The Role of Vitamin D in the Prevention of Type 2 Diabetes: To D or Not to D?

Multifaceted Roles of Vitamin D for Diabetes: From Immunomodulatory Functions to Metabolic Regulations

Numerous studies have established associations between vitamin D and diabetes. The vitamin D receptor is widely distributed throughout the human body, including in pancreatic beta cells (β-cells), hepatocytes, and immune cells. Therefore, vitamin D's effect on the risk, progression, or complications of diabetes may be mediated through various mechanisms. These include the regulation of insulin secretion or sensitivity and modulation of β-cell function and its immunomodulatory and anti-inflammatory effects. This review extensively explores the relationship between vitamin D status and diabetes, as well as the preventive or therapeutic effects of vitamin D supplementation on diabetes from human studies. Additionally, it examines in detail the impact of vitamin D on immune and inflammatory responses in the diabetic milieux and β-cell function to better understand the underlying mechanisms through which vitamin D influences diabetes.

The Correlation Between Vitamin D Levels and the Glycemic Marker HbA1c and Lipid Profile in Patients With Type 2 Diabetes Mellitus: A Study at the King Saud Medical City, Riyadh

Background and objective Vitamin D, a fat-soluble vitamin also called the sunshine vitamin, is produced in plants, and animals when exposed to sunlight. It plays a crucial role in musculoskeletal development, immune system regulation, and glucose metabolism, thereby reducing the risk of diabetes. This study aimed to investigate the association of vitamin D levels with glycemic control markers [glycated hemoglobin (HbA1c)] and lipid profile, as well as sociodemographic factors and comorbidities. Methodology A cross-sectional study was conducted at the King Saud Medical City in Riyadh, Saudi Arabia, among adult diabetic patients aged 20 years and above. The sociodemographic characteristics, vitamin D levels, HbA1c, and lipid profiles of 472 participants were studied. Data were analyzed using SPSS Statistics version 27 (IBM Corp., Armonk, NY). Results The majority of the participants were women (n=296, 62.7%); the mean age of the cohort was 56.5 ±13.13 years. Most participants were Saudi nationals (n=361, 76.5%). Lab tests revealed vitamin D deficiency (71.41 ±36.88 nmol/l) and elevated HbA1c (9.49 ±9.85%) in the cohort. Low-density lipoprotein (LDL) cholesterol levels were higher than normal (2.71 ±4.26 mmol/l), while high-density lipoprotein (HDL) was slightly lower (1.23 ±0.39 mmol/l). Bivariate correlations showed weak negative and positive associations between vitamin D and HbA1c (r=-0.093, p<0.05) and HDL (r=0.114, p<0.05), respectively. HbA1c correlated positively with triglycerides (r=0.168, p<0.01). Conclusions We found an association between deficiency of vitamin D and levels of HbA1c and lipid profile in type 2 diabetes patients. The association was marked by low vitamin D levels and characterized by high HbA1c, LDL cholesterol, and lipid profile. Elevated HbA1c, LDL cholesterol, and triglyceride levels suggested vitamin D's role in lipid homeostasis. Variations in biomarker levels across sociodemographic factors highlight the need for personalized interventions for diabetes prevention and management.

Effects of 1,25-Vitamin D3 and 24,25-Vitamin D3 on Corneal Nerve Regeneration in Diabetic Mice

Corneal nerve homeostasis is essential for the functional integrity of the ocular surface. Vitamin D deficiency (VDD) and vitamin D receptor knockout (VDR KO) have been found to reduce corneal nerve density in diabetic mice. This is the first study to comprehensively examine the influence of vitamin D on nerve regeneration following corneal epithelial injury in diabetic mice. Corneal nerve regeneration was significantly retarded by diabetes, VDR KO, and VDD, and it was accelerated following topical 1,25 Vit D and 24,25 Vit D administration. Furthermore, topical 1,25 Vit D and 24,25 Vit D increased nerve growth factor, glial cell line-derived neurotropic factor, and neurotropin-3 protein expression, and it increased secretion of GDNF protein from human corneal epithelial cells. CD45+ cells and macrophage numbers were significantly decreased, and vitamin D increased CD45+ cell and macrophage recruitment in these wounded diabetic mouse corneas. The accelerated nerve regeneration observed in these corneas following topical 1,25 Vit D and 24,25 Vit D administration may be related to the vitamin D-stimulated expression, secretion of neurotrophic factors, and recruitment of immune cells.

Metabolic and Genetic Association of Vitamin D with Calcium Signaling and Insulin Resistance

Various evidences have unveiled the significance of Vitamin D in diverse processes which include its action in prevention of immune dysfunction, cancer and cardiometabolic disorders. Studies have confirmed the function of VD in controlling the expression of approximately nine hundred genes including gene expression of insulin. VD insufficiency may be linked with the pathogenesis of diseases that are associated with insulin resistance (IR) including diabetes as well as obesity. Thus, VD lowers IR-related disorders such as inflammation and oxidative stress. This review provides an insight regarding the molecular mechanism manifesting, how insufficiency of VD may be connected with the IR and diabetes. It also discusses the effect of VD in maintaining the Ca2+ levels in beta cells of the pancreas and in the tissues that are responsive to insulin.

Association between food intake patterns and serum vitamin D concentrations in US adults

The objective of this study was to investigate the association between food patterns and serum vitamin D concentrations in US adults. Data from two cycles of National Health and Nutrition Examination Surveys, 2003-2004 and 2005-2006, were used (n 6637). Three major food patterns were derived using factor analysis. These three patterns were labeled as meat and processed meat foods (MPF), vegetables, fruit, nuts, and whole grains (VFNW), and sweet, snack, and beverage pattern (SSB). Serum vitamin D was measured with RIA and later standardized to reflect the assay changes over time. In multivariate-adjusted regression analysis, the MPF pattern was significantly, inversely associated with serum vitamin D concentrations (P < 0·001). However, the relation between the VFNW pattern and serum vitamin D was non-linear (P < 0·001). There was no relationship between SSB pattern and serum vitamin D in US adults. In conclusion, persons with a high intake of meat and processed meats are associated with lower serum vitamin D concentrations. Reducing processed foods and emphasizing VFNW will be beneficial from a health perspective.

Association between vitamin D deficiency and the risk of prevalent type 2 diabetes and incident prediabetes: A prospective cohort study using data from The Irish Longitudinal Study on Ageing (TILDA)

BACKGROUND

It is hypothesized that vitamin D contributes to the aetiology of type 2 diabetes mellitus (diabetes). This study's objective was to examine the relationships between baseline vitamin D status (as measured by plasma 25-hydroxyvitamin D concentration) and both prevalent diabetes and prospective risk of developing diabetes, including prediabetes, in a population with historically low levels of vitamin D.

METHODS

In this prospective cohort study, data from The Irish Longitudinal Study on Ageing (TILDA), a nationally representative cohort of adults aged ≥50 years residing in Ireland were analysed, including wave 1 (October 2009-June 2011) (n = 5272) and wave 3 (March 2014-October 2015) (n = 3828). Those aged <50 years at baseline or who did not complete the health assessment were excluded. Logistic regression models examined the associations between baseline vitamin D concentration (nmol/L) with prevalent diabetes status and incident diabetes/prediabetes collected at a 4-year follow-up. Models were adjusted for age, sex, education, body mass index, smoking history, physical activity, use of statins, and the season in which the vitamin D concentration was sampled.

FINDINGS

Deficient baseline vitamin D concentration was cross-sectionally associated with an increased likelihood of having prevalent diabetes (Relative Risk Ratio [RRR] 1·5, 95% CI: 1·03, 2·18; p = 0·037). In longitudinal analyses evaluating diabetes status 4 years later, there was a 62% increased likelihood (RRR: 1·62, 95% CI: 1·12, 2·35; p = 0·011) of developing prediabetes for those with vitamin D <30 nmol/L compared to those with ≥75 nmol/L. The rate of progression from prediabetes to diabetes between wave 1 and 3 was observed to be 32·5%.

INTERPRETATION

Those with lower concentrations of vitamin D, as measured by 25-hydroxyvitamin D, may have different risk profiles with regards to their glycaemic status. Our study had limited power due to the low incidence of diabetes but showed strong associations with incident prediabetes, so further research is required. Optimising vitamin D status at a population level may significantly reduce diabetes.

