NUTRITION AND THE EYE
By Jay Lavine, M.D.
Which one of your five senses would you least like to lose? For most people, it is their vision. Our sight is so precious and we depend upon it so much that we can't imagine what life would be like if we could no longer see. Even when we eat delicious vegetarian food, we "eat" with our eyes: our first impression of the food comes from its appearance, and a bad first impression is hard to overcome no matter how good the food tastes. Not surprisingly, the eye, the delicate and complex end-organ of sight, is influenced by our nutritional status. Let's look at some common eye problems and see how they relate to our diets.
Glaucoma refers to a group of diseases characterized by a progressive loss of the nerve fibers which make up our optic nerves. Glaucoma can result from other eye problems, but we will limit the discussion here to chronic open-angle glaucoma, the most common type. The main risk factor for glaucoma is an elevated intraocular pressure (IOP), the fluid pressure inside the eye, which is different from blood pressure. Some feel that the blood circulation to the optic nerve also plays a role. Nevertheless, the only treatment we have for glaucoma is to lower the IOP. Normally, this is accomplished by drugs in the form of eye drops or pills, and laser or conventional surgery can be performed as a last resort. But drugs, even in eye drop form, have side effects. Therefore, let's explore non-drug, non-surgical alternatives for the lowering of IOP.
A potentially effective therapy is exercise training. One study showed that regular aerobic exercise on an exercise bike lowered the average IOP in patients suspected of having glaucoma by 4-1/2 mm, or about 20%, a significant amount1. Jogging, however, might raise IOP in people who have a less common form of glaucoma called pigmentary glaucoma.
Both eating and drinking can affect IOP. Drinking a large amount of liquid all at once can raise IOP and should be avoided. Dr. Carlo Pissarello published studies in 19152 which showed that the IOP falls right after eating and tends to be highest just before the next meal. This may explain why, in the diurnal variation of IOP, it tends to be highest early in the morning, since we have been fasting overnight. An interesting question would be whether a "grazing" type of diet, which seems to help lower cholesterol and facilitate weight loss, would help keep IOP down. When a glaucoma patient goes to the ophthalmologist to have a pressure check, it might be a good idea occasionally to do it at a time when the IOP is likely to be at its zenith — early in the morning before eating breakfast or else just before supper or lunch.
Can any particular diet lower IOP? The answer appears to be yes. In the late 1940's, Dr. Frederick Stocker and associates studied what they called the "rice diet." This diet had previously proved very effective in lowering blood pressure. The diet was limited to rice, sugar, fruit, and fruit juices, supplemented by vitamins and iron. It contained about 2,000 Calories with 20 gm of protein, 5 gm of fat, 460 gm of carbohydrate, 0.2 gm of sodium, and 0.15 gm of chloride. They found that "reductions [of IOP] of 5 or 7 mm, persisting over long periods, were not un-common3." A reduction of this magnitude is considered quite significant for a glaucoma patient and is about the amount that one would expect to result from a successful laser treatment. The researchers were not sure why the diet was effective but speculated that the very low sodium and chloride content somehow influenced fluid secretion into the eye. I was able to speak with the third author, Dr. James Clower, who was a resident at Duke at the time, and who is still practicing ophthalmology in Florida. He said that no follow-up studies had been done, but he laughingly commented that perhaps Seventh-day Adventists would have the best pressures! [Note from the editors: Many Seventh-day Adventists follow a vegetarian diet. Perhaps Dr. Clower felt this diet would be lower in protein and sodium; although this is not necessarily true.]
A more recent study out of Israel followed people who were placed on intravenous feedings because of intestinal problems4. When the intravenous fluids were fat-free, IOP's were significantly lower than when fat was included. Since certain fat-derived blood chemicals called prostaglandins were greatly reduced in the fat-free phase, and since prostaglandins are known to influence IOP, they theorized that this was the reason for the effect they were observing. Therefore, it may have been the ultra-low fat content of the "rice diet" which was responsible for the lowering of IOP. Certainly, further studies on low-fat diets would be welcome. (Caution: the rice diet as described is nutritionally inadequate and should not be attempted on your own.)
Age-related macular degeneration (AMD) is the leading cause of loss of vision in people over the age of 55. The degeneration involves the central part of the retina where the best vision is, sparing peripheral vision. In a small minority of people with this condition, abnormal blood vessels can grow behind the retina where they can leak and bleed. If this is detected before the blood vessels reach the exact center of the retina, the vessels can sometimes be obliterated with laser.
Nutritional therapy is now all the rage in AMD. Zinc is the most abundant trace mineral in the eye, and a study published in 1988 showed that oral zinc sulfate, 100 mg twice a day, might slow the progression of AMD5. A plethora of zinc/antioxidant supplements has since appeared on the market. The products, promoted by drug companies and often sold by ophthalmologists, are now heavily used.
