Chapter 072. Malnutrition and Nutritional Assessment (Part 7)

Assessment of Circulating (Visceral) ProteinsThe serum proteins most used to assess nutritional status include albumin, total iron-binding capacity (or transferrin), thyroxine-binding prealbumin (or transthyretin), and retinol-binding protein. Because they have differing synthesis rates and half-lives—the half-life of serum albumin is about 21 days whereas those of prealbumin and retinol-binding protein are about 2 days and 12 h, respectively—some of these proteins reflect changes in nutritional status more quickly than others. However, rapid fluctuations can also make shorter-half-life proteins less reliable.Levels of circulating proteins are influenced by their rates of synthesis and catabolism, "third spacing" (loss into interstitial spaces), and,...
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Chapter 072. Malnutrition and Nutritional Assessment (Part 7) Chapter 072. Malnutrition and Nutritional Assessment (Part 7) Assessment of Circulating (Visceral) Proteins The serum proteins most used to assess nutritional status include albumin,total iron-binding capacity (or transferrin), thyroxine-binding prealbumin (ortransthyretin), and retinol-binding protein. Because they have differing synthesisrates and half-lives—the half-life of serum albumin is about 21 days whereas thoseof prealbumin and retinol-binding protein are about 2 days and 12 h,respectively—some of these proteins reflect changes in nutritional status morequickly than others. However, rapid fluctuations can also make shorter-half-lifeproteins less reliable. Levels of circulating proteins are influenced by their rates of synthesis andcatabolism, third spacing (loss into interstitial spaces), and, in some cases,external loss. Although an adequate intake of calories and protein is necessary toachieve optimal circulating protein levels, serum protein levels generally do notreflect protein intake. For example, a drop in the serum level of albumin ortransferrin often accompanies significant physiologic stress (e.g., from infection orinjury) and is not necessarily an indication of malnutrition or poor intake. A lowserum albumin level in a burned patient with both hypermetabolism and increaseddermal losses of protein may not indicate malnutrition. On the other hand,adequate nutritional support of the patients calorie and protein needs is critical forreturning circulating proteins to normal levels as stress resolves. Thus low valuesby themselves do not define malnutrition, but they often point to increased risk ofmalnutrition because of the hypermetabolic stress state. As long as significantphysiologic stress persists, serum protein levels remain low, even with aggressivenutritional support. However, if the levels do not rise after the underlying illnessimproves, the patients protein and calorie needs should be reassessed to ensurethat intake is sufficient. Assessment of Vitamin and Mineral Status The use of laboratory tests to confirm suspected micronutrient deficienciesis desirable because the physical findings for these are often equivocal ornonspecific. Low blood micronutrient levels can predate more serious clinicalmanifestations and may also indicate drug-nutrient interactions. Estimating Energy and Protein Requirements A patients basal energy expenditures (BEE, measured in kilocalories perday) can be estimated from height, weight, age, and gender using the Harris-Benedict equations: where W is weight in kg; H is height in cm, and A is age in years. Aftersolving these equations, total energy requirements are estimated by multiplyingthe BEE by a factor that accounts for the stress of illness. Multiplying by 1.1–1.4yields a range 10–40% above basal that estimates the 24-h energy expenditure ofthe majority of patients. The lower value (1.1) is used for patients withoutevidence of significant physiologic stress; the higher value (1.4) is appropriate forpatients with marked stress such as sepsis or trauma. The result is used as a 24-henergy goal for feeding. When it is important to have a more accurate assessment of energyexpenditure, it can be measured at the bedside using indirect calorimetry. Thistechnique is useful in patients who are believed to be hypermetabolic from sepsisor trauma and whose body weights cannot be obtained accurately. Indirectcalorimetry can also be useful in patients having difficulty weaning from aventilator, as their energy needs should not be exceeded to avoid excessive CO 2production. Patients at the extremes of weight (e.g., obese persons) and/or age aregood candidates as well, because the Harris-Benedict equations were developedfrom measurements in adults with roughly normal body weights. Because urea is a major byproduct of protein catabolism, the amount ofurea nitrogen excreted each day can be used to estimate the rate of proteincatabolism and to determine if protein intake is adequate to offset it. Total proteinloss and protein balance can be calculated from the urinary urea nitrogen (UUN)as follows: The value of 4 g added to the UUN represents a liberal estimate of theunmeasured nitrogen lost in the urine (e.g., creatinine and uric acid), sweat, hair,skin, and feces. When protein intake is low (e.g., less than about 20 g/d), theequation indicates both the patients protein requirement and the severity of thecatabolic state (Table 72-5). More substantial protein intakes can raise the UUNbecause some of the ingested (or infused) protein is catabolized and converted toUUN. Thus at lower protein intakes the ...