Role of vitamin D-dependent and vitamin D-independent mechanisms in absorption of food calcium

Dietary Calcium Intake and Association with Serum Calcium in Healthy Urban North Indian Adults: The Calcium-Chandigarh Urban Bone Epidemiological Study

Introduction

Data on dietary calcium intake (DCI) from healthy North Indian adults are limited. Hence, the present study aims to assess DCI in healthy community-dwelling adults residing in an affluent North Indian city and correlate with serum biochemical parameters.

Methods

Healthy men and women were recruited from the community by door-to-door surveys. Serum total calcium, phosphate, 25-hydroxyvitamin D, and iPTH were estimated. DCI was assessed by recalling the diet consumed in the previous 7 days.

Results

A total of 291 participants were included (mean age = 39.4 ± 12.9 years). The mean (± standard deviation) and median (inter-quartile range) DCI were 392.6 ± 169.1 mg/day and 391 (274-518) mg/day, respectively. DCI was higher in men compared with women. Only 21.3% of participants had DCI more than RDA (600 mg/day). Serum calcium was found to be significantly higher across each quartile of DCI. On multiple linear regression analysis, DCI emerged as an independent positive predictor of serum total calcium.

Conclusion

DCI is low in urban Indians. Promotion of consumption of dairy products and fortification of commonly consumed foods with calcium is needed.

Vitamin D-Mediated Regulation of Intestinal Calcium Absorption

Vitamin D is a critical regulator of calcium and bone homeostasis. While vitamin D has multiple effects on bone and calcium metabolism, the regulation of intestinal calcium (Ca) absorption efficiency is a critical function for vitamin D. This is necessary for optimal bone mineralization during growth, the protection of bone in adults, and the prevention of osteoporosis. Intestinal Ca absorption is regulated by 1,25 dihydroxyvitamin D (1,25(OH)2 D), a hormone that activates gene transcription following binding to the intestinal vitamin D receptor (VDR). When dietary Ca intake is low, Ca absorption follows a vitamin-D-regulated, saturable pathway, but when dietary Ca intake is high, Ca absorption is predominately through a paracellular diffusion pathway. Deletion of genes that mediate vitamin D action (i.e., VDR) or production (CYP27B1) eliminates basal Ca absorption and prevents the adaptation of mice to low-Ca diets. Various physiologic or disease states modify vitamin-D-regulated intestinal absorption of Ca (enhanced during late pregnancy, reduced due to menopause and aging).

Chronic Kidney Disease-Mineral Bone Disease biomarkers in kidney transplant patients

Background:

Chronic Kidney Disease associated with Mineral Bone Disease (CKD-MBD) is frequent in kidney transplant patients. Post-transplantation bone disease is complex, especially in patients with pre-existing metabolic bone disorders that are further affected by immunosuppressive medications and changes in renal allograft function. Main biochemical abnormalities of mineral metabolism in kidney transplantation (KTx) include hypophosphatemia, hyperparathyroidism (HPTH), insufficiency or deficiency of vitamin D, and hypercalcemia.

Objective:

This review aimed to summarize the pathophysiology and main biomarkers of CKD-MBD in KTx.

Methods:

A comprehensive and non-systematic search in PubMed was independently made with an emphasis on biomarkers in mineral bone disease in KTx.

Results:

CKD-MBD can be associated with numerous factors including secondary HPTH, metabolic dysregulations before KTx, and glucocorticoids therapy in post-transplant subjects. Fibroblast growth factor 23 (FGF23) reaches normal levels after KTx with good allograft function, while calcium, vitamin D and phosphorus, ultimately, result in hypercalcemia, persistent vitamin D insufficiency, and hypophosphatemia respectively. As for PTH levels, there is an initial tendency of a significant decrease, followed by a raise due to secondary or tertiary HPTH. In regard to sclerostin levels, there is no consensus in the literature.

Conclusion:

KTx patients should be continuously evaluated for mineral homeostasis and bone status, both cases with successful kidney transplantation and those with reduced functionality. Additional research on CKD-MBD pathophysiology, diagnosis, and management is essential to guarantee long-term graft function, better prognosis, good quality of life, and reduced mortality for KTx patients.

Vitamin D and Gut Health

Vitamin D is a conditionally required nutrient that can either be obtained from skin synthesis following UVB exposure from the diet. Once in the body, it is metabolized to produce the endocrine hormone, 1,25 dihydroxyvitamin D (1,25(OH)2D), that regulates gene expression in target tissues by interacting with a ligand-activated transcription factor, the vitamin D receptor (VDR). The first, and most responsive, vitamin D target tissue is the intestine. The classical intestinal role for vitamin D is the control of calcium metabolism through the regulation of intestinal calcium absorption. However, studies clearly show that other functions of the intestine are regulated by the molecular actions of 1,25(OH)2 D that are mediated through the VDR. This includes enhancing gut barrier function, regulation of intestinal stem cells, suppression of colon carcinogenesis, and inhibiting intestinal inflammation. While research demonstrates that there are both classical, calcium-regulating and non-calcium regulating roles for vitamin D in the intestine, the challenge facing biomedical researchers is how to translate these findings in ways that optimize human intestinal health.

