Physiologic regulation of the serum concentration of 1,25-dihydroxyvitamin D by phosphorus in normal men

Serum phosphorus concentration and its association with the degree and pattern of intracranial arterial calcification

BACKGROUND AND AIM

The aim of this study was to determine whether the serum phosphorus concentrations (SPC) are associated with the degree and pattern of intracranial arterial calcification (IAC) in patients with normal renal function or mild-moderate renal impairment.

METHODS AND RESULTS

A total of 513 patients were enrolled in this study. The degree of IAC measured by IAC scores was evaluated on non-contrast head computed tomography (CT) images and IAC was classified as intimal or medial calcification. Study participants were classified according to IAC degrees (mild, moderate and severe) and patterns (intimal and medial calcification). A multivariate regression model was used to assess the independent relationship of SPC with IAC scores and patterns. Of 513 study participants (mean [SD] age, 68.3 [10.3] years; 246 females [48%]), the mean SPC was 1.07 ± 0.17 mmol/L and IAC scores was 4.0 (3.0-5.0). Multivariate analysis showed that higher serum phosphorus was a significant risk factor for moderate/severe IAC in both patients with eGFR ≥60 ml/min/1.73 m2 (odds ratio [OR], 1.27; 95% confidence interval [CI], 1.01-1.59; P < 0.05) and eGFR <60 ml/min/1.73 m2 (OR, 1.92; 95% CI, 1.04-3.57; P < 0.05), when those with mild IAC were considered as the reference group. However, higher SPC was associated with an increased odds of medial calcification only in patients with eGFR <60 ml/min/1.73 m2 (OR, 1.67; 95% CI, 1.08 to 2.61).

CONCLUSIONS

High levels of serum phosphorus were positively correlated with the degree of IAC, and this significant effect on medial IAC was only present in patients with impaired renal function (eGFR <60 ml/min/1.73 m2).

Hyperparathyroidism in a Large Cohort of Chinese Patients with Tumor-Induced Osteomalacia

CONTEXT

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by excessive production of fibroblast growth factor 23 (FGF23) by a tumor. Hyperparathyroidism (HPT) including secondary HPT (SHPT) and tertiary HPT (THPT) in TIO patients, which is considered to associate with phosphate supplementation, has not been well documented.

OBJECTIVES

To clarify the prevalence, clinical characteristics, and risk factors for HPT in a large cohort of Chinese TIO patients in our hospital.

DESIGN, SETTING AND PARTICIPANTS

This retrospective study enrolled 202 TIO patients.

MAIN OUTCOME MEASUREMENTS

Occurrence of HPT in TIO patients.

RESULTS

HPT was observed in 91 patients (91/202, 45.1%), as 84 patients (41.6%) were diagnosed as SHPT and seven patients (3.5%) were diagnosed as THPT. All patients with THPT underwent parathyroidectomy and only one patient experienced recurrence. Compared with patients without HPT, patients with SHPT had longer disease duration, higher rate of phosphate and calcitriol supplementation, lower serum calcium, lower urine calcium excretion, and higher urine phosphate excretion. Compared with patients with SHPT, patients with THPT had even longer disease duration, higher rate of phosphate and calcitriol supplementation. PTH levels showed positive correlation with intact FGF23 and 1,25(OH)2D levels, but not 25OHD level in TIO patients. Multivariate logistic regression analysis showed that long disease duration and phosphate supplementation were independently associated with occurrence of HPT in TIO patients. Further logistic regression analysis and restricted cubic spline model revealed dose-response relationship between cumulative dose of phosphate supplementation and PTH levels.

CONCLUSIONS

HPT is common in our TIO patients. To avoid the occurrence of HPT in TIO patients, timely diagnosis and tumor resection is necessary, and excessive dose of phosphate supplementation is not suggested before surgery.

Electrolyte’s imbalance role in atrial fibrillation: Pharmacological management

The contribution of the perpetuation of atrial fibrillation is caused by electrical remodeling in which calcium, sodium and potassium channels could refer to changes in the ion channel protein expression, development of fibrosis, gene transcription and ion channel redistribution. Calcium and magnesium could influence the risk of atrial fibrillation which is the leading cause of cardiac death, heart failure and ischemic stroke. The elevated serum concentration of calcium had a higher range of in-patient’s mortality, increased total cost of hospitalization and increased length of hospital stay as compared to those without hypercalcemia in atrial fibrillation patients. Moreover, chloride channels could affect homeostasis, atrial myocardial metabolism which may participate in the development of atrial fibrillation. Up to a 50% risk of incidence of AF are higher in which left ventricular hypertrophy, sudden cardiovascular death and overall mortality relate to a low serum magnesium level. Additionally, magnesium prevents the occurrence of AF after cardiac surgery, whereas greater levels of serum phosphorus in the large population-based study and the related calcium–phosphorus products were linked with a greater incidence of AF. Numerous clinical studies had shown the high preoperative risk of AF that is linked with lower serum potassium levels. The conventional risk factor of increased risk of new onset of AF events could independently link with high dietary sodium intake which enhances the fibrosis and inflammation in the atrium but the mechanism remains unknown. Many drugs were used to maintain the electrolyte imbalance in AF patients.

High sodium reduced the expression of PTH1R and Klotho by inhibiting 1,25(OH)2D3 synthesis in cultured proximal tubule epithelial cells

Background

The proximal tubule is the sensing site of sodium and phosphate and the main place for the synthesis and metabolism of 1,25(OH)2D3. We aimed to investigate the effects of high sodium on the synthesis and function of active vitamin D and local phosphate regulation in proximal tubular epithelial cells.

Methods

Human proximal tubule epithelial (HK-2) cells were treated with different concentrations of sodium/phosphate. The expression of 1α-OHase and 24-OHase was determined. Liquid chromatography/mass spectrometry (LC/MS) and enzyme-linked immunosorbent assay (ELISA) were used to detect the levels of 1,25(OH)2D3. RNA sequencing and bioinformatics analysis was used to probe into the possible pathways. Chromatin samples were immunoprecipitated with antibodies against parathyroid receptor 1 (PTH1R) and Klotho.

Results

We found that high sodium decreased the expression of 1,25(OH)2D3 by reducing 1α-OHase and 24-OHase, reduced the expression of PTH1R and Klotho, and increased the intracellular calcium concentration. These effects were reversed by sodium phosphate transporter inhibitor, sodium hydrogen transporter inhibitor, and a chelator of the extracellular calcium, whereas enhanced by ouabain. Vitamin D receptor (VDR) agonists significantly increased the recruitment of VDR to the vitamin D response element (VDRE) of PTH1R and Klotho promoter, thus increasing the expression of PTH1R and Klotho.

