Nutritional Assessment (Protein Nutriture) A. Michael Spiekennad Scott & White Clinic and Memorial Hospital, Scott, Shewood and Brindley Foundation, Texas A&M University Health Science Center, College of Medicine, Temple, Texas 76508
Protein calorie malnutrition (FCM) is normally associated with very poor people in underdeveloped countries. It is, however, a common undiagnosed disorder among hospitalized patients in the United States (11). Patients enter the hospital with PCM resulting from conditions such as poor diet, liver and gastrointestinal disease, certain types of cancers, and other chronic illnesses (12, 13). Hospital malnutrition has been reported to be 35-40% at the time of admission (14,15). PCM can lead to an increase in the mortality rate, aggravate preexisting disease, impair immune response, and cause a longer hospital stay. This form of malnutrition is difficult to correct. The patient must first break down proteins to amino acids and then use the amino acids to synthesize important proteins in the body that are necessary for life. This differs from other forms of malnutrition in which low levels of vitamins or carbohydrates can be corrected by dietary changes over a short period time (16'). TESTING CRITERIA Laboratory tests to identify PCM should specifically identify the malnourished patient or the patient at risk, be sensitive to short-term dietary changes, and be relatively unaffected by other disease processes. No single approach has proven adequate in assessing PCM. A physical examination with nutritional and medical histones and screening protein assays is considered essential for the initial evaluation (17). Changes in protein levels, preferably those with short half-life and low molecular weights, are more likely to indicate altered nutritional status. PCM can have medical, financial, and even legal consequences. Laboratories are interested in improving the timing of monitoring for nutritional support. A widely held but mistaken view is that body size is a valid indicator of protein deficiency. This viewpoint is reflected in widespread use of anthropometric measurement to assess body composition, although a number of studies have shown poor correlation between anthropometric measurements and body composition (13,14). An assumption also exists that an obese person is unlikely to have inadequate protein calorie status and that a person of small body size is likely to be malnourished. Protein deficiency remains undetected for many reasons. Historically, no markers with the sensitivity and specificity to detect changes in nutritional status have been available. In the last three to five years, however, markers such as prealbumin, retinolbinding protein, and insulin-like growth factor (IGF-l),which are relatively unaffected by nonnutritional factors, are being used for nutritional monitoring (18). PROTEIN MARKERS IN NUTRITIONAL ASSESSMENT The primary objective of nutritional assessment is to identify the patient who is malnourished and then, through nutritional therapy, to preserve or replenish the protein component of the body. Laboratory nutritional assessment is best accomplished by monitoring selected serum proteins. * Please address correspondence and reprint requests to: A. Michael Spiekerman,Ph.D., Professor of Pathology, Scott & White Hospital, 2401 S. 31st St., Temple, TX 76508.
Table 1. Characteristics of Plasma Proteins in Use as Nutritional Markers protein
mol wt
half-life
albumin fibronectin prealbumin (transthyretin) retinol-binding protein somatomedin C (insulin growth factor-1) transfemn interleukin- 1 interleukin-2 interleukin-4 tumor necrosis factor a interleukin-6
65 000 250 000 54 980 21 000 7 650
20 days 15 h 48 h 24 h 2h
33-48 g/L 220-400 mg/L 160-350 m g L 30-60 mg/L 0.10-0.40 m g L
76 000 17 000 15 OOO 20 000 18 500 25 000
10 days a a a a a
1.6-3.6 g/L 0.0001-0.004 pg/L 0.031-2 p g L 0.030-2 p g L 0.0002-0.007 p g L 0.0002-0.007 pg/L
a
ref range
Half-life of interleukins and tumor necrosis factor less than 1
h.
These proteins should have a short biological half-life and should reflect protein status by measurable concentration changes in the serum. The proteins should have a relatively small concentration, a rapid rate of synthesis, and a constant catabolic rate and be responsive only to protein and energy restrictions (18). Protein deficiency states in humans will be prolonged and severe if insensitive markers are used because it takes a long time period before signiticant change occurs in concentrations of these proteins. The concentration of insensitive proteins may be affected not only by protein deficiency but also by other factors such as hepatic density, renal disease, and severe infection. Measurements of the concentration of selected individual proteins may not only provide a sensitive index on protein status but are also a valuable indication of morbidity. Visceral protein measurements remain a reliable and relatively easy method of assessing the energy state of the patient as well (19). Table 1 (110,I l l ) offers detailed information about these protein markers. ALBUMIN Albumin has long been used in the assessment of hospitalized patients. Low levels of serum albumin may reflect low hepatic production or protein loss from the vascular component. Low albumin levels have been identified as a common abnormality in patients in long-term care facilities. Hospitalized patients with low serum albumin levels experienced a 4-fold increase in morbidity and a &fold increase in mortality (112). A low serum albumin in a patient on the nephrology service can be correlated with a longer stay in the hospital with an increased number of infections. The albumin concentration in the body is influenced by albumin synthesis, degradation, and distribution. While albumin has the highest concentration of any protein in the vascular system, over 60% of the protein is present in the extravascular spaces. Albumin from the extravascular pool can be mobilized during periods of protein depletion resulting from the stress of major surgery or infection so that serum albumin concentrations may not decline for a long period (113,114).The degeneration rate of the albumin is proportional to the size of the extravascular pool, which allows the concentration in the serum to be relatively constant. The long biological half-life of albumin (20 days) allows changes in the serum concentration only after long periods of Analytical Chemistry, Vol. 67,No. 12,June 15, 1995
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malnutrition. Since there are many mechanisms that can potentially produce a depressed albumin concentration, an isolated serum albumin level may be of limited value in evaluating liver synthesis rates in patients who are critically ill (115). Serum albumin is not a good indicator of short-term protein and energy deprivation. However, albumin levels are good indicators of chronic deficiency. Traditionally, albumin has been used to help in determining two important nutritional states. First, it helps identify chronic protein deficiency under conditions of adequate non-protein calorie intake, which leads to marked hypoalbuminemia. This may result from the net loss of albumin from both the intravascular and extravascular pools, causing kwashiorkor. Second, albumin concentrations may help define a condition called marasmus. This is caused by caloric insufficiency without protein insufficiency so that, in marasmus, the serum albumin level remains normal, but there is considerable loss of body weight. Studies have classified various levels of malnutrition by using albumin levels. Serum albumin levels of 35 g/L or greater are considered normal (116). Albumin levels of 28-35 g/L indicate mild malnutrition; 21-27 g/L indicates moderate malnutrition;
