Effect of pre-exercise carbohydrate feedings on endurance cycling performance

Six men were studied to compare the effects of pre-exercise carbohydrate feedings on endurance performance and muscle glycogen utilization during prolonged exercise. Trials consisted of a cycling ride to exhaustion at 75% maximal oxygen uptake preceded by the ingestion of either 75 g of glucose in 350 ml of water (GLU), 75 g of fructose in 350 ml of water (FRU), or 350 ml of an artificially sweetened and flavored placebo (CON). No differences were observed between trials for oxygen uptake, respiratory exchange ratio, heart rate, or exercise time to exhaustion (CON = 92.7 +/- 5.2 min, FRU = 90.6 +/- 12.4, and GLU = 92.8 +/- 11.3, mean +/- SE). Blood glucose was elevated as a result of the GLU feeding, but fell rapidly with the onset of exercise, reaching a low of 4.02 +/- 0.34 mmol X l-1 at 15 min of exercise. Serum insulin also increased following the GLU feeding but had returned to pre-drink levels by 30 min of exercise. No differences in blood glucose and insulin were observed between FRU and CON. Muscle glycogen utilization during the first 30 min of exercise (CON = 46.3 +/- 8.2 mmol X kg-1 wet weight, FRU = 56.3 +/- 3.0 mmol X kg-1 wet weight, GLU = 50.0 +/- 4.9 mmol X kg-1 wet weight) and total glycogen use (CON = 93.4 +/- 11.1, FRU = 118.8 +/- 10.9, and GLU = 99.5 +/- 4.3) were similar in the three trials.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of fructose ingestion on muscle glycogen usage during exercise

Eight healthy males were studied to compare the effects of preexercise fructose and glucose ingestion on muscle glycogen usage during exercise. Subjects performed three randomly assigned trials, each involving 30 min of cycling exercise at 75% VO2max. Forty-five min prior to commencing each trial, subjects ingested either 50 g of glucose (G), 50 g of fructose (F), or sweet placebo (C). No differences in VO2 or respiratory exchange ratio were observed between the trials. Blood glucose was elevated (P less than 0.05) as a result of the glucose feeding. With the onset of exercise, blood glucose declined rapidly during G, reaching a nadir of 3.18 +/- 0.15 (SE) mmol X 1(-1) at 20 min of exercise. This value was lower (P less than 0.05) than the corresponding values in F (3.79 +/- 0.20) and C (3.99 +/- 0.18). No differences in exercise blood glucose levels were observed between F and C. Muscle glycogen utilization was greater (P less than 0.05) during G (55.4 +/- 3.3 mmol X kg-1 w.w.) than C (42.8 +/- 4.2). No difference was observed between F (45.6 +/- 4.3) and C. There was a trend (P = 0.07) for muscle glycogen usage to be lower during F than G. These results suggest that the adverse effects of preexercise glucose ingestion are, in general, not observed with either fructose or sweet placebo.     

Caffeine effects on short-term performance during prolonged exercise in the heat

PURPOSE: To determine the effect of water, carbohydrate, and caffeine ingestion on fatigue during prolonged exercise in the heat. METHODS: Seven endurance-trained cyclists (V O2max = 61 +/- 8 mL.kg.min) pedaled for 120 min at 63% V O2max in a hot-dry environment (36 degrees C; 29% humidity), ingesting either no fluid (NF), water (WAT) to replace 97% fluid losses, the same volume of a 6% carbohydrate-electrolyte solution (CES), or each of these treatments along with ingestion of 6 mg of caffeine per kilogram of body weight (NF + CAFF, WAT + CAFF, and CES + CAFF). At regular intervals during exercise, maximal cycling power (PMAX) was measured. Before and after exercise, maximal voluntary contraction (MVC), voluntary activation (VA), and electrically evoked contractile properties of the quadriceps were determined. RESULTS: Without fluid replacement (NF and NF + CAFF), subjects were dehydrated by 3.8 +/- 0.3%, and rectal temperature reached 39.4 +/- 0.3 degrees C, while it was maintained at 38.7 +/- 0.3 degrees C in trials with rehydration (P < 0.05). Trials with caffeine ingestion increased PMAX by 3% above trials without caffeine (P < 0.05). MVC reductions after exercise were larger with NF (-11 +/- 5%) than for the rest of the trials (P < 0.05). MVC was reduced in WAT compared with CES + CAFF (-6 +/- 4 vs 2 +/- 4%; P < 0.05). However, NF + CAFF maintained MVC at the level of the CES trial. VA showed the same treatment response pattern as MVC. There were no differences in electrically evoked contractile properties among trials. CONCLUSION: During prolonged exercise in the heat, caffeine ingestion (6 mg.kg body weight) maintains MVC and increases PMAX despite dehydration and hyperthermia. When combined with water and carbohydrate, caffeine ingestion increases maximal leg force by increasing VA (i.e., reducing central fatigue).     

