Identification of a Vitamin D response element in the human insulin receptor gene promoter

Vitamin D deficiency in women with polycystic ovary syndrome

OBJECTIVE

TO INVESTIGATE: the prevalence of vitamin D deficiency in Korean women with polycystic ovary syndrome (PCOS), and the relationship between vitamin D status and clinical or metabolic features in this group.

METHODS

We recruited 38 women with PCOS using the Rotterdam criteria. A total of 109 premenopausal control women were matched with patients based on age and body mass index. Serum 25-hydroxy vitamin D concentrations less than 20 ng/mL were classified as frank vitamin D deficiency. Since vitamin D may play a significant role in metabolic disturbances in women with PCOS, correlations between clinical or metabolic parameters and vitamin D status were analyzed separately in patients and controls.

RESULTS

Women with PCOS showed no differences in the level of 25-hydroxy vitamin D (19.6±6.6 ng/mL in patients vs. 20.1±7.4 ng/mL in controls, respectively, p=0.696) or prevalence of vitamin D deficiency (57.9% in patients vs. 56.5% in controls, respectively, p=0.880). In addition, we did not find any correlations between serum vitamin D level and clinical or metabolic profiles in either PCOS patients or controls.

CONCLUSION

Our study found no differences in the absolute level of serum vitamin D between PCOS patients and matched controls. Prevalence of vitamin D deficiency was equally common among both patients and controls. Additionally, we did not find any correlations between serum vitamin D level and clinical or metabolic profiles, suggesting that the role of vitamin D in the pathogenesis of PCOS is not yet clear.

Vitamin D: link between osteoporosis, obesity, and diabetes?

Vitamin D (1,25(OH)₂D₃) is a steroid hormone that has a range of physiological functions in skeletal and nonskeletal tissues, and can contribute to prevent and/or treat osteoporosis, obesity, and Type 2 diabetes mellitus (T2DM). In bone metabolism, vitamin D increases the plasma levels of calcium and phosphorus, regulates osteoblast and osteoclast the activity, and combats PTH hypersecretion, promoting bone formation and preventing/treating osteoporosis. This evidence is supported by most clinical studies, especially those that have included calcium and assessed the effects of vitamin D doses (≥800 IU/day) on bone mineral density. However, annual megadoses should be avoided as they impair bone health. Recent findings suggest that low serum vitamin D is the consequence (not the cause) of obesity and the results from randomized double-blind clinical trials are still scarce and inconclusive to establish the relationship between vitamin D, obesity, and T2DM. Nevertheless, there is evidence that vitamin D inhibits fat accumulation, increases insulin synthesis and preserves pancreatic islet cells, decreases insulin resistance and reduces hunger, favoring obesity and T2DM control. To date, there is not enough scientific evidence to support the use of vitamin D as a pathway to prevent and/or treat obesity and T2DM.

Effects of correction of vitamin D insufficiency on serum osteocalcin and glucose metabolism in obese children

OBJECTIVES

Osteocalcin (OCN) and vitamin D insufficiency (VDI) have been shown to be associated with abnormal glucose metabolism (GluMet). Whether correction of VDI affects serum OCN is unknown. This study evaluated the effects of correction of VDI on OCN and GluMet, and determined the associations of OCN with 25-hydroxyvitamin D (25-OHD) and GluMet.

DESIGN, PATIENTS AND MEASUREMENTS

This study involved 230 obese children in a cross-sectional part and 72 participants in a prospective part in which children with VDI were treated with vitamin D2 at a dose of 20 000 IU daily for 28 days. All 230 children underwent an oral glucose tolerance test and had their serum total and undercarboxylated OCNs and 25-OHD measured. Forty of 72 children were reassessed for the GluMet and serum total and undercarboxylated OCNs and 25-OHD after the vitamin D2 treatment.

RESULTS

In the prospective part, correction of VDI by raising mean (SD) 25-OHD of 51·5 (12·3) to 141·8 (40·8) nmol/l resulted in an improvement of their GluMet and increase in their whole-body insulin sensitivity index with no changes in their OCN measures. In the cross-sectional part, after adjustments for age, sex and puberty, the total (β = 0·322) and undercarboxylated OCNs (β = 0·315) were positively associated with insulinogenic index, which is an index of insulin secretion (P = 0·034 and 0·037, respectively) in the group of prediabetic and diabetic children.

CONCLUSIONS

Correction of VDI increased insulin sensitivity and improved GluMet, but had no effect on serum OCN measures. OCN was associated with increased insulin secretion in children with abnormal GluMet.

Inverse relationship between vitamin D status and insulin resistance and the risk of impaired fasting glucose in Korean children and adolescents: the Korean National Health and Nutrition Examination Survey (KNHANES) 2009-2010

OBJECTIVE

To investigate whether low vitamin D status was related to insulin resistance (IR) or impaired fasting glucose (IFG) in Korean adolescents, after adjusting for total body fat mass (FM).

DESIGN

A cross-sectional study.

SETTING

Korea National Health and Nutrition Examination Survey (KNAHNES) 2009-2010.

SUBJECTS

In total, 1466 participants (769 males) aged 10-19 years were assessed for serum 25-hydroxyvitamin D (25(OH)D) levels, for FM by whole-body dual-energy X-ray absorptiometry and for IR by homeostasis model assessment (HOMA-IR) after an 8 h fast.

RESULTS

Age-, sex-, season- and physical-activity-adjusted regression models showed that serum 25(OH)D levels were significantly related to markers of adiposity (P = 0.016 for FM (g), P = 0.023 for FM (%) and P = 0.035 for fat mass index). When the participants were stratified into three 25(OH)D categories (<37.5 nmol/l (n 553), 37.5 to < 50 nmol/l (n 543) and ≥ 50 nmol/l (n 370)), significantly decreasing trends were observed for fasting insulin (all P < 0.001), HOMA-IR (all P < 0.001) and the odds ratios for IFG (all P for trend < 0.05) from the lowest to the highest 25(OH)D category, after adjustments for age, sex, physical activity and all markers of adiposity. In the multivariate logistic regression analysis, the likelihood of participants in the lowest serum 25(OH)D category having IFG was 2.96-3.15 compared with those in the highest 25(OH)D category (all P < 0.05).

CONCLUSIONS

There was a significant inverse relationship between vitamin D status and IR and the risk of IFG, independent of adiposity, in Korean adolescents.

Vitamin D and diabetes

There has been increasing evidence that vitamin D may have a role in modifying risk of diabetes. Vitamin D has both direct and indirect effects on various mechanisms related to the pathophysiology of type 2 diabetes, including pancreatic beta cell dysfunction, impaired insulin action and systemic inflammation. This article describes the biologic plausibility behind the potential association between vitamin D and type 2 diabetes and summarizes the current evidence from human studies that suggests but does not prove a relation between vitamin D and type 2 diabetes, and briefly reports on the potential association between vitamin D and type 1 diabetes.

The study of serum vitamin d and insulin resistance in chinese populations with normal glucose tolerance

Objectives. The aim of this study was to investigate the relationship between serum vitamin D and insulin resistance in Chinese subjects without diabetes mellitus. Methods. Serum 25(OH)D was measured in 897 individuals with normal glucose tolerance (NGT). Oral glucose tolerance tests (OGTTs) were conducted to exclude cases with diabetes, impaired fasting glucose (IFG), and impaired glucose tolerance (IGT). Metabolic parameters were measured and compared between the highest and lowest 25(OH)D quartiles. The relationship between serum 25(OH)D and homeostatic model assessment-insulin resistance (HOMA-IR) was analyzed. Results. Indexes, such as HOMA-IR, FINS, and SBP, were negatively correlated with serum 25(OH)D concentrations. Compared with the lowest quartile, individuals in the highest group had decreased Lg (HOMA-IR), Lg (FINS), and SBP. Pearson correlation analyses showed that serum 25(OH)D was negatively associated with age, BMI, Lg (HOMA-IR), and Lg (FINS). Multivariate linear regression analysis confirmed the negative correlation of Lg (HOMA-IR) and 25(OH)D. Conclusions. This study showed that serum 25(OH)D could be regarded as an independent predictor of insulin resistance for subjects without diabetes mellitus in China. Adequate vitamin D supplementation may improve multiple metabolic disturbances.

