If this is the study I'm thinking of, these are bone/calcium specialists looking at calcium and Vitamin D and are judging required D based on calcium in the body.

Given that D has a lot more functions than that, it's ... limited in utility at best. I have heard that D testing is a bit messy, though.

For a counterpart observation, if you're white, at a low enough latitude and get (nearly) whole body sun exposure for 30 minutes during the summer, your body generates a self-regulated amount of D of 10,000 IU (or maybe a bit more, I forget exactly how much). Regulated in that staying out longer won't increase the amount you get per day.

That suggests various obvious things, especially about safe maximum levels of D in your body.

One final comment: as far as I can tell, with the exception of Vitamin C to a certain extent, you simply cannot study one or two required nutrients in isolation. E.g. if you have too little of X, Y or Z you may not absorb enough D in your body. Ditto for too much of M, N or P. Then there's questions of "how much fiber in the meal?", is there enough fat for the fat soluble stuff, etc. etc.

And our general knowledge of digestion and what goes on in the digestive tract it pathetic. Did you know that you've got more bacteria in your gut than you have cells in your body?

If you eat fortified food and get out in the sun enough, then sure, this may apply to you. But if, like me, you try not to eat food that's been heavily processed/fortified and you live where days are short in the winter, then you still might not be getting enough vitamin D in your diet. If the tests are suspect, you'll never know...