Impact of Vitamin D on Proteinuria, Insulin Resistance, and Cardiovascular Parameters in Kidney Transplant Recipients

Impact of Vitamin D on Proteinuria, Insulin Resistance, and Cardiovascular Parameters in Kidney Transplant Recipients D.R. Lee, J.M. Kong, K.I. Cho, and L. Chan ABSTRACT Low vitamin D levels prevalent in patients with chronic kidney disease have been reported to be associated with proteinuria, insulin resistance, and cardiovascular disease. Kidney transplant recipients are also susceptible to low vitamin D levels but their clinical significance is uncertain. This study investigated the prevalence and association of vitamin D insufficiency with proteinuria, insulin resistance, and cardiovascular parameters among 95 living donor kidney transplant recipients. Levels of 25-hydroxyvitamin D [25(OH)D] were stratified into an insufficient group [25(OH)D ⱕ 30 ng/mL; n ⫽ 19] versus a normal group [25(OH)D ⬎ 30 ng/mL; n ⫽ 76]. Proteinuria (urinary protein-creatinine [P/C] ⱖ 0.2 mg/mg), insulin resistance (homeostasis model assessment of insulin resistance [HOMAIR]) and cardiovascular parameters were compared between groups. Twenty percent of subjects showed vitamin D insufficiency. Proteinuria was higher among the vitamin D insufficient than the normal group (47.4% vs 18.7%; P ⫽ .02). 25(OH)D levels inversely correlated with urinary P/C ratio and intact parathyroid hormone (I-PTH) levels (r ⫽ ⫺.24, P ⫽ .02 and r ⫽ ⫺.23, P ⫽ .02, respectively). No correlations were observed between 25(OH)D levels and HOMA-IR scores or cardiovascular parameters. On univariate analysis, proteinuria and i-PTH levels were independent predictors of vitamin D insufficiency (P ⬍ .01 and P ⫽ .03, respectively). Multivariate analysis demonstrated proteinuria to be a significant predictor of vitamin D insufficiency (odds ratio ⫽ 4.526; P ⫽ .03). In conclusion, vitamin D insufficiency was common and significantly associated with proteinuria among kidney transplant recipients. Additional studies are needed to clarify the causal relationship of vitamin D insufficiency with proteinuria and to determine the role of vitamin D supplementation to attenuate the development of proteinuria. ITAMIN D2 (ERGOCALCIFEROL) comes from dietary sources, such as eggs, fish, or vitamin D containing milk. Vitamin D3 (cholecalciferol), a main source of vitamin D, is produced in the skin upon exposure to ultraviolet B from the sun. This prohormone is hydroxylated in the liver to 25-hydroxyvitamin D [25(OH)D]. The majority of circulating 25(OH)D is bound to vitamin Dbinding protein. Under the influence of parathyroid hormone, 25(OH)D is converted by the 1-alpha hydroxylase (1␣-OHase) in the kidney to form 1,25-dehydroxyvitamin D [1,25(OH)2D]. Other tissues also have the 1␣-OHase and can convert 25(OH)D to 1,25(OH)2D. NKF/KDOQI (The National Kidney Foundation/Kidney Disease Outcomes Quality Initiative) guidelines recommend measurement of 25(OH)D levels in chronic kidney disease (CKD) stages 3 and 4.1 Vitamin D status is usually determined by measuring serum 25(OH)D levels. Because 25(OH)D has a longer

V

half life (⬃3 weeks), it shows 1000 times greater concentrations in the circulation compared to 1,25(OH)2D. The high prevalence of low 25(OH)D levels among patients with chronic kidney disease and end-stage renal disease is attributed to decreased dermal cholecalciferol synthesis, limited dietary intake of vitamin D-containing food, and reduced sun exposure.2,3 From the Divisions of Nephrology (D.R.L., J.M.K.), and Cardiology (K.I.C.), Department of Internal Medicine, Maryknoll General Hospital, Busan, Republic of Korea, and Division of Renal Disease and Hypertension (D.R.L., L.C.), University of Colorado, Aurora, Colorado, USA. Address reprint requests to Dong Ryeol Lee, MD, PhD, Maryknoll General Hospital, Division of Nephrology, Department of Internal Medicine, 4-12 Streets, Daecheung-dong, Jung-ku, Busan, 600-730, Republic of Korea. E-mail: [email protected]

© 2011 Elsevier Inc. All rights reserved. 360 Park Avenue South, New York, NY 10010-1710

0041-1345/–see front matter doi:10.1016/j.transproceed.2011.08.081

Transplantation Proceedings, 43, 3723–3729 (2011)

