You are not addressing the issue at hand, which is whether
all athletes take "supplements" and "vitamin injections,"
i.e., whether they use performance enhancing drugs that purport to be something else, not whether they take
actual supplements and vitamins, injected or not.
I don't think that
all elite athletes do this, though I am sure that many do. I suspect that it probably occurs more in some sports more than others (
e.g., in track and field more than in rowing or sailing).
I could be mistaken, but I am making no more of an assumption that someone who asserts that
all elite athletes take performance enhancing drugs.
As for taking
actual supplements and vitamin injections, I strongly suspect that
not all elite athletes do so. I can think of at least three reasons why not. First, doing so runs the risk of unintentionally injesting a prohibited substance. Second, products marketed as supplements sometimes do not contain the product that is ostensible being sold. Third, it isn't at all clear that a person, particularly an elite athlete, with no vitamin deficiency actually benefits from taking these products.
Indeed, there are considerable risks to taking some vitamins (Vitamin D, for example), if you don't have a deficiency.
http://www.forbes.com/sites/stevens...-six-vitamins-you-shouldnt-take/#32ebc51b4ba4