Vitamin D status and resistance exercise training independently affect glucose tolerance in older adults

We assessed the influence of serum 25-hydroxyvitamin D (25[OH]D) and parathyroid hormone (PTH) concentrations on oral glucose tolerance, body composition, and muscle strength in older, nondiabetic adults who performed resistance exercise training (RT) while consuming diets with either 0.9 or 1.2 g protein kg(-1) d(-1). We hypothesized that individuals with insufficient 25(OH)D and/or high PTH would have less improvement in glucose tolerance after 12 weeks of RT compared with individuals with sufficient 25(OH)D and lower PTH. Sixteen men and 19 women (aged 61 ± 8 years; range, 50-80 years; body mass index, 26.3 ± 3.6 kg/m(2)) performed RT 3 times/wk for 12 weeks, with oral glucose tolerance tests done at baseline and postintervention. Protein intake did not influence the responses described below. Plasma glucose area under the curve (P = .02) and 2-hour plasma glucose concentration (P = .03) were higher for vitamin D-insufficient subjects (25[OH]D <50 nmol/L, n = 7) vs vitamin D-sufficient subjects (25[OH]D ≥50 nmol/L, n = 28). These differences remained significant after adjustment for age and body mass index. Resistance exercise training reduced fat mass (mean ± SD, -6% ± 7%; P < .001) and increased lean body mass (2% ± 3%, P < .001) and whole-body muscle strength (32% ± 17%, P < .001) in these weight-stable subjects but did not affect 25(OH)D or PTH concentrations. Oral glucose tolerance improved after RT (-10% ± 16% in glucose area under the curve and -21% ± 40% in 2-hour glucose, P = .001), but baseline 25(OH)D and PTH did not influence these RT-induced changes. These findings indicate that vitamin D status and RT independently affect glucose tolerance, and a training-induced improvement in glucose tolerance does not offset the negative effect of insufficient vitamin D status in older, nondiabetic adults.

Circulating 25-hydroxyvitamin D, IRS1 variant rs2943641, and insulin resistance: replication of a gene-nutrient interaction in 4 populations of different ancestries

BACKGROUND

Associations of either insulin receptor substrate 1 (IRS1) variants or circulating 25-hydroxyvitamin D [25(OH)D] with type 2 diabetes (T2D) and insulin resistance (IR) are inconsistent. This study sought to determine whether circulating 25(OH)D modulates the association of a potentially functional variant at IRS1 (rs2943641) with insulin resistance.

METHOD

Interaction between IRS1 rs2943641 and circulating 25(OH)D on homeostasis model assessment for IR (HOMA-IR) was examined in the Boston Puerto Rican Health Study (BPRHS) (n = 1144). Replication was performed in the African-American (n = 1126), non-Hispanic white (n = 1967), and Hispanic (n = 1241) populations of the Multi-Ethnic Study of Atherosclerosis (MESA) with genotypes of 3 IRS1 variants, rs2972144, rs1515104, and rs2673142, which are tag single nucleotide polymorphisms (SNPs) and in strong linkage disequilibrium with rs2943641.

RESULTS

Higher circulating 25(OH)D was associated with lower risk of T2D and IR in BPRHS women homozygous for minor allele rs2943641T. Consistently, in each of 3 MESA populations, HOMA-IR and insulin decreased more evidently with higher circulating 25(OH)D in women of the rs2943641TT genotype than in carriers of the major allele (rs2943641C). Metaanalysis indicated significant and consistent interactions between circulating 25(OH)D and IRS1 variants on HOMA-IR (log transformed) [pooled β = -0.008, 95% CI: -0.016 to -0.001, P interaction = 0.004] and insulin (log transformed) (pooled β = -0.006, 95% CI: -0.011 to -0.002, P interaction = 0.023) in 3065 women of the 4 populations.

CONCLUSIONS

Participants with different genotypes of IRS1 rs2943641 exhibit differential benefit from high circulating 25(OH)D for the reduction of insulin resistance and T2D risk. This gene-nutrient interaction, which appears to be limited to women, warrants further examination in randomized controlled trials of vitamin D supplementation.

Enhanced transcription of pancreatic peptide YY by 1α-hydroxyvitamin D3 administration in streptozotocin-induced diabetic mice

Peptide YY (PYY) is a peptide hormone secreted from L cells in the intestine in response to food intake that regulates appetite and gastrointestinal function. PYY is also produced in the pancreatic islets. The vitamin D receptor (VDR) is a nuclear receptor for the active form of vitamin D3 that regulates numerous physiological processes. VDR is expressed in the pancreatic islets and pharmacological VDR activation increases PYY expression in mouse peripheral islet cells. Although VDR is present in insulin-producing β cells as well as non-β cells, the role of β cell VDR in Pyy transcription remains unknown. We treated mice with streptozotocin to ablate β cells in the pancreas. Pancreatic Vdr mRNA expression was decreased in streptozotocin-induced diabetic mice. Interestingly, streptozotocin-treated mice exhibited increased basal Pyy expression and 1α-hydroxyvitamin D3 treatment further increased expression. Moreover, 1α-hydroxyvitamin D3 increased mRNA expression of pancreatic polypeptide and decreased that of neuropeptide Y in streptozotocin-induced diabetic mice but not in control mice. 1α-Hydroxyvitamin D3 slightly increased mRNA expression of insulin but transcript levels were nearly undetectable in the pancreas of streptozotocin-treated mice. Thus, VDR in non-β islet cells is involved in Pyy expression in the mouse pancreas. The findings from this β cell ablation study suggest a hormone transcription regulatory network composed of β cells and non-β cells.

Genetic variation in the vitamin D receptor, plasma 25-hydroxyvitamin D, and biomarkers of cardiometabolic disease in Caucasian young adults

BACKGROUND/AIMS

Vitamin D regulates gene transcription by binding to the vitamin D receptor (VDR), potentially affecting cardiometabolic disease risk. However, studies of 25-hydroxyvitamin D [25(OH)D] and cardiometabolic disease are inconsistent. Inconsistencies may result from unaccounted for interactions between VDR genetic variants and 25(OH)D. We examined the effect of 25(OH)D on the association between VDR variants and cardiometabolic disease biomarkers.

METHODS

The relationship between 25(OH)D, 24 VDR variants, and 10 cardiometabolic biomarkers was examined in 488 Caucasians aged 20-29 years. Covariate-adjusted general linear models were used to examine the interaction effect of 25(OH)D × VDR on each biomarker. When interactions were significant (p < 0.05), relationships were further examined with analysis of covariance, stratified by tertiles of 25(OH)D and adjusted for multiple comparisons.

RESULTS

In the lowest tertile of 25(OH)D, major allele homozygotes for rs3819545 had higher insulin and HOMA-IR than minor allele carriers (p ≤ 0.002). Fasting insulin and HOMA-IR were lower in the highest than the lowest tertile of 25(OH)D among major allele homozygotes (p < 0.0001), but minor allele carriers had similar levels regardless of vitamin D status.

CONCLUSIONS

We identified 25(OH)D-dependent associations between rs3819545 and glycemic dysregulation biomarkers. Major allele homozygotes with low vitamin D status may be at increased risk of insulin resistance.

Vitamin D and type 2 diabetes mellitus (D2)

Based on increasing evidence from animal and human studies, vitamin D deficiency is now regarded as a potential risk factor for Type 2 diabetes mellitus (T2DM). Vitamin D is involved in the pathogenesis of pancreatic β-cell dysfunction, insulin resistance, and systemic inflammation, conditions that contribute to the development of T2DM. Vitamin D can affect the progress of this disease directly through the activation of its own receptor, and indirectly via the regulation of calcium homeostasis. Observational studies have revealed the association between vitamin D deficiency and incident T2DM. More double-blind randomized control studies that investigate the effects of vitamin D supplementation on insulin sensitivity, insulin secretion, and the occurrence of T2DM are needed.

Vitamin D and nonalcoholic fatty liver disease (NAFLD): is it more than just an association?

Vitamin D is a secosteroid with known effects on calcium homeostasis that has recently been shown to have other significant functions regarding immune modulation, cell differentiation and proliferation, and the inflammatory response. As our understanding of the many functions of vitamin D has grown, the presence of vitamin D deficiency (VDD) has become more evident in Western populations. Concomitantly, nonalcoholic fatty liver disease (NAFLD) has become the most common cause of chronic liver disease. NAFLD and VDD are often found together, and while this is not unexpected, given their similar associations with obesity and sedentary lifestyle, a growing body of evidence points to a closely linked and potentially causative relationship between VDD and NAFLD. The epidemiologic association between VDD and NAFLD as well as the role of VDD in the pathogenesis of NAFLD and the available evidence on the clinical utility of vitamin D replacement in NAFLD populations are discussed.