3723

3724

Beyond its classic effects on bone and mineral metabolism, vitamin D receptors are ubiquitously expressed on cardiomyocytes, vascular smooth muscle, endothelium, and pancreas ␤ cells.2,4 – 8 Recent studies have demonstrated a significant association between low vitamin D levels and all-cause mortality and/or cardiovascular mortality among patients free of CKD,9 –12 with CKD,13–16 or on dialysis.17–19 Kidney transplant recipients are susceptible to low vitamin D levels due to reduced sun exposure and the use of corticosteroids. Little data are available on the clinical significance of vitamin D insufficiency.20,21 Furthermore, its relationship to hypertension, insulin resistance, and chronic allograft injury have not been widely explored in kidney transplant recipients. The purpose of our study was to determine the prevalence of vitamin D insufficiency and its association with cardiovascular risk factors in kidney transplantation. We hypothesized that the lack of vitamin D might lead to higher prevalences of proteinuria, insulin resistance, and other cardiovascular disease risk factors. Therefore, in kidney transplant recipients we investigated the associations between vitamin D insufficiency and proteinuria, insulin resistance, and cardiovascular parameters including pulse wave velocity (PWV) for arterial stiffness, flow-mediated vasodilation (FMD) for endothelial dysfunction, and carotid intima-media thickness (IMT). MATERIALS AND METHODS This cross-sectional study included 95 living donor kidney transplant recipients at the Maryknoll General Hospital, Pusan, Harbor city of South Korea (35° 05\?\ north latitude). All patients were over the age of 18 years. To investigate insulin resistance in kidney transplantations showing stable allograft function, we excluded patients with diabetes and/or less than 1 year posttransplantation. The medical ethical committees approved the study; all patients provided written informed consent before inclusion. Serum levels of 25 hydroxyvitamin D were measured by a chemiluminescence immunoassay using the Liaison autoanalyzer (Diasorin Inc). We defined a normal vitamin D level as 25(OH)D ⬎ 30 ng/mL (or ⬎75 nmol/L) and vitamin D insufficiency, ⱕ 30 ng/mL (or 75 nmol/L). Urinary protein-creatinine (P/C) ratios which correlate with the amount of protein in a 24-hour urine,22 were used to define proteinuria as ⬎0.2 mg/mg. Nephrotic-range proteinuria was defined as an urinary P/C ratio ⬎ 3.5 mg/mg.23 HOMA-IR (homeostasis model assessment of insulin resistance) and pancreas ␤-cell function were calculated using: HOMA-IR ⫽ [(fasting insulin (␮U/mL) ⫻ fasting glucose [mmol/l])/22.5] with a value of ⬎ 2.0 representing insulin resistance.

Assessment of PWV PWV was measured using a VP-2000 (Omron Inc) It has multimicro sensors to measure pulse waves based on tonometry at the carotid and femoral arteries. Two continuous waves were used; one to deflect the pulse wave as it reaches the carotid artery and one to detect the pulse wave as it reaches the femoral artery. The time required for the pulse wave to travel from one sensor to the other combined with the distance between the two sensors allowed the calculation of PWV. Three runs of data were used to average the

LEE, KONG, CHO ET AL results be reducing the measurement variability. An abnormal PWV was defined as a value of ⬎0.9.

Assessment of FMD The assessment of flow-mediated, endothelium-dependent vasodilation after reactive hyperemia was examined using two-dimensional ultrasound in the brachial artery 2 cm above the antecubital fossa in the left upper arm. Brachial artery diameter measurements were taken from the anterior to the posterior wall of the lumen during diastole at baseline and at 60 seconds after the release of a 5-minute, 250 mm Hg suprasystolic blood pressure cuff occlusion. FMD was expressed as the percentage change in the brachial artery diameter from baseline to hyperemia after the release of the suprasystolic occlusion. Endothelial-dependent FMD (%) ⫽ (hyperemia-baseline)/base 1 ⫻ 100 was defined as abnormal when ⬍ 0.7% of the median value.

Assessment of Carotid IMT Carotid duplex imaging was used to measure IMT. Using high resolution, we obtained detailed B-mode images of the right and left common carotid arteries, common carotid bifurcation, and the first centimeter of the internal carotid artery. After imaging, the sonographer obtains pulse wave Doppler measures of blood flow velocity. Abnormal carotid IMT was defined as ⱖ1 cm. All subjects were fasting, abstained from smoking, and were resting in the supine position in a quiet room for at least 10 minutes before cardiovascular disease parameter measurements. Blood pressure was measured in the sitting position. All laboratory tests consisted of fasting blood sugar by an enzymatic method, fasting insulin by radioimmunoassay, total cholesterol, intact parathyroid hormone (I-PTH) calcium, phosphorus, creatinine. Glomerular filtration rate (GFR) was estimated by the Modification of Diet in Renal Disease formula: estimated GFR ⫽ (serum creatinine (␮mol/L)/ 88.4) ⫺ 1.154 ⫻ age ⫺ 0.203 ⫻ 0.742 (if the subject is female) ⫻ 1.21 (if black).

