EFFETTI DEL'INSULINA E DEGLI AMINOACIDI SULL'ANABOLISMO PROTEICO POST-PRANDIALE

In type 1 diabetes mellitus, insulin deficiency leads to a loss of protein mass, due to increased protein breakdown and lack of an adequate increase in protein synthesis. Protein anabolism takes place only in the absorptive period. Among hormones and nutrients, insulin and amino acids play a key role in its regulation, but their relative contribution has not been established yet. Thus, we studied 15 healthy subjects, divided in 3 groups. The rates of whole body protein kinetics, and the fractional secretory rates (FSR) of albumin, fibrinogen, antithrombin III (ATIII), and immunoglobulins G (IgG) were measured, during L-[1-¹⁴C]leucine infusion, in the post-absorptive period, and during the intragastric (i.g.) infusion of water (Control n = 5), a liquid mixed meal (Meal + AA n = 5), or during the i.g. infusion of an isocaloric meal without amino acids (Meal - AA n = 5) that induced the same insulin response of the Meal + AA. The results of the present study indicate that post-prandial protein anabolism is mainly sustained (90%) by dietary amino acids (leucine balance: Meal + AA 0.77 ± 0.18, Meal - AA -0.20 ± 0.04, Control -0.30 ± 0.03, μmol·kg^-1·min^-1, p < 0.01 Meal + AA vs Control and Meal - AA). The mechanism responsible for prandial anabolism includes both an increase in whole body protein synthesis (Meal + AA 2.48 ± 0.15, Meal - AA 1.93 ± 0.13, Control 1.73 ± 0.14, μmol·kg^-1·min^-1, p < 0.01 Meal + AA vs Control and Meal - AA), and a decrease in whole body proteolysis (Meal + AA 1.71 ± 0.26, Meal - AA 2.13 ± 0.16, Control 2.02 ± 0.12, μmol·kg^-1·min^-1, p < 0.01 Meal + AA vs Control and Meal - AA). The absorption of the isocaloric glucose-lipid meal slightly reduced protein catabolism decreasing amino acid oxidation (Meal - AA 0.20 ± 0.04 vs Control 0.30 ± 0.03, μmol·kg^-1·min^-1, p = 0.06) and promoting the synthesis of selected proteins (albumin +20% vs Control, p < 0.05), because of the effect of prandial hyperinsulinaemia. The combined action of insulin and amino acids during the absorption of the mixed meal additionally increased albumin FSR (+50% vs Control, p < 0.01, p < 0.03 vs Meal - AA). IgG synthesis was stimulated only by increased amino acid availability (+40%, p < 0.01 vs Control and Meal - AA). Fibrinogen and ATIII FSR did not change during either meal absorption. In conclusion, these data indicate that, during meal absorption in type 1 diabetic patients, appropriate insulinization is required to promote adequate protein anabolism.