Why do so many people have low vitamin d levels?

Low intake, less sun. Most of us don't get enough on our diet. And most of us are indoors most if the time. And if we're smart, when we do go outside, we put sunscreen on. No uv rays, no skin cancer, but also no vitamin d.
Good question. There are many factors that contribute to people having low vitamin d status, low dietary intake, sun avoidance, age and geographic location all play a part. It is important to measure vitamin d in some individuals who may be at risk because of all of variables that may cause low vitamin d. Thank you.