Statistical Analyses Normality was assessed for all variables. Results of the continuous variables were expressed as mean values ⫾ standard deviations with differences at P ⬍ .05 considered significant. We compared the variables with paired Student t test or chi-square test based on the characteristics of the variables. Pearson’s correlation analysis was used to evaluate 25(OH)D correlations. A logistic regression analysis was used for univariate and multivariate analyses. Residual plots and multicollinearity diagnostics were done to ensure adequacy of the model. Multivariate analysis consisted of a stepwise logistic regression between vitamin D insufficiency and variables that were significant upon univariate analysis as independent variables. Statistical analyses employed SPSS software 18.0 (SPSS Inc).

RESULTS Patients and Baseline demographics

Among the 95 living donor kidney transplant recipients with a median duration of 110 months (15–201), preemptive transplantation had been performed in 36 (37.8%) and males comprised 54 subjects (56.8%). Their median age (range) was 48 years (25–70); estimated GFR, 62 mL/min/ 1.73 m2 (26 –99); i-PTH, 47 ng/L (19.2– 66.9); urinary P/C

VITAMIN D IN KIDNEY RECIPIENTS

3725

ratio, 0.08 mg/mg (0 –2.16); and 25(OH)D, 38.6 ng/mL (16.5–76.3). In our population, 19 (20%) recipients showed 25(OH)D insufficiency. The etiology of ESRD included chronic glomerulonephritis (42.1%), undetermined cause (36.8%), hypertensive nephrosclerosis (9.4%), and other causes (11.7%). Most patients were on a calcium channel blocker for blood pressure control. Uses of angiotensin converting enzyme inhibitor/angiotensin receptor blocker, beta blocker, and statin treatments were comparable between the groups. All patients were treated with a calcineurin inhibitor (cyclosporine or tacrolimus) plus mycophenolate mofetil and low-dose steroid (5 mg/d). None of them received vitamin D supplementation. Comparisons of demographic features between the vitamin D insufficient and the normal group 95 are described (Table 1). Duration of transplantation was significantly longer among the vitamin D insufficient than the normal group (125.6 ⫾ 49.4 vs 98.2 ⫾ 52.1 months; P ⫽ .02). Otherwise, no significant differences were shown between the two groups. Comparisons of Biochemical, Metabolic, and Cardiovascular Parameters

1,25(OH)2D levels were significantly lower among the vitamin D insufficient than the normal group: 39.9 ⫾ 17.6 versus 51.2 ⫾ 17.9 pg/mL (P ⬍ .01). No significant differences were observed among i-PTH, calcium, phosphate, serum creatinine or estimated GFR values between the two groups. Urinary P/C ratio was significantly higher in the vitamin D insufficient than the normal group: 0.4 ⫾ 0.6 versus 0.12 ⫾ 0.1 mg/mg (P ⫽ .04). A significantly higher prevalence of proteinuria was observed in the vitamin D insufficient compared to the normal group: 47.4% (9/19) versus 18.7% (14/76; P ⫽ .02). Fasting blood sugar, impaired Table 1. Comparison of Baseline Demographics in 95 Recipients Vitamin D Insufficiency (n ⫽ 19)

Recipient age (y) Recipient gender (M:F) Donor age (y) Donor gender (M:F) BMI Duration of transplantation (mo) SBP (mm Hg) DBP (mm Hg) Acute rejection within 1 y (%) CNI (TAC/CSA) (%) Treatment modality: dialysis/ preemptive (%)

Normal (n ⫽ 76)

P Value

46.6 ⫾ 9.9 9:10 39.1 ⫾ 14.5 12:7 22.6 ⫾ 2.7 125.6 ⫾ 49.4

48.2 ⫾ 10.0 45:35 36.1 ⫾ 10.8 38:38 22.9 ⫾ 2.9 98.2 ⫾ 52.1

.44 .44 .40 .30 .53 .02

119.1 ⫾ 12.1 82.4 ⫾ 8.1 21.1 (4/19)

121.8 ⫾ 11.0 82.2 ⫾ 6.2 19.7 (15/76)

.42 .65 .89

63.2/36.8 73.7/26.3

52.6/47.4 59.2/40.8

.22 .25

Values are expressed as the mean ⫾ standard deviation. BMI, body mass index; CNI, calcineurin inhibitor; CSA, cyclosporine; TAC, tacrolimus; SBP, systolic blood pressure; DBP, diastolic blood pressure; Vitamin D insufficiency, 25(OH)D ⱕ 30 ng/mL; normal, 25(OH)D ⬎ 30 ng/mL; preemptive, transplanted before initiation of dialysis.

