Preexercise carbohydrate ingestion, glucose kinetics, and muscle glycogen use: effect of the glycemic index.

Eight trained men cycled at 70% peak oxygen uptake for 120 min followed by a 30-min performance cycle after ingesting either a high-glycemic index (HGI), low-glycemic index (LGI), or placebo (Con) meal 30 min before exercise. Ingestion of HGI resulted in an elevated (P<0.01) blood glucose concentration compared with LGI and Con. At the onset of exercise, blood glucose fell (P<0.05) such that it was lower (P<0.05) in HGI compared with LGI and Con at 15 and 30 min during exercise. Plasma insulin concentration was higher (P<0.01) throughout the rest period after ingestion of HGI compared with LGI and Con. Plasma free fatty acid concentrations were lower (P<0.05) throughout exercise in HGI compared with LGI and Con. The rates of [6,6-(2)H]glucose appearance and disappearance were higher (P<0.05) at rest after ingestion and throughout exercise in HGI compared with LGI and Con. Carbohydrate oxidation was higher (P<0.05) throughout exercise, whereas glycogen use tended (P = 0.07) to be higher in HGI compared with LGI and Con. No differences were observed in work output during the performance cycle when comparing the three trials. These results demonstrate that preexercise carbohydrate feeding with a HGI, but not a LGI, meal augments carbohydrate utilization during exercise but does not effect exercise performance.     

Ingestion of a high-glycemic index meal increases muscle glycogen storage at rest but augments its utilization during subsequent exercise.

The aim of this study was to compare the effect of preexercise breakfast containing high- and low-glycemic index (GI) carbohydrate (CHO) (2.5g CHO/kg body mass) on muscle glycogen metabolism. On two occasions, 14 days apart, seven trained men ran at 71% maximal oxygen uptake for 30 min on a treadmill. Three hours before exercise, in a randomized order, subjects consumed either isoenergetic high- (HGI) or low-GI (LGI) CHO breakfasts that provided (per 70 kg body mass) 3.43 MJ energy, 175 g CHO, 21 g protein, and 4 g fat. The incremental areas under the 3-h plasma glucose and serum insulin response curves after the HGI meal were 3.9- (P < 0.05) and 1.4-fold greater (P < 0.001), respectively, than those after the LGI meal. During the 3-h postprandial period, muscle glycogen concentration increased by 15% (P < 0.05) after the HGI meal but remained unchanged after the LGI meal. Muscle glycogen utilization during exercise was greater in the HGI (129.1 +/- 16.1 mmol/kg dry mass) compared with the LGI (87.9 +/- 15.1 mmol/kg dry mass; P < 0.01) trial. Although the LGI meal contributed less CHO to muscle glycogen synthesis in the 3-h postprandial period compared with the HGI meal, a sparing of muscle glycogen utilization during subsequent exercise was observed in the LGI trial, most likely as a result of better maintained fat oxidation.     

Reduced oxidation rates of ingested glucose during prolonged exercise with low endogenous CHO availability

Jeukendrup, Asker E., Lars B. Borghouts, Wim H. M. Saris,and Anton J. M. Wagenmakers. Reduced oxidation rates of ingested glucose during prolonged exercise with low endogenous CHO availability. J. Appl. Physiol. 81(5):1952-1957, 1996.This study investigated the effect of endogenouscarbohydrate (CHO) availability on oxidation rates of ingested glucoseduring moderate-intensity exercise. Seven well-trained cyclistsperformed two trials of 120 min of cycling exercise in random order at57% maximal O₂ consumption. Preexercise glycogen concentrations were manipulated byglycogen-lowering exercise in combination with CHO restriction[low-glycogen (LG) trial] or CHO loading[moderate-to-high-glycogen (HG) trial]. In the LG and HGtrials, subjects ingested 4 ml/kg body wt of an 8% corn-derivedglucose solution of high natural¹³C abundance at the start,followed by boluses of 2 ml/kg every 15 min. The third trial, in whichpotato-derived glucose was ingested, served as a control test forbackground correction. Exogenous glucose oxidation rates werecalculated from the ¹³C enrichmentof the ingested glucose and of the breathCO₂. Total CHO oxidation was lowerin the LG trial than in the HG trial during 60-120 min of exercise[84 ± 7 (SE) vs. 116 ± 8 g;P < 0.05]. Exogenous CHOoxidation in this period was 28% lower in the LG trial compared withthe HG trial. Maximal exogenous oxidation rates were also lower(P < 0.05) in the LG trial (0.64 ± 0.05 g/min) than in the HG trial (0.88 ± 0.04 g/min). Thisdecreased utilization of exogenous glucose was accompanied by increased plasma free fatty acid levels (2-3 times higher) and lower insulin concentrations. It is concluded that glycogen-lowering exercise, performed the evening before an exercise bout, in combination with CHOrestriction leads to a reduction of the oxidation rate of ingestedglucose during moderate-intensity exercise.

