a significant (p < 0.05) increase in the proportion of low-frequency activity, and a decrease in the proportion of higher-frequency activity, an effect often observed in response to general malnutrition and heavy metal toxicity. Performance (e.g., response time) on various cognitive and psychomotor tasks also showed an effect of dietary boron. When contrasted with the high boron intake, low dietary boron resulted in significantly poorer performance (p < 0.05) on tasks emphasizing manual dexterity (studies II and III); eye-hand coordination (study II); attention (all studies); perception (study III); encoding and short-term memory (all studies); and long-term memory (study I). Collectively, the data from these three studies indicate that boron may play a role in human brain function and cognitive performance, and provide additional evidence that boron is an essential nutrient for humans.

Biochemical and physiologic consequences of boron deprivation in humans

Boron deprivation experiments with humans have yielded some persuasive findings for the hypothesis that boron is an essential nutrient. In the first nutritional study with humans involving boron, 12 postmenopausal women first were fed a diet that provided 0.25 mg boron/2000 kcal for 119 days, and then were fed the same diet with a boron supplement of 3 mg boron/day for 48 days. The boron supplementation reduced the total plasma concentration of calcium and the urinary excretions of calcium and magnesium, and elevated the serum concentrations of 17 beta-estradiol and testosterone. This study was followed by one in which five men over the age of 45, four postmenopausal women, and five postmenopausal women on estrogen therapy were fed a boron-low diet (0.23 mg/2000 kcal) for 63 days, then fed the same diet supplemented with 3 mg boron/day for 49 days. The diet was low in magnesium (115 mg/2000 kcal) and marginally adequate in copper (1.6 mg/2000 kcal) throughout the study. This experiment found higher erythrocyte superoxide dismutase, serum enzymatic ceruloplasmin, and plasma copper during boron repletion than boron depletion. The design of the most recent experiment was the same as the second study, except this time the diet was adequate in magnesium and copper. Estrogen therapy increased plasma copper and serum 17 beta-estradiol concentrations; the increases were depressed by boron deprivation. Estrogen ingestion also increased serum immunoreactive ceruloplasmin and erythrocyte superoxide dismutase; these variables also were higher during boron repletion than depletion for all subjects, not just those ingesting estrogen.(ABSTRACT TRUNCATED AT 250 WORDS)

Boron influences pollen germination and pollen tube growth in Picea meyeri

To study the role of boron in pollen germination and pollen tube growth of Picea meyeri Rehd. et Wils., pollen grains were cultured in standard medium or boron-deficient medium. Effects of boron on the localization of pectins and callose in the walls of pollen tubes were observed by laser scanning confocal microscopy after staining with aniline blue or immunolabeling with antibodies JIM5 and JIM7. Changes in the structures of pectins and phenolics were investigated by fourier transform infrared (FTIR) microspectroscopy. Pollen germination in boron-deficient medium ranged from 18 to 24%, whereas pollen germination in standard medium reached 61%. Callose accumulated in the tip-regions of pollen tubes cultured in boron-deficient medium, but not in standard medium. Immunolabeling with antibody JIM5 revealed that acidic pectin preferentially accumulated in the tip regions of pollen tubes cultured in boron-deficient medium, whereas acidic pectin was weakly distributed along the entire lengths of pollen tubes cultured in standard medium. Esterified pectin, detected by immunolabeling with antibody JIM7, showed a similar distribution pattern in pollen tubes in both the boron-deficient and standard treatments. The FTIR spectra indicated slight increases in contents of phenolics and carboxylic acids and a substantial decrease in the content of saturated esters in boron-deficient pollen tubes compared with normal pollen tubes. The FTIR spectra confirmed that boron deficiency enhanced acidic pectin accumulation in pollen tubes, which may be associated with the increased content of carboxylic acid. We conclude that boron has a regulatory role in pollen germination and pollen tube growth.

Boron deficiency and transcript level changes

Boron (B) is an essential element for plant growth whose deficiency causes an alteration in the expression of a wide range of genes involved in several physiological processes. However, our understanding of the signal transduction pathways that trigger the B-deficiency responses in plants is still poor. The aims of this review are (i) to summarize the genes whose transcript levels are affected by B deficiency and (ii) to provide an update on recent findings that could help to understand how the signal(s) triggered by B deficiency is transferred to the nucleus to modulate gene expression. In this contribution we review the effects of B deficiency on the transcript level of genes related to B uptake and translocation, maintenance of cell wall and membrane function, nitrogen assimilation and stress response. In addition, we discuss the possible mediation of calcium, arabinogalactan-proteins and other cis-diol containing compounds in the signaling mechanisms that transfer the signal of B deficiency to nuclei. Finally, we conclude that the advance in the knowledge of the molecular basis of B deficiency response in plants will allow improving the tolerance of crops to B deficiency stress.