Table 2. Comparisons of Biochemical Data and Metabolic and Cardiovascular Parameters in 95 Recipients

Biochemical data 1,25(OH)2D (pg/mL) Intact PTH (ng/L) Calcium (mg/dL) Phosphate (mg/dL) Serum creatinine (mg/dL) eGFR (mL/min/1.73 m2) Urinary P/C (mg/mg) Proteinuria (%) Metabolic parameters FBS (mg/dL) IFG (%) Insulin (␮U/mL) HOMA-IR Pancreas ␤-cell function (%) HbA1c Total cholesterol Cardiovascular parameters Arterial stiffness on PWV (%) Endothelial dysfunction on FMD (%) Abnormal IMT on carotid artery (right) (%) Abnormal IMT on carotid artery (left) (%)

Vitamin D Insufficiency (n ⫽ 19)

Normal (n ⫽ 76)

P Value

39.9 ⫾ 17.6 67.6 ⫾ 38.8 9.4 ⫾ 0.5 3.1 ⫾ 0.6 1.3 ⫾ 0.5 58.8 ⫾ 16.8 0.40 ⫾ 0.6 47.4 (9/19)

51.2 ⫾ 17.9 51.9 ⫾ 23.0 9.6 ⫾ 0.5 3.1 ⫾ 0.5 1.3 ⫾ 0.4 63.3 ⫾ 18.1 0.12 ⫾ 0.1 18.7 (14/76)

⬍.01 .11 .15 .83 .83 .49 .04 .02

108.2 ⫾ 15.7 31.6 (6/19) 5.98 ⫾ 3.4 0.81 ⫾ 0.5 56.8 ⫾ 26.4

106.1 ⫾ 21.8 26.3 (20/76) 6.2 ⫾ 3.4 0.85 ⫾ 0.5 60.8 ⫾ 23.9

.35 .77 .73 .63 .29

6.1 ⫾ 0.8 170.3 ⫾ 37.2

6.1 ⫾ 0.8 166.9 ⫾ 27.2

.80 .96

68.8 (11/16)

54 (37/68)

.40

57.9 (11/19)

56 (42/75)

.55

21.1

36.8

.28

31.6

40.8

.60

Values are expressed as the mean ⫾ standard deviation. Vitamin D insufficiency, 25(OH)D ⱕ 30 ng/mL; normal, 25(OH)D ⬎ 30 ng/mL; 1,25(OH)2D, 1.25-dihydroxyvitamin D; PTH, parathyroid hormone; eGFR, estimated glomerular filtration rate; urinary P/C, urine protein/creatinine (proteinuria defined as urine protein/creatinine ⬎ 0.2 mg/mg); FBS, fasting blood sugar; IFG, impaired fasting glucose (⬎110 mg); HOMA-IR, homeostasis model assessment of insulin resistance; HbA1c, glycated hemoglobin; PWV, pulse wave velocity (arterial stiffness was defined as PWV ⬎ 0.9); FMD, flow-mediated vasodilation (endothelial dysfunction was defined as FMD ⬍ 0.7% [⫽ median value]; IMT, intima-media thickness (abnormal carotid IMT was defined as its value ⱖ 1 cm).

fasting glucose, fasting insulin, HOMA-IR, pancreas ␤ cell function, hemoglobin A1c, and total cholesterol were comparable between the groups. No significant differences in the presence of arterial stiffness, endothelial dysfunction, or abnormal IMT were observed between the two groups (Table 2). Predictors for Vitamin D Insufficiency

25(OH)D levels inversely correlated with urinary P/C ratio and i-PTH levels (r ⫽ ⫺.24, P ⫽ .02 and r ⫽ .23, P ⫽ .02, respectively; Fig 1). A positive correlation was observed between 25(OH)D levels and duration of transplantation (months; r ⫽ .21, P ⫽ .04). 25(OH)D levels did not correlate with HOMA-IR score, PWV, FMD, or carotid IMT. On univariate logistic regression analysis, proteinuria and i-PTH independently predicted vitamin D insufficiency (P ⬍ .01 and P ⫽ .03, respectively). Multivariate logistic regression analysis demonstrated proteinuria to be the only significant predictor for vitamin D insufficiency (odds ratio ⫽

3726

LEE, KONG, CHO ET AL

Fig 1. Graphs showing 25(OH)D levels were inversely correlated with urinary P/C ratio (Left) and I-PTH levels (right) (r ⫽ ⫺.24, P ⫽ .02 and r ⫽ ⫺.23, P ⫽ .02, respectively) in kidney transplant recipients. P/C, protein/creatinine; I-PTH, intact parathyroid hormone.

