The role of vitamin D deficiency in the prevalence and severity of diseases: a focus on non-autoimmune outcomes – literature review

Vitamin D is a steroid hormone essential to a broad spectrum of physiological processes. Synthesized in the skin via UVB exposure and obtained from dietary sources, it plays a central role in calcium and phosphorus homeostasis and is critical to cardiovascular, musculoskeletal, neurological, metabolic, and reproductive health. Owing to its pleiotropic effects, vitamin D deficiency can lead to widespread systemic dysfunction and has been associated with increased all-cause mortality. According to a position statement of the European Calcified Tissue Society from 2019 its prevalence is between 30–60% in Western, Southern and Eastern Europe. The 2023 Polish recommendations titled "Vitamin D deficiency in developmental age – the scale of the problem, prevention, treatment, and the latest 2023 guidelines for Poland", state that approximately 90% of adults and nearly all adolescents in Poland are affected by vitamin D deficiency. High-risk groups include older adults, individuals with obesity, pregnant women, and those with limited sunlight exposure or chronic diseases. Although traditionally associated with autoimmune conditions, growing evidence links vitamin D deficiency to a range of non-autoimmune disorders. As a modifiable factor implicated in diverse chronic diseases, vitamin D deficiency underscores the need for standardized diagnostic criteria and targeted clinical trials. This review synthesizes molecular, clinical, and epidemiological findings to evaluate the systemic consequences of vitamin D deficiency. It identifies key research gaps, methodological inconsistencies, and the absence of standardized diagnostic criteria. The need for personalized supplementation strategies and large-scale randomized trials is emphasized to clarify causal relationships and inform evidence-based clinical guidelines.