|Methods for the Evaluation of the Impact of Food and Nutrition Programmes (UNU, 1984, 287 p.)|
|4. Measuring impact using laboratory methodologies|
Biochemical measurements with nutritional implications can be made quite non-invasively on tissues such as hair and nails, or at the extreme of invasiveness on liver and muscle. But in practice, blood and its cellular components and urine are the most readily available tissues and can be obtained with moderate evasiveness, for estimating status in surveys. For most nutrients, urine is unsuitable for nutritional status assessment unless a timed or 24-hour sample can be obtained to compensate for diurnal variations in nutrient excretion rates and in volume. Relating values to creatinine concentrations reduces but does not eliminate this problem. This criticism is most applicable when applied to individuals and to populations of small size. On the other hand, casual urine specimens can be useful if the purpose is to evaluate compliance to a food or nutrient supplementation programme rather than nutritional impact. For example, by including a nutrient marker in the supplement that normally is excreted in the urine, such as riboflavin, increased levels can be detected qualitatively when related to non-participant or non-complying comparison groups.
As already noted, choice of fluid or cellular component in blood that best reflects status rather than immediate dietary intake will vary in relation to how specific nutrients are distributed between extra and intracellular compartments, how responsive this distribution is to dietary change, and how it is influenced by altered physiological conditions. such as acute or chronic infections. drugs, and stressful) circumstances, which are unrelated to diet. For many nutrients, compensatory biochemical mechanisms exist to adjust for short-term fluctuations in dietary intake and to delay the onset of clinical signs of inadequacy. Body reserves of varying size and half-life exist for this purpose. Ideally, assessment of change in the magnitude of the reserve supply of a nutrient (step 2, fig. 4.1.) would be most useful for nutritional surveillance purposes and for measuring the nutritional impact of certain nutrient-specific interventions where homeostatic mechanisms maintain body fluid levels until reserves are depleted, such as for iron and vitamin A. This would represent the earliest stage of dietary inadequacy and indicate when preventive measure should be instituted. However, biochemical indicators of tissue reserves that are present in blood and accessible to laboratory evaluation in surveys are available for only some nutrients, e.g., serum ferritin as a reflector of tissue iron stores. There is no "true" reserve store of protein, only variation in active protein mass that is not easily measured in surveys by laboratory methods.
Blood samples obtained from fasting subjects are preferred to avoid fluctuations in some nutrients that reflect immediate dietary intake. However, under field conditions, especially among children, non-fasting specimens are often all that can be obtained practically. By selection of the appropriate parameter, non-fasting specimens can be used without prejudice in interpretation for estimation of protein and iron status, and for vitamin A status except following a meal that contains a concentrated source of the vitamin (e.g., animal liver). Since blood samples are usually obtained in the morning hours and rich sources of preformed vitamin A are unusual breakfast items in developing countries, this is an unlikely significant confounding variable (1). Fasting specimens may be more critical in the laboratory assessment of certain other nutrients, particularly water-soluble vitamins (2).
Caution must be used in interpreting blood data obtained from subjects with acute infections. These may cause a transient lowering of blood level of some nutrient-specific transport proteins, such as retinol-binding protein and transferrin. Chronic infections too can cause a lowering of circulating levels for some nutrients. These blood concentration changes may not reflect a depletion of the total available body pool, but a temporary redistribution of the nutrient that is without physiological significance.