FUNDING

TILDA is funded by Atlantic Philanthropies, the Irish Department of Health, and Irish Life, while additional funding was provided by the Irish Department of Agriculture, Food and the Marine (13F492) to cover the cost of 25-hydroxyvitamin D analysis.

Vitamin D deficiency associated with risk of prediabetes among older adults: Data from the National Health and Nutrition Examination Survey (NHANES), 2007-2012

AIM

To examine the association between vitamin D (25(OH)D) deficiency and risk of prediabetes in Americans 50+ years of age.

MATERIALS AND METHODS

This was a cross-sectional analysis of NHANES (2007-2012) subjects aged 50+ years, free of kidney/liver diseases and diabetes. Prediabetes was defined as: HbA1c level 5.7%-6.4%, or fasting plasma glucose level 100-125 mg/dL, or Oral Glucose Tolerance Test result 140-199 mg/dL, with no laboratory value in the diabetic range. The comparison group had normal glucose tolerance (NGT) with no marker in the prediabetes/diabetes range. Total serum 25(OH)D levels were deficient at <50 nmol/L, insufficient 50-75 nmol/L, and sufficient >75 nmol/L. Logistic regression included strata, cluster and weight variables. Models were adjusted for body mass index (BMI), ethnicity, age and gender.

RESULTS

The final sample was 2286 adults, predominantly White (80.4%) and female (56.6%), with a mean age of 62.3 years. Within the sample, 1387 had prediabetes (59.1%) and 899 were NGT (40.9%). Individuals classified within a lower serum vitamin D category were more likely to have prediabetes (p = 0.03). Those with 25(OH)D deficiency were more likely to have prediabetes compared to 25(OH)D sufficient individuals (crude OR = 1.48, 95% CI 1.15-1.91), and this association remained significant after adjustment for ethnicity, BMI, age and gender (aOR = 1.39, 95% CI 1.02-1.89). There was no effect modification by BMI, gender or ethnicity.

CONCLUSIONS

Vitamin D status was associated with risk of prediabetes in this sample of Americans 50+ years of age. Future research should seek to understand the potential mechanistic relationship between vitamin D and prediabetes.

Associations of serum amyloid A and 25-hydroxyvitamin D with diabetic nephropathy: A cross-sectional study

BACKGROUND

The present study investigated the relationships between serum amyloid A (SAA), 25-hydroxyvitamin D (25(OH)VD) and diabetic nephropathy (DN) to provide evidence for the prevention and management of DN.

METHODS

A total of 182 patients with type 2 diabetes mellitus (T2DM) were enrolled in this study. The levels of SAA, 25(OH)VD, and other conventional indicators were measured and analyzed. Receiver operating characteristic curve analysis was applied for the combined measurement of SAA and 25(OH)VD, and risk factors for DN were evaluated using binary logistic regression analysis.

RESULTS

The levels of SAA in T2DM patients were significantly higher than those in healthy subjects, and the level significantly increased with the progression of DN (p < 0.05). In contrast, the level of 25(OH)VD in T2DM patients was significantly lower than that in healthy subjects, and the level significantly decreased with the progression of DN (p < 0.05). The combined measurement of SAA and 25(OH)VD distinguished DN patients from T2DM patients better than the measurement of SAA or 25(OH)VD alone. SAA was an independent risk factor for DN, and 25(OH)VD was an independent protective factor for DN.

CONCLUSION

SAA and 25(OH)VD might be used as potential markers to identify patients at increased risk of developing DN.

Vitamin D, respiratory infections, and chronic disease: Review of meta-analyses and randomized clinical trials

BACKGROUND

Observational studies have suggested associations of vitamin D deficiency (VDD) with respiratory tract infections, impaired bone health, and myriad chronic diseases.

OBJECTIVE

To assess potential causal relationships between vitamin D supplementation and a reduced risk of these conditions, a review of the evidence across available meta-analyses of randomized control trials (RCTs) and RCTs was performed.

METHOD

PubMed, Embase, Cochrane Library, and Web of Science were searched from their inception to March 2021. We included only RCTs and meta-analyses of RCTs focusing on the association between vitamin D and respiratory disease, bone health, cardiovascular disease (CVD), diabetes mellitus, and cancer.

RESULTS

A total of 107 RCTs and 62 meta-analysis of RCTs were included. Although most RCTs did not support benefits of vitamin D supplementation, suggestive evidence for benefit was found in populations at greater risk of VDD and for acute respiratory infections, fractures in institutionalized older adults, type 2 diabetes among patients with prediabetes, and cancer mortality. In contrast, no compelling evidence for benefit was found for other respiratory conditions, fractures in community-dwelling adults, falls, cancer incidence, or CVD.

CONCLUSIONS

Current evidence from RCTs and meta-analyses of RCTs is inconsistent regarding the effects of vitamin D supplementation on respiratory infections and chronic diseases. Individuals most likely to benefit are those with baseline VDD or with selected high-risk conditions. Public health initiatives are needed to eliminate VDD globally, and future research will be enhanced by a 'precision prevention' approach to identify those most likely to benefit from vitamin D supplementation.

A Community Based Randomized Controlled Trial to See the Effect of Vitamin D Supplementation on Development of Diabetes Among Women with Prediabetes Residing in A Rural Community of Northern India

Background:

The primary objective of this study was to intervene with vitamin D supplementation in rural-based women with pre-diabetes (impaired fasting glucose or impaired glucose tolerance) to prevent development of type 2 diabetes (T2DM).

Methods:

This was an open-label randomized placebo-controlled trial conducted in rural women with pre-diabetes and vitamin D deficiency (Clinicaltrials.gov NCT02513888). Women aged 20-60 years with pre-diabetes were selected from rural Haryana (north India) and followed up for two years. A semi-structured questionnaire was used to collect information on socio-demographic and behavioral details, like sun exposure, dietary habits, etc., The intervention group received vitamin D supplementation while control group received lactose granules as placebo. Equal doses of calcium carbonate were given to both the groups.

Results:

A total of 132 participants were recruited in the study (58 each in the intervention and control groups). It was observed that there was no statistical significance in the incidence of diabetes in the control group as compared to the intervention group at the end of 2 years (P = 0.701).

Conclusion:

Though during the first year there was some delay in development of DM in the intervention group but at the end of two years there was no significant effect of vitamin D supplementation in delaying the incidence of diabetes in these women after two years.

Trial registration:

(Clinicaltrials.gov NCT02513888).

Mechanisms Involved in the Relationship between Vitamin D and Insulin Resistance: Impact on Clinical Practice

Recent evidence has revealed anti-inflammatory properties of vitamin D as well as extra-skeletal activity. In this context, vitamin D seems to be involved in infections, autoimmune diseases, cardiometabolic diseases, and cancer development. In recent years, the relationship between vitamin D and insulin resistance has been a topic of growing interest. Low 25-hydroxyvitamin D (25(OH)D) levels appear to be associated with most of the insulin resistance disorders described to date. In fact, vitamin D deficiency may be one of the factors accelerating the development of insulin resistance. Vitamin D deficiency is a common problem in the population and may be associated with the pathogenesis of diseases related to insulin resistance, such as obesity, diabetes, metabolic syndrome (MS) and polycystic ovary syndrome (PCOS). An important question is the identification of 25(OH)D levels capable of generating an effect on insulin resistance, glucose metabolism and to decrease the risk of developing insulin resistance related disorders. The benefits of 25(OH)D supplementation/repletion on bone health are well known, and although there is a biological plausibility linking the status of vitamin D and insulin resistance supported by basic and clinical research findings, well-designed randomized clinical trials as well as basic research are necessary to know the molecular pathways involved in this association.

Early detection of macrovascular complications in type 2 diabetes mellitus in Medan, North Sumatera, Indonesia: A cross-sectional study

Background: Macrovascular complications occur very frequently in patients with type 2 diabetes mellitus (T2DM) with a high mortality rate, due to the development of cardiovascular disease (CVD), such as stroke, atherosclerosis acceleration, and atrial fibrillation. T2DM is a significant risk factor for CVD and has become the leading cause of death. The purpose of this study was to detect the early risk of macrovascular complications by using the ankle brachial index (ABI) as a marker. Methods: This study was an analytic study with a cross-sectional approach. The study population was patients with T2DM from several primary health care centers in Medan. In total, 89 subjects who met the inclusion and exclusion criteria were recruited with consecutive sampling. ABI was determined as the ratio of systolic blood pressure in the brachial artery to the posterior tibial artery after the subjects had been relaxed and felt comfortable in a supine position. Examination of vitamin D and lipid profile was derived from examination of venous blood.  Data were processed using SPSS and analyzed with one-way ANOVA. Results: The study found that there was a relationship between LDL-C, triglyceride, and vitamin D (25OH-D) based on the ABI (p > 0.05). Conclusions: ABI can be used for an early detection of macrovascular complications. Apart from being easy to perform, ABI was non-invasive. Some other risk factors that can also be used to assess complications and have relationships with ABI were LDL-C, triglyceride, and vitamin D (25OH-D). Complications in T2DM patients can be prevented with reasonable blood sugar control and lifestyle changes. Education and motivation need to be given to patients so that they become more independent in controlling their disease and improving their quality of life.

