Oral [(13)C]glucose and endogenous energy substrate oxidation during prolonged treadmill running

Long-term physical inactivity exacerbates hindlimb unloading-induced muscle atrophy in young rat soleus muscle

This study investigated the effects of long-term physical inactivity in adolescent on subsequent hindlimb unloading-induced muscle atrophy in rat soleus muscle. First, 3-wk-old male Wistar rats were assigned to an age-matched control (n = 6) or a physical inactivity (n = 8) group. Rats in the physical inactivity group were housed in narrow cages with approximately half the usual floor space for 8 wk to limit range of movement. Whole body energy consumption was measured, and the blood, organs, femoral bone, and hindlimb muscles were removed. We found that long-term physical inactivity did not affect the metabolic and physiological characteristics of growing rats. Then, fifty-six 3-wk-old male Wistar rats were assigned randomly into control (n = 28) and physical inactivity (n = 28) groups. After 8 wk, the rats in both groups underwent hindlimb unloading. The soleus muscles were removed before unloading (0 day), and 1, 3, and 7 days after unloading (n = 7 for each). Although the soleus muscle weight was significantly decreased after 7 days of hindlimb unloading in both groups, the decrease was drastic in the inactive group. A significant interaction between inactivity and unloading (P < 0.01) was observed according to the 4-hydroxynonenal-conjugated protein levels and the histone deacetylase 4 (HDAC4) and NF-κB protein levels. HDAC4 and NF-κB p65 protein levels in the physical inactivity group increased significantly 1 day after hindlimb unloading, along with the mRNA levels of their downstream targets myogenin and muscle RING finger protein 1 (MuRF1). Subsequent protein ubiquitination was upregulated by long-term physical inactivity (P < 0.05).NEW & NOTEWORTHY Long-term physical inactivity exacerbates hindlimb unloading-induced disuse muscle atrophy in young rat soleus muscles, possibly mediated by oxidative stress-induced protein ubiquitination via HDAC4- and NF-κB p65-induced MuRF1 mRNA upregulation.

Perlecan, a heparan sulfate proteoglycan, regulates systemic metabolism with dynamic changes in adipose tissue and skeletal muscle

Perlecan (HSPG2), a heparan sulfate proteoglycan, is a component of basement membranes and participates in a variety of biological activities. Here, we show physiological roles of perlecan in both obesity and the onset of metabolic syndrome. The perinatal lethality-rescued perlecan knockout (Hspg2^−/−-Tg) mice showed a smaller mass and cell size of white adipose tissues than control (WT-Tg) mice. Abnormal lipid deposition, such as fatty liver, was not detected in the Hspg2^−/−-Tg mice, and those mice also consumed more fat as an energy source, likely due to their activated fatty acid oxidation. In addition, the Hspg2^−/−-Tg mice demonstrated increased insulin sensitivity. Molecular analysis revealed the significantly relatively increased amount of the muscle fiber type IIA (X) isoform and a larger quantity of mitochondria in the skeletal muscle of Hspg2^−/−-Tg mice. Furthermore, the perlecan-deficient skeletal muscle also had elevated levels of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α) protein. PGC1α expression is activated by exercise, and induces mitochondrial biosynthesis. Thus, perlecan may act as a mechano-regulator of catabolism of both lipids and glucose by shifting the muscle fiber composition to oxidative fibers. Our data suggest that downregulation of perlecan is a promising strategy to control metabolic syndrome.

Carbon stable-isotope tracking in breath for comparative studies of fuel use

Almost half a century ago, researchers demonstrated that the ratio of stable carbon isotopes in exhaled breath of rats and humans could reveal the oxidation of labeled substrates in vivo, opening a new chapter in the study of fuel use, the fate of ingested substrates, and aerobic metabolism. Until recently, the combined use of respirometry and stable-isotope tracer techniques had not been broadly employed to study fuel use in other animal groups. In this review, we summarize the history of this approach in human and animal research and define best practices that maximize its utility. We also summarize several case studies that use stable-isotope measurements of breath to explore the limits of aerobic metabolism and substrate turnover among several species and various physiological states. We highlight the importance of a comparative approach in revealing the profound effects that phylogeny, ecology, and behavior can have in shaping aerobic metabolism and energetics as well as the fundamental biological principles that underlie fuel use and metabolic function across taxa. New analytical equipment and refinement of methodology make the combined use of respirometry and stable-isotope tracer techniques simpler to perform, less costly, and more field ready than ever before.

