Vitamin D Levels are Associated with Painful Diabetic Peripheral Neuropathy in Chinese Patients with Type 2 Diabetes Mellitus

This study was aimed to evaluate the relationship between vitamin D and painful diabetic peripheral neuropathy (painful-DPN) in Chinese patients with type 2 diabetes mellitus (T2DM). A total of 478 patients with T2DM were enrolled in this cross-sectional study, 127 of whom had painful-DPN and 351 of whom had painless diabetic peripheral neuropathy (painless-DPN). Detailed data including basic information, anthropometric measurements, laboratory examinations and neurological assessments were collected for all patients. Painful-DPN was diagnosed through neurological assessments, including neuropathy symptom score (NSS), neuropathy disability score (NDS), nerve conduction studies and the visual analogue scale. The results showed that serum vitamin D levels were significantly lower in patients with painful-DPN than patients with painless-DPN (p=0.012). Pain scores, NSS and NDS were negatively associated with vitamin D levels, while motor nerve conduction velocities, motor and sensory nerve amplitudes were positively associated with vitamin D levels (all p<0.05). After adjustment for age, sex, diabetes duration, smoking, diabetic kidney disease and diabetic retinopathy, diastolic blood pressure, parathyroid hormone and HbA1c, serum vitamin D levels were independently associated with painful-DPN based on logistic regression assessments (p=0.039). Moreover, diabetic retinopathy and sex were independently associated with painful-DPN (p=0.031; p=0.005, respectively). Receiver operating characteristic analysis demonstrated that serum vitamin D levels <10.3 ng/mL were predictive of the risk of painful-DPN (p<0.05). These findings revealed that lower serum vitamin D levels were significantly associated with painful-DPN. Further studies are needed to explore the causal relationship and pathophysiology of painful-DPN.


Introduction
Painful diabetic peripheral neuropathy (painful-DPN) is one of the most common phenotypes of diabetic neuropathies. According to the literature, the prevalence of painful-DPN ranges from3.3% to 65.3% in patients with diabetes [1][2][3]. Painful-DPN affects patients' health-related quality of life and social function and increases their health care costs [4]. Although many studies have investigated the mechanism of pain in patients with diabetic peripheral neuropathy (DPN), the specific pathophysiology of painful-DPN is not well understood [5].
Vitamin D has a well-known role in the regulation of bone metabolism and calcium homeostasis. During recent years, studies have suggested that vitamin D is associated with dozens of diseases, such as cardiovascular diseases, autoimmune diseases, cancers, and metabolic syndrome, including type 2 diabetes mellitus (T2DM) [6][7][8]. Many studies have further evaluated the relationship between serum vitamin D levels and DPN in patients with type 2 diabetes [9][10][11][12]. A meta-analysis including 1368 individuals with type 2 diabetes showed a significant association between vitamin D deficiency and DPN [13]. Those findings link vitamin D with DPN. Meanwhile, several observational studies have reported that patients with chronic pain have low serum vitamin D levels [14,15].
A number of Randomized Controlled Trails (RCTs) using vitamin D for the treatment of pain have been conducted. A meta-analysis including 19 RCTs founded that vitamin D supplementation led to a significantly greater mean decrease in pain score compared to placebo in patients with chronic pain [16].
Based on these findings, we hypothesize that serum vitamin D levels may be related to painful-DPN. However, most studies reporting the relationship between vitamin D and DPN have not assessed the differences in serum vitamin D levels between painful-DPN and painless diabetic peripheral neuropathy (painless-DPN) in patients with T2DM. There is also a lack of reports in Chinese patients. Therefore, the aim of this study was to investigate the association between serum vitamin D levels and painful-DPN in T2DM in Chongqing, China.

Laboratory Measurements
The laboratory profile included Glycosylated Hemoglobin

Neurological Measurements
The Neuropathy Symptom Score (NSS) and the Neuropathy Disability Score (NDS) have been widely used for screening of DPN in recent years [19][20][21]

Pain Assessment
Pain was evaluated with the Visual Analogue Scale (VAS) [23], a 10 -cm long ruler. We assessed current pain intensity, and minimum and maximum pain intensity during the preceding 2 weeks. The

Statistical Analysis
All analyses were conducted in the statistical package SPSS

Neuropathy parameters
Spearman's correlation analysis showed that the pain scores  Table 2). There is also a significant positive correlation between serum vitamin D levels and amplitude of tibialis, peroneus, ulnar and median motor nerves and amplitude of tibialis, ulnar sensory nerves (p<0.05) ( Table 3).  (Figure 1).

