New Ideas About Food and AMD

Summarized by Robert W. Griffith, MD
June 5, 2006

Summary

What type of carbohydrate you eat may affect your risk of getting age-related macular degeneration (AMD), the leading cause of blindness in the USA.

Introduction

Age-related macular degeneration (AMD) is the leading cause of blindness in people over 50 in developed countries. For the last 15 years there have been reports of dietary influence on the cause and course of the disease. To date, the evidence has favored diets high in antioxidant nutrients (vitamins C and E, carotenoids like lutein and zeaxanthine, fruits and vegetables containing these, and zinc). A high intake of fish or omega-3 fatty acids is also associated with lower rates of the disease. Now a study of the possible effects of a high glycemic index diet has led to an editorial in the American Journal of Clinical Nutrition which we summarize here.

The glycemic index study

The existence of diabetes or a raised blood sugar has not been associated, to date, with the occurrence of AMD. The researchers therefore decided to investigate the quality of the dietary carbohydrate intake in AMD patients. They examined the eyes of over 500 participants in the Nurses' Health Study who had not had AMD diagnosed. The presence and degree of any AMD, along with the presence of any drusen², was classified as either (a) Unaffected, (b) Without AMD but with drusen, or (c) AMD, with or without drusen.

Dietary information came from food-frequency questionnaires collected over 10 years before the eye exams. The women were divided into three groups, according to their dietary glycemic index³ or their total carbohydrate intake. The dietary glycemic index was found to be related to AMD, while the total carbohydrate wasn't. The likelihood of developing AMD was over 2½ times greater in the highest glycemic index group than in the lowest. Neither the glycemic index nor total carbohydrate intake was linked to the presence of drusen.

How relevant are the findings?

As the editorialist says, "the biological plausibility that elevations of blood glucose promote AMD, particularly in the absence of diabetes, remains untested." It's possible that a low glycemic index intake is merely a 'marker' for other aspects of the diet that might be protective for AMD: plenty of fruit, vegetables, whole grains, legumes, and milk, and few refined grains and sugars. And people who eat such a healthy diet may also avoid too much saturated fat, itself a 'promoter' of AMD.

Diagnosing early AMD

The study¹ described above also found that the glycemic index was related to one of the two major earlier signs of AMD - the presence of pigment changes - but not the other (the development of drusen). Notably, zinc intake and dietary lutein and zeaxanthine intake have been linked to pigment changes, while low intake of vitamin E, fruit and vegetables has been associated with the occurrence of large drusen.

It therefore seems likely that we must try to counteract AMD by addressing a number of dietary factors, not just one, or one class of nutrients. One investigator has found that diets high in several antioxidant nutrients lower the risk of AMD more than diets high in just one. To this we must add the need to try to lower the glycemic index of the carbohydrates we eat - a healthy step for many conditions, not just for preventing AMD.

Source

• Diet and age-related macular degeneration: expanding our views. Editorial. JA. Mares, SM. Moeller, Am J Clin Nutr, 2006, vol. 83, pp. 733--734

Footnotes
1. Dietary glycemic index and carbohydrate in relation to early age-related macular degeneration. C-J. Chiu, LD. Hubbard, J. Armstrong,  et al., Am J Clin Nutr, 2006, vol. 83, pp. 880--886
2. Drusen look like specks of yellowish material under the retina. A few small drusen normally form in the human eye, usually after age 40. AMD, in contrast, is almost always associated with a build-up of additional drusen. They occur in two forms: hard drusen are small, solid deposits that apparently do no harm when present in small number; soft drusen are larger with indistinct borders.
3. The glycemic index for a food is defined as the glycemic response (i.e. the amount of glucose released into the blood) after eating a fixed amount of the food, compared to the response after eating a standard food. Low glycemic index carbohydrates produce only small fluctuations in the blood glucose and insulin levels.

Related Links
Age-Related Macular Degeneration (AMD) - An Overview
Lowering Your Risk of AMD
A Test of Risk for AMD?

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