Introduction

Leprosy, or Hansen’s disease (HD), is a chronic infection caused by Mycobacterium leprae and M. lepromatosis, that mainly affects the skin and peripheral nerves. Although global control efforts have reduced incidence, the disease remains endemic in countries like Brazil, India, and Indonesia, that together accounts for approximately 80% of new cases worldwide [1]. Vulnerable populations, particularly refugee and immigrant groups, also carry a disproportionately high leprosy burden. A recent work reported that 25.9% of all identified dermatological conditions in these populations were attributable to leprosy [2]. In 2023, Indonesia reported 14,376 new cases, with 3.0 new grade 2 disability (G2D) cases per million population, far exceeding the World Health Organization (WHO) 2030 target of 0.12 [1]. One of the most common cause of disability among leprosy patients is trophic ulcer (TU), affecting 10–20% of patients [3,4]. These chronic wounds, often on the plantar surface, result from neuropathy, foot deformities, impaired sensation, and comorbidities [5]. They are notoriously difficult to manage and are associated with frequent recurrence, infection, and even malignant transformation [6].

Vitamin D, a secosteroid hormone, has demonstrated a key role in immune modulation and wound healing [7,8]. In damaged tissue, the circulating form, 25-hydroxyvitamin D [25(OH)D], is converted to calcitriol, binds to the vitamin D receptor (VDR), and induces antimicrobial peptides and cytokines involved in tissue repair [9–11]. Vitamin D deficiency has been linked to poor healing in chronic wounds (e.g., diabetic and venous ulcers) [12–14], with some studies showing improvement following supplementation [15,16].

Despite abundant sunlight exposure, vitamin D deficiency is prevalent in the general population in Indonesia and other Southeast Asian countries, with reported rates ranging from 6% to 70% [17–19]. In addition, a study from our center reported a 71.4% prevalence of vitamin D deficiency among newly diagnosed leprosy patients [20]. Furthermore, a previous study conducted in Medan, Indonesia demonstrated lower vitamin D levels among leprosy patients compared to healthy controls [21]. However, whether vitamin D status differs specifically in leprosy patients with TU remains unclear.

Various genetic polymorphisms of the VDR gene may influence receptor function and downstream effects of vitamin D. Among these, the FokI polymorphism (rs2228570) is unique in that it alters the translation initiation site, resulting in structural variation of the receptor [22,23]. The ff genotype produces a longer, less active receptor, and has been associated with leprosy susceptibility and chronic ulcers in other conditions [24–26].

While previous studies have linked vitamin D deficiency and VDR polymorphisms to diabetic foot ulcers [25,27] and leprosy risk [21,28–30], their role in leprosy-related TU remains unexplored. This study aims to assess differences in serum 25(OH)D levels and the distribution of the FokI polymorphism between leprosy patients with and without TU, assess their association with TU incidence, and determine the correlation between vitamin D levels and ulcer severity.

Materials and methods

Results

Sociodemographic and clinical characteristics

A total of 82 participants were recruited between March and July 2024, during the Indonesian dry season, comprising 41 leprosy patients with TU and 41 without. The selection process is summarized in the study flow diagram (Fig 1). Sociodemographic characteristics are presented in Table 1. No statistically significant differences were observed between the groups in terms of sex, age, occupation, marital status, smoking history, or duration of sun exposure. However, patients with TU had significantly lower education levels (p < 0.001) and a higher proportion of household incomes below the regional minimum wage (p = 0.010).

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Table 1. Sociodemographic characteristics of study subjects.

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Fig 1. Study flow diagram.

