Does Endurance Training Protect From Lipotoxicity?

Modest regular exercise and reduction of dietary fat halves the risk of developing type 2 diabetes. In overt type 2 diabetes, higher levels of exercise training improve glycemic control, whereas the impact of diet and optimal dietary composition are presently unknown (1). Thus, one can assume that intensive exercise protects from diet-induced insulin resistance. Intravenous lipid infusion is an established model to increase plasma free fatty acids (FFAs) and induce insulin resistance (2,3). Although plasma FFAs and intramyocellular lipid (IMCL) inversely correlate with insulin sensitivity in sedentary humans, athletes store more IMCL despite greater insulin sensitivity. This has been termed the “athletes’ paradox” (4). In this issue of Diabetes, Phielix et al. (5) hypothesize that, relative to their untrained counterparts, the high oxidative capacity of endurance-trained athletes attenuates lipid-induced insulin resistance during hyperinsulinemicnormoglycemic clamp tests. Results showed that the athletes’ higher VO2max was associated with greater ex vivo muscle mitochondrial capacity, insulin sensitivity, and carbohydrate oxidation. Lipid infusion reduced glucose disposal by 63% in untrained individuals, thereby confirming previous reports (3), but only by 29% in the athletes. The authors explained the athletes’ reduction in glucose disposal exclusively by diminished carbohydrate oxidation. They interpret the concomitant dephosphorylation of muscle glycogen synthase as stimulation of glycogen synthesis reflecting shuttling of glucose into nonoxidative storage as glycogen, in line with the “substrate (glucose:FFA) competition” theory of Randle et al. (6) (Fig. 1). The strength of this article includes combining in vivo and in vitro methods to assess muscle metabolism and signaling without interference from acute exercise effects. Nonetheless, some limitations need to be considered: 1) the nominally higher body weight and plasma FFAs during lipid infusion could have contributed to greater insulin resistance in the untrained participants; 2) indirect calorimetry does not measure tissue-specific nonoxidative metabolism; and 3) assessment of protein expression after prolonged insulin stimulation, which cannot trace the sequence of signaling events. Endurance training causes various adaptations such as increased muscle capillary density, glucose transporter-4 expression, and mitochondrial mass (7). Phielix et al. confirm this in that maximal oxidative phosphorylation expressed per muscle fiber was enhanced in athletes but not different from the untrained individuals when expressed per mitochondrial content. Nevertheless, baseline ATP synthase flux can be lower in relation to tricarboxylic acid cycle flux, thereby indicating less efficient mitochondrial coupling in athletes (8). Without lipid infusion, the athletes’ higher insulin sensitivity resulted from increased oxidative, but not nonoxidative, carbohydrate metabolism. In contrast, a comparable group of athletes had augmented nonoxidative glucose disposal, muscle glycogen synthase activity, and glycogen accumulation (9). Also in sedentary individuals, muscle glycogen synthesis, resulting from increased glucose transport/phosphorylation, accounts for wholebody insulin sensitivity (10). Finally, endurance training improves insulin sensitivity in first-degree relatives of patients with type 2 diabetes by increasing myocellular glucose-6-phosphate and glycogen concentrations (11). These findings indicate that the current study’s observation requires confirmation by direct monitoring of muscle glycogen synthesis and glucose transport/ phosphorylation. Direct monitoring of cellular glucose fluxes would also be important for the article’s main conclusion that lipidinduced insulin resistance is prevented in athletes by shuttling glucose toward glycogen storage. This reasoning favors the substrate competition concept of Randle et al. above the alternative mechanism, which relies on “substrate signaling,” i.e., the interaction of lipids with insulin signaling. Randle et al. (6) inferred from rodent studies that FFAs increase the intramitochondrial acetyl-CoA/CoA and NADH/NAD ratios, leading to pyruvate dehydrogenase inhibition (Fig. 1). Subsequently, glycolytic intermediates and glucose-6-phosphate would accumulate and inhibit hexokinase II (HKII) activity and glucose uptake. The alternative substrate signaling mechanism postulates that myocellular lipid intermediates (diacylglycerol [DAG], ceramides) act as “lipotoxins” to inhibit insulin signaling directly or via activation of novel protein kinase C isoforms (PKC) with subsequent impairment of glucose transport/phosphorylation and reduction in glycogen synthesis (Fig. 1). Indeed, lower increases in glucose-6phosphate precede lipid-induced reduction in insulin sensitivity and glycogen synthesis in sedentary humans (3). Phielix et al. confirm the reduced nonoxidative glucose disposal in untrained volunteers, whereas only glucose oxidation was lower in the athletes during lipid infusion. They speculate that the higher oxidative capacity of trained muscle allows for more efficient shifting from glucose to lipid oxidation. This would imply a rise in glucose-6-phosphate with decreased glucose uptake and continued glycogen synthesis. However, lipid oxidation was comparable between both groups in this study, and the lipid-induced decline of glucose oxidation was similar in another study (12). In the absence of data on glycolytic intermediates and glucose-6-phosphate, the From the Institute for Clinical Diabetology, German Diabetes Center, Leibniz Center for Diabetes Research at Heinrich-Heine University, Düsseldorf, Germany, and the Department of Metabolic Diseases, Heinrich-Heine University, Düsseldorf, Germany. Corresponding author: Michael Roden, [email protected]. DOI: 10.2337/db12-0662 2012 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by -nc-nd/3.0/ for details. See accompanying original article, p. 2472.