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Biomedical subjects

John A Hawley

Publications and source records attributed to John A Hawley.

At least 19 recordsLinked to original sources
PubMed2007

Influence of preexercise muscle glycogen content on transcriptional activity of metabolic and myogenic genes in well-trained humans.

To determine whether preexercise muscle glycogen content influences the transcription of several early-response genes involved in the regulation of muscle growth, seven male strength-trained subjects performed one-legged cycling exercise to exhaustion to lower muscle glycogen levels (Low) in one leg compared with the leg with normal muscle glycogen (Norm) and then the following day completed a unilateral bout of resistance training (RT). Muscle biopsies from both legs were taken at rest, immediately after RT, and after 3 h of recovery. Resting glycogen content was higher in the control leg (Norm leg) than in the Low leg (435 +/- 87 vs. 193 +/- 29 mmol/kg dry wt; P < 0.01). RT decreased glycogen content in both legs (P < 0.05), but postexercise values remained significantly higher in the Norm than the Low leg (312 +/- 129 vs. 102 +/- 34 mmol/kg dry wt; P < 0.01). GLUT4 (3-fold; P < 0.01) and glycogenin mRNA abundance (2.5-fold; not significant) were elevated at rest in the Norm leg, but such differences were abolished after exercise. Preexercise mRNA abundance of atrogenes was also higher in the Norm compared with the Low leg [atrogin: approximately 14-fold, P < 0.01; RING (really interesting novel gene) finger: approximately 3-fold, P < 0.05] but decreased for atrogin in Norm following RT (P < 0.05). There were no differences in the mRNA abundance of myogenic regulatory factors and IGF-I in the Norm compared with the Low leg. Our results demonstrate that 1) low muscle glycogen content has variable effects on the basal transcription of select metabolic and myogenic genes at rest, and 2) any differences in basal transcription are completely abolished after a single bout of heavy resistance training. We conclude that commencing resistance exercise with low muscle glycogen does not enhance the activity of genes implicated in promoting hypertrophy.

Adult↗
PubMed2006

Signalling mechanisms in skeletal muscle: role in substrate selection and muscle adaptation.

Exercise produces a multitude of time- and intensity-dependent physiological, biochemical and molecular changes within skeletal muscle. With the onset of contractile activity, cytosolic and mitochondrial [Ca(2+)] levels are rapidly increased and, depending on the relative intensity of the exercise, metabolite concentrations change (i.e. increases in [ADP] and [AMP], decreases in muscle creatine phosphate and glycogen). These contraction-induced metabolic disturbances activate several key kinases and phosphatases involved in signal transduction. Important among these are the calcium dependent signalling pathways that respond to elevated Ca(2+) concentrations (including Ca(2+)/calmodulin-dependent kinase, Ca(2+)-dependent protein kinase C and the Ca(2+)/calmodulin-dependent phosphatase calcineurin), the 5'-adenosine monophosphate-activated protein kinase, several of the mitogen-activated protein kinases and protein kinase B/Akt. The role of these signal transducers in the regulation of carbohydrate and fat metabolism in response to increased contractile activity has been the focus of intense research efforts during the past decade.

AMP-Activated Protein Kinases↗
PubMed2006

Promoting training adaptations through nutritional interventions.

Training and nutrition are highly interrelated in that optimal adaptation to the demands of repeated training sessions typically requires a diet that can sustain muscle energy reserves. As nutrient stores (i.e. muscle and liver glycogen) play a predominant role in the performance of prolonged, intense, intermittent exercise typical of the patterns of soccer match-play, and in the replenishment of energy reserves for subsequent training sessions, the extent to which acutely altering substrate availability might modify the training impulse has been a key research area among exercise physiologists and sport nutritionists for several decades. Although the major perturbations to cellular homeostasis and muscle substrate stores occur during exercise, the activation of several major signalling pathways important for chronic training adaptations take place during the first few hours of recovery, returning to baseline values within 24 h after exercise. This has led to the paradigm that many chronic training adaptations are generated by the cumulative effects of the transient events that occur during recovery from each (acute) exercise bout. Evidence is accumulating that nutrient supplementation can serve as a potent modulator of many of the acute responses to both endurance and resistance training. In this article, we review the molecular and cellular events that occur in skeletal muscle during exercise and subsequent recovery, and the potential for nutrient supplementation (e.g. carbohydrate, fat, protein) to affect many of the adaptive responses to training.