Effects of caffeine on prolonged intermittent-sprint ability in team-sport athletes

PURPOSE: Caffeine can be a powerful ergogenic aid for the performance of prolonged, submaximal exercise. Little evidence, however, supports an ergogenic effect of caffeine on intermittent-sprint performance. Hence, this study was conducted to examine the effects of acute caffeine ingestion on prolonged intermittent-sprint performance. METHODS: Using a double-blind, placebo-controlled design, 10 male team-sport athletes (amateur level, VO2peak 56.5 +/- 8.0 mL x kg(-1) x min(-1)) completed two exercise trials, separated by 7 d, 60 min after ingestion of either 6 mg x kg(-1) caffeine or placebo. The exercise trial was performed on a front-access cycle ergometer and consisted of 2 x 36-min halves, each composed of 18 x 4-s sprints with 2-min active recovery at 35% VO2peak between each sprint. Urinary caffeine levels were measured after exercise. RESULTS: The total amount of sprint work performed during the caffeine trial was 8.5% greater than that performed during the placebo trial in the first half (75,165.4 +/- 3,902.9 vs 69,265.6 +/- 3,719.7 J, P < 0.05), and was 7.6% greater in the second half (73,978.7 +/- 4,092.6 vs 68,783.2 +/- 3,574.4 J, P < 0.05). Similarly, the mean peak power score achieved during sprints in the caffeine trial was 7.0% greater than that achieved during the placebo trial in the first half (1330.9 +/- 68.2 vs 1244.2 +/- 60.7 W, P < 0.05), and was 6.6% greater in the second half (1314.5 +/- 68.4 vs 1233.2 +/- 59.9 W, P < 0.05). Urinary caffeine levels following the caffeine trial ranged from 3.5 to 9.1 microg x mL(-1) (6.9 +/- 0.6 microg x mL(-1)). CONCLUSION: This study revealed that acute caffeine ingestion can significantly enhance performance of prolonged, intermittent-sprint ability in competitive, male, team-sport athletes.     

Oxidation of combined ingestion of maltodextrins and fructose during exercise

PURPOSE: To determine whether combined ingestion of maltodextrin and fructose during 150 min of cycling exercise would lead to exogenous carbohydrate oxidation rates higher than 1.1 g.min. METHODS: Eight trained cyclists VO2max: 64.1 +/- 3.1 mL.kg.min) performed three exercise trials in a random order. Each trial consisted of 150 min cycling at 55% maximum power output (64.2+/-3.5% VO2max) while subjects received a solution providing either 1.8 g.min of maltodextrin (MD), 1.2 g.min of maltodextrin + 0.6 g.min of fructose (MD+F), or plain water. To quantify exogenous carbohydrate oxidation, corn-derived MD and F were used, which have a high natural abundance of C. RESULTS: Peak exogenous carbohydrate oxidation (last 30 min of exercise) rates were approximately 40% higher with combined MD+F ingestion compared with MD only ingestion (1.50+/-0.07 and 1.06+/-0.08 g.min, respectively, P<0.05). Furthermore, the average exogenous carbohydrate oxidation rate during the last 90 min of exercise was higher with combined MD+F ingestion compared with MD alone (1.38+/-0.06 and 0.96+/-0.07 g.min, respectively, P<0.05). CONCLUSIONS: The present study demonstrates that with ingestion of large amounts of maltodextrin and fructose during cycling exercise, exogenous carbohydrate oxidation can reach peak values of approximately 1.5 g.min, and this is markedly higher than oxidation rates from ingesting maltodextrin alone.     