4.526, P ⫽ .03), independent of recipient factors (recipient age, estimated GFR, calcium, phosphate, i-PTH, proteinuria, and treatment modality prior to transplantation), donor factors (age), or transplantation factors (acute rejection episodes within 1 year, duration of transplantation, and calcineurin inhibitor) (Table 3). DISCUSSION

A high prevalence of vitamin D insufficiency has been documented in two previous studies.20,2 However, our study focused on the association of low vitamin D levels with proteinuria, insulin resistance, and cardiovascular parameters. One striking feature of this study was that the vitamin D insufficient group showed a significantly higher prevalence of proteinuria as compared with the normal group. To assess predictors for vitamin D insufficiency, we performed a multivariate analysis that accounted for recipient, transplantation, and donor factors. Proteinuria remained the only significant predictor for vitamin D insufficiency. In NHANES III (National Health and Nutrition Examination Survey), low 25(OH)D levels were associated with an increased prevalence of albuminuria among the general adult population.24 It is uncertain whether proteinuria leads to low vitamin D levels or vice versa. In patients with nephrotic syndrome, low vitamin D levels are a common manifestation.25,26 An inverse relationship has been reported between vitamin D levels and proteinuria.27,28 Interestingly, our study noted the severity of proteinuria to be in the subnephrotic range. It is unclear whether only urinary loss of vitamin D binding protein or 25(OH)D per se contributed to vitamin D insufficiency. In contrast low vitamin D levels also can cause proteinuria; vitamin D suppresses transcription of renin that activates the renin angiotensin system, which contributes to reduction of proteinuria through hemodynamic mechanisms.29 Low vitamin D content also leads to podocyte loss and glomeruloscerosis

through direct cellular effects.30 Several clinical trials have shown that paricalcitol significantly reduces albuminuria or proteinuria in CKD and proteinuric kidney diseases.31–35 Another aim of our study was to examine the association between vitamin D insufficiency and metabolic parameters, especially insulin resistance. The pancreas also possesses the vitamin D receptor and 1-␣ hydroxylase. Circulating 25(OH)D can be converted to 1,25(OH)2D to work as a paracrine or autocrine hormone. Several observational studies have suggested that low vitamin D levels were associated with insulin resistance or impaired insulin secretion.36 –38 A meta-analysis demonstrated that lower vitamin D levels were associated with abnormal glucose metabolism, type 1 and 2 diabetes mellitus, and metabolic syndrome.39,40 In a study of Finnish men and women, a 40% reduction in the risk of developing type II diabetes was observed among subjects with 25(OH)D levels ⬎ 28 ng/ mL.41 Taken together, these results suggest that vitamin D may have an important role in glycemic control. Though the hyperinsulinemic, euglycemic clamp technique is the gold standard to investigate and quantify insulin resistance, HOMA-IR correlates with this method.42 The major shortcoming of HOMA-IR score is that this model utilizes value generated from lean young adults (less than 35 years old) of Caucasian origin. Our study showed 4.2% (4/95) of subjects to show insulin resistance (HOMA-IR ⬎ 2); no significant association was observed between vitamin D insufficiency and insulin resistance. To demonstrate insulin resistance among kidney transplant recipients, we must consider the possibility of confounding effects of immunosuppressants since calcineurin inhibitors suppress insulin secretion and might mask an increased HOMA-IR score among transplant recipients; however, our study did not demonstrate any effect of calcineurin inhibitors on insulin secretion. Finally, we evaluated the relationship between vitamin D insufficiency and cardiovascular parameters: arterial stiff-

VITAMIN D IN KIDNEY RECIPIENTS

3727

Table 3. Predictors for Vitamin D Insufficiency in Univariate and Multivariate Logistic Regression Analysis Univariate Categories Objects: Reference

Demographics Recipient age (y) Recipient gender Donor age (y) Donor gender Treatment modality for ESRD Calcineurin inhibitor Duration of transplantation SBP (mm Hg) DBP (mm Hg) Acute rejection within 1 y Biochemical data Intact PTH (ng/L) Calcium (mg/dL) Phosphate (mg/dL) Serum creatinine (mg/dL) eGFR (mL/min/1.73 m2) Proteinuria Metabolic parameters FBS IFG Insulin Insulin resistance HbA1c Total cholesterol Cardiovascular disease parameters Arterial stiffness on PWV Endothelial dysfunction on FMD Abnormal carotid IMT (right) Abnormal carotid IMT (left)

Odds Ratio (95% CI)

Yes: no

0.984 (0.935–1.036) 0.620 (0.226–1.702) 1.022 (0.980–1.067) 1.714 (0.609–4.826) 1.929 (0.630–5.905) 0.525 (0.186–1.478) 1.011 (1.000–1.022) 0.978 (0.933–1.024) 1.005 (0.930–1.085) 1.084 (0.314–3.744)

Yes: no

1.019 (1.001–1.036) 0.493 (0.163–1.488) 1.108 (0.401–3.064) 1.494 (0.466–4.786) 0.986 (0.958–1.015) 4.362 (1.480–12.850)

Male: female Male: female Dialysis: preemptive TAC: CSA

Yes: no Yes: no

Yes: no Yes: no Yes: no Yes: no

Multivariate P Value

Odds Ratio (95% CI)

