Association between serum vitamin D and uric acid in the eastern Chinese population: a population-based cross-sectional study

Vitamin D is an essential fat-soluble vitamin for humans. It can be produced in the skin when the epidermis is exposed to ultraviolet B radiation or it can be obtained from the diet, including ergocalciferol (D2) from plants and cholecalciferol (D3) mostly from marine life. The primary source of vitamin D is the keratinocytes of the skin [1]. Vitamin D is activated by hydroxylases, namely, forms of cytochrome P450, to become the active hormone (1,25(OH)₂D). As 25(OH) D has a significantly longer half-life than 1,25(OH)₂D, the serum level of 25(OH) D is considered to be the most stable and reliable indicator of vitamin D status.

Vitamin D is generally regarded as an important pro-hormone, that can participate in regulating the metabolism of calcium phosphate and bone physiology. Recently, an increasing number of studies have shown that vitamin D is also involved in many other diseases. Through its ubiquitously expressed receptor, calcitriol displays potent anti-angiogenic and anti-inflammatory activity [2]. The active vitamin D metabolite can alter DNA transcription through vitamin D receptors (VDRs), heterodimerized with retinoic X receptors, which bind to the regulatory sites in target genes [2].

Serum uric acid (SUA) is the end product of purine metabolism in humans. Most circulating uric acid (UA) is freely filtered in the kidneys, which excrete approximately 60–70% of the total UA from the body [3]. More than 30 years ago, Ames et al. hypothesized that higher SUA levels might have been beneficial during hominoid evolution because of the antioxidant properties of UA [4]. On the other hand, UA in excess may cause nephrolithiasis and gout. And it has been proposed to be linked to many other human diseases [5]. UA is an independent risk factor for insulin resistance, cardiovascular disease (CVD), nonalcoholic fatty liver disease (NAFLD) [6, 7], type 2 diabetes mellitus, metabolic syndrome, and atherosclerosis [8, 9]. Impaired renal function may increase SUA concentration by decreasing renal excretion [10]. Reduced nephron mass and/or 1-α hydroxylase enzyme activity has been shown to be associated with a decline in 1,25(OH)₂D levels in patients with chronic kidney disease (CKD) [11]. Previous studies have found that vitamin D deficiency was a predictor of CKD [12].

Vitamin D insufficiency was found significantly associated with elevated UA among postmenopausal Chinese Han women [13]. Another study showed that lower concentrations of SUA predict lower vitamin D levels in patients with type 2 diabetes and CKD [14]. However, there were few studies focused on vitamin D and UA in the general population. In this study, we intended to investigate the relationship between 25(OH) D and SUA in the general population. Our research will extend our understanding of the relationship between vitamin D and SUA.

We performed a cross-sectional study in the general population. The level of 25(OH) D was higher in hyperuricemic than in normouricemic subjects. Furthermore, 25(OH) D was positively associated with SUA, even after adjustments were made for different variants. The incidence of HUA increased 9.4% for every 10-nmol/L increase in 25(OH)D.

The conclusions of some other studies were similar to ours. Sipahi S et al. found that a decrease in SUA was among the predictors of hypovitaminosis D [14]. However, several previous studies have concluded that HUA is associated with hypovitaminosis D [13, 15, 16]. This finding seems to indicate a complicated relationship between vitamin D status and SUA.

Vitamin D produced in the skin or obtained from the diet should undergo two steps of metabolic activation to become the active hormone (1,25(OH)₂D). The first step, which results in 25-hydroxylated vitamin D, is conducted mostly in the liver by hydroxylases. In the circulation, 25(OH) D is bound to vitamin D-binding protein (DBP). The next hydroxylation occurs after the complexes of 25(OH) D and DBP are reabsorbed from the glomerular filtrate at the proximal tubule of the kidney. The production of 1,25(OH)₂D is regulated by specific hormones on the expression of CYP27B1 and CYP24A1. CYP27B1 activates vitamin D metabolites, while CYP24 A1(24-hydroxylase enzyme) inactivates both 25(OH) D and 1,25(OH)₂D, thus maintaining calcium and phosphate homeostasis [2]. The effect of vitamin D is far more extensive. The nonskeletal effects indicated that vitamin D was involved in a wide variety of pathologic processes. Some studies have reported that plasma 25(OH) D is associated with metabolic syndrome [17]. Additionally, vitamin D controls multiple biological processes, such as the following: cellular growth; angiogenesis or even modulation of the immune [18] and cardiovascular system [19], differentiation of keratinocytes [1]; and inhibition of the proliferation of breast [20], colon [21] and prostate cancer cells [22].

