INTRODUCTION
Type 2 diabetes mellitus is a chronic metabolic disorder characterized by insulin resistance and hyperglycemia, affecting over 500 million adults worldwide and posing a significant public health challenge due to its associated microvascular and macrovascular complications1.
Among these, diabetic peripheral neuropathy (DPN) is one of the most prevalent and debilitating, affecting up to 50% of patients and contributing to sensory loss, pain, and an increased risk of foot ulcers and amputations2. Small fiber neuropathy (SFN), a key subtype of DPN, preferentially damages unmyelinated C fibers and thinly myelinated Aδ fibers3, 4. This disruption manifests early as impairments in pain and temperature perception, with impaired cold sensation being a particularly common and sensitive marker that often precedes large fiber involvement2.
Notably, cold sensitivity deficits are more pronounced in type 2 diabetes mellitus than in type 1 diabetes, affecting up to 60% of patients and correlating with factors such as advanced age, prolonged disease duration, poor glycemic control, and elevated inflammatory markers5.
Alterations in calcium and mineral metabolism have been associated with type 2 diabetes mellitus in population‐based and clinical studies6, 7. For example, Zhai et al. reported an association between serum calcium levels and type 2 diabetes mellitus in an NHANES analysis, although the causal interpretation of this relationship remains complex6. In addition, a recent systematic review and meta‐analysis reported that the pooled prevalence of vitamin D deficiency among patients with type 2 diabetes mellitus was 64.2%, highlighting the frequent disturbance of calcium–vitamin D‐related metabolism in this population8.
In contrast, abnormalities in serum calcium concentrations have been less consistently characterized, and their clinical implications in type 2 diabetes mellitus remain incompletely understood6. These discrepancies may partly reflect the fact that previous studies have assessed different aspects of calcium‐related metabolism, including vitamin D status, circulating serum calcium concentrations, and intracellular calcium signaling, and have examined different clinical endpoints6, 8, 9.
Importantly, calcium dysregulation in diabetes may extend beyond circulating calcium concentrations and involve disturbed intracellular calcium signaling and cytosolic Ca2+ overload, both of which have been proposed as mechanisms contributing to diabetic complications9. In the peripheral nervous system, calcium signaling is essential for sensory neuronal excitability, axonal function, and neuron–glia interactions, and has been specifically implicated in sensory neurons and peripheral glia in the context of diabetic neuropathies10. These findings suggest that disrupted calcium homeostasis may be involved in the pathophysiology of diabetic neuropathy.
In addition, renal function assessment is clinically relevant in patients with diabetes because diabetic kidney disease is a common microvascular complication, and impaired renal function may affect mineral metabolism, including calcium homeostasis11. However, most previous studies have focused on the relationship between calcium/vitamin D‐related metabolism and T2DM or its general metabolic complications, whereas the association between serum calcium levels and quantitative sensory dysfunction, particularly cold perception abnormalities, remains insufficiently studied.
Therefore, we hypothesized that lower serum calcium levels might be associated with impaired sensory perception in patients with Type 2 diabetes mellitus. To address this question, the present study used quantitative sensory testing (QST) in a relatively large sample of patients with Type 2 diabetes mellitus to objectively assess sensory function and to examine the quantitative relationship between serum calcium levels and sensory thresholds, with particular emphasis on cold perception threshold (CPT).
MATERIALS AND METHODS
Patients
This retrospective study included 334 patients with Type 2 diabetes mellitus who were hospitalized in the Department of Endocrinology at a major tertiary care hospital between March 2020 and November 2024, including 204 men (61.1%) and 130 women (38.9%). The study was approved by the Ethics Committee of the participating institution and the participants gave informed written consent.
Inclusion criteria
(a) willingness to participate in the study; (b) diagnosis of type 2 diabetes according to the 1999 World Health Organization diagnostic criteria; and (c) age ≥ 18 years.
Exclusion criteria
(a) peripheral neuropathy other than diabetic origin; (b) pregnancy; (c) coincident major psychiatric disorders; (d) acute complications of diabetes; (e) neurological diseases; (f) vitamin B12 deficiency; (g) alcohol abuse.
