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.