How important is vitamin D? Modern humans started in Africa near the equator, and couldn't successfully move north to Europe till they changed to fairer skin to allow them to make more vitamin D. They also acquired the ability to store vitamin D and ration it over winter, which is not necessary near the equator.
While not completely understood, vitamin D is definitely important for the function of the immune system. And indeed trials have shown that vitamin D supplementation reduces the risks from covid: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9864223/. So why aren't authorities pushing vitamin D around the world?
Trials of vitamin D show a U-shaped curve for health problems. Up to a point it helps, but beyond that point it increases health risks. With vitamin D supplements it is easy to get too much, and effectively impossible to get exactly the right amount. So I presume that is the reason that authorities are unwilling to recommend supplements.
Humans, other than Inuits, are not designed to get their vitamin D orally. We're designed to make it with UV radiation from the Sun on our skin. That also produces a lot of other stuff of unknown value. We could easily be missing some other important product from sun on the skin. Sun on the skin doesn't produce vitamin D overdoses.
So why don't the health authorities recommend increased sun exposure? The reason is that it increases the chance of skin cancer, including melanoma which can be fatal.
Our modern lifestyle leaves many people with low vitamin D levels. Governments need to fix this. There is no easy answer, but the current approach is the worst option.
My choice, since I found out about this 20 years ago, has been to get enough sun (even in winter) to maintain my tan while being careful not to get burnt. For many dark skinned people it would be impossible to get enough sun that way in winter. Either artificial UV or vitamin D supplementation is necessary for those people, and also for those who can't conveniently or culturally acceptably get their skin exposed to the sun. We need to work out the right answers. It's not an exciting research project, and certainly the Pharmaceutical companies are not interested. It is an important part of engineering public health.
Since I wrote this I have learnt that UV produces nitric oxide which improves circulation and is used to attack pathogens. Some are suggesting that this is the reason vitamin D is correlated with good health outcomes, since both are correlated with sun exposure. More research urgently needed.
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