Measuring Food Intake
The many reasons for finding out about the food that people eat fall into three broad categories:
1 Public Health: to evaluate the adequacy and safety of the food that people eat at national or commu-nity level and to identify the need for or to evaluate nutrition-based intervention programs.
2 Clinical: to assist with the prevention, diagnosis, and treatment of diet-related conditions.
3 Research: to study the interrelationships between food intake and physiological function or disease conditions under controlled conditions or in field conditions.
The kind and amount of food intake data required differ in each situation and may require data at the national, community, household, or individual level.
Assessment of nutritional status
Nutritional health is maintained by a state of equilib-rium in which nutrient intake is balanced by nutri-tional requirements. Malnutrition occurs when net nutrient intake is less than requirements (undernutri-tion) or exceeds requirements (overnutrition). Both under- and overnutrition lead to metabolic changes which have acute and chronic consequences for health.
There is no ideal tool to measure a person’s nutri-tional status accurately. Attempts to predict the influ-ence of malnutrition based on single measurements fail to consider the many interacting factors between nutrition and disease state. For this reason, it is necessary to look at several different measurements in order to assess a person’s nutritional status. This process is known as the A, B, C, D of nutritional assessment:
●Biochemical and hematological variables
●Clinical and physical assessment
Obtaining data on food intake is probably the most difficult aspect of nutritional assessment and is asso-ciated with several problems:
●“Food intake” is not a simple measure of one vari-able, such as weight or height, but requires data on the intake of many different food items.
●Food intake data are subject to many sources of variability, since even the same individuals eat dif-ferent foods, at different times, in different places, in many different combinations, and with many different preparation methods. The net effect of all these sources of variability is that more data are needed to generate reliable results than would be the case with a less variable measure.
●We are rarely in a position to know the truth about food intake. With many biological measurements it is possible to check the results obtained against a reference method that is known to give accurate results or by means of an independent measure. For example, we can check an infant’s birth weight by checking the accuracy of the weighing equipment used to measure it by means of a standard weight or, if the information was obtained by means of a questionnaire, we may be able to check the data from official records. With food intake data we have to rely on the individuals who eat the food to provide us with the answers to our questions. We ask individuals to remember what and how much they ate, to estimate how often they eat particular foods, or even, in some situations, to weigh or measure their food intake for a number of days. For this reason one of the most important consider-ations, when obtaining information on food intake from individuals, is to take all possible steps to obtain their full cooperation. It is also extremely important that individuals understand the purpose of the process and what is expected of them. This may well involve much time and effort on the part of the investigator(s), but is essential for high-quality data.
There are a number of different methods to obtain dietary intake data. Each method has its purposes, advantages, and disadvantages. It is, therefore, essential that the purpose of collecting dietary data is clearly defined, so that the most appropriate dietary assessment method is used.
It is also essential to recognize that finding out what people eat requires adequate resources. Appropriately trained personnel must be employed not only for the period of data collection but also for the time it takes to review, enter, and analyze the data. It may not always be necessary to obtain detailed data on food intake in order to answer a particular question. When resources are limited it is probably more useful to collect limited data of high quality than to attempt to collect comprehensive dietary data with inadequate resources. Being able to recognize this situation is important for maximizing available resources. Table lists the different approaches to measurement of food intake.
Finally, it is important that the interpretation and application of data derived from food intake studies take into account the limitations of the data. This clearly does not improve the quality of the data per se, but maximizes their usefulness for the purpose for which they were originally collected. Recognition of the limitations of dietary data involves more than simply stating the limitations. External comparisons to check whether the data are consistent with inde-pendent sources of information on food intake and to determine the likely direction and magnitude of any bias are an integral part of the interpretation of dietary data. Relevant sources of comparative infor-mation may include food supply and expenditure data and physiological or biochemical measures related to nutrient intake.
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