P Value

.54 .35 .31 .31 .25 .22 .04 .34 .91 .89

0.941 (0.876–1.011)

.09

1.033 (0.980–1.090)

.22

2.563 (0.662–9.920) 0.594 (0.160–2.199) 1.010 (0.996–1.024)

.17 .44 .17

0.973 (0.204–4.638)

.97

.03 .21 .84 .49 .33 ⬍.01

1.015 (0.991–1.040) 0.601 (0.174–2.081) 1.089 (0.295–4.017)

.22 .42 .89

1.011 (0.973–1.050) 4.526 (1.152–17.779)

.58 .03

1.005 (0.982–1.027) 1.292 (0.433–3.858) 0.977 (0.838–1.140) 0.648 (0.230–1.825) 1.065 (0.581–1.954) 1.004 (0.987–1.021)

.70 .47 .77 .41 .84 .64

1.843 (0.578–\?\5.879) 0.438 (0.151–1.272) 0.670 (0.230–1.953) 0.457 (0.138–1.514)

.30 .13 .47 .20

CSA, cyclosporine; TAC, tacrolimus; SBP, systolic blood pressure; DBP, diastolic blood pressure; PTH, parathyroid hormone; eGFR, estimated glomerular filtration rate; Proteinuria was defined as urine protein/creatinine ⬎ 0.2 mg/mg; FBS, fasting blood sugar; HbA1c, glycated hemoglobin; IFG, impaired fasting glucose (⬎110 mg); HOMA-IR, homeostasis model assessment of insulin resistance (insulin resistance was defined as HOMA-IR ⬎2); PWV, pulse wave velocity (arterial stiffness was defined as PWV ⬎ 0.9); FMD, flow-mediated vasodilation (endothelial dysfunction was defined as FMD ⬍ 0.7% [⫽ median value]); IMT, intima-media thickness (abnormal carotid IMT was defined as its value ⱖ 1 cm).

ness was confirmed by decreased PWV, endothelial dysfunction, by diminished brachial artery FMD, and abnormally increased carotid artery IMT. Low vitamin D levels have been associated with increased inflammation, reduced endothelial protective factors, and a proatherogenic milieu.43 In our study, no significant differences of these cardiovascular parameters were shown between the vitamin D insufficient and the normal group. Vitamin D levels did not correlate with cardiovascular parameters. These associations have been reported in two previous studies of ESRD patients. Increased aortic PWV and increased carotid IMT were both independent predictors of cardiovascular mortality among patients with ESRD.44 A crosssectional study demonstrated a positive correlation between vitamin D status and arterial compliance measured by FMD and a negative correlation between aortic PWV, 25(OH)D deficiency, and low 1,25(OH)2D could be associated with arteriosclerosis and endothelial dysfunction in ESRD patients on hemodialysis.45 In addition, one study reported that vitamin D receptor agonists displayed protective effects

on cardiovascular risk. A single large dose of vitamin D (100,000 IU) significantly improved endothelial dysfunction as measured by FMD and a decrease in blood pressure.46 NKF/KDOQI guidelines recommend measurement of 25(OH)D levels in all patients with CKD stages 3 to 4. Vitamin D supplementation is also recommended to reduce i-PTH concentrations if 25(OH)D levels are ⱕ75 nmol/L and ⱕ80 nmol/L in patients with secondary hyperparathyroidism, respectively.1 Based on this recommendation, our study demonstrated that 22/95 (23.2%) showed PTH levels above the limit needed to control i-PTH levels (⬍70 ng/L for CKD stage 1–3 and ⬍110 ng/L for CKD stage 4). Our study had some limitations. It was a single-center, cross-sectional evaluation. The lack of correlation between vitamin D insufficiency and insulin resistance or cardiovascular risk may have been related to the small sample size. In addition, it was confined to recipients who resided in the Korean peninsula, and may not be applicable to other populations. There were two previous comparable studies using similar criteria of vitamin D status. A study from

3728

Canada (43° 2 north latitude) of 419 kidney transplant recipients (ⱖ 1 month) with a median estimated GFR of 49.2 mL/min/1.73 m2 showed 75.5% to display vitamin D insufficiency with a mean 25(OH)D of 57.3 ⫾ 26.2 nmol/ L.20 The other study from the UK (52° 97 north latitude) including 140 kidney transplant recipients (ⱖ1 year) with a mean estimated GFR of 45 ⫾ 18 mL/min/1.73 m2 showed a 43% prevalence of vitamin D insufficiency with a mean 25(OH)D level of 16.8 ⫾ 8 ng/mL (42 ⫾ 20 nmol/L).21 These differences from our study may be attributed to the extent of sunshine exposure based on geographical location, to racial differences, to the timing of 25(OH)D measurement, or to the consumption of fatty fish or vitamin D-fortified dairy products by the kidney transplant recipients. In conclusion, vitamin D insufficiency was common. It was significantly associated with proteinuria among kidney transplant recipients. Given the potential risk of proteinuria causing chronic allograft injury and the role of vitamin D in CKD patients, early detection and intervention of vitamin D insufficiency will be of great importance for kidney transplant recipients. Larger randomized controlled trials are necessary to support our results and to determine whether vitamin D supplementation mitigates adverse events, such as proteinuria, new-onset diabetes mellitus, and cardiovascular events, or improves long-termed outcomes among kidney transplant recipients. ACKNOWLEDGMENTS The authors thank Ms Lee, hwa lim, who helped them use perfect medical records and Ms Lee, sang hee, who is a professional sonographer, gave us useful data for this study.

