Archived Comments for:
The role of the carotenoids, lutein and zeaxanthin, in protecting against age-related macular degeneration: A review based on controversial evidence
An Alternative Reason Why Medium-Quantity Lutein might have Similar Effectiveness against AMD to that of Zeaxanthin
John Rokos, Department of Clinical Chemistry, Hammersmith Hospitals NHS Trust
20 October 2004
We know that both the foveal and perifoveal regions take up zeaxanthin, whereas uptake of lutein by the more visually active foveal region is negligible. However, anecdotally it was not the zeaxanthin sources like egg yolk, maize, orange peppers and persimmon that were quoted as being associated with protection against loss of vision due to AMD, but the highly lutein-rich green leafy vegetables like spinach, collard greens and kale. It may be for this reason that retinal conversion of lutein to zeaxanthin is suggested in the literature. I should like to propose a rationale for the possible effectiveness of lutein that does not postulate chemical conversion:
Because of its size, the total binding capacity of the perifoveal region for zeaxanthin might exceed that of the foveal region, with the result that much of the zeaxanthin that gets into the region of the macula could end up in the perifoveal region, thereby failing to benefit the foveal region. When increased amounts of lutein are ingested, perifoveal receptors, that would otherwise bind zeaxanthin, might be occupied by the ingested lutein. Assuming the same overall total level of zeaxanthin in the region, a result would be an increase in the zeaxanthin available to the foveal receptors, thus improving the level of protection to the foveal region.
In this model, lutein additional to that required for saturating the perifoveal receptors would achieve no useful purpose and, indeed, might deletoriously compete with zeaxanthin for transport to the region. Supplements that contain large quantities of lutein without significant quantities of zeaxanthin should perhaps, therefore, be taken at a different time from that of a zeaxanthin-containing meal or snack, or not taken at all (this is additional to the other warnings about supplements in the article).
Competing interests
None declared
Modification of Comment
John Rokos, Imperial College Healthcare NHS Trust
29 January 2014
When making my (first) comment, I was not aware that mesozeaxanthin, not found in the diet or in plasma, can be found in the retina (Bone RA, Landrum JT, Hime GW, Cains A, Zamor J: Stereochemistry of the humanmacular carotenoids. Invest Ophthalmol Vis Sci 1993, 34:2033-2040), let alone that its synthesis in the retina may have been demonstrated directly. If, as seems likely, the predecessor is lutein, than my warning against swamping ingested zeaxanthin with too much lutein becomes inappropriate, and the indirect role to which I assigned lutein, that of making more zeaxanthin available to the fovea by displacing it from binding sites outside the fovea, becomes assignable, to a vastly more limited extent, to zeaxanthin with regard to lutein and/or mesozeaxanthin.
An Alternative Reason Why Medium-Quantity Lutein might have Similar Effectiveness against AMD to that of Zeaxanthin
20 October 2004
We know that both the foveal and perifoveal regions take up zeaxanthin, whereas uptake of lutein by the more visually active foveal region is negligible. However, anecdotally it was not the zeaxanthin sources like egg yolk, maize, orange peppers and persimmon that were quoted as being associated with protection against loss of vision due to AMD, but the highly lutein-rich green leafy vegetables like spinach, collard greens and kale. It may be for this reason that retinal conversion of lutein to zeaxanthin is suggested in the literature. I should like to propose a rationale for the possible effectiveness of lutein that does not postulate chemical conversion:
Because of its size, the total binding capacity of the perifoveal region for zeaxanthin might exceed that of the foveal region, with the result that much of the zeaxanthin that gets into the region of the macula could end up in the perifoveal region, thereby failing to benefit the foveal region. When increased amounts of lutein are ingested, perifoveal receptors, that would otherwise bind zeaxanthin, might be occupied by the ingested lutein. Assuming the same overall total level of zeaxanthin in the region, a result would be an increase in the zeaxanthin available to the foveal receptors, thus improving the level of protection to the foveal region.
In this model, lutein additional to that required for saturating the perifoveal receptors would achieve no useful purpose and, indeed, might deletoriously compete with zeaxanthin for transport to the region. Supplements that contain large quantities of lutein without significant quantities of zeaxanthin should perhaps, therefore, be taken at a different time from that of a zeaxanthin-containing meal or snack, or not taken at all (this is additional to the other warnings about supplements in the article).
Competing interests
None declared
Modification of Comment
29 January 2014
When making my (first) comment, I was not aware that mesozeaxanthin, not found in the diet or in plasma, can be found in the retina (Bone RA, Landrum JT, Hime GW, Cains A, Zamor J: Stereochemistry of the humanmacular carotenoids. Invest Ophthalmol Vis Sci 1993, 34:2033-2040), let alone that its synthesis in the retina may have been demonstrated directly. If, as seems likely, the predecessor is lutein, than my warning against swamping ingested zeaxanthin with too much lutein becomes inappropriate, and the indirect role to which I assigned lutein, that of making more zeaxanthin available to the fovea by displacing it from binding sites outside the fovea, becomes assignable, to a vastly more limited extent, to zeaxanthin with regard to lutein and/or mesozeaxanthin.Competing interests
None declared