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-mTORSer2448, p-AKTSer473, and p-AKTThr308 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-mTORSer2448 by ∼30% above the values observed in the control (no amino acid ingestion) studies; p-AKTSer473 and p-AKTThr308 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.

High-Protein Intake during Weight Loss Therapy Eliminates the Weight-Loss-Induced Improvement in Insulin Action in Obese Postmenopausal Women

“…Data from studies conducted in cultured myotubes, isolated rat skeletal muscles and transgenic mice have demonstrated that amino acids, particularly the branched-chain amino acid leucine, can impair insulin-mediated glucose uptake by AMPK-mediated mTOR phosphorylation and subsequent negative feedback inhibition of PI3K-AKT signaling (Iwanaka et al, 2010, Saha et al, 2011) or possibly downstream signaling to 4E-BP1 (Tsai et al, 2015). In contrast, we recently found these pathways were not involved in the inhibition of insulin-mediated glucose uptake that occurred with acute protein ingestion in people (Smith et al, 2015). In the current study, we also found that chronic HP intake impaired the weight loss-induced improvement in insulin-mediated glucose uptake in the absence of differences in p-AMPK Thr172 , p-mTOR Ser2448 and p-4E-BP1 Thr37/46 in muscle.…”

“…Total plasma free fatty acid concentration was quantified by using an enzymatic colorimetric assay (Wako Diagnostics, Richmond, VA). Plasma glucose and palmitate tracer-to-tracee ratios (TTR) were determined by using GC-MS as previously described (Smith et al, 2015, Mittendorfer et al, 2003b). C3- and C5-acylcarnitine concentrations in plasma were quantified by using liquid chromatography-tandem mass spectrometry (LC-MS/MS, Applied Biosystems Sciex 4000QTRAP with Eclipse C18 column) after adding known amounts of propionyl-L-carnitine ( N-trimethyl -d 3 ) and isovaleryl-L-carnitine ( N,N,N-trimethyl -d 9 ) (both purchased from Cambridge Isotope Laboratories Inc) as internal standards and their conversion to methylesters as described (with minor modifications) by Forni et al (2010).…”

“…However, diet-induced weight loss also decreases lean tissue (including muscle) mass (Wycherley et al, 2012, Leidy et al, 2015), which could have adverse effects on physical function, particularly in populations who are at increased risk of sarcopenia, such as postmenopausal women (Samson et al, 2000, Phillips et al, 1993). Although increased protein intake during diet-induced weight loss is often recommended because it helps preserve lean tissue mass (Wycherley et al, 2012, Leidy et al, 2015), data from a series of studies suggest that high protein (HP) intake could have detrimental metabolic effects; acute intravenous amino acid infusion or protein ingestion reduces insulin sensitivity (Smith et al, 2015, Krebs et al, 2002, Robinson et al, 2014) and habitual HP intake is associated with insulin resistance and an increased risk of developing T2D (Linn et al, 1996, Sluijs et al, 2010, Tinker et al, 2011). The effect of increased protein intake per se on weight-loss induced changes in insulin sensitivity and glucose homeostasis are not known because of the confounding effects of differences in weight loss and food selection and overall diet composition (e.g., consumption of dairy and meat products and saturated and unsaturated fatty acids) between groups in studies that compared HP with standard protein diets (Rietman et al, 2014, Wycherley et al, 2012, Schwingshackl and Hoffmann, 2013).…”

Alterations in 3-Hydroxyisobutyrate and FGF21 Metabolism Are Associated With Protein Ingestion–Induced Insulin Resistance

“…Systemic hyperaminoacidemia, induced by either intravenous amino acid infusion or protein ingestion, acutely reduces insulin-stimulated glucose disposal (1–3), and chronic high protein intake is associated with an increased risk of type 2 diabetes (4–7). The mechanism(s) responsible for the adverse effects of dietary protein intake and the concomitant increase in circulating amino acids on insulin sensitivity is not clear.…”

Comment on Smith et al. Protein Ingestion Induces Muscle Insulin Resistance Independent of Leucine-Mediated mTOR Activation. Diabetes 2015;64:1555–1563

“…Smith et al (1) discussed the resistance to insulin properties of protein ingestion, focusing on the possible role of leucine as a master regulator of mTOR activation. But, finding leucine not guilty of insulin resistance is not a surprise as biochemistry indicates other amino acids as possible offenders.…”