Study Finds Dietary Link to Risk of Eye Disorder

Certain kinds of carbohydrates may play a role in the development of age-related macular degeneration, an incurable degenerative eye disease that is a leading cause of blindness in older adults. A new study has found that eating carbohydrate- rich food with a high glycemic index - a measure of a food's potential to raise blood glucose levels - is associated with the development of the disorder.
The glycemic index is a measure of how fast carbohydrates are metabolized - the faster they are broken down into glucose, the higher the glycemic index. Simple carbohydrates, like those in cakes and cookies, cheese pizza, white bread or other foods sweetened with sugar or corn syrup, are quickly metabolized by the cells, while the complex carbohydrates in brown rice, barley and many other vegetables are broken down more slowly. Heavy consumption of foods with a high glycemic index has been implicated in the development of diabetes, cardiovascular disease and some cancers, according to background information in the paper, which appears in the July issue of The American Journal of Clinical Nutrition.
The researchers examined 4,099 people ages 55 to 80 enrolled in a larger long-term study of eye health. Each participant had 20/32 vision in at least one eye, and the lens of the eye had to be clear enough to allow good photographs that could be used to diagnose macular degeneration. None of the participants had diabetes. Using these criteria, the scientists had 8,125 eyes to analyze. They graded the severity of macular degeneration on a scale of one to five, administered food frequency questionnaires and calculated the dietary glycemic index, a number indicating the quantity of high-glycemic foods consumed, for each participant.
After controlling for age, sex, education level, body mass index, alcohol consumption and other variables, they found that the higher the dietary glycemic index, the more likely a person was to have macular degeneration. Those in the highest one-fifth of the dietary glycemic index had more than a 40 percent increased risk of significant macular degeneration than those in the lowest one-fifth. The amount of carbohydrate consumed was not correlated with disease, suggesting that it is only carbohydrates with a high glycemic index that cause the effect.
"Sugar is fuel for the cells, but too much is destructive, " said Allen Taylor, the senior author of the paper and chief of the Laboratory for Nutrition and Vision Research at Tufts University. "It is known from laboratory and animal studies that carbohydrates can damage the proteins in cells and affect their function. The sugars actually modify things, modify the proteins, and it's the accumulation of this modified stuff that is poisonous to cells."
While the exact mechanism is unknown, the authors suggest that high glucose concentrations are harmful to the retina and the capillaries that supply the eyes, and that a diet of high glycemic index foods causes oxidative stress that increases inflammation.
It may also be that the sharp temporary increase in blood lipid levels that can follow consumption of simple carbohydrates plays a role in damaging the blood vessels. Still, the researchers say, older age, lower education level and smoking are all more significant risk factors for age-related macular degeneration than diet. They also say that the study does not prove a cause-and-effect relationship between a high glycemic diet and macular degeneration, that the study is based on observations made at a single point in time, and that long-term prospective studies will be needed before any firm conclusions can be drawn about the precise relationship between diet and macular degeneration.
Dr. Taylor does not advocate a carbohydrate- free diet. "I'm not an advocate of any extreme diet," he said. "But self-control and limiting exposure to simple sugars is not a bad idea." He added: "People are eating more simple sugar than they used to, and reverting to a diet that is more fruits and vegetables and less sweetened food would help. It doesn't take a lot of change."
[AP]