Serum 25 hydroxyvitamin D in non‐scarring alopecia: A systematic review and meta‐analysis

Non‐scarring alopecia mainly includes androgenetic alopecia (AGA), female pattern hair loss (FPHL), alopecia areata (AA), telogen effluvium (TE), anagen effluvium (AE) and so on. Many studies had investigated the serum 25‐hydroxyvitamin D level and vitamin D deficiency of patients with these diseases, but opinions varied, and no conclusion was reached.

of non-scarring hair loss, while FPHL is the most prevalent form of non-scarring alopecia in female patients.TE is a diffuse hair loss disorder that typically occurs about 3 months after a triggering event and lasts for around 6 months.Moreover, AA is a prevalent type of non-scarring alopecia distinguished by an immunological response leading to abrupt patches of hair loss.It is generally accepted that genetic predisposition, infections, physical and emotional trauma, as well as nutritional status, all have a part to play in the development of these disorders.
When revealed to UVB radiation, the epidermis produces the established an association between the severity of AGA and vitamin D deficiency. 3 Rasheed et al showed that low serum 25(OH) D are associated with female patients with TE and FPHL.Karadag conducted a case-control study in Turkey to examine the effects of 25(OH)D on TE disease and found that serum 25(OH)D levels in TE patients were lower than in the control group. 4All those study are still under debate and are limited in season, region, and small sample.
Thus, In hopes of assessing the serum 25(OH)D levels in individuals who have non-scarring hair loss, such as AGA, FPHL, AA, and TE, this research would conduct a meta-analysis in order to determine its potential involvement in the pathogenesis of the disease and provide data support for vitamin supplementation in the treatment of non-scarring hair loss.

| Search strategy
The meta-analysis was conducted following the guidelines of the PRISMA (Preferred Reporting Project for Systematic Reviews and Meta-Analyses) declaration. 5Two independent investigators conducted a thorough search of several databases, including Web of Science, EMBASE, PubMed, Cochrane Library, and China National Knowledge Infrastructure (CNKI), from conception to November 11, 2022.It is important to note that this meta-analysis exclusively relied on earlier research and did not conduct any research on human participants or animals.4. Sufficient data can be collected to enable the calculation of odds ratios (ORs) and/or weighted mean differences; 5. Studies that were available in all languages.

| Exclusion criteria
The exclusion criteria included the following:  The I 2 test was used to evaluate how much the included research varied from one another.If the I 2 value exceeded 50%, it indicated significant heterogeneity.In this case, we would adopt random-effects model.On the contrary, we used fixed-effects model when I 2 smaller than 50%.In addition, when the p values were less than 0.05, it was considered as a statistically significant difference.Sensitivity analyses were conducted to examine the stability and reliability of the findings.

| Search result and study characteristics
Five hundred and ninety eight studies were found after two researchers conducted a comprehensive search throughout EMBASE, the Web of Science, CNKI, PubMed, and the Cochrane Library databases.After removing duplicates, there were 199 studies identified need to review in full.Of the 199 studies, 156 studies were eliminated, because they did not fulfill criteria.After that, our meta-analysis included a total of 43 studies.We eliminated 20 clinical trials that could not extract the targeted data.As a result, our study included a total 10 670 participants with 3374 patients and 7296 controls from 23 studies.The PRISMA flow diagram for including and excluding these studies is shown in Figure 1.Main characteristics of the included studies are shown in Table 1

| Vitamin D deficiency
Some studies had compared vitamin D deficiency in patients with non-scarring alopecia with those in controls, showing a higher incidence of vitamin deficiency in patients group.We collected data on vitamin D deficiency from nine studies and pooled the data in form of OR to conclude that the patients group had a higher prevalence.
As shown in Figure 3, the value of I 2 was 0% (I 2 of the AA, FPHL, and TE were 0%, 52%, 17% respectively).We used the fixed-effects model because of the low heterogeneity.In detail, the pooled OR of

