The presence of proteinuria is considered a risk predictor of chronic kidney disease (CKD). Several studies have demonstrated that high levels of urinary protein (urine protein-to-creatinine ratio [UPCR], >0.2 g/g) are closely related to both disease progression and cardiovascular outcomes [1–3]. Since albuminuria is reversible in the early stages of kidney damage, it should be identified in high-risk patients, and treatment modalities to reduce proteinuria may delay the onset of CKD.

Angiotensin-converting enzyme inhibitors (ACEi) and angiotensin receptor blockers (ARB) have been shown to reduce proteinuria more effectively than other antihypertensives, which may result from alterations in glomerular hemodynamics and renin-angiotensin system (RAS) inhibition [4,5]. However, in patients with normal blood pressure, since the use of RAS blockers may have negative effects such as hypotension, they still suffer from proteinuria. Therefore, studies were conducted to find various drugs to reduce persistent proteinuria despite the use of ACEi or ARB, such as sulodexide [6], pentoxifylline [7], and vitamin D analogs [8].

There are several hypotheses for how vitamin D might reverse the progression of kidney disease and reduce fibrosis [9]. Activated vitamin D inhibits renin biosynthesis and the endocrine regulation of hyperglycemia by the RAS [10–12]. Emerging evidence in patients with CKD has shown that vitamin D can have a renoprotective effect by reducing proteinuria or albuminuria, even in diabetic nephropathy [13,14]. Other studies conducted in patients with immunoglobulin A (IgA) nephropathy have reported that active vitamin D reduces proteinuria [8,15]. However, this beneficial effect in nondiabetic nephropathy has not yet been clinically demonstrated, and the results of clinical studies estimating the effect of vitamin D on proteinuria have been inconclusive.

Therefore, we conducted this randomized trial to prospectively assess the effectiveness of an active vitamin D analog for the reduction of microalbuminuria in patients with nondiabetic nephropathy. Additionally, we studied the persistent effects of vitamin D treatment on albuminuria, renal function, and blood pressure.

A total of 159 subjects from the three institutions were enrolled and randomly assigned to calcitriol (n = 79) or placebo (n = 80) (Supplementary Fig. 2, available online). A total of 135 subjects completed the study medication requirements during the treatment phase and entered the follow-up phase. During the follow-up phase, eight subjects dropped out. Finally, 67 subjects in the calcitriol group and 60 subjects in the control group completed the study. Of the 32 subjects who dropped out, one subject in the calcitriol group and four subjects in the placebo group stopped participating in the study because of side effects, such as mild headache, hypotension, and slight edema. Another 15 subjects withdrew their voluntary consent, and five were lost to follow-up during the study period. Participant demographics are depicted in Table 1. Baseline characteristics, clinical, and biochemical information, comorbid disease, and concomitant medications were balanced between the calcitriol and placebo groups. In both groups, more than 70% of the participants were hypertensive and most of them were using ACEi or ARB to treat hypertension. Five subjects in the calcitriol group and 12 subjects in the placebo group were treated with other drugs for proteinuria such as pentoxifylline and sulodexide. The mean 25(OH) vitamin D₃ concentrations were 17.16 ± 6.21 ng/mL and 18.01 ± 9.26 ng/mL in the calcitriol and placebo groups, respectively, which indicated vitamin D deficiency status.

In this study, calcitriol administration significantly reduced proteinuria and albuminuria compared to pretreatment. However, we found no evidence of reduced proteinuria compared to placebo. In addition, no significant results were obtained on improving renal function as an expected effect of proteinuria reduction. In terms of safety, no significant abnormalities were found compared with placebo, and most of them were mild and transient.

The purpose of this study was to evaluate the effect of reducing proteinuria with calcitriol, because there are few treatment options available except RAS blockers [17], even though proteinuria is a major factor in the progression of kidney disease [18]. Especially in patients with normal blood pressure, RAS blockers should be used with caution because there is a risk of causing hypotension or hyperkalemia. For this reason, interest in vitamin D analogs as a proteinuria-reducing agent has been increased, and it is being used as an off-label indication for clinical use.

The mechanism by which vitamin D reduces proteinuria is associated with anti-inflammatory action. Several studies have already suggested a potential anti-inflammatory mechanism of vitamin D in CKD. In vitro studies have attenuated the expression of tumor necrosis factor-alpha–induced monocyte chemoattractant protein 1 in proximal tubule cells [19]. Transforming growth factor-beta was inhibited in mesangial and juxtaglomerular cells. Paricalcitol, a synthetic vitamin D receptor (VDR) blocker, inhibits VDR and mediates nuclear factor kappa-B signaling, which inhibits inflammation in the podocyte system of the kidney [20]. In addition, in animal models of primary glomerular diseases, administration of vitamin D reduces albuminuria, improves glomerulosclerosis, and improves tubular function by reducing glomerular infiltration of inflammatory cells [21–23]. Based on the evidence from these in vitro and in vivo studies, decreased inflammation was observed in early CKD patients with higher serum vitamin D levels [24]. Since inflammatory responses increase vascular permeability [25] and glomerular vascular damage eventually results in proteinuria [26], modulating inflammatory responses is associated with reducing proteinuria. In another mechanism, vitamin D influences renin and angiotensin II levels by negatively regulating the renin gene. Li et al. [10] showed that renin expression and plasma angiotensin II production were increased several-fold in VDR-null mice compared to wild-type mice. Park et al. [27] also reported that intervention with calcitriol decreased plasma renin and angiotensin II levels in hemodialysis patients with secondary hyperparathyroidism. According to this mechanism, the combined use of RAAS blockers and vitamin D may better reduce proteinuria.

