Our data indicate that, in terms of characteristic foods (
Table 1 and
Fig.), the compositions of the two major dietary patterns are more diverse than those of the eight minor dietary patterns.
11 Unlike the two major dietary patterns, both of which were significantly associated with both early and advanced AMD, four of the eight minor dietary patterns were significantly associated only with advanced AMD, and the ORs are weaker than those for the two major dietary patterns (
Table 2). It seems that patterns that include more characteristic food items or groups are more strongly associated with risk for AMD. These observations allow finer interrogation of dietary patterns than prior findings.
Only two studies have used PCA to derive dietary patterns and related the patterns to AMD, including our previous study of the major dietary patterns in the AREDS
11 and the Australian Melbourne Collaborative Cohort Study (MCCS).
27 The MCCS identified six major factors (dietary patterns) with minimum eigenvalues of 2.7, which together accounted for 33% of the total dietary variance in the MCCS. In our study of the AREDS, there were only two major dietary patterns with eigenvalues greater than 2.7. These findings suggest that there were a larger number of major dietary patterns in the MCCS than in the AREDS.
Few studies have characterized American minor dietary patterns. In a study of participants aged ≥20 years from the third National Health and Nutrition Examination Survey (NHANES III),
28 two major dietary patterns (explaining 20% of total dietary variance) and four minor dietary patterns (explaining 17% of total dietary variance) with eigenvalues >1.25 were identified. Despite the difference in age between the NHANES III and the AREDS cohorts, the two NHANES III major dietary patterns, named Western and American-healthy patterns, were similar to the two major dietary patterns that we named Western and Oriental patterns in the AREDS.
11 Differences between the studies include higher intakes of tea, potatoes, and salad dressing in the NHANES III American-healthy pattern compared with the AREDS Oriental pattern.
In this study, we used the combination of PCA, logistic analysis, and QCA to visually represent our findings (
Fig.) and to discover previously unidentified foods that may be associated with AMD risk in the context of the overall diet. For example, it is somewhat surprising that the peanut pattern guarded against advanced AMD because the pattern was heavily loaded with sweets, snacks, and high-fat dairy products, which are generally considered detrimental to human health. The peanut pattern was, however, most heavily loaded with peanuts, and although it has been suggested that higher peanut intake is beneficial against CVD,
29,30 no study has reported the association between peanut intake and AMD risk. Studies to investigate the postulated mechanisms for the beneficial effects of higher peanut intake, such as decreasing oxidative stress,
31–33 inhibition of formation of advanced glycation/lipoxidation end products due to their high levels of arginine,
31–38 inhibition of inflammation,
33–35 and improving endothelial function,
33,36–38 are in progress and may be helpful in developing novel therapeutic and prevention strategies against AMD.
Data from our four minor dietary patterns showing no significant associations with AMD are also informative and suggest a need for additional studies into relations between food intake and risk for AMD. For example, the results from the salad pattern implied that a diet higher in green leafy vegetables and tomatoes alone may be insufficient to provide optimal protection against AMD and that salad dressing may be detrimental to the macula. Other foods of interest include pizza and coffee or tea, etc. The results from the alcohol pattern analysis corroborated prior studies that suggested that it may be preferable to evaluate the individual effects of beer, liquor, and wine on disease risk.
39
The strengths of this study include use of the well-characterized AREDS cohort, standardized collection of risk factor information and photographic grading of maculopathy, as well as increased power by using eyes as the unit in our analysis. Recall and selection bias in the AREDS were unlikely to explain our findings, because exposure information was collected before outcome evaluation and our retinal classifications were performed in an independent center, by graders masked to our nutrition data. The cross-sectional nature of this study limits its strength in defining causality and our ability to make dietary recommendations. Although in the present study we evaluated diet as a whole and included all known nondietary confounders in our analysis, residual confounding could still be a concern because dietary patterns may be simply a component of lifestyle in general, which is responsible for the underlying relationship.
In summary, our data suggest that a diet consisting of various healthy foods, including previously unidentified beneficial foods, may be optimal for reducing AMD risk and that the effects of combinations of specific foods in the overall diet warrant further study.