I think a lot of people just don't absorb vitamin D very well. I was out in the sun every day with no sunscreen - enough to get a decent tan but my vitamin D level was in the severely low range of 11. Now I take a total of 45,000 iu every week and the level is finally good. As we age some of us just can't absorb it I guess, plus there are probably other underlying causes. Possibly statins can lower cholesterol too much. We need cholesterol to make vitamin D.