Research Unveils Food Link to Age-related Macular Degeneration (AMD)

Nutrition plays an important role in the occurrence, prevention and treatment of Age-related Macular Degeneration (AMD). Nutrients--which are found in various foods--are commonly advocated for prevention as well as treatment of AMD. However, recent research has shown that some foods can substantially increase a person’s risk for the disease.

According to a study published in the October 2008 issue of the professional journal American Journal of Clinical Nutrition, foods with a high glycemic index can be a risk factor for early or dry AMD (early AMD is known as the precursor for the sight-threatening late or wet AMD), and foods with low glycemic index can protect a person against this disease.

The glycemic index is an index expressed according to a scale of 0-100. This index suggests how much the blood sugar level increases when you eat certain foods. Glucose has the highest glycemic index, 100, whereas this index may be as low as 44 for an orange, 38 for an apple, 18 for soybeans and even nearly 0 for protein-rich foods such as eggs. A high glycemic index diet can cause a nearly instantaneous increase in blood sugar level, thus also increasing your risk for obesity and diabetes. However, a low glycemic index diet causes a slow rise of blood sugar and has little effect on body’s sugar level. Therefore, a low glycemic index diet is important in combating obesity and various other conditions related to increased blood sugar level.

This study indirectly validates the beneficial role of nutritional supplements in cutting the risk and slowing the progression of AMD. The glycemic index for most of the nutritional supplements (used in AMD) is low, which is why it appears that these foods have a natural potential of providing protection against AMD--thanks to their low glycemic index.

What the study says…

The study, Dietary glycemic index and the risk of age-related macular degeneration, was carried out by a University of Sydney (Sydney, Australia) researcher Shweta Kaushik and her colleagues. The researchers noted that for AMD, dietary factors are known risk factors.

“High-glycemic-index diets have been hypothesized as a risk factor for AMD, but prospective data are unavailable,” they wrote in the research paper.
In order to find out a valid association between dietary factors and the risk of AMD, Kaushik and co-workers conducted a population based study, examining the link between dietary glycemic index and the 10-year incidence of AMD, in a group of participants aged 49 years and more. At the commencement of the study, the researchers examined the eyes of 3654 participants. After an interval of 5 years, they reexamined 2335 of the participants, and 1952 participants were reexamined after 10 years. The researchers collected retinal photographs of the participants and these photographs were used to grade the 10-year incidence of early and late AMD. The researchers also collected information on the dietary practice of the participants, with the help of a questionnaire.
Kaushik and co-workers evaluated the data they collected in the research and found that in the study population, the cumulative 10-year incidence for the development of early AMD was 14.1%.

“After age, smoking, other risk factors, and dietary constituents were adjusted for, a higher mean dietary glycemic index was associated with an increased 10-year risk of early AMD,” the researchers noted in the research paper. They also found that greater consumption of lower glycemic index foods could substantially reduce the risk of early AMD in the participants.

What’s important in this study…

Like various other conditions, AMD is connected with the foods you consume, indicates the study by Kaushik and co-workers. According to the study,  foods that instantly increase the blood sugar level (high glycemic index foods) increases the risk of early or dry AMD, whereas foods that cause a slow increase in your blood sugar level (low glycemic index foods) may cut the risk for the disease. The study also suggests that as early or dry AMD is a precursor for late or wet AMD (a condition that is strongly related to loss of vision), eating high glycemic index foods may also indirectly increase the risk for late or wet AMD.
High glycemic index foods contain simple carbohydrates, which get rapidly absorbed (in our body) and cause a rapid rise of blood sugar level. These foods have been linked to increasing body weight; however, their role in diabetes is controversial. Conversely, low glycemic index foods--which contain complex carbohydrates or other nutrients (such as proteins)--get digested and absorbed in our body slowly and raise the blood sugar level gradually. Some experts have recommended these foods to combat obesity and diabetes.