Clinical data collection and laboratory testing
Gender, age, disease course, symptoms, height, weight, blood pressure, medical history, and medication of the patients were collected. Additionally, laboratory and auxiliary test results during hospitalization were also obtained. All tests were conducted at the Department of Endocrinology at a major tertiary care hospital.
Examination of neurological signs and symptoms
Neurological symptoms were assessed through patient interviews and included complaints of burning, numbness, tingling, or spontaneous pain in the extremities. Neurological signs were evaluated through physical examination, which comprised testing for deficits in vibration sense (using a 128‐Hz tuning fork), pinprick sensation, temperature sensation, and the Achilles tendon reflex. Abnormalities were documented using standardized clinical assessment procedures.
For the symptom‐stratified analysis, all enrolled participants were regrouped according to their reported neuropathic symptoms at the time of assessment. Participants were classified into three categories: (1) pain and numbness, defined as the presence of both symptoms; (2) pain or numbness, defined as the presence of either pain or numbness alone; and (3) no symptom, defined as the absence of both pain and numbness. Symptom classification was based on clinical history obtained during the study visit.
QST for thermal perception
Quantitative thermal sensory testing was performed using the Medoc PATHWAY system (Medoc Ltd., Ramat Yishai, Israel) with an ATS thermode (contact area: 900 mm2, 30 mm × 30 mm). Cold threshold and hot threshold were measured bilaterally on the dorsum of the feet using the method of limits. The test sites were selected symmetrically, and areas with callus, scar, ulceration, or other obvious skin lesions were avoided.
Participants were examined in a comfortable seated position. The baseline temperature was set at 32°C. During testing, the temperature increased or decreased at a rate of 1°C/s and returned to baseline at a rate of 3°C/s after each trial.
The safety cut‐off temperatures were set at 10°C and 45°C. Each modality was tested three times on each foot, and the mean value was used for analysis. The thresholds of the right and left feet were recorded separately, and the hot–cold difference was calculated accordingly.
Participants were instructed to respond as soon as they first perceived the stimulus as hotter or colder than the baseline temperature. A video demonstration of this standardized testing procedure in our clinical setting is provided in Supplementary Video 1.
Neurophysiological examination (nerve conduction studies, NCS)
The nerve velocity testing methods have been previously described in detail12. Briefly, an electromyography (EMG) machine (Keypoint4; Tonsbakken 16–18, DK‐2740 Skovlunde, Denmark) was utilized to assess sensory and motor nerve conduction velocity (NCV) by the same neurologist. NCV studies of bilateral median, ulnar, tibial, common peroneal, and superficial peroneal nerves on each subject were conducted.
Diagnosis of DPN
Clinical signs of DPN were evaluated in the feet using the following five tests: (a) activity of ankle reflexes with reinforcement applied. If the reflex of 2 feet decreased or did not appear, it was considered abnormal; (b) vibration sensation at the dorsal aspect of the great toes using a 128‐Hz tuning fork and the on–off method. If the sensation of either foot disappeared, it was considered abnormal; (c) light touch sensation by a 10‐g monofilament on the dorsal aspect of the great toes.
The test was repeated four times on each side, and the total number of times no pressure was recorded for scoring as 1 point per time. If ≥ 5 points, it was considered abnormal; (d) acupuncture pain sensation by a large needle on the patient's foot skin. If the sensation of either foot disappeared, it was considered abnormal; (e) temperature sensation by examination instruments with the metal end that felt cool and the polymer that felt hot.
If the patient could not distinguish the temperature difference between the two ends in either foot, it was considered abnormal. Patients with abnormalities in NCS and at least one symptom or sign of DPN were diagnosed with confirmed DPN2, 13, 14.
Statistical analysis
Group comparisons for continuous variables were conducted using Student's t‐test or One‐way analysis of variance (anova) for normally distributed variables, and the Mann–Whitney U test for nonnormally distributed variables. Categorical variables were compared using the Chi‐square test. Correlations between serum calcium levels and QST were assessed via Pearson correlation analysis, with a corresponding correlation matrix generated.
