fairly strong protective effects were found with higher levels in two large prospective studies, and relative risk was decreased by more than half in the groups with the highest carotene levels.
In the prospective Nurse’s Health Study, risk of breast cancer was 25–35% lower for women in the highest quintiles of plasma carotenoids, versus the lowest quintiles .
however, problems inherent with use of food frequency questionnaires
and the question remains whether the range of intakes
within a cohort is wide enough to detect a protective