Assessment of food and nutrients intake
The assessment of usual food and energy intake in humans is a hard task, requiring the accurate determination of the subject’s routine intake, the conversion of this information to energy and nutrients, and a sufficient duration of food intake measurement to consider the information as a true reflection of habitual food intake. Several methods have been implemented. The Food Frequency Questionnaire (FFQ) and semi-quantitative FFQs are amongst the most used tools in population studies because they allow the collection of dietary information from large numbers of individuals and are normally self-administered. The FFQ is a list of foods and a selection of options relating to the frequency of their consumption in order to capture usual intake. Semi-quantitative FFQs are questionnaires that also collect information about portion size. Where information on portion size is not obtained, standard food portion sizes are often used to calculate nutrient intakes, as is done for many of the FFQs based on the Caerphilly questionnaire [9]. Some of these questionnaires also assess vitamin and PUFA supplementation. A limitation of diet assessment is the potential measurement error in the estimation of food intake. However, any such misclassification is probably non-differential and therefore would lead to an underestimation of the effects of food.