Because the biological response to deprivations of calories and proteins is multifaceted, a variety of techniques and laboratory tests can be used to assess protein energy malnutrition (PEM). Many of the tests, including those described below, are more familiar to dieticians than to clinicians (Blackburn et al. 1977; Antonow and McClain 1985; Figure 1).
Tests of Body Weight
A person's ideal body weight is based on height and can be determined from the Metropolitan Standards table, which is reproduced in any textbook of nutrition. Because of the ease with which height and weight data can be obtained, a person's percentage of the ideal body weight is commonly used as an assessment of PEM. However, caution must be exercised. For example, in the case of an alcoholic with liver disease, fluid retention is common and can mask the severity of weight loss. The total water in the body can be measured by the technique known as isotope dilution; however, this typically is not done. More often, clinicians obtain less reliable estimates of body water by weighing the patient before and after excessive excretion (diuresis).
Anthropometric Tests
Anthropometry is the science of measuring the human body and its parts. Two common anthropometric measurements are skinfold thickness and midarm muscle circumference and area. Skinfold measurements are estimates of stores of body fat. These measurements can be made in a variety of locations throughout the body, but clinicians usually measure the triceps. Researchers have reported significant correlations between skinfold measurements and morbidity and mortality of patients with alcoholic liver disease (Mendenhall et al. 1986). Midarm muscle circumference and area is an indirect estimate of skeletal muscle mass based on the circumference of the midarm and skinfold thickness of the triceps.
Creatinine Height Index
The creatinine height index is another method for estimating muscle mass. This index is based on the amount of creatinine in urine during a 24-hour period. Healthy muscle produces creatinine at a relatively constant rate in proportion to muscle mass. Decreases in muscle mass lead to proportional decreases in the level of creatinine in urine. Factors affecting excretion of creatinine include large amounts of meat in the diet, altered renal function, and drugs, such as steroids, that alter muscle metabolism.
Visceral Proteins
The concentrations of nonmuscle proteins known as visceral proteins also decrease during the course of PEM and therefore can be used to diagnose and monitor malnutrition. Those most commonly measured are the transport proteins circulating in the bloodstream, including albumin, transferrin, prealbumin, and retinol-binding protein. Studies have revealed that diets deficient in protein result in decreases in the rate of synthesis of albumin, but with a concomitant and perhaps compensatory reduction in the rate of degradation of albumin (Rothschild et al. 1969). The observed reduction in concentration of serum albumin develops slowly and has a long recovery time. This finding has led many to conclude that such visceral proteins are more sensitive and responsive to the changes associated with PEM and, hence, are preferred "tools" for monitoring nutrition.
However, changes in concentrations of some proteins can reflect abnormalities that are independent of malnutrition, for example, deficiencies in zinc, iron, and vitamin A. Also, infections, thyroid dysfunction, liver disease, and renal disease can alter visceral protein synthesis and degradation independently of malnutrition. In the case of alcoholic liver disease, the reduction in visceral proteins may be as much a reflection of the liver's inability to synthesize proteins as a result of the shortages in amino acids associated with malnutrition. Despite the lack of specificity, tests of visceral proteins are used widely as indicators of nutritional status and correlate well with morbidity and mortality (Buzby et al. 1980; Mendenhall et al. 1986).
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COPYRIGHT 2004 Gale Group
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