| DISCUSS ION
Although they all belong to non-scarring alopecia, the pathogenesis of AA, AGA, FPHL, and TE are not the same.But multiple studies have shown that serum 25(OH)D levels decreases in several hair loss disorders, although the pathogenesis are varied.An immunological condition called AA is mostly brought on by T cell-mediated inflammation near the anagen hair follicles. 6Vitamin D inhibits the development of specific inflammatory factors in the Th 1 and Th 17 pathways, which has immunosuppressive effects in the skin and aids in preventing automatic attacks and maintaining self-resistance. 7thogenesis of FPHL and AGA is not clear at present, but it is generally believed that they are multifactorial genetically diseases.

TA B L E . 1 (Continued)
Possible pathogenesis can be divided into two types: androgen-dependent and androgen-independent.One of the androgen-independent factors could be vitamin D. A number of studies have revealed that vitamin D is crucial for a healthy hair cycle. 8Some believe that optimal levels of vitamin D are necessary to alleviate aging processes, such as hair loss. 9In addition, Vitamin D receptors (VDRs) are present in tissues such as the small intestine, bones, kidneys, and serum, and are an important part of vitamin D's action.One study compared the VDR in the serum and tissue of AGA patients and FPHL patients and found that the VDR of AGA patients was decreased compared with the normal control group.However, VDR levels were higher in patients with FPHL than patients with AGA. 10 A significant animal study found that the hair follicle cycle, in particular the initiating phase of hair growth, requires on the activation of VDRs.In addition, the absence of VDR may impede keratinocyte differentiation, and disturb the normal hair cycle by affecting the expression of genes essential for cycling of hair follicles. 11,12st studies included in this analysis indicate that individuals with non-scarring alopecia tend to have lower 25(OH)D levels than healthy controls.Some studies have also demonstrated a rela-  not significantly differ from one another, with vitamin D deficiency being the most prevalent deficiency among these individuals. 20e study has several notable strengths.Firstly, previous research has focused primarily on patients with alopecia areata, whereas this is the first meta-analysis to evaluate 25(OH)D levels of non-scarring hair loss, encompassing not only AA but also AGA, FPHL, TE, and other non-scarring hair loss conditions.This wider scope provides more extensive clinical value.Furthermore, the most recent meta-analysis that explored the association between vitamin

| CON CLUS IONS
The available evidence suggests that patients with non-cicatricial alopecia, including AA, FPHL, AGA, and TE, are likely to have sterol hormone vitamin D. Due to its capacity to influence the immune function, vitamin D deficiency has been strongly associated with autoimmune diseases.Recent research indicated that a lack of vitamin D may contribute to non-scarring alopecia, which includes AGA, FPHL, AA, and TE.Vitamin D can have an impact on keratinocyte differentiation and proliferation, which are connected to hair growth and hair cycle.Furthermore, the expression of Vitamin D receptor (VDR) in hair follicles and keratinizing cells may have a significant impact.Jun et al. found a relationship between serum 25-hydroxyvitamin D [25(OH)D] deficiency and AA, FPHL, and AGA among Chinese. 2 Sarita S et al analyzed the serum 25(OH)D levels of 50 AGA patients and 50 controls in Italy,

1 .
Studies of patients diagnosed as having AA, AGA, FPHL, TE, or other non-scarring hair loss; 2. Studies to evaluate the serum 25(OH)D levels or vitamin D deficiency of patients and controls; 3. Original studies compared the patients group with the control group;

Review Manager 5
.3.5 version 15.1 software was used for statistical analysis.The weighted mean difference (WMD) was used to combine the risk ratios (RR, 95% CI) of serum 25(OH)D levels in each study to estimate the mean difference of serum 25(OH)D levels between patients and controls.Each study estimated the ORs of vitamin D deficiency by comparing patients and controls, and then pooled.
. The publication years was 2013 at the earliest and 2022 at the latest.The locations of these trials were Turkey, India, Egypt, China, Poland, Russian, Brazil, Nepal, USA, Philippine, and Israel.Same with most of the studies included, We defined vitamin D deficiency as <20 ng/mL.Of the 23 included studies, most studies aimed to evaluate serum 25(OH)D levels of patients with non-scarring alopecia, while only nine studies focus on vitamin D deficiency, and provide data to analyze.