Kernel density estimation was used to assess and visualize the distribution of CPT in the DPN and DM groups by generating smoothed probability density functions in Stata 18.0 using the command with the default Epanechnikov kernel and an automatically selected bandwidth based on Silverman's rule‐of‐thumb. To investigate potential nonlinear relationships between serum calcium and CPT, multivariable fractional polynomial (MFP) regression modeling was applied adjusting for age, gender, body mass index (BMI), smoking, drinking, systolic blood pressure (SBP), alanine aminotransferase (ALT), serum creatinine (Scr), and fasting plasma glucose (FPG). Post‐estimation graphs for each respective model were generated using the fracplot command.
All statistical analyses were performed with SPSS Statistics (version 26.0) and Stata (version 12.0). A two‐sided P‐value < 0.05 was considered statistically significant across all analyses. Correlation analyses were performed for both side‐specific and mean bilateral thermal thresholds.
Side‐specific analyses were used to preserve potential inter‐limb variability, while mean bilateral values were also examined to reflect overall sensory status.
RESULTS
Baseline clinical characteristics of the study population
A total of 334 patients with type 2 diabetes mellitus were enrolled and divided into two groups: the DM group (n = 146) and the DPN group (n = 188). Baseline characteristics are compared in Table 1. The DPN group was significantly older than the DM group (62.07 ± 11.77 years vs 53.95 ± 15.78 years, P < 0.001) and had a longer duration of diabetes (13.14 ± 9.15 years vs 7.76 ± 7.16 years, P = 0.004).
Notably, no significant differences were observed between the DPN and DM groups in bone metabolism related indicators including serum calcium (2.31 ± 0.12 mmol/L vs 2.30 ± 0.13 mmol/L, P = 0.250), serum phosphorus (1.20 ± 0.21 mmol/L vs 1.15 ± 0.22 mmol/L, P = 0.811), 25‐hydroxy vitamin D [25(OH)D] (52.68 ± 21.84 nmol/L vs 53.38 ± 26.49 nmol/L, P = 0.213), osteocalcin (median 12.05 [9.34–17.10] ng/mL vs 12.50 [9.19–17.89] ng/mL, P = 0.934), Type I collagen carboxyl segment β‐specific sequence [β‐CTX] (median 0.32 [0.20–0.55] ng/mL vs 0.36 [0.24–0.54] ng/mL, P = 0.183) and parathyroid hormone (45.06 ± 43.45 ng/L vs 39.62 ± 27.47 ng/L, P = 0.173). In summary, patients with DPN exhibited a distinct profile characterized by older age, longer disease duration, and altered metabolic and renal parameters compared with those with type 2 diabetes mellitus.
Relationship between quantitative temperature sensation and clinical features
The associations between quantitative sensory testing (QST) parameters and various clinical features are summarized in Table 2. Because CPT represents the temperature at which cold is first perceived, a higher CPT indicates better preserved cold perception, while a lower CPT suggests impaired cold sensitivity. Accordingly, patients with DPN demonstrated significantly impaired cold sensation compared with those with type 2 diabetes mellitus group, as reflected by lower mean CPT values (21.62 ± 7.29°C vs 26.25 ± 4.61°C, P < 0.001).
Similarly, HPT values were elevated in the DPN group (44.39 ± 3.68°C vs 41.55 ± 3.75°C, P < 0.001). Although this mean value was above the approximate thermal pain threshold, no participants reported pain during the test. Consequently, the hot–cold difference was greater in the DPN group (22.76 ± 9.83°C vs 15.30 ± 7.53°C, P < 0.001).