Adaptation, Physiological↗
PubMed2006

Fat and carbohydrate for exercise.

PURPOSE OF REVIEW: To examine the results of new investigations that look at the efficacy of nutrient/training strategies on metabolism and athletic performance. RECENT FINDINGS: 'Dietary periodization' involves the manipulation of macronutrient intake in association with changes in physical training. Such interventions have a major effect on altering patterns of fuel utilization during exercise; however, they often fail to enhance performance capacity. In contrast, the ingestion of a combination of different types of carbohydrate during exercise results in high rates of muscle glucose oxidation (1.5 g/min) and can improve intense, short-duration (approximately 60 min), and prolonged (>90 min) submaximal steady-state exercise, either by metabolic or neural mechanisms. SUMMARY: Further investigation into the responses of specific nutrient/training strategies on metabolic and cellular signaling pathways is warranted to determine the underlying mechanisms by which such interventions exert their effect. Such studies, however, should be coupled with investigations that assess the outcomes of these responses on the 'real life' training adaptations in athletes.

Adaptation, Physiological↗
PubMed2006

Short-term plyometric training improves running economy in highly trained middle and long distance runners.

Fifteen highly trained distance runners VO(2)max 71.1 +/- 6.0 ml.min(-1).kg(-1), mean +/- SD) were randomly assigned to a plyometric training (PLY; n = 7) or control (CON; n = 8) group. In addition to their normal training, the PLY group undertook 3 x 30 minutes PLY sessions per week for 9 weeks. Running economy (RE) was assessed during 3 x 4 minute treadmill runs (14, 16, and 18 km.h(-1)), followed by an incremental test to measure VO(2)max. Muscle power characteristics were assessed on a portable, unidirectional ground reaction force plate. Compared with CON, PLY improved RE at 18 km.h(-1) (4.1%, p = 0.02), but not at 14 or 16 km.h(-1). This was accompanied by trends for increased average power during a 5-jump plyometric test (15%, p = 0.11), a shorter time to reach maximal dynamic strength during a strength quality assessment test (14%, p = 0.09), and a lower VO(2)-speed slope (14%, p = 0.12) after 9 weeks of PLY. There were no significant differences in cardiorespiratory measures or VO(2)max as a result of PLY. In a group of highly-trained distance runners, 9 weeks of PLY improved RE, with likely mechanisms residing in the muscle, or alternatively by improving running mechanics.

Adult↗
PubMed2006

Exercise-induced phosphorylation of the novel Akt substrates AS160 and filamin A in human skeletal muscle.

Skeletal muscle contraction stimulates multiple signaling cascades that govern a variety of metabolic and transcriptional events. Akt/protein kinase B regulates metabolism and growth/muscle hypertrophy, but contraction effects on this target and its substrates are varied and may depend on the mode of the contractile stimulus. Accordingly, we determined the effects of endurance or resistance exercise on phosphorylation of Akt and downstream substrates in six trained cyclists who performed a single bout of endurance or resistance exercise separated by approximately 7 days. Muscle biopsies were taken from the vastus lateralis at rest and immediately after exercise. Akt Ser(473) phosphorylation was increased (1.8-fold; P=0.011) after endurance but was unchanged after resistance exercise. Conversely, Akt Thr(308) phosphorylation was unaltered after either bout of exercise. Several exercise-responsive phosphoproteins were detected by immunoblot analysis with a phospho-Akt substrate antibody. pp160 and pp300 were identified as AS160 and filamin A, respectively, with increased phosphorylation (2.0- and 4.9-fold, respectively; P<0.05) after endurance but not resistance exercise. In conclusion, AS160 and filamin A may provide an important link to mediate endurance exercise-induced bioeffects in skeletal muscle.

