There are people who fail at outdoor sun exposure and formation of Vit D. The basic building block to this sequence is based on cholesterol. If you have a genetic lack (low cholesterol) or take statins to artificially lower it, you might not convert sunlight very well.
Some of the research implies that the older people become, this chemistry may fail. So it may fail also for some genetic reason.
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Weezie looking at petunias 8.25.2017
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