Stratification by symptoms revealed a gradient of impairment: patients reporting both pain and numbness had the lowest CPT values (19.46 ± 7.86°C), indicating the greatest cold‐sensory impairment, together with the highest HPT values (45.13 ± 3.77°C), and the largest hot–cold difference (25.67 ± 10.47°C). Those with pain or numbness alone showed intermediate values (CPT: 22.27 ± 6.72°C, HPT: 44.62 ± 3.71°C, hot–cold difference: 22.35 ± 9.49°C), while asymptomatic patients had the highest CPT values (25.33 ± 5.63°C), indicating relatively preserved cold sensation, along with the lowest HPT values (42.01 ± 3.72°C) and the smallest hot–cold difference (16.68 ± 8.29°C).
No statistically significant associations were observed between HbA1c levels and thermal perception thresholds in the present cohort. For patients with HbA1c ≤ 9%, CPT values were 23.31 ± 7.00 compared with 24.40 ± 5.85 in patients with HbA1c > 9% (P = 0.171). Similarly, HPT values were 43.29 ± 3.86 (≤ 9%) vs 42.88 ± 4.18 (> 9%, P = 0.387), and hot–cold differences were 19.98 ± 9.86 (≤ 9%) vs 18.48 ± 9.09 (> 9%, P = 0.192).
Serum calcium showed a significant graded association with thermal thresholds (all P for trend < 0.05). Across calcium tertiles, CPT increased progressively from T1 (22.84 ± 6.98°C) to T2 (23.43 ± 7.14°C) and T3 (24.66 ± 5.71°C). As higher CPT values indicate less cold‐sensory impairment, these findings suggest that higher serum calcium levels are associated with relatively preserved cold sensation.
In contrast, the hot–cold difference decreased progressively across calcium tertiles, from T1 (20.91 ± 9.81°C) to T2 (19.74 ± 10.31°C) and T3 (17.85 ± 8.46°C), suggesting less pronounced thermal sensory disparity at higher serum calcium levels. HPT showed a similar pattern, decreasing from 43.75 ± 3.84°C in T1 to 43.18 ± 4.11°C in T2 and 42.52 ± 3.83°C in T3. As higher HPT values indicate greater heat sensory impairment, these findings further suggest that higher serum calcium levels are associated with relatively preserved thermal sensory function.
In summary, impaired cold and hot sensation was strongly associated with the presence of DPN, symptom severity, and reduced renal function. Patients with eGFR ≤ 90 mL/min showed poorer cold sensation (CPT: 21.53 ± 7.80°C vs 24.71 ± 5.74°C) and greater hot–cold differences (22.92 ± 10.68°C vs 17.78 ± 8.57°C) compared with those with eGFR >90 mL/min (all P < 0.001). Significant differences in cold sensation, hot sensation, and the hot–cold difference were observed according to renal function status (all P < 0.001).
A modest inverse relationship was also observed between serum calcium levels and hot sensation.
Correlation between serum calcium levels and QST
To investigate the relationship between serum calcium concentrations and quantitative sensory thresholds for temperature perception, we conducted Spearman's rank correlation analyses, and the results are presented in Table 3. Thresholds were assessed bilaterally for cold and hot stimuli on the left (L) and right (R) sides, with combined bilateral values also calculated.
In the total cohort, serum calcium levels were positively correlated with CPT, including L cold (r = 0.176, P = 0.001), R cold (r = 0.134, P = 0.012), and mean cold (r = 0.170, P = 0.001), indicating that higher serum calcium levels were associated with higher CPT values. In subgroup analyses, the association was weaker and did not reach statistical significance in patients with type 2 diabetes mellitus without DPN, with correlation coefficients of r = 0.152 (P = 0.068) for L cold, r = 0.149 (P = 0.074) for R cold, and r = 0.151 (P = 0.054) for combined cold. In contrast, in the DPN subgroup, significant positive correlations were observed for L cold (r = 0.182, P = 0.013) and mean cold (r = 0.160, P = 0.028), whereas the correlation for R cold was not statistically significant (r = 0.097, P = 0.188).
Overall, these findings suggest that the positive association between serum calcium and CPT in the total cohort may be primarily driven by patients with DPN.