Adult↗
PubMed2005

Interaction of contractile activity and training history on mRNA abundance in skeletal muscle from trained athletes.

Skeletal muscle displays enormous plasticity to respond to contractile activity with muscle from strength- (ST) and endurance-trained (ET) athletes representing diverse states of the adaptation continuum. Training adaptation can be viewed as the accumulation of specific proteins. Hence, the altered gene expression that allows for changes in protein concentration is of major importance for any training adaptation. Accordingly, the aim of the present study was to quantify acute subcellular responses in muscle to habitual and unfamiliar exercise. After 24-h diet/exercise control, 13 male subjects (7 ST and 6 ET) performed a random order of either resistance (8 x 5 maximal leg extensions) or endurance exercise (1 h of cycling at 70% peak O2 uptake). Muscle biopsies were taken from vastus lateralis at rest and 3 h after exercise. Gene expression was analyzed using real-time PCR with changes normalized relative to preexercise values. After cycling exercise, peroxisome proliferator-activated receptor-gamma coactivator-1alpha (ET approximately 8.5-fold, ST approximately 10-fold, P < 0.001), pyruvate dehydrogenase kinase-4 (PDK-4; ET approximately 26-fold, ST approximately 39-fold), vascular endothelial growth factor (VEGF; ET approximately 4.5-fold, ST approximately 4-fold), and muscle atrophy F-box protein (MAFbx) (ET approximately 2-fold, ST approximately 0.4-fold) mRNA increased in both groups, whereas MyoD (approximately 3-fold), myogenin (approximately 0.9-fold), and myostatin (approximately 2-fold) mRNA increased in ET but not in ST (P < 0.05). After resistance exercise PDK-4 (approximately 7-fold, P < 0.01) and MyoD (approximately 0.7-fold) increased, whereas MAFbx (approximately 0.7-fold) and myostatin (approximately 0.6-fold) decreased in ET but not in ST. We conclude that prior training history can modify the acute gene responses in skeletal muscle to subsequent exercise.

Adult↗
PubMed2005

Early signaling responses to divergent exercise stimuli in skeletal muscle from well-trained humans.

Skeletal muscle from strength- and endurance-trained individuals represents diverse adaptive states. In this regard, AMPK-PGC-1alpha signaling mediates several adaptations to endurance training, while up-regulation of the Akt-TSC2-mTOR pathway may underlie increased protein synthesis after resistance exercise. We determined the effect of prior training history on signaling responses in seven strength-trained and six endurance-trained males who undertook 1 h cycling at 70% VO2peak or eight sets of five maximal repetitions of isokinetic leg extensions. Muscle biopsies were taken at rest, immediately and 3 h postexercise. AMPK phosphorylation increased after cycling in strength-trained (54%; P<0.05) but not endurance-trained subjects. Conversely, AMPK was elevated after resistance exercise in endurance- (114%; P<0.05), but not strength-trained subjects. Akt phosphorylation increased in endurance- (50%; P<0.05), but not strength-trained subjects after cycling but was unchanged in either group after resistance exercise. TSC2 phosphorylation was decreased (47%; P<0.05) in endurance-trained subjects following resistance exercise, but cycling had little effect on the phosphorylation state of this protein in either group. p70S6K phosphorylation increased in endurance- (118%; P<0.05), but not strength-trained subjects after resistance exercise, but was similar to rest in both groups after cycling. Similarly, phosphorylation of S6 protein, a substrate for p70 S6K, was increased immediately following resistance exercise in endurance- (129%; P<0.05), but not strength-trained subjects. In conclusion, a degree of "response plasticity" is conserved at opposite ends of the endurance-hypertrophic adaptation continuum. Moreover, prior training attenuates the exercise specific signaling responses involved in single mode adaptations to training.

