> Nobody has 0 intake

And thus we have ourselves a quandary, but truth isn't diminished by the practical limits of safe experimentation. We don't have to lie just because.

> it would be harmful to the subjects.

That doesn't make the title precise or the observation inapt.

The experiment itself may be useless control; most people get D vitamins from the sun. How do we know the cohorts got the same D from sun exposure?

If you avoid animal foods the only dietary vitamin D really is from fortified products. However we also synthesize it, albeit poorly as adults, particularly with lower exposure to the sun.

Plenty of people have 0 supplementation though.