Our results agree with evidence from in vitro studies of the role of antioxidants in the lens.
21 Vitamin C is the most important antioxidant vitamin in the lens and found in the aqueous at concentrations of 30- to 50-fold that in plasma.
22 The highly toxic reactive oxygen species that are formed by photochemical reactions of oxygen in the presence of electron donors are converted to less toxic hydrogen peroxide via ascorbic-acid–mediated reduction.
21 22 Lutein and zeaxanthin are the main carotenoids found in the lens, along with tocopherols, α-carotene, lycopene, and retinol but very little β-carotene is present.
21 23 Although vitamin C levels in our population were very low, we observed a significant reduced odds ratio of cataract above 6.5 μmol/L compared with levels below this cutoff. There was no additional benefit for those with blood levels above 15.4 μmol/L. In contrast with our findings the U.S.–India case–control study reported an increased odds ratio for cataract associated with increasing plasma vitamin C although in other analyses an antioxidant index derived from glutathione peroxidase, ascorbic acid, vitamin E, and glucose-6-phosphate dehydrogenase was associated with a substantially reduced odds ratio for nuclear or PSC cataract (OR = 0.12, 95% CI 0.03–0.56).
4 Observational studies conducted in Western populations of dietary or blood antioxidants have shown results which generally support the inverse association of vitamin C with cataract although the evidence is not uniformly consistent. Significant inverse associations for plasma vitamin C have been reported from two studies in Europe. A study in Italy found that vitamin C levels of ≥65.5 μmol/L compared with <30.5 μmol/L were associated with a reduced OR for nuclear (OR = 0.54, 95% CI 0.30–0.97) and PSC cataract (OR = 0.37, 95% CI 0.15–0.93) but not with cortical cataract.
24 A Spanish case–control study found a reduced OR for cataract for vitamin C >49 μmol/L compared with ≤49 μmol/L (OR = 0.32, 95% CI 0.21–0.51).
25 In both these studies, vitamin C was substantially higher than in our study. In contrast, no association was reported in two other European studies, although in both those studies
26 27 the levels of vitamin C were lower than in the studies just earlier in the paragraph. Positive associations for a protective effect of blood measures of vitamin C have also been reported in some observational studies in the United States,
28 29 but not in all.
30 Studies in which dietary measures of antioxidants were used mainly reported a positive protective association,
31 32 33 34 but others showed no association.
35 36