Content of Vitamin D₂ in Alternative Biological and Nutritional Sources and Its Effectiveness as Compared to Vitamin D₃—A Narrative Review

Background/Objectives: Interest in alternative, non-animal sources of vitamin D has increased due to the global prevalence of its deficiency and the growing demand for plant-based dietary options. Mushrooms and algae have emerged as potential sustainable sources of vitamin D₂ and, in selected cases, vitamin D₃ following ultraviolet (UV) exposure. However, the comparative bioavailability and clinical effectiveness of vitamin D₂ relative to vitamin D₃ remain controversial. This review aims to evaluate the content of vitamin D₂ in mushrooms and algae, the impact of UV irradiation on its synthesis, and the effectiveness of vitamin D₂ supplementation compared with vitamin D₃ in humans. Methods: PubMed, ScienceDirect, and Google Scholar were searched (1996–2026). Only human intervention studies were included when assessing clinical efficacy. Data on natural sources and pharmaceutical formulations were analyzed. Results: UV irradiation markedly increases vitamin D₂ content in mushrooms, as compared to cultivated products. Algal vitamin D content varies, depending on species and UV exposure, with no robust clinical trials confirming improvement of serum 25(OH)D after algal supplementation. Across multiple randomized controlled trials, vitamin D₂ consistently increased circulating 25(OH)D₂ but frequently reduced 25(OH)D₃ and demonstrated lower efficacy in raising total 25(OH)D compared with vitamin D₃, as confirmed by a recent meta-analysis. Conclusions: Although UV-enhanced mushrooms represent a quantifiable dietary source of vitamin D₂, clinical evidence consistently indicates lower efficacy of vitamin D₂ compared with vitamin D₃. Algae cannot currently be considered a validated source of vitamin D for improving human vitamin D status. Further mechanistic and long-term clinical studies are required.