Caveats for the Diagnosis of Secondary Undernutrition

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Undernutrition due to disease and dysfunction obviously requires establishment of the following: (1) the existence of deficiencies; and (2) that factors other than underconsumption are influencing the deficiency states. The body composition standard is a body mass index (BMI) of <18.5 kgm~2. With the worldwide pandemic of overweight, recent weight loss of 10% or more of usual body weight may be a more sensitive and reliable indicator of an incipient undernutrition problem. Weight problems diagnosed in this manner would certainly be detectable well before the BMI will have fallen to the aforementioned criterion.

Ill patients with adequate or excessive body mass indices can manifest metabolic substrate metabolism reminiscent of the severe malnutrition syndromes of adult kwashiorkor or marasmus (inanition). Moreover, fluctuations in weight under acute or semia-cute situations often reflect changes in fluid balance. This is also the situation in patients with end-stage renal failure undergoing chronic dialysis. Methods such as bioelectrical impedance, dual X-ray absor-bance, or isotope dilution in association with indirect calorimetry can assess true lean- and fat-mass status and macronutrient metabolism in patients of apparently normal body mass.

Hematological evaluation is important in nutritional assessment. A low hemoglobin, hematocrit, or red cell count signifies anemia, but in individuals with associated diseases, anemia can have a series of origins (hemolytic, hypoproliferative) that are non-nutritional and will not respond to nutritional therapy.

Biochemical evaluation for nutrient deficiency status in patients with associated disease is fraught with caveats and limitations. Michael Golden has defined two classes of nutritional deficiency: in type 1 deficiencies, nutritional desaturation of tissue stores occurs, and circulating levels of nutrients reflect the total body nutrient status; in type 2 deficiency, there is homeostatic conservation of tissue and circulating concentrations of nutrients, such that blood concentrations remain virtually unaltered in the face of depletion. Deficiencies of zinc and magnesium, among others, fall into this second category. Inflammation and infection are stimuli that directly alter the circulating concentrations of nutrient indicators. Ferritin and circulating copper are elevated whereas zinc, iron, and vitamin A concentrations are depressed with activation of the acute-phase response to injury. In liver disease, depressed production of binding proteins can alter the usual indicators of nutritional status as a consequence of hepatic pathophysiology itself, rather than preexisting secondary malnutrition. Finally, it almost goes without saying that attempting biochemical nutrient evaluations from blood samples taken during concurrent infusion of micronutrient solutions in parenteral nutrition regimens - and without a period of distribution and equilibration -will not reflect the tissue stores and total body reserves of the respective nutrients of interest.

Functional indicators of nutritional status have been applied to the assessment of secondary malnutrition and have been plagued by pitfalls. This applies to tests of nitrogen status, immune function, and hepatic protein secretion. Tests such as creati-nine excretion, white blood cell counts, and cutaneous delayed hypersensitivity anergy, as well as decreased serum albumin, transferrin, transthyro-nein (prealbumin), and retinal-binding protein concentrations are sensitive to alteration by stress and injury. Failure to recognize distortion from stress underlies an early fallacy in surgical nutrition, in which low values for albumin, lymphocyte counts, and pre-albumin, together with anergy, predicted poor postoperative outcomes. This misconception justified aggressive preoperative parenteral nutrition and albumin infusions, with little impact on predicted outcomes. In these situations, it was the stress and injury of the underlying disease, rather than nutritional status, that was producing the abnormal values for the biomarkers. Recently, insulin-like growth factor has been advanced as a sensitive indicator of protein status in older patients, but whether it is confounded by nonnutritional features of disease remains to be clarified.

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