Physiology of Calcium Homeostasis: An Overview

Calcium plays a key role in skeletal mineralization and several intracellular and extracellular homeostatic networks. It is an essential element that is only available to the body through dietary sources. Daily acquisition of calcium depends, in addition to the actual intake, on the hormonally regulated state of calcium homeostasis through three main mechanisms: bone turnover, intestinal absorption, and renal reabsorption. These procedures are regulated by a group of interacting circulating hormones and their key receptors. This includes parathyroid hormone (PTH), PTH-related peptide, 1,25-dihydroxyvitamin D, calcitonin, fibroblast growth factor 23, the prevailing calcium concentration itself, the calcium-sensing receptor, as well as local processes in the bones, gut, and kidneys.

Latitude, Vitamin D, Melatonin, and Gut Microbiota Act in Concert to Initiate Multiple Sclerosis: A New Mechanistic Pathway

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS). While the etiology of MS is still largely unknown, scientists believe that the interaction of several endogenous and exogenous factors may be involved in this disease. Epidemiologists have seen an increased prevalence of MS in countries at high latitudes, where the sunlight is limited and where the populations have vitamin D deficiency and high melatonin levels. Although the functions and synthesis of vitamin D and melatonin are contrary to each other, both are involved in the immune system. While melatonin synthesis is affected by light, vitamin D deficiency may be involved in melatonin secretion. On the other hand, vitamin D deficiency reduces intestinal calcium absorption leading to gut stasis and subsequently increasing gut permeability. The latter allows gut microbiota to transfer more endotoxins such as lipopolysaccharides (LPS) into the blood. LPS stimulates the production of inflammatory cytokines within the CNS, especially the pineal gland. This review summarizes the current findings on the correlation between latitude, sunlight and vitamin D, and details their effects on intestinal calcium absorption, gut microbiota and neuroinflammatory mediators in MS. We also propose a new mechanistic pathway for the initiation of MS.

The role of vitamin D in the endocrinology controlling calcium homeostasis

Vitamin D and its' metabolites are a crucial part of the endocrine system that controls whole body calcium homeostasis. The goal of this hormonal control is to regulate serum calcium levels so that they are maintained within a very narrow range. To achieve this goal, regulatory events occur in coordination at multiple tissues, e.g. the intestine, kidney, bone, and parathyroid gland. Production of the vitamin D endocrine hormone, 1,25 dihydroxyvitamin D (1,25(OH)2 D) is regulated by habitual dietary calcium intake and physiologic states like growth, aging, and the menopause. The molecular actions of 1,25(OH)2 D on calcium regulating target tissues are mediated predominantly by transcription controlled by the vitamin D receptor. A primary role for 1,25(OH)2 D during growth is to increase intestinal calcium absorption so that sufficient calcium is available for bone mineralization. However, vitamin D also has specific actions on kidney and bone.

Calcium Balance in Chronic Kidney Disease

PURPOSE OF REVIEW

The kidneys play a critical role in the balance between the internal milieu and external environment. Kidney failure is known to disrupt a number of homeostatic mechanisms that control serum calcium and normal bone metabolism. However, our understanding of calcium balance throughout the stages of chronic kidney disease is limited and the concept of balance itself, especially with a cation as complex as calcium, is often misunderstood. Both negative and positive calcium balance have important implications in patients with chronic kidney disease, where negative balance may increase risk of osteoporosis and fracture and positive balance may increase risk of vascular calcification and cardiovascular events. Here, we examine the state of current knowledge about calcium balance in adults throughout the stages of chronic kidney disease and discuss recommendations for clinical strategies to maintain balance as well as future research needs in this area.

RECENT FINDINGS

Recent calcium balance studies in adult patients with chronic kidney disease show that neutral calcium balance is achieved with calcium intake near the recommended daily allowance. Increases in calcium through diet or supplements cause high positive calcium balance, which may put patients at risk for vascular calcification. However, heterogeneity in calcium balance exists among these patients. Given the available calcium balance data in this population, it appears clinically prudent to aim for recommended calcium intakes around 1000 mg/day to achieve neutral calcium balance and avoid adverse effects of either negative or positive calcium balance. Assessment of patients' dietary calcium intake could further equip clinicians to make individualized recommendations for meeting recommended intakes.