Acute mTOR inhibition induces insulin resistance and alters substrate utilization in vivo

The effect of acute inhibition of both mTORC1 and mTORC2 on metabolism is unknown. A single injection of the mTOR kinase inhibitor, AZD8055, induced a transient, yet marked increase in fat oxidation and insulin resistance in mice, whereas the mTORC1 inhibitor rapamycin had no effect. AZD8055, but not rapamycin reduced insulin-stimulated glucose uptake into incubated muscles, despite normal GLUT4 translocation in muscle cells. AZD8055 inhibited glycolysis in MEF cells. Abrogation of mTORC2 activity by SIN1 deletion impaired glycolysis and AZD8055 had no effect in SIN1 KO MEFs. Re-expression of wildtype SIN1 rescued glycolysis. Glucose intolerance following AZD8055 administration was absent in mice lacking the mTORC2 subunit Rictor in muscle, and in vivo glucose uptake into Rictor-deficient muscle was reduced despite normal Akt activity. Taken together, acute mTOR inhibition is detrimental to glucose homeostasis in part by blocking muscle mTORC2, indicating its importance in muscle metabolism in vivo.

TFEB controls cellular lipid metabolism through a starvation-induced autoregulatory loop

The lysosomal-autophagic pathway is activated by starvation and plays an important role in both cellular clearance and lipid catabolism. However, the transcriptional regulation of this pathway in response to metabolic cues is uncharacterized. Here we show that the transcription factor EB (TFEB), a master regulator of lysosomal biogenesis and autophagy, is induced by starvation through an autoregulatory feedback loop and exerts a global transcriptional control on lipid catabolism via Ppargc1α and Ppar1α. Thus, during starvation a transcriptional mechanism links the autophagic pathway to cellular energy metabolism. The conservation of this mechanism in Caenorhabditis elegans suggests a fundamental role for TFEB in the evolution of the adaptive response to food deprivation. Viral delivery of TFEB to the liver prevented weight gain and metabolic syndrome in both diet-induced and genetic mouse models of obesity, suggesting a new therapeutic strategy for disorders of lipid metabolism.

Effects of ingesting [13C]glucose early or late into cold exposure on substrate utilization

One of the factors limiting the oxidation of exogenous glucose during cold exposure may be the delay in establishing a shivering steady state (approximately 60 min), reducing glucose uptake into skeletal muscle. Therefore, using indirect calorimetry and isotopic methodologies in non-cold-acclimatized men, the main purpose of this study was to determine whether ingesting glucose at a moment coinciding with the maximal shivering intensity could increase the utilization rate of the ingested glucose. (13)C-enriched glucose was ingested (800 mg/min) from the onset (G0) or after 60 min (G60) of cold exposure when the thermogenic rate was stabilized to low-intensity shivering (approximately 2.5 times resting metabolic rate). For the same quantity of glucose ingested, the oxidation rate of exogenous glucose was 35% higher in G60 (159+/-17 vs. 118+/-17 mg/min in G0) between minutes 60 and 90. By the end of cold exposure, exogenous glucose oxidation was significantly greater in G0, reaching 231+/-14 mg/min, approximately 15% higher than the only rates previously reported. This considerably reduced the utilization of endogenous reserves over time and compared with the G60 condition. This study also demonstrates a fall in muscle glycogen utilization, when glucose was ingested from the onset of cold exposure (from approximately 150 to approximately 75 mg/min). Together, these findings indicate the importance of ingesting glucose immediately on exposure to a cold condition, relying on shivering thermogenesis and sustaining that consumption for as long as possible. This substrate not only provides an auxiliary fuel source for shivering thermogenesis, but, more importantly, preserves the limited endogenous glucose reserves.

