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For many thousands of years, the way humans got adequate vitamin D was through exposure to bright sunlight containing ultraviolet light (UV) wavelengths. That UV converts a precursor deep in the skin into vitamin D. Initially, humans lived exclusively in sun-rich areas of the earth near the equator and lived outdoor lives where they got plenty of UV. As populations moved away from the equator and into northern regions like Europe, their dark skin no longer was an advantage against plentiful sunlight and became a hindrance as dark skin filters out so much of the UV and limits vitamin D production. As a result over thousands of years, Europeans with lighter skin faired better health wise and produced more children and so the further north you go in Europe, the lighter the indigenous population’s skins are. The one exception is Inuit peoples in Alaska who could retain dark skin as they consumed so much marine mammal and fish both rich in vitamin D.
Now, we live a primarily indoor lifestyle and use sunscreens to protect against skin cancer which blocks over 99% of D production. As a result over 68% of Americans are low in vitamin D and those with darker skin types even more at risk. A most recent study of American vitamin D levels shows that the only group that has adequate levels of vitamin D are those who regularly take vitamin D supplements. This makes sense as without regular unblocked sun exposure, it is nearly impossible to prevent low vitamin D levels.
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