Measurement error of dietary exposure was a major concern in the current study, as it is in all large cohort studies of diet and chronic disease.
Although we used a widely validated questionnaire, measurement error of dietary intakes, for example, self-reports of long-term dietary supplements, could introduce misclassification in exposure status, but it is likely to be nondifferential.
We also explored whether Hispanic/Latino background (Central American, Cuban, Dominican, Mexican, Puerto Rican, and South American), in addition to other participant characteristics, influences the measurement error of dietary self-report.
Assessing objective recovery biomarkers such as doubly labeled water (DLW) and urinary nitrogen in combination with participant characteristics such as age, body mass index (weight (kg)/height (m)), and background (2) helps to account for the systematic and random measurement error of dietary self-report.
To minimize the impact of random measurement errors of dietary assessment and to better represent long-term diet, we calculated the cumulative averages of nutrient intakes from baseline to the censoring events.
In the context of systematic error, differential reporting of dietary intake and physical activity by weight gain status could distort the interrelation among socioeconomic position, potential mediators and weight gain.
Overall, measurement error in assessment of dietary cadmium would be nondifferential in this prospective cohort study and could have introduced substantial bias toward a finding of no association (Freedman et al. 2011; Kipnis et al. 2003).
Furthermore, the errors of the dietary intake measurements from FFQs and serum biomarkers are independent, so it is feasible to evaluate the correlation of an FFQ in assessing EPA, DHA and other fatty acid intake in a clinical setting.
In addition, our results may be influenced by measurement errors of the dietary questionnaires that may have attenuated our findings, although country-specific validated questionnaires for some flavonoid-rich foods, such as fruits, vegetables, tea, and wine (17, 18), were used.
Random errors in reporting of dietary exposures will also have occurred and may have led to an underestimate of the true associations.
However, there are several limitations in the present study including some measurement error of long-term dietary intake by our FFQ (Ishihara et al, 2003), residual confounding effect that we failed to adjust and influence of improvements in CRC diagnostics on CRC incidence (Holford, 1991; Dubrow et al, 1993; Huang et al, 1999; Minami et al, 2006).
