Protein Ingestion Induces Muscle Insulin Resistance Independent of Leucine-Mediated mTOR Activation

Abstract

Increased plasma branched-chain amino acid concentrations are associated with insulin resistance, and intravenous amino acid infusion blunts insulin-mediated glucose disposal. We tested the hypothesis that protein ingestion impairs insulin-mediated glucose disposal by leucine-mediated mTOR signaling, which can inhibit AKT. We measured glucose disposal and muscle p-mTOR(Ser2448), p-AKT(Ser473), and p-AKT(Thr308) in 22 women during a hyperinsulinemic-euglycemic clamp procedure with and without concomitant ingestion of whey protein (0.6 g/kg fat-free mass; n = 11) or leucine that matched the amount given with whey protein (n = 11). Both whey protein and leucine ingestion raised plasma leucine concentration by approximately twofold and muscle p-mTOR(Ser2448) by ∼30% above the values observed in the control (no amino acid ingestion) studies; p-AKT(Ser473) and p-AKT(Thr308) were not affected by whey protein or leucine ingestion. Whey protein ingestion decreased insulin-mediated glucose disposal (median 38.8 [quartiles 30.8, 61.8] vs. 51.9 [41.0, 77.3] µmol glucose/µU insulin · mL(-1) · min(-1); P < 0.01), whereas ingestion of leucine did not (52.3 [43.3, 65.4] vs. 52.3 [43.9, 73.2]). These results indicate that 1) protein ingestion causes insulin resistance and could be an important regulator of postprandial glucose homeostasis and 2) the insulin-desensitizing effect of protein ingestion is not due to inhibition of AKT by leucine-mediated mTOR signaling.

Trial registration: ClinicalTrials.gov NCT01538836 NCT01757340.

Figure 1

Effects of whey protein and leucine ingestion on whole-body glucose R_d (upper panel) and leg glucose uptake (lower panel) (□, basal; ■, clamp). Data are means ± SEM. Three-way ANOVA revealed a significant group (whey protein vs. leucine groups) × study (control vs. whey protein or leucine ingestion) × condition (basal vs. clamp) interaction (P < 0.001) for whole-body glucose R_d. ANCOVA with plasma insulin concentration as a covariate revealed a significant study (control vs. whey protein) × condition (basal vs. clamp) interaction (P < 0.05) for whole-body glucose R_d and leg glucose uptake. Tukey post hoc analysis revealed the following significant differences. *Significantly different from corresponding basal value (P < 0.01); †significantly different from corresponding control value (P < 0.01); ‡significantly different from corresponding control value (P < 0.05).

Figure 2

Effect of whey protein ingestion on p-AMPK^Thr172, p-ACC^Ser79, p-mTOR^Ser2448, p-p70S6K^Thr389, p-AKT^Ser473, and p-AKT^Thr308 (arbitrary units) in muscle (□, basal; ■, clamp). AMPK, p-ACC, mTOR, and p-p70S6K data are means ± SEM; AKT data were log transformed for ANOVA and are presented as backtransformed geometric means and errors. Three-way ANOVA revealed a significant study (control vs. whey protein or leucine ingestion) × condition (basal vs. clamp) interaction (P < 0.05) for p-mTOR^Ser2448 and p-p70S6K^Thr389 and a significant main effect of clamp (P < 0.001) for p-AKT^Ser473 and p-AKT^Thr308. Tukey post hoc analysis revealed the following significant differences. *Significantly different from corresponding basal value (P < 0.05); †significantly different from corresponding control value (P < 0.05).

Figure 3

Effect of leucine ingestion on p-AMPK^Thr172, p-ACC^Ser79, p-mTOR^Ser2448, p-p70S6K^Thr389, p-AKT^Ser473, and p-AKT^Thr308 (arbitrary units) in muscle (□, basal; ■, clamp). AMPK, p-ACC, mTOR, and p-p70S6K data are means ± SEM; AKT data were log transformed for ANOVA and are presented as backtransformed geometric means and errors. Three-way ANOVA revealed a significant study (control vs. whey protein or leucine ingestion) × condition (basal vs. clamp) interaction (P < 0.05) for p-mTOR^Ser2448 and p-p70S6K^Thr389 and a significant main effect of clamp (P < 0.001) for p-AKT^Ser473 and p-AKT^Thr308. Tukey post hoc analysis revealed the following significant differences. *Significantly different from corresponding basal value (P < 0.05); †significantly different from corresponding control value (P < 0.05).