Neuromuscular fatigue following constant versus variable-intensity endurance cycling in triathletes.

The aim of this study was to determine whether or not variable power cycling produced greater neuromuscular fatigue of knee extensor muscles than constant power cycling at the same mean power output. Eight male triathletes (age: 33+/-5 years, mass: 74+/-4 kg, VO2max: 62+/-5 mL kg(-1) min(-1), maximal aerobic power: 392+/-17 W) performed two 30 min trials on a cycle ergometer in a random order. Cycling exercise was performed either at a constant power output (CP) corresponding to 75% of the maximal aerobic power (MAP) or a variable power output (VP) with alternating +/-15%, +/-5%, and +/-10% of 75% MAP approximately every 5 min. Maximal voluntary contraction (MVC) torque, maximal voluntary activation level and excitation-contraction coupling process of knee extensor muscles were evaluated before and immediately after the exercise using the technique of electrically evoked contractions (single and paired stimulations). Oxygen uptake, ventilation and heart rate were also measured at regular intervals during the exercise. Averaged metabolic variables were not significantly different between the two conditions. Similarly, reductions in MVC torque (approximately -11%, P<0.05) after cycling were not different (P>0.05) between CP and VP trials. The magnitude of central and peripheral fatigue was also similar at the end of the two cycling exercises. It is concluded that, following 30 min of endurance cycling, semi-elite triathletes experienced no additional neuromuscular fatigue by varying power (from +/-5% to 15%) compared with a protocol that involved a constant power.     

Effects of sodium bicarbonate ingestion on prolonged intermittent exercise

PURPOSE: The aim of this study was to determine the effects of sodium bicarbonate ingestion on prolonged intermittent exercise and performance. METHODS: Eight healthy male subjects (mean +/- SD: age 25.4 +/- 6.4 yr, mass 70.9 +/- 5.1 kg, height 179 +/- 7 cm, VO(2max) 4.21 +/- 0.51 L.min-1) volunteered for the study, which had received ethical approval. Subjects undertook two 30-min intermittent cycling trials (repeated 3-min blocks; 90 s at 40% VO(2max), 60 s at 60% VO(2max), 14-s maximal sprint, 16-s rest) after ingestion of either sodium bicarbonate (NaHCO(3); 0.3 g.kg-1) or sodium chloride (NaCl; 0.045 g x kg(-1). Expired air, blood lactate (BLa), bicarbonate (HCO(3)-), and pH were measured at rest, 30 and 60 min postingestion, and during the 40% VO(2max) component of exercise (4, 10, 16, and 29 min). RESULTS: After ingestion, pH increased from rest to 7.46 +/- 0.03 and 7.40 +/- 0.01 for NaHCO(3) and NaCl, respectively (main effect for time and trial; P < 0.05). Values decreased at 15 min of exercise to 7.30 +/- 0.07 and 7.21 +/- 0.06, respectively, remaining at similar levels until the end of exercise. BLa peaked at 15 min (12.03 +/- 4.31 and 10.00 +/- 2.58 mmol.L-1, for NaHCO(3) and NaCl, respectively; P > 0.05) remaining elevated until the end of exercise (P < 0.05). Peak power expressed relative to sprint 1 demonstrated a significant main effect between trials (P < 0.05). Sprint 2 increased by 11.5 +/- 5% and 1.8 +/- 9.5% for NaHCO(3) and NaCl, respectively. During NaHCO(3), sprint 8 remained similar to sprint 1 (0.2 +/- 17%), whereas a decrease was observed during NaCl (-10.0 +/- 16.0%). CONCLUSION: The results of this study suggest that ingestion of NaHCO(3) improves sprint performance during prolonged intermittent cycling.     