     

Effects of 4- and 8-h preexercise feedings on substrate use and performance

Seven well-trained male cyclists were studied during 105 min of cycling (65% of maximal oxygen uptake) and a 15-min "performance ride" to compare the effects of 4- and 8-h preexercise carbohydrate (CHO) feedings on substrate use and performance. A high CHO meal was given 1) 4-h preexercise (M-4), 2) 8-h preexercise (M-8), 3) 4-h preexercise with CHO feedings during exercise (M-4CHO), and 4) 8-h preexercise with CHO feedings during exercise (M-8CHO). Blood samples were obtained at 0, 15, 60, 105, and 120 min and analyzed for lactate, glucose, insulin, and glycerol. Total work output during the performance ride was similar for the M-4 (217,893 +/- 13,348 N/m) and M-8 trials (216,542 +/- 13,905) and was somewhat higher for the M-4CHO (223,994 +/- 14,387) and M-8CHO (224,702 +/- 15,709) trials (P = 0.059, NS). Glucose was significantly elevated throughout exercise, and insulin levels were significantly elevated at 15 and 60 min during M-4CHO and M-8CHO compared with M-4 and M-8 trials. Glycerol levels were significantly lower during the CHO feeding trials compared with placebo and were not significantly different during exercise when the subject had fasted an additional 4 h. The results of this study suggest that when preexercise meals are ingested 4 or 8 h before submaximal cycling exercise, substrate use and performance are similar.     

Effect of carbohydrate ingestion on glucose kinetics and muscle metabolism during intense endurance exercise.

There has been recent interest in the potential performance and metabolic effects of carbohydrate ingestion during exercise lasting approximately 1 h. In this study, 13 well-trained men ingested in randomized order either a 6% glucose solution (CHO trial) or a placebo (Con trial) during exercise to exhaustion at 83+/-1% peak oxygen uptake. In six subjects, vastus lateralis muscle was sampled at rest, at 32 min, and at exhaustion, and in six subjects, glucose kinetics was determined by infusion of [6,6-(2)H]glucose in both trials and ingestion of [6-(3)H]glucose in the CHO trial. Of the 84 g of glucose ingested during exercise in the CHO trial, only 22 g appeared in the peripheral circulation. This resulted in a small (12 g) but significant (P<0.05) increase in glucose uptake without influencing carbohydrate oxidation, muscle glycogen use, or time to exhaustion (CHO: 68.1+/-4.1 min; Con: 69.6+/-5.5 min). Decreases in muscle phosphocreatine content and increases in muscle inosine monophosphate and lactate content during exercise were similar in the two trials. Although endogenous glucose production during exercise was partially suppressed in the CHO trial, it remained significantly above preexercise levels throughout exercise. In conclusion, only 26% of the ingested glucose appeared in the peripheral circulation. Glucose ingestion increased glucose uptake and partially reduced endogenous glucose production but had no effect on carbohydrate oxidation, muscle metabolism, or time to exhaustion during exercise at 83% peak oxygen uptake.     

Effect of increased blood glucose availability on glucose kinetics during exercise

This studyexamined the effect of increased blood glucose availability on glucosekinetics during exercise. Five trained men cycled for 40 min at 77 ± 1% peak oxygen uptake on two occasions. During the second trial(Glu), glucose was infused at a rate equal to the average hepaticglucose production (HGP) measured during exercise in the control trial(Con). Glucose kinetics were measured by a primed continuous infusionofD-[3-³H]glucose.Plasma glucose increased during exercise in both trials and wassignificantly higher in Glu. HGP was similar at rest (Con, 11.4 ± 1.2; Glu, 10.6 ± 0.6µmol · kg¹ · min¹).After 40 min of exercise, HGP reached a peak of 40.2 ± 5.5 µmol · kg¹ · min¹in Con; however, in Glu, there was complete inhibition of the increasein HGP during exercise that never rose above the preexercise level. Therate of glucose disappearance was greater(P < 0.05) during the last 15 min ofexercise in Glu. These results indicate that an increase in glucoseavailability inhibits the rise in HGP during exercise, suggesting thatmetabolic feedback signals can override feed-forward activation of HGPduring strenuous exercise.

