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Melissa J. Benton and Pamela D. Swan

Research suggests that ingesting protein after resistance exercise (RE) increases muscle protein synthesis and results in greater muscle gains. The effect on energy expenditure and substrate utilization, however, is unclear. This study evaluated the effect of RE and post exercise protein on recovery energy expenditure and substrate utilization in 17 women (age 46.5 ± 1.2 y). A whey-protein supplement (120 kcal, 30 g protein) was ingested immediately after 1 bout of RE (PRO) and a non caloric placebo after another (PLA). VO2 and respiratory-exchange ratio (RER) were measured before and for 120 min after each exercise session. RE resulted in a significant increase in VO2 that persisted through 90 min of recovery (P < 0.01) and was not affected by protein supplementation. RE significantly lowered RER, resulting in an increase in fat oxidation for both PLA and PRO (P < 0.01). For PRO, however, RER returned to baseline values earlier than for PLA, resulting in a reduced fat-oxidation response (P = 0.02) and earlier return to pre exercise baseline values than for PLA. Substrate utilization was significantly different between conditions (P = 0.02), with fat contributing 77.76% ± 2.19% for PLA and 72.12% ± 2.17% for PRO, while protein oxidation increased from 17.18% ± 1.33% for PLA to 20.82% ± 1.47% for PRO. Post exercise protein did not affect energy expenditure, but when protein was available as an alternate fuel fat oxidation was diminished. Based on these findings it might be beneficial for middle-aged women to delay protein intake after RE to maximize fat utilization.

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June C. Alberici, Peter A. Farrell, Penny M. Kris-Etherton and Carol A. Shively

This study examined the effects of preexercise candy bar ingestion on glycemic response, substrate utilization, and performance ie 8 trained male cyclists. The cyclists randomly ingested oee large milk chocolate bar (1CB), two large milk chocolate bars (2CB), or a placebo (P) 30 min prior to a 90-min cycle ride at 70% VO2max followed by a 33-W increase every 2 min until exhaustion (~10 min). Glucose decreased after 15 min of exercise but returned to preexercise values by 30 min of exercise. Glucose concentration for 2CB was significantly higher than for P and 1CB at exhaustion, Insulin concentration increased in response to ICB and 2CB and returned to preexercise values within 15 min of exercise. No significant differences were noted for free fatty acid (FFA) concentrations, Jactate concentrations, respiratory exchange ratio, total carbohydrate oxidation, or estimated fat and carbohydrate oxidation rates. Time to exhaustion was similar among the groups. The results suggest that the transient lowering of blood glucose observed with preexercise milk chocolate bar ingestion 30 min prior to exercise may not cause major metabolic perturbations that impair athletic performance in trained athletes performing moderately intense cycle exercise.

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L. Christopher Eschbach, Michael J. Webster, Joseph C. Boyd, Patrick D. McArthur and Tammy K. Evetovich

It has been suggested that Eleutherococcus senticosus (ES). also known as Siberian ginseng or ciwuija. increases fat utilization in humans. The purpose of this study was to examine the physiological responses to supplementation with ES in endurance cyclists. Using arandomized. double-blind crossover design. 9 highly-trained men (28 ± 2 years. V̇O2max 57.3±2.0 ml · kg−1 · min−1) cycled for 120 min at 60% V̇O2max followed by a simulated 10-km lime trial. Diet was controlled, and ES (1,200 mg · day−1) or a placebo (P) were administered for 7 days prior to each of the two trials. Oxygen consumption, respiratory exchange ratio, and heart rate were recorded every 30 min, and rating of perceived exertion. plasma [lactate], and plasma [glucose j were recorded every 20 min during the 120 min of steady state cycling. There were no significant differences (p > .05) between the ES and P groups at any steady-state time interval or during the cycling time trial (ES = 18.10 ± 0.42, P = 17.83 ± 0.47 min). In contrast with previous reports, the results of this study suggest that ES supplementation does not alter steady-state substrate utilization or 10-km cycling performance time.