Substrate source utilization during moderate intensity exercise with glucose ingestion in Type 1 diabetic patients

Substrate oxidation and the respective contributions of exogenous glucose, glucose released from the liver, and muscle glycogen oxidation were measured by indirect respiratory calorimetry combined with tracer technique in eight control subjects and eight diabetic patients (5 men and 3 women in both groups) of similar age, height, body mass, and maximal oxygen uptake, over a 60-min exercise period on cycle ergometer at 50.8% (SD 4.0) maximal oxygen uptake [131.0 W (SD 38.2)]. The subjects and patients ingested a breakfast (containing ∼80 g of carbohydrates) 3 h before and 30 g of glucose (labeled with 13C) 15 min before the beginning of exercise. The diabetic patients also received their usual insulin dose [Humalog = 9.1 U (SD 0.9); Humulin N = 13.9 U (SD 4.4)] immediately before the breakfast. Over the last 30 min of exercise, the oxidation of carbohydrate [1.32 g/min (SD 0.48) and 1.42 g/min (SD 0.63)] and fat [0.33 g/min (SD 0.10) and 0.30 g/min (SD 0.10)] and their contribution to the energy yield were not significantly different in the control subjects and diabetic patients. Exogenous glucose oxidation was also not significantly different in the control subjects and diabetic patients [6.3 g/30 min (SD 1.3) and 5.2 g/30 min (SD 1.6), respectively]. In contrast, the oxidation of plasma glucose and oxidation of glucose released from the liver were significantly lower in the diabetic patients than in control subjects [14.5 g/30 min (SD 4.3) and 9.3 g/30 min (SD 2.8) vs. 27.9 g/30 min (SD 13.3) and 21.6 g/30 min (SD 12.8), respectively], whereas that of muscle glycogen was significantly higher [28.1 g/30 min (SD 15.5) vs. 11.6 g/30 min (SD 8.1)]. These data indicate that, compared with control subjects, in diabetic patients fed glucose before exercise, substrate oxidation and exogenous glucose oxidation overall are similar but plasma glucose oxidation is lower; this is associated with a compensatory higher utilization of muscle glycogen.

Comparison of exogenous glucose, fructose and galactose oxidation during exercise using C-labelling

Six subjects exercised for 120 min on a cycle ergometer (65 (se 3) % VO2max) when ingesting a placebo or glucose, fructose or galactose (100 g in 1000 ml water) labelled with 13C. The oxidation of energy substrates including exogenous hexoses was compared using indirect respiratory calorimetry and 13CO2 production at the mouth. Total carbohydrate progressively decreased and total fat oxidation increased over the 120 min exercise period in the four experimental situations. During the 120 min of exercise, the amount of fructose oxidized (38.8 (se 2.6) g; 9.0 (se 0.6) % energy yield) was not significantly (approximately 4 %) lower than that of exogenous glucose (40.5 (se 3.4) g; 9.2 (se 0.8) % energy yield), while that of galactose (23.7 (se 3.5) g; 5.5 (se 0.9) % energy yield) was only 59 % and 61 % that of glucose and fructose, respectively. When compared with the placebo, the ingestion and oxidation of the three hexoses did not significantly modify fat oxidation or total carbohydrate oxidation, but it significantly reduced (9-13 %) endogenous carbohydrate oxidation. The present data indicate that fructose and exogenous glucose ingested during exercise could be oxidized at a similar rate, but that the oxidation rate of galactose was only approximately 60 % that of the exogenous glucose and fructose, presumably because of a preferential incorporation of galactose into liver glycogen (Leloir pathway). The reduction in endogenous carbohydrate oxidation was, however, similar with the three hexoses.

Influence of age and pubertal status on substrate utilization during exercise with and without carbohydrate intake in healthy boys

Substrate utilization during exercise is known to differ between children and adults, but whether these differences are related to pubertal status is unclear. The objective of this study was to investigate the effects of pubertal status on endogenous (CHOendo) and orally ingested exogenous (CHOexo) carbohydrate and fat oxidation rates during exercise. Twenty boys at the same chronological age (12 y) were divided into three pubertal groups (pre-pubertal, PP: n = 7; early-pubertal, EP: n = 7; mid- to late-pubertal, M-LP: n = 6) and consumed either a placebo or13C-enriched 6% CHO drink while cycling for 60 min at ~70% of their maximal aerobic power (VO2 max). Another group of 14-year-old boys (pubertal, n = 9) completed all procedures. Substrate utilization was calculated for the final 15 min of exercise using indirect calorimetry and stable isotope methodology. CHOexodecreased fat (p < 0.001) and increased total CHO (p < 0.001) oxidation, irrespective of group. Fat oxidation was higher (p = 0.01) in younger boys than in older boys, but similar (p ≥ 0.33) among PP, EP, and M-LP boys. CHOexocontributed to ~30% of energy expenditure (EE) in PP and EP, but to only 24% in M-LP (p = 0.02), which was identical to the older boys (24%). CHOexooxidation rate as a percentage of EE was inversely related to testosterone levels (r = −0.51, p = 0.005, n = 29). It was concluded that reliance on CHOexoduring exercise is particularly sensitive to pubertal status, with the highest oxidation rates observed in pre- and early-pubertal boys, independent of chronological age.

