Dietary Links to AMD and Cataract

Within the next 2 decades, it's anticipated that the number of AMD and cataract cases will each increase by 50%. Prevention remains the best approach for addressing these public health issues, and dietary changes may represent one of the most cost-effective strategies.

Several dietary factors have been linked to AMD, including antioxidant vitamins and minerals, carotenoids such as lutein and beta-carotene, and long chain omega-3 fatty acids. Similarly, certain carotenoids and antioxidants like vitamin C have been associated with some types of cataract. Two new reports suggest that the type of carbohydrates consumed is also related to both visual conditions.

The glycemic index (GI) is a scale that ranks carbohydrate-rich foods by how much they raise blood glucose levels compared to a standard food, usually glucose or white bread. High GI foods include sweets, white rice and bread, processed cereals and French fries, while lower GI foods are generally fiber-rich such as beans, whole grain bread, brown rice and oatmeal.

Glycemic Index and Early AMD

The first study (1) included 1036 eyes from 526 participants in the Nurses' Healthy study without a previous diagnosis of AMD. The presence and degree of AMD were classified by the AREDS system, while dietary information was based on data from an average of 4 food-frequency questionnaires collected over a 10-year period before AMD was assessed. Odds ratios for AMD were estimated in a manner that accounted for the lack of independence between 2 eyes from the same subject.

After multivariate adjustment, GI was related to AMD, specifically to retinal pigment abnormalities. The total amount of carbohydrates consumed showed no relationship. The odds ratio for those in the highest third of GI values compared to the lowest third was 2.71. Neither GI nor total carbohydrate intake was related to drusen.

Commentary

This was the first study to evaluate the relationship between the quality or quantity of carbohydrate intake and AMD. The results showed that retinal pigment abnormalities characteristic of AMD are positively associated with long-term dietary GI, and independent of total carbohydrates and other known or suspected risk factors. The link between GI and AMD could be explained by several hyperglycemia-mediated pathways: glycation, oxidative stress, activation of protein kinase C, direct toxic effects of glucose on the retinal pigment epithelium and capillary endothelium, and associated inflammatory and angiogenic responses. Most notably, a high-GI diet has been proposed to play a role in exacerbating the pro-inflammatory process.

Glycemic Index and Cataract

In the second study (2), dietary information was obtained by food frequency questionnaire from 3,377 participants in the AREDS trial. Lens status was evaluated by using the AREDS System for Classifying Cataracts. Associations were examined for eyes with only a single, or pure, type of lens opacity by using the generalized estimating approach to logistic regression.

For those in the highest fourth of GI values, GI was associated with a higher prevalence of pure nuclear opacities (grade > 2 and odds ratio of 1.29), and moderate nuclear opacities (grade > 4 and odds ratio of 1.43). When comparing the highest vs. the lowest total intake of carbohydrates, the odds ratio was 1.27 for cortical opacities of any severity. In short, GI - or carbohydrate quality - was linked to nuclear cataract, while carbohydrate quantity was related to cortical type.

Commentary

How might higher dietary GI raise the risk of nuclear cataract? After consuming the same amount of high compared to low GI foods, it takes longer for blood glucose levels to return to baseline concentrations. It's possible that higher GI foods may damage the metabolically limited lens nuclear tissue by exposing the tissue to glucose for longer periods. The link between total carbohydrate intake and cortical cataract seen in this study has also been reported in the Blue Mountains Eye Study and the Nutrition and Vision Project.

References

1. Chiu CJ, et al. Dietary glycemic index and carbohydrate in relation to early age-related macular degeneration. AJCN 83:880-86, 2006
2. Chiu CJ, et al. Dietary carbohydrate intake and glycemic index in relation to cortical and nuclear lens opacities in the Age-Related Eye Disease Study. AJCN 83:1177-84, 2006.