Abstract
High-protein diets (HPDs) promote weight loss but other studies implicate these diets and their constituent amino acids (AAs) in insulin resistance. We hypothesized that AA-induced insulin resistance is a temporal and reversible metabolic event. L6 myotubes were serum deprived for 4 h and then incubated in AA and/or insulin (100 nmol/L). Another group of cells was incubated overnight in AA + insulin, starved again, and then reincubated with AA and insulin. Mammalian (mechanistic) target of rapamycin complex 1 (mTORC1) signaling and glucose uptake were then measured. Healthy or insulin-resistant rats were gavaged with leucine (0.48 g/kg) and insulin sensitivity was examined. In myotubes, incubation with AA and insulin significantly (P < 0.05) increased the phosphorylation of the mTORC1 substrate ribosomal protein S6 kinase 1 (S6K1, T389) and of insulin receptor substrate 1 (IRS-1, serine residues), but suppressed insulin-stimulated glucose uptake by 40% (P < 0.01). These modifications were mTORC1-dependent and were reversible. In vivo, leucine gavage reversibly increased S6K1 phosphorylation and IRS-1 serine phosphorylation 5- to 12-fold in skeletal muscle and impaired insulin tolerance of glucose (P < 0.05) in lean rats. In insulin-resistant rats, the impairment of whole blood glucose and AA metabolism induced by leucine gavage (0.001 < P < 0.05) was more severe than that observed in lean rats; however, the impairment was reversible within 24 h of treatment. If these data are confirmed in long-term studies, it would imply that the use of leucine/HPD in treating metabolic diseases is unlikely to have lasting negative effects on insulin sensitivity.