Food frequency questionnaires (FFQs) are designed to assess habitual diet by asking about the frequency with which food items or specific food groups are consumed over a reference period. This method can be used to gather information on a wide range of foods or can be designed to be shorter and focus on foods rich in a specific nutrient or on a particular group of foods e.g. fruit and vegetables. Since FFQs are often designed to assess the ranking of intakes within a study population, it remains controversial whether FFQs can produce accurate estimates of absolute intakes of foods and nutrients. The outcomes measured by FFQs are described in Table D.2.13.
Table D.2.13 Dietary outcomes assessed by food frequency questionnaire.
|Dietary dimension||Possible to assess?|
|Energy and nutrient intake of total diet||Yes|
|Intake of specific nutrients or food||Yes|
|Infrequently consumed foods||Yes|
|Frequency of eating/meal occasions||Yes|
|Adult report of diet at younger age||Yes**|
* possible if repeated measures are collected over time.
** possible if specific questions for a diet during a specific period are included.
An example of an FFQ is displayed in Figure D.2.5. An FFQ can be:
The reference period
The foods list
The length of the list of foods can range from about 20 to 200 items. This depends on how the FFQ is designed (see below). The foods listed should be a combination of:
Some FFQs may include supplementary questions relating to:
Some responses may also help to identify respondents whose diet is very unusual, for whom the FFQ may not be appropriate.
The frequency categories
Use of portion sizes
Some FFQs are known as ‘semi-quantitative’, including portion size estimates. In a semi-quantitative FFQ, respondents are asked to indicate the frequency of consumption of specific quantities of foods (e.g. ½ a cup, ¾ cup etc.). By contrast, quantitative FFQs are used to ask respondents’ usual portion size based on a specified measure.
The potential contribution of questions on portion sizes is debatable for several reasons:
Use of a fixed portion size is not necessarily bad. If a study population is homogenous and sufficiently educated, respondents may readily calculate a frequency of food consumption given a fixed portion size. This capability of respondents is supposed to be verified in a validation study in a study population.
Figure D.2.5 Example of a food frequency questionnaire section for meat and fish.
FFQs are one of the most commonly used retrospective methods in nutritional epidemiology and have become a key research tool in examining the relationship between dietary intake and disease risk. For this purpose, it is more important to rank the intake of individuals relative to others in the population (e.g. high, medium, or low intake) or as quantiles (e.g. fifths of the distribution of intake) than to determine the absolute intake.
FFQs may be a particularly useful method to measure specific dietary behaviours and intakes of particular food groups (e.g. fruit and vegetables), foods not consumed on a daily or weekly basis in a given population (e.g. oily fish), or selected micronutrients present in a limited number of foods (e.g. calcium).
Due to their ease of administration and relatively low respondent burden FFQs have been used extensively in large-scale cohort (prospective) studies such as:
FFQs can be adapted for a particular purpose e.g. to assess calcium intake or habitual consumption of oily fish, but in many cases a comprehensive food list is used so that intakes of all nutrients and total energy intake may be determined. Total energy intake is critical in nutritional epidemiology for energy adjustment and for assessment of misreporting.
Criticism of use of FFQs to assess diet-disease relationships
Results from studies using biomarkers of specific nutrients as the reference method for assessment of dietary intake suggest that the measurement error associated with FFQs is larger than was previously estimated [13, 27] Others have found that the FFQ performs well in comparison with diet records . It is important to remember that not all FFQs are the same and some may be better than others in terms of assessing food and nutrient intakes. For any study, the advantages and disadvantages of using FFQs compared to other dietary assessment methods should be carefully considered .
Key characteristics of food frequency questionnaires are described in Table D.2.14.
Table D.2.14 Characteristics of food frequency questionnaires.
|Number of participants||Any|
|Cost of development||Medium|
|Cost of use||Low|
|Researcher burden of data collection||Low|
|Researcher burden of coding and data analysis||Medium|
|Risk of reactivity bias||No|
|Risk of recall bias||Yes|
|Risk of social desirability bias||Yes|
|Risk of observer bias||No|
|Participant literacy required||Maybe|
|Suitable for use in free living||Yes|
|Requires individual portion size estimation||Maybe|
Considerations relating to the use of FFQs for assessing diet in specific populations are described in Table D.2.15.
Table D.2.15 Dietary assessment by food frequency questionnaire in different populations.
|Infancy and lactation||Requires proxy.|
|Toddlers and young children||May require proxy or adult assistance.|
|Older adults||May require proxy depending on cognitive function .|
|Ethnic groups||Suitable, if developed for the purpose.|
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Adaptation of an existing questionnaire
The development of a new FFQ is costly in terms of time and resources and therefore the use of an existing questionnaire might be appealing. However, several points should be considered before selecting an existing questionnaire for use in a study. These include:
Developing or adapting a food list
It is important to consider carefully the level of grouping of foods. The aggregation of some foods with dissimilar eating patterns (e.g. tomatoes and tomato juice) can make it cognitively difficult for respondents to report frequency of consumption and may lead to an underestimation of intake.
For some food items, asking questions about single items can help respondents differentiate between similar foods with different nutritional profiles (e.g. full fat milk, semi-skimmed milk etc).
In order to achieve a balance between approximation of the absolute intake and burden for the participants, the following steps can be taken:
There may be situations in which the purpose of the FFQ is very specific and a shortened food list is preferable such as the assessment of intakes of calcium and other nutrients potentially involved in bone health, but this limits the use of the questionnaire and the data derived to that use only.