The effect of increasing total parenteral nutrition on protein metabolism

We asked the question, if the amount of nutrients given parenterally is progressively increased, does the rate of whole body protein synthesis rate increase correspondingly and how does the protein breakdown rate change? Eight malnourished patients requiring total parenteral nutrition (TPN) were studied. We measured their whole body protein synthesis and breakdown rate four times at intervals of 3 days using ¹⁵N glycine as the tracer. The first study was done pre-TPN. The 2nd, 3rd, and 4th studies were done at increasing TPN rates. The rates were TPN 1, 1440 kcal/day and 7.9 g/N/day; TPN 2, 2160 kcal/day and 11.9 g/N/day; TPN 3, 2880 kcal/day and 15.8 g/N/day. The protein synthesis rate initially increased as the amount of TPN was increased, but increasing the rate from TPN-2 to TPN-3 did not result in further increase. Increasing the rate of TPN above a certain level does not lead to a concomitant increase in the protein synthesis rate.