Lutein (L) and zeaxanthin (Z) are dietary carotenoids derived from dark green leafy vegetables, orange and yellow fruits that form the macular pigment of the human eyes. The mechanism by which they are involved in the prevention of eye diseases may be due their physical blue light filtration properties and local antioxidant activity. In addition to their protective roles against light-induced oxidative damage, there are increasing evidences that L and Z may also improve normal ocular function by enhancing contrast sensitivity and by reducing glare disability. Studies about L and Z supplementation have indicated that moderate intakes of L and Z are associated with decreased AMD risk and less visual impairment.
The relationship between L, a dietary xanthophyll carotenoid, and visual and cognitive health is particularly compelling because L is taken up selectively into eye and brain tissue (1-3). In part, the beneficial effects of L are thought to be attributable to its antioxidant and anti-inflammatory properties. Given that the eye is an extension of the neural system, L is increasingly recognized as having a role in cognitive function (4). A number of recent studies evinced a possible role for L and Z in cognitive function (2,5,6). For instance, one of the investigations conducted as part of the Irish Longitudinal Study on Aging determined that older adults with higher macular pigment optical density (MPOD) had better results in various indices of cognitive function compared to those with lower MPOD(5). Furthermore, several studies have shown cognitive impairment to be related to age-related eye diseases (AREDs) (7,8), suggesting that similar factors may be involved. These observations are in line with the view that vision and cognition are not easily separable. The rationale supporting a role for L in cognitive function is based on the following observations: (1) L is the predominant carotenoid in human brain tissue in early as well as late life (2,3); (2) primate retinal L concentrations, i.e., macular pigment density, are related to brain L concentrations (9); (3) macular pigment density is related to cognitive function in adults (5,6); and (4) L supplementation in adults improves cognitive function (10).
Among the carotenoids, L and Z are the only two that cross the blood-retina barrier to form macular pigment in the eye (11), and L is the dominant carotenoid in human brain tissue (2,3,9,12-14). Furthermore, only L was consistently associated with a wide range of cognitive measures that included executive function, language, learning, and memory, which are all associated with specific brain regions (2). In a double-blinded, placebo-controlled trial of women who received L supplementation (12 mg/d), docosahexaenoic acid supplementation (800 mg/d), or a combination of the two for 4 months, verbal fluency scores improved significantly in all three treatment groups. Memory scores and rates of learning improved significantly in the combined treatment group, who also displayed a trend toward more efficient learning (15). Terry et al. suggested that individuals with less L may have compensatory neural mechanisms to help them to engage in learning and recalling processes (16). Taken together, these observations suggested that L could influence cognitive function. Hoffmann (17) et al. found that supplementation of Z promoted a better long term delayed memory. Neuroprotection How they influence brain function, however, is less clear. One possibility is simply protection from the accumulated effects of oxidative and inflammatory stress (18). Data linking reduced MPOD to dementia (19) and cognitive impairment (20) is consistent with that possibility. Another possibility, more relevant to younger individuals and palliative approaches, is a direct improvement by some type of local interaction with neural cells (the so-called neural efficiency hypothesis) (21). Besides, it has also been suggested that the carotenoids may play a beneficial role by enhancing gap junctional communication in the brain and there is a growing body of evidence that the macular carotenoids may have a favourable effect on neuronal processing (22, 23). These carotenoids have been shown to improve communication through cell-to-cell channels, modulate the dynamic instability of microtubules (structural units of neurons), and prevent degradation of synaptic vesicle proteins (24,25).
Blue light protection
The eye is a major sensory organ that requires special care for a healthy and productive lifestyle. Numerous studies have identified L and Z to be important components for eye health. Their role in human health, in particular the health of eyes, is established from epidemiological, clinical and interventional studies (26,27). They constitute the main MPs found in the yellow spot of the human retina which protect the macula from damage by blue light, improve visual acuity and scavenge harmful reactive oxygen species. They have also been linked with reducing the risk of AMD and ARC. They may also enhance visual performance by decreasing chromatic aberration and enhancing contrast sensitivity (28).
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