Diabetes mellitus (DM) is a chronic disease. According to the International Diabetes Federation (IDF), approximately 366 million people worldwide were living with DM in 2021, and this number is expected to rise rapidly to 783 million by 2045 (1). In China, the prevalence of DM has increased rapidly over the past few decades, rising from 0.67% in the 1980s to 12.8% in 2017, affecting an estimated 130 million people. This represents the highest prevalence rate of DM globally, making it a public health issue in China (2). Diabetic peripheral neuropathy (DPN) is one of the most common chronic complications of type 2 diabetes mellitus (T2DM), affecting >90% of patients. Its clinical manifestations range from mild paresthesia to severe conditions such as ulcers, infections, muscle atrophy, neuralgia, neurodegenerative fractures, and Charcot joint disease. DPN is a major risk factor for foot ulcers, infection, gangrene, and amputations. As DPN advances, it diminishes patients’ capacity to work, significantly lowers their quality of daily life, and imposes a considerable financial strain on individuals and families.

Vitamin D, a steroidal hormone, exists primarily in two forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Previous studies have identified nuclear receptors for vitamin D in neurons and glial cells, suggesting its role in the synthesis of neurotrophic factors and enzymes (3). These findings indicate that vitamin D may delay the progression of DPN through multiple mechanisms. Reduced levels of vitamin D have been associated with an increased risk of DPN in patients with diabetes. A large-scale clinical study conducted abroad revealed that approximately 81% of patients with diabetes have vitamin D deficiency, which is closely associated with the development of DPN (4).

This study aimed to further explore the correlation between serum 25(OH)D levels and DPN in patients with T2DM. By analyzing these relationships, we hope to provide valuable insights for the prevention and treatment of DPN in clinical practice.

The incidence of T2DM, a complex metabolic disease affecting both developing and developed countries, has been reported to be increasing at an alarming rate (6). With rapid economic development, industrialization, lifestyle changes, and an aging population, the prevalence of diabetes in China has risen rapidly. It has become a major chronic non-communicable disease, ranking among the leading health threats alongside cardiovascular diseases and cancer. According to the WHO, China’s economic losses due to diabetes and related cardiovascular diseases amounted to $557.7 billion between 2005 and 2015 (7). Globally, the prevalence of diabetes has increased dramatically, reaching 8.3% in 2014, which corresponds to approximately 3.87 million cases (8). In the early stages, diabetes may present with frequent symptoms such as dry mouth, polydipsia, polyuria, or weight loss, which often do not affect daily life and may be overlooked. However, as the disease progresses, poor blood glucose control and long-term metabolic disorders can lead to chronic microvascular complications, including nerve, eye, and kidney damage, as well as an increased risk of cardiovascular and cerebrovascular diseases. Diabetic microvascular complications, such as diabetic neuropathy, retinopathy, and nephropathy, typically occur in patients who have had diabetes for several years or even decades (9). T2DM is associated with systemic chronic inflammation, characterized by increased concentrations of acute-phase response proteins and inflammatory markers. These inflammatory mediators contribute to insulin resistance and β-cell dysfunction (10, 11). Since vitamin D possesses anti-inflammatory and immunomodulatory properties, it can help alleviate low-grade chronic inflammation in diabetes by regulating cytokine production (12). The main pathogenesis of T2DM involves insulin resistance. Vitamin D not only enhances insulin secretion by regulating insulin receptor gene expression but also promotes the repair of damaged islet cells and stimulates β-cell function, thereby improving insulin sensitivity and glycemic control of patients with T2DM (5). Therefore, vitamin D has a protective effect against T2DM, and vitamin D deficiency can lead to an increased risk of T2DM (6).

