Objective:
To evaluate modifiable risk and protective factors for age-related macular degeneration (AMD) among elderly twins.

Methods:
The US Twin Study of Age-Related Macular Degeneration comprises elderly male twins from the National Academy of Sciences-National Research Council World War II Veteran Twin Registry.
To determine genetic and environmental risk factors for AMD, twins were surveyed for a prior diagnosis of AMD and underwent an eye examination, fundus photography, and food frequency and risk factor questionnaires.
This environmental component of the study includes 681 twins: 222 twins with AMD (intermediate or late stages) and 459 twins with no maculopathy or early signs.

Risk for AMD according to cigarette smoking and dietary fat intake was estimated using logistic regression analyses.

Results:
Current smokers had a 1.9-fold increased risk (95% confidence interval, 0.99-3.68, P = .06) of AMD while past smokers had about a 1.7-fold increased risk (95% confidence interval, 1.2-2.6, P = .009). Increased intake of fish reduced risk of AMD, particularly for 2 or more servings per week (P trend = .04).

Dietary omega-3 fatty intake was inversely associated with AMD (odds ratio, 0.55; 95% confidence interval, 0.32-0.95) comparing the highest vs lowest quartile.

Reduction in risk of AMD with higher intake of omega-3 fatty acids was seen primarily among subjects with low levels (below median) of linoleic acid intake, an omega-6 fatty acid (P trend<.001).
The attributable risk percentage was 32% for smoking and the preventive fraction was 22% for higher omega-3 intake.

Conclusions:
This study of twins provides further evidence that cigarette smoking increases risk while fish consumption and omega-3 fatty acid intake reduce risk of AMD.