It's endemic in Western civilizations where most people work inside. Even if you think you get a lot of sun, your body may not necessarily convert it well to D3. It is an important thing to get checked and strive for 50 ng/ml or higher since low D3 levels are tied to cancer, heart disease, and many other issues.

Anyway, "Wheat Belly" is written by a cardiologist and he has a lot to say on the subject of cholesterol tests and numbers. That book can answer your questions far better than I can.