Time Trends within Countries

The analysis of cancer trends over time can lead to useful findings in the study of diet and cancer. By looking at the change in cancer rates in a specific population over time and comparing these rates with changes in specific factors over the same period (e.g., changes in dietary habits), investigators can uncover possible associations supporting the dietary factors hypotheses. For example, researchers have examined vital statistics for Japanese natives and US whites to unveil changes in cancer mortality and related antecedent patterns of lifestyle in the two populations. These investigations have uncovered that animal fat consumption in Japan steadily increased from a daily level of 6.5 g per person in 1955 to 27.6 g in 1987; at the same time the Japanese rate of colon cancer in men rose at a rapid pace; in fact, the mortality rates owing to colon cancer in men almost trebled over this time. This evidence lends more support to the hypothesis that mortality from colon cancer in men is influenced by high dietary fat consumption.

Similar data were collected in Singapore to determine trends in the incidence of breast cancer: in 1996 an average annual increase in breast cancer incidence of 3.6% over a 25-year period for all women was reported. The most convincing evidence that the observed trend was real was that it was clearly cohort-related rather than period-related. The risk was observed to increase in successive birth cohorts from the 1890s to the 1960s. Changes in dietary consumption patterns (e.g., the adoption of a more 'Western' diet) fall among other factors cited as having a possible effect on the continuing increase in rates of breast cancer among women in Singapore. Like descriptive studies, time-trend studies are a valuable source for hypotheses generation, but more definitive evidence is required from analytical epidemiology to uncover any real associations between dietary factors and cancer rates.

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