Substrate utilization during prolonged exercise with ingestion of 13C-glucose in acute hypobaric hypoxia (4,300 m)

Energy substrate oxidation was measured using indirect respiratory calorimetry combined with tracer technique in five healthy young male subjects, during a 80-min exercise period on ergocycle with ingestion of 140 g of (13)C-labelled glucose, in normoxia and acute hypobaric hypoxia (445 mmHg or 4,300 m), at the same relative [77% V(.-)((O)(2)(max))] and absolute workload (161+/-8 W, corresponding to 77 and 54% V(.-)((O)(2)(max)) in hypoxia and normoxia). The oxidation rate of exogenous glucose was not significantly different in the three experimental situations: 21.4+/-2.9, 20.2+/-1.2 and 17.2+/-0.6 g over the last 40 min of exercise at approximately 77 and approximately 54% V(.-)((O)(2)(max)) in normoxia and in hypoxia, respectively, providing 12.5+/-1.5, 16.8+/-1.1 and 14.9+/-1.1% of the energy yield, although ingestion of glucose during exercise resulted in a higher plasma glucose concentration in hypoxia than normoxia. The contribution of carbohydrate (CHO) oxidation to the energy yield was significantly higher in hypoxia (92.0+/-2.1%) than in normoxia for both a given absolute (75.3+/-5.2%) and relative workload (78.1+/-1.8%). This greater reliance on CHO oxidation in hypoxia was entirely due to the significantly larger contribution of endogenous glucose oxidation to the energy yield: 75.9+/-1.7% versus 66.6+/-3.3 and 55.2+/-3.7% in normoxia at the same relative and absolute workload.

Metabolic fate of a large amount of 13C-glycerol ingested during prolonged exercise

We have shown that the oxidation rate of exogenous glycerol and glucose during prolonged exercise were similar when ingested in small amounts (0.36 g/kg) (J Appl Physiol 90:1685,2001). The oxidation rate of exogenous carbohydrate increases with the amount ingested. We, thus, hypothesized that the oxidation rate of exogenous glycerol would also be larger when ingested in large amount. The study was conducted on six male subjects exercising for 120 min at 64 (2)% VO(2)max while ingesting 1 g/kg of (13)C-glycerol. Substrate oxidation was measured using indirect respiratory calorimetry corrected for protein oxidation, and from V(13)CO(2) at the mouth. The (13)C enrichment of plasma glucose was also measured in order to follow the possible conversion of (13)C-glycerol into glucose. In spite of the large amount of glycerol ingested and absorbed (plasma glycerol concentration = 8.0 (0.3) mmol/l at min 100), exogenous glycerol oxidation over the last 80 min of exercise [8.8 (1.6) g providing 4.1 (0.7)% of the energy yield] was similar to that observed when 0.36 g/kg was ingested. The comparison between the (13)C enrichment of plasma glucose and the oxidation rate of (13)C-glycerol showed that a portion of exogenous glycerol was converted into glucose before being oxidized, but also suggested that another portion could have been directly oxidized in peripheral tissues.

