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Carotenoids are a family of 700 compounds found in fruits, vegetables and green plants and provide much of the color to the human diet (particularly yellows, oranges, and reds.) These colors reveal themselves in all their glory in the autumn when the green chlorophyll degrades revealing the beautiful colors of autumn leaf foliage.
Of these 700 colorful compounds only about 20 have been detected in human plasma and tissues. For this review we can divide the carotenoids found in humans into vitamin A precursors, like beta-carotene and the non-vitamin A precursors that have hydroxyl groups attached on the end-ring structure like the macular pigments, lutein and zeaxanthin (also called xanthophylls). See Table 1.
The unique structures of carotenoids (40 carbon long molecules along with centrally located conjugated double bonds) are responsible for their color spectra and their ideal performance as antioxidants that can quench free radical reactions. Lutein and zeaxanthin are remarkably similar in structure. While lutein and zeaxanthin have the same number of double bonds, zeaxanthin has 11 conjugated double bonds while lutein's eleventh double bond (10 conjugated) forms a more chemically reactive allylic hydroxyl end group. Conjugated double bonds are particularly effective at quenching singlet oxygen that produces Reactive Oxygen Species (ROS). A three dimensional view of zeaxanthin shows it to be a straight molecule that is able to easily transverse a biological cell membrane because of its hydrophilic end groups and lipophilic hydrocarbon chain. Lutein has a much more bent structure and one less conjugated double bond making it a poorer membrane antioxidant. Lutein is more prevalent in nature because of this bent structure. Lutein plays a prominent role in green leafs (and thus more prominent in our diet) because the bent structure makes it the perfect chemical molecule to fit into the three dimensional view of the photoreaction center of a chloroplast.
Lutein sitting in this structure is perfectly designed for its role in harvesting and transferring energy from light and supporting the chloroplast's generation of energy to support the plant cell.
Because of these differences in structures, lutein is probably 10-20 times more prevalent in the average US diet than zeaxanthin. Table 2 shows the content of lutein and zeaxanthin in fruits and vegetables. From this table it becomes obvious that dark green leafy vegetables contain high levels of the two pigments but the ratios of lutein/zeaxanthin are 20:1 - 40:1. Some non-green leafy vegetables like corn, oranges and orange peppers have lower total xanthophyll contents but higher ratios of zeaxanthin to lutein.
Dietary Intake. Determining the daily intake of lutein and zeaxanthin is problematic because food composition databases often did not analyze lutein and zeaxanthin separately and comparisons of published data are often inconsistent. (See Table 2.) Despite many health agencies recommendation of eating at least five servings of fruits and vegetables per day, there remains wide variation in consumption among various human sub-populations. Lutein intake ranges are between 0.5 to 6mg/day with an average of probably 1mg/day. Zeaxanthin intake ranges are probably between 0.1 mg/day to 2mg/day with an average of probably 0.2 - 0.5mg/day.
Dietary Gap. There are several major epidemiological studies attempting to link dietary carotenoid consumption with risks of AMD and cataract. This dietary gap between the low and high risks individuals equilibrated to around 6mg/day of lutein and zeaxanthin. These studies do not directly show causal relationship with zeaxanthin and lutein but rather a strong relationship with fruits and vegetables high in these xanthophylls. These data suggests that there may be a dietary gap of 4-5mg/day of these xanthophylls that could influence the risks of eye disease. This level of consumption probably relates to a daily dietary consumption for prevention or reducing the risks of eventual eye disease and might be a basis for a maintenance dosage.
These analyses do not address intervention in an ongoing eye-disease state. Perhaps, hints for appropriate xanthophylls levels may be found from the 2001 Age-Related Eye Disease Study (AREDS) reports. The AREDS trial is the largest eye-disease intervention trial completed to date. The AREDS intervention trial used dietary intervention levels of 7-15 times the normal recommended daily allowance (RDAs) of established dietary antioxidants. This dietary antioxidant combination reduced risks in advanced AMD but did not show statistical relevance in intervention of cataracts. Two cautions are relevant, however. Copper was added to the formula because of concerns about the high level of zinc used and concerns about high levels of beta-carotene became apparent during these trials when two unrelated intervention trials showed a significant increased risk of lung cancer with high doses of beta-carotene in smokers.
These analyses suggest that while caution should be exercised, doses significantly greater than average daily intake should be considered for intervention in the disease process. |