Validity of a semi-quantitative food frequency questionnaire for estimating dietary intake of antioxidant vitamins and fatty acids

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To evaluate the validity of a semi-quantitative food frequency questionnaire (FFQ) which we developed for and used in a large scale prospective study, we compared the estimated intake level of antioxidant vitamins (e.g. alpha- and beta-carotene, vitamin C) and fatty acids (e.g. EPA, DHA) by FFQ with those by 4 time 7-day dietary records (DR). Serum antioxidant vitamins and fatty acids were also measured. The correlation coefficients between FFQ and DR for carotene and vitamin C intake were both 0.44 in men, and 0.39 and 0.23 in women, respectively, after adjusting for energy intake. Those values for EPA and DHA were 0.47 and 0.46 in men and 0.52 and 0.47 in women, after adjusting for total fatty acid intake. The correlation coefficient between FFQ with serum level of beta-, alpha-carotene, and vitamin C were -0.02, 0.21, and-0.13, respectively, and those with serum composition of EPA and DHA were 0.47 and 0.41, respectively. We conclude that the estimates of carotene, vitamin C and fatty acids by our FFQ are reasonable to estimate usual dietary intake level of each nutrient in large scale epidemiological study.