Despite the fact that approximately 25 to 30 million people worldwide are affected by AMD, awareness of the condition is low, says AMD Alliance International. And as the generation of Baby Boomers gets older, the Alliance expects incidence to be on the rise and triple by 2025.
The macula is a yellow spot of about five millimeters diameter on the retina. As we age, levels of the pigments in the macula decrease naturally, thereby increasing the risk of AMD. The yellow color is due to the content of the carotenoids lutein and zeaxanthin, which we derive from the diet.
These compounds are the only carotenoids capable of filtering the harmful blue light than can damage cells in the eye, the rods and the cones, explains Holger Becker, PhD, Xangold Global Product Manager at Cognis.
A thin macular pigment can allow the blue light through and destroy the cells. Maintaining high levels of both carotenoids, and therefore the macular pigment, is a valid approach to maintaining eye health and reducing the risk of AMD, adds Becker.
In 1994, Dr Johanna Seddon and her co-workers at Harvard University reported a link between the intake of carotenoid-rich food, particularly dark green leafy vegetables like spinach, and a significant reduction in AMD.
“Although these observational nutritional data do not establish causality, it seems prudent to concur with the recommendation of increasing the consumption of vegetables in the diet and, in particular, to include dark green, leafy vegetables that are rich in lutein and zeaxanthin,” concluded the researchers in the Journal of the American Medical Association (Vol. 272, pp. 1413-1420).
Since then scores of studies have come out supporting the benefits, and elucidating the mechanisms involved. Commenting on the strength of the lutein-AMD science, Dr Diane Alexander, technical service manager, R&D for Kemin Health said: "I think the science is conclusive. There is a lot of epidemiological evidence, as well as many supplementation studies, which show that increased serum levels of lutein and zeaxanthin are associated with increased macular pigmentation."
Dr Alexander pointed to a recent review by five experts that concluded that macular pigment optical density (MPOD) - an indicator of xanthophyll levels in the eye - may also "potentially serve as a biomarker not only for predicting the risk for eye disease but also for visual function" (Vision Research, 2010, Vol. 50, pp 716–728).
Ratios of lutein to zeaxanthin are important, said Abhijit Bhattacharya from OmniActive Health Technologies. “When you look at data of levels in healthy diets – based on serum concentrations we typically need 5:1 lutein to zeaxanthin,” he said. “So we probably need to be supplementing at this 5:1 ratio.”
Final support for the link may be a few years away if the high profile AREDS2 (Age-Related Eye Disease Study) reports positive results. The intervention, expected to end in 2013, is using a formulation which includes lutein and zeaxanthin, and the omega-3 fatty acids DHA and EPA. The nutrients for the new study are supplied by DSM Nutritional Products using Kemin’s FloraGlo lutein ingredient.
Another carotenoid, astaxanthin, has also been linked to eye health. A patent by the University of Illinois’ Mark Tso in 1996 reports a method “to prevent, retard or treat eye and central nervous system diseases or injuries, such as age-related macular degeneration” using astaxanthin (US Patent #5527533).
While the science appears substantial, the European Food Safety Authority’s (EFSA) Panel on Dietetic Products, Nutrition and Allergies (NDA) is not yet convinced. The NDA’s lutein opinion found that while the carotenoid had the ability to raise macular pigment density, this effect did not necessarily benefit, “maintenance of normal vision”. A similar response was issued for meso-zeaxanthin.
Lutein producers are not accepting the decision lightly and there is some collaboration to ensure future health claims success. NutraIngredients will cover the regulatory aspects of eye health in the fourth parts of its series.
Looking to omega-3
Beyond lutein and zeaxanthin, a growing body of science supports potential benefits for omega-3 fatty acids in AMD. The mechanism behind omega-3's putative effect was proposed by researchers from the University of Sydney to be down to insufficient fatty acid intake causing abnormal metabolism in the retina, which affects cell renewal (Archives of Opthamology, Vol. 124, pp 981-986).
"Omega-3 DHA is a major structural and functional component of the photoreceptor of the retina (93 percent of the omega 3 fatty acids and 30 percent of total fatty acids) and is in need of constant replenishment due to oxidative stress in those cells," explained Rob Winwood, director of scientific affairs Europe, Martek Biosciences, producers of algal DHA. "DHA increases the MPOD in the central region of the macula," he added noting a 2008 study published in the American Journal of Clinical Nutrition (Vol. 87, pp. 1521-1529).
