Vitamin D and Muscle Status in Inflammatory and Autoimmune Rheumatic Diseases: An Update

Background and Objectives: Vitamin D is a secosteroid hormone essential for calcium homeostasis and skeletal health, but established evidence highlights its significant roles also in muscle health and in the modulation of immune response. This review aims to explore the impact of impaired vitamin D status on outcomes of muscle function and involvement in inflammatory and autoimmune rheumatic diseases damaging the skeletal muscle efficiency both with direct immune-mediated mechanisms and indirect processes such as sarcopenia. Methods: A comprehensive literature search was conducted on PubMed and Medline using Medical Subject Headings (MeSH) terms: "vitamin D, muscle, rheumatic diseases." Additionally, conference abstracts from The European Alliance of Associations for Rheumatology (EULAR) and the American College of Rheumatology (ACR) (2020-2023) were reviewed, and reference lists of included papers were scanned. The review emphasizes the evidence published in the last five years, while also incorporating significant studies from earlier years, structured by the extent of evidence linking vitamin D to muscle health in the most commonly inflammatory and autoimmune rheumatic diseases encountered in clinical practice. Results: Observational studies indicate a high prevalence of vitamin D serum deficiency (mean serum concentrations < 10 ng/mL) or insufficiency (<30 ng/mL) in patients with idiopathic inflammatory myopathies (IIMs) and polymyalgia rheumatica, as well as other autoimmune connective tissue diseases such as rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and systemic sclerosis (SSc). Of note, vitamin D insufficiency may be associated with reduced muscle strength (2 studies on RA, 2 in SLE and 1 in SSc), increased pain (1 study on SLE), fatigue (2 studies on SLE), and higher disease activity (3 studies on IIMs and 1 on SLE) although there is much heterogeneity in the quality of evidence and different associations for the different investigated diseases. Therefore, linked to the multilevel biological intervention exerted by vitamin D, several translational and clinical studies suggest that active metabolites of this secosteroid hormone, play a role both in reducing inflammation, but also in enhancing muscle regeneration, intra-cellular metabolism and mitochondrial function, although interventional studies are limited. Conclusions: Altered serum vitamin D status is commonly observed in inflammatory and autoimmune rheumatic diseases and seems to be associated with adverse muscle health outcomes. While maintaining adequate serum vitamin D concentrations may confer muscle-protective effects, further research is needed to confirm these findings and establish optimal supplementation strategies to obtain a safe and efficient serum threshold.