Involvement of auxin and CKs in boron deficiency induced changes in apical dominance of pea plants (Pisum sativum L.)

It has previously been shown that boron (B) deficiency inhibits growth of the plant apex, which consequently results in a relatively weak apical dominance, and a subsequent sprouting of lateral buds. Auxin and cytokinins (CKs) are the two most important phytohormones involved in the regulation of apical dominance. In this study, the possible involvement of these two hormones in B-deficiency-induced changes in apical dominance was investigated by applying B or the synthetic CK CPPU to the shoot apex of pea plants grown in nutrient solution without B supply. Export of IAA out of the shoot apex, as well as the level of IAA, Z/ZR and isopentenyl-adenine/isopentenyl-adenosine (i-Ade/i-Ado) in the shoot apex were assayed. In addition, polar IAA transport capacity was measured in two internodes of different ages using 3H-IAA. In B-deficient plants, both the level of auxin and CKs were reduced, and the export of auxin from the shoot apex was considerably decreased relative to plants well supplied with B. Application of B to the shoot apex restored the endogenous Z/ZR and IAA level to control levels and increased the export of IAA from the shoot apex, as well as the 3H-IAA transport capacity in the newly developed internodes. Further, B application to the shoot apex inhibited lateral bud growth and stimulated lateral root formation, presumably by stimulated polar IAA transport. Applying CPPU to the shoot apex, a treatment that stimulates IAA export under adequate B supply, considerably reduced the endogenous Z/ZR concentration in the shoot apex, but had no stimulatory effect on IAA concentration and transport in B-deficient plants. A similar situation appeared to exist in lateral buds of B-deficient plants as, in contrast to plants well supplied with B, application of CKs to these plants did not stimulate lateral bud growth. In contrast to the changes of Z/ZR levels in the shoot apex, which occurred after application of B or CPPU, the levels of i-Ade/i-Ado stayed more or less constant. These results suggest that there is a complex interaction between B supply and plant hormones, with a B-deficiency-induced inhibition of IAA export from the shoot apex as one of the earliest measurable events.

Use of phenylboronic acids to investigate boron function in plants. Possible role of boron in transvacuolar cytoplasmic strands and cell-to-wall adhesion

The only defined physiological role of boron in plants is as a cross-linking molecule involving reversible covalent bonds with cis-diols on either side of borate. Boronic acids, which form the same reversible bonds with cis-diols but cannot cross-link two molecules, were used to selectively disrupt boron function in plants. In cultured tobacco (Nicotiana tabacum cv BY-2) cells, addition of boronic acids caused the disruption of cytoplasmic strands and cell-to-cell wall detachment. The effect of the boronic acids could be relieved by the addition of boron-complexing sugars and was proportional to the boronic acid-binding strength of the sugar. Experiments with germinating petunia (Petunia hybrida) pollen and boronate-affinity chromatography showed that boronic acids and boron compete for the same binding sites. The boronic acids appear to specifically disrupt or prevent borate-dependent cross-links important for the structural integrity of the cell, including the organization of transvacuolar cytoplasmic strands. Boron likely plays a structural role in the plant cytoskeleton. We conclude that boronic acids can be used to rapidly and reversibly induce boron deficiency-like responses and therefore are useful tools for investigating boron function in plants.

Boron deficiency and concentrations and composition of phenolic compounds in Olea europaea leaves: a combined growth chamber and field study

Boron deficiency is the most frequent micronutrient disorder in olive (Olea spp.) orchards. We tested the hypothesis that plant boron status affects phenolic metabolism, which, in turn, influences several ecophysiological traits of olive (Olea europaea L.) trees, by studying the effects of boron deficiency on leaf phenolic compounds of olive in a growth chamber experiment (CE) and a field experiment (FE). In the CE, a semi-hydroponic system was used to control nutrient supply. Plants received complete nutrient solution containing either 23 (control) or 0 microM H3BO3 (boron-deficient treatment). In the FE, boron-deficient trees were chosen based on visible boron-deficiency symptoms and analysis of their leaf boron concentration. Boron deficiency caused significant accumulation of phenolic compounds in leaves of CE plants (1.7 to 5.8 times more, depending on leaf age), but not in leaves of FE plants. However, in both experiments, the concentration of an unidentified phenolic compound, with a UV-spectrum resembling that of caffeic acid, increased in response to boron deficiency (by a factor of 40 to 184 in the CE and by a factor of three in the FE). Regression analysis showed that the concentration of this compound was negatively correlated to leaf boron concentration, irrespective of growth conditions and treatment. We conclude that, under field conditions, boron deficiency may not be the only factor determining the concentration of total phenolics, but it may be responsible for the accumulation of a distinct phenolic metabolite in olive leaves.