Association of plasma calcium concentrations with alcohol craving: New data on potential pathways

Recently, calcium was suggested to be the active moiety of acamprosate. We examined plasma calcium concentrations in association with severity of alcohol dependence and its interaction with regulating pathways and alcohol craving in alcohol-dependent patients. 47 inpatient alcohol-dependent patients undergoing detoxification treatment underwent laboratory testing, including calcium, sodium, liver enzymes as well as serum concentrations of calcitonin, parathyroid hormone and vitamin D. The psychometric dimension of craving was analyzed with the Obsessive Compulsive Drinking Scale (OCDS). The severity of withdrawal was measured with the Alcohol Dependence Scale (ADS) and with the Alcohol Dependence Scale for high-risk sample (ADS-HR). The main findings of our investigation are: a) a negative correlation of plasma calcium concentrations with alcohol craving in different dimensions of the OCDS; b) a negative correlation of plasma calcium concentrations with breath alcohol concentration; c) lowered calcitonin concentration in the high-risk sample of alcoholics; d) lowered plasma vitamin D concentrations in all alcoholic subjects. Our study adds further support for lowered plasma calcium concentrations in patients with high alcohol intake and especially in patients with increased craving as a risk factor for relapse. Lowered calcitonin concentrations in the high-risk sample and lowered vitamin D concentrations may mediate these effects. Calcium supplementation could be a useful intervention for decreasing craving and relapse in alcohol-dependent subjects.

25-Hydroxyvitamin D level does not reflect intestinal calcium absorption: an assay using strontium as a surrogate marker

There is conflicting evidence as to the optimal serum 25-hydroxyvitamin D [25(OH)D] concentration for intestinal calcium absorption (Abs-Ca). Our purpose was to assess the relationship between vitamin D status and Abs-Ca in postmenopausal women. Fifty volunteers with low bone mass were grouped according to their serum 25(OH)D concentration as follows: mild deficient, <50 nmol/L (DEF) and sufficient, ≥75 nmol/L (SUF). The subjects were submitted to an oral strontium overload test to assess their Abs-Ca. Fasting blood samples were obtained to perform the relevant hormonal and biochemical tests. After the subjects received the test solution, blood samples were drawn at 30, 60, 120, and 240 min to determine the strontium concentrations. Abs-Ca was indirectly expressed as the area under the serum strontium concentration curve (AUC). A repeated measures ANOVA was performed to determine the differences among the groups. Pearson's correlation and multiple linear regression analysis were used to study the associations between the variables. The mean 25(OH)D and 1,25-dihydroxyvitamin D [1,25(OH)2D] concentrations differed between the groups (SUF vs. DEF) as follows: 98.7 ± 18.2 vs. 38.4 ± 8.5 nmol/L (p < 0.001) and 36.2 ± 10.2 vs. 24.9 ± 4.6 pg/mL (p < 0.001), respectively. There was no statistically significant difference between the groups for parathyroid hormone and AUC. Only 1,25(OH)2D influenced the strontium absorption in the last 2 h of the test. In the studied population, no correlation between levels of 25(OH)D and Abs-Ca was found. Only 1,25(OH)2D influenced Abs-Ca as measured by a strontium absorption test.

[Disorders of calcium metabolism]

The majority of clinical complaints derive from disorders of calcium metabolism and are associated with a wide variety of clinical symptoms caused by numerous diseases with entirely different types of pathophysiology. The prognosis varies from favorable to fatal depending on the pathophysiology of the underlying disorder of calcium metabolism; therefore, the diagnostic work-up aims to quickly identify the underlying disease causing the disturbance in calcium homeostasis. Every clinical situation with a diminished state of calcium absorption is treated with calcium and vitamin D in varying doses whereas every disorder with an increased calcium absorptive or resorptive state is treated with improved diuresis in addition to antiresorptive drugs, such as bisphosphonates. In many situations the management of a disturbed calcium balance requires an interdisciplinary approach in order to treat the underlying disease in parallel with correction of the calcium homeostasis.

Mechanisms and regulation of renal magnesium transport

Magnesium's most important role is in the release of chemical energy. Although most magnesium is stored outside of the extracellular fluid compartment, the regulated value is blood magnesium concentration. Cellular magnesium and bone magnesium do not play a major role in the defense of blood magnesium concentration; the major role is played by the kidney, where the renal tubule matches the urinary magnesium excretion and the net entry of magnesium into the extracellular fluid. In the kidney, magnesium is reabsorbed in the proximal tubule, the thick ascending limb of the loop of Henle, and the distal convoluted tubule. Magnesium absorption is mainly paracellular in the proximal tubule and in the thick ascending limb of the loop of Henle, whereas it is transcellular in the distal convoluted tubule. Several hormones and extracellular magnesium itself alter the distal tubular handling of magnesium, but the hormone(s) regulating extracellular magnesium concentration remains unknown.

