Enteral Versus Parenteral Feeding in Critical Illness

Hypermetabolic, critically ill patients rapidly mobilize lean tissue, releasing large quantities of amino acids into the circulating amino acid pool for distribution to the liver, intestine, bone marrow, and injured or healing tissues. Within these sites, gluconeogenesis, immune cell proliferation, and fibroblast proliferation support the body’s attempts to seal and heal its injuries while maintaining defenses against bacterial challenges. Glutamine and alanine synthesis up-regulated through the metabolism of branched-chain amino acids by skeletal muscle provide specific fuels for enterocytes and rapidly proliferating immunocytes as well as sources of carbon skeletons for gluconeogenesis [1]. In addition, amino acids from the general amino acid pool are synthesized by the liver into acute-phase production to upregulate effectiveness of the reticuloendothelial system in clearing foreign material from the bloodstream [2]. While current opinion suggests that well-nourished patients can maintain a vigorous metabolic response for a week or perhaps longer, certain mechanisms of mucosal defenses may be influenced by nutrition support even in well-nourished patients.