To fully address the role of lifestyle in cancer etiology and prevention, we must understand when in the course of disease exposure is most important. Examples from breast cancer with the follow-up of survivors from the atomic bomb in Japan attest to the value of cohort studies to define the exposure–disease relation in the time course of life and etiology [27
]. Findings such as this also point to the need for studies to address lifestyle during childhood and adolescence in relation to breast cancer risk [28
]. Cohorts addressing diet, growth, and cancer risk work to fill this gap in knowledge [29
]. Follow-up of childhood cancer survivors offers one important example of such cohort findings relating to clinical practice guidelines [31
]. On the other hand, cohorts have been able to supplement existing data resources to fill in gaps in exposure over the life course. For example, the validated measures of self-reported body shape and adiposity [32
] added to several cohorts attest to the value of these measures in relation to cancer risk [33
]. Likewise, we added recall of high school diet to the Nurses’ Health Study and then Nurses’ Health Study II [34
] and a more detailed assessment of physical activity [35
] to further refine our ability to address adolescent exposure and breast cancer risk [36
]. Validation shows these measures can be included with adequate performance to detect important relations. The California Teachers Study took this approach from its beginning with assessment of key exposures over the life course and relating these exposures to risk of cancers [38
Clearly in the setting of established data collection, storage, and participant follow-up, the addition of data collection strategies to fill in periods of the life course that may be particularly important offers a valuable and cost-efficient strategy to uncover details of exposure–disease relations.
Understanding temporal details within exposure–disease relations is essential to inform the timing and population characteristics for prevention strategies. Cohort studies have developed and validated measures of intermediate endpoints such as colon polyps and benign breast lesions, adding these endpoints to the existing cancer endpoints that were of primary importance for initiating the cohort studies [41
]. With these endpoints, we can evaluate the timing of diet and activity in relation to precursor lesions, progression to invasive disease, and progression from invasive disease to recurrence and mortality.
Another approach to filling gaps in knowledge has been the establishment of additional cohorts to address specific populations. As noted above, these have often been defined by race or ethnicity or occupation. Another example relates to adolescent exposure and the creation of the Growing Up Today Study. Initial attempts to expand our assessment of diet, physical activity, and growth in relation to subsequent cancer risk were turned down as not innovative or lacking validated measures of diet. With non-federal funds, I led the creation and validation of the adolescent diet assessment tool [42
] prior to being awarded funds from NICHD to study diet, adiposity, and excess weight gain for gain in height among adolescent children of participants in the Nurses’ Health Study II [45
]. Again with the mother already participating in the study, additional data sources were available (and tracking of participant’s long term) at little marginal cost.
Based on these examples, I conclude we should not abandon existing cohorts to reallocate funds to the creation of a new final cohort.