# Investigating the Inflammatory Link Between Vitamin D and Hidradenitis Suppurativa: A Systematic Review and Causal Inference Analysis

## TL;DR

This study explores the relationship between vitamin D levels and hidradenitis suppurativa, finding that low vitamin D is likely a marker of inflammation rather than a cause.

## Contribution

The study uses Mendelian randomization to investigate causality, finding no direct causal link between vitamin D and HS.

## Key findings

- Low vitamin D levels are common in HS patients and correlate with disease severity and inflammation.
- Mendelian randomization found no causal effect of vitamin D on HS risk.
- Vitamin D supplementation improves clinical outcomes, suggesting potential therapeutic value.

## Abstract

An inverse correlation between serum vitamin D levels and hidradenitis suppurativa (HS) severity is frequently reported, yet the causal nature and direction of this association remain unresolved. A systematic review was conducted following PRISMA guidelines, identifying 12 relevant studies. A two-sample Mendelian randomization (MR) analysis using the inverse-variance weighted (IVW) method was subsequently performed using genetic instruments for vitamin D from the UK Biobank (n = 417,580) and HS summary statistics from FinnGen (n = 1420). The systematic review confirmed a high prevalence of vitamin D deficiency (<20 ng mL−1) among HS patients (weighted mean 17.90 ng mL−1) and identified inverse correlations between vitamin D levels and disease severity, active lesions, and C-reactive protein (CRP), while supplementation improved clinical outcomes. A null MR estimate consistent with the absence of a detectable average linear causal effect of lifelong genetically predicted 25(OH)D levels on HS risk in the analyzed population was observed. Sensitivity analyses yielded consistent null results with no significant horizontal pleiotropy. The results suggest that hypovitaminosis D is likely a marker of the systemic inflammatory state rather than a direct causative factor. The observed clinical benefits of vitamin D supplementation warrant further interventional studies to define its potential therapeutic role.