Early identification of metabolic syndrome risk: A review of reviews and proposal for defining pre-metabolic syndrome status

AIMS

a) To analyze the relationship of known and emerging biomarkers/indicators for early risk identification of cardiometabolic health risk; b) to identify early risk markers to be used in both clinical and nonclinical settings; and c) to propose a definition of early risk identification in terms of pre-metabolic syndrome (PreMetSyn).

DATA SYNTHESIS

Pubmed/Medline, Web of Science, Embase, and Cochrane were searched for Systematic Reviews and Meta-analysis. Selected studies were evaluated, and relevant data were extracted and synthesized.

CONCLUSIONS

Serum uric acid is a good predictive biomarker of metabolic syndrome (MetSyn) and has been associated with non-alcoholic liver fat disease (NAFLD) and type 2 diabetes. NAFLD emerges as an early risk indicator of PreMetSyn by itself. Muscle strength should also be included as an early risk marker of cardiometabolic health. High serum triglycerides and waist circumference confirm their predictive value regarding MetSyn. Indicators related to an inflammatory/pro-inflammatory status usually linked to MetSyn showed limited evidence as robust biomarkers for PreMetSyn. Authors suggest defining PreMetSyn related to cardiometabolic risk. It is also necessary to determine how close people are to the cut-off point of MetSyn components, including emerging indicators proposed by our review. Some biomarkers could be used as indicators of PreMetSyn, before any of the MetSyn components appear, allowing early health interventions to prevent its development. Defining a PreMetSyn status might consider both emerging indicators and those variables already included in the definition of MetSyn. New indicators should be considered to create a new risk score specifically meant for PreMetSyn.

The effect of daily intake of vitamin D-fortified yogurt drink, with and without added calcium, on serum adiponectin and sirtuins 1 and 6 in adult subjects with type 2 diabetes

BACKGROUND

Some evidence suggests indirect ameliorating effects of vitamin D in diabetes via adiponectin and sirtuins. This study aimed to evaluate the effects of daily intake of vitamin D-fortified yogurt drink, either with or without added calcium, on serum adiponectin, sirtuins (SIRT)1 and 6.

METHODS

Briefly, 75 adults aged 30-60 years from both sexes with type 2 diabetes were randomly allocated to one of the three groups: (i) D-fortified-yogurt drink (DY; containing 1000 IU vitamin D and 300 mg calcium), (ii) Ca+D-fortified-yogurt drink (CDY; containing 1000 IU vitamin D and 500 mg calcium) and (iii) plain yogurt drink (PY; containing no detectable vitamin D and 300 mg calcium). All assessments were performed initially and after 12 weeks.

RESULTS

A significant within-group increment in serum adiponectin concentrations was observed in both DY and CDY groups (+60.4 ± 8.6, +57.5 ± 6.4 µg/L, respectively; p < 0.001 for both). The concentrations of SIRT1 and SIRT6 had a significant within-group increment only in the CDY group (p = 0.003, p = 0.001 respectively). Being in CDY group was more favorable predictor of improvement in SIRT6 concentrations. Changes of 25(OH)D were a significant predictor of changes of adiponectin. However, this association disappeared following adjustment for changes of SIRT1. In contrast, the association between changes of 25(OH)D and HbA1c remained significant even after adjustment for SIRT1.

CONCLUSIONS

Daily consumption of vitamin D-fortified yogurt drink for 12 weeks resulted in an increase in circulating concentrations of SIRT1 and SIRT6 in T2D subjects and D+Ca-fortified yogurt drink was more in favor of SIRT6 increment.

Association of serum vitamin D levels with chronic disease and mortality

Introduction

Introduction: whether hypovitaminosis D is an overarching cause of increased mortality or a prognostic marker of poor health has not been well elucidated. Objectives: we sought to determine the association of serum 25-hydroxyvitamin D [25-(OH)-D3] levels with the clinical biochemical parameters and mortality risk in chronic diseases. Methods: we reviewed the clinical charts and collected the clinical biochemical parameters of patients diagnosed with chronic conditions who had at least one 25-(OH)-D3 determination, with or without calcium and vitamin D supplementation, and who were selected using a cluster random sampling design (n = 1,705). The analysis was focused on metabolic disorders (type-2 diabetes mellitus [T2DM] and obesity), autoimmune disorders, and mortality. Multivariate logistic regression analyses were performed. Results: low 25-(OH)-D3 levels were reported in 1,433 (84.0%) patients, of which 774 (45.4%) had insufficiency (20-29 ng/mL) and 659 (38.6%) patients had deficiency (< 20 ng/mL). Lower 25-(OH)-D3 levels in T2DM patients were associated with higher glycosylated hemoglobin levels (p < 0.001). Patients with 25-(OH)-D3 levels < 12.5 ng/mL had a higher mortality risk than those with levels ≥ 12.5 ng/mL (HR: 3.339; 95% CI: 1.342-8.308). We observed lower 25-(OH)-D3 levels in patients with grade-III obesity (p = 0.01). We found a higher risk of 25-(OH)-D3 deficiency in rheumatoid arthritis, type-1 diabetes, and systemic lupus erythematosus (p = 0.032, p = 0.002, p = 0.049, respectively). Conclusions: we found a significant relationship between 25-(OH)-D3 levels and glycemic control, body mass index, autoimmune disease, and mortality risk. Nevertheless, whether hypovitaminosis D plays a causal role or is a consequence of chronic disease remains controversial.

Cardiovascular Risk in Patients with Primary Hyperparathyroidism

Primary hyperparathyroidism (PHPT) is one of the most common endocrine disorders characterized by parathyroid hormone (PTH)-dependent hypercalcemia. Cardinal features include low trauma fractures, nephrolithiasis, and chronic kidney disease. Several experimental studies established that parathyroid hormone exerts actions on the cardiovascular (CV) system, including vasodilatation and positive inotropic and chronotropic effects. Observational studies, especially in severe cases, report a higher prevalence of hypertension, diabetes mellitus, lipid abnormalities, endothelial dysfunction, arrhythmias, and left ventricular hypertrophy in patients with PHPT, while the risk of CV events seems to be increased in severe cases. However, the effect of surgery is inconsistent on CV abnormalities and, more importantly, on CV disease (CVD) events, especially in mild cases. In the current review, we describe the available evidence linking PHPT and CVD, as well as the effect of surgical management and pharmacological treatment on CVD manifestations in patients with PHPT. Based on the current evidence, CVD is not considered an indication for surgery.

Vitamin D and new insights into pathophysiology of type 2 diabetes

Deficiency in vitamin D plays a role in the onset and development of insulin resistance (IR) and type 2 diabetes (T2DM). A normal level of vitamin D is able to reduce low grade inflammation, which is a major process in inducing insulin resistance. It is also engaged in maintaining low resting levels of reactive species and radicals, normal Ca²⁺ signaling, a low expression of pro-inflammatory cytokines but increased formation of anti-inflammatory cytokines. Vitamin D is also able to prevent hypermethylation (of DNA) and consequent functional inactivation of many genes, as well as other epigenetic alterations in β cells and in other insulin-sensitive peripheral tissues, mainly liver, adipose tissue and muscle. Vitamin D deficiency thus belongs to key factors accelerating the development of IR and consequently T2DM as well. However, vitamin D supplementation aimed at the control of glucose homeostasis in humans showed controversial effects. As a result, further studies are running to gain more detailed data needed for the full clinical utilization of vitamin D supplementation in the prevention and treatment of T2DM. Until new results are published, supplementation with high doses of vitamin D deficiency is not recommended. However, prevention of vitamin D deficiency and its correction are highly desired.

