Shining D' light on chronic kidney disease: mechanisms that may underpin the cardiovascular benefit of vitamin D

Vitamin D supplementation as a prophylactic therapy in the management of pre-eclampsia: Focus on VEGF, Ki67, oxidative stress markers in correlation to placental ultra structure

BACKGROUND

Pre-eclampsia (PE) is a progressive hypertension condition that manifests in the second or third trimester of pregnancy and causes significant proteinuria. A lack of vitamin D (Vit. D) is linked to different pregnancy problems, including impaired placental development. Vitamin D has been shown to enhance fetal growth and lower the incidence of PE.

AIM OF THE WORK

To better understand the pathophysiological mechanisms behind the PE disease and the therapeutic approaches used to manage it, this study examines the role of Vit. D in placental ischemia and its regulatory effects in Nitro L-arginine Methyl Ester (L-NAME) animal model of PE.

METHODS

Fifty female rats in the estrus stage were mated with 30 male rats. Thirty female rats were pregnant and divided into three equal groups: control, Preeclampsia group (PE); using L-NAME for induction of PE, and Vit. D group from 7th day then induction by L-NAME at 10th day till end of pregnancy. Mean arterial Bp, proteinuria, oxidative stress markers, histological structure and immunohistochemical expression of Ki67 and VEGF, Morphometric study, and transmission electron microscopy(TEM) were assessed. The results of the current study suggested that, Vit. D supplementation could lower blood pressure, reduce oxidative stress, and restore angiogenic balance through vascular endothelial growth factor (VEGF) and Ki67.

CONCLUSION

For the first time, we conclude that vitamin D supplementation may not only have direct effects on blood pressure regulation and angiogenic hemostasis but also recover placental function, actually contributing to the prevention or management of PE.

The association between obesity and vitamin D deficiency modifies the progression of kidney disease after ischemia/reperfusion injury

Acute kidney injury (AKI) alters renal hemodynamics, leading to tubular injury, activating pathways of inflammation, proliferation, and cell death. The initial damage caused to renal tissue after an ischemia/reperfusion (I/R) injury exerts an important role in the pathogenesis of the course of AKI, as well as in the predisposition to chronic kidney disease. Vitamin D deficiency has been considered a risk factor for kidney disease and it is associated with tubulointerstitial damage, contributing to the progression of kidney disease. Obesity is directly related to diabetes mellitus and hypertension, the main metabolic disorders responsible for the progression of kidney disease. Furthermore, the expansion of adipose tissue is described as an important factor for increased secretion of pro-inflammatory cytokines and their respective influence on the progression of kidney disease. We aimed to investigate the influence of vitamin D deficiency and obesity on the progression of renal disease in a murine model of renal I/R. Male Wistar rats underwent renal I/R surgery on day 45 and followed until day 90 of the protocol. We allocated the animals to four groups according to each diet received: standard (SD), vitamin D-depleted (VDD), high fat (HFD), or high fat vitamin D-depleted (HFDV). At the end of 90 days, we observed almost undetectable levels of vitamin D in the VDD and HFDV groups. In addition, HFD and HFDV groups presented alterations in the anthropometric and metabolic profile. The combination of vitamin D deficiency and obesity contributed to alterations of functional and hemodynamic parameters observed in the HFDV group. Moreover, this combination favored the exacerbation of the inflammatory process and the renal expression of extracellular matrix proteins and phenotypic alteration markers, resulting in an enlargement of the tubulointerstitial compartment. All these changes were associated with an increased renal expression of transforming growth factor β and reduced expression of the vitamin D receptor. Our results show that the synergistic effect of obesity and vitamin D deficiency exacerbated the hemodynamic and morphological changes present in the evolution of renal disease induced by I/R.

