Introduction: Dietary assessment methods
Dietary assessment methods can be estimated through the use of various tools, which differ depending on study objectives, design and resources. One way is to classify them into prospective and retrospective methods. Another way of classification is into the current diet and food/dietary habits. With current diet, you mean the diet that you actually eat or have eaten. With food habits, the usual diet you mean the diet that you usually eat, but not necessarily the diet that you eat right now, for instance, today. The current diet methods are the various record methods, the duplicate portion technique and 24/48 hour recall methods. The dietary habits methods are the various diet history methods and food frequency questionnaires.
Prospective methods and Retrospective methods are briefly described below.
1. Prospective methods
In the prospective methods the diet are recorded when you consume the foods. The various methods differ in how precise you are in estimating the portion sizes. For this method, one records food for 3 to 7 days. Some types of prospective methods
- Menu records A menu recording is a record of the respondent’s food intake at the time of consumption regarding type of food, but does not indicate any portion sizes.
- Estimated food records In an estimated food record, respondents record the foods eaten at the time of consumption and estimate the portion sizes, for instance with the aid of pictures of foods, rulers, standard household measuring cups and spoons. Detailed descriptions of all foods and beverages, including brand names, and their method of preparation and cooking are recorded. For composite dishes the amount of each raw ingredient used in the recipe and, if possible, the final weight of the composite dish are recorded.
- Weighed food records In a weighed record all foods and beverages consumed are weighed by the subject, parent or caretaker at the time of consumption. Details of methods of food preparation, description of foods, and brand names should also be recorded. For composite dishes, weights of all raw ingredients used in the recipe should be noted, as well as the weight of the portion consumed. It can be both an advantage and a disadvantage to weigh foods and beverages since you need a weighing scale. The advantage is that you do not need to estimate portion sizes and the portion sizes will be more accurate compared to the other methods. The primary disadvantage is that it is necessary to bring the scale with you all the time during the recording period.
- The duplicate portion technique In this method duplicate portions of all foods and beverages consumed are collected in a container and later analyzed. A great disadvantage with the duplicate portion technique is of course that one must bring the container everywhere. This restricts the number of days of collection, as well as limiting the selection of people to those willing to put up with this inconvenience. Besides these disadvantages for the volunteers, the duplicate portion technique is a very expensive method and more expensive the more analyses one performs. Validation studies on the duplicate portion technique indicate an underestimation of the total food intake.
Advantages with prospective methods
One of the major advantages with the prospective methods is that they are not affected by memory since the foods and beverages are recorded at the time of consumption. Another advantage is that portion sizes can be more accurately estimated compared to retrospective methods. Concerning the duplicate portion technique, it is an advantage as nutrients can be analyzed. An advantage, that is valid for all current methods, is that information regarding irregularities in food intake can be obtained.
Disadvantages with prospective methods
Probably the greatest disadvantage of prospective methods is that they often affect the consumption of foods and beverages. Several studies have demonstrated that the total intake will be low and that there is a selective under reporting. Another disadvantage is that you can only perform this method during a short time period, normally not more than a week. Since records and the duplicate portion technique are demanding methods for the subjects the drop-out rate may be high and one will get a selection of people who are probably more health and food conscious.
Prospective methods are not affected by the memory, except for the duplicate portion technique since subjects already at the grocery store have to remember to by the double amount of foods and beverages that they normally need.
2. Retrospective methods
In retrospective methods information is collected on foods and beverages already consumed. Among the retrospective methods are the 24 and 48 hour recalls current diet methods and diet histories and food frequency questionnaires dietary habits methods.
Quantities of foods consumed can be obtained by food models, pictures of foods, rulers, standard household measuring cups and spoons. Some types of retrospective methods are given below.
- 24 hour and 48 hour recalls
- Diet histories With this method, or rather methods, one will obtain an estimate of the individual’s dietary habits during a long time period, from weeks up to one year. However, it has been demonstrated that describing food habits during a year is almost impossible, due to seasonal variations. Therefore, it is important to define the time period covered.
The method exists in many forms, which has led to that researchers often talk about the diet history family. They have in common that they describe habitual dietary habits. The diet history is considered to be a difficult method to perform. The method is very demanding for both the volunteers and the interviewers. For instance, the method is very time consuming (often several hours) and you need to have a good memory of what you have eaten and quantities consumed.
The method, first developed by Bertha Burke in 1947 is made up of three parts. The first part consists of a collection of general information on the overall eating pattern. The general information obtained includes detailed descriptions of foods, their frequency of consumption, and usual portion sizes in common household measures. This first part also consists of a 24 hour recall.
The second part consists of a questionnaire on the frequency of consumption of specific food items, used to verify and clarify the information on the kinds and amounts of foods given as the usual intake in the first component. This second part serves as a “cross-check” for the information on usual intake obtained from the first part.
The third part consists of a three-day food record using household measures. Since this last part showed to be the least helpful it is often abandoned.
An advantage with these methods is that relatively long time periods can be studied and thereby the intake on the individual level can be obtained. Another advantage is that the interviewer can contribute to a good communication; thus, a low drop-out rate can be obtained. It is also likely that these methods give a more valid intake compared to other methods.
Disadvantages with the methods are that they are very labour intensive, which affects both the interviewer and the volunteer, and unsuitable for large surveys. Also, the results obtained depend on the skill of the interviewer, which impacts the quality of the interview. Researchers will not obtain information on the day-to-day variation in food intake and, as with all retrospective methods, good memory is important for estimating frequencies and portion sizes.
- Food frequency questionnaires
- Assessment of intake in the distant past These data are derived from some form of food frequency questionnaire or diet history interview. The methods are primarily used in case-control studies when dietary information is needed from the distant past. Results from studies in which current as well as past diet were assessed reveal that the recalled diet agrees more closely with the current diet than with the past diet. This demonstrates that the current diet is having a strong influence on the recalled diet.
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