A high level of UA is considered to be associated with impaired renal function [23] and gouty arthritis [24]. Additionally, HUA may increase the risk of some diseases, such as CVD [25] or insulin resistance [26]. On the other hand, UA is a strong antioxidant. Nabipour I et al. found that a high level of UA was positively associated with higher bone mineral density (BMD) at all skeletal sites, serum calcium and 25(OH) D, as well as a lower prevalence of fractures in older men [27]. It is hypothesized that when liver function is impaired, both the production of UA and 25(OH) D decreases, because UA is produced in hepatocytes by xanthine oxidase, and vitamin D is hydroxylated in the liver to become 25(OH)D.

Estrogen may have influences on vitamin D and SUA [28, 29]. There was a different relationship between vitamin D and SUA in premenopausal women and postmenopausal women [13]. Vitamin D insufficiency was significantly associated with elevated UA among postmenopausal Chinese Han women, but no significant association was found among premenopausal women. It has been hypothesized that estradiol (E2) may affect SUA through mechanisms involving renal clearance, secretion and reabsorption [30]. Our study population included men, premenopausal women and postmenopausal women. We adjusted for gender and age, but we did not adjust for menopausal status, so the effect of estradiol may be confounded.

In addition, other factors may affect the SUA and 25(OH) D levels, such as sun exposure, vitamin D supplementation, and the use of certain drugs. Elevated parathyroid hormone (PTH) levels are thought to reduce renal urate excretion, although the exact mechanism remains unclear [31]. It was found that teriparatide therapy increased incidence of HUA in postmenopausal women [32]. SUA level returned to the pretreatment level after stopping treatment of PTH [33]. On the other hand, PTH can induce the expression of CYP27B1 and inhibit CYP24A1, as a result, the production of 1,25(OH)₂D increases [2]. Therefore, hyperparathyroidism or PTH replacement can influence both SUA and vitamin D. We excluded the subjects who took anti-osteoporosis drugs, thus, no participant used PTH replacement.

UA is initially filtered in the kidney. Acute renal failure is associated with increased circulating SUA concentration as a decrease of renal excretion [10]. Reduced nephron mass and/or 1α-hydroxylase enzyme activity has been shown to be associated with a decline in 1, 25(OH)₂D levels in patients with CKD [11]. As the substrate of 1,25(OH)₂D, levels of 25(OH) D might be increased. Chen W et al. found that hyperuricemia suppresses 1α-hydroxylase, leading to lower 1,25(OH)₂D and higher PTH in rats [30]. However, vitamin D is converted to 25(OH) D in the liver by 25-hydroxylase. Some studies have shown that treatment of HUA increases 1,25(OH)₂D levels with no change in 25(OH)D [16, 34, 35]. In our study, we only measured the serum level of 25(OH) D to reflect vitamin D status. Thus, impaired renal function might raise SUA and 25(OH) D levels simultaneously.

Osteoporosis is a common public health problem in China. The prevalence of osteoporosis in China has increased over the past years, affecting more than one-third of people aged 50 years and older [36]. The most common prevention and treatment of osteoporosis is vitamin D supplementation. Although the causality between SUA and vitamin D was not clear, we should pay attention to the risk of hyperuricemia induced by excessive vitamin D supplements. More clinical trials are necessary to investigate the effect of vitamin D supplementation on serum UA.

Our study has several limitations that must be considered. First, we did not consider seasonal variation in 25(OH) D concentrations. Second, data on sun exposure were not available. Third, we did not measure serum calcium and parathyroid hormone, and we could not determine whether the association of 25(OH) D with SUA was partly mediated by calcium or secondary hyperparathyroidism, although individuals using anti-osteoporosis drugs were excluded. Fourth, the exact type and dose of alcohol were not available. In our questionnaire, we only recorded drinking status as never drinking and past or current drinking. In addition, diet-related data were not available in our study, so the influence of diet on SUA levels was not considered.

Our findings in the eastern China population revealed that serum UA was positively associated with 25(OH) D, and the incidence of HUA increased 9.4% for every 10-nmol/L increase in 25(OH)D. Higher levels of serum 25(OH) D may be a potential predictor of HUA.