It might also be worth mentioning that measuring 25-hydroxy is also an approximation at best. That still has to be converted to 1,25-dihydroxyvitamin by the kidneys. People with high insulin resistance (diabetics, pre-diabetics) or kidney damage tend to have problems converting 25-hydroxy. Measuring 25-hydroxy is primarily used for toxicity and even that has variables that are difficult to measure, so a statistical baseline is applied.

> That kind of throws the whole study into question.

Well, that turns it into a study that checks if extra vitamin will have an effect, even when there is no deficiency. It doesn't throw the study into question, but it may not apply to what the people on this thread are talking about (what's never clear).

I'm not so sure it does, as something like 800 IU of Vitamin D is the daily minimum.