Serum, Dietary, and Supplemental Vitamin D Levels and Insulin Resistance in 6294 Randomly Selected, Non-Diabetic U.S. Adults

The primary aim of this study was to determine the associations between serum, dietary, and supplemental vitamin D levels and insulin resistance in 6294 non-diabetic U.S. adults. A total of 8 years of data from the 2011−2018 National Health and Nutrition Examination Survey (NHANES) and a cross-sectional design were utilized to answer the research questions. Serum vitamin D levels were quantified using high-performance liquid chromatography−tandem mass spectrometry. Dietary and supplemental vitamin D intakes were assessed using the average of two 24 h dietary recalls taken 3−10 days apart. The homeostatic model assessment (HOMA), based on fasting glucose and fasting insulin levels, was employed to index insulin resistance. Demographic covariates were age, sex, race, and year of assessment. Differences in physical activity, body mass index (BMI), cigarette smoking, body weight, season, and energy intake were also controlled statistically. Serum levels of vitamin D differed significantly, and in a dose−response order, across quartiles of HOMA-IR, after adjusting for year, age, sex, and race (F = 30.3, p < 0.0001) and with all the covariates controlled (F = 5.4, p = 0.0029). Dietary vitamin D levels differed similarly across HOMA-IR quartiles, but to a lesser extent, respectively (F = 8.1, p = 0.0001; F = 2.9, p = 0.0437). Likewise, supplemental vitamin D levels also differed across the HOMA-IR quartiles, respectively (F = 3.5, p = 0.0205; F = 3.3, p = 0.0272). With all the covariates controlled, the odds of having insulin resistance were significantly greater for those in the lowest quartile of serum and supplemental vitamin D intake compared to the other quartiles combined. In conclusion, in this nationally representative sample, serum, dietary, and supplemental vitamin D were each predictive of insulin resistance, especially in those with low serum levels and those with no supplemental intake of vitamin D.

The Restoration of Vitamin D Levels Slows the Progression of Renal Ischemic Injury in Rats Previously Deficient in Vitamin D

Chronic kidney disease (CKD) remains a global public health problem. The initial damage after ischemia/reperfusion (I/R) injury plays an important role in the pathogenesis of acute kidney injury (AKI) and predisposition to CKD. Several studies have been showing that nontraditional risk factors such as AKI and hypovitaminosis D could also be involved in CKD progression. Vitamin D deficiency (VDD) is associated with hemodynamic changes, activation of inflammatory pathways and renal disease progression (RDP) following I/R-AKI. Strategies for prevention and/or slowing RDP have been determined and the sufficiency of vitamin D has been emerging as a renoprotective factor in many diseases. Therefore, we investigated the effect of the restoration of vitamin D levels in the progression of I/R injury (IRI) in rats previously deficient in vitamin D. On day 30, male Wistar rats were submitted to bilateral 45 min IRI and divided into three groups: IRI, standard diet for 120 days; VDD+IRI, vitamin D-free diet for 120 days; and VDD+IRI+R, vitamin D-free diet in the first 30 days and just after I/R, we reintroduced the standard diet in the last 90 days. After the 120-day protocol, VDD+IRI+R rats presented an improvement in the renal function and renal protein handling followed by a smaller fractional interstitial area. Furthermore, those animals exhibited a reestablishment regarding the hemodynamic parameters and plasma levels of aldosterone, urea and PTH. In addition, the restoration of vitamin D levels reestablished the amount of MCP1 and the renal expressions of CD68+ and CD3+ cells in the VDD+IRI+R rats. Also, VDD+IRI+R rats showed a restoration regarding the amount of collagen type III and renal expressions of fibronectin, vimentin and α-SMA. Such changes were also accompanied by a reestablishment on the renal expression of VDR, Klotho, JG12, and TGF-β1. Our findings indicate that the restoration of vitamin D levels not only improved the renal function and hemodynamics but also reduced the inflammation and fibrosis lesions observed in I/R-AKI associated with VDD. Thus, monitoring of vitamin D status as well as its replacement in the early stages of kidney injury may be a therapeutic alternative in the mitigation of renal disease progression.

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.

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.