Insulin-sensitizing effects of vitamin D repletion mediated by adipocyte vitamin D receptor: Studies in humans and mice

Objective

Adipose tissue inflammation and fibrosis appear to contribute to insulin resistance in obesity. Vitamin D receptor (Vdr) genes are expressed by adipocytes, macrophages, and fibroblasts, all of which could potentially play a role in adipose tissue inflammation and fibrosis. As vitamin D has been shown to have direct anti-inflammatory effects on adipocytes, we determined whether specific vitamin D receptor-mediated effects on adipocytes could impact adipose tissue inflammation and fibrosis and ultimately insulin resistance.

Methods

We examined the effects of repleting vitamin D in 25(OH)D-deficient, insulin resistant, overweight-to-obese human subjects (n = 19). A comprehensive assessment of whole-body insulin action was undertaken with stepped euglycemic (∼90 mg/dL) hyperinsulinemic clamp studies both before and after the administration of vitamin D or placebo. Adipose tissue fibrosis and inflammation were quantified by real-time rt-PCR and immunofluorescence in subcutaneous abdominal adipose tissue.

To determine whether vitamin D's effects are mediated through adipocytes, we conducted hyperinsulinemic clamp studies (4 mU/kg/min) and adipose tissue analysis using an adipocyte-specific vitamin D receptor knockout (VDR-KO) mouse model (adiponectin-Cre + VDR+/fl) following high-fat diet feeding for 12 weeks.

Results

25(OH)D repletion was associated with reductions in adipose tissue expression of pro-inflammatory and pro-fibrotic genes, decreased collagen immunofluorescence, and improved hepatic insulin sensitivity in humans. Worsening trends after six months on placebo suggest progressive metabolic effects of 25(OH)D deficiency. Ad-VDR-KO mice mirrored the vitamin D-deficient humans, displaying increased adipose tissue fibrosis and inflammation and hepatic insulin resistance.

Conclusions

These complementary human and rodent studies support a beneficial role of vitamin D repletion for improving hepatic insulin resistance and reducing adipose tissue inflammation and fibrosis in targeted individuals, likely via direct effects on adipocytes. These studies have far-reaching implications for understanding the role of adipocytes in mediating adipose tissue inflammation and fibrosis and ultimately impacting insulin sensitivity.

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Vitamin D repletion improved hepatic insulin sensitivity in obese insulin-resistant and vitamin D deficient human.

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Correcting vitamin D deficiency concomitantly reduced adipose tissue expression of pro-inflammatory and pro-fibrotic genes.

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Worsening trends in these metabolic parameters were observed following 6 months of uncorrected vitamin D deficiency.

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Adipocyte-specific depletion of VDR in mice induced adipose tissue inflammation and fibrosis and hepatic insulin resistance.

Understanding the Biological Activities of Vitamin D in Type 1 Neurofibromatosis: New Insights into Disease Pathogenesis and Therapeutic Design

Vitamin D is a fat-soluble steroid hormone playing a pivotal role in calcium and phosphate homeostasis as well as in bone health. Vitamin D levels are not exclusively dependent on food intake. Indeed, the endogenous production-occurring in the skin and dependent on sun exposure-contributes to the majority amount of vitamin D present in the body. Since vitamin D receptors (VDRs) are ubiquitous and drive the expression of hundreds of genes, the interest in vitamin D has tremendously grown and its role in different diseases has been extensively studied. Several investigations indicated that vitamin D action extends far beyond bone health and calcium metabolism, showing broad effects on a variety of critical illnesses, including cancer, infections, cardiovascular and autoimmune diseases. Epidemiological studies indicated that low circulating vitamin D levels inversely correlate with cutaneous manifestations and bone abnormalities, clinical hallmarks of neurofibromatosis type 1 (NF1). NF1 is an autosomal dominant tumour predisposition syndrome causing significant pain and morbidity, for which limited treatment options are available. In this context, vitamin D or its analogues have been used to treat both skin and bone lesions in NF1 patients, alone or combined with other therapeutic agents. Here we provide an overview of vitamin D, its characteristic nutritional properties relevant for health benefits and its role in NF1 disorder. We focus on preclinical and clinical studies that demonstrated the clinical correlation between vitamin D status and NF1 disease, thus providing important insights into disease pathogenesis and new opportunities for targeted therapy.

Association of vitamin D3 and its metabolites in patients with and without type 2 diabetes and their relationship to diabetes complications

Background

Epidemiological studies have suggested that vitamin D deficiency is associated with the development of type 2 diabetes (T2DM) and is related to diabetes complications. This study was undertaken to determine the relationship between diabetes complications and cardiovascular risk factors with vitamin D₃ and its metabolites: 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), 25-hydroxyvitamin D₃ (25(OH)D₃), 24,25-dihydroxyvitamin D₃ (24,25(OH)₂D₃); and 25-hydroxy-3epi-vitamin D₃ (3epi25(OH)D₃).

Methods

750 Qatari subjects, 460 (61.3%) with and 290 (38.7%) without T2DM, who were not taking vitamin D₃ supplements, participated in this cross-sectional, observational study. Plasma concentrations of vitamin D₃ and its metabolites were measured by liquid chromatography tandem mass spectrometry analysis.

Results

T2DM subjects had lower concentrations of all vitamin D₃ metabolites (p < 0.001) except 3epi25(OH)D₃ (p < 0.071). Males had higher concentrations of all vitamin D₃ metabolites (p < 0.001). In the T2DM subjects, lower 25(OH)D₃ was associated with retinopathy (p < 0.03) and dyslipidemia (p < 0.04), but not neuropathy or vascular complications; lower 1,25(OH)₂D₃ was associated with hypertension (p < 0.009), dyslipidemia (p < 0.003) and retinopathy (p < 0.006), and coronary artery disease (p < 0.012), but not neuropathy; lower 24,25(OH)₂D₃ concentrations were associated with dyslipidemia alone (p < 0.019); 3epi25(OH)D₃ associated with diabetic neuropathy alone (p < 0.029). In nondiabetics, 25(OH)D₃, 1,25(OH)₂D₃ and 24,25(OH)₂D₃ were associated with dyslipidemia (p < 0.001, p < 0.001, p < 0.015, respectively) and lower 1,25(OH)₂D₃ was associated with hypertension (p < 0.001). Spearman's correlation showed 1,25(OH)₂D₃ to be negatively correlated to age and diabetes duration.

Conclusions

Different diabetes complications were associated with differing vitamin D parameters, with diabetic retinopathy related to lower 25(OH)D₃ and 1,25(OH)₂D₃ levels, hypertension significantly associated with lower 1,25(OH)₂D₃, while dyslipidemia was associated with lower 25(OH)D₃, 1,25(OH)₂D₃ and 24,25(OH)₂D_3. While 25(OH)D metabolites were lower in females, there was not an exaggeration in complications.

Correlating Circulating Vitamin D3 with Aspects of the Metabolic Syndrome and Nonalcoholic Fatty Liver Disease in Healthy Female Volunteers

Background: Significant inverse correlations between circulating vitamin D3 and the presence and strength of common clinical entities influenced by insulin resistance (IR) have been reported. Among these entities are common maladies such as the metabolic syndrome (MS) and nonalcoholic fatty liver disease (NAFLD). Since hypovitaminosis D3 is considered a common worldwide health disturbance, the consequences emanating from such relationships once soundly established might reasonably be ameliorated by vitamin replacement. Accordingly, a need exists for definitive confirmatory support for the existence of these linkages particularly in ordinary, relatively healthy individuals.Methodology: Many aspects of MS and NAFLD were examined in healthy females via linear regression analyses to determine significant correlations mainly using vitamin D3 and fasting blood glucose (FBG), the latter a surrogate for IR, as independent variables.Results: A descending linear regression between vitamin D3 and FBG did not reach full statistical significance, but a negative correlation of vitamin D3 with HbA1C was statistically significant and that with circulating insulin concentrations showed a downward statistical trend. These findings are consistent with previous ones by others suggesting an ameliorating effect of vitamin D3 on IR. Body weight, body mass index (BMI), and body fat correlated statistically significantly with vitamin D3 levels, as did systolic blood pressure. Again, these correlations were negative instead of positive unlike the case when FBG was the independent variable. The significant negative correlation of ALT with vitamin D3 was also consistent with previous reports that higher circulating vitamin D3 favorably influences the extent and intensity of NAFLD.Conclusions: Employing FBG as a surrogate for IR and ALT for status of NAFLD, linear correlations reasonably suggest that increasing circulating vitamin D3 can favorably influence the initial development of and/or strength of risk factors for MS including NAFLD in relatively fit females.