Conclusions

High sodium can decrease the synthesis of active vitamin D in the proximal tubules, affect the gene regulation of 1,25(OH)2D3/VDR, and significantly reduce the expression of PTH1R and Klotho. It revealed the influence of a high-sodium diet on mineral metabolism and the core role of vitamin D in kidney mineral metabolism.

The human pathogenic 91del7 mutation in SLC34A1 has no effect in mineral homeostasis in mice

Kidneys are key regulators of phosphate homeostasis. Biallelic mutations of the renal Na⁺/phosphate cotransporter SLC34A1/NaPi-IIa cause idiopathic infantile hypercalcemia, whereas monoallelic mutations were frequently noted in adults with kidney stones. Genome-wide-association studies identified SLC34A1 as a risk locus for chronic kidney disease. Pathogenic mutations in SLC34A1 are present in 4% of the general population. Here, we characterize a mouse model carrying the 91del7 in-frame deletion, a frequent mutation whose significance remains unclear. Under normal dietary conditions, 12 weeks old heterozygous and homozygous males have similar plasma and urinary levels of phosphate as their wild type (WT) littermates, and comparable concentrations of parathyroid hormone, fibroblast growth factor 23 (FGF-23) and 1,25(OH)₂ vitamin D₃. Renal phosphate transport, and expression of NaPi-IIa and NaPi-IIc cotransporters, was indistinguishable in the three genotypes. Challenging mice with low dietary phosphate did not result in differences between genotypes with regard to urinary and plasma phosphate. Urinary and plasma phosphate, plasma FGF-23 and expression of cotransporters were similar in all genotypes after weaning. Urinary phosphate and bone mineral density were also comparable in 300 days old WT and mutant mice. In conclusion, mice carrying the 91del7 truncation do not show signs of impaired phosphate homeostasis.

Use of Micro-CT Imaging to Assess Ventral Mandibular Cortical Thickness and Volume in an Experimental Rodent Model With Chronic High-Phosphorus Intake

Adverse effects of high dietary phosphorus on bone health have been observed in both animal and human studies. The aim of the investigation was to examine chronic effects of high phosphorus diet on the apical mandibular cortical thickness and volume in a hystricomorph rodent (Octodon degus) using microcomputed tomography. Male degus were randomly divided into two groups fed by different mineral contents from the age of 12 weeks till the age of 17 months. The micro-CT scanning and wall thickness analysis were applied on the region of the mandible exactly under the apices of the 4th premolar tooth, first molar tooth, and second molar tooth in two animals from each group. General overview and mapping of the ventral mandibular bone thickness revealed pronounced bony mandibular protrusions in all the animals fed a high-phosphorus diet with obvious bone thinning apically to the 4th premolar and first and second molar tooth apices. Mandibular bone volume and thickness located apically to the premolar and molars were statistically significantly smaller/thinner in the group fed by a high phosphorus diet. The thinnest bone measured 0.004 mm, where the mandibular 4th premolar tooth almost perforated the mandibular cortex. Similar studies of metabolic bone disease and its influence on alveolar bone were also published in rats and mice. The influence of different environmental, infectious, or metabolic factors on the growing tooth, alveolar bone formation, and bone pathologies must be done experimentally on growing animals. In contrast, degus have continuously growing dentition, and the effect of any of the above listed factors can be studied in this animal model at any age and for longer time periods.

Dietary phosphorus level regulates appetite through modulation of gut and hypothalamic expression of anorexigenic genes in broiler chickens

Two experiments were designed to elucidate gut and hypothalamic molecular regulation of appetite by dietary phosphorus (P) concentration in broiler chickens. Birds (192 Cobb-500 broiler chickens) were randomly assigned to 3 experimental diets in experiment 1 (Exp. 1) and 24 broiler chickens were randomly assigned to 3 treatment groups in Exp. 2. Each diet comprised 8 replicate cages, with either 8 birds (Exp. 1) or 1 bird (Exp. 2) per replicate cage. In Exp. 1, diets contained 1.2 (P-deficient), 2.8 (P-marginal) or 4.4 (P-adequate) g/kg non-phytate P (nPP). In Exp. 2, birds fed the P-adequate diet were pair-fed (PF) to the feed consumption levels of birds fed the P-deficient diet. Feed intake and BW gain (P < 0.001) decreased in birds fed the P-deficient diet in Exp. 1. Birds fed the P-deficient diet had similar feed intake and BW gain with PF group fed the P-adequate diet (Exp. 2) but was significantly lower (P < 0.001) than birds fed the P-adequate diets. Sodium-phosphate cotransporter (NaPi-IIb) mRNA was upregulated (P < 0.05) in both experiments. Conversely, cholecystokinin (CCK) mRNA was downregulated (P < 0.01) in birds fed P-deficient diets. Anorexia-related hypothalamic cholecystokinin receptor (CCKAR) and melanocortin receptors (MC3R and MC4R) were upregulated (P < 0.05) in birds fed P-deficient diets, in both experiments. The current data show that dietary P deficiency decreases feed intake in broiler chickens by altering the expression of anorexigenic genes in the gut and hypothalamus of broiler chickens.

Adverse effects on growth performance and bone development in nursery pigs fed diets marginally deficient in phosphorus with increasing calcium to available phosphorus ratios

The objective of this experiment was to evaluate the growth performance and bone mineral content (BMC) of nursery pigs in response to increasing total calcium (Ca) to available phosphorus (aP) ratios in diets containing phytase (250 FTU/kg; Natuphos E, BASF, Florham Park, NJ). A total of 480 nursery pigs (body weight (BW) = 5.7 ± 0.6 kg) with 10 pigs per pen and 7 pens per treatment (6 pens fed 2.75:1 diet) were allotted to seven treatments consisting of increasing ratios of calcium to available phosphorus (Ca:aP): 1.25, 1.50, 1.75, 2.00, 2.25, 2.50, and 2.75. From day −7 to 0, pigs were fed a common diet. They were then fed the treatment diets during two experimental phases from day 1 to 14 and 15 to 28, respectively. Available P was formulated to 0.33% and 0.27% (approximately 90% of requirement) in dietary phases 1 and 2, respectively. BW, average daily gain (ADG), average daily feed intake (ADFI), and gain-to-feed ratio (G:F) were determined. BMC of the femur was measured on day 28 on one pig per pen using dual x-ray absorptiometry. Data were analyzed as a linear mixed model using PROC MIXED (SAS, 9.3). Orthogonal polynomial contrasts were used to determine the linear and quadratic effects of increasing the Ca:aP. Over the 28-d experimental period, increasing Ca:aP resulted in a linear decrease in ADG (353, 338, 328, 304, 317, 291, and 280 g/d; P < 0.01), ADFI (539, 528, 528, 500, 533, 512, and 489 g/d; P < 0.05), and G:F (0.68, 0.66, 0.64, 0.62, 0.61, 0.59, and 0.58; P < 0.01). Increasing Ca:aP also resulted in decreased BW on days 14 and 28 (P < 0.01). The BMC of the femur decreased with increasing Ca:aP (6.2, 6.3, 5.7, 5.9, 5.5, 5.6, and 5.3 g; P < 0.05). Regression analysis explained the impact of Ca:aP as follows on ADG (ADG [g/d] = 339 − 36x; r² = 0.81), G:F (G:F = 0.61 – 0.03x; r² = 0.72), and BMC (BMC [g] = 6.4 – 0.27x; r² = 0.43), where x is the Ca:aP. In conclusion, all outcomes indicated that any level of calcium above the minimum used in this experiment impaired growth performance and skeletal development. Further research using even lower levels of dietary Ca is warranted.