Vitamin D and aging

Age-related changes affect vitamin D metabolism and increase the requirement for vitamin D in the elderly. Also there is an age related decrease in calcium absorption and a higher calcium intake is needed. Increasing calcium from dietary sources may be better than supplements, and requires increasing the intake of dairy products or other and calcium-fortified foods. Evidence suggests that vitamin D and calcium nutrition can be improved in the elderly by increasing the vitamin D intake to 800 IU daily together with a total calcium intake of 1000 mg daily. This combination is a simple, inexpensive strategy that can reduce fractures in institutionalized individuals by 30%.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Maternal vitamin D status and calcium intake interact to affect fetal skeletal growth in utero in pregnant adolescents

BACKGROUND

Maternal calcium intake and vitamin D status may affect fetal bone development.

OBJECTIVE

This study was designed to examine relations between maternal calcium intake, 25-hydroxyvitamin D [25(OH)D] status, and fetal bone growth across pregnancy.

DESIGN

This was a prospective longitudinal design. Maternal 25(OH)D, parathyroid hormone, and 1,25-dihydroxyvitamin D [1,25(OH)(2)D] were determined at midgestation (∼26 wk) and at delivery in 171 adolescents (≤ 18 y). Dietary recalls and fetal sonograms were performed up to 3 times across gestation, and fetal femur and humerus z scores were generated.

RESULTS

Fetal femur and humerus z scores and neonatal birth length were significantly greater (P < 0.03) in adolescents consuming ≥ 1050 mg than in those consuming <1050 mg Ca/d. Maternal 25(OH)D > 50 nmol/L was significantly positively associated with fetal femur and humerus z scores (P < 0.01). When maternal smoking, height, race, weight gain, and gestational age were controlled for, these relations remained significant. Interactions between calcium intake and 25(OH)D were evident. Calcium intake was associated with fetal femur z scores and birth length only when maternal 25(OH)D was ≤ 50 nmol/L (P < 0.05). Similarly, maternal 25(OH)D was associated with fetal femur and humerus z scores only when maternal calcium intake was <1050 mg/d (P < 0.03).

CONCLUSIONS

Optimal calcium intake and adequate maternal vitamin D status are both needed to maximize fetal bone growth. Interactions between these nutrients were evident when either calcium or vitamin D status was limited. Improving maternal calcium intake and/or vitamin D status during pregnancy may have a positive effect on fetal skeletal development in pregnant adolescents.

A brief review of external mass balance and internal calcium redistribution in dialysis patients--is calcium a uremic toxin?

Recent debates between 2 schools of thought on calcium mass balance in dialysis patients and its relevance to disease--one emphasizing external calcium mass balance, and the other, internal calcium redistribution--have created controversy. Due to decreased ability to excrete calcium and loss of endocrine function by the kidney, patients suffering from chronic kidney disease, particularly when requiring dialysis, demonstrate varying degrees of positive or negative calcium balance, vitamin D deficiency, and secondary hyperparathyroidism. Consequently, patients are prone to bone demineralization, with diminished bone strength, and are thus prone to fractures that substantially worsen morbid outcomes in this population. However, intra- and interdialytic positive calcium mass balance creates complications of a different kind, which include the occurrence of vascular and cardiac disease and reduced survival. This review aims to shed light on the mechanisms of and relationships between external calcium mass balance and internal calcium redistribution and their consequences. It also discusses the potential to improve current regimens by means of diffusive and convective calcium mass transfer for the achievement of neutral calcium mass balance.

Calcium citrate and vitamin D in the treatment of osteoporosis

The combination of calcium with vitamin D (vitamin D(3) [colecalciferol]) forms the basis of preventive and therapeutic regimens for osteoporosis. A number of studies have suggested that the combination of calcium and vitamin D is effective when administered at respective dosages of at least 1200 mg and 800 IU per day, although efficacy is, as expected, affected by patient compliance. Overall, treatment with this combination appears to be effective in reducing the incidence of non-vertebral and hip fractures. Also, in all drug studies (of antiresorptive and anabolic agents and strontium ranelate) that demonstrated a reduction in risk of osteoporotic fractures, patients also took calcium and vitamin D supplements. An important finding in this regard is that vitamin D levels have been demonstrated to be inadequate in more than half of women treated for osteoporosis in the US and Europe. The capacity of the small intestine to absorb calcium salts depends on the solubility and ionization of the salts. These properties vary for different salts, with fasting calcium citrate absorption being greater than that of calcium lactogluconate and calcium carbonate. Calcium citrate formulations taken between meals may help to prevent abdominal distension and flatulence, as well as minimize the risk of renal calculus formation, thus helping to optimize patient compliance. Therefore, calcium citrate combined with vitamin D is the combination of choice for the prevention or treatment of osteoporosis.

