Positive association between serum silicon levels and bone mineral density in female rats following oral silicon supplementation with monomethylsilanetriol.

UNLABELLED: Observational (epidemiological) studies suggest the positive association between dietary silicon intake and bone mineral density may be mediated by circulating estradiol level. Here, we report the results of a silicon supplementation study in rats that strongly support these observations and suggest an interaction between silicon and estradiol. INTRODUCTION: Epidemiological studies report strong positive associations between dietary silicon (Si) intake and bone mineral density (BMD) in premenopausal women and indicate that the association may be mediated by estradiol. We have tested this possibility in a mixed-gender rodent intervention study. METHODS: Tissue samples were obtained from three groups of 20-week-old Sprague Dawley rats (five males and five females per group) that had been supplemented ad libitum for 90 days in their drinking water with (i) <0.1 mg Si/L (vehicle control), (ii) 115 mg Si/L (moderate dose) or (iii) 575 mg Si/L (high dose). All rats received conventional laboratory feed, whilst supplemental Si was in the form of monomethylsilanetriol, increasing dietary Si intakes by 18 and 99 %, for the moderate- and high-dose groups, respectively. RESULTS: Fasting serum and tissue Si concentrations were increased with Si supplementation (p < 0.05), regardless of gender. However, only for female rats was there (i) a trend for a dose-responsive increase in serum osteocalcin concentration with Si intervention and (ii) strong significant associations between serum Si concentrations and measures of bone quality (p < 0.01). Correlations were weaker or insignificant for tibia Si levels and absent for other serum or tibia elemental concentrations and bone quality measures. CONCLUSIONS: Our findings support the epidemiological observations that dietary Si positively impacts BMD in younger females, and this may be due to a Si-estradiol interaction. Moreover, these data suggest that the Si effect is mediated systemically, rather than through its incorporation into bone.

Silicon and bone health.

Low bone mass (osteoporosis) is a silent epidemic of the 21st century, which presently in the UK results in over 200,000 fractures annually at a cost of over one billion pounds. Figures are set to increase worldwide. Understanding the factors which affect bone metabolism is thus of primary importance in order to establish preventative measures or treatments for this condition. Nutrition is an important determinant of bone health, but the effects of the individual nutrients and minerals, other than calcium, is little understood. Accumulating evidence over the last 30 years strongly suggest that dietary silicon is beneficial to bone and connective tissue health and we recently reported strong positive associations between dietary Si intake and bone mineral density in US and UK cohorts. The exact biological role(s) of silicon in bone health is still not clear, although a number of possible mechanisms have been suggested, including the synthesis of collagen and/or its stabilization, and matrix mineralization. This review gives an overview of this naturally occurring dietary element, its metabolism and the evidence of its potential role in bone health.

Moderate alcohol consumption and increased bone mineral density: potential ethanol and non-ethanol mechanisms.

Mounting epidemiological evidence indicates an association between the moderate ingestion of alcoholic beverages and higher bone mineral density (v. abstainers). More limited findings provide some evidence for translation of this association into reduced fracture risk, but further studies are required. Here, these data are reviewed and caveats in their assimilation, comparison and interpretation as well as in the use and application of bone health indices are discussed. Whilst it is concluded that evidence is now strong for the moderate alcohol-bone health association, at least in relation to bone mineral density, mechanisms are less clear. Both ethanol and non-ethanol components have been implicated as factors that positively affect bone health in the light of moderate consumption of alcoholic beverages, and four particular areas are discussed. First, recent findings suggest that moderate ethanol consumption acutely inhibits bone resorption, in a non-parathyroid hormone- and non-calcitonin-dependent fashion, which can only partly be attributed to an energy effect. Second, critical review of the literature does not support a role for moderate ethanol consumption affecting oestrogen status and leading to a knock-on effect on bone. Third, Si is present at high levels in certain alcoholic beverages, especially beer, and may have a measurable role in promoting bone formation. Fourth, a large body of work indicates that phytochemicals (e.g. polyphenols) from alcoholic beverages could influence bone health, but human data are lacking. With further work it is hoped to be able to model epidemiological observations and provide a clear pathway between the magnitude of association and the relative contribution of these mechanisms for the major classes of alcoholic beverage.

Effect of humic acid on water chemistry, bioavailability and toxicity of aluminium in the freshwater snail, Lymnaea stagnalis, at neutral pH.