Deterioration of CPT with increasing disease severity
Kernel density estimation was employed to visualize the distribution of CPT among patients with type 2 diabetes mellitus and those with DPN. The CPT ranged from approximately 0°C–30°C across both groups, with the density peaking between 20°C and 30°C. The overlapping distributions suggest similarities in central tendencies, although subtle differences in shape and spread may indicate variations in sensory thresholds between the type 2 diabetes mellitus and DPN cohorts.
Kernel density estimation (Figure 1) demonstrated a leftward shift and increased dispersion in the distribution of CPT in the DPN group compared with the DM group (21.62 ± 7.29°C vs 26.25 ± 4.61°C, P < 0.001). This pattern indicates diminished cold sensitivity and greater inter‐individual variability in patients with DPN, reflecting the progression of neuropathic involvement. The corresponding distribution of HPT between the two groups is presented in Supplementary Figure 1, providing complementary evidence of thermal sensory alterations across both cold and warm modalities.
Relationship between serum calcium and CPT
As illustrated in Figure 2, in the total study population, multivariable fractional polynomial (MFP) regression adjusted for age, sex, BMI, smoking, drinking, SBP, ALT, SCr, and FPG showed a positive association between serum calcium and CPT. The fitted curve demonstrated a gradual increase in CPT as serum calcium concentrations rose from approximately 1.80 mmol/L to 2.60 mmol/L. The 95% confidence interval surrounding the fitted line indicated statistical precision across the range of observed values.
These findings indicate that higher serum calcium levels were associated with better preserved cold perception, whereas lower serum calcium levels were associated with impaired temperature sensitivity, suggesting a potential dose–response relationship. To further validate the robustness of this association, we performed a sensitivity analysis using albumin‐corrected calcium values in patients with serum albumin < 35 g/L (N = 6). The results remained consistent with the primary analysis, as shown in Supplementary Figure 2, supporting the independent association between serum calcium and cold perception thresholds.
DISCUSSION
This study is the first to discover a significant positive correlation between serum calcium levels and CPT in patients with type 2 diabetes mellitus. The study showed that lower serum calcium levels are associated with reduced cold sensitivity, a correlation particularly pronounced in patients with DPN. After adjusting for multiple covariates, the correlation between CPT and serum calcium remained robust.
Furthermore, fractional polynomial regression analysis revealed a clear dose–response pattern. Although a negative correlation was observed between serum calcium and HPT (higher calcium levels correspond to a lower HPT, implying increased thermal sensitivity), this correlation was significantly weaker and only reached statistical significance at specific sites. These results suggest that the effect of calcium has a certain pattern specificity, preferentially regulating cold‐sensitive small fibers (Aδ and C fibers).
Although both cold and warm perception thresholds were assessed, the observed association with serum calcium was more evident for cold perception, which is why CPT was emphasized as the primary sensory outcome in the present analysis. HPT data are presented in Supplementary Figure 1.
There was no significant difference in mean serum calcium levels between the DPN group and the non‐DPN type 2 diabetes mellitus group. However, even within the normal reference range, relatively high serum calcium levels may have a neuroprotective effect on small fibers, preventing hyperglycemic damage, while slightly low serum calcium levels may exacerbate transient receptor potential melastatin 8 (TRPM8) channel dysfunction and impair cold sensation.
Cold sensation, mediated predominantly by thinly myelinated Aδ and unmyelinated C fibers via the TRPM8 ion channel, is recognized as one of the earliest and most severely affected modalities in diabetic neuropathy15. In diabetic patients, especially those with DPN, the TRPM8 channel is downregulated and its function is suppressed due to chronic hyperglycemia and overactivation of the protein kinase C/protein kinase A (PKC/PKA) pathway, leading to decreased baseline cold sensation. Therefore, when these patients develop hypocalcemia, the reduced number of functional channels prevents paradoxical cold hypersensitivity; instead, cold sensation is further weakened or completely lost.
DPN directly reflects preexisting TRPM8 dysfunction16, 17, 18.
QST studies have shown a higher incidence of abnormal CPT compared with temperature, heat pain, or vibration thresholds3, 15. Consistent with this, the mean CPT in the cohort of this study was significantly lower in patients with DPN than in patients with type 2 diabetes mellitus without DPN. Kernel density estimation showed a significant leftward shift and increased dispersion of CPT values in the DPN group, with 15%–20% of patients experiencing almost complete loss of cold sensation (CPT < 15°C).