Meta-analysis: vitamin D and non-alcoholic fatty liver disease

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is a highly prevalent condition. Emerging evidence suggests that vitamin D may play a role in the pathogenesis of NAFLD.

AIM

To review systematically the association between vitamin D levels, measured as serum 25-hydroxy vitamin D [25(OH)D], and NAFLD.

METHODS

We used PubMed and EMBASE databases to identify all studies that assessed the association between vitamin D and NAFLD up until 22 April 2013, without language restrictions. We included studies that compared vitamin D levels between NAFLD cases and controls and also those that compared the odds of vitamin D deficiency by NAFLD status. Pooled standardised differences and odds ratios were calculated using an inverse variance method.

RESULTS

Seventeen cross-sectional and case-control studies have evaluated the association between vitamin D and NAFLD. NAFLD was diagnosed using biopsy (4 studies), ultrasound or CT (10 studies) and liver enzymes (3 studies). Nine studies provided data for a quantitative meta-analysis. Compared to controls, NAFLD patients had 0.36 ng/mL (95% CI: 0.32, 0.40 ng/mL) lower levels of 25(OH)D and were 1.26 times more likely to be vitamin D deficient (OR 1.26, 95% CI: 1.17, 1.35).

CONCLUSIONS

NAFLD patients have decreased serum 25(OH)D concentrations, suggesting that vitamin D may play a role in the development of NAFLD. The directionality of this association cannot be determined from cross-sectional studies. Demonstration of a causal role of hypovitaminosis D in NAFLD development in future studies could have important therapeutic implications.

Endothelial relaxation mechanisms and nitrative stress are partly restored by Vitamin D3 therapy in a rat model of polycystic ovary syndrome

AIMS

In polycystic ovary syndrome (PCOS), metabolic and cardiovascular dysfunction is related to hyperandrogenic status and insulin resistance, however, Vitamin D3 has a beneficial effect partly due to its anti-oxidant capacity. Nitrative stress is a major factor in the development of cardiovascular dysfunction and insulin resistance in various diseases. Our aim was to determine the effects of vitamin D3 in a rat model of PCOS, particularly the pathogenic role of nitrative stress.

MAIN METHODS

Female Wistar rats weighing 100-140g were administered vehicle (C), dihydrotestosterone (DHT) or dihydrotestosterone plus vitamin D3 (DHT+D) (n=10 per group). On the 10th week, acetylcholine (Ach) induced relaxation ability of the isolated thoracic aorta rings was determined. In order to examine the possible role of endothelial nitric oxide synthase (eNOS) and cyclooxygenase-2 (COX-2) pathways in the impaired endothelial function, immunohistochemical labeling of aortas with anti-eNOS and anti-COX-2 antibodies was performed. Leukocyte smears, aorta and ovary tissue sections were also immunostained with anti-nitrotyrosine antibody to determine nitrative stress.

KEY FINDINGS

Relaxation ability of aorta was reduced in group DHT, and vitamin D3 partly restored Ach induced relaxation. eNOS labeling was significantly lower in DHT rats compared to the other two groups, however COX-2 staining showed an increment. Nitrative stress showed a significant increase in response to dihydrotestosterone, while vitamin D3 treatment, in case of the ovaries, was able to reverse this effect.

SIGNIFICANCE

Nitrative stress may play a role in the pathogenesis of PCOS and in the development of the therapeutic effect of vitamin D3.

Correcting vitamin D insufficiency improves insulin sensitivity in obese adolescents: a randomized controlled trial

BACKGROUND

Obese adolescents are at a greater risk of vitamin D deficiency because vitamin D is thought to be sequestered by excess adipose tissue. Poor vitamin D status has been associated with a higher prevalence of the metabolic syndrome, type 2 diabetes, or both in adults and adolescents.

OBJECTIVE

The objective was to determine in obese adolescents the efficacy and safety of 4000 IU vitamin D3/d and whether subsequent increased circulating concentrations of 25-hydroxyvitamin D [25(OH)D] are associated with improved markers of insulin sensitivity and resistance and reduced inflammation.

DESIGN

Obese adolescent patients [n = 35; mean ± SD age: 14.1 ± 2.8 y; BMI (in kg/m(2)): 39.8 ± 6.1; 25(OH)D: 19.6 ± 7.1 ng/mL] were recruited from the University of Missouri Adolescent Diabetes and Obesity Clinic and were randomly assigned to receive either vitamin D3 (4000 IU/d) or placebo as part of their standard care. Anthropometric measurements, inflammatory markers (IL-6, TNF-α, C-reactive protein), adipokines (leptin, adiponectin), fasting glucose, fasting insulin, and HOMA-IR values were measured at baseline and at 2 follow-up visits (3 and 6 mo).

RESULTS

After 6 mo, there were no significant differences in BMI, serum inflammatory markers, or plasma glucose concentrations between groups. Participants supplemented with vitamin D3 had increases in serum 25(OH)D concentrations (19.5 compared with 2.8 ng/mL for placebo; P < 0.001), fasting insulin (-6.5 compared with +1.2 μU/mL for placebo; P = 0.026), HOMA-IR (-1.363 compared with +0.27 for placebo; P = 0.033), and leptin-to-adiponectin ratio (-1.41 compared with +0.10 for placebo; P = 0.045). Inflammatory markers remained unchanged.

CONCLUSION

The correction of poor vitamin D status through dietary supplementation may be an effective addition to the standard treatment of obesity and its associated insulin resistance. This trial was registered at clinicaltrials.gov as NCT00994396.

The effect of vitamin D on insulin resistance in patients with type 2 diabetes

INTRODUCTION

Over the past decade, numerous non-skeletal diseases have been reported to be associated with vitamin D deficiency including type2 diabetes mellitus (T2DM). Different studies provide evidence that vitamin D may play a functional role in glucose tolerance through its effects on insulin secretion and insulin sensitivity. This study evaluates the effects of vitamin D supplementation on insulin resistance in T2DM.

METHOD

Through a before-after study, 100 patients with T2DM, 30-70 years old, were recruited from an Arak diabetes clinic as consecutive attenders. Participants were assessed for clinical and biochemistry. Serum insulin and, 25(OH)D concentration, and HOMA-IR was calculated. All measurements were performed at the beginning and the end of the study. Patients received 50,000 unit of vitamin D3 orally per week for eight weeks, Statistical analysis was made using SPSS17. The results were analyzed by descriptive tests, and a comparison between variables were made using paired T-tests or Wilcoxon tests, as appropriate.

RESULTS

100 participants including 70 women (70%) and 30 men (30%) took part in the study. All results were presented as Mean±SD, or medians of non-normally distributed.24% of the participants were Vitamin D deficient {serum 25(OH)D ≤ 20 ng/ml(50 nmol/l)}.Mean serum 25 (OH) D concentration was 43.03± 19.28 ng/ml (107.5±48.2 nmol/l).The results at baseline and at the end, for FPG were 138.48±36.74 and 131.02±39 mg/dl (P=0.05), for insulin, 10.76±9.46 and 8.6±8.25 μIu/ml (P=0.028) and for HOMA-IR, 3.57±3.18 and 2.89±3.28 (P=0.008) respectively.

CONCLUSION

Our data showed significant improvements in serum FPG, insulin and in HOMA-IR after treatment with vitamin D, suggested that vitamin D supplementation could reduce insulin resistance in T2DM.

The roles of vitamin D in skeletal muscle: form, function, and metabolism

Beyond its established role in bone and mineral homeostasis, there is emerging evidence that vitamin D exerts a range of effects in skeletal muscle. Reports of profound muscle weakness and changes in the muscle morphology of adults with vitamin D deficiency have long been described. These reports have been supplemented by numerous trials assessing the impact of vitamin D on muscle strength and mass and falls in predominantly elderly and deficient populations. At a basic level, animal models have confirmed that vitamin D deficiency and congenital aberrations in the vitamin D endocrine system may result in muscle weakness. To explain these effects, some molecular mechanisms by which vitamin D impacts on muscle cell differentiation, intracellular calcium handling, and genomic activity have been elucidated. There are also suggestions that vitamin D alters muscle metabolism, specifically its sensitivity to insulin, which is a pertinent feature in the pathophysiology of insulin resistance and type 2 diabetes. We will review the range of human clinical, animal, and cell studies that address the impact of vitamin D in skeletal muscle, and discuss the controversial issues. This is a vibrant field of research and one that continues to extend the frontiers of knowledge of vitamin D's broad functional repertoire.

