Taylor and colleagues analyzed data from a sub-group of participants in the Nurses' Health Study (NHS) who were enrolled in the Nutrition and Vision Program. The researchers looked at the total amount of carbohydrates consumed over 10 years and the dietary glycemic index, which is a measure of the quality of overall dietary carbohydrate.
"Women who consumed diets with a relatively high dietary glycemic index had greater risk of developing signs of early age-related macular degeneration when compared with women who consumed diets with a lower dietary glycemic index," says lead 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. High total carbohydrate intake, however, did not significantly increase the risk factor for AMD.
"In other words, the types of carbohydrates being consumed were more important than the absolute amount," explains Taylor, senior author. A high-glycemic-index diet is one that is rich in high-glycemic-index foods, which are converted more rapidly to blood sugar in the body than are low-glycemic-index foods.
Chiu, Taylor, and colleagues examined the eyes of more than 500 women between 53 and 73 years of age, looking for changes indicative of early AMD. The researchers also analyzed the participants' diets, as reported in questionnaires that had been administered periodically over the course of 10 years preceding their eye exams.
"Dietary glycemic index may be an independent and modifiable risk factor for early AMD," concludes Taylor, who is also a nutrition, ophthalmology and biochemistry professor on the Tufts health sciences campus in Boston. "The likelihood of having abnormalities characteristic of AMD on eye exam more than doubled for women who consumed diets with the highest glycemic index, regardless of other factors already known or suspected to increase the risk of AMD, such as age, high blood pressure, cigarette smoking, and obesity."
AMD primarily and irreversibly affects central vision, which is critical for many activities, such as reading and driving. The disease is caused by the gradual breakdown of light-sensitive cells in the region of the eye's retina called the macula. It is estimated that 1.75 million Americans 40 years of age and older have some manifestation of AMD.
Prior to the current study, the association between AMD and dietary carbohydrate had not been evaluated. "We are interested in studying the role of glucose in age-related diseases like AMD," Taylor says, "because evidence suggests that problems with glucose metabolism, as in diabetes, may cause damaging by-products to accumulate in sensitive tissues and contribute to disease."
"We cannot say, based on these data, whether or not consuming a diet with a high glycemic index causes AMD," says Taylor. He points out that there are other possible explanations for the relationship he and his colleagues observed. "Perhaps a high-glycemic-index diet is a marker for an overall dietary or lifestyle pattern that increases the risk of developing AMD." A diet high in high-glycemic-index foods like white bread and french fries has a higher overall glycemic index than a diet based more heavily on low-glycemic-index foods, such as lentils and yams.
Taylor is cautious in his interpretation of this data, but he believes that further research is critical, as it may ultimately prove helpful in preventing or delaying the onset of such potentially debilitating and costly diseases.