Confusion on the complexity of calcium balance

Calcium balance is an overall assessment of the net calcium taken in minus the net calcium taken out. It can only be assessed when patients are in steady state and requires complicated isotope methods that can simultaneously assess intestinal absorption and endogenous secretion, urinary and stool excretion, bone calcium uptake and removal, and dialysate calcium removal. By virtue of the need for steady state, formal balance studies cannot be accurately carried out in patients on dialysis. However, many of the components of calcium balance have been assessed. Importantly, because 99% of calcium is in bone, studies must accurately assess both the rapidly exchangeable calcium from the bone surface and the net bone calcium balance that results from the difference in bone formation minus resorption. While it is tempting to adjust the dialysate calcium concentration to correct the net positive calcium balance that is likely present in patients who receive calcium-based phosphate binders, the reality is that the highly variable, yet important, role of bone cannot be easily assessed at the bedside. Thus, it is best to prevent the calcium overload in the first place by avoiding high-dose calcium-based phosphate binders and optimizing bone remodeling.

Sparkle lamp ingestion: a rare cause of death

A 51-year-old man was brought to the emergency department after he had drunk 200 mL of fluid from a decorative sparkle lamp. His calcium level was 4.99 mmol/L with a blood gas pH of 7.21. His calcium levels were corrected and his acidosis improved, but his stay in the intensive care department was complicated on day 10 by upper gastrointestinal hemorrhage because of caustic erosions in his stomach. Ten days after admission, he had an emergency right hemicolectomy during which residual luminescent particles were found to have eroded through his cecal mucosa. He eventually died 15 days after admission because of disseminated intravascular coagulopathy and refractory septic shock. This case demonstrates the potential for fatal toxicity associated with ingestion of a common household item.

Calcium and iron regulate swarming and type III secretion in Vibrio parahaemolyticus

Here, we probe the response to calcium during growth on a surface and show that calcium influences the transcriptome and stimulates motility and virulence of Vibrio parahaemolyticus. Swarming (but not swimming) gene expression and motility were enhanced by calcium. Calcium also elevated transcription of one of the organism's two type III secretion systems (T3SS1 but not T3SS2) and heightened cytotoxicity toward host cells in coculture. Calcium stimulation of T3SS gene expression has not been reported before, although low calcium is an inducing signal for the T3SS of many organisms. EGTA was also found to increase T3SS1 gene expression and virulence; however, this was demonstrated to be the consequence of iron rather than calcium chelation. Ectopic expression of exsA, encoding the T3SS1 AraC-type regulator, was used to define the extent of the T3SS1 regulon and verify its coincident induction by calcium and EGTA. To begin to understand the regulatory mechanisms modulating the calcium response, a calcium-repressed, LysR-type transcription factor named CalR was identified and shown to repress swarming and T3SS1 gene expression. Swarming and T3SS1 gene expression were also demonstrated to be linked by LafK, a σ(54)-dependent regulator of swarming, and additionally connected by a negative-feedback loop on the swarming regulon propagated by ExsA. Thus, calcium and iron, two ions pertinent for a marine organism and pathogen, play a signaling role with global consequences on the regulation of gene sets that are relevant for surface colonization and infection.

Molecular mechanisms for regulation of intestinal calcium absorption by vitamin D and other factors

Optimal intestinal calcium (Ca) absorption is necessary for the protection of bone and the prevention of osteoporosis. Ca absorption can be represented as the sum of a saturable pathway and a non-saturable pathway that is primarily dependent upon luminal Ca concentration. While models have been proposed to describe these transport components, significant gaps still exist in our understanding of these processes. Habitual low intake of Ca up-regulates the saturable transport pathway, a process mediated by increased renal production of 1,25 dihydroxyvitamin D (1,25(OH)(2)D). Consistent with this, low vitamin D status as well as deletion/mutation of the vitamin D receptor (VDR) or 25 hydroxyvitamin D-1α hydroxylase (CYP27B1) genes limit Ca absorption by reducing the saturable pathway. There is some evidence that non-saturable Ca absorption in the ileum is also regulated by vitamin D status, but the mechanism is unclear. Treatment with a number of hormones can regulate Ca absorption in vivo (e.g. parathyroid hormone (PTH), thyroid hormone, growth hormone (GH)/insulin-like growth factor I (IGF-1), estrogen, testosterone). However, some of these actions are indirect (i.e. mediated through the regulation of vitamin D metabolism or signaling), whereas only a few (e.g. estrogen, IGF-1) have been shown to persist in the absence of vitamin D signaling.