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Elizabeth M. Broad, Ronald J. Maughan and Stuart D.R. Galloway

In a randomized, placebo-controlled, double-blind crossover design, 15 trained males undertook exercise trials during two 4 wk supplementation periods, with either 3 g L-Carnitine L-tartrate (LCLT) or 3 g placebo (P) daily. Total carbohydrate and fat oxidation during 90 min steady state cycling were not different between 0 or 4 wk within LCLT or P trials (mean ± standard deviation: carbohydrate oxidation P0 99 ± 36, P4W 111 ± 27, LCLT0 107 ± 33, LCLT4W 112 ± 32 g, respectively; fat oxidation P0 99 ± 28, P4W 92 ± 21, LCLT0 94 ± 18, LCLT4W 90 ± 22 g, respectively). Subsequent 20 km time trial duration was shorter after P (P0 31:29 ± 3:50, P4W 29:55 ± 2:58 min:s, P < 0.01), with no significant change over LCLT (LCLT0 31:46 ± 4:06, LCLT4W 31.19 ± 4.08 min:s). Four weeks LCLT supplementation had no effect on substrate utilization or endurance performance.

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Emma Stevenson, Clyde Williams, Maria Nute, Peter Swaile and Monica Tsui

The present study investigated the effect of the glycemic index of an evening meal on responses to a standard high glycemic index (HGI) breakfast the following morning. The metabolic responses to exercise 3 h after breakfast were also investigated. Seven active males completed 2 trials. In each trial, participants were provided with an evening meal on day 1, which was composed of either HGI or LGI (high or low glycemic index) carbohydrates. On day 2, participants were provided with a standard HGI breakfast and then performed a 60 min run at 65% VO2max 3 h later. Plasma glucose and serum insulin concentrations following breakfast were higher in the HGI trial compared to the LGI trial (P < 0.05). During exercise, there were no differences in substrate utilization. The results suggest that consuming a single LGI evening meal can improve glucose tolerance at breakfast but the metabolic responses to subsequent exercise were not affected.

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Robert G. McMurray and Peter A. Hosick

The study evaluated the interactions of puberty and obesity on substrate oxidation of overweight girls (n = 38) and boys (N = 35; BMI > 85th percentile) matched for gender, age, and puberty (pre/pubertal) with normal weight girls and boys. Metabolic rates (VO2) were obtained during rest and at 4, 5.6 and 8 k/h. Carbohydrate oxidation rates (mg/kgFFM/min) adjusted for % predicted VO2max, were higher for prepubertal OW children than pubertal children (p < .03). Fat oxidation rates were higher for NW prepubertal boys compared with other boys. Results indicate that OW children, regardless of gender or pubertal status, increase their carbohydrate oxidation rate to compensate for higher than normal metabolic rates. The effects of obesity on the substrate use is marginally related to puberty.

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Nancy L. Keim, Amy Z. Belko and Teresa F. Barbieri

Energy expenditure (EE) was measured at specific steady-state work rates to determine if body fat percentage or gender was associated with exercise EE, substrate oxidation, or work efficiency. Body fat percentage (leaner vs. fatter men, 9-15% vs. 20-25% fat; leaner vs. fatter women, 16-24% vs. 32-48% fat) was not related to work efficiency or submaximal EE. Fatness affected substrate oxidation in men but not in women. Compared to fatter men, leaner men had higher fat oxidation (6.7 ± 1.6 vs. 1.4 ± 2.0 mg · kg fat-free mass [FFM]1 · min1; p < .01) and lower carbohydrate oxidation (26.6 ± 4.2 vs. 39.3 ± 5.0 mg ⋅ kg FFM1min1; p< .01) at 60% V˙O2max. When men and women of similar fatness and relative aerobic capacity were compared, men had higher EE measured as kilojoules per minute but similar rates of EE and substrate oxidation per kilogram of FFM at 40-60% V˙O2max. It was concluded that body FFM, not fatness, is a determinant of exercise EE, whereas fatness is associated with differences in exercise substrate oxidation in men. Along with aerobic fitness, gender and fatness should be considered in future studies of exercise substrate oxidation.

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Rebecca T. McLay, Christine D. Thomson, Sheila M. Williams and Nancy J. Rehrer

This study compared 3 d of carbohydrate loading (CHOL; 8.4 g·kg−1·d−1 carbohydrate) in female eumenorrheic athletes with 3 d of an isoenergetic normal diet (NORM; 5.2 g·kg−1·d−1 carbohydrate) and examined the effect of menstrual-cycle phase on performance, muscle-glycogen concentration [glyc], and substrate utilization. Nine moderately trained eumenorrheic women cycled in an intermittent protocol varying in intensity from 45% to 75% VO2max for 75 min, followed by a 16-km time trial at the midfollicular (MF) and midluteal (ML) phases of the menstrual cycle on NORM and CHOL. Time-trial performance was not affected by diet (CHOL 26.10 ± 1.04 min, NORM 26.16 ± 1.35 min; P = 0.494) or menstrual-cycle phase (MF 26.05 ± 1.10 min, ML 26.23 ± 1.33 min; P = 0.370). Resting [glyc] was lowest in the MF phase after NORM (575 ± 145 mmol·kg−1·dw−1), compared with the MF phase after CHOL (728 mmol·kg−1·dw−1) and the ML phase after CHOL and NORM (756 and 771 mmol·kg−1·dw−1, respectively). No effect of phase on substrate utilization during exercise was observed. These data support previous observations of greater resting [glyc] in the ML than the MF phase of the menstrual cycle and suggest that lower glycogen storage in the MF phase can be overcome by carbohydrate loading.

