spinach per day) or lutein and zeaxanthin (1 cup of corn per day) (Hammond et al., 1997). Interestingly, when MPOD was enhanced following dietary modification, it was maintained at that level for several months despite resumption of an unmodified diet.
In summary, results of studies that have investigated MPOD as a biological indicator of carotenoid adequacy suggest that it has substantial potential as an indicator for estimating the requirements for lutein and zeaxanthin. Because of the unique metabolism of carotenoids in the macula, this technique will be useful in associating dietary intakes of lutein and zeaxanthin with the health of the macula. However, insufficient MPOD studies have been conducted to date to make recommendations relative to the dietary intakes of lutein and zeaxanthin.
Cataracts are also problematic, with cataract extraction being the most frequently performed surgical procedure in the elderly (Taylor, 1993). Although the etiology of this condition is not known, oxidative processes may play a role. Cataracts are thought to result from photo-oxidation of lens proteins, resulting in protein damage, accumulation, aggregation, and precipitation in the lens (Taylor, 1993). The cornea and lens filter out ultraviolet light, but visible blue light reaches the retina and may contribute to photic damage or other oxidative insults (Seddon et al., 1994).
Higher dietary intake of carotenoids or higher blood concentrations of carotenoids have been found to be inversely associated with the risk of various forms of cataract in some, but not all, studies. Jacques and Chylack (1991) reported that subjects with low plasma carotenoid concentrations (those with concentrations less than the twentieth percentile: less than 1.7 µmol/L [90 µg/dL]) had a 5.6-fold increased risk of any senile cataract and a 7.2-fold increased risk of cortical cataract, compared with subjects with high plasma total carotenoid concentrations (greater than the eightieth percentile; more than 3.3 µmol/L [177 µg/dL]). Mares-Perlman et al. (1995) performed a cross-sectional analysis of serum α-carotene, β-carotene, β-cryptoxanthin, lutein and zeaxanthin, and lycopene versus the severity of nuclear and cortical opacities, and found that higher concentrations of individual or total carotenoids were not associated with the severity of nuclear or cortical opacities overall. However, higher serum β-carotene (highest quintile median concentration 0.32 µmol/L [17 µg/dL] ) was associated with less opacity in men, and higher concentrations of α-carotene (highest quintile