Metabolic response to prolonged cycling with (13)C-glucose ingestion following downhill running

The purpose of this study was to describe the effect of muscle damage and delayed-onset muscle soreness (DOMS) on the metabolic response during a subsequent period of prolonged concentric exercise (120 min, approximately 61% V(.)O(2max), on a cycle ergometer), with ingestion of 3 g of (13)C-glucose/kg body mass. We hypothesized that the oxidation of plasma and exogenous glucose would be reduced, while the oxidation of glucose arising from muscle glycogen would be increased. Six male subjects were studied during exercise in a control situation and 2 days following downhill running, at a time when plasma creatine kinase (CK) activity was increased, and DOMS was present. Carbohydrate and lipid oxidation were computed from indirect respiratory calorimetry corrected for protein oxidation, while the oxidation of plasma glucose and muscle glycogen were computed from V(.)(13)CO(2) and the ratio of (13)C/(12)C in the plasma glucose. All data were presented as the mean and the standard error of the mean. The oxidation of protein (approximately 6% energy yield, in the control and the experimental trial), lipid (approximately 15 and approximately 18%), and carbohydrate (approximately 79 and approximately 76%), as well as that of plasma glucose (approximately 41 and approximately 46%), glucose from the liver (approximately 12 and approximately 14%), and glucose from muscle glycogen (approximately 38 and approximately 31%) were not significantly different between the control and experimental (DOMS) trials. The response of the plasma glucose, insulin, lactate, and free fatty acid concentrations was not modified by the previous eccentric exercise. These results indicate that the metabolic response to prolonged concentric exercise is not modified by muscle damage and DOMS resulting from a bout of eccentric exercise performed 2 days before.

Gender difference in the metabolic response to prolonged exercise with [13C]glucose ingestion

The metabolic response to a 120-min cycling exercise with ingestion of [(13)C]glucose (3 g kg(-1)) was compared in women in the follicular phase of the cycle [ n=6; maximum rate of oxygen uptake (VO(2max)) 44.7 (2.6) ml kg(-1) min(-1)] and in men [ n=6; VO(2max) 54.2 (4.3) ml kg(-1) min(-1)] working at the same relative workload (approximately 65% VO(2max): 107 and 179 W in women and men, respectively). We hypothesized that the contribution of endogenous substrate oxidations (indirect respiratory calorimetry corrected for protein oxidation) to the energy yield will be similar in men and women, but that women will rely more than men on exogenous glucose oxidation. Over the exercise period, the respective contributions of protein, lipid and carbohydrate oxidation to the energy yield, were similar in men [3.7 (0.9), 21.7 (2.9) and 74.6 (3.5)%] and women [3.4 (0.8), 21.5 (2.2), 75.1 (2.5)%]. The rate of exogenous glucose oxidation was approximately 45% lower in women than men (0.5 and 0.6 g min(-1) vs 0.7 and 0.9 g min(-1), between min 40 and 80, and min 80 and 120, respectively). However, when the approximately 39% difference in absolute workload and energy expenditure was taken into account, the contribution of exogenous glucose oxidation to the energy yield was similar in men and women: 22.5 vs 24.2% between min 40 and 80, and 25.7 and 28.5% between min 80 and 120, respectively. These data indicate that when fed glucose, the respective contributions of the oxidation of the various substrates to the energy yield during prolonged exercise at the same % VO(2max) are similar in men and in women in the follicular phase of the cycle.

Oxidation of exogenous glucose, sucrose, and maltose during prolonged cycling exercise

The purpose of the present study was to investigate whether combined ingestion of two carbohydrates (CHO) that are absorbed by different intestinal transport mechanisms would lead to exogenous CHO oxidation rates of >1.0 g/min. Nine trained male cyclists (maximal O(2) consumption: 64 +/- 2 ml x kg body wt(-1) x min(-1)) performed four exercise trials, which were randomly assigned and separated by at least 1 wk. Each trial consisted of 150 min of cycling at 50% of maximal power output (60 +/- 1% maximal O(2) consumption), while subjects received a solution providing either 1.8 g/min of glucose (Glu), 1.2 g/min of glucose + 0.6 g/min of sucrose (Glu+Suc), 1.2 g/min of glucose + 0.6 g/min of maltose (Glu+Mal), or water. Peak exogenous CHO oxidation rates were significantly higher (P < 0.05) in the Glu+Suc trial (1.25 +/- 0.07 g/min) compared with the Glu and Glu+Mal trials (1.06 +/- 0.08 and 1.06 +/- 0.06 g/min, respectively). No difference was found in (peak) exogenous CHO oxidation rates between Glu and Glu+Mal. These results demonstrate that, when a mixture of glucose and sucrose is ingested at high rates (1.8 g/min) during cycling exercise, exogenous CHO oxidation rates reach peak values of approximately 1.25 g/min.