To be clear: Serious vitamin deficiencies can cause serious problems (scurvy in the case of vitamin C, rickets in the case of vitamin D, beriberi for vitamin B).1 But if you live in the developed world and eat a normal diet — even a pretty unhealthy one — you will be nowhere near this kind of deficiency.
I live in Canada and eat a normal diet. My blood 25-hydroxyvitamin D level was 22 nM. I had symptoms of hypovitaminosis D which went away after taking 150 kIU over the course of a month, at which point my blood level of 25-hydroxyvitamin D had increased to 76 nM.
Sure, this is pure anecdote, but it proves that it is possible to be deficient.
I'm in Canada and the UV index (which is based on UVB levels) is still hitting 4 or 5 on clear days. That is above the level where you are recommended to wear clothing, hats, sunscreen, etc to avoid burning. How do you figure that is not enough to produce vitamin D?
Considering that vitamin D is typically measured in nanograms, it would be pretty difficult to take pure vitamin D! But to answer the question I think you're asking: I take pills. Now that I'm on a "maintenance dose" of 1000 IU/day I could theoretically get it from dietary sources if I worked hard; but when I was placed on 5000 IU/day there was no possible way to get enough that way.
I live in Canada and eat a normal diet. My blood 25-hydroxyvitamin D level was 22 nM. I had symptoms of hypovitaminosis D which went away after taking 150 kIU over the course of a month, at which point my blood level of 25-hydroxyvitamin D had increased to 76 nM.
Sure, this is pure anecdote, but it proves that it is possible to be deficient.