Eye Benefits
(See Table 6 for a summary of the following eye related studies.)
The eye's job is to convert light (electromagnetic radiation) into neuroelectric signals so the brain can interpret them to form images of the rest of the world. Along with the visual frequencies of light are more energetic forms of light such as ultraviolet radiation. These have enough energy to produce free radicals in the eye, and if insufficient antioxidant protection is present, the free radicals can destroy eye tissue, potentially leading to cataracts and other eye diseases. This section deals with studies focusing on how vitamin E and other antioxidants might help protect the eye.
In this study at Baylor College of Medicine, Houston, Texas, human eyes from 30 donors were analyzed for vitamin E concentrations. (The eyes were given to science after death as part of an organ donation program.) Both the alpha- and gamma- forms of tocopherol were measured. Most of the alpha-tocopherol was found in the retina and pigment epithelium-choroid, and the remainder was in the iris and ciliary body. Gamma-tocopherol was present only at a small fraction of the alpha-tocopherol levels. The concentration of alpha-tocopherol in the eye did not change significantly in terms of the age of the donor, which ranged from 23 to 100, except for a large reduction in the oldest donor. However, the number of samples in this study did not provide enough statistical strength for a definite correlation with age.
The researchers also examined how alpha-tocopherol was transported in the eye and in experiments on cattle eyes, found that a binding protein usually associated with vitamin A transport also binds to vitamin E. This may be a way vitamin E is transported in the interphotoreceptor matrix between the retina and pigment epithelium.
The authors comment that the data shows alpha-tocopherol in the adult human eye is present at a concentration similar to other tissues, such as the liver. Also, the gamma- to alpha-tocopherol ratio is comparable to those found in lymphocytes.
Alvarez, R. Liou, G., Fong, S., Bridges, C. Levels of Alpha- and Gamma-Tocopherol in Human Eyes: Evaluation of the Possible Role of IRBP in Intraocular Alpha-Tocopherol Transport. American Journal of Clinical Nutrition. Vol. 46, pp.481-7, 1987.
One of the major causes of blindness in the elderly is cataract formation. Some researchers believe that cataracts develop due to the accumulation of free radical damage. The reduction of the antioxidant defenses in aging results in free radicals cross-linking and destroying proteins in the lens. The lens has three main parts - the nucleus (center), the cortex (middle), and the capsule (outer wrapping). Any of these areas may become more opaque, and form cataracts. If excessive free radical activity in the lens indeed leads to cataracts, then increased levels of antioxidants may offer some protection.
In this study from the University of Kuopio, Finland, 410 hypercholesterolemic men underwent eye exams to measure lens opacities every 18 months for four-and-a-half years. Plasma levels of vitamin E were also measured. Those with plasma vitamin E concentrations in the lowest 25% had 3.7 times the risk of early cortical lens opacity progression compared with those in the highest 25%. Cigarette smoking also significantly predicted cortical lens opacity progression. However, risk of nuclear lens opacity progression was not related to vitamin E concentrations or smoking, suggesting that different mechanisms may be involved in different parts of the lens.
The authors conclude, "The data suggest that low plasma vitamin E content may be associated with increased risk of the progression of early cortical lens opacity."
Rouhiainen, R., Rouhiainen, H., Salonen, J. Association Between Low Plasma Vitamin E Concentration and Progression of Early Cortical Lens Opacities. American Journal of Epidemiology. Vo. 144, pp.496-500, 1996.
In Beaver Dam, a mostly white community in south-central Wisconsin, 4,926 residents participated in an ongoing study from the University of Wisconsin. A randomly selected subsample of 400 participants over 50 years of age were involved in this part of the study. Subjects' serum levels of alpha- and gamma-tocopherol and various carotenoid were measured. The opacities of their lenses were also examined and graded. After 5 years, a follow-up involved re-examining lens opacity. The researchers found subjects with the highest alpha- and gamma-tocopherol concentrations had 60% lower risk of developing nuclear cataracts. Carotenoid concentrations were not associated with cataract risk, but lutein and cryptoxanthin showed some protection for subjects older than 65 years.
