Vitamin D3 is called the vitamin of the decade. Never came in such a short time so many new research out of a single vitamin kind.
Literally thousands of publications came out with every time new aspects in all areas of health. In summary; the thought of reference values in blood tests are much too low, the RDA (recommended daily allowance) is so low that it has minimum effect and just about the entire population is deficient.
The only exception is the group of sunbathers in the summer, but they also have large deficits in the winter. In Europe you can because in the wintertime total vitamin D3 building for lack of UVB light in the sun. Supplementation is therefore highly advisable.
And if you do not regularly in the summer in the middle of the day comes out, use in that period is also very advisable.