AMP-Activated Protein Kinases↗
PubMed2005

PGC-1alpha gene expression is down-regulated by Akt- mediated phosphorylation and nuclear exclusion of FoxO1 in insulin-stimulated skeletal muscle.

There are multiple binding domains on the promoter region of the peroxisome proliferator activator receptor gamma coactivator-1 alpha (PGC-1alpha) gene, including a trio of insulin responsive elements that are activated by the forkhead box class-O (FoxO1) winged helix transcription factor, which is known to be regulated by acute transforming retrovirus thymoma (Akt). Here we show that in skeletal muscle biopsy specimens from healthy humans and cultured human skeletal myotubes, insulin phosphorylates Akt (Ser473) and FoxO1 (Thr24, Ser256), leading to reduced nuclear abundance of FoxO1 total protein. This is associated with an insulin-mediated repression of the mRNA expression PGC-1alpha and downstream genes associated with oxidative phosphorylation. In contrast, in muscle taken from insulin resistant humans or in palmitate-treated insulin resistant myotubes, neither Akt nor FoxO1 was phosphorylated by insulin, resulting in a failure for nuclear exclusion of FoxO1 total protein, and an inability for insulin to repress the mRNA expression of PGC-1alpha and down-stream genes. To determine whether the regulation of FoxO1 was Akt dependent, we next treated Akt2 -/- and wild-type mice with or without insulin. Insulin phosphorylated Akt and FoxO1 (Thr24, Ser256) resulting in a reduced nuclear expression of FoxO1 total protein in wild-type but not Akt2 -/- skeletal muscle. We conclude that insulin decreases the expression of genes involved in oxidative metabolism in healthy but not insulin resistant muscle, due to a decrease in FoxO1 phosphorylation and nuclear exclusion secondary to reduced Akt activity.

Animals↗
PubMed2005

Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration.

Five days of a high-fat diet while training, followed by 1 day of carbohydrate (CHO) restoration, increases rates of whole body fat oxidation and decreases CHO oxidation during aerobic cycling. The mechanisms responsible for these shifts in fuel oxidation are unknown but involve up- and downregulation of key regulatory enzymes in the pathways of skeletal muscle fat and CHO metabolism, respectively. This study measured muscle PDH and HSL activities before and after 20 min of cycling at 70% VO2peak and 1 min of sprinting at 150% peak power output (PPO). Estimations of muscle glycogenolysis were made during the initial minute of exercise at 70% VO2peak and during the 1-min sprint. Seven male cyclists undertook this exercise protocol on two occasions. For 5 days, subjects consumed in random order either a high-CHO (HCHO) diet (10.3 g x kg(-1) x day(-1) CHO, or approximately 70% of total energy intake) or an isoenergetic high-fat (FAT-adapt) diet (4.6 g x kg(-1) x day(-1) FAT, or 67% of total energy) while undertaking supervised aerobic endurance training. On day 6 for both treatments, subjects ingested an HCHO diet and rested before their experimental trials on day 7. This CHO restoration resulted in similar resting glycogen contents (FAT-adapt 873 +/- 121 vs. HCHO 868 +/- 120 micromol glucosyl units/g dry wt). However, the respiratory exchange ratio was lower during cycling at 70% VO2peak in the FAT-adapt trial, which resulted in an approximately 45% increase and an approximately 30% decrease in fat and CHO oxidation, respectively. PDH activity was lower at rest and throughout exercise at 70% VO2peak (1.69 +/- 0.25 vs. 2.39 +/- 0.19 mmol x kg wet wt(-1) x min(-1)) and the 1-min sprint in the FAT-adapt vs. the HCHO trial. Estimates of glycogenolysis during the 1st min of exercise at 70% VO2peak and the 1-min sprint were also lower after FAT-adapt (9.1 +/- 1.1 vs. 13.4 +/- 2.1 and 37.3 +/- 5.1 vs. 50.5 +/- 2.7 glucosyl units x kg dry wt(-1) x min(-1)). HSL activity was approximately 20% higher (P = 0.12) during exercise at 70% VO2peak after FAT-adapt. Results indicate that previously reported decreases in whole body CHO oxidation and increases in fat oxidation after the FAT-adapt protocol are a function of metabolic changes within skeletal muscle. The metabolic signals responsible for the shift in muscle substrate use during cycling at 70% VO2peak remain unclear, but lower accumulation of free ADP and AMP after the FAT-adapt trial may be responsible for the decreased glycogenolysis and PDH activation during sprinting.

