Hypocaloric lipid emulsions and amino acid metabolism in injured rats

The present study was designed to determine the degree and mechanism by which administration of medium-chain triglyceride emulsions spare body protein after injury. Forty male rats underwent venous catheterization and received nonsterile bilateral femur fractures. All rats received 2.5 g/day amino acids and either no additional calories (group I) or 20 kcal/day of either glucose (group II), a long-chain triglyceride emulsion (group III), a medium-chain triglyceride emulsion (group IV), or a structured lipid emulsion composed of 40% sunflower oil and 60% medium-chain triglycerides (group V). The diets were administered for 3 days, and rates of plasma leucine flux, oxidation, and incorporation into protein as well as tissue protein synthetic rates in liver and muscle were measured using the constant infusion of L-[1-14C]leucine. Results demonstrated that the administration of glucose or various lipid emulsions improved cumulative nitrogen balance significantly when compared to a diet containing amino acids alone. In addition, the administration of glucose or lipid emulsions significantly stimulated protein synthesis in liver and muscle. Moreover, a structured lipid emulsion of medium- and long-chain fatty acids produced significant increases in liver protein synthesis greater than that observed with either glucose or long-chain triglyceride emulsions. We conclude that added energy as fat or glucose reduces net protein catabolism and improves tissue protein synthesis in these injures rats and that lipid emulsions are as effective as dextrose. A structured triglyceride emulsion synthesized from medium- and long-chain fatty acids appears to better support hepatic protein synthesis.