Could Exposure Assessment Problems Give Us Wrong Answers to Nutrition and Cancer Questions?

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Whether fruits and vegetables provide protection from cancer is of considerable public health importance—but it remains an open question. In this issue of the Journal, Hung et al. report findings from two cohort studies: fruit and vegetable intake is associated with a “modest” reduction in risk of noncommunicable (“chronic”) disease ( 1 ) . This reduction is confined to cardiovascular disease. The association for cancer is null.

Most evidence for a protective role of fruits and vegetables against cancer has comes from case–control studies. The authors rightly note the possibility of recall bias in such studies. Indeed, prospective cohort studies have tended to demonstrate weak or no associations between fruit and vegetable intake and malignant disease, including large-bowel cancer ( 2 ) .

So the question arises: does total fruit and vegetable intake really confer little or no cancer protection? (In this editorial we address only the relation of total fruit and vegetable intake to all cancer. However, similar issues can be raised for associations between individual fruits or vegetables and cancers at specific sites.) Hung et al. consider—and largely dismiss—the possibility that problems with exposure assessment have caused their study to give the wrong answer to this question. However, more consideration of this possibility is in order.