Humans are meant to obtain vitamin D via activation of a skin reaction with exposure to sunlight that yields the active hormone.
But there are common situations in which this does not occur or occurs to an inadequate level. This becomes increasingly true as we age, as we gradually lose the capacity to activate vitamin D in the skin as we get older. It means that at age, say, 70 you can have a dark leather tan over much of the body’s surface area yet have a 25-hydroxy vitamin D blood level of 16 ng/ml—far below our target level of 60-70 ng/ml. I have personally witnessed this phenomenon countless times.
Recognize this, as restoration of healthy levels of 25-hydroxy vitamin D is such a crucial aspect of overall health.