Antioxidants and cancer: what is the evidence?

The validity of questionnaire-based micronutrient intake estimates is increased by including dietary supplement use in Swedish men

Although many validation studies of FFQ have been reported, only a few addressed the effect of inclusion of dietary supplement use on the validity of micronutrient estimates. The purpose of the present study was to assess reproducibility and validity of a self-administered FFQ, intended to measure total nutrient intake from diet and from dietary supplements, in a random population-based sample of 248 middle-aged and elderly men (40-74 y old) in central Sweden. Fourteen 24-h recall interviews were used as the reference method. Participants were telephoned about once a month for 1 y, covering every day of the week, twice in random order. Spearman correlation coefficients increased 13% (from 0.49 to 0.62), between FFQ-based micronutrient estimates and the fourteen 24-h recall interviews (reference method) when supplement use was included. Correct classification into highest quintiles was also improved by 14% (from 37 to 51%). There were no differences in mean intraclass correlation coefficients for 1-y reproducibility between total micronutrient intake and micronutrients from foods only. The increase in the validity of micronutrient estimates due to inclusion of supplements in nutrient intake assessment has important implications for sample size in epidemiologic studies. Because a dramatic increase in dietary supplement use has occurred in recent decades in many countries, the issue of including supplements in dietary assessment should be of highest priority in nutritional epidemiologic studies of chronic diseases.