The KDIGO guideline for dialysate calcium will result in an increased incidence of calcium accumulation in hemodialysis patients

The recently published KDIGO (Kidney Disease: Improvement of Global Outcomes) guideline (GL) for dialysate calcium suggests a narrow range of dialysate inlet calcium concentrations (C(di)Ca(++)) of 2.50-3.00 mEq/l. The work group's primary arguments supporting the GL were (1) there is a negligible flux of body Ca(++) during dialysis and (2) C(di)Ca(++) of 2.50 mEq/l will generally result in neutral Ca(++) mass balance (Ca(MB)). We believe we have shown that both of these arguments are incorrect. Kinetic modeling and analysis of dialyzer Ca(++) transport during dialysis (J(d)Ca(++)) demonstrates that more than 500 mg of Ca can be transferred during a single dialysis and that on average 76% of this Ca flux is from the miscible calcium pool rather than plasma pool. Kinetic modeling of intestinal calcium absorption (Ca(Abs)) shows a strong dependence of Ca(Abs) on the dose of vitamin D analogs and weaker dependence on the level of Ca intake (Ca(INT)). We used the Ca(Abs) model to calculate Ca(Abs) as a function of total Ca(INT) and prescribed doses of vitamin D analogs in 320 hemodialysis patients. We then calculated total dialyzer calcium removal (TJ(d)Ca(++)) and the C(di)Ca(++) that would be required to achieve TJ(d)Ca(++)=Ca(Abs), that is, Ca(MB)=0 over the whole dialysis cycle (that is, covering both the intra- and the inter-dialytic period). The results indicate that 70% of patients on Ca-based binders and 20-50% of patients on non-Ca-based binders would require C(di)Ca(++) <2.50 mEq/l to prevent long-term Ca accumulation.

Update on the pharmacogenetics of the vitamin D receptor and osteoporosis

Vitamin D and calcium are essential for normal skeletal growth and for maintaining the mechanical and structural integrity of the skeleton. Reduced intake of calcium and vitamin D may be associated with reduced bone mass and osteoporosis while a chronic and severe vitamin D deficiency may lead to osteomalacia. Given the importance of vitamin D in bone homeostasis, common polymorphisms in the vitamin D receptor gene were the first to be investigated as possible determinants of bone mass and fracture risk. Even though results are still conflicting and the molecular mechanisms by which these polymorphisms influence receptor activity remain in part to be investigated, an additional important issue is represented by their potential pharmacogenomic and pharmacogenetic implications. This review analyzes major pharmacogenetic studies of polymorphisms in the vitamin D receptor gene and osteoporosis.

Phosphate is a uremic toxin

Hyperphosphatemia is one of the more prevalent metabolic disturbances in kidney failure. Phosphate can be considered a uremic toxin based on the accumulation of phosphate during chronic kidney disease, the effects of phosphate on biological systems, and the adverse effects of hyperphosphatemia. The renal clearance of phosphate is maintained until later stages of chronic kidney disease, when the remaining nephrons are no longer able to excrete sufficient phosphate to offset dietary phosphate absorption. Clearance of phosphate by conventional forms of dialysis is insufficient to prevent hyperphosphatemia in most endstage kidney-disease patients. Phosphate contributes to metabolic disturbances such as hyperparathyroidism, vitamin D resistance, and hypocalemia. In combination with these and other factors, hyperphosphatemia damages many organs, including the parathyroid glands, bones, and most importantly the cardiovascular system. Elevated phosphorus is associated with arterial and valvular calcification, arteriosclerosis, and an increased risk of cardiovascular death. Importantly, the adverse effects of hyperphosphatemia are partially preventable with the effective treatments available today.

Calcium metabolism and vitamin D in the extreme longevity

Skeletal remodelling is a continuous process during life and is still active also in extreme senescence. In the elderly, bone resorption often prevails over bone formation, causing bone loss and fragility. Elderly subjects are exposed to the risk of fractures, and loss of self-sufficiency, if considering that the proximal femur is the most frequently involved site. Bone remodelling can maintain circulating calcium within physiological ranges, at the expense of a substantial loss of this ion from the skeleton, particularly during senescence. Calcium metabolism is regulated at cellular/molecular level by a network of cytokines, growth factors, systemic hormones that act on bone in paracrine/autocrine/systemic fashion. Among the molecules involved in bone metabolism, parathyroid hormone (PTH) and vitamin D present some peculiar aspects during senescence. The osteometabolic features in a consistent group of centenarians have been evaluated. It results that a severe hypovitaminosis D was present in 99 out of 104 centenarians (25-OH vitamin D below 5 nmol/L), and that it plays an important role as a factor inducing a vicious circle involving hypocalcemia, secondary hyperparathyroidism, together with biochemical features indicating a consistent bone loss. Serum C-terminal cross-linking telopeptide, a specific marker of bone resorption was elevated in 92% of these subjects. Moreover, it has been found that several femoral fractures had occurred after 90 years of age. These data offer a rational for the possible prevention of elevated bone turnover, bone loss and consequently the reduction of osteoporotic fractures and fractures-induced disability, in the oldest olds, through the simple supplementation with calcium and vitamin D.

