Descriptive epidemiology, particularly regarding the
cancer pattern in Japanese and Koreans in the United States, indicates that lifestyle factors contribute substantially to the development of common
cancers such as gastric, colorectal, breast and
prostate cancers. Sex and age are important determinants of many
cancers, and the variation in
cancer incidence according to these factors is also indicative of the role of environmental factors. While
cancer of first-degree relatives or parental
cancer was related to an approximately 2-fold increased risk for most site-specific
cancers, a large Scandinavian twin study suggested that the contribution of genetic factors was generally small and that a statistically significant effect of hereditable factors was observed only for prostate, colorectal and breast
cancers. It was roughly estimated in this article that infectious agents contributed to 20% of incident cases of
cancer in Japan. In a recent cohort study in Japan, it is estimated that 29% of male
cancers and 3% of female
cancers can be ascribed to smoking. Among other lifestyle factors, alcohol consumption and
obesity have provided convincing evidence as factors conferring increased risks of various
cancers. The increased risks of
colorectal cancer and
breast cancer associated with alcohol drinking have been recently acknowledged internationally. Among dietary factors, red meat,
aflatoxin and
beta-carotene are considered to increase risks of colorectal, liver and
lung cancers, respectively. Vegetables and fruits probably decrease the risk of
cancer at various sites, and
calcium specifically decreases the risk of
colorectal cancer. Evidence for increased risk of
gastric cancer associated with salted foods is judged to be not sufficient, although a high-
salt diet enhanced
gastric cancer in animals infected with Helicobacter pylori. The role of dietary factors in
cancer development will be more clearly established by research on gene-environment interaction focusing on functional genetic polymorphisms.