Vitamin D, sub-inflammation and insulin resistance. A window on a potential role for the interaction between bone and glucose metabolism

Vitamin D is a key hormone involved in the regulation of calcium/phosphorous balance and recently it has been implicated in the pathogenesis of sub-inflammation, insulin resistance and obesity. The two main forms of vitamin D are cholecalciferol (Vitamin D3) and ergocalciferol (Vitamin D2): the active form (1,25-dihydroxyvitamin D) is the result of two hydroxylations that take place in liver, kidney, pancreas and immune cells. Vitamin D increases the production of some anti-inflammatory cytokines and reduces the release of some pro-inflammatory cytokines. Low levels of Vitamin D are also associated with an up-regulation of TLRs expression and a pro-inflammatory state. Regardless of the effect on inflammation, Vitamin D seems to directly increase insulin sensitivity and secretion, through different mechanisms. Considering the importance of low grade chronic inflammation in metabolic syndrome, obesity and diabetes, many authors hypothesized the involvement of this nutrient/hormone in the pathogenesis of these diseases. Vitamin D status could alter the balance between pro and anti-inflammatory cytokines and thus affect insulin action, lipid metabolism and adipose tissue function and structure. Numerous studies have shown that Vitamin D concentrations are inversely associated with pro-inflammatory markers, insulin resistance, glucose intolerance and obesity. Interestingly, some longitudinal trials suggested also an inverse association between vitamin D status and incident type 2 diabetes mellitus. However, vitamin D supplementation in humans showed controversial effects: with some studies demonstrating improvements in insulin sensitivity, glucose and lipid metabolism while others showing no beneficial effect on glycemic control and on inflammation. In conclusion, although the evidences of a significant role of Vitamin D on inflammation, insulin resistance and insulin secretion in the pathogenesis of obesity, metabolic syndrome and type 2 diabetes, its potential function in treatment and prevention of type 2 diabetes mellitus is unclear. Encouraging results have emerged from Vitamin D supplementation trials on patients at risk of developing diabetes and further studies are needed to fully explore and understand its clinical applications.

A therapeutic role for vitamin D on obesity-associated inflammation and weight-loss intervention

Vitamin D plays an essential role in the regulation of skeletal metabolism as well as calcium and phosphate homeostasis, while vitamin D receptor (VDR) regulates de novo lipid synthesis, thereby contributing to the development of obesity. Furthermore, obese individuals are at a greater risk for vitamin D deficiency which may increase the potential risk for chronic inflammation, insulin resistance, and metabolic syndrome. While acute exercise enhances the activation of inflammatory signaling pathways, chronic exercise training may attenuate elevated pro-inflammatory cytokine production, resulting in the improvement of cardiovascular and metabolic health in obese individuals. Supplementation with vitamin D coupled with exercise or mild caloric restriction has been shown to improve markers of fitness and inflammation as well as cholesterol. Therefore, this review primarily addresses the impact of vitamin D deficiency in obesity-related inflammatory imbalances and how exercise and weight-loss interventions may enhance the beneficial effects on vitamin D-mediated inflammation in obesity.

Effect of oral vitamin D analogs on mortality and cardiovascular outcomes among adults with chronic kidney disease: a meta-analysis

Background

Vitamin D deficiency is highly prevalent in patients with chronic kidney disease (CKD) and has been associated with all-cause and cardiovascular mortality in observational studies. However, evidence from randomized controlled trials (RCTs) supporting vitamin D supplementation is lacking. We sought to assess whether vitamin D supplementation alters the relative risk (RR) of all-cause and cardiovascular mortality, as well as serious adverse cardiovascular events, in patients with CKD, compared with placebo.

Methods

PubMed/MEDLINE, EMBASE, Cochrane Library, and selected nephrology journals and conference proceedings were searched in October 2013. RCTs considered for inclusion were those that assessed oral vitamin D supplementation versus placebo in adults with CKD (≤60 mL/min/1.73 m²), including end-stage CKD requiring dialysis. We calculated pooled RR of mortality (all-cause and cardiovascular) and that of cardiovascular events and stratified by CKD stage, vitamin D analog and diabetes prevalence.

