>>510006681 (OP)Unless you go out into the sun daily you're vitamin D deficient if you live a western lifestyle where you drive to work, work in an office and spend the majority of your day indoors. You don't develop vitamin D from sunlight which hits you through windows.
Most governmental bodies say that 10-20ng/ml is adequate but this is just enough to prevent you from getting bone deformities. Optimal is more like 40-50 ng/ml, which is about the amount that native tribes in Africa have or people that work all day in the sun.
A substantial amount of westerners have inadequate amounts of vitamin D <20ng/ml. An inadequate amount is linked to a lot of problems as it's require for immune system function, testosterone production, proper calcium depositing (in bone vs arteries) and a lot of disease related health outcomes are directly associated with vitamin D levels.
This is really easy to solve however, D3 supplements are dirt cheap and the sun is free but clothing inhibits D3 production significantly. 2000 IU / day is about the amount that I need to get in the 40-50ng/ml range. Some need more, some less, but blood tests are pretty affordable.