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Consumption of carbohydrate or medium- or long-chain triglycerides by unfed rats exerts different protein-sparing effects.

Consumption of nonprotein energy is reported to promote nitrogen retention (the protein-sparing effect). The present study was conducted to determine whether consumption of medium-chain triglycerides (MCT), long-chain triglycerides (LCT) or carbohydrate by unfed rats would exert different protein-sparing effects over 48 h. The amounts of MCT, LCT and carbohydrate were adjusted to be isoenergetic. The control group received only water. The urinary excretion of urea in the groups fed carbohydrate or MCT was significantly lower than in the LCT and control groups within the first 24 h. Urinary excretion of urea in the group fed LCT was similar to that of the control group for the first 24 h but was significantly lower after 48 h. Liver serine dehydratase activities in the groups fed MCT or LCT were similar, although they were significantly lower than in the control group. Serine dehydratase activity in the carbohydrate-fed group was the lowest. Liver L-lysine-2-oxoglutarate reductase activity was significantly lower in the groups fed carbohydrate, MCT or LCT than in the control group. Rats fed MCT had the lowest plasma concentrations of lysine and branched-chain amino acids. However, the plasma concentration of glutamic acid in the rats fed MCT was the highest. These results indicate that rats fed MCT, LCT or carbohydrate exhibit different protein-sparing effects and that the difference between rats fed MCT and those fed LCT is not attributable to differences in amino acid-degrading enzymes. Consumption of MCT may conserve some amino acid-derived nitrogen as glutamic acid.