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Background: Vitamin D can be synthesised following exposure to ultraviolet radiation (UVR), ingested in the diet or provided through oral supplementation. The medical literature frequently states that humans obtain most of their vitamin D from sunshine and that UVR exposure is essential to maintain vitamin D levels.Objective: A systematic review was conducted to determine the requirement for UVR in maintaining adequate (>50nmol/L) serum 25-hydroxyvitamin D (25OHD) levels.Method: Studies reporting serum 25OHD during situations of negligible UVR exposure were sought. 41 studies (from a search yielding 42,698 articles) totalling 4, 211 healthy adults met the inclusion criteria, providing 56 datasets from different population groups.Results: Over 50% of subjects had >50nmol/L 25OHD in 10 of 19 datasets reporting winter levels in areas with limited UVR. In addition, >50% of subjects had adequate 25OHD levels in 4 of 12 datasets from polar regions during periods of negligible UVR, 1 of 9 datasets documenting clothing related minimal UVR and 2 of 8 datasets detailing employment related minimal UVR.Conclusion: The data demonstrate that many adults maintain adequate serum vitamin D levels despite negligible UVR exposure for several months. However, we acknowledge that preceding UVR exposure leading to vitamin D storage and delayed release may account for this maintenance of adequate serum vitamin D levels. There remains a need for further research on whether UVR exposure is required for longer-term maintenance of adequate vitamin D levels.