Results

The search identified 4246 articles, of which 13 were included. No significant treatment effect of oral vitamin D on all-cause mortality (RR: 0.84; 95% CI: 0.47, 1.52), cardiovascular mortality (RR: 0.79; 95% CI: 0.26, 2.28) or serious adverse cardiovascular events (RR: 1.20; 95% CI: 0.49, 2.99) was observed. The pooled analysis demonstrated large variation in trials with respect to dosing (0.5 ug–200 000 IU/week) and duration (3–104 weeks).

Conclusions

Current RCTs do not provide sufficient or precise evidence that vitamin D supplementation affects mortality or cardiovascular risk in CKD. While its effect on biochemical endpoints is well documented, the results demonstrate a lack of appropriate patient-level data within the CKD literature, which warrants larger trials with clinical primary outcomes related to vitamin D supplementation.

Vitamin D deficiency aggravates nephrotoxicity, hypertension and dyslipidemia caused by tenofovir: role of oxidative stress and renin-angiotensin system

Vitamin D deficiency (VDD) is prevalent among HIV-infected individuals. Vitamin D has been associated with renal and cardiovascular diseases because of its effects on oxidative stress, lipid metabolism and renin-angiotensin-aldosterone system (RAAS). Tenofovir disoproxil fumarate (TDF), a widely used component of antiretroviral regimens for HIV treatment, can induce renal injury. The aim of this study was to investigate the effects of VDD on TDF-induced nephrotoxicity. Wistar rats were divided into four groups: control, receiving a standard diet for 60 days; VDD, receiving a vitamin D-free diet for 60 days; TDF, receiving a standard diet for 60 days with the addition of TDF (50 mg/kg food) for the last 30 days; and VDD+TDF receiving a vitamin D-free diet for 60 days with the addition of TDF for the last 30 days. TDF led to impaired renal function, hyperphosphaturia, hypophosphatemia, hypertension and increased renal vascular resistance due to downregulation of the sodium-phosphorus cotransporter and upregulation of angiotensin II and AT1 receptor. TDF also increased oxidative stress, as evidenced by higher TBARS and lower GSH levels, and induced dyslipidemia. Association of TDF and VDD aggravated renovascular effects and TDF-induced nephrotoxicity due to changes in the redox state and involvement of RAAS.

Sunlight exposure and cardiovascular risk factors in the REGARDS study: a cross-sectional split-sample analysis

Background

Previous research has suggested that vitamin D and sunlight are related to cardiovascular outcomes, but associations between sunlight and risk factors have not been investigated. We examined whether increased sunlight exposure was related to improved cardiovascular risk factor status.

Methods

Residential histories merged with satellite, ground monitor, and model reanalysis data were used to determine previous-year sunlight radiation exposure for 17,773 black and white participants aged 45+ from the US. Exploratory and confirmatory analyses were performed by randomly dividing the sample into halves. Logistic regression models were used to examine relationships with cardiovascular risk factors.

Results

The lowest, compared to the highest quartile of insolation exposure was associated with lower high-density lipoprotein levels in adjusted exploratory (−2.7 mg/dL [95% confidence interval: −4.2, −1.2]) and confirmatory (−1.5 mg/dL [95% confidence interval: −3.0, −0.1]) models. The lowest, compared to the highest quartile of insolation exposure was associated with higher systolic blood pressure levels in unadjusted exploratory and confirmatory, as well as the adjusted exploratory model (2.3 mmHg [95% confidence interval: 0.8, 3.8]), but not the adjusted confirmatory model (1.6 mg/dL [95% confidence interval: −0.5, 3.7]).

Conclusions

The results of this study suggest that lower long-term sunlight exposure has an association with lower high-density lipoprotein levels. However, all associations were weak, thus it is not known if insolation may affect cardiovascular outcomes through these risk factors.

Paricalcitol effects on activities and metabolism of platelet activating factor and on inflammatory cytokines in hemodialysis patients

PURPOSE

Paricalcitol improves the inflammatory status of hemodialysis patients. PAF is a strong inflammatory mediator which is produced during hemodialysis. We studied the effects of paricalcitol on PAF and other inflammatory mediators implicated in chronic kidney disease (CKD).

