Manipulation of dietary carbohydrate and muscle glycogen affects glucose uptake during exercise when fat oxidation is impaired by beta-adrenergic blockade

Pioglitazone does not enhance exogenous glucose oxidation or metabolic clearance rate during aerobic exercise in men under acute high-altitude exposure

Insulin insensitivity decreases exogenous glucose oxidation and metabolic clearance rate (MCR) during aerobic exercise in unacclimatized lowlanders at high altitude (HA). Whether use of an oral insulin sensitizer before acute HA exposure enhances exogenous glucose oxidation is unclear. This study investigated the impact of pioglitazone (PIO) on exogenous glucose oxidation and glucose turnover compared with placebo (PLA) during aerobic exercise at HA. With the use of a randomized crossover design, native lowlanders (n = 7 males, means ± SD, age: 23 ± 6 yr, body mass: 84 ± 11 kg) consumed 145 g (1.8 g/min) of glucose while performing 80 min of steady-state (1.43 ± 0.16 V̇o2 L/min) treadmill exercise at HA (460 mmHg; [Formula: see text] 96.6 mmHg) following short-term (5 days) use of PIO (15 mg oral dose per day) or PLA (microcrystalline cellulose pill). Substrate oxidation and glucose turnover were determined using indirect calorimetry and stable isotopes ([13C]glucose and 6,6-[2H2]glucose). Exogenous glucose oxidation was not different between PIO (0.31 ± 0.03 g/min) and PLA (0.32 ± 0.09 g/min). Total carbohydrate oxidation (PIO: 1.65 ± 0.22 g/min, PLA: 1.68 ± 0.32 g/min) or fat oxidation (PIO: 0.10 ± 0.0.08 g/min, PLA: 0.09 ± 0.07 g/min) was not different between treatments. There was no treatment effect on glucose rate of appearance (PIO: 2.46 ± 0.27, PLA: 2.43 ± 0.27 mg/kg/min), disappearance (PIO: 2.19 ± 0.17, PLA: 2.20 ± 0.22 mg/kg/min), or MCR (PIO: 1.63 ± 0.37, PLA: 1.73 ± 0.40 mL/kg/min). Results from this study indicate that PIO is not an effective intervention to enhance exogenous glucose oxidation or MCR during acute HA exposure. Lack of effect with PIO suggests that the etiology of glucose metabolism dysregulation during acute HA exposure may not result from insulin resistance in peripheral tissues.NEW & NOTEWORTHY Short-term (5 days) use of the oral insulin sensitizer pioglitazone does not alter circulating glucose or insulin responses to enhance exogenous glucose oxidation during steady-state aerobic exercise in young healthy men under simulated acute (8 h) high-altitude (460 mmHg) conditions. These results indicate that dysregulations in glucose metabolism in native lowlanders sojourning at high altitude may not be due to insulin resistance at peripheral tissue.

Impact of Dietary Carbohydrate Restriction versus Energy Restriction on Exogenous Carbohydrate Oxidation during Aerobic Exercise

Individuals with high physical activity levels, such as athletes and military personnel, are likely to experience periods of low muscle glycogen content. Reductions in glycogen stores are associated with impaired physical performance. Lower glycogen stores in these populations are likely due to sustained aerobic exercise coupled with sub-optimal carbohydrate or energy intake. Consuming exogenous carbohydrate during aerobic exercise may be an effective intervention to sustain physical performance during periods of low glycogen. However, research is limited in the area of carbohydrate recommendations to fuel performance during periods of sub-optimal carbohydrate and energy intake. Additionally, the studies that have investigated the effects of low glycogen stores on exogenous carbohydrate oxidation have yielded conflicting results. Discrepancies between studies may be the result of glycogen stores being lowered by restricting carbohydrate or restricting energy intake. This narrative review discusses the influence of low glycogen status resulting from carbohydrate restriction versus energy restriction on exogenous carbohydrate oxidation and examines the potential mechanism resulting in divergent responses in exogenous carbohydrate oxidation. Results from this review indicate that rates of exogenous carbohydrate oxidation can be maintained when glycogen content is lower following carbohydrate restrictions, but may be reduced following energy restriction. Reductions in exogenous carbohydrate oxidation following energy restriction appear to result from lower insulin sensitivity and glucose uptake. Exogenous carbohydrate may thus be an effective intervention to sustain performance following short-term energy adequate carbohydrate restriction, but may not be an effective ergogenic aid when glycogen stores are low due to energy restriction.