Double blind carbohydrate ingestion does not improve exercise duration in warm humid conditions

The positive effects of carbohydrate (CHO) supplementation on endurance exercise are well documented but the placebo (PLA(c)) effect can make the ergogenic qualities of substances more difficult to determine. Therefore, this study tested the effect of double blind ingestion of PLA(c) and CHO(c) in capsules versus known capsule (CHO(k)) ingestion on prolonged exercise heat stress. Nine well trained male volunteers (mean+/-S.D.: 23+/-3 years; 62.4+/-6.5 kg and 65.8+/-5.2 mL kg(-1) min(-1) peak oxygen consumption) exercised at 60% of maximum power output until volitional exhaustion (TTE) in the three different conditions. Capsules were ingested with 252+/-39 mL of water. Blood glucose in CHO(c) and CHO(k) was similar but higher (p<0.05) than PLA(c) from 45 min to end of exercise. There were no differences in TTE between PLA(c) (125.2+/-37.1 min) or CHO(c) (138.8+/-47.0 min) or between CHO(c) and CHO(k) (155.8+/-54.2 min). Time to volitional exhaustion was different between PLA(c) and CHO(k) (p<0.05). Increased TTE resulted when participants and researchers knew the capsule content, but not in the double blind condition. The difference could be related to a combined effect of CHO ingestion and knowledge of what was ingested possibly acting as a potent psychological motivator.     

The effect of glucose ingestion on endurance upper-body exercise and performance

The physiological responses to glucose supplementation during arm crank exercise were investigated. Ten subjects of mean age 28 +/- 8 years; stature 180.8 +/- 6.5 cm; mass 82.7 +/- 11.5 kg, .VO(2) peak 3.10 +/- 0.50 l x min(-1) were tested on two occasions separated by a week. A 7.6% glucose drink or placebo was administered in a blind crossover design 20 min prior to exercise. Subject's arm cranked for 60 min at an exercise intensity of 65% .VO(2)peak followed by a 20 min performance test. Rate of ventilation, oxygen uptake, RER, heart rate and blood lactate demonstrated similar responses between trials throughout the course of the hour. The blood glucose concentrations at rest were similar between trials increasing after glucose ingestion to show a significant difference (p < 0.05) to the placebo trial at the onset of exercise, then returning to resting values after 20 min. The 20 min performance tests revealed that after glucose ingestion athletes achieved a greater mean distance of 12.55 +/- 1.29 km than in the placebo trial of 11.50 +/- 1.68 km (p < 0.05). In conclusion, the results showed that after one-hour of arm crank exercise, performance over a further twenty minutes was improved when glucose was ingested twenty minutes prior to exercise.     

Effects of acute modafinil ingestion on exercise time to exhaustion

PURPOSE: The purpose of this study was to investigate the effect of acute ingestion of modafinil (M) on time to exhaustion during high-intensity exercise. Modafinil (M) is a psychostimulant developed to treat narcolepsy, with "arousal" properties attributed to an increased release of dopamine in the CNS. Because other stimulants with similar properties have ergogenic effects, it was hypothesized that acute treatment with M would enhance physical performance. METHODS: Fifteen healthy male subjects, with a maximal aerobic power (VO2max) of 47 +/- SD 8 mL x kg x min, exercised on a cycle ergometer for 5 min at 50% VO2max and then at approximately 85% VO2max to exhaustion. They did this weekly for 3 wk: a control trial (C) the first week, and then 3 h after ingesting either placebo (P) or M (4 mg x kg) during the remaining 2 wk. The P and M trials were conducted with a balanced order, double-blind design. RESULTS:: Mean +/- SD times to exhaustion at 85% VO2max (TE) were 14.3 +/- 2.8, 15.6 +/- 3.8 and 18.3 +/- 3.5 min for the C, P, and M trials, respectively. TE for M was significantly longer than for the C and P trials. Oxygen uptake at exhaustion was slightly but significantly greater for M compared with P and C. HR increased with time and was further elevated by M. Subjective ratings of perceived exertion (RPE) were significantly lower for M compared with C and P but only after 10 min of exercise at 85% VO2max. CONCLUSION: Acute ingestion of modafinil prolonged exercise time to exhaustion at 85% VO2max and reduced RPE. The RPE results suggest that the dampening of the sensation of fatigue was likely a factor responsible for the enhanced performance.     