High-phosphorus diet controlled for sodium elevates blood pressure in healthy adults via volume expansion

Whether increasing exposure to dietary phosphorus can lead to adverse clinical outcomes in healthy people is not clear. In this open‐label prospective cross‐over study, we are to explore the impact of various dietary phosphorus intake on mineral, sodium metabolisms and blood pressure in young healthy adults. There were 3 separate study periods of 5 days, each with a 5 days washout period between different diets interventions. Six young healthy male volunteers with normal nutrition status were recruited in Phase I Clinical Research Center and sequentially exposed to the following diets: (a) normal‐phosphorus diet (NPD): 1500 mg/d, (b) low‐phosphorus diet (LPD): 500 mg/d, (c) high‐phosphorus diet (HPD): 2300 mg/d. HPD induced a significant rise in daily average serum phosphate (1.47 ± 0.02 mmol/L [4.56 ± 0.06 mg/dl]) compared to NPD (1.34 ± 0.02 mmol/L [4.15 ± 0.06 mg/dL]) and LPD (1.17 ± 0.02 mmol/L [3.63 ± 0.06 mg/dL]) (p < .05). Daily average levels of serum parathyroid hormone and fibroblast growth factor 23 in HPD were significantly higher, and serum 1,25(OH)₂D₃ was remarkably lower than those in LPD. HPD induced a significant decrease in daily average serum aldosterone and an increase in daily average atrial natriuretic peptide level compared to LPD. The 24‐hour urine volume in HPD subjects was less than that in LPD subjects. HPD significantly increased daily average systolic blood pressure by 6.02 ± 1.24 mm Hg compared to NPD and by 8.58 ± 1.24mm Hg compared to LPD (p < .05). Our study provides the first evidence that 5‐day high‐phosphorus diet can induce elevation in SBP in young healthy adults, which may due to volume expansion.

Modulating phosphate consumption, a novel therapeutic approach for the control of cancer cell proliferation and tumorigenesis

Phosphorus, often in the form of inorganic phosphate (Pi), is critical to cellular function on many levels; it is required as an integral component of kinase signaling, in the formation and function of DNA and lipids, and energy metabolism in the form of ATP. Accordingly, crucial aspects of cell mitosis - such as DNA synthesis and ATP energy generation - elevate the cellular requirement for Pi, with rapidly dividing cells consuming increased levels. Mechanisms to sense, respond, acquire, accumulate, and potentially seek Pi have evolved to support highly proliferative cellular states such as injury and malignant transformation. As such, manipulating Pi availability to target rapidly dividing cells presents a novel strategy to reduce or prevent unrestrained cell growth. Currently, limited knowledge exists regarding how modulating Pi consumption by pre-cancerous cells might influence the initiation of aberrant growth during malignant transformation, and if reducing the bioavailability or suppressing Pi consumption by malignant cells could alter tumorigenesis. The concept of targeting Pi-regulated pathways and/or consumption by pre-cancerous or tumor cells represents a novel approach to cancer prevention and control, although current data remains insufficient as to rigorously assess the therapeutic value and physiological relevance of this strategy. With this review, we present a critical evaluation of the paradox of how an element critical to essential cellular functions can, when available in excess, influence and promote a cancer phenotype. Further, we conjecture how Pi manipulation could be utilized as a therapeutic intervention, either systemically or at the cell level, to ultimately suppress or treat cancer initiation and/or progression.

Mineral and bone physiology in the foetus, preterm and full-term neonates

Mother is the major source of minerals in foetal life with placenta actively transporting against a concentration and electrochemical gradient. The foetal serum mineral concentration is thereby higher as compared to maternal values, which possibly help in its rapid accretion in developing bones and for counteracting postnatal fall in calcium levels at birth. Parathyroid hormone related peptide (PTHrP) and parathyroid hormone (PTH) play a major role in mineral physiology during foetal life with hormones like calcitriol, calcitonin, FGF-23 and sex steroids having minimal role. PTHrP and PTH also play a major role in endochondral bone formation and mineralization of skeleton. At the birth, as the cord is clamped, there is loss of active transport of minerals through placenta and the neonate has to rely on enteral intake of minerals to meet the demands of growing bones and metabolisms. The calcium levels fall after birth, reaching a nadir at 24-48 h and gradually rise to adult values over several days, probably resulting from a fall in PTHrP levels and hyporesponsiveness of parathyroid glands. As PTH and calcitriol levels increase postnatally, there is a rise in calcium levels with maturation in functioning of kidneys and intestines. However, there may be significant delay in intestinal maturation in preterm infants along with an increased demand for mineral accretion, which predispose them to osteopenia of prematurity.

Habitual confectionery intake is associated with serum phosphorus levels : A cross-sectional study on healthy subjects

Hyperphosphatemia is associated with an increased risk of developing cardiovascular disease. Recently, it has been shown that high serum phosphorus levels are associated with increased cardiovascular events in healthy subjects, but the dietary factors determining serum phosphorus level have not been fully investigated. The study investigated the influence of habitual dietary factors on serum phosphorus levels in healthy young participants. This cross-sectional study conducted fasting blood sampling in 109 healthy young people and used a brief-type self-administered diet history questionnaire to evaluate the subject's habitual meals. Since the bioavailability of dietary phosphorus depends on the food sources, habitual phosphorus intakes from different food groups and intake frequency (times/month) of processed foods were calculated. The mean serum phosphorus level was 3.9 0.5 mg/dl ; 10.1% of the subjects had serum phosphorus levels that exceeded the reference levels of 4.5 mg/dl. Total phosphorus intake and phosphorus intake from animal-based food did not differ between serum phosphorus quartiles. Higher intake of confectionery was associated with increased serum phosphorus levels. This study showed that frequent consumption of confectionery was associated with elevated serum phosphorus levels. Additional studies are needed to determine whether this is a causal relationship. J. Med. Invest. 66 : 134-140, February, 2019.

