I've been surprised by the amount of vitamin C I've been able to consume without reaching bowel tolerance. The consensus is that absorption increases with infection or inflammation in the body. Over the past weekend I was out of town and used a brand that was available, that is, other than the Vitamin C Foundation, not-from-China product. I didn't take as much of the off-brand as I usually do of the Vitamin C Foundation product because I was too busy. However I noticed that I was running to the bathroom, something that never happens even though I take upwards of 30 grams per day of the not-made-in-China product.
This seems to me to indicate that bowel tolerance is relative to quality of vitamin C.
I'm getting regular blood work to monitor my CRP, serum ferritin, fibrinogen, Homocysteine, Lp(a), and a few other things, trying to find out if my high tolerance for vitamin C indicates infection or inflammation. Because I've been taking at least 15 grams per day for about thirty years, maybe my tolerance is simply high.