Adaptation, Physiological↗
PubMed2005

Chronic rosiglitazone treatment restores AMPKalpha2 activity in insulin-resistant rat skeletal muscle.

Rosiglitazone (RSG) is an insulin-sensitizing thiazolidinedione (TZD) that exerts peroxisome proliferator-activated receptor-gamma (PPARgamma)-dependent and -independent effects. We tested the hypothesis that part of the insulin-sensitizing effect of RSG is mediated through the action of AMP-activated protein kinase (AMPK). First, we determined the effect of acute (30-60 min) incubation of L6 myotubes with RSG on AMPK regulation and palmitate oxidation. Compared with control (DMSO), 200 microM RSG increased (P < 0.05) AMPKalpha1 activity and phosphorylation of AMPK (Thr172). In addition, acetyl-CoA carboxylase (Ser218) phosphorylation and palmitate oxidation were increased (P < 0.05) in these cells. To investigate the effects of chronic RSG treatment on AMPK regulation in skeletal muscle in vivo, obese Zucker rats were randomly allocated into two experimental groups: control and RSG. Lean Zucker rats were treated with vehicle and acted as a control group for obese Zucker rats. Rats were dosed daily for 6 wk with either vehicle (0.5% carboxymethylcellulose, 100 microl/100 g body mass), or 3 mg/kg RSG. AMPKalpha1 activity was similar in muscle from lean and obese animals and was unaffected by RSG treatment. AMPKalpha2 activity was approximately 25% lower in obese vs. lean animals (P < 0.05) but was normalized to control values after RSG treatment. ACC phosphorylation was decreased with obesity (P < 0.05) but restored to the level of lean controls with RSG treatment. Our data demonstrate that RSG restores AMPK signaling in skeletal muscle of insulin-resistant obese Zucker rats.

AMP-Activated Protein Kinases↗
PubMed2005

Sleep disturbance at simulated altitude indicated by stratified respiratory disturbance index but not hypoxic ventilatory response.

At high altitudes, the clinically defined respiratory disturbance index (RDI) and high hypoxic ventilatory response (HVR) have been associated with diminished sleep quality. Increased RDI has also been observed in some athletes sleeping at simulated moderate altitude. In this study, we investigated relationships between the HVR of 14 trained male endurance cyclists with variable RDI and sleep quality responses to simulated moderate altitude. Blood oxygen saturation (SpO2%), heart rate, RDI, arousal rate, awakenings, sleep efficiency, rapid eye movement (REM) sleep, non-REM sleep stages 1, 2 and slow wave sleep as percentages of total sleep time (%TST) were measured for two nights at normoxia of 600 m and one night at a simulated altitude of 2,650 m. HVR and RDI were not significantly correlated with sleep stage, arousal rate or awakening response to nocturnal simulated altitude. SpO2 was inversely correlated with total RDI (r = -0.69, P = 0.004) at simulated altitude and with the change in arousal rate from normoxia (r = -0.65, P = 0.02). REM sleep response to simulated altitude correlated with the change, relative to normoxia, in arousal (r = -0.63, P = 0.04) and heart rate (r = -0.61, P = 0.04). When stratified, those athletes at altitude with RDI >20 h(-1) (n = 4) and those with <10 h(-1) (n = 10) exhibited no difference in HVR but the former had larger falls in SpO2 (P = 0.05) and more arousals (P = 0.03). Neither RDI (without stratification) nor HVR were sufficiently sensitive to explain any deterioration in REM sleep or arousal increase. However, the stratified RDI provides a basis for determining potential sleep disturbance in athletes at simulated moderate altitude.

