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close this bookMethods for the Evaluation of the Impact of Food and Nutrition Programmes (UNU, 1984, 287 p.)
close this folder4. Measuring impact using laboratory methodologies
View the document(introduction...)
View the documentIntroduction
View the documentDevelopment of a primary nutritional deficiency
View the documentChoice of tissue for laboratory assessment
View the documentSelection of laboratory methodologies for nutritional impact evaluation
View the documentLaboratory methods for assessment of nutritional impact
View the documentAnnex A. Laboratory evaluation of protein nutriture
View the documentAnnex B. Laboratory evaluation of vitamin A nutriture
View the documentAnnex C. Suggested methods for hematology
View the documentReferences

Development of a primary nutritional deficiency

Figure 4.1. (see

FIG. 4.1. Generalized Scheme of the Development of a Primary Nutritional Deficiency. ) depicts a generalized scheme of the sequence that occurs from the time diets become inadequate to when clinical signs and symptoms become evident. This sequence will vary for different nutrients, as will the sequence of reversal in response to re-supply of the nutrient. An understanding of these time sequences in depletion/repletion of specific nutrients of concern is crucial, therefore, in selecting a methodology that will provide the information required to evaluate nutritional impact of various types of programmes. Hence laboratory methodology, judiciously selected for a particular nutrient, can be useful in detecting changes in subclinical levels of nutriture; or it can be an expensive, non-informative exercise.

It is important to keep in mind that in practice, the relative availability of some nutrients to support biochemical functions varies from day to day with fluctuations in intake. Hence, flow in figure 4.1 may be upward or downward and a single laboratory measurement or clinical observation will not reveal the direction of events. Furthermore, the rapidity with which the direction of flow responds to alterations in food or nutrient supply will vary. For example, anatomical signs, though the last to appear, may take substantially longer to disappear than will restoration of the activity of nutrient-dependent enzymes or the concentration of the nutrient in blood. Thus the biochemical assessment of nutritional status for a specific nutrient does not always directly correlate with the findings from dietary or clinical assessment, particularly when applied on an individual basis or among populations of small sample size. Generally, when applied to populations for assessment of nutritional status, the trend will be in the same direction for dietary, biochemical, and clinical findings.

To use cross-sectionally obtained measurements for purposes of evaluating the nutritional impact of nutrition interventions, a laboratory measurement should be chosen that represents the cumulative effects on nutriture. i.e., nutritional status, rather than immediate responses to dietary intake. On the other hand (as noted earlier) acute changes in the physiological environment, acute infection, hormonal balances etc., as well as diet, can shift the distribution of nutrients among compartments, thus affecting biochemical events. Therefore it is often also useful to have an indicator of the immediate situation, particularly when dealing with individuals or populations of small size. In all cases, an appropriate comparison group is necessary to evaluate associations among laboratory measurements with nutritional intervention.