DPN is one of the most common chronic complications of DM, affecting a wide range of patients with varying degrees of severity (13). There is usually an increased risk of physical disability, cardiovascular disease, and mortality. At least 20% of patients with type 1 diabetes develop distal symmetric polyneuropathy within 20 years, while 10% to 20% of newly diagnosed type 2 diabetic patients develop the condition, with prevalence rising to 50% within 10 years. The neurological complications associated with DPN place a huge burden on both the patient and society (14). DPN mainly leads to decreased quality of life due to pain, sensory loss, gait instability, foot ulceration, and amputation (15). Furthermore, its progression can lead to vision loss, neurological impairment, reduced mobility and cognitive function, decreased quality of life, limited employment, lower productivity, and increased healthcare costs. Without treatment, irreversible damage or even death may occur (16). DPN development has been attributed to various possible etiologies. Over the past few decades, it has primarily been believed that prolonged hyperglycemia leads to several damaging mechanisms, including the activation of the polyol pathway, the generation of oxygen radicals, the formation of advanced glycation end products, inflammation, impaired microcirculation, oxidative stress, and both direct and indirect nerve damage. However, recent studies have suggested that, in addition to glycemic factors, factors such as age-related neuronal degradation, hypertension, blood lipid levels, body weight, and decreased neurotrophic factors interact, leading to the occurrence and development of peripheral neuropathy. Later studies have found that vitamin D levels were substantially reduced in patients with DPN. It has been suggested that vitamin D deficiency is a risk factor for DPN and may be involved in its development, such as in oxidative stress caused by hyperglycemia, inflammation, and neuronal ischemia. The mechanism of DPN is still not completely understood, and in addition to strict glycemic control, effective modified therapy for DPN is lacking (17, 18). The identification of DPN risk factors is crucial for a better understanding of the mechanisms underlying DPN and for more effective treatments.

Vitamin D, like vitamins A, E, and K, is a fat-soluble vitamin that exists in two forms: vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Vitamin D3 is synthesized in the skin upon exposure to ultraviolet B (UV-B) irradiation, while vitamin D2 is obtained from dietary sources and converted to ergocalciferol under UV irradiation. In the liver, both cholecalciferol and ergocalciferol are hydroxylated to form 25(OH)D2 and 25(OH)D3. These metabolites are further converted in the kidneys by 1-α-hydroxylase into the biologically active form, 1.25(OH)D (19). Among these forms, 25(OH)D is the most abundant vitamin D metabolite in circulation, serving as the primary storage form in the body. Due to its long plasma half-life and stability, 25(OH)D is considered the gold standard for assessing serum vitamin D levels in clinical studies (20). In this study, 25(OH)D levels were measured using LC–MS/MS. The study examined serum 25(OH)D, serum 25(OH)D3, and serum 25(OH)D2, with 25(OH)D being the primary study index.

In addition to participating in bone metabolism, vitamin D is also involved in the pathogenesis of various diseases, including cardiovascular diseases, metabolic diseases, cancers, multiple sclerosis, biological infections, and autoimmune conditions (21). It also plays a key role in the proliferation and differentiation of immune cells and other immune regulations of the body (22). Vitamin D deficiency is widespread, affecting 30%–87% of the population. 25(OH)D concentration between 20 and 30– ng/mL (50–75 nmol/L) is considered insufficient, while levels below 20 ng/mL (<50 nmol/L) indicate vitamin D deficiency. Vitamin D also plays a critical role in insulin secretion by binding to vitamin D receptors expressed on pancreatic β-cells (23). It enhances insulin sensitivity by binding to the insulin receptor expression by binding to the vitamin D response element present in the promoter of the human insulin receptor gene (23, 24). It also influences fatty acid metabolism in insulin-responsive tissues by activating transcription factors (25) and protects against cytokine-induced apoptosis (26–28). Therefore, there is an inverse relationship between vitamin D levels and the risk of diabetic complications.