Furthermore, a meta-analysis by Australian scientists reported that a high intake of omega-3 fatty acids and fish may reduce the risk of AMD by up to 38 per cent (Archives of Ophthalmology, 2008, Vol. 126, pp. 826-833).
This was followed by a study published in the American Journal of Clinical Nutrition (2009, Vol. 90, pp. 1601-1607), which found that increased intakes of omega-3 fatty acids may reduce the risk of developing age-related blindness by 30 percent. The results were derived from a sub-section of 1,837 people participating in AREDS. All the participants were considered to be at a moderate-to-high risk of advanced AMD.
Vision loss beyond AMD
Eye health is not all about AMD, however. The benefits of lutein may also extend to rentinitis pigmentosa – a group of inherited eye diseases that affect the retina. It causes the degeneration of photoreceptor cells in the retina, bringing progressive vision loss to about one in 4,000 people worldwide.
A study published in the Archives of Ophthalmology, found that a combined supplement of lutein and vitamin A may slow vision loss associated with these diseases. A daily supplement containing 12 milligrams of lutein in combination with 15,000 International Units of vitamin A (retinyl palmitate) was associated with a preservation of mid-peripheral vision.
There is also significant evidence for the benefits of lutein and zeaxanthin for cataracts and cataract-related conditions, said OmniActive’s Bhattacharya.
This makes sense, said Kemin’s Alexander since lutein is present in the lens of the eye, and cataracts are caused in part by oxidation of the lens.
Healthy eyes in healthy people
Beyond AMD, there is also data that lutein may also protect against the detrimental effects of long-term computer display light exposure, according to a Chinese study published in the British Journal of Nutrition (2009, Vol. 10, pp 186-190).
“Visual function in healthy subjects who received the lutein supplement improved, especially in contrast sensitivity, suggesting that a higher intake of lutein may have beneficial effects on the visual performance,” wrote the researchers from Peking University.
Another study with subjects aged between 22 and 45 found that a combination of lutein, zeaxanthin and blackcurrant extract may reverse signs of visual fatigue. Researchers from Japan and Singapore reported that visual fatigue, caused by many factors, not least staring at computer monitors for long hours, may be eased a daily supplement containing blackcurrant fruit extract (200 mg), lutein (5 mg), and zeaxanthin (1 mg) (Applied Ergonomics, 2009, Vol. 40, pp. 1047-1054).
Furthermore, a study published in the journal Optometry and Vision Science (2008, Vol. 85, pp. 82-88) reported that lutein and zeaxanthin may reduce the deleterious effects of glare on a test group of people with normal eyesight.
Researchers from the University of Georgia, Athens recruited 40 healthy subjects with an average age of 23.9 and assigned them to receive daily supplements of lutein (10 mg, FloraGlo, Kemin) and zeaxanthin (2 mg, OptiSharp, DSM) for six months.
"The positioning of lutein is changing," said Kemin’s Dr Alexander. "[These new studies show] it is an essential nutrient that everyone needs everyday to protect their vision."
Another area not to be over-looked in eye health is dry eyes. Keratoconjunctivitis sicca, or dry eye syndrome, is a condition where the eyes do not make enough tears, or the tears evaporate too quickly. This produces dry eyes and increases the risk of inflammation. According to a recent study by Finnish researchers, the prevalence of dry eye can be up to 30 per cent in people aged 50 y and older.
Fatty acids are again linked to a potential benefit, and the omega-3 and omega-6 content of sea buckthorn was recently touted as the main bioactives behind sea buckthorn’s ability to potentially reduce symptoms of dry eye syndrome.
Scientists from the University of Turku reported in the Journal of Nutrition (doi: 10.3945/jn.109.118901) that the linolenic acid contained in the oil is a basis for anti-inflammatory compounds, which could reduce inflammation. The researchers also note that sea buckthorn oil is a rich source of vitamin E and that antioxidants “may protect the eye from oxidative damage leading to activation of inflammatory cascades”.