METHODS

We examined the in vitro effects of paricalcitol on PAF/thrombin-induced aggregation as well as on the activities of PAF-basic metabolic enzymes, lyso-PAF acetyltransferase (Lyso-PAF-AT), DTT-insensitive CDP-choline: 1-alkyl-2-acetyl-sn-glycerol cholinephospho-transferase (PAF-CPT) and PAF-acetylhydrolase (PAF-AH) in blood cells from healthy volunteers. In addition, the in vivo effects of paricalcitol on the above these enzymes were examined in plasma and blood cells of hemodialysis patients who had not received any type of vitamin D treatment during the last three months before and after receiving paricalcitol for a month. Finally, IL-12p70, IL-1β, IL-6, IL-8 and TNF-α were measured.

RESULTS

Paricalcitol inhibited in vitro PAF/thrombin-induced platelet aggregation and the inhibitory effect was comparable with that of PAF/thrombin antagonists. In addition, paricalcitol inhibited in vitro PAF-CPT activity in platelets and leukocytes and increased PAF-AH activity in leukocytes, while much higher concentrations of paricalcitol were needed to inhibit Lyso-PAF-AT activity. Similarly, in hemodialysis patients, paricalcitol treatment reduced PAF-CPT activity in platelets and leukocytes and increased PAF-AH activity in leukocytes, while it could not influence Lyso-PAF-AT activity. On the other hand, paricalcitol therapy reduced IL-8, IL-1β, and TNF-α.

CONCLUSIONS

These results further support the beneficial effects of vitamin D treatment in hemodialysis patients, since it strongly affects PAF/thrombin activities, PAF-metabolism, and IL-8, IL-1β and TNF-α circulating levels.

Vitamin D and inflammation

Recent studies have provided evidence for an important role of vitamin D deficiency in the pathogenesis of cardiovascular disease (CVD) in the general population, and of an association with markers of inflammation. Vitamin D deficiency, defined by low serum levels of 25-hydroxyvitamin D, is especially prevalent in patients with chronic kidney disease (CKD). Chronic low-grade inflammation is a hallmark of CKD and has been disclosed as one important factor contributing to the progression of CKD and a high cardiovascular comorbidity. This review highlights clinical and experimental studies that could potentially explain a link between vitamin D and inflammation. Whether correction of vitamin D deficiency has beneficial effects on markers of inflammation and cardiovascular outcome should be investigated by controlled clinical trials.

Vitamin D supplementation in pre-dialysis chronic kidney disease: A systematic review

Vitamin D deficiency is associated with a variety of skeletal, cardiometabolic, and immunologic co-morbidities that are present in chronic kidney disease (CKD). We performed a systematic review to investigate the effects of vitamin D supplementation, in the form of ergocalciferol or cholecalciferol, on various health outcomes in early CKD. Seventeen clinical trials were identified, only two of which were randomized, placebo controlled trials. The majority of studies supplementing with > 2,000 IU/day of cholecalciferol achieved optimal vitamin D status, whereas studies supplementing with ergocalciferol were less consistent. Studies varied widely in their effects on lowering serum parathyroid hormone concentrations. Few studies investigated effects of vitamin D treatment on other clinical health indicators in early CKD. Rigorous studies are necessary to investigate optimal vitamin D dosing strategies in early CKD for the maintenance of adequate vitamin D status, management of secondary hyperparathyroidism and improvement of non-skeletal related clinical outcomes.

High-dose cholecalciferol reduces parathyroid hormone in patients with early chronic kidney disease: a pilot, randomized, double-blind, placebo-controlled trial

BACKGROUND

Vitamin D deficiency contributes to secondary hyperparathyroidism, which occurs early in chronic kidney disease (CKD).