Pathogenesis of Secondary Hyperparathyroidism

Chronic renal failure is the primary cause of secondary hyperparathyroidism (SHPT). Patients with mineral metabolism disorders commonly present with low serum calcium levels, hyperphosphatemia, and calcitriol deficiency. In normal renal function subjects, parathyroid cells have a low turnover and rarely undergo mitoses. In uremic conditions, however, parathyroid glands become hyperplasic and leave quiescence. During the last ten years, new molecular mechanisms have been investigated to better understand the pathogenesis of SHPT: the emerging role of the Calcium Sensing Receptor (CaSR); the importance of the parathyroid expression of the Vitamin D receptor (VDR); the growing evidence on the central role of the Fibroblast Growth Factor 23 (FGF-23). In contrast, the discovery of a parathyroid phosphate sensor or receptor has yet to be made.

Hyperphosphatemia in Dialysis Patients: The Therapeutic Role of Lanthanum Carbonate

Phosphate overload is a dramatic consequence in end-stage renal disease (ESRD) patients. Recent studies have well documented that abnormalities in mineral and bone metabolism in these patients are associated with increased cardiovascular morbidity and mortality. Elevated serum phosphate and calcium-phosphate product levels play an important role in the pathogenesis of secondary hyperparathyroidism and extra-skeletal calcification in dialysis patients. Furthermore, inorganic phosphate may cause vascular calcification directly through a real “ossification” of the tunica media in the vasculature of ESRD patients.

The “classical” treatment of secondary hyperparathyroidism and hyperphosphatemia in ESRD patients consists of either calcium- or aluminum-based phosphate binders and calcitriol administration. Unfortunately, this “old generation” therapy is not free of complications. This review paper suggests that new calcium- and aluminum-free phosphate binders, such as lanthanum carbonate, can be used to treat hyperphosphatemia and secondary hyperparathyroidism in ESRD patients.

Kidney function, bone-mineral metabolism markers, and future risk of peripheral artery disease

BACKGROUND AND AIMS

Reduced kidney function is a risk factor for lower-extremity peripheral artery disease (PAD). However, the associations of novel filtration markers with PAD are yet to be quantified. Moreover, little is known on whether bone-mineral metabolism (BMM) markers are related to incident PAD beyond kidney function.

METHODS

Using data from 12,472 participants at baseline (1990-1992) of the Atherosclerosis Risk in Communities (ARIC) study, we comprehensively quantified the associations of kidney related markers with incident PAD (defined as hospitalizations with diagnosis of lower-extremity atherosclerosis, revascularization, or amputation). Kidney related markers of interest included estimated glomerular filtration rate (eGFR) (based on creatinine, cystatin C, and both), cystatin C, beta-2 microglobulin (B2M), and BMM markers (serum fibroblast growth factor 23, parathyroid hormone, calcium, and phosphorus).

RESULTS

During a median follow-up of 21 years, 471 participants developed incident PAD. Low eGFR was significantly associated with future PAD risk, with slightly stronger relationship when cystatin C was used (adjusted hazard ratio [HR] 6.3-8.3 for eGFR <30 and 2.4-3.5 for eGFR 30-59 vs. eGFR ≥90 mL/min/1.73 m2). Among all filtration markers, B2M had the strongest association with incident PAD (HR for top vs. bottom quartile 2.60 [95% CI: 1.91-3.54] for B2M vs. 1.20 [0.91-1.58] for creatinine-based eGFR). Among BMM markers, only phosphorus remained significant for PAD risk beyond potential confounders, including kidney function (HR 1.47 [1.11-1.94] in top quartile).

CONCLUSIONS

Kidney dysfunction was independently associated with future PAD risk, particularly when assessed with cystatin C and B2M. Among the BMM markers tested, phosphorus was most robustly associated with incident PAD beyond kidney function. Our results suggest the potential usefulness of novel filtration markers for PAD risk assessment and the possible role of phosphorus in the pathophysiology of PAD.

Interaction of Serum Phosphate with Age as Predictors of Cardiovascular Risk Scores in Stable Renal Transplant Recipients

We calculated rate of changes in the cardiovascular risk calculator for renal transplant recipients (CRCRTR) major adverse cardiac events (MACE) in clinically stable renal transplant recipients (RTRs) to identify covariables that associate with fast cardiovascular (CV) risk progression. CRCRTR-MACE scores were calculated on 139 patients in 2011 and 2014. Score changes above and below median changes in scores were labeled fast or slow CV risk progression. Multivariate analysis (MVA) was performed to identify variables significant to percentage changes in scores. Receiver-operating characteristic (ROC) analysis was performed to define sensitivity and specificity of factors significant to fast score progression. Follow-up was 2.61 (2.02-4.47) years. Slow and fast progressions were present in 50.4 and 49.6% of patients, with a median change of 25.8% (- 92.1 to 1,444.7%). MVA showed percentage changes in age and serum phosphate were the only significant variables impacting fast progression in scores. ROC showed 2011 serum phosphate of 1.15 mmol/L to predict fast progression (area under the curve [AUC] of 0.628, p > 0.0126). Age older than 45 years combined with 2011 serum phosphate above 1.15 mmol/L had a significant AUC of 0.781, p < 0.0010 interleukin (IL)-1A and IL-28A were significant associates with serum phosphate above 1.1 mmol/L in the MVA. Changes in CV risk in RTR over time are highly variable. Serum phosphate, even within upper normal levels, predicts worsening of CV risk scores in stable RTR.