The authors conclude, "Findings were compatible with the possibility that nuclear cataract may be linked inversely to vitamin E status, but neither strongly supported nor negated the hypothesized inverse association of nuclear cataract with serum carotenoids."
Lyle, B., Mares-Perlman, J., Klein, B. et al. Serum Carotenoids and Tocopherols and Incidence of Age-Related Nuclear Cataract. American Journal of Clinical Nutrition. Vol. 69, pp.272-7, 1999.
To begin exploring the possibility antioxidant supplementation might help reduce risk of cataracts, researchers from the University of Western Ontario in Canada conducted a case-control study of 500 subjects. Aged over 55, half of the subjects had cataracts and the other half did not. The researchers conducted personal interviews inside the subjects' homes, where the atmosphere is much more relaxed and open for the participant. The subjects answered questions regarding consumption of vitamin supplements.
The investigation revealed that those who did not have cataracts reported taking significantly more vitamin E and vitamin C supplements. The results suggested those who supplemented with these antioxidants had half the risk of cataracts. While no causal relationship between antioxidant supplements and cataract prevention could be drawn from the design of this study, the results supported further clinical trials to more carefully examine the possible benefits.
Robertson, J., Donner, A., Trevithick, J. A Possible Role for Vitamins C and E in Cataract Prevention. American Journal of Clinical Nutrition. Vol. 53(suppl), pp.346S-51S, 1991.
Funded by the National Eye Institute, the Longitudinal Study of Cataract studied the natural history of lens opacities in 764 elderly (median age 65) subjects. At the start of the study, subjects completed detailed dietary intake surveys, including vitamin supplement use. Plasma vitamin E levels were also measured. For the next 5 years, annual follow-up eye exams determined eye health and progression of any ocular diseases. The study found:
- Risk of developing nuclear opacity was 1/3 less for regular multivitamin users.
- Risk was 1/2 for regular users of vitamin E supplements and those who had higher vitamin E plasma levels.
The authors point out that while the results are encouraging, other factors may be involved. For instance, vitamin supplement usage may merely be associated with some other factor not explicitly studied, such as geographic location, societal and economic status, or lifestyle, which may be the true risk determinants. The nature of this type of study does not rule out other possibilities, and more concrete associations must come from the results of direct clinical trials using antioxidant supplements. They conclude, "Because these data are based on observational studies only, the results are suggestive but inconclusive. The possible effect of nutritional supplements on the lens requires confirmation by ongoing clinical trials."
Leske, M., Chylack, L., He, Q. Antioxidant Vitamins and Nuclear Opacities. The Longitudinal Study of Cataract. Ophthalmology. Vol. 105. pp.831-6, 1998.
From the Postgraduate Institute of Medical Sciences, Rohtak, India, this study involves 50 cataract patients, half of whom had cortical cataracts and the other half, nuclear cataracts. The patients were randomly assigned to take either 100 mg vitamin E twice a day (total of 200 mg per day) or placebo. The results:
- 40% reduction in cortical lens opacity in patients with cortical cataract taking vitamin E.
- 14% decrease in nuclear lens opacity in patients with nuclear cataract taking vitamin E.
- 25% increase of vitamin E in cortical lens tissue and 21% increase in nuclear lens tissue among supplementers.
- 52% increase of glutathione in cortical lens tissue and 16% increase in nuclear lens tissue among supplementers.
The sparing of glutathione observed in the present study is also shown in many other studies (see diabetes section of this article). The authors conclude, "A daily oral intake of antioxidants (vitamin C and vitamin E) in low doses may therefore be protective against the process of cataractogenesis, specifically cortical cataract."
Seth, R., Kharb, S. Protective Function of Alpha-Tocopherol Against the Process of Cataractogenesis in Humans. Annals of Nutrition and Metabolism. Vol. 43, pp.286-9, 1999.
