Adjusting for energy intake in dietary pattern investigations using principal components analysis

Objective: The effect of energy adjustment on variables entered into principal component analysis (PCA) to derive dietary patterns has received little attention. This study determines the effect of adjusting for energy on dietary patterns resulting from PCA and the subsequent effect on future outcomes. Design and methods: As part of regular self-completion questionnaires, used in the Avon Longitudinal Study of Parents and Children, pregnant women were asked to record the frequency of consumption of a variety of food items. A total of 12 053 women completed the questionnaire. Individual dietary types were identified using PCA, before and after adjusting the food variables for energy intake. Associations with estimated nutrient intakes and with birthweight were examined for the two solutions and when energy adjustment was performed at a later stage of the analysis. Results: Slight differences were seen in terms of the components extracted and the factor loadings obtained. The associations with nutrient intakes showed that there was a general reduction in the size of the correlation coefficients for the energy-adjusted components compared to the unadjusted components. There did not appear to be any difference in the size of the effects of the dietary pattern scores on birthweight, whether energy was adjusted for before entry into the PCA or after. Conclusions: In this sample, it is not necessary to adjust for energy intake before entry into a PCA analysis to determine dietary patterns when using food frequency questionnaire data. Effects of energy intake can be determined at a later stage in the analytical process.