The nutritional support is essential in the treatment of the septic patient, but the optimal substrates, have not been well defined, although several studies advise the use of solutions with a high proportion of branched amino acids. The use of lipids in the Parenteral Nutrition (PN) of these patients is presently accepted by all authors, although there is no consensus of the composition of the mix to be used. We have done a prospective, randomized study in a group of critical septic patients who received PN. All cases were used a solution with a high proportion of branched amino acids and glucose. 40% of the non-protein calories were administered as lipids. Based on this, two groups were formed. Group 1 medium chain triglycerides and long chain triglycerides (MCT/LCT) in a ratio of 1:1, and Group II LCT. After ten days, both groups showed an increase in hepatic cholestasis enzymes and an improvement of the parameters of protein anabolism and catabolism. Group 1 showed a significantly higher increase in the retinol birdring protein and the improvement of the nitrogen balance. Besides this, the global analysis of all the parameters of protein anabolism and catabolism showed a more favorable evolution, with statistical significance, in those who received MCT/LCT. Therefore, the MCT/LCT in the PN of the septic patient who is given a solution rich in branched amino acids, achieves a greater protein saving and a faster recevesy of the markers of protein synthesis, than those with LCT. Both groups show an increase in cholestasis enzymes.