Substrate utilization during exercise with glucose and glucose plus fructose ingestion in boys ages 10--14 yr.

We measured substrate utilization during exercise performed with water (W), exogenous glucose (G), and exogenous fructose plus glucose (FG) ingestion in boys age 10-14 yr. Subjects (n = 12) cycled for 90 min at 55% maximal O(2) uptake while ingesting either W (25 ml/kg), 6% G (1.5 g/kg), or 3% F plus 3% G (1.5 g/kg). Fat oxidation increased during exercise in all trials but was higher in the W (0.28 +/- 0.023 g/min) than in the G (0.24 +/- 0.023 g/min) and FG (0.25 +/- 0.029 g/min) trials (P = 0.04). Conversely, total carbohydrate (CHO) oxidation decreased in all trials and was lower in the W (0.63 +/- 0.05 g/min) than in the G (0.78 +/- 0.051 g/min) and FG (0.74 +/- 0.056 g/min) trials (P = 0.009). Exogenous CHO oxidation, as determined by expired (13)CO(2), reached a maximum of 0.36 +/- 0.032 and 0.31 +/- 0.030 g/min at 90 min in G and FG, respectively (P = 0.04). Plasma insulin levels decrease during exercise in all trials but were twofold higher in G than in W and FG (P < 0.001). Plasma glucose levels decreased transiently after the onset of exercise in all trials and then returned to preexercise values in the W and FG (approximately 4.5 mmol/l) trials but were elevated by approximately 1.0 mmol/l in the G trial (P < 0.001). Plasma lactate concentrations decreased after the onset of exercise in all trials but were lower by approximately 0.5 mmol/l in W than in G and FG (P = 0.02). Thus, in boys exercising at a moderate intensity, the oxidation rate of G plus F is slightly less than G alone, but both spare endogenous CHO and fat to a similar extent. In addition, compared with flavored W, the ingestion of G alone and of G plus F delays exhaustion at 90% peak power by approximately 25 and 40%, respectively, after 90 min of moderate-intensity exercise.     

Epinephrine infusion during moderate intensity exercise increases glucose production and uptake

The glucoregulatory response to intense exercise [IE, >80% maximum O(2) uptake (VO(2 max))] comprises a marked increment in glucose production (R(a)) and a lesser increment in glucose uptake (R(d)), resulting in hyperglycemia. The R(a) correlates with plasma catecholamines but not with the glucagon-to-insulin (IRG/IRI) ratio. If epinephrine (Epi) infusion during moderate exercise were able to markedly stimulate R(a), this would support an important role for the catecholamines' response in IE. Seven fit male subjects (26 +/- 2 yr, body mass index 23 +/- 0.5 kg/m(2), VO(2 max) 65 +/- 5 ml x kg(-1) x min(-1)) underwent 40 min of postabsorptive cycle ergometer exercise (145 +/- 14 W) once without [control (CON)] and once with Epi infusion [EPI (0.1 microg x kg(-1) x min(-1))] from 30 to 40 min. Epi levels reached 9.4 +/- 0.8 nM (20x rest, 10x CON). R(a) increased approximately 70% to 3.75 +/- 0.53 in CON but to 8.57 +/- 0.58 mg x kg(-1) x min(-1) in EPI (P < 0.001). Increments in R(a) and Epi correlated (r(2) = 0.923, P 相似文献   

(-)-hydroxycitrate ingestion increases fat oxidation during moderate intensity exercise in untrained men

We examined the effects of (-)-Hydroxycitrate (HCA) ingestion on fat oxidation during moderate intensity exercise in untrained men. Six subjects ingested 500 mg of HCA or a placebo for 5 days and did endurance exercise. Blood FFA concentrations were significantly increased and respiratory exchange ratio (RER) decreased by HCA ingestion. These results suggested short-term HCA ingestion increases fat oxidation in untrained men.     

Glucose ingestion and substrate utilization during exercise in boys with IDDM.