The olfactory system in Alzheimer’s disease: Pathology, pathophysiology and pathway for therapy

Olfaction is frequently mentioned as a “neglected sense”, although the olfactory system has several interesting and unique anatomical and physiological features. Olfactory involvement is present in several degenerative disorders, especially in Alzheimer’s disease (AD). The peripheral and central parts of the olfactory system are damaged even in the early stages of AD, manifesting in profound olfactory deficits. Besides the early pathology, the olfactory system may be involved in the pathogenesis of AD by providing a route of entry for pathological agents still unknown. In contrast to this olfactory vector hypothesis, the olfactory system can be used to deliver therapeutic agents in AD, such as nerve growth factor and insulin, by decreasing the side-effects of the therapy or providing a non-invasive method of delivery.

Sunlight and Vitamin D: A global perspective for health

Vitamin D is the sunshine vitamin that has been produced on this earth for more than 500 million years. During exposure to sunlight 7-dehydrocholesterol in the skin absorbs UV B radiation and is converted to previtamin D3 which in turn isomerizes into vitamin D3. Previtamin D3 and vitamin D3 also absorb UV B radiation and are converted into a variety of photoproducts some of which have unique biologic properties. Sun induced vitamin D synthesis is greatly influenced by season, time of day, latitude, altitude, air pollution, skin pigmentation, sunscreen use, passing through glass and plastic, and aging. Vitamin D is metabolized sequentially in the liver and kidneys into 25-hydroxyvitamin D which is a major circulating form and 1,25-dihydroxyvitamin D which is the biologically active form respectively. 1,25-dihydroxyvitamin D plays an important role in regulating calcium and phosphate metabolism for maintenance of metabolic functions and for skeletal health. Most cells and organs in the body have a vitamin D receptor and many cells and organs are able to produce 1,25-dihydroxyvitamin D. As a result 1,25-dihydroxyvitamin D influences a large number of biologic pathways which may help explain association studies relating vitamin D deficiency and living at higher latitudes with increased risk for many chronic diseases including autoimmune diseases, some cancers, cardiovascular disease, infectious disease, schizophrenia and type 2 diabetes. A three-part strategy of increasing food fortification programs with vitamin D, sensible sun exposure recommendations and encouraging ingestion of a vitamin D supplement when needed should be implemented to prevent global vitamin D deficiency and its negative health consequences.

Low 25-hydroxyvitamin D and risk of type 2 diabetes: a prospective cohort study and metaanalysis

BACKGROUND

Vitamin D deficiency has been implicated in decreased insulin secretion and increased insulin resistance, hallmarks of type 2 diabetes mellitus. We tested the hypothesis that low plasma 25-hydroxyvitamin D [25(OH)D] is associated with increased risk of type 2 diabetes in the general population.

METHODS

We measured 25(OH)D in 9841 participants from the general population, of whom 810 developed type 2 diabetes during 29 years of follow-up. Analyses were adjusted for sex, age, smoking status, body mass index, income, physical activity, HDL cholesterol, and calendar month of blood draw.

RESULTS

Lower 25(OH)D concentrations, by clinical categories or seasonally adjusted quartiles, were associated with higher cumulative incidence of type 2 diabetes (trend, P = 2×10(-7) and P = 4×10(-10)). Multivariable adjusted hazard ratios of type 2 diabetes were 1.22 (95% CI 0.85-1.74) for 25(OH)D <5 vs ≥20 μg/L and 1.35 (1.09-1.66) for lowest vs highest quartile. Also, the multivariable adjusted hazard ratio of type 2 diabetes for a 50% lower concentration of 25(OH)D was 1.12 (1.03-1.21); the corresponding hazard ratio for those ≤58 years old was 1.26 (1.15-1.41). Finally, in a metaanalysis of 16 studies, the odds ratio for type 2 diabetes was 1.50 (1.33-1.70) for the bottom vs top quartile of 25(OH)D.

CONCLUSIONS

We observed an association of low plasma 25(OH)D with increased risk of type 2 diabetes. This finding was substantiated in a metaanalysis.

Vitamin D deficiency: a new risk factor for type 2 diabetes?

Recent compelling evidence suggests a role of vitamin D deficiency in the pathogenesis of insulin resistance and insulin secretion derangements, with a consequent possible interference with type 2 diabetes mellitus. The mechanism of this link is incompletely understood. In fact, vitamin D deficiency is usually detected in obesity in which insulin resistance is also a common finding. The coexistence of insulin resistance and vitamin D deficiency has generated several hypotheses. Some cross-sectional and prospective studies have suggested that vitamin D deficiency may play a role in worsening insulin resistance; others have identified obesity as a risk factor predisposing individuals to exhibit both vitamin D deficiency and insulin resistance. The available data from intervention studies are largely confounded, and inadequate considerations of seasonal effects on 25(OH)D concentrations are also a common design flaw in many studies. On the contrary, there is strong evidence that obesity might cause both vitamin D deficiency and insulin resistance, leaving open the possibility that vitamin D and diabetes are not related at all. Although it might seem premature to draw firm conclusions on the role of vitamin D supplementation in reducing insulin resistance and preventing type 2 diabetes, this manuscript will review the circumstances leading to vitamin D deficiency and how such a deficiency can eventually independently affect insulin sensitivity.

Vitamin D receptor activation induces peptide YY transcription in pancreatic islets

Peptide YY (PYY) is a peptide hormone secreted from L cells in the intestine after food intake and regulates appetite and intestinal function. PYY is also expressed in the pancreas, but the mechanisms of regulation of pancreatic PYY expression have not been elucidated. The vitamin D receptor (VDR) is a nuclear receptor for the active form of vitamin D(3) and regulates numerous physiological processes. Because VDR is expressed in the pancreas, we investigated the role of pancreatic VDR activation and found that Pyy is a VDR target gene in the mouse pancreas. Treatment of mice with 1α-hydroxyvitamin D(3) increased plasma PYY levels. VDR activation increased mRNA and protein expression of PYY in the pancreatic islets of mice and pancreatic endocrine cell lines but did not change intestinal PYY expression. 1α-Hydroxyvitamin D(3)-dependent induction of pancreatic and plasma PYY was abolished in VDR-null mice. We identified a functional vitamin D-responsive element in the mouse Pyy promoter using chromatin immunoprecipitation assay, EMSA, and luciferase promoter assay. Thus, Pyy is a tissue-specific VDR target gene. The pancreatic VDR-PYY pathway may mediate a regulatory function of vitamin D in the neuroendocrine system.

Vitamin D, type 2 diabetes and other metabolic outcomes: a systematic review and meta-analysis of prospective studies

Vitamin D status may influence the risk of developing metabolic diseases such as Type 2 diabetes (T2D), metabolic syndrome (MetS) and insulin resistance (IR). Several studies have assessed vitamin D in relationship with metabolic outcomes; however, results remain inconsistent. A systematic review and meta-analysis using multiple databases (MEDLINE, Web of Science and EMBASE), was performed up to 10 August 2012. Prospective studies reporting association of circulating or dietary vitamin D with incident T2D, MetS and IR outcomes were included. Relative risks (RR) were pooled using random effects and subgroup analysis by pertinent study-level characteristics was performed. A total of seventeen articles based on eighteen unique prospective studies, and comprising 210 107 participants with 15 899 metabolic events, collected during a median follow up of 10 years (range 3-22 years), were included. RR for individuals in top v. bottom thirds of baseline vitamin D were 0·81 (95% CI 0·71, 0·92); 0·86 (95% CI 0·80, 0·92); and 0·84 (95% CI 0·64, 1·12) for T2D, MetS and IR outcomes, respectively. Moderate heterogeneity was found between fourteen studies (I 2 = 67%, P < 0·001) reporting on T2D. Findings were generally consistent across various study-level characteristics. In conclusion, vitamin D status at baseline in apparently healthy adults is inversely associated with future risks of T2D and MetS. Interventions aimed at maintaining adequate levels of vitamin D in addition to preventing deficiency may be a useful preventive measure for metabolic diseases. However, reliable evidence from carefully designed intervention studies, particularly those based on healthy populations, is needed to confirm observational findings.

