Early life events and later risk of colorectal cancer:: age-period-cohort modelling in the Nordic countries and Estonia

Background: A lowering of colorectal cancer risk for the birth cohorts born around World War II (WWII) has previously been observed in Norway, a country which suffered some 20% caloric restriction during the war. The purpose of the study was to conduct a similar kind of analysis in the other Nordic countries and Estonia, which were also subjected to various degrees of energy restriction during WWII. Methods: All new cases of colorectal cancer in the Nordic countries and Estonia diagnosed between 40 and 84 years of age and born between 1874 and 1953, were collected from the national cancer registries. The incidence data were fitted to an age-period-cohort model. Results: A transient drop in the estimated colorectal cancer incidence rate was observed for the birth cohorts born around WWII in Estonia, together with a tendency of decreased risk in Sweden and Denmark. Conclusion: The previously observed lowering of colorectal cancer risk for persons born during WWII in Norway also prevails in Estonia. Energy restriction is a possible explanation for these findings, since the countries suffered from varying nutritional conditions during the war. Exogenous factors acting during periods early in life may have an impact on later colorectal cancer risk.