(22-10-09) Low Glycemic Index Diet and High DHA Intakes Slow Progress of AMD

The Age-Related Eye Disease Study (AREDS), a randomized, placebo-controlled intervention trial in the U.S., showed that the consumption of high-dose antioxidant vitamins and zinc reduces the chance of developing age-related macular degeneration (AMD) and its progression from early to advanced disease.
A continuation of this trial is currently examining whether the addition of
lutein, zeaxanthin and long-chain omega-3 polyunsaturated fatty acids (n-3 LC-PUFAs) to the supplement will further reduce the chance of AMD. An initial report from AREDS indicated that n-3 LC-PUFAs were associated with a significant reduction in the progression of AMD from bilateral drusen to central geographic atrophy.

In this report based on data from AREDS, investigators examined the
relationships of different antioxidant supplements, n-3 LC-PUFAs, lutein and zeaxanthin and the glycemic index?the extent to which carbohydrates raise blood glucose levels after eating?on the rate of progression of AMD in the AREDS participants over 8 years. The glycemic index for each participant?s diet was estimated from the amount of carbohydrate consumed and the weighted average of
each glycemic index value for each food item. Dietary intakes were assessed at baseline.

The assessment of AMD was based on the number of eyes rather than the number of participants. Participants were excluded from the study if they had diabetes, invalid energy intakes, missing data, advanced AMD or were lost to followup. There were 2,924 eligible participants and 5,146 eyes (72 participants had only one functioning eye). Early AMD was defined has having no or intermediate drusen, while late AMD included those with large drusen.

For some nutrients, there were significant interactions with the particular AREDS intervention. These were observed for docosahexaenoic acid (DHA) consumption and the progression of early, but not late, AMD and for low glycemic index foods and progression to late AMD (Table). These interactions
were not observed for the consumption of eicosapentaenoic acid (EPA) and beta-carotene. The investigators also reported a greater reduction in the risk of advanced AMD with higher intakes of DHA and EPA in combination with low glycemic index foods. This synergistic effect of both dietary variables was also associated with an approximately 50% lower risk of progression to advanced
AMD compared with low intakes of these fatty acids and a greater intake of high glycemic index foods.

For progression of early AMD, higher intakes of DHA were associated with significantly less progression in participants consuming the placebo
supplement, but not in any other treatment group (antioxidants, zinc and both).
The significant interaction between DHA consumption and the various dietary interventions indicates that the influence of DHA was affected by other dietary variables as well. For progression to advanced AMD, higher intakes of DHA were associated with less progression in the placebo group, with a tendency for
slower progression in participants consuming the antioxidant supplement (P =
0.06). Higher intakes of EPA were associated with slower progression to
advanced AMD in the antioxidant group and with a lower glycemic index in the zinc group and antioxidants plus zinc group.

The authors commented that high intakes of beta-carotene might actually increase the chance of progression to advanced AMD, at least in participants consuming the antioxidants plus zinc. However, this tendency was only marginally significant (P = 0.06), although the antioxidants and daily multi-vitamin supplement provided high levels of beta-carotene. The potential hazard of higher intakes of beta-carotene on lung cancer among smokers is a reminder that nutrient supplements can sometimes cut two ways.

The association of a high glycemic index diet with an increased chance of developing advanced AMD adds another variable to the dietary characteristics affecting AMD and its progression. The investigators discussed the ?antagonistic? interaction between DHA and the AREDS supplement in the progression of early AMD. However, data to support this assertion were reported
only for participants taking the antioxidants and zinc where the trend was not statistically significant, unlike the overall interaction for all participants.
The authors occasionally treated marginally significant associations, e.g. P = 0.07 as though they were statistically significant. Overall, the study
confirmed the value of higher intakes of DHA in slowing the progression of early and advanced AMD, although the particular supplement affected these relationships. The placebo and AREDS supplement groups had the greatest reduction in the chance of advanced AMD progression with higher intakes of DHA.

Chiu CJ, Klein R, Milton RC, Gensler G, Taylor A. Does eating particular
diets alter risk of age-related macular degeneration in users of the age-
related eye disease study supplements?

Source: Br J Ophthalmol 2009;93:1241-1246.

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