A Prospective Study Evaluates the Relationship Between Regular Antioxidant Supplement

A prospective study evaluates the relationship between regular antioxidant supplement use (vitamins C and E) and the risk of stroke in healthy and physically active men age 40-60.  The study compares the risk of stroke among men who consumed antioxidant supplements for ≥5 years and among men who consumed antioxidant supplements for <5 years, as compared to a reference group of men who never consumed antioxidant supplements.  According to the study results, men who consumed antioxidant supplements for <5 years and men who consumed antioxidant supplements for ≥5 years have stroke relative risks of 0.95 (p = 0.45) and 0.75 (p < 0.01) , respectively, when compared to the reference group.  The results of the study were adjusted to account for baseline differences related to healthy behaviors and overall health.  Which of the following factors most likely explains why the relative risk of stroke is lower with longer antioxidant use?

A) Accumulation effect
B) Lead-time bias
C) Observer bias
D) Rare disease assumption
E) Selection bias

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