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The clinical characteristics of both groups are detailed in Table 2. Subjects in the TU group had significantly longer disease duration (median 10 vs. 2.25 years, p < 0.001), higher prevalence of sensory neuropathy in the hands (97.6% vs. 51.2%, p < 0.001) and feet (100% vs. 78%, p = 0.002), and higher prevalence of non-ulcer deformities (70.7% vs. 12.2%, p < 0.001). No significant differences were observed in nutritional status, disease classification (WHO and Ridley-Jopling), treatment status, history of leprosy reactions, and hypertension

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Table 2. Clinical characteristics of study subjects.

https://doi.org/10.1371/journal.pntd.0014205.t002

Ulcer characteristics

There were 69 ulcers among the 41 subjects with TU. For consistency in analysis, only the largest ulcer per patient was evaluated, with aggregated data presented in Table 3. The median ulcer area was 1.89 cm², and the majority (78.1%) extended to the subcutaneous tissue. Most ulcers were located on the lower extremities (90.2%), particularly on the forefoot (41.5%). The median ulcer duration was four months, and the median PUSH score was 7 (range 3–14). The majority of ulcers exhibited mild to moderate exudate (70.7%) and had a granulation tissue base (73.2%). Callus formation surrounding the ulcer was observed in 82.9% of cases.

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Table 3. Characteristics of the largest ulcer among the trophic ulcer group (n = 41).

https://doi.org/10.1371/journal.pntd.0014205.t003

Serum 25(OH)D levels

The mean serum 25(OH)D concentration was notably lower in individuals with trophic ulcers (TU) compared to those without (13.14 ± 5.88 ng/mL vs. 20.18 ± 7.19 ng/mL, p < 0.001), as represented in Fig 2. Furthermore, vitamin D deficiency or severe deficiency was significantly more prevalent among patients in the TU group (82.9% vs. 36.6%, p < 0.001), as shown in Table 4. Exploratory subgroup analyses did not demonstrate significant differences in serum 25(OH)D levels between BT/BB and BL/LL subtypes in the overall cohort or within TU and non-TU groups.

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Table 4. Vitamin D status and FokI polymorphism genotype and allelle frequencies in leprosy patients with and without trophic ulcers.

https://doi.org/10.1371/journal.pntd.0014205.t004

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Fig 2. Scatter plot showing serum 25(OH)D levels in leprosy patients with and without trophic ulcer (***, p < 0.001).

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Association between vitamin D and FokI polymorphism with TU

In bivariate analysis, vitamin D status, FokI polymorphism, educational level, household income, leprosy duration, and of non-ulcer deformity were associated with TU (Table 5). These variables, together with nutritional status, were included in the multivariable logistic regression to control for potential confounding. After adjustment, low vitamin D status remained independently associated with TU (adjusted odds ratio [aOR] 4.15; 95% CI: 1.41–12.24; p = 0.010), as did the presence of the FokI polymorphism (aOR 7.86; 95% CI: 1.80–34.36; p = 0.006). Educational level showed a protective association with TU, while non-ulcer deformity remained positively associated. Leprosy duration did not remain statistically significant after adjustment but was retained in the model due to its clinical relevance as a potential confounder.

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Table 5. Multivariate analyses of variables associated with trophic ulcers.

https://doi.org/10.1371/journal.pntd.0014205.t005

Correlation between vitamin D levels and ulcer severity

A weak but statistically significant inverse correlation was found between serum 25(OH)D levels and PUSH scores among leprosy patients with TU (Pearson’s r = –0.312, p = 0.047), as shown in Fig 3A. Exploratory subgroup analyses (Fig 3B, 3C) demonstrated a moderate inverse correlation in BT/BB subjects (n = 18; r = –0.605, p = 0.008), whereas no significant correlation was observed in BL/LL subjects (n = 23; r = –0.079, p = 0.719).

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Fig 3. Correlation between serum 25(OH)D levels and trophic ulcer severity, as assessed using the Pressure Ulcer Scale for Healing (PUSH) scoring.

(A) Overall, a weak inverse correlation was observed. In exploratory subgroup analyses, a stronger inverse correlation was found in BT/BB subjects (B), whereas no correlation was observed in BL/LL subjects (C).

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Genetic model analysis

Different inheritance models of the FokI polymorphism were evaluated for their association with TU (Table 6). Statistically significant associations were observed under the codominant (p = 0.037), recessive (p = 0.016), and log-additive (p = 0.017) models. The recessive model showed the best fit based on the lowest Akaike and Bayesian Information Criterion values. In this model, patients with the homozygous mutant genotype (ff) had a 4.64-fold higher risk of developing TU compared to those with FF or Ff genotypes.

