Well, I remember I pinged you to some of the recent data on Vitamin D and you at least, at the time, didn't seem interested. And let me make it clear, I am not a doctor, or have any affilliation with pharmaceuticals, or with any nutbar snake oil salesmen MSM group.
Regardless of what might be on the net, and you must admit, plenty could be found to support either extreme, I do know this: My dog is (was) 13 years old and getting in pretty bad shape. His movement was slow, his rear quarters were bothering him quite a bit, same old story.
Now, at 14, he has basically shaved a couple years off his age. He moves better, his spontaneous energy level is much better, the little snot comes back in and teases me and won't let well enough alone.
And all I did was started giving him 1000 mg C a day, and a bit of glucosamine. About once a week, I'll give him an 800 IU Vitamin D.
So no matter what, I have to believe what I actually see with my eyes. And I know that personally, I do way better if I take 4 to 6 grams a day, and if I'm sick or stressed (which happens less it seems) than I up it to 8 or 10.
Too bad humans lack the gene, then we would be able to measure serum levels and have a clue. But we can take hints from animals that make their own. And all those studies seem to indicate that even 1000 mg is nowhere near enough.
Metabolic response during early stages of surgically-induced osteoarthritis in mature beagles.
Dietary supplementation with vitamin C resulted in increased serum protein content and increased cartilage formation, although ascorbate had no effect on enzyme activities.
Since that time, Glucosamine has also come into being.