REFERENCES 1. National Kidney Foundation: K/DOQI clinical practice guidelines for bone metabolism and disease in chronic kidney disease. Am J Kidney Dis 42:1, 2003 2. Holick MF: Vitamin D deficiency. N Engl J Med 357:266, 2007 3. Jacob AI, Sallman A, Santz Z, et al: Defective photoproduction of cholecalciferol in normal and uremic humans. J Nutr 114:1313, 1984 4. Bouillon R, Carmeliet G, Verlinden L, et al: Vitamin D and human health: lessons from vitamin D receptor null mice. Endocr Rev 29:726, 2008 5. Nibbelink KA, Tishkoff DX, Hershey SD, et al: 1,25(OH)2vitamin D3 actions on cell proliferation, size, gene expression, and receptor localization, in the HL-1 cardiac myocyte. J Steroid Biochem Mol Biol 103:533, 2007 6. Wu Wong JR, Nakane M, Ma J, et al: Effects of Vitamin D analogs on gene expression profiling in human coronary artery smooth muscle cells. Atherosclerosis 186:20, 2006 7. Merke J, Milde P, Lewicka S, et al: Identification and regulation of 1,25-dihydroxyvitamin D3 receptor activity and biosynthesis of 1,25-dihydroxyvitamin D3. Studies in cultured bovine aortic endothelial cells and human dermal capillaries. J Clin Invest 83:1903, 1989 8. Maestro B, Davila N, Carranza MC, et al: Identification of a vitamin D response element in the human insulin receptor gene promoter. J Steroid Biochem Mol Biol 84:223, 2003 9. Dobnig H, Pilz S: Independent association of low serum 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D levels with

LEE, KONG, CHO ET AL all-cause and cardiovascular mortality. Arch Intern Med 168:1340, 2008 10. Wang TJ, Penina MJ, Booth SL, et al: Vitamin D and risk of cardiovascular disease. Circulation 117:503, 2008 11. Melamed ML, Michos ED, Post W, et al: 25-Hydroxyvitamin D levels and the risk of mortality in the general population. Arch Intern Med 168:1629, 2008 12. Semba RD, Houston DK, Bandinelli S, et al: Relationship of 25-hydroxyvitamin D with all cause and cardiovascular disease mortality in older community-dwelling adults. Eur J Clin Nutr 64:203, 2010 13. Barreto DV, Baretto FC, Liabeuf S, et al: Vitamin D affects survival independently of vascular calcification in chronic kidney disease. Clin J Am Soc Nephrol 4:1128, 2009 14. Mehrotra R, Kermah DA, Salusky IB, et al: Chronic kidney disease, hypovitaminosis D, and mortality in the United States. Kidney Int 76:931, 2009 15. Ravani P, Malberti F, Tripepi G, et al: Vitamin D levels and patient outcome in chronic kidney disease. Kidney Int 75:88, 2009 16. Melamed ML, Astor B, Michos ED, et al: 25-Hydroxyvitamin D levels, race, and the progression of kidney disease. J Am Soc Nephrol 20:2631, 2009 17. Wolf M, Shah A, Gutierrez O, et al: Vitamin D levels and early mortality among incident hemodialysis patients. Kidney Int 72:1004, 2007 18. Wang AY, Lam CW, Sanderson JE, et al: Serum 25hydroxyvitamin D status and cardiovascular outcomes in chronic peritoneal dialysis patients: a 3-y prospective cohort study. Am J Clin Nutr 87:1631, 2008 19. Drechsler C, Verduijn M, Pilz S, et al: Vitamin D status and clinical outcomes in incident dialysis patients: results from the NECOSAD study. Nephrol Dial Transplant 26(3):1024 –1032, 2011 20. Boudville NC, Hodsman AB: Renal function and 25hydroxyvitamin D concentrations predict parathyroid hormone levels in renal transplant patients. Nephrol Dial Transplant 21:2621, 2006 21. Stavroulopoulosa A, Cassidya MJD, Porter CJ, et al: Vitamin D status in renal transplant recipients. Am J Transplant 7:2546, 2007 22. Lemann J Jr, Doumas BT: Proteinuria in health and disease assessed by measuring the urinary protein/creatinine ratio. Clin Chem 33:297, 1987 23. Kashif W, Siddiqi N, Dincer HE, et al: Proteinuria: How to evaluate an important finding. Cleve Clin J Med 70:535, 2003 24. De Boer IH, Ioannou GN, Kestenbaum B, et al: 25Hydroxyvitamin D levels and albuminuria in the Third National Health and Nutrition Examination Survey (NHANES III). Am J Kidney Dis 50:69, 2007 25. Malluche HH, Goldstein DA, Massry SG: Osteomalacia and hyperparathyroid bone disease in patients with nephrotic syndrome. J Clin Invest 63:494, 1979 26. Massry SG, Goldstein DA: Calcium metabolism in patients with nephrotic syndrome: a state with vitamin D deficiency. Am J Clin Nutr 31:1572, 1978 27. Goldstein DA, Oda Y, Kurokawa K, et al: Blood levels of 25-hydroxyvitamin D in patients with nephrotic syndrome. Ann Intern Med 87:664, 1977 28. Kano K, Nonoda A, Yoneshida H, et al: Serum concentrations of 25-hydroxyvitamin D and 24, 25-dihydroxyvitamin D in patients with various types of renal diseases. Clin Nephrol 14:274, 1980 29. Freundlich M, Quiroz Y, Zhang Z, et al: Suppression of renin-angiotensin gene expression in the kidney by paricalcitol. Kidney Int 74:1394, 2008 30. Kuhlmann A, et al: 1,25-Dihydroxyvitamin D3 decreases podocyte loss and podocyte hypertrophy in the subtotally nephrectomized rat. Am J Physiol Renal Physiol 286:526, 2004

