Researchers from Tufts and Harvard Universities report that, although they observed an association between AMD and low GI, low GI diets are frequently richer in micronutrients essential for eye health that were not accounted for in their analysis.
AMD affects the central part of the retina called the macula, which controls fine vision, leaving sufferers with only limited sight. AMD affects over 10 million people in the United States, with a further 15 to 20 million worldwide, and is the leading cause of blindness in people over 50.
The new study, published in the April issue of the American Journal of Clinical Nutrition (Vol. 83, pp. 880-886), followed 526 women from the Nurses' Health Study with an average age of 62 for over ten years.
Dietary patterns were evaluated using validated 126-item food frequency questionnaires (FFQ) that were completed by the subjects every four years from 1984. Baseline eye examinations were performed in 1995.
"According to our data, when total carbohydrate intake was held constant, there was a more than two-fold higher odds for ARM [an early form of AMD] in participants with dietary GI in the highest versus lowest third of the study sample," wrote lead author Chung-Jung Chiu from Tufts University.
When the researcher looked at the relationship between total carbohydrate intake and the risk of age-related maculopathy (ARM) they found that a high GI diet was associated with a 50 per cent increased risk.
Interestingly, the occurrence of drusen - white or yellow deposits on the retina or head of the optic nerve that are thought to be an early sign of AMD - was not significantly affected by total carbohydrate intake.
"Our failure to find an association, if it does exist, may imply that carbohydrate exposure before the time in life when the dietary data were collected could have been responsible for drusen formation," suggested Chiu.
In an accompanying editorial by Julie Mares and Suzen Moeller from the University of Wisconsin - Madison, the link between AMD risk and glycemic index is suggested to be due to a broader diet pattern that could protect against AMD.
"It may be that a low GI score is a marker for other combined aspects of diet that could protect against AMD. Diets with a low GI often include plenty of fruit, vegetables, whole grains, legumes, and milk and few refined grains and sugars," said Mares and Moeller.
"Obviously, such diets are richer in a variety of nutritive and non-nutritive antioxidants," they said.
Indeed, this may be where the real explanation lies. Antioxidant-rich diets containing vitamins C and E, and the carotenoids zeaxanthin and lutein, are strongly associated with a decrease in the development of AMD.
Chiu and colleagues admit this limitation with using GI index as a measure, explaining: "Dietary GI is based on an arithmetic sun of GIs of individual foods. It does not account for the possible effects of other nutrients."
This study supports earlier Dutch research that reported that dietary intake of vitamins C and E, zinc, and beta-carotene could reduce the risk of AMD by a third.
Mares and Moeller appear to agree with the Dutch conclusions that dietary approaches are more important than nutritional supplements.
"Studies of diet patterns in relation to AMD increase the likelihood that solutions will emphasize healthy lifelong dietary practices rather than the consumption of high amounts of a narrow range of specific nutrients in supplements that are costly and potentially harmful," concluded the editorial.
Doubts were cast on the safety of both lutein and zeaxanthin last year when Harvard researchers reported that the carotenoids may increase the risk of heart attack. The role of both carotenoids in eye health however is backed up by more than 130 peer-reviewed studies.
The Harvard study does not seem to have impacted on the launch of new products aimed at the eye health market. Although only 26 products were launched in Europe in 2003, 2004 saw 69 launches, with similar numbers reported in 2005. Data source: Mintel's Global New Product Database.