Protein and fat metabolism in rats during repletion with total parenteral nutrition (TPN)

We investigated the effect of non-protein calorie source on the repletion by total parenteral nutrition (TPN) of protein-depleted rats. Rats were depleted by feeding them a protein free diet for 4 weeks. Control (non-depleted) and depleted rats were given one of three isocaloric, isonitrogenous TPN regimens for 6 days. They differed in the proportion of non-protein calories given as glucose and fat: diet 1, 100% glucose; diet 2, 75% glucose, 25% fat; and diet 3, 100% fat. The liver, skeletal muscle and whole body protein synthesis and break-down rates were measured by using [15N]glycine as a tracer. The fatty acid distribution in the liver was measured by gas chromatography-mass spectrometry.

(a) Omission of fat leads to a decrease in the fraction of linoleate in the liver. (b) Giving all the non-protein calories as a fat emulsion causes fatty acid accumulation in the liver and a significant increase in the liver protein fractional synthesis rate in the control rats and killed about half the depleted rats. (c) For TPN regimens 1 and 2 both the muscle and whole body synthesis rates were increased during repletion. (d) Of the three TPN regimens, the second one which contained both fat and glucose was superior to the other two.