Use of biological markers to validate self-reported dietary intake in a random sample of the European Prospective Investigation into Cancer United Kingdom Norfolk cohort

Background: The validity of dietary assessment methods should be establishe d before diet-disease associations are reported. Objective: Our objective was to validate a 7-d food diary and a food-freque ncy questionnaire (FFQ) against independent biomarkers of intake in urine ( nitrogen. potassium. and sodium) and blood (plasma ascorbic acid). Design: A total of 146 healthy middle-aged men and women were recruited fro m the European Prospective Investigation into Cancer UK Norfolk cohort. a f ree-living cohort of approximate to 25000 persons. Over a 9-mo period. urin ary nitrogen, potassium. and sodium were estimated from 2-6 complete 24-h u rine collections in 134 subjects and plasma ascorbic acid was estimated fro m 2-3 fasting blood samples in 118 subjects. Subjects completed 2 FFQs and two 7-d food diaries. Results: In men and women combined, correlations between 24-h urinary nitro gen excretion and dietary intake from the 7-d food diary were high (r = 0.5 7-0.67) compared with those for the FFQ (r = 0.21-0.29). Similarly, correla tions between urinary potassium and dietary potassium were higher for the 7 -d food diary (r = 0.51-0.55) than for the FFQ (r = 0.32-0.34). There was n o overall difference in correlations between plasma ascorbic acid and dieta ry vitamin C between the 7-d food diary (r = 0.40-0.52) and the FFQ (r = 0. 44-0.45). Conclusions: These data indicate that. despite increased subject burden, th e 7-d food diary provided a better estimate of nitrogen and potassium intak es than did the FFQ in this study population. However, with respect to plas ma ascorbic acid. both the FFQ and 7-d food diary provided a similar rankin g of subjects according to vitamin C intake.