The influence of humic acid on the water chemistry of environmentally relevant concentrations of Al at neutral pH was studied, together with its effect on the bioavailability and toxicity of Al in Lymnaea stagnalis. Humic acid significantly reduced the loss of Al from the water and increased the fraction of filterable Al, although this was a relatively small fraction of total Al. Filterable Al concentration in the presence or absence of humic acid was independent of initial Al concentration. Humic acid only partly reduced toxicity, as observed by a reduction in behavioural suppression, and had no effect on the level of Al accumulated in tissues. These results suggest that humic acid maintains Al in a colloidal form that is bioavailable to L. stagnalis. However, these colloidal Al-humic acid species were less toxic since behavioural toxicity was reduced. Humic acid may play an important role in limiting the toxicity of Al to freshwater organisms.

A provisional database for the silicon content of foods in the United Kingdom.

Si may play an important role in bone formation and connective tissue metabolism. Although biological interest in this element has recently increased, limited literature exists on the Si content of foods. To further our knowledge and understanding of the relationship between dietary Si and human health, a reliable food composition database, relevant for the UK population, is required. A total of 207 foods and beverages, commonly consumed in the UK, were analysed for Si content. Composite samples were analysed using inductively coupled plasma-optical emission spectrometry following microwave-assisted digestion with nitric acid and H(2)O(2). The highest concentrations of Si were found in cereals and cereal products, especially less refined cereals and oat-based products. Fruit and vegetables were highly variable sources of Si with substantial amounts present in Kenyan beans, French beans, runner beans, spinach, dried fruit, bananas and red lentils, but undetectable amounts in tomatoes, oranges and onions. Of the beverages, beer, a macerated whole-grain cereal product, contained the greatest level of Si, whilst drinking water was a variable source with some mineral waters relatively high in Si. The present study provides a provisional database for the Si content of UK foods, which will allow the estimation of dietary intakes of Si in the UK population and investigation into the role of dietary Si in human health.

Dietary silicon intake in post-menopausal women.

Si has been suggested as an essential element, and may be important in optimal bone, skin and cardiovascular health. However, there are few estimates of dietary Si intakes in man, especially in a UK population. Following the development of a UK food composition database for Si, the aim of the present study was to investigate dietary intakes of Si amongst healthy women aged over 60 years and to identify important food sources of Si in their diet. Healthy, post-menopausal female subjects (>60 years of age; n 209) were recruited from the general population around Dundee, Scotland as part of an unrelated randomised controlled intervention study where dietary intake was assessed using a self-administered, semi-quantitative food-frequency questionnaire at five time-points over a 2-year period. Food composition data on the Si content of UK foods was used to determine the Si content of food items on the food-frequency questionnaire. Mean Si intake was 18.6 (sd 4.6) mg and did not vary significantly across the 2 years of investigation. Cereals provided the greatest amount of Si in the diet (about 30 %), followed by fruit, beverages (hot, cold and alcoholic beverages combined) and vegetables; together these foods provided over 75 % about Si intake. Si intakes in the UK appear consistent with those reported previously for elderly women in Western populations, but lower than those reported for younger women or for men.

Role of exogenous and endogenous silicon in ameliorating behavioural responses to aluminium in a freshwater snail.

Aluminium accumulation by the freshwater snail Lymnaea stagnalis is correlated with behavioural depression which is ameliorated by addition of orthosilicic acid. We hypothesised that Si is relocated to the digestive gland in response to Al, leading to the formation of non-toxic hydroxyaluminosilicates (HAS). Exposure to 500 microg l(-1) Al for 30 days was associated with an initial period of behavioural depression, followed by apparent tolerance and subsequent depression, suggesting saturation of the cellular detoxification pathway during prolonged exposure. Exogenous Si (7.77 mg l(-1)) completely ameliorated all behavioural effects of Al but did not prevent its accumulation. In the presence of added Al, significantly more of this Si was accumulated by the tissues, compared to controls and snails exposed to Si alone. In snails exposed to Al plus Si, Al and Si concentrations were significantly correlated, with a ratio around 3:1 Al:Si, consistent with the presence of the non-toxic HAS protoimogolite.

Orthosilicic acid stimulates collagen type 1 synthesis and osteoblastic differentiation in human osteoblast-like cells in vitro.