This pattern is consistent with clinical observations of early cold sensation reduction in patients with DPN and highlights the vulnerability of cold‐sensitive fibers3. Further analysis based on symptom stratification showed that patients with both pain and numbness symptoms exhibited the lowest CPT (19.46 ± 7.86°C), highlighting that small fibers are the earliest and most susceptible subtype in the pathogenesis of DPN, and QST is the most sensitive test for small fiber neuropathy15.
A possible explanation for the association between serum calcium and temperature perception thresholds is that calcium plays a fundamental role in sensory neuronal excitability, neurotransmitter release, axonal conduction, and intracellular signal transduction19. Thermal perception is mainly mediated by small sensory nerve fibers, and dysfunction of these fibers is a well‐recognized feature of DPN2, 20. In patients with diabetes, chronic hyperglycemia, oxidative stress, mitochondrial dysfunction, and microvascular injury may contribute to peripheral nerve damage and may also disturb calcium homeostasis, thereby impairing calcium‐dependent neuronal signaling21, 22.
Lower serum calcium levels may therefore reflect, or potentially contribute to, altered sensory nerve function and abnormal cold and heat perception. In addition, renal dysfunction, which is common in patients with diabetes, may influence calcium balance and mineral metabolism, potentially further aggravating sensory abnormalities11. Taken together, these findings suggest that altered calcium homeostasis may be involved in impaired temperature perception in patients with type 2 diabetes mellitus.
However, given the cross‐sectional design of the present study, causality cannot be inferred, and further mechanistic and longitudinal studies are needed to confirm this hypothesis.
No significant association was found between a single HbA1c measurement and QST thermal perception thresholds in our hospital‐based cross‐sectional study of patients with type 2 diabetes mellitus, with or without DPN, unlike reports linking long‐term and short‐term glycemic variability to nerve dysfunction23. A single HbA1c measurement may not adequately capture cumulative exposure or fluctuations that potentially contribute to neuropathy. Thermal thresholds are influenced by multiple factors, including diabetes duration, neuropathy severity, microvascular injury, renal function, and mineral metabolism.
The lack of association thus reflects differences in study population, design, and glycemic metrics, rather than excluding a role for dysglycemia.
Side‐specific differences were noted in the correlation analyses of thermal perception thresholds. In the DPN subgroup, serum calcium correlated significantly with left‐foot CPT and mean bilateral CPT, but the correlation with right‐foot CPT did not reach statistical significance. Although the direction of association was consistent across sides, this discrepancy may be attributable to inter‐limb variability in sensory impairment, inherent QST measurement variability, differences in lower‐limb vasculopathy, and limited statistical power in subgroup analyses, rather than a distinct biological effect.
Therefore, these side‐specific findings should be interpreted with caution and validated in larger studies.
The effect of serum calcium on CPT has been explained mechanistically and is supported by substantial clinical evidence. Hyperglycemia impairs extracellular Ca2+ influx in sensory neurons, downregulates TRPM8 membrane expression, and desensitizes this channel to cold and menthol stimuli24. Methylglyoxal, an active dicarbonyl compound elevated in diabetes, covalently modifies TRPM8 and induces channel inactivation‐a key mechanism in diabetic cold‐sensory analgesia18.
Importantly, extracellular Ca2+ concentration directly determines the temperature activation threshold of TRPM8. Experimental studies indicate that a decrease of only 0.2–0.3 mmol/L can shift the threshold several degrees toward lower temperatures25, 26. This quantitative relationship is consistent with our observation that for every 0.1 mmol/L increase in serum calcium concentration, the CPT increases by approximately 1–2°C.