This study was intended to compare exogenous [(13)C]glucose (Glu(exo)) oxidation in boys with insulin-dependent diabetes mellitus (IDDM) and healthy boys of similar age, weight, and maximal O(2) uptake. In a control trial with water intake (CT) and in a (13)C-enriched glucose trial (GT), subjects cycled for 60 min (58.8 +/- 0.9% maximal O(2) uptake) while the utilization of total glucose, total fat, and Glu(exo) was assessed. In CT, total glucose was 84.7 +/- 9.2 vs. 91.3 +/- 6.6 g/60 min (not significantly different) and total fat was 13.3 +/- 2.2 vs. 11.1 +/- 1.7 g/60 min (not significantly different) in IDDM vs. healthy boys, respectively. In GT, Glu(exo) was 10.4 +/- 1.7 vs. 14.8 +/- 1.1 g/60 min, corresponding to 9.0 +/- 1.0 vs. 12.4 +/- 0.5% of the total energy supply in IDDM and healthy boys, respectively (P < 0.05). Endogenous glucose was spared in both groups by 12.6 +/- 3.5% (P < 0.05). Blood glucose and plasma insulin concentrations were two- to threefold higher in IDDM vs. healthy boys in both trials. In conclusion, Glu(exo) is impaired in exercising boys with IDDM, even when plasma insulin levels are elevated.     

Substrate utilization during exercise in formerly morbidly obese women.

The purpose of this study was to compare substrate utilization during fasting and submaximal exercise in morbidly obese women after weight loss (WL) with that in weight-matched controls (C). WL were studied in the weight-stable condition approximately 24 mo after gastric bypass surgery. Energy intake (self-reported) and expenditure ((2)H(2)(18)O) were also compared. The respiratory exchange ratio during exercise at the same absolute (15 W) workload was significantly (P < or = 0.05) elevated in WL vs. C (0.90 +/- 0.02 vs. 0.83 +/- 0.03); this was reflected as lower fat utilization in WL (29.7 +/- 4.8 vs. 53.2 +/- 9.7% of energy from fat). Respiratory exchange ratio during exercise at the same relative (65% of maximal O(2) uptake) intensity was also significantly (P < 0.05) elevated in WL (0.96 +/- 0.01 vs. 0.89 +/- 0.02), and fat use was concomitantly depressed (12.4 +/- 3.0 vs. 34.3 +/- 9.9% of energy from fat). Resting substrate utilization, daily energy expenditure, and self-reported relative macronutrient intake did not differ between groups. These data suggest that lipid oxidation is depressed during physical activity in WL. This defect may, at least in part, contribute to a propensity for the development of morbid obesity.     

Oxidation of combined ingestion of glucose and fructose during exercise.

The purpose of the present study was to examine whether combined ingestion of a large amount of fructose and glucose during cycling exercise would lead to exogenous carbohydrate oxidation rates >1 g/min. Eight trained cyclists (maximal O(2) consumption: 62 +/- 3 ml x kg(-1) x min(-1)) performed four exercise trials in random order. Each trial consisted of 120 min of cycling at 50% maximum power output (63 +/- 2% maximal O(2) consumption), while subjects received a solution providing either 1.2 g/min of glucose (Med-Glu), 1.8 g/min of glucose (High-Glu), 0.6 g/min of fructose + 1.2 g/min of glucose (Fruc+Glu), or water. The ingested fructose was labeled with [U-(13)C]fructose, and the ingested glucose was labeled with [U-(14)C]glucose. Peak exogenous carbohydrate oxidation rates were approximately 55% higher (P < 0.001) in Fruc+Glu (1.26 +/- 0.07 g/min) compared with Med-Glu and High-Glu (0.80 +/- 0.04 and 0.83 +/- 0.05 g/min, respectively). Furthermore, the average exogenous carbohydrate oxidation rates over the 60- to 120-min exercise period were higher (P < 0.001) in Fruc+Glu compared with Med-Glu and High-Glu (1.16 +/- 0.06, 0.75 +/- 0.04, and 0.75 +/- 0.04 g/min, respectively). There was a trend toward a lower endogenous carbohydrate oxidation in Fruc+Glu compared with the other two carbohydrate trials, but this failed to reach statistical significance (P = 0.075). The present results demonstrate that, when fructose and glucose are ingested simultaneously at high rates during cycling exercise, exogenous carbohydrate oxidation rates can reach peak values of approximately 1.3 g/min.     

