The results confuse me. Taking 4000 or 10,000 IU results in lower bone mineral density than 400. Is lower BMD better? Otherwise it sounds like vitamin D is actually bad for you.
The study is saying that too much MIGHT be bad for you. 400IUs was the control (not 0). Some vitamins have adverse effects in high doses. Vitamin D might be one of them, but the study just posed the question, it didn't conclude that that was the case. It would take more studies to confirm that.
The reduction is very small, but the 400 group also had lower mean BMD.
Yes, it appears that too much vitamin D may decrease bone density.