Link between carbohydrate quality and vision loss is strengthened by new data

BOSTON -- Age-related macular degeneration (AMD) and its associated vision loss may be connected to the quality of carbohydrates an individual consumes. In a study published in the July issue of the American Journal of Clinical Nutrition, Allen Taylor, PhD, director of the Laboratory for Nutrition and Vision Research at the Jean Mayer USDA Human Nutrition Research Center on Aging (USDA HNRCA) at Tufts University, and colleagues confirmed earlier findings linking dietary glycemic index with the risk of developing AMD.

"Men and women who consumed diets with a higher glycemic index than average for their gender and age-group were at greater risk of developing advanced AMD," corresponding author Taylor says. "The severity of AMD increased with increasing dietary glycemic index."

Glycemic index is a scale applied to foods based on how quickly the carbohydrates in foods are converted to blood sugar, or glucose. Foods like white rice, pasta and bread are examples of foods with a high-glycemic-index, meaning that these foods are associated with a faster rise and subsequent drop in blood sugar. Whole wheat versions of rice, pasta and bread are examples of foods that have a low-glycemic-index. These foods are often considered higher quality carbohydrates because they are associated with a slower and less dramatic rise and fall of blood sugar.

"Our results build upon findings from an earlier, smaller study in which we determined that consuming a diet with a high glycemic index, but not one with a high total amount of carbohydrate, increased the risk of developing early AMD," says first author Chung-Jung Chiu, DDS, PhD, scientist in the Laboratory for Nutrition and Vision Research at the HNRCA and an assistant professor at Tufts University School of Medicine.

In the current study, Taylor, Chiu, and colleagues analyzed data from 4,099 men and women participating in the nationwide Age-Related Eye Disease Study (AREDS). Detailed dietary histories were obtained at the start of the study when participants were 55 to 80 years of age and had varying degrees of AMD. The AREDS was designed to assess the effect of high-dose antioxidant vitamins and zinc on the progression of AMD and cataracts, two of the leading causes of vision loss in older adults.

"Although carbohydrate quality was not the main focus in the AREDS, we were fortunate that the investigators had collected the dietary carbohydrate information we needed to do our analyses," says Taylor, who is also a professor at the Friedman School of Nutrition Science and Policy at Tufts and the Tufts University School of Medicine. "Our findings suggest that 20 percent of the cases of advanced AMD might have been prevented if those individuals had consumed a diet with a glycemic index below the average for their age and gender," notes Taylor.

AMD typically occurs after middle age, although the events which cause it may begin earlier. A leading cause of irreversible blindness, AMD results from the gradual breakdown of light-sensitive cells in the central region of the eye's retina, called the macula. Although there is no effective therapy for AMD, dietary intervention may delay its progress. Identifying modifiable risk factors for AMD is becoming increasingly important as the population ages. As Taylor and colleagues point out, the number of people in the US with visually impairing AMD is expected to double and reach three million by 2020.

"Our results support our hypothesis," says Taylor, "that dietary glycemic index, which has been related to the risk of diabetes, is also associated with the risk and severity of AMD." Taylor speculates that carbohydrates that comprise a high-glycemic-index diet may provide eye tissue"It is possible that the type of damage produced by poor quality carbohydrates on eye tissue is similar in both diabetic eye disease and AMD."

Taylor and colleagues conclude that the risk for AMD may be diminished by improving dietary carbohydrate quality, as defined by dietary glycemic index. This may be achieved by relatively simple dietary alterations, such as replacing white bread with whole grain bread. "However," Taylor cautions, "additional studies are needed before we can recommend dietary carbohydrate management as a prevention strategy for AMD."

The study was supported by the U.S. Department of Agriculture Agricultural Research Service and by grants from the National Institutes of Health, Johnson and Johnson Focused Giving Program, the American Health Assistance Foundation, and individuals.

Chiu C-J, Milton RC, Gensler G, Taylor A. American Journal of Clinical Nutrition. 2007 (July); 86(1):180-188. "Association between dietary glycemic index and age-related macular degeneration in nondiabetic participants in the Age-Related Eye Disease Study."

The Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy at Tufts University is the only independent school of nutrition in the United States. The school's eight centers, which focus on questions relating to famine, hunger, poverty, and communications, are renowned for the application of scientific research to national and international policy. For two decades, the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University has studied the relationship between good nutrition and good health in aging populations. Tufts research scientists work with federal agencies to establish the USDA Dietary Guidelines, the Dietary Reference Intakes, and other significant public policies.

If you are a member of the media interested in learning more about this topic, or speaking with a faculty member at the Friedman School of Nutrition Science and Policy at Tufts University, or another Tufts health sciences researcher, please contact Siobhan Gallagher at 617-636-6586 or Christine Fennelly at 617-636-3707.