From my coming-soon article on glutathione and metabolic disease:

Let’s look at the impact of vitamin C on blood glutathione levels. Two excellent studies performed 30 years apart paint a fascinating picture. The first took 200 healthy subjects and put them on a vitamin C restricted diet for 6 weeks and then gave half the group a placebo and the other half 500 mg per day of vitamin C, then measured their blood levels of glutathione (gsh) every week for the 6 week duration of the study. The only difference in the two groups was either placebo or C, the diet was the same, and they were not taking glutathione precursors like NAC. The C group experienced a 50% increase in blood glutathione levels.