VITAMIN D IN KIDNEY RECIPIENTS 31. Agarwal R, Acharya M, Tian J, et al: Antiproteinuric effect of oral paricalcitol in chronic kidney disease. Kidney Int 68:2823, 2005 32. Szeto C, Chow KM, Kwan B, et al: Oral calcitriol for the treatment of persistent proteinuria in immunoglobulin A nephropathy: an uncontrolled trial. Am J Kidney Dis 51:724, 2008 33. Fishbane S, Chittineni H, Packman M, et al: Oral paricalcitol in the treatment of patients with CKD and proteinuric: a randomized trial. Am J Kidney Dis 54:647, 2009 34. Alborzi P, Patel N, Peterson C, et al: Paricalcitol reduces albuminuria and inflammation in chronic kidney disease: a randomized double blind pilot trial. Hypertension 52:249, 2008 35. Lambers Heerspink HJ, Agarwal R, Coyne DW, et al: The selective vitamin D receptor activator for albuminuria lowering (VITAL) study: study design and baseline characteristics. Am J Nephrol 30:280, 2009 36. Norman AW, Frankel JB, Heldt AM, et al: Vitamin D deficiency inhibits pancreatic secretion of insulin. Science 209:823, 1980 37. Liu E, Meigs JB, Pittas AG, et al: Plasma 25-hydroxyvitamin d is associated with markers of the insulin resistant phenotype in nondiabetic adults. J Nutr 139:329, 2009 38. Mathieu C, Gysemans C, Giulietti A, et al: Vitamin D and diabetes. Diabetologia 48:1247, 2005

3729 39. Chonchol M, Scragg R: 25-Hydroxyvitamin D, insulin resistance, and kidney function in the Third National Health and Nutrition Examination Survey. Kidney Int 71:134, 2007 40. Pittas AG, Lau J, Hu FB, et al: Review: the role of vitamin D and calcium in type 2 diabetes. A systematic review and meta-analysis. J Clin Endocrinol Metab 92:2017, 2007 41. Mattila C, Knekt P, Männistö S, et al: Serum 25-hydroxyvitamin D concentration and subsequent risk of type 2 diabetes. Diabetes Care 30:2569, 2007 42. Matthews DR, Hosker JP, Rudenski AS, et al: Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia 28:412, 1985 43. Zittermann A, Schleithoff S, Koerfer R: Vitamin D and vascular calcification. Curr Opin Lipidol 18:41, 2007 44. Safar ME, Levy BI, Struijker-Boudier H: Systolic blood pressure, pulse pressure and arterial stiffness as cardiovascular risk factors. Curr Opin Nephrol Hypertens 10:257, 2001 45. London GM, Guérin AP, Verbeke FH, et al: Mineral metabolism and arterial functions in end-stage renal disease: potential role of 25-hydroxyvitamin D deficiency. J Am Soc Nephrol 18:613, 2007 46. Sugden JA, Davies JI, Witham MD, et al: Vitamin D improves endothelial function in patients with type 2 diabetes mellitus and low vitamin D levels. Diabet Med 25:320, 2008