Age and risk factors for colon cancer (United States and Australia): Are there implications for understanding differences in case-control and cohort studies?

Data from two population-based case-control studies were used to investigate the effect of age on colon cancer risk. Dietary intake data were assessed from a study conducted in Utah (United States) between 1979 and 1983; reproductive data were assessed from a study conducted in Adelaide (Australia) between 1979 and 1980. Data from both studies were assessed for their impact on those less than 65 years of age and those 65 or more years of age. Intake of energy, fat, and protein had a greater impact on risk among older men than among younger men. Risk estimates for the upper quartile of intake relative to the lowest quartile of intake were 8.5 (95 percent confidence interval [CI]=1.7–43.0) for energy, 8.2 (CI=1.6–41.3) for protein, and 7.2 (CI=1.6–31.4) for total fat for older men, while comparable risk estimates were 2.4 (CI=0.6–9.1) for energy, 3.0 (CI=0.7–13.6) for protein, and 1.9 (CI=0.5–7.1) for total fat among younger men. Similar trends were seen for older women for energy and protein. β-carotene decreased colon cancer risk among younger men (odds ratio [OR]=0.4, CI=0.1–1.2) and women (OR=0.1, CI=0.1–0.5), although not among older men (OR=1.2, CI=0.3–4.9) and women (OR=1.9, CI=0.6–64). Calcium decreased risk of colon cancer among older men (OR=0.1, CI=<0.1–0.8) and younger women (OR=0.2, CI=<0.1–0.7). Women who were diagnosed at age 65 or older and were nulliparous had a tenfold increase in colon cancer risk (CI=2.4–47.9) relative to women who had an early age at first birth. Women diagnosed with colon cancer before age 65 did not experience an increase of colon cancer risk associated with being nulliparous. These data suggest that age at diagnosis may interact with other factors to alter risk of colon cancer.