Does sunlight prevent cancer? A systematic review.

Accumulating evidence for beneficial effects of sunlight on several types of cancer with a high mortality rate makes it necessary to reconsider the health recommendations on sun exposure, which are now mainly based on the increased risks for skin cancer. We reviewed all published studies concerning sun exposure and cancer, excluding skin cancer. All selected studies on prostate (3 ecologic, 3 case-control and 2 cohort), breast (4 ecologic, 1 case-control and 2 cohort) and ovary cancer (2 ecologic and 1 case-control) showed a significantly inverse correlation between sunlight and mortality or incidence. Two ecologic, 1 case-control and 2 prospective studies showed an inverse relation between sunlight and colon cancer mortality; 1 case-control study found no such association. Ecologic studies on non-Hodgkin lymphoma (NHL) mortality and sunlight gave conflicting results: early studies showing mostly positive and later studies showing mostly negative correlations. Three case-control studies and 1 cohort study found a significant inverse association between the incidence of NHL and sunlight. The question of how to apply these findings to (public) health recommendations is discussed.