Substrate utilization during acute exercise in obese Zucker rats

The purpose of the present study was to compare the carbohydrate use of insulin-resistant obese Zucker rats with that of their lean littermates during steady-state exercise. Obese and lean rats were randomly assigned to a sedentary group or to a run group in which rats ran at 72-73% of their maximal O2 consumption, with the duration of exercise set to require an energy expenditure of 2.1-2.2 kcal. During the run the respiratory exchange ratio was significantly higher in the obese than in the lean rats [0.94 +/- 0.01 (SE) and 0.86 +/- 0.01, respectively], which indicate that the obese rats required 54% more carbohydrate than the lean rats. Total muscle glycogen utilization in the soleus, plantaris, and red and white gastrocnemius was not different between groups. Obese rats had total liver glycogen values five times greater than those of lean rats (833.38 +/- 101.4 and 152.8 +/- 37.5 mg, respectively) and utilized twice as much liver glycogen as their lean littermates (193.5 and 90.4 mg, respectively). The obese rats exhibited higher blood glucose and insulin concentrations than the lean rats during the run. These findings indicate that, despite their characteristic insulin resistance, the obese Zucker rats had a greater dependency on carbohydrate as a substrate during exercise than their lean littermates and that the major source of this carbohydrate was liver glycogen.     

Aerobic exercise is necessary to improve glucose utilization with moderate weight loss in women

Objective: To determine the effects of weight loss (WL) alone and combined with aerobic exercise on visceral adipose tissue (VAT), intramuscular fat, insulin‐stimulated glucose uptake, and the rate of decline in free fatty acid (FFA) concentrations during hyperinsulinemia. Research Methods and Procedures: We studied 33 sedentary, obese (BMI = 32 ± 1 kg/m²) postmenopausal women who completed a 6‐month (three times per week) program of either WL alone (n = 16) or WL + aerobic exercise (AEX) (n = 17). Glucose utilization (M) was measured during a 3‐hour hyperinsulinemic‐euglycemic clamp (40 mU/m² per minute). M/I, the amount of glucose metabolized per unit of plasma insulin (I), was used as an index of insulin sensitivity. Results: Body weight, total fat mass, and percentage fat decreased similarly in both groups (p < 0.01). VAT, subcutaneous abdominal adipose tissue, mid‐thigh subcutaneous fat, and intramuscular fat decreased to a similar extent in both groups and between 14% and 27% after WL and WL+AEX (p < 0.05). WL alone did not change M or M/I; however, M and M/I increased 15% and 21% after WL+AEX (p < 0.05). Fasting concentrations and rate of decline of FFA did not change in either group. In stepwise regression models to determine the independent predictors of changes in M and M/I, the change in VAT was the single independent predictor of M (r² = 0.30) and M/I (r² = 0.33). Discussion: Intramuscular fat decreases similarly with 6 months of moderate WL alone or with aerobic exercise in postmenopausal women. In contrast, only WL combined with exercise results in increased glucose utilization and insulin sensitivity. These findings should be validated in a larger population.     

Effect of training status on fuel selection during submaximal exercise with glucose ingestion.

In this study, an oral glucose load was enriched with a [U-(13)C]glucose tracer to determine differences in substrate utilization between endurance-trained (T) and untrained (UT) subjects during submaximal exercise at the same relative and absolute workload when glucose is ingested. Six highly trained cyclists/triathletes [maximal workload (Wmax), 400 +/- 9 W] and seven UT subjects (Wmax, 296 +/- 8 W) were studied during 120 min of cycling exercise at 50% Wmax ( approximately 55% maximal O(2) consumption). The T subjects performed a second trial at the mean workload of the UT group (148 +/- 4 W). Before exercise, 8.0 ml/kg of a (13)C-enriched glucose solution (80 g/l) was ingested. During exercise, boluses of 2.0 ml/kg of the same solution were administered every 15 min. Measurements were made in the 90- to 120-min period when a steady state was present in breath (13)CO(2) and plasma glucose (13)C enrichment. Energy expenditure was higher in T than in UT subjects (58 vs. 47 kJ/min, respectively; P < 0.001) at the same relative intensity. This was completely accounted for by an increased fat oxidation (0.57 vs. 0.40 g/min; P < 0.01). At the same absolute intensity, fat oxidation contributed more to energy expenditure in the T compared with the UT group (44 vs. 33%, respectively; P < 0.01). The reduction in carbohydrate oxidation in the T group was explained by a diminished oxidation rate of muscle glycogen (indirectly assessed by using tracer methodology at 0.72 +/- 0.1 and 1.03 +/- 0.1 g/min, respectively; P < 0.01) and liver-derived glucose (0.15 +/- 0.03 and 0.22 +/- 0.02 g/min, respectively; P < 0.05). Exogenous glucose oxidation rates were similar during all trials (+/-0.70 g/min).     

beta-adrenergic blockade augments glucose utilization in horses during graded exercise.