The Molecular Mechanisms by Which Vitamin D Prevents Insulin Resistance and Associated Disorders

Numerous studies have shown that vitamin D deficiency is very common in modern societies and is perceived as an important risk factor in the development of insulin resistance and related diseases such as obesity and type 2 diabetes (T2DM). While it is generally accepted that vitamin D is a regulator of bone homeostasis, its ability to counteract insulin resistance is subject to debate. The goal of this communication is to review the molecular mechanism by which vitamin D reduces insulin resistance and related complications. The university library, PUBMED, and Google Scholar were searched to find relevant studies to be summarized in this review article. Insulin resistance is accompanied by chronic hyperglycaemia and inflammation. Recent studies have shown that vitamin D exhibits indirect antioxidative properties and participates in the maintenance of normal resting ROS level. Appealingly, vitamin D reduces inflammation and regulates Ca²⁺ level in many cell types. Therefore, the beneficial actions of vitamin D include diminished insulin resistance which is observed as an improvement of glucose and lipid metabolism in insulin-sensitive tissues.

Vitamin D supplementation rescues simvastatin induced myopathy in mice via improving mitochondrial cristae shape

Statin induced myopathy (SIM) is a main deleterious effect leading to the poor treatment compliance, while the preventive or therapeutic treatments are absent. Mounting evidences demonstrated that vitamin D plays a vital role in muscle as a direct modulator. The deficiency of vitamin D was considered as a cause of muscle dysfunction, whereas the supplementation resulted in a remission. However, there is no causal proof that vitamin D supplementation rescues SIM. Here, using the mice model of simvastatin-induced myopathy, we investigated the role of vitamin D supplementation and the mechanisms associated with mitochondria. Results indicated that simvastatin administration (80 mg/kg) impaired skeletal muscle with the increased serum creatine kinase (CK) level and the declined grip strength, which were alleviated by vitamin D supplementation. Moreover, vitamin D supplementation rescued the energy metabolism dysfunction in simvastatin-treated mice gastrocnemius by reducing the abnormal aggregation of muscular glycogen and lactic acid. Mitochondrial homeostasis plays a key role in the process of energy metabolism. Thus, the mitochondrial dysfunction is a mortal damage for the highly energy-requiring tissue. In our study, the mitochondrial cristae observed under transmission electron microscope (TEM) were lytic in simvastatin-treated gastrocnemius. Interestingly, vitamin D supplementation improved the mitochondrial cristae shape by regulating the expression of mitofusin-1/2 (MFN1/2), optic atrophy 1 (OPA1) and dynamin-related protein 1 (Drp1). As expected, the mitochondrial dysfunction and oxidative stress was mitigated by vitamin D supplementation. In conclusion, these findings suggested that moderate vitamin D supplementation rescued simvastatin induced myopathy via improving the mitochondrial cristae shape and function.

Effects of Vitamin D Receptor Knockout and Vitamin D Deficiency on Corneal Epithelial Wound Healing and Nerve Density in Diabetic Mice

Diabetic keratopathy occurs in ∼70% of all people with diabetes. This study was designed to examine the effects of vitamin D receptor knockout (VDR-/-) and vitamin D deficiency (VDD) on corneal epithelial wound healing and nerve density in diabetic mice. Diabetes was induced using the low-dose streptozotocin method. Corneal epithelial wounds were created using an Algerbrush, and wound healing was monitored over time. Corneal nerve density was measured in unwounded mice. VDR-/- and VDD diabetic mice (diabetic for 8 and 20 weeks, respectively) had slower healing ratios than wild-type diabetic mice. VDR-/- and VDD diabetic mice also showed significantly decreased nerve density. Reduced wound healing ratios and nerve densities were not fully rescued by a supplemental diet rich in calcium, lactose, and phosphate. We conclude that VDR-/- and VDD significantly reduce both corneal epithelial wound healing and nerve density in diabetic mice. Because the supplemental diet did not rescue wound healing or nerve density, these effects are likely not specifically related to hypocalcemia. This work supports the hypothesis that low vitamin D levels can exacerbate preexisting ophthalmic conditions, such as diabetes.

Association between vitamin D and prediabetes: A PRISMA-compliant meta-analysis

BACKGROUND

Previous studies showed conflicting results for associations between vitamin D and prediabetes. The study aimed to make a systematic review and meta-analysis for the association between vitamin D and prediabetes.

METHODS

We searched for articles identifying associations between vitamin D and prediabetes published in English until July 2019 in following databases (PubMed, Web of Science, EMBASE, Medline, Google Scholar, and Cochrane databases). Finally, we conducted these analyses (heterogeneities examination, meta-regression analyses, sensitivity analysis, and publication bias examination) using STATA 12.0 software (Stata Corporation, College Station, TX, USA). Q test and I were applied to examine heterogeneities between studies.

RESULTS

Twelve studies were finally included in the present study. The study included 4 studies to explore the association between serum levels of 25-hydroxy (OH) vitamin D and risks of prediabetes (including 3094 participants). Additionally, the present study included 8 studies (including 865 individuals with prediabetes treated with vitamin D supplementation and 715 patients treated with placebo) to assess differences in therapeutic effects between individuals with prediabetes treated with vitamin D supplementation and those treated with placebo. The present study showed no significant associations between low serum levels of 25(OH) vitamin D and high risk of prediabetes. Additionally, the study showed no significant differences in changes of hemoglobin A1c (HbA1c), fasting plasma glucose (FPG), and homeostatic model assessment of insulin resistance (HOMA-IR) between individuals with prediabetes treated with vitamin D and those patients given placebo, whereas meta-analysis showed significantly greater changes in 2-hour oral glucose tolerance test (2HPG) in individuals with prediabetes treated with vitamin D, compared with individuals with prediabetes treated with placebo.

CONCLUSION

The study supported that low serum levels of 25(OH) vitamin D increased the risk of prediabetes. In addition, vitamin D supplementation improves impaired glucose tolerance in prediabetes. However, more large-scale clinical trials are essential to explore the association between vitamin D and prediabetes.

Association of Differing Qatari Genotypes with Vitamin D Metabolites

OBJECTIVE

Genetic studies have identified four Qatari genotypes: Q1 Arab, Bedouin; Q2 Asian/Persian; Q3 African; and a fourth admixed group not fitting into the previous 3 groups. This study was undertaken to determine if there was an increased risk of deficiency of vitamin D and its metabolites associated with differing genotypes, perhaps due to genetic differences in skin pigmentation.

METHODS

398 Qatari subjects (220 type 2 diabetes and 178 controls) had their genotype determined by Affymetrix 500 k SNP arrays. Total values of 1,25-dihydroxyvitamin D (1,25(OH)2D), 25-hydroxyvitamin D (25(OH)D), 24,25-dihydroxyvitamin D (24,25(OH)2D), and 25-hydroxy-3epi-vitamin D (3epi-25(OH)D) concentrations were measured by the LC-MS/MS analysis.

RESULTS

The distribution was as follows: 164 (41.2%) genotyped Q1, 149 (37.4%) genotyped Q2, 31 (7.8%) genotyped Q3, and 54 (13.6%) genotyped "admixed." Median levels of 25(OH)D and 3epi-25(OH)D did not differ across Q1, Q2, Q3, and "admixed" genotypes, respectively. 1,25(OH)2D levels were lower (p < 0.04) between Q2 and the admixed groups, and 24,25(OH)2D levels were lower (p < 0.05) between Q1 and the admixed groups. Vitamin D metabolite levels were lower in females for 25(OH)D, 1,25(OH)2D (p < 0.001), and 24,25(OH)2D (p < 0.006), but 3epi-25(OH)D did not differ (p < 0.26). Diabetes prevalence was not different between genotypes. Total 1,25(OH)2D (p < 0.001), total 24,25(OH)2D (p < 0.001), and total 3epi-25(OH)D (p < 0.005) were all significantly lower in diabetes patients compared to controls whilst the total 25(OH)D was higher in diabetes than controls (p < 0.001).

CONCLUSION

Whilst 25(OH)D levels did not differ between genotype groups, 1,25(OH)2D and 24,25(OH)2D were lower in the admixed group, suggesting that there are genetic differences in vitamin D metabolism that may be of importance in a population that may allow a more targeted approach to vitamin D replacement. This may be of specific importance in vitamin D replacement strategies with the Q2 genotype requiring less, and the other genotypes requiring more to increase 1,25(OH)2D. Whilst overall the group was vitamin D deficient, total 25(OH)D was higher in diabetes, but 1,25(OH)2D, 24,25(OH)2D, and 3epi-25(OH)D were lower in diabetes that did not affect the relationship to genotype.