Adult↗
PubMed2005

Failure to repeatedly supercompensate muscle glycogen stores in highly trained men.

PURPOSE: It is not known whether it is possible to repeatedly supercompensate muscle glycogen stores after exhaustive exercise bouts undertaken within several days. METHODS: We evaluated the effect of repeated exercise-diet manipulation on muscle glycogen and triacylglycerol (IMTG) metabolism and exercise capacity in six well-trained subjects who completed an intermittent, exhaustive cycling protocol (EX) on three occasions separated by 48 h (i.e., days 1, 3, and 5) in a 5-d period. Twenty-four hours before day 1, subjects consumed a moderate (6 g.kg)-carbohydrate (CHO) diet, followed by 5 d of a high (12 g.kg.d)-CHO diet. Muscle biopsies were taken at rest, immediately post-EX on days 1, 3, and 5, and after 3 h of recovery on days 1 and 3. RESULTS: Compared with day 1, resting muscle [glycogen] was elevated on day 3 but not day 5 (435+/-57 vs 713+/-60 vs 409+/-40 mmol.kg, P<0.001). [IMTG] was reduced by 28% (P<0.05) after EX on day 1, but post-EX levels on days 3 and 5 were similar to rest. EX was enhanced on days 3 and 5 compared with day 1 (31.9+/-2.5 and 35.4+/-3.8 vs 24.1+/-1.4 kJ.kg, P<0.05). Glycogen synthase activity at rest and immediately post-EX was similar between trials. Additionally, the rates of muscle glycogen accumulation were similar during the 3-h recovery period on days 1 and 3. CONCLUSION: We show that well-trained men cannot repeatedly supercompensate muscle [glycogen] after glycogen-depleting exercise and 2 d of a high-CHO diet, suggesting that the mechanisms responsible for glycogen accumulation are attenuated as a consequence of successive days of glycogen-depleting exercise.

Adult↗
PubMed2004

Hypoxic ventilatory response is correlated with increased submaximal exercise ventilation after live high, train low.

This study tested the hypothesis that live high, train low (LHTL) would increase submaximal exercise ventilation (V(E)) in normoxia, and the increase would be related to enhanced hypoxic ventilatory response (HVR). Thirty-three cyclists/triathletes were divided into three groups: 20 consecutive nights of hypoxia (LHTLc, n = 12), 20 nights of intermittent hypoxia (4x5-night 'blocks' of hypoxia interspersed by two nights of normoxia, LHTLi, n = 10), or control (CON, n = 11). LHTLc and LHTLi slept 8-10 h per night in normobaric hypoxia (2,650 m), and CON slept under ambient conditions (600 m). Resting, isocapnic HVR (DeltaV(E)/Deltablood oxygen saturation) was measured in normoxia before (PRE) and after 15 nights (N15) hypoxia. Submaximal cycle ergometry was conducted PRE and after 4, 10, and 19 nights of hypoxia (N4, N10, and N19 respectively). Mean submaximal exercise V(E) was increased (P < 0.05) from PRE to N4 in LHTLc [74.4 (5.1) vs 80.0 (8.4) l min(-1); mean (SD)] and in LHTLi [69.0 (7.5) vs 76.9 (7.3) l min(-1)] and remained elevated in both groups thereafter, with no changes observed in CON at any time. Prior to LHTL, submaximal V(E) was not correlated with HVR, but this relationship was significant at N4 (r = 0.49, P = 0.03) and N19 (r = 0.77, P < 0.0001). Additionally, the increases in submaximal V(E) and HVR from PRE to N15-N19 were correlated (r = 0.51, P = 0.02) for the pooled data of LHTLc and LHTLi. These results suggest that enhanced hypoxic chemosensitivity contributes to increased exercise V(E) in normoxia following LHTL.