Genetic Variants Associated with Circulating Parathyroid Hormone

Parathyroid hormone (PTH) is a primary calcium regulatory hormone. Elevated serum PTH concentrations in primary and secondary hyperparathyroidism have been associated with bone disease, hypertension, and in some studies, cardiovascular mortality. Genetic causes of variation in circulating PTH concentrations are incompletely understood. We performed a genome-wide association study of serum PTH concentrations among 29,155 participants of European ancestry from 13 cohort studies (n=22,653 and n=6502 in discovery and replication analyses, respectively). We evaluated the association of single nucleotide polymorphisms (SNPs) with natural log-transformed PTH concentration adjusted for age, sex, season, study site, and principal components of ancestry. We discovered associations of SNPs from five independent regions with serum PTH concentration, including the strongest association with rs6127099 upstream of CYP24A1 (P=4.2 × 10-53), a gene that encodes the primary catabolic enzyme for 1,25-dihydroxyvitamin D and 25-dihydroxyvitamin D. Each additional copy of the minor allele at this SNP associated with 7% higher serum PTH concentration. The other SNPs associated with serum PTH concentration included rs4074995 within RGS14 (P=6.6 × 10-17), rs219779 adjacent to CLDN14 (P=3.5 × 10-16), rs4443100 near RTDR1 (P=8.7 × 10-9), and rs73186030 near CASR (P=4.8 × 10-8). Of these five SNPs, rs6127099, rs4074995, and rs219779 replicated. Thus, common genetic variants located near genes involved in vitamin D metabolism and calcium and renal phosphate transport associated with differences in circulating PTH concentrations. Future studies could identify the causal variants at these loci, and the clinical and functional relevance of these variants should be pursued.

Effects of gender and body weight on fibroblast growth factor 23 responsiveness to estimated dietary phosphorus

Fibroblast growth factor 23 (FGF23) is a molecule involved in regulating phosphorus homeostasis. Although some studies indicated an association between serum FGF23 levels and sex, the association has not been fully investigated. The purpose of this study was to evaluate whether sex could influence FGF23 responsiveness to dietary phosphorus intake in healthy individuals. Thirty two healthy subjects between 21 and 28 years were recruited for this study. Subjects performed 24-hour urine collection and blood samples were collected. We estimated phosphorus intake (UC-P) from the urine collection (UC), and evaluated any association between UC-P and serum FGF23 levels. Subsequently, we compared serum FGF23 levels between males and females. Positive correlation was observed between UC-P and serum FGF23 levels. Serum FGF23 levels were significantly higher in males than in females. Serum FGF23 levels/UC-P was significantly higher in females than in males. There was no significant difference in serum FGF23 levels/UC-P/BW between the male and female groups. Our results indicate that there was no gender difference between FGF23 responsiveness to phosphorus intake per body weight.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

Renal Phosphate Reabsorption is Correlated with the Increase in Lumbar Bone Mineral Density in Patients Receiving Once-Weekly Teriparatide

In order to assess the changes in serum calcium and phosphate and the changes in renal tubular phosphate reabsorption (TmP/GFR) and to evaluate the association between these indices and the increase in bone mineral density (BMD) with once-weekly intermittent administration of teriparatide (TPTD), the results from the teriparatide once-weekly efficacy research (TOWER) trial were re-analyzed. The TOWER trial studied postmenopausal women and older men with osteoporosis. Patients were randomly assigned to receive TPTD 56.5 μg or placebo for 72 weeks. Of these patients, the present study investigated those whose calcium and phosphate levels and lumbar BMD (L-BMD) were measured (TPTD group, n = 153 and Placebo group, n = 137). The TPTD group had significantly lower serum phosphate, calcium-phosphate product, and TmP/GFR at weeks 4, 24, 48, and 72 and urinary fractional calcium excretion (FECa) at weeks 12, 48, and 72 (p < 0.05). In the TPTD group, the serum phosphate and TmP/GFR during early treatment (4, and 12 weeks) showed a significant positive correlation with the percent change in L-BMD at weeks 48 and 72. Based on multivariate analysis corrected for age, BMI, and L-BMD at the start of treatment, serum phosphate and TmP/GFR at week 4 showed a significant correlation with the percent change in L-BMD. This study suggests that the L-BMD response to once-weekly long-term TPTD treatment is associated with circulating phosphate or with the status of its renal reabsorption. Preventing decrease in serum phosphate levels may be important in acquiring greater L-BMD with once-weekly TPTD.

Nocturnal eating disturbs phosphorus excretion in young subjects: a randomized crossover trial

BACKGROUND

Nocturnal eating have recently increased. Serum phosphorus levels and regulators of phosphorus have circadian variations, so it is suggested that the timing of eating may be important in controlling serum phosphorus levels. However, there have been no reports on the effects of nocturnal eating on phosphorus metabolism. The objective was to evaluate the effects of nocturnal eating on phosphorus metabolism.

METHODS

Fourteen healthy men participated in two experimental protocols with differing dinner times. The design of this study was a crossover study. The subjects were served test meals three times (breakfast; 07:30 h, lunch; 12:30 h, dinner; 17:30 or 22:30 h) a day. Blood and urine samples were collected to assess diurnal variation until the following morning.

RESULTS

The following morning, fasting serum phosphorus levels in the late dinner group were markedly higher than those in the early dinner group (p < 0.001), although serum calcium levels were maintained at approximately constant levels throughout the day in both groups. Fluctuations in urinary calcium excretion were synchronized with the timing of dinner eating, however, fluctuations in urinary phosphorus excretion were not synchronized. Urinary phosphorus excretions at night were inhibited in the late dinner group. In the late dinner group, intact parathyroid hormone levels didn't decrease, and they were significantly higher in this group compared with the early dinner group at 20:00 h (p = 0.004). The following morning, fasting serum fibroblast growth factor 23 levels in the late dinner group had not changed, but those in the early dinner group were significantly increased (p = 0.003). Serum free fatty acid levels before dinner were significantly higher in the late dinner group compared with the early dinner group.

CONCLUSIONS

Our results indicate that nocturnal eating inhibits phosphorus excretion. It is suggested that nocturnal eating should be abstained from to manage serum phosphorus levels to within an adequate range.

Vascular Calcification and Stone Disease: A New Look towards the Mechanism

Calcium phosphate (CaP) crystals are formed in pathological calcification as well as during stone formation. Although there are several theories as to how these crystals can develop through the combined interactions of biochemical and biophysical factors, the exact mechanism of such mineralization is largely unknown. Based on the published scientific literature, we found that common factors can link the initial stages of stone formation and calcification in anatomically distal tissues and organs. For example, changes to the spatiotemporal conditions of the fluid flow in tubular structures may provide initial condition(s) for CaP crystal generation needed for stone formation. Additionally, recent evidence has provided a meaningful association between the active participation of proteins and transcription factors found in the bone forming (ossification) mechanism that are also involved in the early stages of kidney stone formation and arterial calcification. Our review will focus on three topics of discussion (physiological influences-calcium and phosphate concentration-and similarities to ossification, or bone formation) that may elucidate some commonality in the mechanisms of stone formation and calcification, and pave the way towards opening new avenues for further research.