Figure 1:
Receiver-operating characteristic curve analysis for predicting the risk of painful-DPN.

Discussion
This study demonstrated that patients with painful-DPN had lower serum vitamin D levels than patients with painless-DPN. Pain severity of NCV impairment, respectively [27,28].
In patients with painful-DPN, Shillo et al. did not find a significant association between nerve conduction studies, DAN, and serum vitamin D levels, although HRV during deep breathing was significantly correlated with serum vitamin D levels [25]. However, our results showed that serum vitamin D levels were significantly associated with VAL, motor nerve conduction velocity and motor and sensory nerve amplitude, but not with DAN and sensory nerve conduction velocity. In addition, NSS and NDS showed a negative correlation with serum vitamin D levels in our study. The discrepancies between the results of our study and previous studies may be due to the different races and sample sizes.
Furthermore, a research letter and an interventional study (without control group) have reported that vitamin D supplementation can relieve neuropathic pain in patients with T2DM, and vitamin D may play a role in the treatment of painful-DPN [29,30]. In addition, a recent observational study found that lower vitamin D levels were significantly associated with painful-DPN, compared to painless-DPN, no-DPN, and healthy volunteers [25].
The findings of our study are consistent with those results.
Consequently, vitamin D may play a role in the pathophysiology of painful-DPN. Some possible mechanisms are as follows. Nerve growth factor (NGF) is well known to be beneficial for neurons [31,32]. A vitamin D3 derivative (CB1093) has been found to increase NGF concentrations in diabetic rats [33], and active Vitamin D3 (tacalcitol) has been shown to promote expression of NGF [34]. After correction of the vitamin D deficiency, NGF production increases. Therefore, vitamin D3 may be useful for preventing and treating neurotrophic deficits. Importantly, vitamin D3 (cholecalciferol) supplementation has been found to relieve neuropathic pain in rats by inducing the dysregulation of multiple genes, an effect associated with modulating opioid signaling [35].
Interestingly, another cross-sectional study using the PainDETECT questionnaire to assess neuropathic pain showed that serum vitamin D was not associated with neuropathic pain in patients with T2DM [26]. The inconsistency may be due to the different races, sample sizes and different diagnosis criteria of painful-DPN.
Diabetic retinopathy was independently associated with

painful-DPN in the present study. A previous study in the Middle
East has demonstrated that diabetic retinopathy was significantly related to painful-DPN, using the Douleur Neuropathique-4 [36].
Microangiopathy is an important basis of DPN [37]. Diabetic retinopathy is one of the most common microangiopathies in diabetes mellitus. These findings suggest that microangiopathy may also play a role in the development of painful-DPN. In agreement with our results, sex was independently associated with painful-DPN in several studies [2,26,36,38,39]. Females experienced a higher frequency of neuropathic pain among diabetic patients, possibly because they have higher sensitivity and exhibit protective effects in multiple sensory domains [40]. Therefore, sex should also be considered in the detection and treatment of painful-DPN.
He et al. reported the cut-off value of vitamin D for predicting the occurrence of DPN to be 16.01ng/mL [11]. However, our study First, this study was a lack of more sensitive measures for small fiber neuropathy (than HRV and VAL) such as skin biopsy. Due to the lack of translated and sinicized neuropathic pain questionnaires in China, no neuropathic pain questionnaires were used to screen for painful-DPN in this study. Moreover, because of the cross-sectional design, a conclusion of cause and effect between the variables and painful-DPN cannot be inferred. In addition, the cutoff value of vitamin D in predicting painful-DPN could not be confirmed.
Furthermore, we did not assess the effects of confounding factors, including sunlight exposure and diet on serum vitamin D levels.
Finally, this was also a single-center study, and more studies are necessary to verify our findings.

Conclusion
In this cross-sectional study, we found that higher serum vitamin D levels may play a protective role in painful-DPN in Chinese individuals with type 2 diabetes. Serum vitamin D levels <10.3 ng/mL may predict the risk of painful-DPN. Further large studies are needed to verify our findings and explore the underlying mechanisms.