F I G U R E 2
among 25(OH)D levels and disease severity.For instance, Sanke et al 3 found that 25(OH)D levels in patients with premature AGA were significantly associated with the severity of the disease.Similarly, Rasheed et al 13 reported a decrease in 25(OH)D levels as the severity of FPHL increased.Moreover, Unal et al 14 found that 25(OH)D levels were correlated with the severity of AA.Forest plot showing vitamin D in non-scarring alopecia.In contrast, Tran et al15 reported that patients with alopecia such as AA, FPHL or AGA did not show reduced serum 25(OH)D levels in comparison with healthy controls.A study found no significant differences in mean serum 25(OH)D levels or incidence of vitamin D deficiency between healthy controls and patients with alopecia areata (AA).25Many variables can affect the level of 25(OH)D in human serum, including country of residence, age, sex, exposure to sunlight, season, and sample size.In our analysis, we observed high heterogeneity and performed multiple subgroup analyses.Our findings suggest that these factors have a more significant effect on 25(OH)D levels but a smaller impact on the incidence of vitamin D deficiency.Overall, we maintain that there is a significant difference in 25(OH)D levels or vitamin D deficiency between patients with alopecia such as AA, FPHL and AGA, compared to healthy controls.This insight can help us better understand the pathogenesis of these conditions and provide new directions for treatment.However, we acknowledge the need for more large-scale studies to confirm these findings, particularly regarding FPHL and AGA.In two separate studies, Siddappa, an Indian author, investigated 25(OH)D levels in 30 AA patients under 18 years of age and 100 adult AA patients, both of which exhibited lower 25(OH)D levels and higher incidence of vitamin D deficiency.

D
and hair loss was published in 2019.This study included newly published research on vitamin D from the past 2 years, combining this with the latest research progress to provide new ideas for future clinical research and clinical application practices.Finally, the study conducted subgroup analyses according to several factors that may influence 25(OH)D levels, including disease type, country, age, gender, and sample size, ensuring the reliability and clinical application value of the analysis.There were several limitations to this meta-analysis that need to be considered.First, the high heterogeneity in the original studies may have contributed to the heterogeneity observed in the meta-analysis, despite measures taken to adjust for errors.This may be due to factors such as country, age, sex, disease species, and other factors that affect serum 25(OH)D levels, such as diet and sun exposure.These factors need to be taken into account and controlled for in future studies.In addition, the random-effects model was used to reduce the risk of error.Second, only two of the included studies had a sample size of more than 1000, and up to 15 studies had a sample size of less than 100, which limited the statistical power of the analysis.Third, Few studies included assessments of VDR levels, which are critical for vitamin D's effects, and the available data were limited.Therefore, more studies on VDR are needed to obtain more reliable results.

F I G U R E 3
Forest plot showing vitamin D deficiency in non-scarring alopecia.| 1139 CHEN et al. insufficient serum 25(OH)D levels and elevated incidence of vitamin D deficiency.Therefore, monitoring for vitamin D deficiency and vitamin D supplement may be helpful in the management of nonscarring alopecia.
AA, FPHL, TE, and diffuse hair loss were 3.32 (95% CI 2.25, 4.91), However, we found that the heterogeneity of FPHL and diffuse hair loss was high (>50%) relatively, so we took a series of measures to analyze and reduce heterogeneity.It was worth mentioning that we also conducted several subgroup analyses for vitamin D deficiency (mean age, gender, country, sample size), shown in Table2.
Subgroup analysis of serum vitamin D level and vitamin D deficiency.
hair loss.The study found that the four types of hair loss diseases did TA B L E .2