BACKGROUND: Physical activity appears to be inversely related to cancer risk, although the evidence is convincing for colon cancer only. As physical activity levels are difficult to measure in the general population, we aimed to investigate how physical activity influences cancer risk using a cohort of Norwegian world class athletes. METHODS: The cohort includes 3,428 athletes. Individual questionnaires gave information about physical activity and lifestyle variables until attained age. To elucidate the level of cancer risk, groups of athletes were compared to the general population (external comparisons) and to each other (internal comparisons). RESULTS: A slightly reduced risk of total cancer was observed in the cohort of athletes, but stronger effects were observed for subgroups. The risk reduction was most pronounced for lung cancer and for leukemia/lymphoma. In women, a threefold increased risk of thyroid cancer was observed. CONCLUSION: This cohort of athletes seems to have a reduced risk of cancer. The beneficial association, however, is weak and may be attributed to healthy lifestyle as well as to physical activity. Prolonged strenuous exercise may also increase the risk of thyroid cancer in women.
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