I spend lots of time outside and I live in California. I still had life-long vit D deficiency, probably since birth. No idea why, might be something genetic.

The only way I could figure it out was via blood tests, spaced a few years apart. First time the doctor said "hm, you're pretty low on vit D, you should supplement it". Second time, it was more like "you're really low and you should take XXX amount each day, NOW!"

Big changes in terms of mood and "energy". I'm now kicking myself for not taking it earlier.