Impact of genetic variation on metabolic response of bone to diet

There is compelling evidence to suggest that both the development of bone to peak bone mass at maturity and subsequent loss depend on the interaction between genetic, hormonal, environmental and nutritional factors. The major part (≤80%) of the age-specific variation in bone turnover and bone density is genetically determined. However, the notion of genetic determinant is of little value unless the specific genes that are involved can be identified. Most work in this area of osteoporosis research has focused on the candidate gene approach, which has identified several candidate genes for osteoporosis, including genes encoding the vitamin D receptor (VDR), oestrogen receptors (α and β), apolipoprotein E, collagen type I α 1 and methylenetetrahydrofolate reductase, amongst many others. However, in general, findings from numerous studies of the association between such genes and various bone variables have been inconsistent. In addition to possible gene—gene interactions it is likely that there are interactions between these genes and certain environmental factors, especially nutrition, that may mediate expression of bone-related phenotypes. While these potential interactions add a level of complexity to our understanding of these apparent genetic effects on bone, identification of a role for genetic factors without knowledge of their interaction with nutrients can do little to advance prevention and treatment of osteoporosis. This information is especially important because, unlike genotype, diet and nutrition can be modified. The aim of the present review is to critically evaluate current knowledge relating to candidate genes for osteoporosis, with particular emphasis on their interaction with nutrients and dietary factors in determining bone health.

Large-scale population-based study shows no evidence of association between common polymorphism of the VDR gene and BMD in British women

The VDR is a candidate gene for osteoporosis. Here we studied five common polymorphisms of VDR in relation to calcium intake and vitamin D status in a population-based cohort of 3100 British women, but found no significant association with bone mass, bone loss, or fracture.

INTRODUCTION

Population studies of vitamin D receptor (VDR) polymorphisms have produced conflicting results. We performed a comprehensive study dealing with all potential confounders in a large population to determine whether polymorphisms in the VDR gene influence bone health.

MATERIALS AND METHODS

We studied 3100 women (50-63 years old) with bone markers, 25-hydroxyvitamin D, calcium, PTH, diet, and physical activity collected in 1998-2000. BMD was measured in 1990-1994 and 1998-2000. Fracture prevalence was assessed in 2002. Women were genotyped for five polymorphisms in the VDR gene: Cdx-2, Fok1, Bsm1, Apa1, and Taq1. The relationship between VDR and BMD, and interactions between VDR genotype, dietary calcium, and 25-hydroxyvitamin D, were examined using analysis of covariance.

RESULTS

Compared with carriers of the G allele, homozygotes for the rare Cdx-2 A polymorphism (n = 136) had less bone loss (-0.5 +/- 1.2 versus -0.7 +/- 1.0%/year [SD]; p = 0.01) and lower PTH (3.0 +/- 1.6 versus 3.4 +/- 2.0 pM; p = 0.03) despite similar vitamin D status. The association was not significant after correction for multiple testing or adjustment for confounders. At low calcium intakes, AA homozygotes had greater femoral neck (FN) BMD compared with carriers of the G allele, but at higher calcium intakes, the association was reversed. At low calcium intake, homozygotes for the b allele of Bsm1 had greater BMD compared with carriers of the B allele, but at higher calcium intakes, there was no difference. Similar results were seen for the Taq1 polymorphism. There was no evidence of gene-nutrient interaction when adjusted for body weight. No interactions between genotypes and vitamin D status on BMD were observed.

CONCLUSIONS

VDR does not seem to influence BMD or bone turnover in early postmenopausal white women with adequate calcium intake. Gene-nutrient interactions on BMD may be an indirect consequence of interactions between genotype and calcium intake on weight.