Hyperphosphatemic familial tumoral calcinosis: genetic models of deficient FGF23 action

Hyperphosphatemic familial tumoral calcinosis (hFTC) is a rare disorder of phosphate metabolism defined by hyperphosphatemia and ectopic calcifications in various locations. To date, recessive mutations have been described in three genes involving phosphate metabolism: FGF23, GALNT3, and α-Klotho, all of which result in the phenotypic presentation of hFTC. These mutations result in either inadequate intact fibroblast growth factor-23 (FGF23) secretion (FGF23 or GALNT3) or resistance to FGF23 activity at the fibroblast growth factor receptor/α-Klotho complex (α-Klotho). The biochemical consequence of limitations in FGF23 activity includes increased renal tubular reabsorption of phosphate, hyperphosphatemia, and increased production of 1,25-dihydroxyvitamin D. The resultant ectopic calcifications can be painful and debilitating. Medical treatments are targeted toward decreasing intestinal phosphate absorption or increasing phosphate excretion; however, results have been variable and generally limited. Treatments that would increase FGF23 levels or signaling would more appropriately target the genetic etiologies of this disease and perhaps be more effective.

Phosphate balance in ESRD: diet, dialysis and binders against the low evident masked pool

Phosphate metabolism is crucial in the pathophysiology of secondary hyperparathyroidism and vascular calcification. High phosphate levels have been consistently associated with unfavorable outcomes in dialysis patients, but several limitations are still hampering a resolutive definition of the optimal targets of phosphate serum levels to be achieved in this cohort. Nonetheless, hyperphosphatemia is a late marker of phosphate overload in humans. Clinical nephrologists routinely counteract the positive phosphate balance in dialysis patients through nutritional counseling, stronger phosphate removal by dialysis and prescription of phosphate binders. However, the superiority against placebo of phosphate control by diet, dialysis or binders in terms of survival has never been tested in dedicated randomized controlled trials. The present review discusses this conundrum with particular emphasis on the rationale supporting the value of a simultaneous intervention against phosphate overload in dialysis patients via the improvement of dietary intakes, dialysis efficiency and an individualized choice of phosphate binders.

Calcium, phosphate and calcium phosphate product are markers of outcome in patients with chronic heart failure

BACKGROUND

Serum calcium (Ca) and inorganic phosphate (Pi) concentrations and calcium-phosphate product (CPP) levels are positively associated with worse outcomes in patients with chronic kidney disease, but there are few data for Pi or Ca and none for CPP in patients with chronic heart failure (CHF).

METHODS

Unselected, consecutive patients with CHF (left ventricular ejection fraction, LVEF ≤45%) were enrolled in a prospective observational study for the occurrence of hospitalisation and mortality. Blood samples were collected at the time of recruitment and analysed immediately.

RESULTS

Patients (n = 713) were on contemporary optimal treatment and mean (standard error, SE) follow-up was 765 (18.9) days. Mean (SE) Ca was 2.29 (0.004) mmol/l. Median (interquartile range, IQR) Pi was 1.11 (0.98-1.23) mmol/l and median CPP 2.53 (2.21-2.88) mmol(2)/l(2). LVEF correlated inversely with Ca, natural log-transformed (Ln)Pi, and LnCPP. There was no difference in CPP between classes of symptom severity or diabetes status. Ca and LnCPP (but not LnPi) were associated with total mortality. Ca was significantly associated with progressive HF and non-cardiovascular death but not with sudden death. Binary logistic regression analyses showed that LnPi and LnCPP were associated with risk of hospitalisation.

CONCLUSIONS

Ca, Pi and CPP could be useful additional variables in determining risk in CHF patients. Further work is required to elucidate the mechanisms underlying the adverse influence and determine whether lowering phosphate levels per se in CHF patients is of benefit.

Increasing dietary phosphorus intake from food additives: potential for negative impact on bone health

It is important to consider whether habitual high phosphorus intake adversely affects bone health, because phosphorus intake has been increasing, whereas calcium intake has been decreasing in dietary patterns. A higher total habitual dietary phosphorus intake has been associated with higher serum parathyroid hormone (PTH) and lower serum calcium concentrations in healthy individuals. Higher serum PTH concentrations have been shown in those who consume foods with phosphorus additives. These findings suggest that long-term dietary phosphorus loads and long-term hyperphosphatemia may have important negative effects on bone health. In contrast, PTH concentrations did not increase as a result of high dietary phosphorus intake when phosphorus was provided with adequate amounts of calcium. Intake of foods with a ratio of calcium to phosphorus close to that found in dairy products led to positive effects on bone health. Several randomized controlled trials have shown positive relations between dairy intake and bone mineral density. In our loading test with a low-calcium, high-phosphorus lunch provided to healthy young men, serum PTH concentrations showed peaks at 1 and 6 h, and serum fibroblast growth factor 23 (FGF23) concentrations increased significantly at 8 h after the meal. In contrast, the high-calcium, high-phosphorus meal suppressed the second PTH and FGF23 elevations until 8 h after the meal. This implies that adequate dietary calcium intake is needed to overcome the interfering effects of high phosphorus intake on PTH and FGF23 secretion. FGF23 acts on the parathyroid gland to decrease PTH mRNA and PTH secretion in rats with normal kidney function. However, increased serum FGF23 is an early alteration of mineral metabolism in chronic kidney disease, causing secondary hyperthyroidism, and implying resistance of the parathyroid gland to the action of FGF23 in chronic kidney disease. These findings suggest that long-term high-phosphorus diets may impair bone health mediated by FGF23 resistance both in chronic kidney disease patients and in the healthy population.

Phosphorus and the kidney: What is known and what is needed

A major role of the kidneys is to maintain phosphorus homeostasis. High serum phosphorus has been linked to all-cause and cardiovascular mortality in chronic kidney disease (CKD) both before and after initiation of renal replacement therapy. Considering the clinical implications of uncontrolled hyperphosphatemia, maintenance of phosphorus concentrations within an optimum range is standard of care in this patient population. Recently, the epidemiologic associations between serum phosphorus and worse outcome have been extended to the general population. This becomes even more important in view of the increasing dietary phosphorus intake in the American diet due in large part to the greater consumption of foods processed with phosphate additives. A greater understanding of mechanisms and epidemiology of altered phosphorus metabolism and disease in CKD may help clarify the possible role of excess dietary phosphorus as a health risk factor in the general population.