Differential associations of ferritin and 25-hydroxyvitamin D with fasting glucose and diabetes risk in community dwelling older men

AIMS

We examined associations of ferritin and 25-hydroxyvitamin D with fasting glucose and prevalent diabetes in older men.

METHODS

Cross-sectional analysis of 4153 community-dwelling men aged 70 to 89 years in Western Australia. Plasma ferritin, 25-hydroxyvitamin D, and glucose were assayed. Diabetes was ascertained from self-report, medications, and fasting glucose.

RESULTS

There were 577 men with diabetes (13.9%). In the whole cohort, ferritin was associated with fasting glucose (0.051 mmol/L per 1 SD increase in ferritin, P = .006) and 25-hydroxyvitamin D was inversely associated (-0.085 mmol/L per 1 SD, P < .001). Ferritin was not associated with prevalent diabetes (highest vs. lowest quartile; >225 vs <66 μg/L: adjusted odds ratio [OR] 0.97, 95% confidence interval [CI], 0.74-1.27, P = .83). Higher vitamin D was associated with decreased odds of prevalent diabetes (highest vs lowest quartile; >82 nmol/L vs <53 nmol/L: OR = 0.57, 95% CI = 0.43-0.75, P < .001). There was no interaction between ferritin and vitamin D on diabetes risk.

CONCLUSIONS

In older men, ferritin is associated with fasting glucose but not prevalent diabetes. Higher 25-hydroxyvitamin D concentrations are independently associated with lower fasting glucose and reduced risk of diabetes. Clinical trials are required to determine whether interventions, which raise vitamin D concentrations, would reduce incidence of diabetes in this expanding demographic group.

Type 2 Diabetes Mellitus, Insulin Resistance, and Vitamin D

PURPOSE OF REVIEW

There is a growing, largely inconsistent, literature on the role of vitamin D in association with type 2 diabetes, insulin resistance/insulin secretion, glycemic indices, and complications of type 2 diabetes. Pathophysiologic, bystander, preventive, and treatment roles of vitamin D have all been proposed. In this focused review, we attempt to organize and clarify our current information in this area.

RECENT FINDINGS

Clinical study interpretation is difficult because of variability in dosage, dosage form, study duration, and populations studied, as well as recently reported normal human polymorphisms in vitamin D synthesis and catabolism, vitamin D-binding protein, and vitamin D receptors in addition to a host of potential epigenetic confounders. Low vitamin D status appears to be associated with type 2 diabetes and most other insulin resistance disorders reported to date. The extraskeletal benefits of supplementation/repletion in these disorders in our species, with a few highlighted exceptions, remain to be established. This focused review attempts to summarize our current knowledge in this burgeoning area through a review of key meta-analyses, observational studies, randomized control trials, and Mendelian randomization studies and will hopefully serve as a guide to indicate future research directions and current best practice.

Skeletal and Extraskeletal Actions of Vitamin D: Current Evidence and Outstanding Questions

The etiology of endemic rickets was discovered a century ago. Vitamin D is the precursor of 25-hydroxyvitamin D and other metabolites, including 1,25(OH)2D, the ligand for the vitamin D receptor (VDR). The effects of the vitamin D endocrine system on bone and its growth plate are primarily indirect and mediated by its effect on intestinal calcium transport and serum calcium and phosphate homeostasis. Rickets and osteomalacia can be prevented by daily supplements of 400 IU of vitamin D. Vitamin D deficiency (serum 25-hydroxyvitamin D <50 nmol/L) accelerates bone turnover, bone loss, and osteoporotic fractures. These risks can be reduced by 800 IU of vitamin D together with an appropriate calcium intake, given to institutionalized or vitamin D-deficient elderly subjects. VDR and vitamin D metabolic enzymes are widely expressed. Numerous genetic, molecular, cellular, and animal studies strongly suggest that vitamin D signaling has many extraskeletal effects. These include regulation of cell proliferation, immune and muscle function, skin differentiation, and reproduction, as well as vascular and metabolic properties. From observational studies in human subjects, poor vitamin D status is associated with nearly all diseases predicted by these extraskeletal actions. Results of randomized controlled trials and Mendelian randomization studies are supportive of vitamin D supplementation in reducing the incidence of some diseases, but, globally, conclusions are mixed. These findings point to a need for continued ongoing and future basic and clinical studies to better define whether vitamin D status can be optimized to improve many aspects of human health. Vitamin D deficiency enhances the risk of osteoporotic fractures and is associated with many diseases. We review what is established and what is plausible regarding the health effects of vitamin D.

Analysis of Association between Vitamin D Deficiency and Insulin Resistance

Recent evidence revealed extra skeleton activity of vitamin D, including prevention from cardiometabolic diseases and cancer development as well as anti-inflammatory properties. It is worth noting that vitamin D deficiency is very common and may be associated with the pathogenesis of insulin-resistance-related diseases, including obesity and diabetes. This review aims to provide molecular mechanisms showing how vitamin D deficiency may be involved in the insulin resistance formation. The PUBMED database and published reference lists were searched to find studies published between 1980 and 2019. It was identified that molecular action of vitamin D is involved in maintaining the normal resting levels of ROS and Ca2+, not only in pancreatic β-cells, but also in insulin responsive tissues. Both genomic and non-genomic action of vitamin D is directed towards insulin signaling. Thereby, vitamin D reduces the extent of pathologies associated with insulin resistance such as oxidative stress and inflammation. More recently, it was also shown that vitamin D prevents epigenetic alterations associated with insulin resistance and diabetes. In conclusion, vitamin D deficiency is one of the factors accelerating insulin resistance formation. The results of basic and clinical research support beneficial action of vitamin D in the reduction of insulin resistance and related pathologies.

The association between 25-hydroxy vitamin D deficiency and diabetic complications in patients with type 2 diabetes mellitus

AIMS

To evaluation the relationship between serum 25-hydroxy vitamin D [25-(OH)D] deficiency and diabetic complications in patients with type 2 diabetes (T2DM).

METHODS

One hundred and sixty three patients with T2DM [DM + uncomplicated (n = 36), DM + nephropathy (n = 31), DM + neuropathy (n = 30), DM + retinopathy (n = 30), DM + cardiovascular disease (CAD) (n = 36)], 35 CAD and 40 healthy volunteers were included.

RESULTS

Serum 25-(OH)D levels were found as significantly lower in all patients compared to the control group (p < 0.05). 25-(OH)D in patients with DM + retinopathy (p < 0.006), DM + nephropathy (p < 0.001) and DM + neuropathy (p < 0.001) was significantly lower than that of the control group. 25-(OH)D in patients with DM + nephropathy (p < 0.001), DM + neuropathy (p < 0.01) and DM + retinopathy (p < 0.001) was significantly lower than in the DM + uncomplicated group. 25-(OH)D levels were found as significantly lower in DM + CAD compared to the CAD group (p < 0.01). Serum 25-(OH)D and HbA1c and parathyroid hormone (PTH) were found to be negatively correlated with each other in DM + all complications.

CONCLUSIONS

Low serum 25-OHD levels were found to be associated with the development of diabetes and complications. Low serum 25-OHD levels may be a consequence of even worse metabolic control of diabetes.

