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Study seeks answers to nutrition’s effects on eye
It is hoped AREDS will find out what earlier, smaller studies have left in doubt about vitamins’ effects on cataracts, AMD.
Studies to date on the effects of vitamins and antioxidants on ocular disease, though numerous, have often been inconclusive or contradictory. (See accompanying chart.) A large-scale prospective study now in progress aims to eliminate some of the questions surrounding nutrition and ocular health.
To find definitive answers, the National Institutes of Health (NIH) is sponsoring the Age-Related Eye Disease Study (AREDS), a 10-year multicenter natural history study of age-related macular degeneration (AMD) and cataract.
Despite the studies published to date, there is still little known about why vitamins play the role they do in health. Whereas some physicians will wait for definite conclusions, others are beginning to form opinions based on the information available.
“It makes sense that the things we eat affect the organs of our bodies,” said Robert Cykiert, MD, an associate professor of ophthalmology at New York University Medical Center in New York. “It’s similar to the discoveries about diet and cholesterol and their effects on heart disease. Unfortunately, we don’t have the definitive answers yet like we do with cholesterol. At this point all we can do is pay attention to what is statistically valid or invalid and then make recommendations to people.”
The NIH states in the study protocol for AREDS that “it is particularly difficult to evaluate nutritional risk factors in an observational study because of the confounding of these factors with other variables (both known and unknown) that are associated with the development and progression of either AMD or cataract.”
Dr. Cykiert said, “If you speak to a lot of ophthalmologists, they say that this is all nonsense, and then you speak to others and they’re prescribing vitamins to their patients. It’s just not clear. I think the results of this study are going to clear up a lot of these questions that we have about nutrients.”
Conflicting results
The role of nutrients on ocular health has only been investigated over the past 10 years or so. In the late 1980s, a study by David Newsome, MD, and colleagues examined the effects of zinc on macular degeneration. The study paved the way for other researchers to take closer looks at the effects of vitamins and minerals on the development and progression of ocular disorders.
More recent studies have investigated the role of antioxidant vitamins in protection against cataracts. One of these, “Diet and cataract: the Blue Mountains Eye Study,” found that vitamin A intake was associated with reduced risk of cataract. The study also found that levels in the eye of both retinol and carotenoids were inversely associated with cataract. These results contrasted with an earlier study done in Massachusetts, which found a protective association only for carotenoids.
“A potential problem in any study of diet and cataract is that people with the ‘better’ diet might also have healthier lifestyles in general,” said Robert G. Cumming, PhD, first author of the Blue Mountains Eye Study. “For example in this case, it appears that people with higher niacin intakes were less likely to be smokers, were better educated and had lower sun exposure. After taking these variables into account in a statistical model, we found no association between niacin and cortical cataract.”
One thing that the Blue Mountains Eye Study did not find was a difference in results based on gender. Other studies however, such as the Beaver Dam Eye Study and two studies done on carotenoid and vitamin A intakes in association with cataract extraction, did note gender differences in their results, especially in regard to lutein metabolism. The Beaver Dam Eye Study also found that lutein metabolism may play a role in tissue interactions and in determining macular pigment (MP) density.
The authors of the Blue Mountains Eye Study do not feel that the difference in their results are due to lower statistical power, as the numbers of male and female subjects in their nuclear cataract analyses were very similar to the numbers in the Beaver Dam Eye Study.
“It may be a sample size issue,” Dr. Cumming said. “I must admit, however, that I cannot think of a good biological reason for different effects of the same nutrients on the lens in men compared to women.”
The two cataract extraction studies, completed by Johanna M. Seddon, MD, noted a 3% difference in risk of cataract extraction between men and women when looking at lutein intake.
The studies state that, “Men in the highest fifth of lutein and zeaxanthin intake had a 19% lower risk of cataract relative to men in the lowest fifth,” and “those [women] with the highest intake of lutein and zeaxanthin had a 22% decreased risk of cataract extraction compared with those in the lowest quintile.”
Lutein and zeaxanthin
Most studies seem consistent in their findings on lutein’s protective effect against cataract. All of the studies previously mentioned identified a link between spinach consumption and a lower incidence of cataract. Dr. Seddon’s study, “A prospective study of carotenoid intake and risk of cataract extraction in U.S. men,” states that “among specific foods, high intake of spinach, which is rich in lutein, was most consistently associated with a lower risk of cataract, whereas carrot intake (a major source of a and b-carotene) showed no consistent relation with cataract.”
It has also been suggested that lutein plays a role in the prevention of AMD. The Institute of Medicine’s 2000 report, “Dietary Reference Intakes for vitamin C, vitamin E, selenium and carotenoids,” summarized several studies examining these nutrients. The report suggests that the same nutrients protect against both AMD and cataract, and that dietary and plasma levels of lutein and zeaxanthin in men and women respectively were positively associated with MP density.
The attention that lutein has been given, however, may be unwarranted, according to the National Eye Institute. A statement issued in March, 2000, advised the public that the possible benefits of lutein on eye health are still uncertain, and any association between the two should be regarded with caution.
The statement reports that green, leafy vegetables are rich in lutein, but they also provide other nutrients that are beneficial for overall health. It then goes on to warn that there is little direct scientific evidence to support the claim of benefit from lutein, and further research is needed before conclusions can be drawn.
Literature review: Nutrition and the eye | |
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For Your Information:
- Robert Cykiert, MD, can be reached at 345 East 37th St., Ste. 210, New York, NY 10016; (212) 922-1430; fax: (212) 922-1436; email: LaserDoc@LasikGo.com.
- Robert G. Cumming, PhD, can be reached at Department of Public Health and Community Medicine, Bldg. A27, University of Sydney, Sydney, NSW 2006, Australia; (61) 2-9351-4822; fax: (61) 2-9351-7420; e-mail: bobc@pub.health.usyd.edu.au.