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Before discussing these results, the reader should know of three inherent problems in this area. In dietary studies, the subjects are attempting to recall broad dietary histories on questionnaires, and then dietary intakes are calculated from inconsistent food composition tables. Can you recall how much of what fruits and vegetables you ate last year? Early studies focused on beta-carotene and not the xanthophylls, and the separate analyses of foods and serum for lutein and zeaxanthin has only recently been initiated. In addition, blood serum analyses are a measurement of a transient and highly variable analyate. Xanthophylls can rise several fold in the serum and then drop to baseline in one day. This is analogous to trying to interpret a film from one still shot taken from a movie.
To summarize, the relationship between serum levels of the xanthophylls and AMD and cataract appear to be inconsistent and researchers are now hoping that Macular Pigment Optical Density (MPOD) measurement may be a better way of assessing tissue history of xanthophyll long-term intake.
At first glance, dietary intake based epidemiological studies also appear to be inconsistent, however, when studies are segregated by studies that had quintiles of sub-populations with very low consumption of xanthophyll containing foods are compared versus quintiles of people eating very high consumption of these same foods, a strong consistent pattern emerges. In populations consuming foods containing approximately 6 mg xanthophylls/day (lots of fruits and vegetables, particularly dark green leafy, the risk reductions are very strong.
These very consistent studies showed lower risk for prevalence of nuclear cataracts, cataract extractions, lens opacity and AMD, particularly wet AMD. Unfortunately while these were strong and compelling studies, all of them only show a direct link with the fruits and vegetables (not distinctively the xanthophylls) and reduced risks of eye disease.
The ability to influence the concentration of the xanthophylls in the target eye tissues is an important piece of evidence that the target organ is responsive to modification. There are now numerous studies showing the desired dose response. Macular Pigment Optical Density can be measured indirectly and non-invasively by at least six different techniques. A discussion of measurement techniques is beyond the scope of this review and has been subject to some criticism on reproducibility. No non-invasive technique is currently available to measure lens pigment content in-vivo. The MPOD human volunteer supplementation trials can be summarized as follows:
- Both foods and supplements containing lutein and zeaxanthin are capable of raising retinal levels of the xanthophylls in most, but not all volunteers. The reason for non-responsive volunteers is not yet delineated (measurement artifact or truly physiologically non-responsive)
- The retinal response is very slow relative to blood serum response (months versus days) but appears to remain stable for months upon cessation of supplementation. This suggests that an intervention dosage may be significantly higher than a preventative or maintenance dosage.
- There appears to be a relationship between peak serum levels and ability to increase MPOD suggesting high dietary intake may raise retinal levels faster and more effectively.
- There are factors other than peak blood serum levels that appear to affect the ability to increase MPOD. These include many of the risk factors for AMD but may also include genetics, obesity, and other serum or retinal transport/binding proteins for the xanthophylls.
- There has been only one small trial to date directly comparing lutein's and zeaxanthin's ability to influence MPOD in humans where bioavailability was directly controlled (Garnett, et. al. 2002). In this trial, blood serum responses were equal but more individuals retinally responded to zeaxanthin.
We can conclude that for most people the retina does accumulate xanthophylls upon supplementation. While the human lens contains the xanthophylls, there is little else known about the responsiveness of human lens. The first report of dietary manipulation of lens xanthophylls content in an animal model was reported this year. |