In diabetic rats, the transient amplitude of sensory neuron Ca2+ begins to decline as early as week 4, prior to axonal degeneration27, 28, accompanied by endoplasmic reticulum Ca2+ depletion, mitochondrial Ca2+ overload, and activation of the Ca2+‐dependent apoptosis pathway10, 29. Knockout of the mitochondrial calcium uniporter (MCU) gene prevents painful diabetic neuropathy in mice30, suggesting that Ca2+ homeostasis disturbance is not only a consequence of DPN but also a driving factor.
Clinical studies have confirmed these findings. Serum total calcium and ionized calcium levels were lower in patients with DPN than in diabetic patients without neuropathy, and vitamin D deficiency (< 50 nmol/L) was independently associated with impaired CPT, which was reversed by high‐dose vitamin D supplementation31. Meta‐analyses of vitamin D supplementation trials (including a single intramuscular injection of 600,000 IU) have shown that vitamin D deficient individuals have significant analgesic effects and partial recovery of QST31, 32.
In this study, there were no differences in the levels of 25(OH)D, PTH, osteocalcin, or β‐CTX among the groups, suggesting that the protective effect of serum calcium may be partially independent of the classic vitamin D‐PTH axis. The mechanism may involve the dysregulation of calcium‐sensitive channels and pathways in peripheral sensory endings. This could include direct modulation of TRPM8, the activity of T‐type Ca2+ 3.2 channels (which are upregulated in painful DPN)33, or other Ca2+‐dependent signaling cascades.
From a translational medicine perspective, routine monitoring of total calcium in the management of type 2 diabetes mellitus may help in the early identification of high‐risk individuals for small fiber dysfunction. For patients diagnosed with cold sensation loss, targeted calcium supplementation may be superior to glycemic control alone in improving QST results and reducing the risk of foot ulcers34, 35, especially considering the close association between cold sensation impairment and cold‐induced injury or winter ulcers34.
This study has several limitations. First, its retrospective design precludes causal inference. Second, ionized calcium, the biologically active form of calcium, was not measured; although albumin correction was performed in participants with albumin levels below 35 g/L, the results remained unchanged.
Third, no age‐ and sex‐matched nondiabetic control group undergoing the same thermal sensory testing protocol was included. In addition, local skin temperature was not recorded before testing, which may have contributed to variability in thermal thresholds36. Future prospective studies incorporating ionized calcium measurements, skin temperature monitoring, and matched healthy controls are needed.
In summary, this study found that serum calcium is an independent regulator of cold sensation in patients with type 2 diabetes mellitus, as cold‐sensitive fibers are more sensitive to calcium homeostasis disturbances at this time. These findings enhance our understanding of the mechanisms of diabetic small fiber neuropathy and suggest that serum calcium optimization is a promising new therapeutic target for prospective interventional trials.
AUTHOR CONTRIBUTIONS
Yexuan Hao (First Author): Writing – original draft, validation, formal analysis, data curation, visualization. Qiaoli Cui (First Author): Writing – original draft, formal analysis, visualization, methodology, funding acquisition. Lang Qin: Data curation, resources.
Wanwan Sun: Data curation, investigation, funding acquisition. Shiwei Zhou: Data curation, software. Yuanpin Zhang: Data curation, investigation.
Hangping Zheng: Data curation, resources, funding acquisition. Lijin Ji: Data curation, resources, investigation. Xiaoxia Liu: Data curation, investigation.
Shuo Zhang: Data curation, resources, investigation. Yiming Li: Funding acquisition, resources. Yanpei Cao (Corresponding Author): Methodology, writing – review and editing, supervision, project administration, conceptualization.
Xiaoming Zhu (Corresponding Author): Methodology, writing – review and editing, funding acquisition, resources, supervision, project administration, conceptualization.
DISCLOSURE
The authors declare no conflict of interest.
Approval of the research protocol: The study protocol was approved by the Ethics Committee of Huashan Hospital, Fudan University (No. 2016‐395) and was conducted in accordance with the Declaration of Helsinki, as revised in Fortaleza, Brazil, in October 2013.
Informed Consent: Written informed consent was obtained from all participants.
Approval date of the Registry and the Registration No number of the study/trial: N/A.
Animal Studies: N/A.
Supporting information