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

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Abstract
Background: The validity of dietary assessment methods should be established before diet-disease associations are reported. Objective: Our objective was to validate a 7-d food diary and a food-frequency 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 from the European Prospective Investigation into Cancer UK Norfolk cohort, a free-living cohort of ≈25000 persons. Over a 9-mo period, urinary nitrogen, potassium, and sodium were estimated from 2–6 complete 24-h urine collections in 134 subjects and plasma ascorbic acid was estimated from 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 nitrogen excretion and dietary intake from the 7-d food diary were high (r = 0.57–0.67) compared with those for the FFQ (r = 0.21–0.29). Similarly, correlations 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 no overall difference in correlations between plasma ascorbic acid and dietary 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, the 7-d food diary provided a better estimate of nitrogen and potassium intakes than did the FFQ in this study population. However, with respect to plasma ascorbic acid, both the FFQ and 7-d food diary provided a similar ranking of subjects according to vitamin C intake.

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