Proposed physiologic functions of boron in plants pertinent to animal and human metabolism

Boron has been recognized since 1923 as an essential micronutrient element for higher plants. Over the years, many roles for boron in plants have been proposed, including functions in sugar transport, cell wall synthesis and lignification, cell wall structure, carbohydrate metabolism, RNA metabolism, respiration, indole acetic acid metabolism, phenol metabolism and membrane transport. However, the mechanism of boron involvement in each case remains unclear. Recent work has focused on two major plant-cell components: cell walls and membranes. In both, boron could play a structural role by bridging hydroxyl groups. In membranes, it could also be involved in ion transport and redox reactions by stimulating enzymes like nicotinamide adenine dinucleotide and reduced (NADH) oxidase. There is a very narrow window between the levels of boron required by and toxic to plants. The mechanisms of boron toxicity are also unknown. In nitrogen-fixing leguminous plants, foliarly applied boron causes up to a 1000% increase in the concentration of allantoic acid in leaves. In vitro studies show that boron inhibits the manganese-dependent allantoate amidohydrolase, and foliar application of manganese prior to application of boron eliminates allantoic acid accumulation in leaves. Interaction between borate and divalent cations like manganese may alter metabolic pathways, which could explain why higher concentrations of boron can be toxic to plants.

The significance of dietary boron, with particular reference to athletes

Ergogenic substances and synthetic steroids have a wide spread use, particularly among non-professional athletes. To avoid the side-effect of drug abuse, it is suggested that the key to success is a proper athletic nutrition. It is a balanced intake of nutritional wholesome foods that contain a proper blend of essential nutrients. Knowledge of human physiology and nutrition has increased greatly, and so has application of dietary alterations and supplementation with specific nutrients. Modulation of dietary composition and/or supplementation with specific nutrients with the intent of improving human physical performance is a working definition of nutritional ergogenic aids. Boron is a trace element nutrient, and recently its supplements have been shown to increase the concentration of plasma steroid hormones. In a single blind cross-over trial, it resulted in a significant increase in plasma 17-B estradiol (E2) concentration (P < 0.004) and there was a trend for plasma testosterone (T) levels to be increased. The ratio of E2/T increased significantly. However, there was no perturbation in plasma lipids. Furthermore, the effect of boron on steroidogenesis and its mechanism was also investigated in two more studies conducted on adult male rats. The elevation of endogenous steroid hormones as a result of boron supplementation suggest that boron may be used as an ergogenic safe substance for athletes which should be further investigated.

Relation of boron to the composition and mechanical properties of bone

A review of the experimental studies relating boron to biological effects on appendicular and axial bones in animal models suggests that numerous influences, known and unknown, affect the responsiveness of bone to dietary boron. Degrees of skeletal response to boron are modified by other nutritional variables that include calcium, magnesium, vitamin D, and fluoride. Evidence suggests that appendicular and axial bones may differ in their responses. Tests of the mechanical properties of bones may provide useful criteria for assessing the impacts of boron status on bone. These tests might resolve questions about optimal intakes of boron because mechanical properties sometimes respond to boron when composition of bones does not. Difficulty in interpreting some of the existing research arises because of the incipient state of knowledge regarding boron nutriture, to analytical problems associated with determining accurately the small quantities of boron in feed and tissues, and to technological difficulties in controlling extraneous exposure of experimental animals to boron. Yet there is considerable evidence that both compositional and functional properties of bone are affected by boron status.

Effects of dietary boron in rats fed a vitamin D-deficient diet

Although boron has long been known to be a required nutrient for plants, it was not until recently that there was any suggestion of a nutritional requirement for animals and humans. Addition of boron to the diet of vitamin D-deficient chicks indicated that boron may play a role in animal nutrition. Studies with rats have demonstrated that supplemental dietary boron has most marked effects when the diet is deficient in known nutrients. We observed higher apparent-balance values of calcium, magnesium, and phosphorus for rats fed a vitamin D-deprived diet with dietary supplemental boron (2.72 ppm), than for rats fed the same diet without added boron (0.16 ppm). The treatment group with dietary supplemental boron demonstrated a high degree of variability in response to boron. We hypothesize that relatively large and variable vitamin D stores in weanling rats from a colony supplemented with 3000 IU vitamin D/kg diet accounted for the observed variable response. A recent, unpublished study using weanling rats from a low-vitamin D colony appears to support this hypothesis.