Metabolomic profiles are reflective of hypoxia-induced insulin resistance during exercise in healthy young adult males

Hypoxia-induced insulin resistance appears to suppress exogenous glucose oxidation during metabolically matched aerobic exercise during acute (<8 h) high-altitude (HA) exposure. However, a better understanding of this metabolic dysregulation is needed to identify interventions to mitigate these effects. The objective of this study was to determine if differences in metabolomic profiles during exercise at sea level (SL) and HA are reflective of hypoxia-induced insulin resistance. Native lowlanders (n = 8 males) consumed 145 g (1.8 g/min) of glucose while performing 80-min of metabolically matched treadmill exercise at SL (757 mmHg) and HA (460 mmHg) after 5-h exposure. Exogenous glucose oxidation and glucose turnover were determined using indirect calorimetry and dual tracer technique ([¹³C]glucose and [6,6-²H₂]glucose). Metabolite profiles were analyzed in serum as change (Δ), calculated by subtracting postprandial/exercised state SL (ΔSL) and HA (ΔHA) from fasted, rested conditions at SL. Compared with SL, exogenous glucose oxidation, glucose rate of disappearance, and glucose metabolic clearance rate (MCR) were lower (P < 0.05) during exercise at HA. One hundred and eighteen metabolites differed between ΔSL and ΔHA (P < 0.05, Q < 0.10). Differences in metabolites indicated increased glycolysis, tricarboxylic acid cycle, amino acid catabolism, oxidative stress, and fatty acid storage, and decreased fatty acid mobilization for ΔHA. Branched-chain amino acids and oxidative stress metabolites, Δ3-methyl-2-oxobutyrate (r = -0.738) and Δγ-glutamylalanine (r = -0.810), were inversely associated (P < 0.05) with Δexogenous glucose oxidation. Δ3-Hydroxyisobutyrate (r = -0.762) and Δ2-hydroxybutyrate/2-hydroxyisobutyrate (r = -0.738) were inversely associated (P < 0.05) with glucose MCR. Coupling global metabolomics and glucose kinetic data suggest that the underlying cause for diminished exogenous glucose oxidative capacity during aerobic exercise is acute hypoxia-mediated peripheral insulin resistance.

Carbohydrate supplementation: a critical review of recent innovations

PURPOSE

To critically examine the research on novel supplements and strategies designed to enhance carbohydrate delivery and/or availability.

METHODS

Narrative review.

RESULTS

Available data would suggest that there are varying levels of effectiveness based on the supplement/supplementation strategy in question and mechanism of action. Novel carbohydrate supplements including multiple transportable carbohydrate (MTC), modified carbohydrate (MC), and hydrogels (HGEL) have been generally effective at modifying gastric emptying and/or intestinal absorption. Moreover, these effects often correlate with altered fuel utilization patterns and/or glycogen storage. Nevertheless, performance effects differ widely based on supplement and study design. MTC consistently enhances performance, but the magnitude of the effect is yet to be fully elucidated. MC and HGEL seem unlikely to be beneficial when compared to supplementation strategies that align with current sport nutrition recommendations. Combining carbohydrate with other ergogenic substances may, in some cases, result in additive or synergistic effects on metabolism and/or performance; however, data are often lacking and results vary based on the quantity, timing, and inter-individual responses to different treatments. Altering dietary carbohydrate intake likely influences absorption, oxidation, and and/or storage of acutely ingested carbohydrate, but how this affects the ergogenicity of carbohydrate is still mostly unknown.

CONCLUSIONS

In conclusion, novel carbohydrate supplements and strategies alter carbohydrate delivery through various mechanisms. However, more research is needed to determine if/when interventions are ergogenic based on different contexts, populations, and applications.

Acute hypoxia reduces exogenous glucose oxidation, glucose turnover, and metabolic clearance rate during steady-state aerobic exercise

BACKGROUND

Exogenous carbohydrate oxidation is lower during steady-state aerobic exercise in native lowlanders sojourning at high altitude (HA) compared to sea level (SL). However, the underlying mechanism contributing to reduction in exogenous carbohydrate oxidation during steady-state aerobic exercise performed at HA has not been explored.

OBJECTIVE

To determine if alterations in glucose rate of appearance (R_a), disappearance (R_d) and metabolic clearance rate (MCR) at HA provide a mechanism for explaining the observation of lower exogenous carbohydrate oxidation compared to during metabolically-matched, steady-state exercise at SL.

METHODS

Using a randomized, crossover design, native lowlanders (n = 8 males, mean ± SD, age: 23 ± 2 yr, body mass: 87 ± 10 kg, and VO_2peak: SL 4.3 ± 0.2 L/min and HA 2.9 ± 0.2 L/min) consumed 145 g (1.8 g/min) of glucose while performing 80-min of metabolically-matched (SL: 1.66 ± 0.14 V̇O₂ L/min 329 ± 28 kcal, HA: 1.59 ± 0.10 V̇O₂ L/min, 320 ± 19 kcal) treadmill exercise in SL (757 mmHg) and HA (460 mmHg) conditions after a 5-h exposure. Substrate oxidation rates (g/min) and glucose turnover (mg/kg/min) during exercise were determined using indirect calorimetry and dual tracer technique (¹³C-glucose oral ingestion and [6,6-²H₂]-glucose primed, continuous infusion).

