Antioxidants in tears and plasma: Inter-relationships and effect of vitamin C supplementation

Purpose. To investigate inter-relationships between total antioxidant capacity and ascorbate concentration in plasma and tears, and the effect of antioxidant supplementation with reference to these variables. Methods. Twenty-one subjects were studied in this placebo-controlled, cross-over intervention trial. Fasting plasma and tear ascorbate concentrations and total antioxidant capacity (as Ferric Reducing/Antioxidant Power (FRAP)) were measured pre- and post-supplementation with vitamin C (1 g/day). Results. Mean ± SD ascorbate in tears and plasma at entry were 17 ± 6 and 52 ± 13 μM, respectively; FRAP values were, respectively, 273 ± 94 and 1101 ± 168 μM. There was no significant correlation between tear and plasma levels (r = -0.068; P = 0.771 for ascorbate; r = 0.418; P = 0.059 for FRAP). Neither was significant correlation seen between the two variables in plasma (r = 0.162; P = 0.483) or tears (r = 0.353; P = 0.117). Acute responses (up to 3 hours) showed a similar pattern of increase in both fluids, however, peak response in tears (33 ± 4 μM) was much smaller and slightly later than in plasma (125 ± 13 μM). After 4 weeks' supplementation, ascorbate increased (P < 0.001) in both fluids, however, the increase in tear ascorbate was small (5 μM), compared to plasma (38 μM). The increase in tear ascorbate appeared to plateau after 2 days' supplementation; plasma levels were still increasing. Higher tear ascorbate at entry was associated (P < 0.05) with smaller supplementation-related response. No significant changes in FRAP were seen in either fluid (P > 0.05). Conclusions. Ascorbate concentration in both plasma and tears increased with vitamin C supplementation, but the total antioxidant capacity of these fluids did not. Furthermore, the increase in tear ascorbate was modest in comparison to that in plasma, and is suggestive of a "ceiling" for tear ascorbate of under 40 μM. Results support the concept of a control mechanism for an integrated antioxidant defense system, and suggest that the amount of ascorbate in tears is both actively controlled and purposefully limited.