As Food Frequency Questionnaires (FFQs) are subject to measurement error, associations between self-reported intake by FFQ and outcome measures should be adjusted using correction factors obtained from a validation study. Whether the correction is adequate depends on the characteristics of the reference method used in the validation study. Preferably, reference methods should 1) be unbiased and 2) have errors uncorrelated with those in the FFQ. We aimed to assess the validity of the duplicate portion (DP) technique as a reference method for protein and potassium intake and compare its validity with that of a commonly used reference method, the 24 hour recall (24hR) using urinary biomarkers as the unbiased reference method.
For 198 Dutch subjects, two DPs, two FFQs, two urinary biomarkers and between one and fifteen 24hRs (web-based and/or telephone-based) were collected within 1.5 years. Multivariate measurement error models were used to estimate proportional scaling bias, error correlations between FFQ and DP or 24hR and attenuation factors of these methods.
The DP was less influenced by proportional scaling bias (0.58 for protein and 0.72 for K) and correlated errors between DP and FFQ were lowest (protein 0.28 and K 0.17) compared to the 24hRs. Attenuation factors (protein 0.74 and K 0.54) also indicated that the DP performed better than the 24hRs.
Therefore the DP is probably the best available reference method for FFQ validation, especially for (micro)nutrients for which information from food composition databases is of questionable validity and that currently have no generally accepted recovery biomarker.