Macular pigment optical density and its relationship with serum and dietary levels of lutein and zeaxanthin
Section snippets
Dietary lutein and zeaxanthin
Mean daily intake of L and Z, combined, varies from 0.8 to 4 mg per day, depending on the population studied and the method of dietary assessment employed [7], [8], [9]. However, daily intake of carotenoids such as L varies widely between individuals, as illustrated by a standard deviation of 2.45 mg/day in a recently published study [9]. Approximately 78% of dietary L and Z is sourced from vegetables, spinach (30 g contains 3659 mg of lutein and zeaxanthin), and orange pepper being particularly
Serum lutein and zeaxanthin
Carotenoids enter the circulation via the lymphatic duct as a component of the chylomicrons formed in the enterocyte, and analysis of the chylomicron composition is required if intestinal absorption of the carotenoids is to be studied prior to hepatic metabolism, uptake into tissues, and exchange with other lipoproteins. Gartner et al. [18] have demonstrated a peak rise in carotenoid composition of the chylomicron fraction 9 h following a loading dose of multiple carotenoids, with preferential
Macular pigment optical density
Macular pigment (MP) refers to the accumulation at the macula of a single isomer of L, and 3 stereoisomers of Z (RRZ, meso-Z, and SSZ or [{3S,3′S}-β,β-carotene-3,3′diol]), to the exclusion of all other carotenoids which are found in human blood [1], [23]. Snodderly and co-workers [1], [24], [25] have eloquently described the anatomical distribution of MP in the primate retina, and demonstrated that its optical density peaks at the centre of the fovea, representing a concentration of almost 1 mM,
Lutein and zeaxanthin in adipose tissue, liver, and spleen
L and Z are known to accumulate in liver, spleen, and adipose tissue [21]. There is evidence of an inverse relationship between body fat and macular pigment optical density in humans [34], [35], and a similar relationship with retinal L (but not Z) in female quail [21], suggesting that fat and retina compete for L. This hypothesis is supported by the observed preferential uptake by quail fat of serum lutein, when compared with zeaxanthin, by a margin of 4:1 [36].
Observational studies
Of the seven observational studies analysing the relationship between dietary intake of L and Z and serum levels of these carotenoids, all have demonstrated significant and positive relationships (p<0.05; r=0.21–0.74) [34], [37], [38], [39], [40], [41], [42]. The largest of these studies included 2786 subjects, and found that demographic characteristics, dietary L and Z intake, serum cholesterol concentration, and lifestyle factors explained 24% of the variance in serum L concentration, and
Relationship between serum levels of L and Z and macular pigment
Studies investigating the relationship between retinal and serum carotenoids should also be interpreted with caution, primarily because serum levels of L and Z reflect recent nutritional intake only. In contrast, MP has a slow biological turnover, and therefore reflects the local balance between pro-oxidant stresses and antioxidant defences in the retina. In other words, a dramatic change in diet is unlikely to affect MP for several weeks, but will be reflected in much more rapid changes of
The relationship between macular pigment optical density and dietary intake of lutein and zeaxanthin
Cross-sectional studies investigating the relationship between dietary intake of L and Z and MP should be interpreted with full appreciation of their limitations. First, dietary assessment by questionnaire is vulnerable to many sources of error including a subject's recall bias, as well as his/her digestive and absorptive idiosyncracies. Also, the use of different sources of carotenoid data by investigators can introduce inconsistencies which prevent meaningful comparisons between studies.
Conclusion
There is a growing body of scientific evidence which suggests that MP may protect against ARM, thus rendering our need to comprehend the relationships between L and Z concentrations in the diet, serum, and retina, as well as other tissues, all the more urgent. We need, and should support, studies designed to enhance our understanding of the bioavailability of carotenoids, and their distribution into various tissues. Such projects will require a team of researchers with a diverse array of
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