Silicon deficiency in animals leads to bone defects. This element may therefore play an important role in bone metabolism. Silicon is absorbed from the diet as orthosilicic acid and concentrations in plasma are 5-20 microM. The in vitro effects of orthosilicic acid (0-50 microM) on collagen type 1 synthesis was investigated using the human osteosarcoma cell line (MG-63), primary osteoblast-like cells derived from human bone marrow stromal cells, and an immortalized human early osteoblastic cell line (HCC1). Collagen type 1 mRNA expression and prolyl hydroxylase activity were also determined in the MG-63 cells. Alkaline phosphatase and osteocalcin (osteoblastic differentiation) were assessed both at the protein and the mRNA level in MG-63 cells treated with orthosilicic acid. Collagen type 1 synthesis increased in all treated cells at orthosilicic acid concentrations of 10 and 20 microM, although the effects were more marked in the clonal cell lines (MG-63, HCCl 1.75- and 1.8-fold, respectively, P < 0.001, compared to 1.45-fold in the primary cell lines). Treatment at 50 microM resulted in a smaller increase in collagen type 1 synthesis (MG-63 1.45-fold, P = 0.004). The effect of orthosilicic acid was abolished in the presence of prolyl hydroxylase inhibitors. No change in collagen type 1 mRNA level was seen in treated MG-63 cells. Alkaline phosphatase activity and osteocalcin were significantly increased (1.5, 1.2-fold at concentrations of 10 and 20 microM, respectively, P < 0.05). Gene expression of alkaline phosphatase and osteocalcin also increased significantly following treatment. In conclusion, orthosilicic acid at physiological concentrations stimulates collagen type 1 synthesis in human osteoblast-like cells and enhances osteoblastic differentiation.

Bioavailability and toxicity of freshly neutralized aluminium to the freshwater crayfish Pacifastacus leniusculus.

Freshly neutralized aluminium (Al) is toxic to a variety of freshwater organisms despite its insolubility at circumneutral pH. Insoluble Al acts exogenously--for example, on the fish gill--thereby impairing respiratory function, and endogenously in grazing and filter-feeding invertebrates following ingestion during drinking and feeding. This paper examines the bioavailability and behavioral toxicity of freshly neutralized Al to the freshwater crayfish Pacifastacus leniusculus exposed to 500 microg L(-1) added Al for 20 days under controlled conditions. We test the hypothesis that aqueous Al is toxic to the crayfish and that this is largely due to the metal's association with the gill rather than following accumulation in the body. Little Al was accumulated in the digestive gland (hepatopancreas) or flexor muscle, but large amounts were associated with the gills, resulting in concentration factors of up to 1 x 10(4). Histochemistry showed that much of this metal was extracellular to the gill epithelium and associated with the mucus layer. Behavioral dysfunction was observed following exposure to Al for five days. Reduction in the amount of Al in the water column, due to binding to snail trail mucus attached to the substrate, reduced the amount of Al associated with the gill and delayed the onset of behavioral dysfunction. We conclude that freshly neutralized Al is toxic to the crayfish and that main site of Al action is the gill.

Influence of oligomeric silicic and humic acids on aluminum accumulation in a freshwater grazing invertebrate.

This study examined the influence of oligomeric silicic acid and humic acid on aluminum in the water column and its accumulation in the freshwater snail Lymnaea stagnalis. Forty-eight hours after addition of Al (500 microg L(-1)), 83% of the metal was lost from the water column. This loss was reduced by oligomeric silica (20 mg L(-1)) and by humic acid (10 mg L(-1)). Aluminum accumulated in the digestive gland and, to a lesser extent, in the remaining soft tissues, and this accumulation was reduced by oligomeric silica. In the presence of humic acid, Al accumulation in the digestive gland was unaffected, though less was accumulated in the remaining tissues. Snails accumulated Si preferentially in the digestive gland and this accumulation was increased in the presence of added Al. Thus, both oligomeric silica and humic acid influence Al bioavailability and Si is upregulated in the digestive gland in the presence of Al.

Rapid non-equilibrium aluminium-ligand interactions: studies on the precipitation of aluminium by laser light scattering, ultrafiltration and centrifugation.

The study aimed to develop simple assays to study aluminium-ligand interactions in natural/biological systems where equilibrium is rarely reached and thus where the initial seconds or hours of interactions are important. The immediate and non-equilibrium precipitation of aluminium hydroxide, in aqueous solution at neutral pH, was therefore studied by laser light scattering (diffraction), ultrafiltration and centrifugation. The interaction of weak ligands, present in the gastrointestinal lumen, on the precipitation of aluminium hydroxide was also investigated. The initial kinetics and particle sizes of precipitated aluminium hydroxide were sensitive to a number of external factors, including the presence of weak ligand (bicarbonate), sheer force (stirring), electrolyte concentration and initial (i.e. added) aluminium concentration. However, after a few seconds (no weak ligand), or several hundred seconds (with weak ligand), the subsequent observed changes to the solid phase were of small magnitude and occurred slowly. Thus, a 25-min window, within 5 and 30 min of pH adjustment, can be used to study the interactions of aluminium-ligand. This may approximate better to most natural systems where unperturbed aluminium-ligand equilibrium must rarely exist.