To examine the role of beta-adrenergic mechanisms in the regulation of endogenous glucose (Glu) production [rate of appearance (R(a))] and utilization [rate of disappearance (R(d))] and carbohydrate (CHO) metabolism, six horses completed consecutive 30-min bouts of exercise at approximately 30% (Lo) and approximately 60% (Hi) of estimated maximum O(2) uptake with (P) and without (C) prior administration of the beta-blocker propranolol (0.22 mg/kg iv). All horses completed exercise in C; exercise duration in P was 49.9 +/- 1.2 (SE) min. Plasma Glu was unchanged in C during Lo but increased progressively in Hi. In P, plasma Glu rose steadily during Lo and Hi and was higher (P < 0.05) than in C throughout exercise. Plasma insulin declined during exercise in P but not in C; beta-blockade attenuated (P < 0.05) the rise in plasma glucagon and free fatty acids and exaggerated the increases in epinephrine and norepinephrine. Glu R(a) was 8.1 +/- 0.8 and 8.4 +/- 1.0 micromol. kg(-1). min(-1) at rest and 30.5 +/- 3.6 and 42.8 +/- 4.1 micromol. kg(-1). min(-1) at the end of Lo in C and P, respectively. During Hi, Glu R(a) increased to 54.4 +/- 4.4 and 73.8 +/- 4.7 micromol. kg(-1). min(-1) in C and P, respectively. Similarly, Glu R(d) was approximately 40% higher in P than in C during Lo (27.3 +/- 2.0 and 39.5 +/- 3.3 micromol. kg(-1). min(-1) in C and P, respectively) and Hi (37.4 +/- 2.6 and 61.5 +/- 5.3 micromol. kg(-1). min(-1) in C and P, respectively). beta-Blockade augmented CHO oxidation (CHO(ox)) with a concomitant reduction in fat oxidation. Inasmuch as estimated muscle glycogen utilization was similar between trials, the increase in CHO(ox) in P was due to increased use of plasma Glu. We conclude that beta-blockade increases Glu R(a) and R(d) and CHO(ox) in horses during exercise. The increase in Glu R(d) under beta-blockade suggests that beta-adrenergic mechanisms restrain Glu R(d) during exercise.     

Effect of carbohydrate ingestion on glucose kinetics during exercise in the heat.

Six endurance-trained men [peak oxygen uptake (V(O(2))) = 4.58 +/- 0.50 (SE) l/min] completed 60 min of exercise at a workload requiring 68 +/- 2% peak V(O(2)) in an environmental chamber maintained at 35 degrees C (<50% relative humidity) on two occasions, separated by at least 1 wk. Subjects ingested either a 6% glucose solution containing 1 microCi [3-(3)H]glucose/g glucose (CHO trial) or a sweet placebo (Con trial) during the trials. Rates of hepatic glucose production [HGP = glucose rate of appearance (R(a)) in Con trial] and glucose disappearance (R(d)), were measured using a primed, continuous infusion of [6,6-(2)H]glucose, corrected for gut-derived glucose (gut R(a)) in the CHO trial. No differences in heart rate, V(O(2)), respiratory exchange ratio, or rectal temperature were observed between trials. Plasma glucose concentrations were similar at rest but increased (P < 0.05) to a greater extent in the CHO trial compared with the Con trial. This was due to the absorption of ingested glucose in the CHO trial, because gut R(a) after 30 and 50 min (16 +/- 5 micromol. kg(-1). min(-1)) was higher (P < 0.05) compared with rest, whereas HGP during exercise was not different between trials. Glucose R(d) was higher (P < 0.05) in the CHO trial after 30 and 50 min (48.0 +/- 6.3 vs 34.6 +/- 3.8 micromol. kg(-1). min(-1), CHO vs. Con, respectively). These results indicate that ingestion of carbohydrate, at a rate of approximately 1.0 g/min, increases glucose R(d) but does not blunt the rise in HGP during exercise in the heat.     

