The prevalence of chronic kidney disease has been progressively increasing over the last two decades, with 40,385 Canadians being treated for end-stage renal disease in December 2011 [1]. Of these patients, 58% were receiving dialysis while 42% had a functioning renal transplant [1]. Studies have shown that patients who have received a kidney transplant have improved overall survival, less cardiovascular disease and improved quality of life compared to similar patients on dialysis [2]. Unfortunately, kidney transplants can still fail due to acute rejection and chronic allograft nephropathy; transplant failure can result in a threefold greater risk of death compared to patients with functioning grafts [3, 4]. Given the increasing demand for renal transplants, it is important to identify modifiable risk factors implicated in graft failure to improve patient survival and quality of life.

Vitamin D is a steroid hormone involved in the regulation of calcium, phosphorus and bone metabolism. It is obtained from the diet and sun exposure and requires two hydroxylation steps for conversion to its physiologically active form, 1,25-dihydroxyvitamin D (calcitriol) [5]. The second hydroxylation step is performed by the enzyme 1α-hydroxylase, which is found predominantly in the kidney [5]. Many patients with a GFR less than 30 mL/min/1.73 m² have an inadequate calcitriol concentration due to hyperphosphatemia-mediated inhibition of 1α-hydroxylase [6].

It is estimated that 17% of patients with chronic kidney disease have vitamin D insufficiency, which is defined as a vitamin D concentration between 15 and 30 ng/mL (40 to 70 nmol/L), and approximately 80% have vitamin D deficiency (vitamin D concentration <15 ng/mL or 37 nmol/L) [7]. Renal transplant patients are no exception. Stavroulopoulos et al. measured the vitamin D concentration for 104 patients within one year of a transplant and for 140 patients more than one year after a transplant [8]. They found that 97% and 94% of these patients, respectively, had vitamin D concentrations below 30 ng/mL [8]. Possible reasons for this include the intentional avoidance of sun exposure by patients on immunosuppressants and accelerated vitamin D catabolism by glucocorticoids [8]. Furthermore, there is a reluctance among physicians to prescribe vitamin D supplements for fear of precipitating hypercalcemia, hyperphosphatemia or hypoparathyroidism in patients with chronic kidney disease [9]. Aside from the general recommendations for those with chronic kidney disease, there are no specific guidelines for vitamin D supplementation for renal transplant patients.

Vitamin D deficiency may impact kidney allograft function, as vitamin D has known renoprotective properties. Vitamin D negatively regulates the renin-angiotensin-aldosterone system (RAAS) and has been shown to reduce RAAS-mediated renal fibrogenesis in a rat model of obstructive nephropathy [10, 11]. Furthermore, vitamin D supplementation has been shown to decrease proteinuria in both human and animal models, which is a recognized risk factor for progressive renal failure. In rats, 1,25-dihydroxyvitamin D supplementation was found to reduce glomerulosclerosis and albuminuria through its anti-proliferative effects [12]. In humans, calcitriol and paricalcitol supplementation in addition to RAAS inhibition have been associated with significant proteinuria reductions in the settings of IgA nephropathy and diabetic nephropathy, respectively [13, 14]. A significant association between vitamin D insufficiency and proteinuria has also been shown for renal transplant recipients; however, a causal relationship has not been established [15]. Considering these findings, RAAS inhibition and proteinuria reduction are two mechanisms by which vitamin D could positively affect graft function.

The immunomodulatory effects of vitamin D provide an additional basis for a role in renal transplantation. Calcitriol receptors are present on various immune cells, including T cells, B cells, monocytes and antigen-presenting cells [16]. In T cells, calcitriol suppresses helper T cell proliferation and differentiation, alters cytokine production and causes a shift from a pro-inflammatory Th1 response to a tolerogenic Th2 response [16, 17]. Calcitriol also inhibits dendritic cell differentiation and maturation into antigen-presenting cells, which may be protective in transplantation [17]. In a rat model of chronic allograft nephropathy, administration of 1,25-dihydroxyvitamin D₃ prolonged allograft survival, decreased episodes of acute rejection, reduced proteinuria and prevented histologic changes associated with chronic allograft nephropathy [18]. It follows that vitamin D supplementation may reduce acute rejection and chronic allograft nephropathy in humans through its interactions with the immune system.

Given the prevalence of vitamin D deficiency in patients with chronic kidney disease and the possible benefit of vitamin D in organ transplantation, it is important to establish recommendations to guide further therapy and research.

In this systematic review, we will assess the impact of serum vitamin D concentration and vitamin D supplementation on renal transplant outcomes. While our methodology is designed to minimize selection bias, we do anticipate several limitations with this review: clinical and methodological heterogeneity, the quality of existing studies and the paucity of randomized controlled trials. Therefore, we anticipate that we may not be able to combine studies for a meta-analysis, but rather present findings using descriptive methods.

Our primary outcome is the GFR determined by direct measurement or estimation from serum creatinine concentrations. This outcome is subject to methodological heterogeneity, as the GFR is unlikely to be measured at the same time in all studies. We have not specified time points as inclusion criteria due to the risk of eliminating potentially relevant studies. Including studies with different time points may complicate data extraction and analysis; however, we feel it is important to increase the sensitivity of our review. If there are sufficient studies at a certain time point (e.g. 1-year post transplantation), we plan to pool the data where appropriate.

Another source of heterogeneity is the use of vitamin D compounds with different physiological mechanisms (e.g. cholecalciferol vs calcitriol vs paricalcitol), which may or may not have different effects on graft function. Previous data suggest that calcitriol may have greater parathyroid hormone (PTH)-lowering effects and a greater risk of hypercalcemia than nutritional compounds [21, 22]. There may also be important differences between paricalcitol, a selective vitamin D receptor agonist, and the nonselective calcitriol regarding PTH-lowering, risk of hypercalcemia and overall survival [23, 24]. We must consider that nutritional vitamin D and selective and non-selective vitamin D receptor agonists may affect transplant function differently, which may limit opportunities for pooling treatment effects. To maximize the yield of our literature search, however, we will include studies with all vitamin D formulations, including nutritional compounds, receptor agonists and vitamin D analogs. Including different formulations will increase the comprehensiveness of our review and may allow us to compare the effects of various vitamin D compounds.

Cross-sectional studies that compare serum vitamin D concentration with transplant outcome may provide important information; however, they also introduce the possibility of reverse causation. It has been well established that vitamin D levels decline with chronic kidney disease. Thus, in a cross-sectional study showing an association between low vitamin D concentration and reduced GFR, one cannot confidently exclude the possibility that reduced kidney function was the cause of vitamin D deficiency, rather than the result. Therefore, associations between serum vitamin D and GFR must be interpreted with caution.

Lastly, our results may be confounded by our inability to control for sun exposure and dietary vitamin D intake. Neither can be measured accurately in the typical outpatient setting. Studies that assess vitamin D supplementation without baseline serum concentrations are unlikely to capture a patient’s vitamin D status completely. Measuring baseline vitamin D concentrations will partially reduce this bias, though incompletely due to ongoing potential differences in vitamin D exposure via sunlight and diet. Although we recognize this as a limitation, after a renal transplant, patients are cautioned about sun exposure secondary to concerns about skin cancer. If patients follow this recommendation, differences in sun exposure should be minimized. The impact of nutritional vitamin D intake on serum vitamin D levels is unlikely to be significant.

In summary, our systematic review will provide insight into the effects of vitamin D insufficiency and/or replacement on renal transplant function. The results may have an immediate impact on patient care and facilitate guideline development for nutritional supplements for renal transplant patients. The data will also serve as the starting point for further research on vitamin D and kidney transplantation.