Vitamin D deficiency, insulin resistance, serum adipokine, and leptin levels in peritoneal dialysis patients

OBJECTIVE

Associations between 25 hydroxy vitamin D [25(OH)D], adipokines levels, and insulin resistance have been reported. The aim of this study was to explore the effects of cholecalciferol supplementation on vitamin D levels, insulin resistance, leptin, and adiponectin levels in vitamin D-deficient peritoneal dialysis (PD) patients.

METHODS

In nineteen vitamin D-deficient PD patients, who were treated with cholecalciferol, fasting serum glucose, insulin, adiponectin, leptin, 25(OH)D and parathyroid hormone (PTH) were measured before and after cholecalciferol replacement therapy. Eighteen (94.7 %) PD patients with vitamin D deficiency were receiving active vitamin D compounds (alphacalciferol) for PTH control. Alphacalciferol dosing was kept constant during treatment with cholecalciferol.

RESULTS

While mean 25(OH)D significantly increased from (10.2 ± 4.9 ng/ml) to (82.9 ± 56.5 ng/ml) (p < 0.05), mean homeostatic model assessment-insulin resistance index significantly decreased from (4.6 ± 3.6) to (2.8 ± 2.0) after cholecalciferol replacement therapy (p < 0.05). Serum leptin levels (12.9 ± 17.6 ng/ml) significantly increased (18.1 ± 19.5 ng/ml) (p < 0.05), while there was no change in serum adiponectin, calcium, and phosphate after vitamin D replacement. Serum PTH levels significantly decreased from 551.9 ± 276.6 pg/ml to 434.0 ± 273.4 ng/ml.

CONCLUSIONS

Cholecalciferol replacement therapy significantly decreases PTH levels and insulin resistance. The results of this study need to be confirmed in larger clinical trials.

Joint effects of obesity and vitamin D insufficiency on insulin resistance and type 2 diabetes: results from the NHANES 2001-2006

OBJECTIVE

The possible interaction of serum 25-hydroxyvitamin D [25(OH)D] and obesity in regard to type 2 diabetes and insulin resistance has not been well studied. To explore the effect modification of obesity on the association between 25(OH)D and insulin resistance/type 2 diabetes, data were examined from a nationally representative sample.

RESEARCH DESIGN AND METHODS

The analytic sample for the type 2 diabetes analysis (n = 12,900) was limited to participants from the National Health and Nutrition Examination Survey (NHANES) 2001-2006 over 20 years of age. Participants >20 years of age assigned to the morning session and free of diabetes were limited to the insulin resistance analysis (n = 5,806). Multiplicative interaction was assessed through a cross-product interaction term in a multiple logistic regression model. The presence of additive interaction between insufficient 25(OH)D and obesity (indicated by BMI or waist circumference) was evaluated by calculation of the relative excess risk due to interaction (RERI) and attributable proportion due to interaction (AP).

RESULTS

There was no multiplicative interaction of insufficient 25(OH)D and obesity on type 2 diabetes or insulin resistance. Furthermore, none of the RERI or AP values were statistically significant in the diabetes analysis. However, there was strong additive interaction between abdominal obesity and insufficient 25(OH)D (RERI 6.45 [95% CI 1.03-11.52]) in regard to insulin resistance. In addition, 47% of the increased odds of insulin resistance can be explained by interaction between insufficient 25(OH)D and high BMI (AP 0.47 [95% CI 0.08-0.87]).

CONCLUSIONS

Within a cross-sectional, nationally representative sample, abdominal obesity and insufficient 25(OH)D interact to synergistically influence the risk of insulin resistance.

Synergism of CYP2R1 and CYP27B1 polymorphisms and susceptibility to type 1 diabetes in Egyptian children

CYP2R1 (25α-hydroxylase) catalyzes vitamin D(3) to 25-hydroxyvitamin D(3), while the CYP27B1 (1α-hydroxylase) catalyzes the 25(OH)D(3) to 1, 25(OH)(2)D(3). 1, 25(OH)(2)D(3) prevents the development of autoimmune diabetes. We aimed to investigate CYP2R1 and CYP27B1 genes polymorphisms and susceptibility to type 1 diabetes in children. One hundred and twenty type 1 diabetic patients and One hundred and twenty controls were genotyped for CYP2R1 (rs10741657) and CYP27B1 (rs10877012) polymorphism. GG genotype of CYP2R1 increased risk to develop type 1 diabetes, and CC genotype of CYP27B1 increased risk to develop type 1 diabetes. Our finding suggested that GG genotype of CYP2R1 polymorphism and/or CC genotype of CYP27B1 polymorphism increased the risk of developing of type 1 diabetes in Egyptian children. In addition there was a synergism between GG genotype of CYP2R1 and CC genotype of CYP27B1 regarding the risk of development of type 1 diabetes.

Effect of 1, 25(OH)₂ vitamin D₃ on glucose homeostasis and DNA damage in type 2 diabetic mice

AIMS

The purpose of the study was to examine the effect of 1, 25(OH)₂ Vitamin D₃ supplementation on type 2 diabetic (T2DM) mice.

MATERIALS AND METHODS

A total of 24 mice were taken and divided into three groups of control; diabetic and diabetic + vitamin D supplemented ones. Serum calcium level, fasting blood glucose level (FBG), hexokinase activity, glucose-6-phosphatse and fructose 1,6 bisphosphatase activity were measured to establish a relevant correlation between vitamin D supplementation and hyperglycemia in T2DM.

RESULTS

There occurred an increase in FBG levels (250 ± 0.41 mg/dl) and a significant decrease in serum calcium levels in the diabetic group (8.63 ± 0.40 mg/ml) both of which reached near control levels on vitamin D₃ supplementation. The activity of the glucose metabolic enzymes was also assayed in diabetic group and was found to be deviated from control group; hexokinase (0.0241 ± 0.014 μg/mg/ml) FBPase (0.433 ± 0.002 μg/mg/ml) and G6Pase (0.918 ± 0.02 μg/mg/ml). However, the activity of these enzymes returned to near control values with hexokinase activity reaching 0.717 ± 0.003 μg/mg/ml on vitamin D₃ supplementation. The FBPase and G6Pase activities were decreased to 0.2733 ± 0.008 μg/mg/ml and G6Pase 0.71 ± 0.01 μg/mg/ml respectively. In addition to enzymatic analysis, the organs of all three groups of mice were subjected to comet assay. The diabetic group receiving vitamin D supplementation showed a marked recovery exhibiting shorter tail length both in liver (21.80 ± 2.40 μm) and pancreatic cells (19.25 ± 1.90 μm) as compared to the diabetic group exhibiting a tail length of 30.41 ± 2.50 μm and 32.45 ± 2.87 μm in liver and pancreatic cells respectively.

CONCLUSION

The present study shows that vitamin D₃ supplementation is positively correlated with decrease in blood glucose level and serum calcium level in fasting condition. This suggests a positive influence of vitamin D on glucose homeostasis. Besides, the activity of various glucose metabolic enzymes (hexokinase, FBPase and G6Pase) as shown by our results and the remarkable shortening of DNA tail length in vitamin D supplemented diabetic group as compared to diabetic group without supplementation further support the idea that vitamin D supplementation might be an add-on therapy for patients with T2DM.

Extraskeletal effects of vitamin D

The presence of vitamin D receptors in diverse tissues like immune cells, beta-cells in the pancreas, and cardiac myocytes has prompted research to evaluate the impact of vitamin D deficiency on the occurrence of immune diseases, diabetes, and cardiovascular disease (CVD). The expression of receptors not only in normal cells, but also in cancer cells including breast, prostate, and colon cancer cells has moreover opened the path to therapeutic exploitation of vitamin D or its metabolites and hypocalcemic structural analogues as pharmaceutical tools in the fight against chronic non-communicable diseases like diabetes, CVD, and cancer.

Effect of activated vitamin D on glucoparameters in HCV seropositive and seronegative patients on chronic hemodialysis

INTRODUCTION

Many studies support the role of vitamin D in the pathogenesis of both types of diabetes. Pancreatic tissues express the vitamin D receptor (VDR) and vitamin D-binding protein; some allelic variations in genes involved in vitamin D metabolism and VDR are associated with glucose intolerance, defective insulin secretion, and sensitivity. Epidemiological links have been established between type 2 diabetes mellitus (DM) and hepatitis C virus (HCV) infection.