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Brian J. Martin, Rachel B. Tan, Jenna B. Gillen, Michael E. Percival and Martin J. Gibala

Supplementation with green tea extract (GTE) in animals has been reported to induce numerous metabolic adaptations including increased fat oxidation during exercise and improved performance. However, data regarding the metabolic and physiological effects of GTE during exercise in humans are limited and equivocal.

Purpose:

To examine the effects of short-term GTE treatment on resting energy expenditure (REE), wholebody substrate utilization during exercise and time trial performance.

Methods:

Fifteen active men (24 ± 3 y; VO2peak = 48 ± 7 ml·kg·min−1; BMI = 26 ± 3 kg·m2(–1)) ingested GTE (3x per day = 1,000 mg/d) or placebo (PLA) for 2 day in a double-blind, crossover design (each separated by a 1 week wash-out period). REE was assessed in the fasted state. Subjects then ingested a standardized breakfast (~5.0 kcal·kg-1) and 90 min later performed a 60 min cycling bout at an intensity corresponding to individual maximal fat oxidation (44 ± 11% VO2peak), followed by a 250 kJ TT.

Results:

REE, whole-body oxygen consumption (VO2) and substrate oxidation rates during steady-state exercise were not different between treatments. However, mean heart rate (HR) was lower in GTE vs. PLA (115 ± 16 vs. 118 ± 17 beats·min−1; main effect, p = .049). Mixed venous blood [glycerol] was higher during rest and exercise after GTE vs. PLA (p = .006, main effect for treatment) but glucose, insulin and free-fatty acids were not different. Subsequent time trial performance was not different between treatments (GTE = 25:38 ± 5:32 vs. PLA = 26:08 ± 8:13 min; p = .75).

Conclusion:

GTE had minimal effects on whole-body substrate metabolism but significantly increased plasma glycerol and lowered heart rate during steady-state exercise, suggesting a potential increase in lipolysis and a cardiovascular effect that warrants further investigation.

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Michael C. Riddell, Sara L. Partington, Nicole Stupka, David Armstrong, C. Rennie and Mark A. Tarnopolsky

Compared to males, females oxidize proportionately more fat and less carbohydrate during endurance exercise performed in the fasted state. This study was designed to test the hypothesis that there may also be gender differences in exogenous carbohydrate (CHOexo) oxidation during exercise. Healthy, young males (n = 7) and females (n = 7) each completed 2 exercise trials (90 min cycle ergometry at 60% VO2peak), 1 week apart. Females were eumenorrheic and were tested in the midfollicular phase of their menstrual cycle. Subjects drank intermittently either 8% CHOexo (1 g glucose · kg · h−1) enriched with U-13C glucose or an artificially sweetened placebo during the trial. Whole-body substrate oxidation was determined from RER, urinary urea excretion, and the ratio of 13C:12C in expired gas during the final 60 min of exercise. During the placebo trial, fat oxidation was higher in females than in males (0.42 · 0.07 vs. 0.32 · 0.09 g · min−1 · kg LBM–1 × 10–2) at 30 min of exercise (p < .05). When averaged over the final 60 min of exercise, the relative proportions of fat, total carbohydrate, and protein were similar between groups. During CHOexo ingestion, both the ratio of 13C:12C in expired gas (p < .05) and the proportion of energy derived from CHOexo relative to LBM (p < .05) were higher in females compared to males at 75- and 90-min exercise. When averaged over the final 60 min of exercise, the percentage of CHOexo to the total energy contribution tended to be higher in females (14.3 · 1.2%) than in males (11.2 · 1.2%; p = .09). The reduction in endogenous CHO oxidation with CHOexo intake was also greater in females (12.9 · 3.1%) than in males (5.1 · 2.0%; p = .05). Compared to males, females may oxidize a greater relative proportion of CHOexo during endurance exercise which, in turn, may spare more endogenous fuel. Based on these observations, ingested carbohydrate may be a particularly beneficial source of fuel during endurance exercise for females.