Methods for measuring intake over the longer term
Food frequency questionnaires consist of a list of foods and options to indicate how frequently each food is consumed. Respondents indicate the frequency of consumption during a specified period by marking the appropriate option column. The food lists may contain only a few food items or may contain up to 200 foods. The type and number of foods included is determined by the purpose of the study and the target population. For example, a food frequency question-naire designed to determine calcium intake would contain only foods which provide calcium, while a questionnaire to measure overall dietary adequacy would need to contain all foods known to be con-sumed by the target population. Likewise, a food fre-quency questionnaire designed to assess dietary intakes of a homogeneous target population with a diet of limited variety will be shorter than one designed to assess food intakes of a heterogeneous population with a variety of food intake patterns. There are several types of food frequency question- naires. The type of food frequency questionnaire used depends on the purpose of the study, the target population, and the required level of accuracy of food portion estimation.
The period of recall depends on the study objec-tives. In the past, most food frequency questionnaires used the preceding year or 6 months as the reference period. Theoretically, this should eliminate the effects of season. In practice, however, respondents tend to answer according to what is in season or available at the time of the study. For example, intake of oranges was found to be higher when interviews were carried out during the citrus season than at other times of the year. Information may be more reliable when recall period is shorter. Recent recommendations are that recall periods should not be longer than 1 month. If annual intakes are required, the food frequency ques-tionnaire must be repeated in different seasons. It is very important that the respondent understands what the recall period is and that only this period should be considered when giving frequencies of intake.
The frequency of consumption is usually indicated by options such as:
● more than once a day
● 3–4 times per week
● 1–2 times per week
● 1–2 times per month
This type of response format requires only that the appropriate columns be marked and is most suitable for self-administered questionnaires. When appropri-ately designed, such questionnaires can be optically scanned, which saves time on data entry and checking procedures (Figure 10.5).
Figure 10.5 Typical layout of a food frequency questionnaire suitable for optical scanning (reproduced with permission of Anti Cancer Council of Victoria, Melbourne, Australia).
Closed response options, however, treat the fre-quency of consumption as a categorical variable and assume that frequency of consumption is constant throughout the recall period. The choice of categories may bias the results: too few categories may underes-timate frequencies whereas too many may overesti-mate frequencies. Respondents may have difficulty in matching their food intake to the available categories. For example, when food is purchased on a monthly basis food items such as fresh fruit and vegetables may be consumed every day while the stocks last, but, once used up, will not be consumed until purchased in the following month.
An alternative method for recording responses is to provide columns headed as the number of times per day, per week and per month, seldom, and never. From this, the average frequency of consumption and the amount of food consumed per day can be calcu-lated. Table 10.3 shows an extract from such a food frequency questionnaire. The advantage of this response format is that it allows the respondent to describe the frequency of consumption in detail. Responses such as consumption of a food twice a day for 6 days of the week can be recorded. The disadvantages are that clear instructions must be given, making this method more appropriate for interviewer-administered questionnaires than self-administered questionnaires, and the interview takes longer and requires more writing and calcula-tions than the closed format, making more room for errors.
Most food frequency questionnaires obtain infor-mation only on the frequency of consumption of a food over a given period and not on the context in which the foods were eaten, i.e., on meal patterns. Meal-based food frequency questionnaires have been used on the basis that it may be easier for respondents to provide the information in the context of meals.
The information on meal patterns obtained from such questionnaires is, however, more limited than that which can be obtained from a dietary history. Additional questions, if included, can provide some information on cooking methods.
Some food frequency questionnaires also attempt to quantify the frequency information by obtaining data on portion size. Information on the quantity of each food consumed may be obtained by asking respondents to indicate on the questionnaire whether their usual portions are small, medium, or large relative to those eaten by others, by asking subjects to describe their usual portion size in terms of a stan-dard portion size described on the questionnaire (semiquantitative food frequency questionnaire), or by reference to a picture atlas of food portions, food models, or actual food portions (quantitative food frequency questionnaire). When portion sizes are used, it is important that these reflect the consump-tion patterns of the population.
Food frequency questionnaires are mainly used in studies designed to look for associations between food intake and disease or risk of disease, particularly when specific foods rather than the level of consump-tion of a nutrient are thought to be the important factor. Since the cost of administration and respon-dent burden are relatively low, they are suitable for use when sample sizes are large, particularly if a postal method is used.
The success of a food frequency questionnaire depends on how closely the food list and portion size descriptions reflect the food patterns of the target population. This is sometimes referred to as being culture sensitive. Much time and care must be put into the development of a food frequency question-naire in order to ensure that it provides an accurate reflection of the dietary intakes of a population. Pre-liminary studies, using 24 hour recalls, food records, or indirect methods may be needed to obtain infor-mation on food items, frequency of consumption, and portion sizes in the target population. Since food frequency questionnaires are usually developed for use in specific target populations, a food frequency questionnaire developed for use in one population may not be appropriate for use with another popula-tion with different food intake patterns. It is also extremely important that the questionnaire be tested for repeatability and validity in the target population before being used, even if it has been tested in a different population.
Detailed reviews of the design and evaluation of food frequency methods for use in epidemiological studies are given by Willett (1998) and Cade et al. (2002).
The principal objective of the diet history is to obtain detailed information on the habitual intake of an individual. As first proposed by Burke in the 1940s, the method had several components (Burke 1947):
● an interview to obtain usual diet
● a cross-check of this information by food group
● a 3 day record of food consumed in household measures.
The 3 day record component is now seldom used as a regular component of a diet history. Its purpose originally was as a way of checking the data obtained from the diet history interview. A diet history is usually obtained by an experienced nutritionist by means of an open interview followed by some kind of cross-
check using a standard list of commonly consumed foods. The interview usually begins with a review of the food that was eaten in a specific time-frame (e.g., yesterday) or on a typical day, and then moves on to explore the variations in food intake that occur for each meal over a given period. Information on the usual size of food portions is obtained with the aid of food models or photographs in the same way as for a 24 hour recall. The time-frame for a diet history can range from the previous month to the previous year.
In practical terms it is easier for respondents to reconstruct the immediate past, but the past year is often used to capture seasonal variation. Whatever the time-frame used, it is important for it to be clearly specified. In the literature the term “diet history” is sometimes used loosely to describe any form of diet recall, including the 24 hour recall and self-administered food frequency questionnaires, as well as interviewer-administered recalls of habitual or longer term intake. This broader use can be confusing and is best avoided. The dependence of the diet history on both respondent and interviewer skills may make the results obtained less comparable between individuals than those obtained from other methods, and for this reason it is often considered more appropriate to categorize diet history data (e.g., as high, medium, low) rather than to treat them as intakes expressed in terms of absolute units per day.
The diet history is favored in Scandinavia and the Netherlands, where a structured interview may be used. The structured interview is more standardized but may miss elements specific to the individual or bore the respondents with irrelevant questions. The open-ended interview allows for tailoring to the indi-vidual, but risks missing important items.