Avoidance responses to aluminium in the freshwater bivalve Anodonta cygnea.

This study examined the effect of aluminium (Al) on the filtering behaviour (shell opening or gape) of the freshwater bivalve Anodonta cygnea L in neutral fresh water. Parallel measurements of Al concentration in the soft tissues were made to examine the relationship between changes in behaviour and accumulation of Al. The number of lysosomal granules in the gill, kidney and digestive gland were counted, as lysosomes are known to be involved in the excretion and detoxification of trace metals. The bivalves were exposed to two environmentally relevant concentrations of added Al i.e. 250 and 500 microg l(-1) (9.25 and 18.5 microM l(-1)) at neutral pH for 15 days and shell movement monitored continuously. Aluminium affected the mussels' filtering activity, producing an avoidance reaction whose magnitude was concentration-dependent; 250 microg l(-1) added Al produced no detectable change, while 500 microg l(-1) Al reduced mean duration of shell opening by 50%. This effect was irreversible over a 15 day recovery period. Tissue levels of Al after 15 days exposure were an order of magnitude higher in animals exposed to 250 microg l(-1) added Al than in those exposed to 500 microg l(-1). This was consistent with the inhibition of filtering activity due to valve closure at the higher concentration, which may have prevented uptake of Al. In addition, probable different chemical speciation of Al in the water column (soluble for 250 and colloidal for 500 microg l(-1)) may lead to marked differences in tissue uptake. The kidney and digestive gland were the main sites of accumulation of Al and concentrations remained significantly elevated 15 days after transfer of animals to clean water. It is suggested that mucus plays a role in the exclusion of Al as elevated concentrations were measured in the pseudofaeces of animals during and after exposure. Lysosomal granules may be involved in the intracellular handling and detoxification of Al as numbers increased significantly in all organs during exposure and continued to increase after the animals were transferred to clean water. The present study provides evidence for the bioavailability and toxicity of Al to mussels at neutral pH and at concentrations which are known to enter neutral freshwaters when mobilised by natural or anthropogenic acidity. The changes in behaviour and uptake of Al in the mussel observed in this investigation are, therefore, likely to be reflected in the natural environment and the degree to which Al affects the 'fitness' of the mussel populations and the transfer of Al through the food chain merit investigation.

Bioaccumulation and toxicity of aluminium in the pond snail at neutral pH.

The low solubility of aluminium (Al) at neutral pH means that it largely exists as colloidal particulates in aquatic systems. However, the pond snail Lymnaea stagnalis accumulates significant amounts of Al following exposure to water containing added Al (up to 500 microg l(-1)) at pH 7. This is accompanied by depression of behavioural activity (locomotion, feeding) which subsequently recovers, suggesting tolerance to the metal. The presence of silica ameliorates behavioural toxicity of Al, but does not prevent uptake of the metal. In vitro studies using the isolated central nervous system demonstrate toxicity at the cellular level. Extracellular application of Al (100 microM) led to membrane depolarisation, bursts of action potentials and action potential broadening. The chemical form in which Al is applied influences the extent of bioaccumulation and toxicity. Detailed knowledge of its solution chemistry is therefore essential.

Oligomeric but not monomeric silica prevents aluminum absorption in humans.

BACKGROUND: Soluble silica, a ubiquitous component of the diet, may be the natural ligand for dietary aluminum and may prevent its accumulation and toxicity in animals. However, previous studies on the inhibition of aluminum absorption and toxicity by soluble silica have produced conflicting results. We recently identified a soluble silica polymer, oligomeric silica, that has a much higher affinity for aluminum than does monomeric silica and that may be involved in the sequestration of aluminum. OBJECTIVE: By using (26)Al as a tracer, we investigated the effects of oligomeric and monomeric silica on the bioavailability of aluminum (study 1) and compared the availability of silicon from oligomeric and monomeric silica in the human gastrointestinal tract (study 2). DESIGN: In study 1, three healthy volunteers each ingested aluminum alone (control), aluminum with oligomeric silica (17 mg), and aluminum with monomeric silica (17 mg). In study 2, five healthy volunteers ingested both the oligomeric and monomeric forms of silica (34 mg). Serum and urine samples were analyzed for aluminum and silicon. RESULTS: Oligomeric silica reduced the availability of aluminum by 67% (P = 0.01) compared with the control, whereas monomeric silica had no effect (P = 0.40). Monomeric silica was readily taken up from the gastrointestinal tract and then excreted in urine (53%), whereas oligomeric silica was not detectably absorbed or excreted. CONCLUSIONS: The oligomeric, high-aluminum-affinity form of soluble silica reduces aluminum availability from the human gastrointestinal tract. Its potential role in the amelioration of aluminum toxicity in other biological systems requires attention.

