After seeing an ER attending physician recommend Vitamin C supplements for an upper respiratory illness, URI, aka the common cold, I probably did a few days worth of research on the topic on what was probably then called MedLine at the time. This is what I recall about Vitamin C supplementation:
Pharmacologic doses, as opposed to physiologic doses, on a routine basis only helped patients with an absolute vitamin C deficiency, aka scurvy. It had no effect on the the incidence of URI or other illness.
Pharmacologic doses decreased the incidence of flu-like illness suffered by runners of marathons compared to those runners who took only placebos.
Pharmacologic doses decreased the duration of symptoms of those with URIs.
I use and recommend Vitamin C when I have a cold.
IMHO, Vitamin C supplementation with pharmacologic doses only seems to be good advice when a person or animal has certain abnormal stressors. I would be leary of what you find on PubMed if the results have not yet been replicated, even more so about what you find unsolicited, and linked on the Internet.
Here's the search strategy that I used to find that citation on PubMed. (flu-like illness or influenza or URI or upper respiratory illness or viral syndrome or stress) and (vitamin C supplementation or ascorbic acid supplementation)
Click on "Related Articles", near to the right of the title, and you'll find a bunch of articles about anti-oxidants with contradictory results. Caveat emptor.
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.