Antioxidants in tears and plasma: Inter-relationships and effect of vitamin C supplementation.
OBJECTIVE
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 micro M, respectively; FRAP values were, respectively, 273 +/- 94 and 1101 +/- 168 micro M. 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 micro M) was much smaller and slightly later than in plasma (125 +/- 13 micro M). After 4 weeks' supplementation, ascorbate increased (P < 0.001) in both fluids, however, the increase in tear ascorbate was small (5 micro M), compared to plasma (38 micro M). 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 micro M. 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.
Choy
Camus
C
Department of Optometry and Radiography, The Hong Kong Polytechnic University, Hong Kong SAR, Kowloon, China. orrsch3@inet.polyu.edu.hk
Benzie
Iris
I
Cho
Pauline
P
D000328:Adult; D000975:Antioxidants / Q000008:administration & dosage / Q000378:metabolism* / Q000494:pharmacology*; D001205:Ascorbic Acid / Q000008:administration & dosage / Q000097:blood / Q000378:metabolism / Q000494:pharmacology*; D001769:Blood / Q000378:metabolism*; D018592:Cross-Over Studies; D004334:Drug Administration Schedule; D005215:Fasting / Q000502:physiology; D005260:Female; D005290:Ferric Compounds / Q000378:metabolism; D006801:Humans; D008297:Male; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D016037:Single-Blind Method; D013666:Tears / Q000378:metabolism*; D013997:Time Factors
D000975:Antioxidants; D005290:Ferric Compounds; D001205:Ascorbic Acid