Dietary phosphorus and kidney disease

High serum phosphate is linked to poor health outcome and mortality in chronic kidney disease (CKD) patients before or after the initiation of dialysis. Therefore, maintenance of normal serum phosphate levels is a major concern in the clinical care of this population with dietary phosphorus restriction and/or use of oral phosphate binders considered to be the best corrective care. This review discusses (1) evidence for an association between serum phosphate levels and bone and cardiovascular disease (CVD) in CKD patients as well as progression of kidney disease itself; (2) the relationship between serum phosphate and dietary phosphorus intake; and (3) implications from these data for future research. Increasing our understanding of the relationship between altered phosphorus metabolism and disease in CKD patients may clarify the potential role of excess dietary phosphorus as a risk factor for disease in the general population.

Public health impact of dietary phosphorus excess on bone and cardiovascular health in the general population

This review explores the potential adverse impact of the increasing phosphorus content in the American diet on renal, cardiovascular, and bone health of the general population. Increasingly, studies show that phosphorus intakes in excess of the nutrient needs of a healthy population may significantly disrupt the hormonal regulation of phosphate, calcium, and vitamin D, which contributes to disordered mineral metabolism, vascular calcification, impaired kidney function, and bone loss. Moreover, large epidemiologic studies suggest that mild elevations of serum phosphate within the normal range are associated with cardiovascular disease (CVD) risk in healthy populations without evidence of kidney disease. However, few studies linked high dietary phosphorus intake to mild changes in serum phosphate because of the nature of the study design and inaccuracies in the nutrient composition databases. Although phosphorus is an essential nutrient, in excess it could be linked to tissue damage by a variety of mechanisms involved in the endocrine regulation of extracellular phosphate, specifically the secretion and action of fibroblast growth factor 23 and parathyroid hormone. Disordered regulation of these hormones by high dietary phosphorus may be key factors contributing to renal failure, CVD, and osteoporosis. Although systematically underestimated in national surveys, phosphorus intake seemingly continues to increase as a result of the growing consumption of highly processed foods, especially restaurant meals, fast foods, and convenience foods. The increased cumulative use of ingredients containing phosphorus in food processing merits further study given what is now being shown about the potential toxicity of phosphorus intake when it exceeds nutrient needs.

Nutritional rickets around the world

Nutritional rickets is a major public health problem in many countries of the world. The disease is characterized by deformities of the long bones, enlargement of the wrists and costochondral junctions, hypotonia and, in infants, craniotabes and delayed fontanelle closure. Predominantly caused by severe vitamin D deficiency, rickets can also be associated with hypocalcemic seizures and cardiac failure. First presentation is typically at 6-24 months of age, although hypocalcemia may be evident in younger infants. In many affluent industrialized countries, the prevalence of rickets in the general population diminished after the introduction of clean-air legislation and dietary supplementation. However, in such countries, vitamin-D deficiency rickets has re-emerged in recent years, particularly among groups with limited exposure to UVB-containing sunshine. Infants at risk of rickets tend to be those whose mothers had poor vitamin D status during pregnancy and those exclusively breast-fed for a prolonged period with little skin exposure to UVB. In other countries of the world, the prevalence of rickets can be high, even in regions with abundant year-round UVB-containing sunshine. In general, this is also due to vitamin D deficiency related to limited sun exposure. However, reports from Africa and Asia suggest that there may be other etiological factors involved. Studies in South Africa, Nigeria, The Gambia and Bangladesh have identified rickets in children, typically 3-5 years old at first presentation, in whom plasma 25-hydroxyvitamin D concentrations are higher than those characteristic of primary vitamin D deficiency. Calcium deficiency has been implicated, and in some, but not all, disturbances of phosphate metabolism, renal compromise and iron deficiency may also be involved. Continuing studies of the etiology of nutritional rickets will provide evidence to underpin guidelines for the prevention and treatment of rickets world-wide. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Relation of serum phosphorus levels to the incidence of atrial fibrillation (from the Atherosclerosis Risk In Communities [ARIC] study)

High serum phosphorus levels have been linked with vascular calcification and greater cardiovascular morbidity and mortality. We assessed whether serum phosphorus was associated with the atrial fibrillation (AF) incidence in a large community-based cohort in the United States. Our analysis included 14,675 participants (25% black, 45% men) free of AF at baseline (1987 to 1989) and with measurements of fasting serum phosphorus from the Atherosclerosis Risk In Communities (ARIC) study. The incidence of AF was ascertained through the end of 2008 from study visit electrocardiograms, hospitalizations, and death certificates. Cox proportional hazard models were used to estimate the hazard ratios of AF by the serum phosphorus levels, adjusting for potential confounders. During a median follow-up of 19.7 years, we identified 1,656 incident AF cases. Greater serum phosphorus was associated with a greater AF risk: the hazard ratio of AF with a 1-mg/dl increase in serum phosphorus was 1.13 (95% confidence interval 1.02 to 1.26). No significant interaction was seen by race (p = 0.88) or gender (p = 0.51). The risk of AF was increased in association with greater serum phosphorus in those with an estimated glomerular filtration rate of ≥90 ml/min/1.72 m(2) but not among those with an estimated glomerular filtration rate of <90 ml/min/1.72 m(2). The total corrected calcium levels were not related to AF risk; however, greater levels of the calcium-phosphorus product were associated with greater AF risk. In conclusion, in the present large population-based study, greater levels of serum phosphorus and the related calcium-phosphorus product were associated with a greater incidence of AF.

Factors related to nephrotic-range proteinuria in late-stage chronic kidney disease patients with diabetes mellitus

PURPOSE

Diabetic nephropathy and proteinuria are important risk factors for both end-stage renal disease and cardiovascular events. The present study aimed to identify the factors associated with nephrotic-range proteinuria in patients with advanced diabetic nephropathy.

METHODS

This cross-sectional study enrolled 386 diabetic patients with chronic kidney disease (CKD) stages 3-5, from our outpatient Department of Nephrology. Urinary protein-to-creatinine ratio was recorded. Additionally, other laboratory parameters, body mass index, blood pressure, comorbidities, and medications were also reviewed.

RESULTS

The mean age of the patients was 65.1 ± 11.6 years. Among patients with CKD stage 3 and 4, the odds ratio (OR) for nephrotic-range proteinuria in relation with systolic blood pressure significantly increased starting from 121 mmHg (OR 7.04 and 11.79 for systolic blood pressure of 121-140 and ≥141 mmHg, respectively, in comparison with systolic blood pressure below 121 mmHg). In addition, serum phosphorus ≥4.7 mg/dl was associated with significantly higher risk (OR 15.45) for severe proteinuria, compared with a phosphorus level ≤2.6 mg/dl. Finally, hypertriglyceridemia ≥241 mg/dl was also associated with higher OR for severe proteinuria, compared with a triglyceride level ≤200 mg/dl. Similar associations were found in patients with CKD stage 5.