AIM

To explore the possible therapeutic potential of pharmacologic doses of 1-α-hydroxy vitamin D therapy in improving pancreatic β-cell function in HCV seropositive hemodialysis (HD) patients.

PATIENTS AND METHODS

Twenty HCV seropositive HD patients and 20 HCV seronegative patients as control group were randomly selected from HD units. 1-α-Hydroxy vitamin D therapy was administrated in the dose ranged from 0.25 to 0.5 μg/day for 3 months. Corrected total serum calcium, phosphorus, intact parathyroid hormone (iPTH), 25-hydroxy vitamin D [25(OH) vitamin D], 1,25-dihydroxy vitamin D, and glucoparameters [fasting blood glucose, glycohemoglobin test (HbA1c%), homeostatic model assessment (HOMA)-insulin resistance, and HOMA-β-cell function% (B%)] were measured under basal conditions and after 3 months of therapy.

RESULTS

There was highly significant improvement in the concentrations of fetal bovine serum (FBS), serum insulin, HbA1c%, 25(OH) vitamin D, and HOMA-β-cell function in HCV seropositive and HCV seronegative groups after oral 1-alphacalcidiol therapy (p < 0.001). Positive correlation exists between the percentage increase in serum insulin and that in HOMA-β-cell function versus 25(OH) vitamin D (p < 0.021 and p < 0.027, respectively) in HCV negative group.

CONCLUSION

1-α-Hydroxy vitamin D oral therapy may improve glycemic control in HCV seropositive and HCV seronegative HD patients.

Vitamin D responsive elements within the HLA-DRB1 promoter region in Sardinian multiple sclerosis associated alleles

Vitamin D response elements (VDREs) have been found in the promoter region of the MS-associated allele HLA-DRB1*15:01, suggesting that with low vitamin D availability VDREs are incapable of inducing *15:01 expression allowing in early life autoreactive T-cells to escape central thymic deletion. The Italian island of Sardinia exhibits a very high frequency of MS and high solar radiation exposure. We test the contribution of VDREs analysing the promoter region of the MS-associated DRB1 *04:05, *03:01, *13:01 and *15:01 and non-MS-associated *16:01, *01, *11, *07:01 alleles in a cohort of Sardinians (44 MS patients and 112 healthy subjects). Sequencing of the DRB1 promoter region revealed a homozygous canonical VDRE in all *15:01, *16:01, *11 and in 45/73 *03:01 and in heterozygous state in 28/73 *03:01 and all *01 alleles. A new mutated homozygous VDRE was found in all *13:03, *04:05 and *07:01 alleles. Functionality of mutated and canonical VDREs was assessed for its potential to modulate levels of DRB1 gene expression using an in vitro transactivation assay after stimulation with active vitamin D metabolite. Vitamin D failed to increase promoter activity of the *04:05 and *03:01 alleles carrying the new mutated VDRE, while the *16:01 and *03:01 alleles carrying the canonical VDRE sequence showed significantly increased transcriptional activity. The ability of VDR to bind the mutant VDRE in the DRB1 promoter was evaluated by EMSA. Efficient binding of VDR to the VDRE sequence found in the *16:01 and in the *15:01 allele reduced electrophoretic mobility when either an anti-VDR or an anti-RXR monoclonal antibody was added. Conversely, the Sardinian mutated VDRE sample showed very low affinity for the RXR/VDR heterodimer. These data seem to exclude a role of VDREs in the promoter region of the DRB1 gene in susceptibility to MS carried by DRB1* alleles in Sardinian patients.

Vitamin D as a predictor of insulin resistance in polycystic ovarian syndrome

INTRODUCTION

Polycystic Ovary Syndrome (PCOS) is the most common endocrinological disorder in women in the reproductive age group. The salient features of this condition include hyperandrogenic features, infertility and insulin resistance among others. Mechanisms behind these features are a matter of debate. Vitamin D has been implicated lately in the etiology of many disorders. The aim of our study was to assess the role of vitamin D as an etiological and predictive factor in PCOS.

MATERIALS AND METHODS

The study comprised 60 proven cases of PCOS diagnosed on the basis of Rotterdam criteria. The parameters assessed include HOMA-IR, vitamin D besides the routine anthropometric and biochemical parameters.

RESULTS

The study population was divided into 3 groups according to vitamin D status. Insulin resistance was most severe in the sub group with vitamin D deficiency. Multiple regression analysis established the role of vitamin D as the best predictor of insulin resistance in our study.

CONCLUSION

Vitamin D has an important role in the pathogenesis of insulin resistance in PCOS.

Vitamin D and fertility: a systematic review

BACKGROUND

Vitamin D has been well-known for its function in maintaining calcium and phosphorus homeostasis and promoting bone mineralization. There is some evidence that in addition to sex steroid hormones, the classic regulators of human reproduction, vitamin D also modulates reproductive processes in women and men.

AIM

The aim of this review was to assess the studies that evaluated the relationship between vitamin D and fertility in women and men as well as in animals.

METHODS

We performed a systematic literature search in Pubmed for relevant English language publications published until October 2011.

RESULTS AND DISCUSSION

The vitamin D receptor (VDR) and vitamin D metabolizing enzymes are found in reproductive tissues of women and men. Vdr knockout mice have significant gonadal insufficiency, decreased sperm count and motility, and histological abnormalities of testis, ovary and uterus. Moreover, we present evidence that vitamin D is involved in female reproduction including IVF outcome (clinical pregnancy rates) and polycystic ovary syndrome (PCOS). In PCOS women, low 25-hydroxyvitamin D (25(OH)D) levels are associated with obesity, metabolic, and endocrine disturbances and vitamin D supplementation might improve menstrual frequency and metabolic disturbances in those women. Moreover, vitamin D might influence steroidogenesis of sex hormones (estradiol and progesterone) in healthy women and high 25(OH)D levels might be associated with endometriosis. In men, vitamin D is positively associated with semen quality and androgen status. Moreover, vitamin D treatment might increase testosterone levels. Testiculopathic men show low CYP21R expression, low 25(OH)D levels, and osteoporosis despite normal testosterone levels.

Association of vitamin D receptor gene polymorphisms with insulin resistance and response to vitamin D

The objectives of the study were to determine associations between single nucleotide polymorphisms (SNPs) of the vitamin D receptor (VDR) gene and insulin resistance and the effects of these SNPs on changes in insulin sensitivity in response to vitamin D supplementation. The research described here was an extension of the Surya study. Genotyping of the Cdx-2, FokI, BsmI, ApaI, and TaqI SNPs was carried out on 239 South Asian women in New Zealand using polymerase chain reaction-based techniques. Associations of these genotypes and 3' end haplotypes with insulin resistance were determined using multiple regression analysis. Associations between SNP genotypes and responses in insulin sensitivity to vitamin D supplementation (4000 IU vitamin D(3) per day) were also determined for a subset (81) of these women. BsmI BB, ApaI AA, and TaqI tt genotypes were significantly associated with lower insulin resistance compared with BsmI bb, ApaI aa, and TaqI TT, respectively, in the cohort of 239 women. Furthermore, homozygosity of the haplotypes baT and BAt was associated with higher and lower insulin resistance, respectively, compared with no copies of their respective alleles. Of the 81 subjects who were supplemented with vitamin D, women with the FokI Ff genotype showed a significantly greater improvement in insulin sensitivity (increase of 29.4 [2.9, 38.1]) compared with women with the FokI FF genotype (increase of 2.3 [-11.5, 10.1]). This study has highlighted the association of vitamin D responsiveness and insulin resistance with VDR gene polymorphisms. This is the first study to determine associations between all three. Genotyping of the VDR gene may provide a predictive measure for insulin resistance in response to vitamin D intervention.

