Parenteral nutrition, biochemistry and respiratory gas exchange.

Lipid-based total parenteral nutrition (TPN) may have an advantage over glucose-based TPN in certain patients because less carbon dioxide is produced. This conclusion, however, does not properly derive from respiratory quotient (RQ) data. Stoichiometric analysis of glucose and lipid metabolism leads to conclusions similar clinically but at variance conceptually with those based on RQ. Thus, changing from glucose to lipid oxidation is associated with an 11% increase in oxygen consumption, a 23% decrease in carbon dioxide production, and a fall in RQ. In contrast, the diversion of glucose metabolism to synthesis of palmitylstearyloleyl triglyceride results in an 8-fold increase in RQ, but contrary to widely published concepts, the rate of carbon dioxide production falls by 68%. However, even this reduced carbon dioxide production associated with lipogenesis is undesirable because it is not associated with significant ATP production.