CONCLUSIONS

Higher systolic blood pressure, serum phosphorus, and triglyceride levels are associated with nephrotic-range proteinuria in patients with diabetic nephropathy and CKD stage 3-5. Further studies should clarify whether a reduction in serum phosphorus would lead to a decrease in proteinuria in these patients.

Rickets-vitamin D deficiency and dependency

Rickets is an important problem even in countries with adequate sun exposure. The causes of rickets/osteomalacia are varied and include nutritional deficiency, especially poor dietary intake of vitamin D and calcium. Non-nutritional causes include hypophosphatemic rickets primarily due to renal phosphate losses and rickets due to renal tubular acidosis. In addition, some varieties are due to inherited defects in vitamin D metabolism and are called vitamin D dependent rickets. This chapter highlights rickets/osteomalacia related to vitamin D deficiency or to inherited defects in vitamin D metabolism. Hypophosphatemic rickets and rickets due to renal tubular acidosis are discussed in other sections of the journal.

Vitamin D in HIV-Infected Patients

Observational studies have noted very high rates of low 25(OH)D (vitamin D) levels in both the general and HIV-infected populations. In HIV-infected patients, low 25(OH)D levels are likely a combination of both traditional risk factors and HIV-specific and antiretroviral therapy-specific contributors. Because of this unique risk profile, HIV-infected persons may be at greater risk for low 25(OH)D levels and frank deficiency and/or may respond to standard repletion regimens differently than HIV-uninfected patients. Currently, the optimal repletion and maintenance dosing regimens for HIV-infected patients remain unknown, as do potential benefits of supplementation that may be unique to the HIV-infected population. This paper reviews data published on HIV infection and vitamin D health in adults over the last year.

Vitamin D metabolism in the kidney: regulation by phosphorus and fibroblast growth factor 23

1,25-Dihydroxyvitamin D (1,25(OH)(2)D) plays a critical role in calcium and phosphorus (Pi) metabolism, bone growth, and tissue differentiation. The synthesis of 1,25(OH)(2)D in the proximal renal tubule is the primary determinant of its circulating concentration and is mediated by the mitochondrial enzyme, 25-hydroxyvitamin D-1α-hydroxylase, CYP27B1). Enzyme activity in the kidney is tightly regulated by several factors, of which Pi and fibroblast growth factor 23 (FGF-23) are important determinants. In healthy human subjects and experimental animals, dietary Pi restriction and resultant hypophosphatemia stimulate renal 1,25(OH)(2)D production by transcriptional up regulation of the 1α-hydroxylase gene, and this effect is independent of serum concentrations of PTH. Dietary Pi intake and serum Pi concentration also are important determinants of the circulating concentration of FGF-23, itself a potent regulator of Pi and vitamin D metabolism. In several inherited human hypophosphatemic diseases, including X-linked hypophosphatemia, serum FGF-23 concentrations are increased, resulting in renal Pi wasting, hypophosphatemia, inappropriately low serum concentrations of 1,25(OH)(2)D, and growth retardation and rickets in children. Experimental studies demonstrate that direct administration of recombinant FGF-23 or its over-expression in mice induces a dose-dependent decrease in renal CYP27B1 mRNA expression, an increase in renal 24-hydroxylase mRNA expression, and a consequent decrease in serum 1,25(OH)(2)D concentrations. Studies in vitro and in vivo demonstrate that activation of MEK/ERK1/2 signaling in the kidney is necessary for the suppression of CYP27B1 gene expression by FGF-23. Thus, phosphorus and FGF-23 are important physiologic determinants of the renal metabolism of 1,25(OH)(2)D.

Vitamin D metabolism, cartilage and bone fracture repair

The 1,25-(OH)(2)D metabolite mediates the endocrine actions of vitamin D by regulating in the small intestine the expression of target genes that play a critical role in intestinal calcium absorption. The major role of the vitamin D hormone on bone is indirect and mediated through its endocrine function on mineral homeostasis. However, genetic manipulation of the expression of Cyp27b1 or the VDR in chondrocytes strongly support a direct role for locally synthesized 1,25(OH)(2)D, acting through the VDR, in vascular invasion and osteoclastogenesis during endochondral bone development. Cells from the growth plate respond to the 24,25-(OH)(2)D and 1,25-(OH)(2)D metabolites in a cell maturation-dependent manner and the effects of 1,25-(OH)(2)D are thought to be mediated through binding to the membrane-associated receptor PDIA3 (protein disulfide isomerase associated 3). The physiological relevance of membrane-mediated 1,25-(OH)(2)D signaling is emerging and is discussed. Finally, preliminary results suggest that mice deficient for Cyp24a1 exhibit a delay in bone fracture healing and support a role for 24,25-(OH)(2)D in mammalian fracture repair.

Lower concentrations of serum phosphorus within the normal range could be associated with less calcification of the coronary artery in Koreans with normal renal function

BACKGROUND

Serum phosphorus concentrations are associated with an increased risk of cardiovascular disease (CVD) and mortality in patients with renal insufficiency. This association has also been reported in Western individuals without chronic kidney disease (CKD).

OBJECTIVE

It is unclear, however, whether this correlation occurs in Korean individuals without CKD, who usually ingest less phosphorus than do Western individuals.

DESIGN

We reviewed the findings in 402 healthy Korean adults with a mean (±SD) age of 50.8 ± 8.5 y (n = 257 men and 145 women) and a glomerular filtration rate of 83.5 ± 14.1 mL/min, who underwent health screening with electron-beam computed tomography (EBCT). The study population was separated into 4 groups on the basis of the coronary calcium concentration (Agatston score: 0, >0 to ≤10, >10 to ≤100, and >100). Mean serum phosphorus concentrations, measured ≥10 y before EBCT, were compared.

RESULTS

Multivariate analysis showed that age (P = 0.001), male sex (P = 0.002), family history of CVD (P = 0.006), serum glucose (P = 0.003), and serum phosphorus >3.6 mg/dL (P = 0.008) were significant factors influencing the coronary calcification group with an Agatston score >100, when those with an Agatston score of 0 were considered as the reference group. Compared with the group with a serum phosphorus concentration ≤3.3 mg/dL, the OR of an Agatston score >100 in individuals with a serum phosphorus concentration >3.6 to ≤3.9 mg/dL was 3.89 (95% CI: 1.43, 10.63; P = 0.008) and in those with a serum phosphorus concentration >3.9 mg/dL was 3.17 (95% CI: 1.19, 8.41; P = 0.021).

CONCLUSION

A lower concentration of serum phosphorus within the normal range could be associated with less calcification of the coronary artery in Koreans with normal renal function.