Effect of vitamin D3 in reducing metabolic and oxidative stress in the liver of streptozotocin-induced diabetic rats

Diabetes mellitus is a growing health problem worldwide and is associated with severe liver complications. The aim of the present study is to analyse the status of metabolic and free-radical-scavenging enzymes and second messengers in the liver of streptozotocin (STZ)-induced diabetic rats, and to determine the hepatoprotective role of vitamin D(3). All studies were performed using the liver of adult male Wistar rats. Gene expression studies were carried out using real-time PCR with specific probes. Second messenger levels were determined using (3)H-labelled Biotrak assay kits, and glucose uptake assay with D-[(14)C]glucose. The present results show that there was a decrease in hepatic glucose uptake, malate dehydrogenase activity, glycogen content, inositol triphosphate (IP(3)) and cyclic GMP levels, and superoxide dismutase, glutathione peroxidase, phospholipase C, cyclic AMP-responsive element-binding protein, vitamin D receptor (VDR) and insulin receptor (INSR) gene expression in the diabetic rats when compared with the controls (all P < 0·05), while cyclic AMP levels and GLUT2 expression were increased (P < 0·05). Treatment of the diabetic rats with vitamin D(3) and insulin reversed the altered parameters to near control values. In conclusion, the data suggest a novel role of vitamin D(3) in restoring impaired liver metabolism in STZ-induced diabetic rats by regulating glucose uptake, storage and metabolism. We demonstrated that the restoring effect of vitamin D(3) is mediated through VDR modulation, thereby improving signal transduction and controlling free radicals in the liver of diabetic rats. These data suggest a potential role for vitamin D(3) in the treatment of diabetes-associated hepatic complications.

Vitamin D deficiency in obese children and its relationship to glucose homeostasis

OBJECTIVES

The aim of the study was to compare the prevalence of vitamin D deficiency in obese and non-overweight children in North Texas, to examine relationships between dietary habits and 25-hydroxyvitamin D [25(OH)D] level in obese children, and to examine the relationship between 25(OH)D level and markers of abnormal glucose metabolism and blood pressure.

PATIENTS AND METHODS

Using a cross-sectional design, systolic and diastolic blood pressure, dietary information, serum 25(OH)D, fasting glucose and insulin, 2-h glucose from oral glucose tolerance test, hemoglobin A1c, and homeostasis model assessment of insulin resistance were recorded for 411 obese subjects (6-16 yr old) at an obesity referral clinic. 25(OH)D was also obtained from 87 control non-overweight subjects (6-16 yr old).

RESULTS

Ninety-two percent of obese subjects had a 25(OH)D level below 75 nmol/liter, and 50% were below 50 nmol/liter. Among non-overweight subjects, these frequencies were 68 and 22%, respectively (both P < 0.01 compared with obese subjects). 25(OH)D was negatively associated with soda intake (P < 0.001), juice intake (P = 0.009), and skipping breakfast (P < 0.001). 25(OH)D was negatively correlated with homeostasis model assessment of insulin resistance (r = -0.19; P = 0.001) and 2-h glucose (r = -0.12; P = 0.04) after adjustment for body mass index and age but was not correlated with hemoglobin A1c, systolic blood pressure Z score, or diastolic blood pressure Z score.

CONCLUSIONS

Vitamin D deficiency is common in children in this southern United States location and is significantly more prevalent in obese children. Lower 25(OH)D level is associated with risk factors for type 2 diabetes in obese children.

Is dietary fat, vitamin D, or folate associated with pancreatic cancer?

Although potentially modifiable risk factors for pancreatic cancer include smoking, obesity, and diabetes, less is known about the extent to which diet affects cancer risk. Recent studies have demonstrated some consistency for dietary fat being associated with elevated pancreatic cancer risk, particularly from animal sources. However, less is known about which fatty acids pose the greatest risk. Vitamin D, due to its endogenous production following UV-B exposure, is a unique risk factor in that researchers have created several methods to assess its exposure in humans. Studies that measured vitamin D exposure differently have shown inconsistent results. Dietary studies suggest protective associations, whereas studies of circulating 25-hydroxyvitamin D status show null or positive associations with low or very high concentrations, respectively. Several, but not all epidemiologic studies provide evidence of an inverse relationship between total and/or dietary folate and risk of pancreatic cancer. Protective associations for circulating folate are more often observed among populations with inadequate status. This article reviews the current epidemiological and experimental evidence investigating the relationship of dietary fat, vitamin D, and folate with pancreatic cancer. Additionally the mechanisms by which these risk factors may contribute to cancer, the methodological challenges involved with assessing risk, and other obstacles encountered when ascertaining the magnitude and direction of these three exposures are discussed.

Identification of virtual signal transducers and activators of transcription response elements in the human insulin receptor gene promoter

In this study, we look for the existence of signal transducers and activators of transcription response elements (STATREs) in the human insulin receptor (hIR) gene promoter and their possible relation with the estradiol-provoked transcriptional repression of the hIR gene and cellular insulin resistance in U-937 human promonocytic cells. Potential STATREs in the region from -1819 to -271 bp of the hIR gene promoter were identified by their homology with the consensus STATRE (5'TTCnnnGAA3') using the SEQFIND programme developed in our laboratory. We located five virtual STATRE-like sites: [(I): -1472/-1464], [(II): -1548/-1540], [(III): -1552/-1544], [(IV): -1587/-1579] and [(V): -1678/-1670] showing a difference of only one base from this consensus. These STATREs-like sites were situated between 33 bp upstream the 5' half-element of the estrogen response element 1 (ERE1)-like (-1430/-1418) and 102 bp upstream the 5' half-element of the ERE2-like (-1567/-1555) complexed with AP-1-like sites. A principal complex constituted by STATREs (II-IV) the ERE2 and AP-1 sites (IV and V) was located between -1587/-1540 bp of the hIR gene promoter. In conclusion, these results represent the first identification of virtual STATREs in the hIR gene promoter. These STATREs appear to be specifically located in the surroundings of the two EREs overlapped by various AP-1 sites. These complexes could mediate crosstalk among STATs, estrogen receptor β (ERβ), and AP-1 regulating the ERβ-mediated transcriptional repression of the hIR gene and insulin resistance in U-937 cells.

Vitamin D and diabetes: its importance for beta cell and immune function

Experimental evidence indicates that vitamin D may play a role in the defense against type 1 diabetes (T1D) as well as type 2 diabetes (T2D). Epidemiological data have established a link between vitamin D deficiency and an increased incidence of both T1D and T2D, whereas early and long-term vitamin D supplementation may decrease the risk of these disorders. The protective effects of vitamin D are mediated through the regulation of several components such as the immune system and calcium homeostasis. However, an increasing amount of evidence suggests that vitamin D also affects beta cells directly thereby rendering them more resistant to the types of cellular stress encountered during T1D and T2D. This review evaluates the role of vitamin D signaling in the pathogenesis of T1D and T2D with a special emphasis on the direct effects of vitamin D on pancreatic beta cells.

Diabetes, pancreatic cancer and vitamin D. Is there a link?

The role of vitamin D is not merely limited to maintaining skeletal health but also extends to maintaining glucose homeostasis by preserving insulin secretion and sensitivity and thus deficiency of vitamin D plays an important role in aetiopathogenesis of T2 diabetes. In addition to its many other roles, vitamin D has recently been found to have growth inhibiting affects on pancreatic cancer cells. Ecological studies have shown that there exists an inverse correlation between sun exposure and death rates for pancreatic cancer. Since vitamin D has promising role in both type 2 diabetes mellitus and pancreatic cancer, its deficiency may be associated to any or both of these chronic diseases. The present review thus aims to find correlation between diabetes and pancreatic cancer and if vitamin D is a common link between the two.

Modulation of vitamin D signaling is a potential therapeutic target to lower cardiovascular risk in chronic kidney disease

While it is true that many traditional cardiovascular risk factors are amenable to intervention in chronic kidney disease (CKD), the results of intervention may not be as efficacious as those obtained in the general population. Thus, there may also be a unique milieu established in CKD, which causes excess cardiovascular disease (CVD) burden by mechanisms that are as yet not fully recognized. Recently, vitamin D has sparked widespread interest because of its potential favorable benefits on CVD. However, the mechanisms for how vitamin D may improve CVD risk markers and outcomes have not been fully elucidated. Furthermore, hypovitaminosis D is highly prevalent in the CKD cohort. Given this background, we hypothesize that low vitamin D status may act as a new CVD risk marker, and modulation of vitamin D signaling may be a potential therapeutic target to lower cardiovascular risk in CKD. The data presented in this review support that the low vitamin D status may be linked with the high cardiovascular risk in CKD, based on both the biological effects of vitamin D itself on the cardiovascular system, and the cross-actions between vitamin D signaling and the multiple metabolic pathways. Considering the high prevalence of hypovitaminosis D, limited natural vitamin D food sources, and reduced sun exposure in CKD patients, recommendations for treatment of hypovitaminosis D mainly focus on exogenous supplementation with vitamin D and its analogues. Although promising, when to start therapy, the route of administration, the dose, and the duration remain need to be discussed.

