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Nutrition's blind spot: Macular degeneration

The retina is the site for the capturing of visual images in the eye and it is often likened to the camera film of the eye. The central part of the retina is called the macula. It is a tiny spot of about 1.4 millimeters and is yellow in colour due to the high concentration of the pigment, lutein. Although it is tiny, the macula contains the largest concentration of cone cells, which are responsible for central high-resolution vision. With aging, the macula becomes prone to damage and this age-related macular degeneration (ARMD) is the major cause of loss of central vision in older persons. Effectively, the central field of vision becomes dysfunctional while the peripheral vision remains unaffected. Reading or using a computer, becomes impossible while other activities of daily life remain unaffected. Some types of ARMD are amenable to treatment with drugs while for others there is no treatment and thus there has been a drive to seek out preventative measures.

In 2001, researchers from the US National Eye Institute were the lead authors on a paper which reported that a daily oral supplement with certain minerals and vitamins, reduced the risk of developing advanced ARMD by 25% over a 5 year period[1]. The research was prompted by several small studies, which suggested a role for certain antioxidant micronutrients in preventing ARMD. However whereas these small studies were insufficient to build up a solid base of evidence, they spurned a phenomenal growth in the sales of food supplements with very questionable claims. Thus the study, known as The Age Related Eye Disease Study (AREDS), enrolled 3,640 subjects with ARMD to test the hypothesis that certain micronutrients might reduce the progressions of the disease. The results showed a reduction in ARMD progression with daily supplements of vitamin C, vitamin E, beta-carotene, zinc and copper.

In May of this year, a second AREDS paper was reported in the Journal of the American Medical Association[2]. This study sought to further explore whether additional micronutrients might enhance the effects of the first AREDS micronutrient cocktail. The first of the micronutrients was lutein, which is found in high concentration in the macula. It is synthesised only by plants, particularly green leafy plants and in the eye, serves to protect the retina from damage arising through the high-energy photons of blue light.  Zeaxanthin, the second micronutrient is also involved in photo quenching in the macula and, again, it is a plant derived compound. The final two additional compounds examined were the very long chain polyunsaturated fatty acids found in high concentration in oily fish and which also form a main element of the surrounding of nerve calls (the myelin sheath). The eye is a very heavily innervated organ. The outcome of this second AREDS study was that there was no additional benefit to be had over and above what had been achieved with the original AREDS cocktail (vitamin C, vitamin E, beta-carotene, zinc and copper).

Yet another paper[3]recently focused on other micronutrients and their role in ARMD, specifically two B vitamins, folic acid and vitamin B12. Volunteers had retinal photographs taken 5 to 10 years apart and the incidence of ARMD was ascertained using these retinal photographs. At entry into the study, subjects complete a food frequency questionnaire, which was used to ascertain habitual folic acid and B12 intakes. Blood samples were taken to ascertain the biochemical status of the vitamins. The use of nutritional supplements was also recorded. After statistical adjustment for such confounding factors as age, smoking, gender, and so on, those deemed to be deficient in folic acid status using blood measures, had an 89% increased risk of ARMD. B12 deficiency increased the 10-year risk of ARMD by about 60%. Those subjects who were regular users of food supplements had a 47% lower risk of macular degeneration. This study was observational and cannot prove cause and effect for which a long-term intervention study (such as AREDS 1 and 2) is needed.

 ARMD accounts for more than 50% of all blindness in the US and the numbers affected will grow from about 2 to 4 million by 2025. Clearly, ARMD will continue to be a major area of public health nutrition. And once again, all this research shows that the old adage that all your nutrients can be had derived entirely from a health diet isn’t true. No doubt it is true for some but not all and nutrigenetics will soon reveal who is most sensitive to low micronutrient intakes in relation to ARMD.

[1] Age Related Eye Disease Study (AREDS 1) Group (2001) Arch Opthalmol, 119, 1417-1436
[2] Age Related Eye Disease Study (AREDS 2) Group (2013) JAMA, 309, 2005-2015
[3] Gopinath B et al (2013) AJCN, 98, 129-135


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