9. Conclusions

The dietary conditions to get the best protein-sparing effect require that energy balance be achieved in providing carbohydrate and fat in sufficient amounts. When severely depleted patients undergo nutritional rehabilitation, a positive energy balance (not exceeding + 20%) is advisable, since an energy excess may improve N retention. The proportion of carbohydrate and fat energy depends upon the age and clinical condition of the patients. Most studies show that a proportion of 25 to 50% of fat as non-protein energy is to be recommended for patients who need nutritional rehabilitation or for growing children. It is well established that glucose and free fatty acids have a protein-sparing action. When the clinical conditions do not allow (or do not require) to reach energy balance, dietary glucose has a greater protein-sparing action than dietary lipids. This is probably mainly due to the increased glucose-induced secretion of insulin, which inhibits muscle proteolysis and hepatic gluconeogenesis. When energy balance is reached, or under conditions of positive energy balance, glucose and lipids appear to have a similar protein-sparing action. For patients requiring TPN, the advantage of a balanced mixture of glucose and lipid is a smaller thermogenic response to nutrients, than when glucose is the only non-protein energy source.

When energy balance cannot be obtained, it is not reasonable to increase N intake to improve N balance, since protein oxidation will be increased as a result and the dietary protein will be used mainly as energy source with the disadvantage of an increased ureogenesis and an enhanced thermogenic response.