Effects of vitamin D and calcium supplementation on pancreatic β cell function, insulin sensitivity, and glycemia in adults at high risk of diabetes: the Calcium and Vitamin D for Diabetes Mellitus (CaDDM) randomized controlled trial

BACKGROUND

A suboptimal vitamin D and calcium status has been associated with higher risk of type 2 diabetes in observational studies, but evidence from trials is lacking.

OBJECTIVE

We determined whether vitamin D supplementation, with or without calcium, improved glucose homeostasis in adults at high risk of diabetes.

DESIGN

Ninety-two adults were randomly assigned in a 2-by-2 factorial-design, double-masked, placebo-controlled trial to receive either cholecalciferol (2000 IU once daily) or calcium carbonate (400 mg twice daily) for 16 wk. The primary outcome was the change in pancreatic β cell function as measured by the disposition index after an intravenous-glucose-tolerance test. Other outcomes were acute insulin response, insulin sensitivity, and measures of glycemia.

RESULTS

Participants had a mean age of 57 y, a body mass index (BMI; in kg/m(2)) of 32, and glycated hemoglobin (Hb A(1c)) of 5.9%. There was no significant vitamin D × calcium interaction on any outcomes. The disposition index increased in the vitamin D group and decreased in the no-vitamin D group (adjusted mean change ± SE: 300 ± 130 compared with -126 ± 127, respectively; P = 0.011), which was explained by an improvement in insulin secretion (62 ± 39 compared with -36 ± 37 mU · L(-1) · min, respectively; P = 0.046). Hb A(1c) increased less, but nonsignificantly, in the vitamin D group than in the no-vitamin D group (0.06 ± 0.03% compared with 0.14 ± 0.03%, respectively; P = 0.081). There was no significant difference in any outcomes with calcium compared with no calcium.

CONCLUSION

In adults at risk of type 2 diabetes, short-term supplementation with cholecalciferol improved β cell function and had a marginal effect on attenuating the rise in Hb A(1c). This trial was registered at clinicaltrials.gov as NCT00436475.

Vitamin D-associated polymorphisms are related to insulin resistance and vitamin D deficiency in polycystic ovary syndrome

INTRODUCTION

Women with polycystic ovary syndrome (PCOS) frequently suffer from metabolic disturbances including insulin resistance (IR), which might be related to vitamin D metabolism. We aimed to investigate the association of polymorphisms in the vitamin D receptor (VDR) gene as well as vitamin D level-associated genes with metabolic and endocrine parameters in PCOS women. Moreover, we examined whether there are associations with PCOS susceptibility.

METHODS

Metabolic, endocrine, and anthropometric measurements and oral glucose tolerance tests were performed in 545 PCOS and 145 control women. Genotyping of VDR (Cdx2, Bsm-I, Fok-I, Apa-I, and Taq-I), GC, DHCR7, and CYP2R1 polymorphisms was performed.

RESULTS

25-Hydroxyvitamin D (25(OH)D) levels showed significant negative correlation with IR and positive correlation with insulin sensitivity (P<0.05 for all) in PCOS women. In PCOS women, the VDR Cdx2 'AA' genotype was associated with lower fasting insulin (P=0.039) and homeostatic model assessment-IR (P=0.041) and higher quantitative insulin-sensitivity check index (P=0.012) and MATSUDA index (P=0.003). The VDR Apa-I 'AA' genotype was associated with lower testosterone (P=0.028) levels. In PCOS women, 170 women (31.2%) presented with 25(OH)D levels <20 ng/ml. PCOS women carrying the GC 'GG' genotype and the DHCR7 'GG' genotype had a significantly higher risk for 25(OH)D levels <20 ng/ml (OR 2.53 (1.27-5.06), P=0.009, and OR 2.66 (1.08-6.55), P=0.033 respectively) compared with PCOS women carrying the GC 'TT' genotype and DHCR 'TT' genotype in multivariate analyses. We observed no association of genetic variations and PCOS susceptibility.

CONCLUSION

VDR and vitamin D level-related variants are associated with metabolic and endocrine parameters including 25(OH)D levels in PCOS women.

An update on the islet renin-angiotensin system

The traditional renin-angiotensin system (RAS) components have been studied extensively since the rate-limiting component of RAS, renin, was first characterized. The ongoing identification of various novel RAS components and signaling pathways continues to elaborate the complexity of this system. Regulation of RAS according to the conventional and contemporary views of its functions in various tissues under pathophysiological conditions is a main treatment strategy for many metabolic diseases. The local pancreatic RAS, first proposed to exist in pancreatic islets two decades ago, could regulate islet function and glycemic control via influences on islet cell mass, inflammation, and ion channels. Insulin secretion, the major function of pancreatic islets, is controlled by numerous factors. Among these factors and of particular interest are glucagon-like peptide-1 (GLP-1) and vitamin D, which may regulate islet function by directly binding receptors on islet beta cells. These factors may work with local RAS signaling in islets to protect and maintain islet function under diabetic and hyperglycemic conditions. In this concise review, the local islet RAS will be discussed with particular attention being paid to recent notable findings.

Effect of supplementation with vitamin D₃ on glucose production pathways in human subjects

SCOPE

Research reports suggest that vitamin D affects glucose and insulin metabolism; however, the exact mechanisms are unclear. ²H NMR analysis of monoacetone glucose (MAG) after tracer administration provides a non-invasive method of profiling hepatic glucose metabolism. This study examined the effects of supplementation with vitamin D₃ on contribution of glycogenolysis to glucose production.

METHODS AND RESULTS

Tracer administration and biofluid collections were performed with eight healthy females before and following a 4-wk vitamin D₃ administration period. Following an overnight fast subjects ingested deuterated water and acetaminophen. Full void urine samples were collected after 4 h. ²H NMR spectra of urinary monoacetone glucose were acquired to determine the contribution of glycogenolysis to glucose production. The mean contribution of glycogenolysis to glucose production was 60±13%. Supplementation with vitamin D₃ had no effect on hepatic glucose production. Regression analysis revealed a significant relationship between carbohydrate intake and the contribution of glycogenolysis (β=0.914, p=0.004).

CONCLUSION

In conclusion, we saw no changes in the percentage contribution of glycogenolysis following supplementation with vitamin D₃. The reproducibility of our results and the non-invasive nature of the method highlight the potential for this method in assessing mechanistic modes of action in future nutritional interventions.

Severe hypoglycemia with endogenous hyperinsulinemia in a nondiabetic hemodialysis patient following parathyroidectomy

Insulin resistance and abnormal carbohydrate metabolism is a recognized complication in patients with advanced chronic kidney disease. We describe a rare case of severe hypoglycemia with inappropriate endogenous hyperinsulinemia in a hemodialysis patient following parathyroidectomy for severe secondary hyperparathyroidism. The patient required intravenous dextrose infusions for 3 days to prevent neuroglycopenic symptoms. Extensive workup for hypoglycaemia revealed high C-peptide and insulin levels in the absence of insulinoma. It is postulated that reversal of parathyroid hormone (PTH)-mediated inhibition of insulin secretion, along with large doses of 1,25-dihydroxyvitamin D3 (calcitriol) were responsible for the increase in insulin secretion and consequent hypoglycemia in this patient.

Vitamin d and pancreatic cancer-an update

The non-classical actions of vitamin D, namely anti-proliferation, pro-differentiation, immune function modulation, and anti-inflammation, have received great attention during the past decade, in particular, the potential of vitamin D analogs alone or in combination with other anticancer agents for the treatment of a variety of cancers. The association between vitamin D status and the higher incidence of many forms of cancer has suggested that vitamin D may play a role in the etiology of these types of cancer. Although it is still controversial whether this association exists for pancreatic cancer, biochemical evidence clearly indicates pancreatic cancer cells are responsive to the inhibitory effect of vitamin D and its analogs. In this review, we discuss briefly the origin and current therapy of pancreatic cancer, the history, source, metabolism and functions of vitamin D, the recent progress in the epidemiological studies of sunlight, and vitamin D status, and biochemical studies of vitamin D analogs in the prevention and treatment of pancreatic cancer.