The serum vitamin D metabolome: What we know and what is still to discover

Vitamin D, referring to the two forms, D2 from the diet and D3 primarily derived from phototransformation in the skin, is a prohormone important in human health. The most hormonally active form, 1α,25-dihydroxyvitamin D (1α,25(OH)₂D), formed from vitamin D via 25-hydroxyvitamin D (25(OH)D), is not only important for regulating calcium metabolism, but has many pleiotropic effects including regulation of the immune system and has anti-cancer properties. The major circulating form of vitamin D is 25(OH)D and both D2 and D3 forms are routinely measured by LC/MS/MS to assess vitamin D status, due to their relatively long half-lives and much higher concentrations compared to 1α,25(OH)₂D. Inactivation of both 25(OH)D and 1α,25(OH)₂D is catalyzed by CYP24A1 and 25-hydroxyvitamin D3 3-epimerase. Initial products from these enzymes acting on 25(OH)D3 are 24R,25(OH)₂D3 and 3-epi-25(OH)D3, respectively, and both of these can also be measured routinely in some clinical laboratories to further document vitamin D status. With advances in LC/MS/MS and its increased availability, and with the help of studies with recombinant vitamin D-metabolizing enzymes, many other vitamin D metabolites have now been detected and in some cases quantitated, in human serum. CYP11A1 which catalyzes the first step in steroidogenesis, has been found to also act on vitamins D3 and D2 hydroxylating both at C20, but with some secondary metabolites produced by subsequent hydroxylations at other positions on the side chain. The major vitamin D3 metabolite, 20S-hydroxyvitamin D3 (20S(OH)D3), shows biological activity, often similar to 1α,25(OH)₂D3 but without calcemic effects. Using standards produced enzymatically by purified CYP11A1 and characterized by NMR, many of these new metabolites have been detected in human serum, with semi-quantitative measurement of 20S(OH)D3 indicating it is present at comparable concentrations to 24R,25(OH)₂D3 and 3-epi-25(OH)D3. Recently, vitamin D-related hydroxylumisterols derived from lumisterol3, a previtamin D3 photoproduct, have also been measured in human serum and displayed biological activity in initial in vitro studies. With the current extensive knowledge on the reactions and pathways of metabolism of vitamin D, especially those catalyzed by CYP24A1, CYP27A1, CYP27B1, CYP3A4 and CYP11A1, it is likely that many other of the resulting hydroxyvitamin D metabolites will be measured in human serum in the future, some contributing to a more detailed understanding of vitamin D status in health and disease.

The Role of Vitamin D Binding Protein, Total and Free 25-Hydroxyvitamin D in Diabetes

Vitamin D is important for bone health, but may also have extra-skeletal effects. Vitamin D and its binding protein DBP have immunological effects and may therefore be important in the development of type 1 diabetes (T1DM), and low serum levels of 25-hydroxyvitamin D (25(OH)D) are associated with later development of type 2 diabetes (T2DM). However, it has so far been difficult to convincingly show an effect of vitamin D supplementation on prevention or treatment of diabetes. The serum level of 25(OH)D has traditionally been used as a marker of a subject's vitamin D status. This measurement includes both 25(OH)D bound to DBP and albumin as well as the free from of 25(OH)D. However, according to the free hormone hypothesis, the free form is the biologically active. Previously the free form of 25(OH)D had to be calculated based on measurements of 25(OH)D, DBP, and albumin, but recently a method for direct measurement of free 25(OH)D has become commercially available. This is important in clinical conditions where the amount of DBP is affected, and has caused a renewed interest in which vitamin D metabolite to measure in clinical situations. In the present review the relations between DBP, total and free 25(OH)D in T1DM and T2DM are described.

Effects of Lifestyle Modification and Anti-diabetic Medicine on Prediabetes Progress: A Systematic Review and Meta-Analysis

Background: Pre-diabetes is a risk factor for full-blown diabetes; it presents opportunities to prevent the actual diseases. It is therefore essential to identify effective preventive strategies, and to clarify the direction of future research. Methods: PubMed, Embase and Cochrane Central Register of Controlled Trials were searched using key terms (Supplementary Table 1). We applied network meta-analysis to multiple comparisons among various diabetic preventive strategies, including lifestyle and pharmacological interventions; traditional meta-analysis for the synthesis of basal metabolic changes after interventions; and trial sequential analysis for determinations as to whether analysis conclusions meet expectations. Results: We included 32 randomized controlled trials comprising 43,669 patients and 14 interventions in the meta-analysis. Both lifestyle modifications and anti-diabetic medications improved physical conditions, including weight loss, blood glucose, and blood pressure. Network meta-analysis suggested that the progression of diabetes could be delayed to varying degrees by lifestyle and pharmacological interventions, except for angiotensin-converting enzyme inhibitors, statins, sulfonylureas and vitamin D. The risk ratios (RR) [95% credible interval (CrI)] compared with control were: GLP-1RAs 0.28 (0.15, 0.50), Orlistat 0.33 (0.18, 0.55), TZM 0.33 (0.16, 0.63), TZD 0.39 (0.27, 0.53), LST 0.54 (0.32, 0.88), lifestyle 0.58 (0.49, 0.67), LSM 0.62 (0.45, 0.80), GI 0.66 (0.46, 0.88), SU 0.67 (0.40, 1.00), Vitamin D 0.91 (0.59, 1.40), ACEI 0.93 (0.62, 1.40), statins 1.20 (0.84, 1.60). Conclusions: In adults with pre-diabetes, firm evidence supports the notion that lifestyle modifications and metformin reduces the incidence of diabetes with an average of 20% relative risk reduction, while statins increase the relative risk 20%. We found that lifestyle modifications, promising long-term strategies involving three factors (nutrition, exercise, and weight loss) contribute to health by reducing BMI, body weight, waist and hip circumference, systolic and diastolic pressure, fasting, and 2-h postprandial blood glucose, total cholesterol and by increasing HDL. We made this determination using TSA, avoiding further waste of experimental resources.

Sex-Specific Association of Serum 25-Hydroxyvitamin D3 with Insulin Resistance in Chinese Han Patients with Newly Diagnosed Type 2 Diabetes Mellitus

This study aimed to explore the association between serum 25-hydroxyvitamin D (25(OH)D) and insulin resistance as well as β-cell function in Chinese Han patients with newly diagnosed type 2 diabetes mellitus (T2DM). A total of 264 patients was included in this study. Serum 25(OH)D, plasma glucose, serum insulin and other biochemical parameters were assayed. Postprandial venous blood was collected after a mixed-nutrient load. Insulin resistance was assessed by the homeostasis model assessment for insulin resistance (HOMA-IR) and Matsuda insulin sensitivity index (Matsuda ISI). The β-cell function was assessed by the homeostasis model assessment for insulin secretion (HOMA-β) and the change in insulin divided by change in glucose from 0 to 30 min (ΔI_0-30/ΔG_0-30). Patients were divided into three groups according to tertiles of serum 25(OH)D levels. There were significant differences in HOMA-IR and Matsuda ISI among the three groups (HOMA-IR, p=0.005; Matsuda ISI, p=0.009). Pearson correlation analyses showed that serum 25(OH)D was negatively correlated with fasting serum insulin (FIns) (r=-0.209, p=0.012) and HOMA-IR (r=-0.273, p=0.001), and positively correlated with Matsuda ISI (r=0.219, p=0.009) only in the male population. Multiple stepwise regression analyses showed that in the male population, serum 25(OH)D was an independent predictor for both HOMA-IR and Matsuda ISI before and after adjustment for confounding factors, respectively (p<0.05 for both). This study indicates the association of vitamin D with insulin resistance in male patients with newly diagnosed T2DM, which may contribute to the understanding of the mechanism underlying the onset of T2DM in the Chinese Han population.

Differential and Overlapping Effects of 20,23(OH)₂D3 and 1,25(OH)₂D3 on Gene Expression in Human Epidermal Keratinocytes: Identification of AhR as an Alternative Receptor for 20,23(OH)₂D3

A novel pathway of vitamin D activation by CYP11A has previously been elucidated. To define the mechanism of action of its major dihydroxy-products, we tested the divergence and overlap between the gene expression profiles of human epidermal keratinocytes treated with either CYP11A1-derived 20,23(OH)₂D3 or classical 1,25(OH)₂D3. Both secosteroids have significant chemical similarity with the only differences being the positions of the hydroxyl groups. mRNA was isolated and examined by microarray analysis using Illumina’s HumanWG-6 chip/arrays and subsequent bioinformatics analyses. Marked differences in the up- and downregulated genes were observed between 1,25(OH)₂D3- and 20,23(OH)₂D3-treated cells. Hierarchical clustering identified both distinct, opposite and common (overlapping) gene expression patterns. CYP24A1 was a common gene strongly activated by both compounds, a finding confirmed by qPCR. Ingenuity pathway analysis identified VDR/RXR signaling as the top canonical pathway induced by 1,25(OH)₂D3. In contrast, the top canonical pathway induced by 20,23(OH)₂D3 was AhR, with VDR/RXR being the second nuclear receptor signaling pathway identified. QPCR analyses validated the former finding by revealing that 20,23(OH)₂D3 stimulated CYP1A1 and CYP1B1 gene expression, effects located downstream of AhR. Similar stimulation was observed with 20(OH)D3, the precursor to 20,23(OH)₂D3, as well as with its downstream metabolite, 17,20,23(OH)₃D3. Using a Human AhR Reporter Assay System we showed marked activation of AhR activity by 20,23(OH)₂D3, with weaker stimulation by 20(OH)D3. Finally, molecular modeling using an AhR LBD model predicted vitamin D3 hydroxyderivatives to be good ligands for this receptor. Thus, our microarray, qPCR, functional studies and molecular modeling indicate that AhR is the major receptor target for 20,23(OH)₂D3, opening an exciting area of investigation on the interaction of different vitamin D3-hydroxyderivatives with AhR and the subsequent downstream activation of signal transduction pathways in a cell-type-dependent manner.