Effect of creatine loading on long-term sprint exercise performance and metabolism

PURPOSE: This study examined whether creatine (Cr) supplementation could enhance long-term repeated-sprint exercise performance of approximately 80 min in duration. METHODS: Fourteen active, but not well-trained, male subjects initially performed 10 sets of either 5 or 6 x 6 s maximal bike sprints, with varying recoveries (24, 54, or 84 s between sprints) over a period of 80 min. Work done (kJ) and peak power (W) were recorded for each sprint, and venous blood was collected preexercise and on four occasions during the exercise challenge. Muscle biopsies (vastus lateralis) were obtained preexercise as well as 0 min and 3 min postexercise. Subjects were then administered either 20 g.d-1 Cr.H2O (N = 7) or placebo (N = 7) for 5 d. Urine samples were collected for each 24 h of the supplementation period. Subjects were then retested using the same procedures as in test 1. RESULTS: Total work done increased significantly (P < 0.05) from 251.7 +/- 18.4 kJ presupplementation to 266.9 +/- 19.3 kJ (6% increase) after Cr ingestion. No change was observed for the placebo group (254.0 +/- 10.4 kJ to 252.3 +/- 9.3 kJ). Work done also improved significantly (P < 0.05) during 6 x 6 s sets with 54-s and 84-s recoveries and approached significance (P = 0.052) in 5 x 6 s sets with 24-s recovery in the Cr condition. Peak power was significantly increased (P < 0.05) in all types of exercise sets after Cr loading. No differences were observed for any performance variables in the placebo group. Resting muscle Cr and PCr concentrations were significantly elevated (P < 0.05) after 5 d of Cr supplementation (Cr: 48.9%; PCr: 12.5%). Phosphocreatine levels were also significantly higher (P < 0.05) immediately and 3 min after the completion of exercise in the Cr condition. CONCLUSION: The results of this study indicate that Cr ingestion (20 g.day-1 x 5 d) improved exercise performance during 80 min of repeated-sprint exercise, possibly due to an increased TCr store and improved PCr replenishment rate.     

Acute formoterol administration has no ergogenic effect in nonasthmatic athletes

PURPOSE: To determine the effect of formoterol (formoterol fumarate dihydrate) on the aerobic and anaerobic capacities of highly trained athletes. METHODS: 10 male athletes (age = 26.2 +/- 0.9, VO(2max) = 65.6 +/- 2.4 mL x kg(-1) x min(-1)) with minimal bronchial reactivity to aerosols (i.e., negative methacholine challenge test) completed three identical exercise sessions differing only by the medication administered. Formoterol (F) a long-acting beta(2)-agonist, presently not approved for international competition by the I.O.C. Medical committee, was compared with salbutamol (S), an accepted bronchodilator, and a placebo (P). Formoterol (12 microg), salbutamol (400 microg), or placebo was administered by a Turbuhaler, 10 min before exercise testing in a double-blind, randomized, three-way crossover design. Testing sessions included a Wingate anaerobic test followed 15 min later by an incremental cycle ergometer test to exhaustion. RESULTS: There were no significant differences between the groups in VO(2max) (F = 66.5 +/- 2.7; S = 67.8 +/- 2.5; P = 67.5 +/- 2.1 mL x kg(-1) x min(-1)) or Wingate peak power (F = 885 +/- 40; S = 877 +/- 40; P = 885 +/- 44 W) values. During the maximal aerobic test, no differences were observed in maximum minute ventilation, respiratory exchange ratio, heart rate, or work between the three experimental conditions. Also, there were no differences in the Wingate anaerobic test variables, total work, or fatigue index. CONCLUSION: Formoterol, administered in one aerosolized therapeutic dose, does not have an ergogenic effect in elite athletes without asthma.     

Caffeine, maximal power output and fatigue.

The purpose of this investigation was to determine the effects of caffeine ingestion on maximal power output and fatigue during short term, high intensity exercise. Nine adult males performed 15 s maximal exercise bouts 60 min after ingestion of caffeine (7 mg.kg-1) or placebo. Exercise bouts were carried out on a modified cycle ergometer which allowed power output to be computed for each one-half pedal stroke via microcomputer. Peak power output under caffeine conditions was not significantly different from that obtained following placebo ingestion. Similarly, time to peak power, total work, power fatigue index and power fatigue rate did not differ significantly between caffeine and placebo conditions. These results suggest that caffeine ingestion does not increase one's maximal ability to generate power. Further, caffeine does not alter the rate or magnitude of fatigue during high intensity, dynamic exercise.     