Aluminum toxicity in a molluscan neuron: effects of counterions.

Previous studies using the freshwater snail Lymnaea stagnalis have indicated significant accumulation of aluminum (Al) from simple salts (chloride or nitrate) or Al lactate [Al(lactate)3 preparations, but not from the Al maltol complex [Al(maltol)3]. This is in contrast to findings in mammalian systems, where uptake and neurotoxicity are greatest for the soluble and lipophilic Al(maltol)3 complex. This study was undertaken to investigate the direct effects of extracellular Al (100 microM) from three Al preparations [AlCl3, Al(lactate)3 and Al(maltol)3] on electrophysiological parameters of an identified neuron, the right parietal dorsal 1 (RPD1) neuron, of L. stagnalis in vitro. The effects of the corresponding counterion/ligand on the solubility and availability of Al in solution were also examined. Significant effects of Al on electrical properties, including membrane depolarization, increased firing activity, and abnormal firing patterns, were seen in the presence of AlCl3 and Al(lactate)3, which formed polyhydroxy and labile Al species in aqueous solution, but not with Al(maltol)3, which remained as the soluble monomeric complex. Qualitative differences were also observed between the response to AlCl3 and Al(lactate)3, despite their similar chemistry. The extent of action potential broadening was greater with Al(lactate)3, suggesting some interaction between Al and lactate in their cellular uptake and/or toxicity. It is suggested that polyhydroxy Al species are toxic to molluscan neurons, possibly via disruption of intracellular Ca2+ homeostasis.

Silicic acid: its gastrointestinal uptake and urinary excretion in man and effects on aluminium excretion.

Silicon (Si), as silicic acid, is suggested to be the natural antidote to aluminium (Al) toxicity, and was recently shown to promote the urinary excretion of Al from body stores. The metabolism of Si in man, however, remains poorly investigated. Here we report on the pharmacokinetics and metabolism of Si in healthy volunteers following ingestion of orthosilicic acid (27-55 mg/l Si) in water. We also investigated whether orthosilicic acid promotes the urinary excretion of endogenous Al. Minimum, median uptake of Si from the ingested dose was 50.3% (range: 21.9-74.7%, n = 8) based on urinary analysis following dosing. Significant correlations were observed between creatinine clearance and Si levels in serum or urine (r = 0.95 and 0.99, respectively). Renal clearance of Si was 82-96 ml/min suggesting high renal filterability. These results suggest that orthosilicic acid is readily absorbed from the gastrointestinal tract of man and then readily excreted in urine. There was no significant increase in Al excretion, over 32 h, following ingestion of the orthosilicic acid dose (P = 0.5; n = 5).

The regulation of mineral absorption in the gastrointestinal tract.

The absorption of metal ions in the mammalian single-stomached gut is fortunately highly selective, and both luminal and tissue regulation occur. Initially, assimilation of metal ions in an available form is facilitated by the intestinal secretions, chiefly soluble mucus (mucin) that retards hydrolysis of ions such as Cu, Fe and Zn. Metal ions then bind and traverse the mucosally-adherent mucus layer with an efficiency M+ > M2+ > M3+. At the mucosa Fe3+ is probably uniquely reduced to Fe2+, and all divalent cations (including Fe2+) are transported by a membrane protein (such as divalent cation transporter 1) into the cell. This minimizes absorption of toxic trivalent metals (e.g. Al3+). Intracellular metal-binding molecules (such as mobilferrin) may be present at the intracellular side of the apical membrane, anchored to a transmembrane protein such as an integrin complex. This mobilferrin would receive the metal ion from divalent cation transporter 1 and, with part of the integrin molecule, transport the metal to the cytosol for safe sequestration in a larger complex such as ferritin or 'paraferritin'. beta 2-Microglobulin and HFE (previously termed human leucocyte antigen H) may be involved in stabilizing metal mobilferrin-integrin to form this latter complex. Finally, a systemic metal-binding protein such as transferrin may enter the antiluminal (basolateral) side of the cell for binding of the sequestered metal ion and delivery to the circulation. Regulatory proteins, such as HFE, may determine the degree of ion transport from intestinal cells to the circulation. Gradients in pH and perhaps pCa or even pNa could allow the switching of ions between the different transporters throughout this mechanism.