Missing portion sizes in FFQ--alternatives to use of standard portions.
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.


METHODS
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).


METHODS
The Danish Health Examination Survey 2007-2008.


METHODS
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.
1The Research Unit for General Practice and Section of General Practice,Department of Public Health,University of Copenhagen,Øster Farimagsgade 5,1014 Copenhagen,Denmark.
Rasmus
R
Køster-Rasmussen
1The Research Unit for General Practice and Section of General Practice,Department of Public Health,University of Copenhagen,Øster Farimagsgade 5,1014 Copenhagen,Denmark.
Volkert
V
Siersma
3Faculty of Food Science and Nutrition,School of Health Sciences,University of Iceland,Reykjavik,Iceland.
Thorhallur I
TI
Halldorsson
1The Research Unit for General Practice and Section of General Practice,Department of Public Health,University of Copenhagen,Øster Farimagsgade 5,1014 Copenhagen,Denmark.
Niels
N
de Fine Olivarius
2Clinical Institute,University of Southern Denmark,Odense,Denmark.
Jan E
JE
Henriksen
6Institute of Preventive Medicine,Capital Region,Bispebjerg and Frederiksberg Hospital,Copenhagen,Denmark.
Berit L
BL
Heitmann
D003718:Denmark; D004032:Diet; D015930:Diet Records; D004034:Diet Surveys / Q000379:methods*; D002149:Energy Intake; D005247:Feeding Behavior; D005260:Female; D005502:Food; D006801:Humans; D008297:Male; D008875:Middle Aged; D010781:Photography; D064787:Portion Size; D012044:Regression Analysis; D011795:Surveys and Questionnaires
Bias; FFQ; Missing values; Multiple imputation; Portion sizes