The leading cause of blindness throughout the world is a degenerative disease called age-related macular degeneration (ARMD). The macula is the central region of the retina, where the concentration of nerve fibers is great. Researchers believe accumulated free radical damage to the macula destroys sensitive nerve tissue, degrading vision over time. Potentially, antioxidants may help prevent such damage.
In this study from the University of Wisconsin Medical School, Madison, Wisconsin, 167 ARMD patients and 167 healthy controls underwent eye exams and blood tests. Serum levels of various dietary carotenoids (alpha- and beta-carotene, beta-cryptoxanthin, lycopene, lutein, zeaxanthin) and vitamin E (alpha- and gamma-tocopherol) were measured. The average vitamin E levels in ARMD patients were slightly lower than healthy controls, but the difference was not significant after adjusting for cholesterol levels. Of the carotenoids, only very low lycopene levels was associated with increased risk of having ARMD. Whether the lower levels of lycopene and vitamin E in ARMD resulted from the disease or was due to a dietary deficiency that promoted the disease could not be determined and requires further research to answer.
Mares-Perlman, J., Brady, W., Klein, R. Serum Antioxidants and Age-Related Macular Degeneration in a Population-Based Case-Control Study. Archives of Ophthalmology. Vol. 113, pp.1518-23, 1995.
The Age Related Macular Degeneration Study Group involved 71 patients from 8 medical centers across the United States. The placebo group had 32 subjects, and the supplemented group, 39. The supplemented group took two capsules per day of an over-the-counter, multiple antioxidant vitamin supplement. Each capsule contained 14 nutrients including 100 IU of vitamin E. Dietary assessments and eye exams were made at 6, 12, and 18 months.
Eye exams showed that those in the supplemented group maintained their distance vision and sensitivity in the left eye compared to the placebo group. Also, patients taking the supplements reported subjectively greater vision stability. However, the cortical opacity of the right lens of supplemented patients increased more than those taking placebo. Thus, while the supplementation appears to have stabilized the progression of macular degeneration for 18 months, it did not improve it.
Age Related Macular Degeneration Study Group. Multicenter Ophthalmic and Nutritional Age-Related Macular Degeneration Study - Part 1: Design, Subjects and Procedures. Journal of the American Optometric Association. Vol. 67, pp.12-29, 1996.
Age Related Macular Degeneration Study Group. Multicenter Ophthalmic and Nutritional Age-Related Macular Degeneration Study - Part 2: Antioxidant Intervention and Conclusions. Journal of the American Optometric Association. Vol. 67, pp.30-49, 1996.
Age-related maculopathy is pathology of the macula due to aging. In the early stages, the presence of soft, indistinct abnormalities occurs in the macula. In the late stages, the disorder is known as age-related macular degeneration.
In the Blue Mountains Eye Study from the Australian National University, 2,900 Australians (age 49 and older) were given eye exams to diagnose early and late age-related maculopathy. They also completed detailed dietary surveys consisting of 145 food items and reported vitamin supplement usage. Researchers then calculated the nutritional value of the diets using the computerized version of the Australian Tables of Food Composition, 1990. Daily food intake of carotene, zinc, vitamin C, and vitamin A were established. However, no correlation between dietary levels of these antioxidants and the risk of having age-related maculopathy was found. No correlation between antioxidant supplement usage existed either.
Careful consideration of the data suggest that the results are statistically substantial. The authors acknowledge they could only measure total carotene content of foods, whereas some studies indicate only certain carotenes such as lycopene show definite correlations. The cross-sectional design of the study has inherent weaknesses, however similar studies (considered above) had positive results. The authors find no obvious explanation why their results differed, and conclude, "We found no associations between age-related maculopathy and dietary antioxidant intake, either from diet alone or including supplements, in the Blue Mountain Eye Study population."
Smith, W., Mitchell, P., Webb, K., Leeder, S. Dietary Antioxidants and Age-Related Maculopathy. The Blue Mountains Eye Study. Ophthalmology. Vol. 106, pp.761-7, 1999.