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Table 6. Genetic models of FokI polymorphism associated with trophic ulcer among leprosy patients.

https://doi.org/10.1371/journal.pntd.0014205.t006

Discussion

This study highlights an observed association between vitamin D status and receptor polymorphism with TU development among leprosy patients. Serum 25(OH)D levels were lower and the FokI polymorphism was more frequent in patients with TU, and both remained associated with TU after adjustment for confounders. A weak inverse correlation was also observed between vitamin D levels and ulcer severity. However, causality cannot be inferred due to the case-control design.

Despite the absence of sample matching, comparable age and sex distributions between groups likely minimized confounding. Some sociodemographic disparities persisted, with significantly more patients in the TU group having lower educational attainment and household incomes below the regional minimum wage. These findings align with prior evidence that low socioeconomic status impairs access to healthcare and delays ulcer prevention and management [5]. Clinically, patients with TU had a significantly longer disease duration and higher prevalence of sensory neuropathy and deformities. These features are well-established risk factors for ulcer formation due to repeated unnoticed trauma and altered biomechanical pressure points [3,5,36], particularly on weight-bearing areas like the forefoot, which was the most common ulcer site in this study. The median ulcer duration of four months, with a substantial portion persisting for several years, underscores the chronic and refractory nature of TU in leprosy.

Patients with TU exhibited significantly lower serum 25(OH)D levels, supporting our primary hypothesis. Vitamin D status also remained independently associated with TU after adjustment for potential confounders. To our knowledge, this is the first investigation to explore this association specifically in individuals with leprosy. As this was a case-control study, temporal direction cannot be determined, and the findings should not be interpreted as evidence of causality. Low vitamin D levels have similarly been associated with impaired healing in other chronic ulcers, including diabetic, venous, and pressure ulcers, suggesting a broader role of vitamin D in wound biology [12–14,37,38]. However, given the multifactorial nature of these conditions, vitamin D is more appropriately regarded as a contributory factor rather than a sole causal determinant.

In our study, lower vitamin D levels were weakly correlated with increased TU severity, consistent with findings in diabetic foot ulcers (DFU) [12,39,40]. Although the observed correlation was weak and of uncertain clinical significance, it may reflect a potential contribution of vitamin D status to ulcer progression. In exploratory subgroup analyses, the inverse correlation between vitamin D levels and ulcer severity appeared stronger among BT/BB subjects but was not observed in BL/LL patients, suggesting greater clinical relevance in the former group. One possible explanation for this observation is that tuberculoid leprosy forms are immunologically characterized by relatively preserved cell-mediated immunity, whereas lepromatous disease is associated with impaired cellular responses, in which vitamin D-mediated effect might be less clinically apparent [28]. In addition, ulcer severity in BL/LL patients may be more strongly influenced by greater disease burden, including nerve and structural damage, thereby limiting the contribution of vitamin D. However, given the modest subgroup size and post-hoc nature of this analysis, these findings should be considered hypothesis-generating.

Vitamin D, through its active form that binds to the cytoplasmic vitamin D receptor, regulates genes involved in antimicrobial peptide production, inflammatory response, and tissue repair. Various vitamin D-dependent pathways are engaged throughout these processes, thus disturbances caused by low vitamin D levels or receptor polymorphisms can interfere with these functions. In leprosy, two Brazilian studies have shown that both vitamin D deficiency and VDR polymorphisms are associated with increased disease susceptibility [41,42]. Evidence from chronic ulcer research also indicates that vitamin D contributes to wound healing through effects on innate immunity, cytokine regulation, angiogenesis, and keratinocyte proliferation [7,9,10]. Vitamin D supplementation has been linked to improved outcomes in chronic ulcers [15,43], and agents acting downstream of vitamin D have shown clinical benefit. In a randomized trial, Miranda et al. [44] reported that topical LL-37, a peptide induced by vitamin D signaling, accelerated wound granulation in DFU.