Pre-exercise oral creatine ingestion does not improve prolonged intermittent sprint exercise in humans

BACKGROUND: This investigation determined whether pre-exercise oral Cr ingestion could enhance prolonged intermittent sprint exercise performance. METHODS: EXPERIMENTAL DESIGN: a randomised, double-blind crossover design was employed. SETTING: testing was performed at the Western Australian Institute of Sport and participants were monitored and treated by both scientific and medical personnel. PARTICIPANTS: eight active, but not well-trained males with a background in multiple-sprint based sports acted as subjects for this investigation. INTERVENTIONS: subjects ingested either 15 g Cr.H2O or placebo 120 min and 60 min prior to the start of an 80-min maximal sprint cycling task (10 sets of multiple 6-sec sprints with varying active recoveries). Subjects were retested 14 days later, being required to ingest the alternate supplement and repeat the exercise test. MEASURES: performance variables (work done and peak power) were obtained throughout the exercise challenge. Muscle biopsies (vastus lateralis) were raised to a peak of 2348+/-223 micromol x l(-1) prior to the commencement of exercise after Cr ingestion. There were no significant changes in any cycling performance parameters following Cr ingestion, although blood La- was significantly lower (p<0.05) than placebo at all time points during were taken preexercise as well as immediately and 3 min post-exercise in order to determine concentrations of ATP, PCr, Cr, La- and glycogen. Venous blood was drawn prior to and on four occasions during the exercise test, and analysed for Cr, NH3+, La- and pH. RESULTS: Serum Cr concentrations exercise, and plasma NH3+ accumulation was also significantly reduced (p<0.05) in the Cr condition, but only in the second half of the 80-min exercise test. Muscle ATP and TCr levels as well as postexercise PCr replenishment were unaffected following Cr administration. CONCLUSIONS: The data suggest that although the pre-exercise ingestion of a large Cr dose was shown to have some impact on blood borne metabolites, it does not improve maximal prolonged intermittent sprint exercise performance, possibly due to an insufficient time allowed for uptake of serum Cr by skeletal muscle to occur. Therefore, this form of loading does not provide an alternative method of Cr supplementation to the traditional five-day supplementation regimes established by previous research.     

Oxidation and metabolic effects of fructose or glucose ingested before exercise

The aim of this study was to compare the effects of fructose (F) and glucose (G) intake before exercise on oxidation of the ingested substrate, glycogen utilization, work output, and metabolic changes. Ten trained subjects ingested F or G (1 g/kg), both of which were naturally enriched in 13C. After 1 h of rest, they exercised on an ergometer at 61% of their maximal oxygen uptake (VO2 max) for 45 min, which was immediately followed by 15 min at their maximal voluntary output. During the resting hour, blood insulin and glucose were lower (p less than 0.05) and respiratory quotient and blood lactate higher (p less than 0.01) after F. During exercise, the differences disappeared, apart from a transient but moderate (4.3 mmol/l) hypoglycemia after G compared to F. No difference between F and G was observed for uric acid, glycerol, FFA, and glucagon. Glycogen decrements in the vastus lateralis muscle were 67 +/- 9 (F) and 97 +/- 15 (G) mmol/kg, values not significantly different from each other (P greater than 0.05). The maximal voluntary work produced during the last 15 min did not differ between treatments. During the 2 h after sugar ingestion, 30 +/- 3 g of F and 26 +/- 3 g of G were oxidized to 13CO2. These findings indicate that fructose ingested before exercise was utilized at least as well as glucose, allowed a more stable glycemia, and did not modify performance.     

