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The most natural way that humans get vitamin D into their body is through exposure to sunlight. If one is at the equator in the summer without sunscreen, human skin will produce approximately 10 000 IU of vitamin D over one hour, a testament to the incredible reserve that human skin has to translate ultraviolet light B (UVB) exposure into vitamin D3 levels in serum.
However, modern culture has made sun exposure an inefficient transducer of vitamin D. First, humans spend upwards of 90% of their time indoors and, when outside, have clothes and sunscreen on, both of which reduce the exposure to UVB radiation and, consequently, the production of vitamin D in the skin. Finally, in countries such as the UK, that are far north of the equator, for at least 6 months of the year, there is significantly less UVB radiation that reaches the earth's surface given the larger solar zenith angle of the sun's rays with respect to the earth's surface.1 We are a long way from where modern humans originated, 10 000 years ago, naked, at the equator and outdoors 100% of the time.
Thus, to get enough vitamin D, humans must rely on supplementation of their diet to attain appropriate intake. The fact that we have this dual mechanism is testament to the critical biological role that vitamin D plays in human immunity and physiology. This dietary supplementation has been used since the middle of the 18th century when cod-liver oil was regularly …