Lifestyle factors and colorectal cancer: The Norwegian Women and Cancer Study

Abstract

Colorectal cancer (CRC) is a major global disease. The incidence rate among Norwegian women is currently the highest in the world. Lifestyle factors have a substantial influence on CRC susceptibility. However, it is not clear whether these factors are responsible for the high incidence in Norwegian women, or whether they play a role in CRC survival. This doctoral project investigated lifestyle factors in relation to CRC incidence and survival.

We used self-reported information from the Norwegian Women and Cancer (NOWAC) Study, linked with Cancer Registry of Norway and Statistics Norway. We used Cox proportional hazards models to calculate hazard ratios for CRC risk by physical activity levels. We used the Karlson, Holm, and Breen method of decomposition to examine the extent to which the risk factors accounted for the observed geographical differences in CRC incidence. We performed competing mortality risks analyses to determine the associations between pre-diagnostic lifestyle factors and CRC survival.

We found no association between physical activity level and the risk of CRC. Adult height, being a former smoker, or a current smoker, were associated with increased CRC risk; and a duration of education of >12 years, and a fruit and vegetable intake of >300 g/day were associated with reduced CRC risk. However, these factors combined, did not account for the geographical variations in CRC incidence. Finally, we found that a pre-diagnostic vitamin D intake of >10 μg/day was associated with 25% reduction in CRC death.

Our data suggest that women may need to look further than physical activity in order to reduce their risk of CRC; and lifestyle factors did not explain geographical variations in CRC incidence in Norwegian women. A pre-diagnostic vitamin D intake could improve CRC survival.