Effect of intermittent high-intensity compared with continuous moderate exercise on glucose production and utilization in individuals with type 1 diabetes

Previously, the decline in glycemia in individuals with type 1 diabetes has been shown to be less with intermittent high-intensity exercise (IHE) compared with continuous moderate-intensity exercise (MOD) despite the performance of a greater amount of total work. The purpose of the present study was to determine whether this lesser decline in glycemia can be attributed to a greater increment in endogenous glucose production (Ra) or attenuated glucose utilization (Rd). Nine individuals with type 1 diabetes were tested on two separate occasions, during which either a 30-min MOD or IHE protocol was performed under conditions of a euglycemic clamp in combination with the infusion of [6,6-(2)H]glucose. MOD consisted of continuous cycling at 40% VO2 peak, whereas IHE involved a combination of continuous exercise at 40% VO2 peak interspersed with additional 4-s maximal sprint efforts performed every 2 min to simulate the activity patterns of intermittent sports. During IHE, glucose Ra increased earlier and to a greater extent compared with MOD. Similarly, glucose Rd increased sooner during IHE, but the increase by the end of exercise was comparable with that elicited by MOD. During early recovery from IHE, Rd rapidly declined, whereas it remained elevated after MOD, a finding consistent with a lower glucose infusion rate during early recovery from IHE compared with MOD (P<0.05). The results suggest that the lesser decline in glycemia with IHE may be attributed to a greater increment in Ra during exercise and attenuated Rd during exercise and early recovery.     

Substrate utilization by the inactive leg during one-leg or arm exercise.

Substrate utilization by the nonexercising leg was studied in healthy subjects during one-leg exercise at an average work load of 105 W for 40 min (n equals 8) or during arm exercise at 65 W for 20 min (n equals 5). During one-leg exercise both the blood flow and the A-FV difference of oxygen for the non exercising leg rose, resulting in an approximately five fold increment in oxygen uptake. EMG activity of the leg was increased above basal. Despite unchanged or falling arterial levels of insulin, the A-FV difference for glucose across the nonexercising leg rose during exercise and the estimated glucose uptake increased approximately fourfold. Release of lactate in the basal state reverted to a significant net uptake of lactate by the nonexercising leg. During arm exercise there was a 20-70% rise in leg blood flow and the leg oxygen uptake rose 25-45% in spite of minimal EMG activity from the thigh muscles. There was a large uptake of lactate by the legs during arm exercise. We conclude that several important metabolic alterations take place in the nonexercising leg tissues during physical exertion: 1) blood flow and oxygen uptake rise, partly as a consequence of motor activation; 2) substrate utilization shifts from a predominant FFA uptake in the basal state to a greater utilization of carbohydrate; 3) nonexercising muscle, and possibly adipose tissue, play an important role in the removal of lactate during exercise.     

Cardiovascular responses to facial cooling during low and moderate intensity exercise

To investigate the hypothesis that facial cooling (FC) exerts a greater influence on the cardiovascular system at lower versus higher levels of exercise, this study examined the effect of facial cooling [mean (SE): 0 (2)°C at 0.8 m·s^–1 wind velocity] during 30 min low [35% maximum oxygen consumption ( O_2max)] and moderate (70% O_2max) levels of cycle ergometry in the supine position. Five male subjects were assigned in random order to four exercise conditions: (1) FC at 35% O_2max(FC35), (2) no cooling (NFC35), (3) FC at 70% O_2max(FC70), and (4) no cooling (NFC70). Heart rate (f _c), stroke volume (V _s), and cardiac output ( _c) were measured at rest and every 10 min of exercise using impedance cardiography. During FC35, the change in f _c [mean (SE)] was significantly lower (P < 0.05) than NFC35 at 10 [22 (5) vs 31 (3) beats· min^–1], 20 [29 (6) vs 35 (3) beats·min^–1], and 30 [29 (5) vs 38 (4) beats·min^–1] min. No differences in f _c were observed between FC70 and NFC70. Furthermore, FC had no effect on V _s or _cat either exercise intensity. However, when comparing the FC70 and NFC70 conditions, there was a significant main effect (P<0.05) in mean arterial pressure (P _a) response with cooling despite the fact that neither V _s or _cwere different from the NFC70 control. The increase (P < 0.05) in the estimated change in systemic vascular resistance ( _a· _c ^–1) could partly explain the relative rise in _aat FC70. No pressor effect of cooling was observed at 35% O_2max. The results suggest that the FC condition promotes exercise bradycardia at low levels of exercise and exerts a greater pressor response during moderate exercise.