Vitamin D insufficiency: Definition, diagnosis and management

Severe vitamin D deficiency can be defined as the dose of vitamin D or serum 25OHD concentrations needed to prevent nutritional rickets or osteomalacia. There is large international consensus that these diseases can be prevented by 400 IU of vitamin D/d and 25OHD above 30 nmol/l (12 ng/ml). Vitamin D deficiency can also accelerate the risk of fractures and probably also of falls in elderly subjects but there is no consensus on the required daily doses or minimal 25OHD threshold for these endpoints. The majority of experts consider 800 IU/d and serum 25OHD above 50 nmol/l (20 ng/ml) as sufficient, with a minority opinion aiming for 75 nmol/l or even higher. For other extra-skeletal endpoints, no hard evidence is available to define whether or not this is causally related to vitamin D status. Therefore, for these endpoints no minimal dosage or 25OHD threshold can be defined.

Metabolically healthy obesity, vitamin D, and all-cause and cardiometabolic mortality risk in NHANES III

BACKGROUND & AIMS

Previous studies assessing the prognosis of metabolically healthy obesity (MHO) have been limited by a lack of a harmonized definition of MHO phenotype. Furthermore, obesity is a risk factor for vitamin D deficiency and low vitamin D status has been associated with a higher risk of mortality; however, few studies have evaluated the joint association between vitamin D, metabolic health phenotype, and mortality risk. Using a harmonized definition, we investigated whether MHO is associated with subsequent all-cause and cardiometabolic mortality, and whether serum 25-hydroxyvitamin D [25(OH)D] modifies these associations.

METHODS

This study included participants aged ≥20 years from the Third National Health and Nutrition Examination Survey (NHANES III). MHO phenotype was defined as a combination of obesity (≥30 kg/m²) and zero component of metabolic syndrome. Multivariable Cox regression was used to assess the risk of mortality across metabolic phenotypes, and the joint association between metabolic phenotype and 25(OH)D. Fine and Gray regression was performed to account for competing risk events.

RESULTS

Among 11,333 participants, a total of 2980 deaths (937 cardiometabolic death outcomes) occurred during a median follow-up of 19.1 years. In the absence of any metabolic abnormality, obesity (MHO) was not associated with a higher risk of all-cause (hazard ratio [HR], 0.89 [95% CI, 0.52-1.51]) or cardiometabolic mortality (cause-specific HR, 1.21 [95% CI 0.33-4.46]). Similar results were obtained from competing risk analysis. No significant differences in average 25(OH)D levels were observed between MHO and non-MHO participants; however, there was a significant interaction between metabolic health phenotype and serum 25(OH)D in relation to cardiometabolic mortality such that levels of serum 25(OH)D < 50 nmol/L were associated with increased risk of cardiometabolic mortality, particularly in participants within the normal-weight and obese BMI ranges.

CONCLUSIONS

Our results support the hypothesis that MHO phenotype is a benign health condition. Vitamin D deficiency may exacerbate the risk of cardiometabolic death outcomes associated with metabolic dysfunction in normal weight and obese individuals. Further research is warranted to validate our findings.

Nutraceutical approaches to metabolic syndrome

Metabolic Syndrome (MetS), affecting at least 30% of adults in the Western World, is characterized by three out of five variables, from high triglycerides, to elevated waist circumference and blood pressure. MetS is not characterized by elevated cholesterolemia, but is rather the consequence of a complex interaction of factors generally leading to increased insulin resistance. Drug treatments are of difficult handling, whereas well-characterized nutraceuticals may offer an effective alternative. Among these, functional foods, e.g. plant proteins, have been shown to improve insulin resistance and reduce triglyceride secretion. Pro- and pre-biotics, that are able to modify intestinal microbiome, reduce absorption of specific nutrients and improve the metabolic handling of energy-rich foods. Finally, specific nutraceuticals have proven to be of benefit, in particular, red-yeast rice, berberine, curcumin as well as vitamin D. All these can improve lipid handling by the liver as well as ameliorate insulin resistance. While lifestyle approaches, such as with the Mediterranean diet, may prove to be too complex for the single patient, better knowledge of selected nutraceuticals and more appropriate formulations leading to improved bioavailability will certainly widen the use of these agents, already in large use for the management of these very frequent patient groups. Key messages Functional foods, e.g. plant proteins, improve insulin resistance. Pro- and pre-biotics improve the metabolic handling of energy-rich foods. Nutraceutical can offer a significant help in handling MetS patients being part of lifestyle recommendations.

Association of Vitamin D Deficiency and Degree of Coronary Artery Disease in Cardiac Patients with Type 2 Diabetes

Several modifiable factors may influence cardiac function in diabetic patients. The aim of the study was to evaluate the influence of vitamin D level on the stage of coronary atherosclerosis in cardiac patients diagnosed with type 2 diabetes. The study was performed in 337 consecutive patients undergoing coronarography. The stage of atherosclerosis was evaluated using Coronary Artery Surgery Study Score. The plasma 25(OH)D concentration was determined by an electrochemiluminescence method. Patients without significant lesions in coronary arteries presented the highest 25(OH)D level, significantly higher than patients with one-, two-, and three-vessel coronary artery disease (CAD) (p < 0.01). Significantly lower level of the 25(OH)D was observed in patients hospitalized due to acute coronary syndrome (ACS) in comparison to patients hospitalized due to stable CAD (p < 0.001). Lower 25(OH)D levels were observed in patients with the history of myocardial infarction (MI) in comparison to patients without previous MI (p < 0.001). In cardiac patients with diabetes, the higher number of stenotic coronary arteries is associated with lower values of the 25(OH)D. A group of male cardiac patients with diabetes with significant stenosis in three coronary arteries, hospitalized due to acute coronary syndrome, with a history of previous MI and hyperlipidemia presented the lowest vitamin D level.

Vitamin D Supplementation Enhances C18(dihydro)ceramide Levels in Type 2 Diabetes Patients

Sphingolipids are characterized by a broad range of bioactive properties. Particularly, the development of insulin resistance, a major pathophysiological hallmark of Type 2 Diabetes mellitus (T2D), has been linked to ceramide signaling. Since vitamin D supplementation may slow down T2D progression by improving glucose concentrations and insulin sensitivity, we investigated whether vitamin D supplementation impacts on plasma sphingolipid levels in T2D patients. Thus, plasma samples of 59 patients with non-insulin-requiring T2D from a placebo-controlled, randomized, and double-blind study were retrospectively analyzed. Once per week, patients received either 20 drops of Vigantol oil, corresponding to a daily dose of 1904 IU/d vitamin D (verum: n = 31), or a placebo oil consisting of medium chain triglycerides (placebo: n = 28). Blood samples were taken from all of the participants at three different time points: 1) at the beginning of the study (baseline), 2) after 6 months supplementation, and 3) after an additional 6 months of follow-up. Plasma sphingolipids were measured by high-performance liquid chromatography tandem mass spectrometry. At baseline and 6 months follow-up, no significant differences in plasma sphingolipid species were detected between the placebo and verum groups. After 6 months, vitamin D supplementation significantly enhanced plasma C18dihydroceramide (dhCer; N-stearoyl-sphinganine (d18:0/18:0)) and C18ceramide (Cer; N-stearoyl-sphingosine (d18:1/18:0)) levels were observed in the verum group compared to the placebo group. This was accompanied by significantly higher 25-hydroxyvitamin D₃ (25(OH)D₃) blood levels in patients receiving vitamin D compared to the placebo group. Taken together, vitamin D supplementation induced changes of the C18 chain-length-specific dhCer and Cer plasma levels in patients with T2D. The regulation of sphingolipid signaling by vitamin D may thus unravel a novel mechanism by which vitamin D can influence glucose utilization and insulin action. Whether this acts favorably or unfavorably for the progression of T2D needs to be clarified.