Control of Hyperphosphatemia among Patients with ESRD

Derangements of mineral metabolism occur during the early stages of chronic kidney disease (CKD). Hyperphosphatemia develops in the majority of patients with ESRD and has long been associated with progression of secondary hyperparathyroidism and renal osteodystrophy. More recent observational data have associated hyperphosphatemia with increased cardiovascular mortality among dialysis patients. Adequate control of serum phosphorus remains a cornerstone in the clinical management of patients with CKD not only to attenuate the progression of secondary hyperparathyroidism but also possibly to reduce the risk for vascular calcification and cardiovascular mortality. These measures include dietary phosphorus restriction, dialysis, and oral phosphate binders. Dietary restriction is limited in advanced stages of CKD. Phosphate binders are necessary to limit dietary absorption of phosphorus. Aluminum hydroxide is an efficient binder; however, its use has been nearly eliminated because of concerns of toxicity. Calcium salts are inexpensive and have been used effectively worldwide as an alternative to aluminum. Concerns of calcium overload have led to the investigation of alternatives. Currently, only two Food and Drug Administration-approved noncalcium, nonaluminum binders are available. Sevelamer hydrochloride is an exchange resin and was not as effective as calcium acetate in meeting new guideline recommendations in one double-blind clinical trial. Lanthanum carbonate is a rare earth element and has been studied less extensively. Concerns of long-term administration and toxicity exist. Furthermore, these agents are significantly more expensive than calcium salts, which may contribute to patient noncompliance.

Adapting to the transition between gestation and lactation: differences between rat, human and dairy cow

Adequate blood calcium concentrations are vital for the normal function of mammals. Mechanisms for maintaining normal blood calcium function adequately most of the time; however, occasionally they fail and calcium homeostasis is compromised. Milk fever or periparturient hypocalcemia in dairy cattle is a well-documented example of a breakdown in the mechanisms of calcium homeostasis. This disease occurs at the time of parturition and is unique to adult dairy animals. The disease results from the inability of animals to cope with the sudden demand for calcium in support of colostrum formation. Animals developing the disease become hypocalcemic and require intravenous calcium to survive. The precise metabolic disorder(s) responsible for the onset of milk fever is still being debated. This report will highlight some of the current concepts related to the causes and prevention of milk fever in dairy cattle, as well as contrasting differences in calcium demands that exist between dairy cattle, humans and rats at the onset of lactation.

Plasma changes in micronutrients following a multivitamin and mineral supplement in healthy adults

OBJECTIVE

To estimate the micronutrient (riboflavin, folate, vitamin C, vitamin B(12), iron, zinc and copper) bioavailability in healthy adults from a multi-micronutrient dietary supplement to assess the possible influence on it by the tablet disintegration properties and by the relative intestinal permeability of subject.

METHODS

The bioavailability of seven micronutrients from a single brand of multi-micronutrient dietary supplement was measured on two separate occasions in the presence of a standardized test meal in 15 healthy adult subjects. Each subject visited the Metabolic Research Unit on four separate randomized occasions for an absorption test. One test measured the intestinal permeability. The other three tests measured the postprandial changes in plasma or serum concentrations after consuming a test meal alone (control:placebo effect), or the test meal with either whole or crushed and powdered dietary supplements. 15 healthy Caucasian adult volunteers, aged 42 +/- 14 years.

RESULTS

The 12 hour-post-dose AUC for riboflavin, folate and vitamin C (whole and crushed tablet), and that for vitamin B(12) (only for the crushed tablet treatment) and iron (only for the whole tablet treatment) were all significantly (p < 0.001) higher than after a test meal alone. In contrast there was no significant increase in the AUC after supplement intake for zinc and copper. Neither the form of the supplement for all micronutrients tested nor intestinal permeability of the subject for riboflavin, folate, vitamin C, iron, zinc and copper influenced the postdose nutrient AUC. In contrast, for vitamin B(12) the intestinal permeability of the subject influenced significantly the nutrient AUC (p = 0.003).

CONCLUSION

Tablet disintegration characteristics of this dietary supplement did not limit absorption of these seven micronutrients. The intestinal permeability of subject was only positively correlated with the B(12) bioavailability. Results are suggestive of using multi-micronutrients dietary supplements as a vehicle to decrease the prevalence of multiple micronutrient deficiencies overall for vitamins in healthy adults.

Passive diffusion does not play a major role in the absorption of dietary calcium in normal adults

Several authors have indicated recently that passive diffusion is the dominant process by which dietary calcium is absorbed by normal adults. These conclusions have important implications in the maintenance of normal calcium status in humans. They imply that increasing luminal calcium concentration by increasing intake is the most important strategy in promoting calcium absorption. I present several studies that dispute this contention and two studies that suggest that passive diffusion is of little practical significance, even in young children. The contribution of passive diffusion is estimated to be between 8 and 23% of the total calcium absorbed by normal adults. Thus, it clearly does not represent the major path by which they absorb calcium. Children lacking the vitamin D receptor system do not absorb enough calcium by passive diffusion to prevent signs of rickets even when diets contain very large amounts of dietary calcium (>2 g/d). The conclusion drawn from this analysis is that passive diffusion (dictated by calcium intake) is not the major mechanism by which dietary calcium is absorbed by normal adult humans. The vitamin D-dependent processes are more important quantitatively and thus constitute a major determinant of calcium status. Individuals who are not exposed to sunlight may be especially at risk.