Enhancement of 2000-m rowing performance after caffeine ingestion

PURPOSE: To investigate the effect of caffeine ingestion on short-term endurance performance in competitive rowers. METHODS: In this randomized double-blind crossover study, eight competitive oarsmen (peak oxygen uptake [VO2peak] 4.7+/-0.4 L x min(-1), mean +/- SD) performed three familiarization trials of a 2000-m rowing test on an air-braked ergometer, followed by three experimental trials at 3- to 7-d intervals, each 1 h after ingesting caffeine (6 or 9 mg x kg(-1) body mass) or placebo. Trials were preceded by a standardized warm-up (6 min at 225+/-39 W; 75+/-7.7% VO2peak). RESULTS: Urinary caffeine concentration was similar before ingestion (approximately 1 mg x L(-1)) but rose to 6.2+/-3.6 and 14.5+/-7.0 mg x L(-1) for the low and high caffeine doses, respectively. Plasma free fatty acid concentration before exercise was higher after caffeine ingestion (0.29+/-0.17 and 0.39+/-0.20 mM for 6 and 9 mg x kg(-1), respectively) than after placebo (0.13+/-0.05 mM). Respiratory exchange ratio during the warm-up was also substantially lower with caffeine (0.94+/-0.09 and 0.93+/-0.06 for the low and high dose) than with placebo (0.98+/-0.12). Subjects could not distinguish between treatments before or after the exercise test. Both doses of caffeine had a similar ergogenic effect relative to placebo: performance time decreased by a mean of 1.2% (95% likely range 0.4-1.9%); the corresponding increase in mean power was 2.7% (0.4-5.0%). Performance time showed some evidence of individual differences in the effect of caffeine (SD 0.9%; 95% likely range 1.5 to -0.9%). CONCLUSIONS: Ingestion of 6 or 9 mg x kg(-1) of caffeine produces a worthwhile enhancement of short-term endurance performance in a controlled laboratory setting.     

Pre-exercise feeding does not affect endurance cycle exercise but attenuates post-exercise starvation-like response

The effects of ingesting a mixed-snack food (CB), fructose (FRU), or placebo (PBO) prior to exercise (70% peak VO2) on the metabolic response during and after cycle exercise were studied in eight normal healthy volunteers with a wide range of peak VO2 (30-70 cc.kg-1.min-1). The study was designed to minimize the impact of confounding factors by using various strategies. First, the volunteers were grouped in teams with stratification by peak VO2, and the tests were randomized by a Latin-square design. Second, subjects received two acclimation trials in the cycle ergometer to diminish the effect of learning experiences and allow them to get used to the room and equipment. In addition, financial incentives were offered for team and individual endurance times. The test meals were administered 30 min prior to the beginning of exercise, and the subjects exercised to exhaustion, which was defined with clear-cut endpoints. Gas and blood samples were taken at regular intervals before, during, and for 60 min after each exercise bout. CB and FRU induced higher pre-exercise glucose and insulin concentrations. Blood lactate increased 100% with FRU ingestion. Despite these differences; endurance time, substrate, and hormone concentrations as well as rates of substrate oxidation during exercise were identical among the three conditions. During the post-exercise recovery period, PBO was associated with a starvation-like pattern of substrate utilization in which lipid oxidation was 60% greater and carbohydrate oxidation 50% less than following either CB (75 +/- 11, 248 +/- 27 mg.min-1, P less than 0.05) or F ingestion (93 +/- 4, 221 +/- 14 mg.min-1).(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of carbohydrate ingestion on counterregulatory hormones during prolonged exercise

This study was undertaken to determine the effects of ingesting 5.0 (CHO-5), 6.0 (CHO-6), and 7.5 g/100 ml (CHO-7.5) carbohydrate (CHO) solutions on blood glucose and counterregulatory hormonal responses during prolonged intermittent exercise. Eight well-trained cyclists performed four trials consisting of seven 12-min cycling bouts at 70% of VO2max with 3 min rest between each ride. A final 12 min ride was an all-out self-paced performance ride. During the rest interval the subjects ingested either a water placebo (WP) or one of the CHO solutions at a rate of 8.5 mg/kg/h (approx. 150 ml). Blood samples were taken at 0, 25, 55, 85, and 115 min of exercise and were assayed for glucose, glucagon (GG), cortisol (CT), insulin (IN), epinephrine (EP), and norepinephrine (NE). Blood glucose levels were significantly lower in the WP trial compared to the CHO trials at 25 (4.6 +/- 0.2 vs 5.7 +/- 0.5 mmol/l) and 55 min (4.4 +/- 0.3 vs 5.0 +/- 0.8 mmol/l). At 85 min blood glucose was significantly lower in the WP compared to the CHO-6 and CHO-7.5 trials. GG and IN levels were not significantly different between trials; however, the GG:IN molar ratio was significantly higher in the WP than in the CHO-7.5 trial. CT was significantly elevated in the WP trial compared to the CHO-7.5 trial. EP and NE levels were not affected by CHO ingestion. These data suggest that CHO feedings prevent the typical hormonal responses which are responsible for hepatic glucose release, thus eliciting a possible hepatic glycogen sparing.     