Acclimatization↗
PubMed2004

Rosiglitazone enhances glucose tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle.

We hypothesized that improved glucose tolerance with rosiglitazone treatment would coincide with decreased levels of i.m. triacylglycerol (IMTG), diacylglycerol, and ceramide. Obese Zucker rats were randomly divided into two experimental groups: control (n = 9) and rosiglitazone (n = 9), with lean Zucker rats (n = 9) acting as a control group for obese controls. Rats received either vehicle or 3 mg/kg rosiglitazone for 6 wk. Glucose tolerance was impaired (P < 0.01) in obese compared with lean rats, but was normalized after rosiglitazone treatment. IMTG content was higher in obese compared with lean rats (70.5 +/- 5.1 vs. 27.5 +/- 2.0 micromol/g dry mass; P < 0.05) and increased an additional 30% (P < 0.05) with rosiglitazone treatment. Intramuscular fatty acid composition shifted toward a higher proportion of monounsaturates (P < 0.05) in obese rosiglitazone-treated rats due to an increase in palmitoleate (16:1; P < 0.05). Rosiglitazone treatment increased (P < 0.05) skeletal muscle diacylglycerol and ceramide levels by 65% and 100%, respectively, compared with obese rats, but elevated muscle diacylglycerol was not associated with changes in the total or membrane contents of the diacylglycerol-sensitive protein kinase C isoforms theta;, delta, alpha, and beta. In summary, we observed a disassociation among skeletal muscle IMTG, diacylglycerol and ceramide content, and glucose tolerance with rosiglitazone treatment in obese Zucker rats. Our data suggest, therefore, that rosiglitazone enhances glucose tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle.

Animals↗
PubMed2004

Regulation of fuel metabolism by preexercise muscle glycogen content and exercise intensity.

To date, the results of studies that have examined the effects of altering preexercise muscle glycogen content and exercise intensity on endogenous carbohydrate oxidation are equivocal. Differences in the training status of subjects between investigations may, in part, explain these inconsistent findings. Accordingly, we determined the relative effects of exercise intensity and carbohydrate availability on patterns of fuel utilization in the same subjects who performed a random order of four 60-min rides, two at 45% and two at 70% of peak O(2) uptake (Vo(2 peak)), after exercise-diet intervention to manipulate muscle glycogen content. Preexercise muscle glycogen content was 596 +/- 43 and 202 +/- 21 mmol/kg dry mass (P < 0.001) for high-glycogen (HG) and low-glycogen (LG) conditions, respectively. Respiratory exchange ratio was higher for HG than LG during exercise at both 45% (0.85 +/- 0.01 vs. 0.74 +/- 0.01; P < 0.001) and 70% (0.90 +/- 0.01 vs. 0.79 +/- 0.01; P < 0.001) of Vo(2 peak). The contribution of whole body muscle glycogen oxidation to energy expenditure differed between LG and HG for exercise at both 45% (5 +/- 2 vs. 45 +/- 5%; P < 0.001) and 70% (25 +/- 3 vs. 60 +/- 3%; P < 0.001) of Vo(2 peak). Yet, despite marked differences in preexercise muscle glycogen content and its subsequent utilization, rates of plasma glucose disappearance were similar under all conditions. We conclude that, in moderately trained individuals, muscle glycogen availability (low vs. high) does not influence rates of plasma glucose disposal during either low- or moderate-intensity exercise.

Adult↗