Furthermore, vitamin D influences several pathways that may contribute to TU formation in leprosy. Reduced vitamin D activity weakens vitamin D-dependent innate immune pathways, lowering LL-37 and other antimicrobial peptides [11,45]. This impaired response may compromise cutaneous host defence and contribute to the microbiome dysbiosis and metabolic disturbances reported in TU [46]. Adequate vitamin D signalling also supports TLR2/1-mediated macrophage killing of M. leprae [47] and promotes nerve repair through Schwann cells [48]; therefore, deficiency may exacerbate neural damage. As neuropathy progresses, sensory loss permits repeated unrecognised trauma, while autonomic fiber involvement can reduce local perfusion and sweating, weakening the epidermal barrier [49]. These fibers also release neuropeptides that promote vasodilation, chemotaxis, and keratinocyte repair, and loss of these signals has been shown to delay wound healing in diabetic neuropathy [50]. While a similar neuropeptide-related disturbance is plausible in leprosy, it remains understudied.

Although seasonal variation in vitamin D levels is well recognized in higher-latitude countries, its impact in this study is likely minimal. Indonesia, an equatorial country, receives relatively constant daylight duration and high UV indices throughout the year, including in Jakarta where seasonal fluctuations are modest [51]. Studies from Indonesia and other Southeast Asian countries have not demonstrated meaningful seasonal differences in serum 25(OH)D levels [52,53]. In this study, all participants were recruited during the dry season, reducing intra-study variation in UV exposure. While the wet season may theoretically reduce sun exposure, these factors indicate that seasonal bias is unlikely to have significantly affected our findings.

The homozygous mutant (ff) genotype and allele (f) of the FokI polymorphism more frequent among TU patients. Although the FokI polymorphism remained associated with TU after multivariable adjustment, the association appeared less robust than that of vitamin D status. suggesting that its clinical relevance may be modulated by modifiable factors such as vitamin D levels. Similar patterns have been reported in DFU, where associations observed in unadjusted analyses were attenuated after accounting for vitamin D status and other covariates [25].

Mechanistically, the ff genotype encodes a longer VDR isoform with reduced transcriptional activity [22]. Under the recessive model, individuals with the Ff genotype still produce the shorter, more active isoform, which may explain the limited biological effect of FokI heterozygosity. Reduced VDR activity in ff homozygotes could diminish vitamin D-dependent antimicrobial, immunomodulatory, and tissue repair pathways [7,9], contributing to neural damage or delayed wound healing in leprosy. Still, these effects may be insufficient to influence ulcer development without concurrent vitamin D deficiency. Finally, TU development is multifactorial. While vitamin D status represents one potentially modifiable factor, educational level likely reflects broader social determinants of health. Non-ulcer deformities may be partially mitigated through strengthened prevention of disability (POD) care, although established deformities are largely irreversible.

While the recessive model showed the strongest association in our analysis, genetic model testing was exploratory and should therefore be interpreted cautiously. The additional significance observed under the log-additive model may suggest a potential incremental effect of the mutant allele and could be consistent with a broader polygenic mechanism on ulcer susceptibility. These findings require confirmation in larger cohorts. Future genome-wide association studies (GWAS) are warranted to identify additional genetic loci involved in immune regulation, nerve damage, and wound repair as potential risk factors for TU in leprosy patients.

Conclusion

This study demonstrates that lower serum vitamin D status and the FokI polymorphism were independently associated with the presence of TU in patients with leprosy. Furthermore, a weak inverse correlation was observed between serum vitamin D levels and ulcer severity. While genetic variation may contribute to ulcer susceptibility, vitamin D status represents a potentially modifiable parameter in clinical practice. These findings support consideration of vitamin D status assessment within comprehensive ulcer care in leprosy. However, prospective cohort studies and interventional trials are required to clarify temporal relationships and to determine whether correction of vitamin D deficiency can reduce ulcer incidence or severity in this population.

Supporting information