When Michael Holick, in a series of landmark experiments, showed that UVB radiation from sunlight triggers vitamin D production in humans, nowhere in his writings was I able to find that he said UVB radiation was the only ambient radiation that could do so. But that is what nearly everyone believes.
In fact, Holick’s own lab showed (in an ignored finding) that when directing UVC radiation to purified epidermal lipids, UVC radiation (in an ignored finding) triggered robust production of vitamin D when he directed UVC radiation to purified epidermal lipids (MacLaughlin JA, Anderson RR, Holick MF) . Close reading of MacLaughlin et al reveals that significant photosynthesis of vitamin D from 7-dehydrohydrocholesterol (7-DHC) occurs at UVC wavelengths in the epidermal lipids the authors extracted [as illustrated in MacLaughlin et al’s figure 1(B)]. Indeed, per photon, UVC photosynthesis is slightly greater than UVB [as illustrated in MacLaughlin et al’s figure 1(C)].
Furthermore, several animal studies indicate that UVC radiation, which should never be used in man due to its carcinogenic properties, is highly effective in both producing vitamin D and in treating rachitic rats (Bunker et al, 1940, JACS). Knudson A and Benford F. studied numerous UV wavelengths in rats, finding UVC as effective as any of the UVB wavelengths studied in curing rickets (Quantitative studies of the effectiveness of ultraviolet radiation of various wavelengths in rickets. J Biol Chem. 1938;124:287–299).
The belief that only UVB radiation can produce vitamin D appears to be a widespread misconception. Fortunately for humans, as UVC is highly carcinogenic, UVC radiation does not penetrate the atmosphere and certainly does not trigger cutaneous vitamin D during normal circumstances human affairs. However, under the right conditions, it can trigger robust vitamin D synthesis.
What good is any of this knowledge? Only to empathize that how we think and how we frame concepts is important. Holick discovered that UVB was one ambient radiation that triggers vitamin D production in humans. Holick never discovered that ambient UVB radiation is the only such ambient radiation that can do so.
John Cannell, MD. Is UVB the only source of natural vitamin D production? The Vitamin D Council Blog & Newsletter, 5/2018.