According to a document by the American Diabetes Association, “The glycemic index (GI) ranks carbohydrate foods based on how they affect the body's blood glucose levels. "High GI" foods such as corn flakes, instant potatoes, and white bread greatly affect blood glucose levels. "Low GI" foods such as oatmeal, most fruits and vegetables, legumes, and nuts produce less of an effect. Recently, some weight loss diets have popularized the concept of the glycemic index, linking low GI foods to weight loss and high GI foods to weight gain.”

The study by Kaushik and co-workers indicates that you’d stay protected against AMD, if you become fond of low glycemic index foods. Foods with a glycemic index of 70 and more are known as high glycemic index foods, and foods which have a glycemic index of 55 and less are called low glycemic index foods. The GI for steamed white rice is 98, white bread it’s 70, cornflakes has a GI of 84, for French fries the GI is 75, the GI for mashed and Jacket potatoes are respectively 70 and 85, puffed wheat has a GI of 89, for bran flakes the GI is 74, and for watermelon it’s 72. Low glycemic foods include noodles, which have a GI of 40, the GI  of sweet corn is 55, the GI for porridge (made with water) is 42, low-fat yogurt with sweetener has a GI of 14, the GI for whole and skimmed milk are 27 and 32 respectively, and for wholemeal and white spaghetti the GI are 37 and 41 respectively. Nutritional Supplements for AMD and Glycemic Index….

The findings of this study by Kaushik and co-workers emphasizes the importance of healthy eating habits and the use of nutritional supplements in the prevention and treatment of AMD.  Nutritional supplements including lutein, zeaxanthin, beta-carotene, vitamin E, Glutathione, zinc and selenium work as antioxidants in the retina and play an important role in protecting the macula against oxidative damage. Amid these nutrients, two major nutrients--lutein and zeaxanthin--which are naturally present in human retina--reduce the risk of AMD by scavenging free radicals and filtering the harmful blue light from the macula. Beta-carotene, a precursor of vitamin A, is a known antioxidant and has a protective role in AMD. The dietary sources for lutein, zeaxanthin and beta-carotene include carrots, squash, peaches, papaya, broccoli, spinach and peas.

The glycemic index for most of these foods are less than 55, for example the GI for Peas is 48, carrots have a GI of 49, the GI for both spinach and broccoli is 15, and for peaches and squash the GI are 42 and 15, respectively. The GI for papaya is only 58. Glutathione---another important antioxidant used for risk reduction and slowing the progression of AMD--can be found in foods such as broccoli (GI=15), spinach (GI=15), asparagus (GI =15), avocadoes (GI=15), squash (GI=15), cauliflower (GI=15), walnuts (GI=15), and others.

All these foods are low glycemic index foods, with a glycemic index far less than 55. Vitamin E is a powerful biological antioxidant used in the nutritional therapy for AMD. The dietary sources for this vitamin include tomato (GI=38), peanuts (GI=15), hazelnuts (GI=25), almonds (GI=15), avocadoes (GI=15), and carrot juice (GI=49). These foods too have a substantially low glycemic index. Zinc and selenium are important trace elements used in various nutritional supplements for AMD. Selenium incorporates with proteins and fights against cell damage (in retina) caused by free radicals, while zinc protects against AMD by inhibiting the oxidative reactions in the macula. Zinc may be obtained from various foods, such as plain low-fat yogurt (GI=14), cashews (GI=15), baked beans (GI=48), milk (GI=27), walnuts (GI=15) and almonds (GI=15). Selenium can be found in oatmeal (GI=15), walnut (GI=15), spaghetti (GI=37), noodles (GI=40), and eggs (GI=nearly 0). All these food sources have a substantially low glycemic index.

In Conclusion…

While high glycemic index foods increase the risk of developing AMD, various nutrients that help cut the risk and slow the progression of AMD are found in foods that have a low glycemic index. There are 3 key mechanisms by which various nutritional supplements advocated in AMD act—(1) working as scavengers for free radicals, (2)inhibiting oxidative damage to retina, and (3) filtering the harmful blue light from the retina.

However, the recent research by Kaushik and co-workers indicates that apart from these 3 mechanisms of action, nutritional supplements may also have a fourth mechanism for providing protection against AMD, thanks to a substantially low glycemic index of their natural food sources.