Vitamin C in human and guinea pig aqueous, lens and plasma in relation to intake

Given the recent correlation between nutrition and risk for eye disease, there is keen interest in a possible correlation between nutrient intake and eye-tissue nutrient levels. In this work, the objective was (1) to determine, for the first time, the relation between dietary intake of vitamin C and eye tissue levels of the vitamin in free-living humans, (2) to determine the relation between levels of the vitamin in plasma, lens and aqueous, and (3) to compare this information to data gathered for a carefully reared group of guinea pigs that were fed different levels of vitamin C.

Two hundred sixty-five cataract patients (mean age = 72 years) from a clinical practice were recruited for this study. One hundred thirty-two patients provided the dietary intake data via a food frequency questionnaire, which we used for this work. Plasma, aqueous humor, and lens samples were obtained at the time of lentectomy and preserved for vitamin C analysis. Comparable samples were obtained from male Hartley white guinea pigs that were fed known amounts of vitamin C. Linear and log10-linear statistical models were also used to characterize the relation between vitamin C intake and human ocular tissue levels of the vitamin and to examine potential confounding and the effect of modification by age and sex.

In humans, plasma and aqueous vitamin C concentrations were related to intake in a log-linear fashion, with slopes of 0.03 mM plasma vitamin C/log10-mg daily vitamin C intake and 0.41 mM aqueous vitamin C/log10-mg daily vitamin C intake. The best fit of vitamin C levels in lens and diet predicts a linear relationship with a sex-adjusted slope of 0.00094 mM lens vitamin C/mg daily vitamin C intake, although a log-linear relation can also be modeled. In guinea pigs, diet was related to eye tissue and plasma levels of the vitamin by a log10 linear relationship in all cases. Vitamin C in human lens was linearly related to plasma and aqueous vitamin C with slopes of 8.8 and 0.23, respectively. Vitamin C in aqueous was related to plasma in a log10-linear fashion with a slope of 1.6 mM aqueous vitamin C/log10 mM plasma vitamin C. In guinea pigs, vitamin C in plasma was related to aqueous and lens vitamin C by log10-linear relationships, whereas lens and aqueous vitamin C were clearly linearly related.

Plasma and aqueous appear to be saturated in humans with intakes of < 250 mg vitamin C/day. However, a saturating relationship between lens vitamin C and dietary intake in humans was not indicated in this study, although such a relationship is seen in guinea pigs. Intertissue relations between vitamin C levels in humans and guinea pigs are similar for some but not all relations.