title: 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.
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).
  
  
  METHODS
  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 approximately 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.
authors.affiliation: Medical Research Council, Dunn Human Nutrition Unit, Cambridge, United Kingdom.
authors.forename: N M
authors.initials: NM
authors.lastname: McKeown
authors.forename: N E
authors.initials: NE
authors.lastname: Day
authors.forename: A A
authors.initials: AA
authors.lastname: Welch
authors.forename: S A
authors.initials: SA
authors.lastname: Runswick
authors.forename: R N
authors.initials: RN
authors.lastname: Luben
authors.forename: A A
authors.initials: AA
authors.lastname: Mulligan
authors.forename: A
authors.initials: A
authors.lastname: McTaggart
authors.forename: S A
authors.initials: SA
authors.lastname: Bingham
mesh_terms: D000368:Aged; D015415:Biomarkers / Q000097:blood / Q000652:urine; D015331:Cohort Studies; D015930:Diet Records; D004034:Diet Surveys; D002149:Energy Intake; D005260:Female; D005492:Folic Acid / Q000097:blood*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009584:Nitrogen / Q000652:urine*; D015596:Nutrition Assessment; D011188:Potassium / Q000652:urine*; D011446:Prospective Studies; D015203:Reproducibility of Results; D012650:Self Disclosure; D012964:Sodium / Q000652:urine*; D011795:Surveys and Questionnaires
chemical_list: D015415:Biomarkers; D005492:Folic Acid; D012964:Sodium; D009584:Nitrogen; D011188:Potassium