In examining whether high-dose zinc supplementation is justified, we encounter some problems and uncertainties. First, only this one study has been published in a peer-reviewed journal. Generally, a study, no matter how well done, should be confirmed by additional studies. Second, only one dosage of zinc was studied. Perhaps a much smaller dose would also be effective. Third, large amounts of zinc can impair the immune system by affecting white blood cell function6. This was studied using 150 mg of elemental zinc twice a day. Whether the amount of zinc currently being prescribed for AMD can impair immune function remains to be determined. Our immune systems protect our bodies against cancer and infections. Fourth, zinc in high doses can interfere with the ab-sorption of other minerals, such as copper and iron.
A copper deficiency anemia can occur7, and copper deficiency has also been theorized to be a cause of atherosclerosis (hardening of the arteries), which results in heart disease8. To lessen that risk, the supplements generally contain some copper. We cannot be sure, though, that they contain enough copper to prevent copper deficiency. On the other hand, some have speculated that perhaps it is not the zinc which is helping the AMD but a copper deficiency induced by the high zinc dose. (Subjects in the AMD study did not take copper along with the zinc.) If that is the case, then taking copper with the zinc may nullify the beneficial effect initially observed.
As you can see, there are no clear cut answers at present. We eagerly await further studies.
Oxidation of the polyunsaturated fatty acids found in the membranes of the rods and cones of the retina has been proposed as a possible cause of AMD. This is the rationale for the use of antioxidant vitamins, such as beta-carotene and other carotenoids, vitamin C, and vitamin E. One recent study did show, in fact, a reduced risk of visual loss from the bleeding from AMD in people with high blood levels of these antioxidants9. Another study showed that higher blood cholesterol levels seemed to reduce the risk of the dry, or non-bleeding, form of AMD10. The authors did not have a good explanation for this phenomenon. My theory is that since beta-carotene and vitamin E travel in the blood with cholesterol, people who are genetically predisposed to lower cholesterol levels carry a lesser amount of these antioxidants to their tissues. Vegetarians, however, have a higher antioxidant/cholesterol ratio than non-vegetarians11, 12, and so they probably would not share the higher risk associated with low cholesterol levels. Again, studies are needed.
A small, controlled French study found that ginkgo biloba extract (50:1) had a beneficial effect on the vision of patients with AMD13. Ginkgo is a most interesting herb with many potential applications. It contains unique compounds called ginkgolides which are potent inhibitors of platelet-activating factor (PAF), a body chemical involved in inflammatory processes. PAF inhibitors have been shown to combat inflammation and to increase blood flow to areas with reduced circulation. The ginkgo extract also has anti-oxidant properties. Whether it was one component or a synergistic effect among several components of this extract which had the beneficial effect is not certain. In any case, a much larger study needs to be done. Ginkgo should not be used by anyone who takes Coumadin or who has a bleeding tendency. Also, PAF inhibitors may impair natural killer cell (a type of white blood cell) function somewhat. Herbs, like any drug, should be used only with the consent of your physician.
Cataract refers to a cloudiness of the eye's lens. It is caused by changes in the protein which composes the lens. Since the lens and the fluid surrounding it are high in antioxidants, it is thought that the antioxidants may help the lens maintain its clarity. People with higher than average intakes of beta-carotene, vitamin C, and vitamin E appear to have a reduced risk of cataract. More definitive studies are now being conducted.
Diabetes increases the risk of cataract, but it can also cause more severe visual loss by affecting the blood vessels in the retina, a condition called retinopathy. The walls of the blood vessels are weakened, causing them to leak, which blurs vision. Abnormal, fragile blood vessels may also grow in. They can bleed into the eye, causing severe problems. Type II diabetes, the milder adult-onset form, is virtually absent in populations consuming high fiber diets14. Thus, a low-fat, high fiber vegetarian diet may be the best way to prevent or reverse this illness. Type I diabetes, the juvenile insulin-dependent form, may be triggered by a reaction to a cow's milk protein15. A high fiber, vegetarian-type diet can lower insulin requirements and improve control, which may retard the progression of retinopathy. (Caution: diabetics should not change their diets without the consent of their physicians.)
Both types of diabetes can cause retinopathy. One study showed that diabetics without retinopathy had significantly higher carbohydrate and fiber intakes than did diabetics with retinopathy16. Furthermore, dietary or other treatment which aggressively lowers blood cholesterol levels can sometimes clear up the fat-rich leakage called hard exudates which many diabetics develop in their retinas17. This could conceivably eliminate the need for laser treatments in some individuals. In a small pilot study, an extract of the herb ginkgo biloba (see above) showed some promise in improving vision in patients with very mild retinopathy18.
To summarize, the ideal nutritional approach to maintain the health of the eye would appear to be a high fiber, high carbohydrate, high antioxidant, low fat, low protein type of diet, typified by — you guessed it — a vegetarian diet.
Jay Lavine, M.D., is an opthalmologist and resides in Phoenix, Arizona.
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