[Mistletoe]

It is known that vitamin D is best obtained from sunlight. I was once at a dinner and sitting with the head of the office for supplements for the United States and asked him how much vitamin D someone should get. His answer was quite simple, get it from sunlight. The skin autoregulates vitamin D dosage that way.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3897598/

>Thus when the skin is exposed to sunlight it can only convert approximately 15% of 7-dehydrocholesterol to previtamin D3 (Fig. 18).32 Any further exposure will result in a photoequilibrium whereby previtamin D3 is converted into lumisterol3 and tachysterol3 as well as revert back to 7-dehydrocholesterol (Fig. 17). In addition when vitamin D3 is made from previtamin D3 in the skin if it is exposed to solar UVB radiation it will absorb UVB radiation and be converted into several suprasterols and 5,6-trans-vitamin D3 (Figs. 17 and and19).19). In addition previtamin D3 can also be converted to several toxisterols (Fig. 20).33-36 Therefore no matter how much sun a human is exposed to vitamin D intoxication will not occur because any excess previtamin D3 and vitamin D3 is photodegraded into products that have no calcemic activity.31,32

[mannykannot]

No doubt, but it is an empirical fact that this is not working too well for a nontrivial number of people - and if it were working well, there would still be no reason to bring up the issue in the article about sunbeds, especially as no evidence for vitamin D intoxication being a difficult-to-avoid problem in practice was presented either in that article or above. If sunlight is the optimal solution, then outdoor activity is better for you than lolling on a sunbed.

I am a case in point: I am frequently outdoors in all seasons, well beyond the point where I have to be careful to avoid sunburn, yet I have a significant all-seasons vitamin D deficiency. The conclusion of the abstract to the article you link to says "a three-part strategy of increasing food fortification programs with vitamin D, sensible sun exposure recommendations and encouraging ingestion of a vitamin D supplement when needed should be implemented to prevent global vitamin D deficiency and its negative health consequences" [my emphasis.]