To investigate the relationship between DPN and 25(OH)D, this study categorized patients with T2DM into the DPN and NDPN groups based on diagnostic criteria. A comparison of general data and biochemical indicators between the two groups revealed that 25(OH)D and 25(OH)D3 levels were reduced in both groups. However, the mean 25(OH)D level in the DPN group (23.12 ± 0.74 ng/mL) was significantly lower than that in the NDPN group (25.68 ± 0.67 ng/mL). Similarly, the mean 25(OH)D3 level in the DPN group (22.10 ± 0.77 ng/mL) was significantly lower than that in the NDPN group (24.45 ± 0.66 ng/mL). In addition, 81.8% of patients in the DPN group exhibited reduced 25(OH)D and 25(OH)D3 levels, compared to 73% in the NDPN group, indicating that 25(OH)D reduction is more prevalent in DPN. An observational study on patients with T2DM aged >5 years demonstrated a significant reduction in 25(OH)D levels. This suggests that lower 25(OH)D levels serve not only as a relevant risk factor for DPN but also as a new therapeutic option for DPN. Furthermore, studies have shown that vitamin D levels are negatively correlated with the presence and severity of DPN. The severity of nerve conduction velocity decreased as vitamin D levels increased. After 3 months of vitamin D supplementation, neuropathic pain symptoms were reduced by approximately 50% (29). However, some scholars have raised doubts. Therefore, there is still controversy about whether to supplement vitamin D and what is its best dosage and course of treatment, which need to be verified by larger clinical trials (30).

Serum 25(OH)D levels were further categorized based on “Consensus on the clinical application of vitamin D and its analogs” into group A (deficiency group) with serum 25(OH)D < 20 ng/mL, group B (insufficient group) with levels between 20–30 ng/mL, and group C (normal group) with levels >30 ng/mL; their incidence of peripheral neuropathy was 70.6%, 47.8%, and 42.6%, respectively. As serum 25(OH)D levels increased, the incidence of DPN gradually decreased. Logistic regression analysis was used to assess the effects of 25(OH)D3, 25(OH)D, FT3, TG, BMI, RHR, and BUN on DPN. The results indicated that serum 25(OH)D and FT3 were protective factors against DPN in T2DM.

The ROC curve analysis demonstrated that the optimal cutoff value of 25(OH)D3 for predicting DPN in T2DM was 18.85 ng/mL, yielding an AUC of 0.76. In addition, the optimal cutoff for 25(OH)D for predicting T2DM was 19.94 ng/mL, with 90% sensitivity and 50% specificity, resulting in an AUC of 0.765.

The results indicated that patients with T2DM with lipid disorders (especially TG) are at higher risk of developing DPN. Even without T2DM, hypertriglyceridemia remained significantly associated with peripheral neuropathy. Patients with elevated TG levels were 2.1 times more likely to develop DPN, highlighting it as an independent risk factor, especially for small unmyelinated axons (31). However, in our study, TG levels were higher in the NDPN group, potentially due to poor recent blood glucose control, lipid-lowering medication use, dietary differences, smoking, and alcohol consumption, which may have influenced the results.

In addition, this study identified FT3 as a protective factor against DPN. T3 plays a role in inhibiting sensory neuron axonal expansion while stimulating the formation of new axons, contributing to nerve repair and regeneration. Animal studies have previously shown that T3 promotes dorsal root ganglia and neuron growth. In a chronic hyperglycemic state, reduced T3 levels lead to vascular endothelial dysfunction, resulting in insufficient nerve blood supply and damage to the peripheral nerves. Thyroid hormones protect microvascular endothelial cells, thereby reducing hyperglycemia-induced nerve damage. Furthermore, studies have shown that abnormal thyroid function may affect vitamin D metabolism, and patients with subclinical hypothyroidism often exhibit concurrent vitamin D deficiency, with both factors synergistically increasing the risk of DPN (32).In T2DM patients, the simultaneous evaluation of thyroid function (such as TSH and FT3) and vitamin D levels may help to identify high-risk groups of DPN in the early stage. However, the interaction mechanism between thyroid hormone sensitivity index and vitamin D is not clear, and more studies are needed to confirm it.

A reduction in 25(OH)D levels in patients with T2DM is closely related to the occurrence of DPN. 25(OH)D serves as a protective factor against DPN, and when serum levels drop below 19.94 ng/mL, the risk of developing DPN increases. Therefore, monitoring 25(OH)D levels may aid in the early diagnosis of DPN and provide a basis for timely intervention.