Objective: Standard portions or substitution of missing portion sizes with medians may generate bias when quantifying the dietary intake from FFQ. The present study compared four different methods to include portion sizes in FFQ.
Design: We evaluated three stochastic methods for imputation of portion sizes
based on information about anthropometry, sex, physical activity and age. Energy
intakes computed with standard portion sizes, defined as sex-specific medians
(median), or with portion sizes estimated with multinomial logistic regression
(MLR), ‘comparable categories’ (Coca) or k-nearest neighbours (KNN) were
compared with a reference based on self-reported portion sizes (quantified by a
photographic food atlas embedded in the FFQ).
Setting: The Danish Health Examination Survey 2007–2008.
Subjects: The study included 3728 adults with complete portion size data.
Results: Compared with the reference, the root-mean-square errors of the mean
daily total energy intake (in kJ) computed with portion sizes estimated by the four
methods were (men; women): median (1118; 1061), MLR (1060; 1051), Coca
(1230; 1146), KNN (1281; 1181). The equivalent biases (mean error) were (in kJ):
median (579; 469), MLR (248; 178), Coca (234; 188), KNN (−340; 218).
Conclusions: The methods MLR and Coca provided the best agreement with the
reference. The stochastic methods allowed for estimation of meaningful portion
sizes by conditioning on information about physiology and they were suitable for
multiple imputation. We propose to use MLR or Coca to substitute missing portion size values or when portion sizes needs to be included in FFQ without portion size data.