We are constantly told of the problems of an ageing population and in particular the impacts on health. If current trends of eye disease were to continue, in the years to come a very large proportion of the population could be suffering loss of vision due to eye diseases such as age-related macular degeneration (AMD), cataracts, glaucoma, and diabetic retinopathy. This will have a devastating fiscal impact on society in addition to the large scale suffering of individuals including loss of independence in otherwise healthy elderly people. Improvements in surgery and pharmaceuticals along with technological advances in the diagnosis of eye disorders certainly have a key role to play but with the advancing tide of numbers of people likely to be affected there has also to be a preventative strategy.
Also prevention is cheaper.
According to the online journal Medscape, “a recent survey found that baby boomers (born between 1945 and 1964) fear vision loss almost as much as they fear heart disease or cancer. Still, almost half do not receive an annual eye examination, and few are knowledgeable about how dietary factors influence eye health. Furthermore, supplement use to promote eye health is low, even in patients already diagnosed with age-related eye disease. Eye care providers, primary care providers and specialists in diabetes can point to a burgeoning body of research to convince their patients to increase their dietary intake of key nutrients.”
There is already an impressive but as yet far from complete body of knowledge regarding the effects of nutrition on eye diseases. The ball started rolling with the landmark AREDS (Age Related Eye Disease Study) sponsored by the National Eye Institute in the USA which concluded that AMD was responsive to nutrition. This was a controlled randomised trial that showed a 25% reduction in risk for progression to advanced AMD in high-risk patients who took a combination of antioxidant vitamins (vitamins C and E), beta-carotene, zinc and copper.
A number of laboratory studies, epidemiological studies as well as smaller clinical studies also raised the possible effects of other groups of compounds in the diet such as carotenoids (including the xanthophylls Lutein and Zeaxanthin) and omega-3 fatty acids. There have been some conflicting results with one study (the CARMA study not showing any protective effect of carotenoids with regard to AMD.)
This is not to say they are of no use. In fact this illustrates many of the problems in planning and caution required in interpreting data from epidemiological and basic laboratory studies.
Anatomical studies show that different carotenoids are concentrated in different parts of the retina; Lutein is found more in the peripheral retina and Zeaxanthin in the central retina or Macula Lutea so a study that groups differently acting carotenoids together as one agent is likely to show a smaller effect when looking at a disease affecting the macula.
AREDS2 is now under way to establish the benefits of xanthophylls and omega-3 fatty acids, and initial results are expected in 2013.
Even though the results of AREDS2 are not yet known, manufacturers of oral supplements are jumping the gun and including xanthophylls and omega-3 fatty acids.
I will briefly summarise some of the nutrients that are currently thought to play a role and also their sources in food. There will be more included in later blogs.
Lutein and Zeaxanthin are xanthophyll carotenoids found in green leafy vegetables that have antioxidant and light-screening mechanisms. They cannot be synthesised in the human body so they must be derived from food. In the plant they probably protect the tissue from the free radicals produced as a by-product of photosynthesis where there is a high concentration of oxygen in the presence of light. In the eye, where the blood flow relative to the weight of tissue is the highest in the body there is also a high oxygen environment in the presence of light giving rise to free radicals. It is therefore not surprising that these xanthophylls are therefore concentrated in the retina.
As mentioned above Lutein is found in green leafy vegetables and also eggs, and major dietary sources of Zeaxanthin are corn, spinach, collard greens, lettuce and tangerines.
Zinc is an antioxidant and anti-inflammatory mineral, and it was included in the AREDS study. It probably acts as a cofactor in enzyme systems involved in inactivation of free radicals. Note that high dietary or supplemental levels of zinc can interfere with copper absorption and metabolism so supplements containing zinc must also have copper.
Good dietary sources of zinc include oysters, crab, toasted wheat germ, low-fat roast beef, mutton, pumpkin seeds, dark chocolate (but check the label for the fats added to ensure no hydrogenated fats AKA trans-fats are added) and peanuts.
Omega-3 Fatty Acids
This is an exciting area of research. It is known that these compounds are highly concentrated in the retina, especially docosahexaenoic acid (DHA). Prospective observational studies looking at the effect of DHA in the diet suggest a protective role in relation to AMD and there is also a beneficial effect in dry eye conditions.
Dietary sources are fatty fish (salmon, tuna, mackerel & sardines) or through omega-3 fatty acid supplements in the form of oil or capsules. For those intolerant of fish products there are high quality supplements derived by fermentation such as Nuique.
There is less evidence that this slows or prevents AMD in studies looking at Vitamin E supplementation but there is an effect on slowing down advanced AMD when used in combination with other nutrients i.e. beta-carotene, vitamin C and zinc.
Dietary sources of vitamin E include fortified cereals, wheat germ, sunflower seeds and vegetable oils.
In the next blog I’ll outline the roles of Vitamin C, the flavonoids and Vitamin A including beta-carotene.