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INTRODUCTION

The practice of clinical nutrition, particularly in the setting of acute illness, is somewhat complex due to the multiple interactions between disease and the ability to nourish patients. Much of clinical practice is based on observation or on flawed, biased, or severely underpowered studies. Furthermore, terminology related to malnutrition is extremely confusing: terms referring to problems attributable to alterations in nutritional intake are intermixed with those phenomena secondary to inflammatory illness, such as disease-related muscle wasting, which are not responsive to changes in nutrition intake.

Many excellent clinical publications1,2 review general and specific nutritional concepts. This chapter introduces newly adopted definitions for malnutrition and their clinical utility, and reviews approaches to the clinical challenges specific to selected pulmonary diseases, acute respiratory failure, and the critical care setting. Chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF) serve as the prototypes for nutrition-related issues encountered in chronic pulmonary disease.

MALNUTRITION REDEFINED

Malnutrition has long been known to be associated with poor outcomes in medical and surgical patients.3 However, for decades, the terminology describing malnutrition has created great confusion, incorporating biomarkers, such as albumin, which have since been disproven as reflective of dietary intake. While new efforts focus on defining malnutrition based on etiology,4 the use of clinical parameters unrelated and unresponsive to nutrient intake5 remain entrenched. The new definition of malnutrition (Table 150-1), adopted by major nutrition organizations in the United States, describes two forms: malnutrition due to nutrient imbalance or malnourishment (e.g., starvation, marasmus, obesity), and that which occurs due to systemic illness (e.g., catabolism, cachexia).6 Only malnutrition due to nutrient imbalance resulting from alterations in intake or uptake can be treated by alterations in nourishment. It remains challenging, however, to decipher whether alterations in body habitus are due to illness, or due to altered intake, or both, and even whether and when changes in intake or nutrient levels represent deficiency or disease epiphenomena. While the new definitions have not been validated to predict which patients will respond to nutritional interventions, they are highly predictive of overall outcomes.7

TABLE 150-1Findings in the New Definition of Malnutrition

Inflammatory markers, such as C-reactive protein, and serum albumin and prealbumin, do not correlate with nutrient intake, and the belief that most traditional markers of malnutrition (e.g., albumin, prealbumin [transthyretin], measures of immune function) reflect adequacy of nourishment has been disproved.5 Despite this, not all of these markers have been removed from the new definition.4 While it still may be surprising to many clinicians that many so-called nutritional markers are, in truth, ...

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