This chapter is not written in the usual question and answer format of the rest of the book. No attempt has been made to make the chapter simple and easy to understand. It is meant to impress the reader that the author is actually capable of research (even if only on the net and by going through various scientific journals). It is also meant to impress the reader with the author’s command over complex technical jargon. The average reader may skip the chapter in its entirety. The above average reader may however, read on.
I knew you would consider yourself above average! Well then read on, but don’t say I did not warn you.
Retinal diseases, including age related macular degeneration (AMD or ARMD) are the leading cause of blindness in the developed world, while cataracts are the leading cause of avoidable or preventable blindness in developing countries such as India. Epidemiological studies show some evidence of the role of nutrition in the prevention and treatment of these potentially blinding conditions. Though most proponents of alternative medicine will jump to the conclusion that every person at risk should modify his or her diet and take sometimes expensive nutritional supplements to maintain healthy eyesight, most eye surgeons remain skeptics. The truth is somewhere in between these two extremes. Let us examine trends emerging from recent research. I leave readers to draw their own conclusions.
Age related macular degeneration or AMD is an affection of the centre of the retina, called the macula. This region is responsible for all our near vision and most of our fine vision. Barely 20% of these cases, the so-called ‘wet’ types of AMD are amenable to laser treatment. For the rest there is no known cure.
Though the exact cause of AMD is not known, the macular area is known to be rich in caratenoids such as lutein and zeaxanthin. Zeaxanthin is closer to the centre of the macula called the fovea, than lutein. Both contribute to the normal yellow appearance of the macula. Lower density of these pigments has been found experimentally to increase the chances of light toxicity to the retina and macula. These act as scavengers of free radicals. Free radicals cause lipid peroxidation of the photoreceptor cell membranes, hastening cell death. In ‘wet’ or neovascular AMD, persons with medium to high caratenoids levels were found to have significantly lowered risk of losing vision compared to those with low serum levels of these caratenoids. One of the theories for why smokers are at increased risk of AMD is because smoking causes lowering of serum caratenoids levels.
Vitamin E, in the form of a-tocopherol is protective for AMD. The physician Health study evaluated 21000 people for 12 years. They found 13.5% decreased risk, when vitamin E supplements were given. This was however, not statistically significant. In experimental animal studies, chances of AMD increased if vitamin A was simultaneously deficient in addition to deficiency of vitamin E.
Vitamin C. In rats, if large quantities of vitamin C were given prior to prolonged light exposure, it appeared to protect the macula from light damage. Its protective action probably stems from its anti-oxidant properties.
Effect of Diet. In the Beaver Dam Study, 2000 people in the age group of 43-84 years were asked to answer a food questionnaire. There was a significant inverse association between dietary intakes of caratenoids (fresh fruits and vegetables) and vitamin E and risk of AMD.This was corroborated by several similar studies across the globe. Spinach, collard greens, zucchini, green leafy vegetables (high lutein but low zeaxanthin content) and most fruits (especially kiwi, grapes, oranges, yellow squash) are rich sources of caratenoids. Egg yolk and corn have highest lutein and zeaxanthin (85% of total caratenoids content). Corn is the vegetable with the highest lutein content (60% of total caratenoids content). Orange pepper is the vegetable with the highest zeaxanthin content (37% of total caratenoids content). Moderate amounts of red wine (rich in flavinoids) even if taken only once a month reduces the risk of AMD according to the Beaver dam Study, while beer increases the risk.
Zinc. Zinc is normally found in high concentrations in the retina, as it is a requirement of several retinal enzyme systems. Low dietary intake of zinc increases risk of AMD, while there is an isolated report of a double blind study showing zinc supplementation slowed progress of AMD compared to placebo. Post-mortem examination of the retina of those with AMD found decreased zinc compared to those without AMD.
One of the causes of diabetic changes in the retina, or diabetic retinopathy is development of glycosylated proteins i.e. attachment of sugar to proteins. These generate free radicals, which cause oxidative tissue damage and depletion of glutathione (GSH). Besides, these combine with lipids to form advanced glycated end products (AGE), which are deposited in blood vessels, damaging them. These processes are blocked by anti-oxidants. Besides anti-oxidants also reduce permeability of the capillaries (small blood vessels) in the retina.
Vitamin C, E and gingko biloba are anti-oxidants that prevent glycosylation. Magnesium and Vitamin B6 are also supposed to help in slowing progress of diabetic retinopathy.
Cataract is the opacification of the lens of the eye. This occurs when the soluble proteins, which are in reduced form and therefore transparent are converted by oxidation to their insoluble oxidized form. Glutathione levels in the lens decrease with age, resulting in faulty oxidative defence systems within the lens, causing cataract. In cataracts removed by surgery, glutathione (GSH) levels showed a dramatic decrease by 80% compared to normals. This is due to decreased synthesis. Cataract patients are also deficient in Vitamin A and the caratenoids, lutein and zeaxanthin. Smoking, Ultraviolet B light, ionizing radiation, obesity, family history, oxidative stresses and several medications such as steroids, gout medications and heavy metals increase the risks of cataracts.
In the Nurses health Study, over 50,000 women were given a food questionnaire. Those with higher consumption of lutein, zeaxanthin and Vitamin A had significantly less cataract risk. This is not true for beta-carotene, alpha-carotene or other caratenoids, however.
Vitamin E & cataract. Low levels of vitamin e are associated with increased risk of cortical cataract in clinical studies. Rats given subcutaneous injections of 100I.U. of alpha-tocopherol (vitamin E0 did not develop cataract when exposed to ionizing radiation unlike controls, who all developed cataracts. The same effect was noticed on substituting the injection with frequent eye drops of vitamin E. In the longitudinal study of cataract, among 764 participants, risk of development of cataract were 30% less in regular users of multivitamins and 57% less in regular users of supplemental vitamin E. The conclusions of the study were: 1. Vitamin E decreases oxidative stress in cataractous lens. 2. Part of the vitamin E effect is due to its ability to enhance glutathione levels. 3. Vitamin E seems to protect against cortical rather than nuclear cataracts.
Vitamin C & cataract. In animal experiments, steroid induced cataracts in chick embryos were prevented by prior administration of high doses of vitamin C. Cortisone causes decrease in GSH levels, which is slowed by vitamin C. In the Nurses health Study, supplementation of diet with vitamin C for greater than 10 years was found to have 77% lower incidence of early lens opacities.
Riboflavin & cataract. In a trial in China involving 20,000 people, there was 44% decease in risk for persons taking a riboflavin/niacin combination compared to controls. Riboflavin is a precursor of FAD, a co-enzyme for GSH reductase, which recycles GSH.
In conclusion, there is perhaps something to be said for greater attention to be paid to nutrition in the management and prevention of potentially blinding eye diseases.