RESULTS

Total carbohydrate oxidation was higher (P < 0.05) at HA (2.15 ± 0.32) compared to SL (1.39 ± 0.14). Exogenous glucose oxidation rate was lower (P < 0.05) at HA (0.35 ± 0.07) than SL (0.44 ± 0.05). Muscle glycogen oxidation was higher at HA (1.67 ± 0.26) compared to SL (0.83 ± 0.13). Total glucose R_a was lower (P < 0.05) at HA (12.3 ± 1.5) compared to SL (13.8 ± 2.0). Exogenous glucose R_a was lower (P < 0.05) at HA (8.9 ± 1.3) compared to SL (10.9 ± 2.2). Glucose R_d was lower (P < 0.05) at HA (12.7 ± 1.7) compared to SL (14.3 ± 2.0). MCR was lower (P < 0.05) at HA (9.0 ± 1.8) compared to SL (12.1 ± 2.3). Circulating glucose and insulin concentrations were higher in response carbohydrate intake during exercise at HA compared to SL.

CONCLUSION

Novel results from this investigation suggest that reductions in exogenous carbohydrate oxidation at HA may be multifactorial; however, the apparent insensitivity of peripheral tissue to glucose uptake may be a primary determinate.

Exercising with low muscle glycogen content increases fat oxidation and decreases endogenous, but not exogenous carbohydrate oxidation

BACKGROUND

Initiating aerobic exercise with low muscle glycogen content promotes greater fat and less endogenous carbohydrate oxidation during exercise. However, the extent exogenous carbohydrate oxidation increases when exercise is initiated with low muscle glycogen is unclear.

PURPOSE

Determine the effects of muscle glycogen content at the onset of exercise on whole-body and muscle substrate metabolism.

METHODS

Using a randomized, crossover design, 12 men (mean ± SD, age: 21 ± 4 y; body mass: 83 ± 11 kg; VO_2peak: 44 ± 3 mL/kg/min) completed 2 cycle ergometry glycogen depletion trials separated by 7-d, followed by a 24-h refeeding to elicit low (LOW; 1.5 g/kg carbohydrate, 3.0 g/kg fat) or adequate (AD; 6.0 g/kg carbohydrate, 1.0 g/kg fat) glycogen stores. Participants then performed 80 min of steady-state cycle ergometry (64 ± 3% VO_2peak) while consuming a carbohydrate drink (95 g glucose +51 g fructose; 1.8 g/min). Substrate oxidation (g/min) was determined by indirect calorimetry and ¹³C. Muscle glycogen (mmol/kg dry weight), pyruvate dehydrogenase (PDH) activity, and gene expression were assessed in muscle.

RESULTS

Initiating steady-state exercise with LOW (217 ± 103) or AD (396 ± 70; P < 0.05) muscle glycogen did not alter exogenous carbohydrate oxidation (LOW: 0.84 ± 0.14, AD: 0.87 ± 0.16; P > 0.05) during exercise. Endogenous carbohydrate oxidation was lower and fat oxidation was higher in LOW (0.75 ± 0.29 and 0.55 ± 0.10) than AD (1.17 ± 0.29 and 0.38 ± 0.13; all P < 0.05). Before and after exercise PDH activity was lower (P < 0.05) and transcriptional regulation of fat metabolism (FAT, FABP, CPT1a, HADHA) was higher (P < 0.05) in LOW than AD.

CONCLUSION

Initiating exercise with low muscle glycogen does not impair exogenous carbohydrate oxidative capacity, rather, to compensate for lower endogenous carbohydrate oxidation acute adaptations lead to increased whole-body and skeletal muscle fat oxidation.

Adipose triglyceride lipase deletion from adipocytes, but not skeletal myocytes, impairs acute exercise performance in mice

Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme mediating triacylglycerol hydrolysis in virtually all cells, including adipocytes and skeletal myocytes, and hence, plays a critical role in mobilizing fatty acids. Global ATGL deficiency promotes skeletal myopathy and exercise intolerance in mice and humans, and yet the tissue-specific contributions to these phenotypes remain unknown. The goal of this study was to determine the relative contribution of ATGL-mediated triacylglycerol hydrolysis in adipocytes vs. skeletal myocytes to acute exercise performance. To achieve this goal, we generated murine models with adipocyte- and skeletal myocyte-specific targeted deletion of ATGL. We then subjected untrained mice to acute peak and submaximal exercise interventions and assessed exercise performance and energy substrate metabolism. Impaired ATGL-mediated lipolysis within adipocytes reduced peak and submaximal exercise performance, reduced peripheral energy substrate availability, shifted energy substrate preference toward carbohydrate oxidation, and decreased HSL Ser(660) phosphorylation and mitochondrial respiration within skeletal muscle. In contrast, impaired ATGL-mediated lipolysis within skeletal myocytes was not sufficient to reduce peak and submaximal exercise performance or peripheral energy substrate availability and instead tended to enhance metabolic flexibility during peak exercise. Furthermore, the expanded intramyocellular triacylglycerol pool in these mice was reduced following exercise in association with preserved HSL phosphorylation, suggesting that HSL may compensate for impaired ATGL action in skeletal muscle during exercise. These data suggest that adipocyte rather than skeletal myocyte ATGL-mediated lipolysis plays a greater role during acute exercise in part because of compensatory mechanisms that maintain lipolysis in muscle, but not adipose tissue, when ATGL is absent.

Long-term dietary effects on substrate selection and muscle fiber type in very-long-chain acyl-CoA dehydrogenase deficient (VLCAD(-/-)) mice

Dietary fat restriction and increased carbohydrate intake are part of treatment in very-long-chain acyl-CoA dehydrogenase (VLCAD)-deficiency, the most common defect of long-chain fatty acid oxidation. The long-term impact of these interventions is unknown. We characterized here the effects of a fat-reduced, carbohydrate-enriched diet and an increased fat intake on energy metabolism in a mouse model of VLCAD-deficiency. Wild-type and VLCAD(-/-) mice were fed one year either with a normal (5.1%), a high fat (10.6%) or a low-fat, carbohydrate-enriched (2.6%) diet. Dietary effects on genes involved in lipogenesis, energy homeostasis and substrate selection were quantified by real-time-PCR. Acylcarnitines as sign of impaired energy production were determined in dried blood spots and tissues. White skeletal muscle was analyzed for muscle fiber type as well as for glycogen and triglyceride content. Both dietary modifications induced enhanced triacylglyceride accumulation in skeletal muscle and inhibition of glucose oxidation. This was accompanied by an up-regulation of genes coding for oxidative muscle fiber type I and a marked accumulation of acylcarnitines, especially prominent in the heart (164±2.8 in VLCAD(-/-) vs. 82.3±2.1 in WT μmol/mg) under a low-fat, carbohydrate-enriched diet. We demonstrate here that both dietary interventions with respect to the fat content of the diet reverse endogenous compensatory mechanisms in muscle that have evolved in VLCAD(-/-) mice resulting in pronounced energy deficiency. In particular, the low-fat carbohydrate-enriched diet was not effective in the long term. Further experiments are necessary to define the optimal energy provision for fatty acid oxidation defects.

Energy deficit without reducing dietary carbohydrate alters resting carbohydrate oxidation and fatty acid availability

Reduced carbohydrate (CHO) availability after exercise has a potent influence on the regulation of substrate metabolism, but little is known about the impact of fat availability and/or energy deficit on fuel metabolism when dietary CHO availability is not reduced. The purpose of this study was to determine the influence of a postexercise energy deficit, independent of CHO availability, on plasma substrate concentrations and substrate oxidation. Seven moderately trained men (peak oxygen uptake: 56 ± 2 ml·kg−1·min−1) performed exhaustive cycling exercise on two separate occasions. The two trials differed only by the meals ingested after exercise: 1) a high-fat diet designed to maintain energy balance or 2) a low-fat diet designed to elicit energy deficit. The CHO and protein contents of the diets were identical. The next morning, we measured plasma substrate and insulin concentrations and CHO oxidation, and we obtained muscle biopsies from the vastus lateralis for measurement of pyruvate dehydrogenase kinase (PDK)-2 and PDK-4 mRNA expression by using RT-PCR. Despite identical blood glucose (5.0 ± 0.1 and 4.9 ± 0.1 mM) and insulin (7.9 ± 1.1 and 8.4 ± 0.9 μU/ml) concentrations, plasma fatty acid and glycerol concentrations were elevated three- to fourfold during energy deficit compared with energy balance and CHO oxidation was 40% lower ( P < 0.01) the morning after energy deficit compared with energy balance (328 ± 69 and 565 ± 89 μmol/min). The lower CHO oxidation was accompanied by a 7.3 ± 2.5-fold increase in PDK-4 mRNA expression after energy deficit ( P < 0.05), whereas PDK-2 mRNA was similar between the trials. In conclusion, energy deficit increases fatty acid availability, increases PDK-4 mRNA expression, and suppresses CHO oxidation even when dietary CHO content is not reduced.