In clinical practice, the antiproteinuric effect of vitamin D has been reported in various patient groups, including diabetic kidney disease and IgA nephropathy. In one study of patients with diabetic nephropathy (UACR, >0.03 g/g), UACR was significantly decreased in the treatment and control groups after administration of vitamin D at 50,000 IU/week, and there was a significant difference between the groups (p = 0.028) [28]. In another study by Liyanage et al. [29], high-dose vitamin D (50,000 IU/week) was also associated with a significant reduction in UPCR compared to distilled water (p = 0.001). The eGFR was significantly increased in the vitamin D-administered group only (p = 0.03 for the between-group difference). One of the reasons for the discrepancy between our findings and theirs is that the patient groups were different. In patients with diabetic nephropathy, there may be more glomerular vascular injury due to hyperglycemia and more proteinuria, which may result in a greater antiproteinuric effect of calcitriol. Another reason is the different doses of vitamin D used in clinical trials. In the previous study, when high-dose vitamin D was injected intramuscularly at 50,000 IU/week, a renoprotective effect was shown in CKD patients [30]. The dose of the study is equivalent to intravenous administration of 1,050 μg/week in terms of its bioavailability. Since the weekly dose administered in our study was 2.45 μg/week (0.5 μg/day for 7 days and given 70% bioavailability [31]), the dose in those two studies was 400 times higher than in our study. In addition, the bioavailability of vitamin D is reduced in CKD patients. Vitamin D-binding protein (DBP) excretion is promoted and DBP reabsorption through the proximal tubule is inhibited in patients with proteinuria, leading to a reduction of DBP [32,33]. In patients with low DBP levels, the clearance of active vitamin D may have been enhanced, resulting in less efficacy of calcitriol to decrease proteinuria.

Additionally, several studies were conducted in nondiabetic CKD patients. In patients with IgA nephropathy, Liu et al. [15] used a calcitriol dose similar to ours in IgA nephropathy patients with persistent proteinuria of protein excretion >0.8 g/day. After 48 weeks of 0.5 μg calcitriol twice weekly, the UPCR was significantly reduced compared to that of the control group (p = 0.03). Although the study showed a decrease in proteinuria, calcitriol did not affect recovery of renal function or changes in blood pressure. Despite the small number of patients (n = 50), the effect of reducing UPCR appears to be due to the longer treatment duration of 48 weeks in patients with a single disease. A meta-analysis of seven randomized controlled trials (RCTs; n = 310) also reported the antiproteinuric effects of calcitriol in patients with IgA nephropathy [8]. In that study, calcitriol contributed to a significant decrease in proteinuria, but no significant differences were observed in the change in serum creatinine, serum calcium, or serum phosphorus level. Most of the studies have been conducted in patients with IgA nephropathy, so the evidence is limited in their application to other nondiabetic patients. Another RCT, conducted in nondiabetic stage III–IV CKD patients, reported that paricalcitol did not alter plasma levels of renin and UACR [34]. The duration of treatment was shorter than in our study, but the treatment dose (2 µg) was equivalent to 0.5 µg/day in our study. However, there is still insufficient evidence to demonstrate the effect of calcitriol on reducing albuminuria in nondiabetic patients. In this regard, our results are meaningful in that the effects of calcitriol were evaluated in various nondiabetic patients.

Nevertheless, there were several limitations to this clinical trial. In particular, unlike other studies, the antiproteinuric effect of calcitriol could not be determined because proteinuria was significantly reduced in the placebo group. This can be explained as follows: first, the subjects included in this study had microalbuminuria, which is a relatively small amount of albumin in the urine. Since the probability of diagnosing albuminuria in microalbuminuric patients (UACR, ≥0.03 g/g) was 47.2% based on a dipstick analysis [35], it was difficult to define whether the test results in the participants were an actual increase or decrease in albuminuria or errors in the test. In particular, we tried to collect samples of the first urine in the morning to measure proteinuria, but the urinalysis time of patients varied. In addition, our ability to measure the exact effect of the test drug was limited because the protein in the urine was diluted according to the patient’s water or sodium intake. Second, we recommended that the participants have a low-salt diet, but the amount of sodium intake per day was not evaluated. Many prior studies have reported that low-salt diets significantly affect proteinuria reduction. Hwang et al. [36] reported that the amount of daily albuminuria was significantly decreased in the intensive low-salt diet education group compared to the conventional education group (p < 0.001). In the Keyzer et al. study [37], paricalcitol had no significant effect on albuminuria in patients, but low-salt diets significantly reduced albuminuria and decreased urinary excretion of sodium in patients with ACEi (p < 0.001). It seems that there was a difference in the treatment effect according to compliance with the low-salt diet in our study, but we could not consider the diet effect when assessing the changes in proteinuria. The urinary sodium excretion is closely correlated with dietary salt intake; therefore, it would be necessary to consider changes in urinary sodium concentration as a confounding factor in the relationship between vitamin D and proteinuria in further studies. Finally, the short treatment period and follow-up period did not allow us to determine the long-term antiproteinuric effect of vitamin D, especially on the recovery of renal function. In this study, both calcitriol and placebo reduced proteinuria over 6 months, but a sustained effect on proteinuria was observed only in the calcitriol group. Therefore, calcitriol’s effect on reducing proteinuria can be measured more clearly under long-term administration. A long-term study is needed to evaluate the reduction of proteinuria and recovering renal function after dosing longer than 12 months.

In conclusion, while a significant reduction in proteinuria was observed following calcitriol use without notable adverse events, no significant difference in proteinuria reduction was found between the calcitriol and the placebo. Therefore, calcitriol treatment for reducing albuminuria requires careful consideration due to insufficient evidence. Further real-world studies are needed to evaluate the antiproteinuric effects considering confounding factors.