OBJECTIVES

We aimed to determine whether high-dose cholecalciferol supplementation for 1 y in early CKD is sufficient to maintain optimal vitamin D status (serum 25-hydroxyvitamin D [25(OH)D] concentration ≥30 ng/mL) and decrease serum parathyroid hormone (PTH). A secondary aim was to determine the effect of cholecalciferol on blood pressure and serum fibroblast growth factor-23 (FGF23).

DESIGN

This was a double-blind, randomized, placebo-controlled trial. Forty-six subjects with early CKD (stages 2-3) were supplemented with oral cholecalciferol (vitamin D group; 50,000 IU/wk for 12 wk followed by 50,000 IU every other week for 40 wk) or a matching placebo for 1 y.

RESULTS

By 12 wk, serum 25(OH)D increased in the vitamin D group only [baseline (mean ± SD): 26.7 ± 6.8 to 42.8 ± 16.9 ng/mL; P < 0.05] and remained elevated at 1 y (group-by-time interaction: P < 0.001). PTH decreased from baseline only in the vitamin D group (baseline: 89.1 ± 49.3 to 70.1 ± 24.8 pg/mL; P = 0.01) at 12 wk, but values were not significantly different from baseline at 1 y (75.4 ± 29.5 pg/mL; P = 0.16; group-by-time interaction: P = 0.09). Group differences were more pronounced in participants with secondary hyperparathyroidism (group-by-time interaction: P = 0.004). Blood pressure and FGF23 did not change in either group.

CONCLUSIONS

After 1 y, this oral cholecalciferol regimen was safe and sufficient to maintain serum 25(OH)D concentrations and prevent vitamin D insufficiency in early CKD. Furthermore, serum PTH improved after cholecalciferol treatment, particularly in patients who had secondary hyperparathyroidism.

Predictors of vitamin D status in predialysis chronic kidney disease patients: a cross-sectional analysis in a high ultraviolet climate

OBJECTIVE

To determine vitamin D status in a subtropical climate among an unselected, referred predialysis chronic kidney disease (CKD) population; assess risks and correlates; and review whether higher 25-hydroxyvitamin D (25-OHD) concentration can mitigate the decrement in circulating 1,25-dihydroxyvitamin D (1,25-OHD) normally encountered with advancing CKD.

DESIGN

Prospective cross-sectional cohort study.

SETTING

Renal unit in Brisbane, Australia (27°28' S).

SUBJECTS

Five hundred ninety-three consecutive CKD patients (stage 1 to 5).

MAIN OUTCOME MEASURE

25-OHD insufficiency (concentrations: 15 to 30 ng/mL) and deficiency (<15 ng/mL), bone-mineral parameters, including 1,25-OHD, calcium, and phosphate.

RESULTS

Despite potentially higher environmental ultraviolet (UV) exposure, only 48% of patients with CKD were 25-OHD sufficient. Traditional risks for hypovitaminosis D were maintained, and sufficiency was independently predicted by testing in the summer/autumn period (odds ratio [OR]: 2.77, 95% confidence interval [CI]: 1.88 to 4.08, P < .001), male gender (OR: 2.18, 95%CI: 1.46 to 3.24, P < .001), Caucasian race (OR: 2.28, 95%CI: 1.37 to 3.78, P = .001), hypoalbuminemia (OR: 0.47, 95%CI: 0.25 to 0.85, P = .01), macroalbuminuria (OR: 0.60, 95%CI: 0.39 to 0.92, P = .02), and normal body mass index (OR: 1.94, 95%CI: 1.22 to 3.07, P = .005). Vitamin D sufficiency was also associated with higher corrected calcium (0.4 mg/dL increments; OR: 1.29, 95%CI: 1.08 to 1.55, P = .005). Although circulating 25-OHD concentrations were relatively maintained across the range of renal function observed, 1,25-OHD concentrations decreased with advancing CKD.

CONCLUSION

25-OHD insufficiency is mitigated but still highly prevalent in patients with CKD in a high ambient UV environment. Despite the maintenance of relatively higher 25-OHD concentrations with advancing CKD, substrate availability does not appear to be a major determinant of circulating 1,25-OHD.