CNS fatigue and prolonged exercise: effect of glucose supplementation

INTRODUCTION: Ingestion of carbohydrates during prolonged exercise may improve endurance, whereas an insufficient supply of glucose results in hypoglycemia and fatigue. Fatigue, defined as a loss of force-generating capacity, may develop for a variety of reasons and involve both central and peripheral factors. This study investigated whether CNS activation of the skeletal muscles was affected by prolonged exercise with or without glucose supplementation. METHODS: Voluntary force production and central activation ratios, assessed by the twitch interpolation technique, were determined during a 2-min sustained maximal knee extension in eight endurance-trained males in a baseline condition and immediately after 3 h of cycling randomized to be with or without glucose supplementation. RESULTS: The exercise bout without glucose supplementation (placebo trial) reduced the blood glucose concentration from 4.5 +/- 0.2 to 3.0 +/- 0.2 mM, whereas blood glucose homeostasis was maintained during the glucose trial. The average force during the sustained maximal voluntary muscle contraction was 248 +/- 23 N at baseline, 222 +/- 20 N in the glucose trial, and 197 +/- 21 N in the placebo trial (P < 0.05 between conditions). In the placebo trial, the lowered force production was accompanied by a reduced level of CNS activation compared with the other two conditions (P < 0.05), whereas the central activation ratios were similar in the glucose trial as compared with baseline. CONCLUSION: Exercise-induced hypoglycemia attenuates CNS activation during a sustained maximal muscle contraction, whereas central activation appears to be unaffected by 3 h of moderately intense exercise in endurance-trained athletes when euglycemia is maintained by carbohydrate ingestion.     

Pre-exercise glucose ingestion at different time periods and blood glucose concentration during exercise

The purpose of this study was to investigate the effects of glucose ingestion (GI) at different time periods prior to exercise on blood glucose (BG) levels during prolonged treadmill running. Eight subjects (X+/-SD), age 20+/-0.5yr, bodymass 70.7+/-4.1 kg, height 177+/-4 cm, VO2max 52.8+/-7.8 ml x kg(-1) x min(-1) who underwent different experimental conditions ingested a glucose solution (1 g/kg at 350 ml) 30 min (gl-30), 60 min (gl-60), 90 min (gl-90), and a placebo one 60 min (pl-60) prior to exercise in a counterbalanced design. Afterwards they ran at 65% of VO2max for 1 hour and then at 75 % of VO2max till exhaustion. Fingertip blood samples (10 microl) were drawn every 15 min before and during exercise for the determination of BG levels. Oxygen uptake (VO2), heart rate (HR), and blood lactate (La) were also measured every 15 min during exercise. Peak BG values were reached within 30 min after GI but were different (p < 0.01) at the onset of exercise (gl-30: 147+/-22, gl-60: 118+/-25, gl-90: 109+/-22, pl-60: 79+/-5mg/dl). The two-way ANOVA repeated measures and the Tukey post-hoc test revealed a higher BG concentration (p < 0.05) for the gl-30 and the pl-60 as compared to the gl-60 and gl-90 during running (e.g. 15min run: 82+/-11, 68+/-5, 64+/-3, 78+/-7, and 60min run: 98+/-12, 85+/-12, 83+/-11, 94+/-11 mg/dl for gl-30, gl-60, gl-90, and pl-60, respectively). However, this did not significantly affect the duration of treadmill running. The La levels were higher (p < 0.05) after GI as compared to placebo throughout exercise (values at exhaustion: 4.6+/-0.2, 5.0+/-1.5, 4.8+/- 1.7 mmol/l for gl-30, gl-60, gl-90, and 3.5+/-0.8 mmol/l for placebo). The gl-30 and the placebo fluctuated closer to normoglycaemic levels. The glucose ingestion (60 to 90 min) prior to exercise lowered the blood glucose levels without affecting the duration of running performance at 75% VO2max. Thus, in order to maintain normoglycaemic levels, pre-exercise glucose supplementation should be given 30 min before the onset of exercise.