     

Influence of differing macronutrient intakes on muscle glycogen resynthesis after resistance exercise

The provision of additional protein (Pro)to a carbohydrate (CHO) supplement resulted in an enhanced rate ofmuscle glycogen resynthesis after endurance exercise (Zawadzki et al.,J. Appl. Physiol. 72: 1854-1859,1992). A comparison of isoenergetic CHO and CHO/Pro formula drinks onmuscle glycogen resynthesis has not been examined after eitherendurance or resistance exercise. We studied the effect of isoenergeticCHO (1 g/kg) and CHO/Pro/fat (66% CHO, 23% Pro, 11% fat) definedformula drinks and placebo (Pl) given immediately(t = 0 h) and 1 h(t = +1 h) after resistance exercisein 10 healthy young men. They performed a whole body workout (9 exercises/3 sets at 80% 1 repetition maximum) with unilateral kneeextension exercise [exercise (Ex) and control (Con) leg].The CHO/Pro/fat and CHO trials resulted in significantly greater(P < 0.05) plasma insulin andglucose concentration compared with Pl. Muscle glycogen wassignificantly lower (P < 0.05) for the Ex vs. Con leg immediately postexercise for all three conditions. The rate of glycogen resynthesis was significantly greater(P < 0.05) for both CHO/Pro/fat andCHO (23.0 ± 4.5 and 19.3 ± 6.1 mmol · kg drymuscle¹ · h¹,respectively) vs. Pl (Ex = 2.8 ± 2.3 and Con = 1.4 ± 3.6 mmol · kg drymuscle¹ · h¹).These results demonstrated that a bout of resistance exercise resultedin a significant decrease in muscle glycogen and that consumption of anisoenergetic CHO or CHO/Pro/fat formula drink resulted in similar ratesof muscle glycogen resynthesis after resistance exercise. This suggeststhat total energy content and CHO content are important in theresynthesis of muscle glycogen.

     

Improvements in exercise performance: effects of carbohydrate feedings and diet

In an effort to determine the effects of carbohydrate (CHO) feedings immediately before exercise in both the fasted and fed state, 10 well-trained male cyclists [maximum O2 consumption (VO2 max), 4.35 +/- 0.11 l/min)] performed 45 min of cycling at 77% VO2 max followed by a 15-min performance ride on an isokinetic cycle ergometer. After a 12-h fast, subjects ingested 45 g of liquid carbohydrate (LCHO), solid carbohydrate confectionery bar (SCHO), or placebo (P) 5 min before exercise. An additional trial was performed in which a high-CHO meal (200 g) taken 4 h before exercise was combined with a confectionery bar feeding (M + SCHO) immediately before the activity. At 10 min of exercise, serum glucose values were elevated by 18 and 24% during SCHO and LCHO, respectively, compared with P. At 0 and 45 min no significant differences were observed in muscle glycogen concentration or total use between the four trials. Total work produced during the final 15 min of exercise was significantly greater (P less than 0.05) during M + SCHO (194,735 +/- 9,448 N X m), compared with all other trials and significantly greater (P less than 0.05) during LCHO and SCHO (175,204 +/- 11,780 and 176,013 +/- 10,465 N X m, respectively) than trial P (159,143 +/- 11,407 N X m). These results suggest that, under conditions when CHO stores are less than optimal, exercise performance is enhanced with the ingestion of 45 g of CHO 5 min before 1 h of intense cycling.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of glucose supplement timing on protein metabolism after resistance training

Roy, B. D., M. A. Tarnopolsky, J. D. MacDougall, J. Fowles,and K. E. Yarasheski. Effect of glucose supplement timing onprotein metabolism after resistance training. J. Appl.Physiol. 82(6): 1882-1888, 1997.We determinedthe effect of the timing of glucose supplementation on fractionalmuscle protein synthetic rate (FSR), urinary urea excretion, and wholebody and myofibrillar protein degradation after resistance exercise.Eight healthy men performed unilateral knee extensor exercise (8 sets/~10 repetitions/~85% of 1 single maximal repetition). Theyreceived a carbohydrate (CHO) supplement (1 g/kg) or placebo (Pl)immediately (t = 0 h) and 1 h(t = +1 h) postexercise. FSR wasdetermined for exercised (Ex) and control (Con) limbs by incrementalL-[1-¹³C]leucineenrichment into the vastus lateralis over ~10 h postexercise. Insulinwas greater (P < 0.01) at 0.5, 0.75, 1.25, 1.5, 1.75, and 2 h, and glucose was greater(P < 0.05) at 0.5 and 0.75 h for CHO compared with Pl condition. FSR was 36.1% greater in the CHO/Ex leg than in the CHO/Con leg(P = not significant) and6.3% greater in the Pl/Ex leg than in the Pl/Con leg(P = not significant). 3-Methylhistidine excretion was lower in the CHO (110.43 ± 3.62 µmol/g creatinine) than Pl condition (120.14 ± 5.82, P < 0.05) as was urinary ureanitrogen (8.60 ± 0.66 vs. 12.28 ± 1.84 g/g creatinine,P < 0.05). This suggests that CHOsupplementation (1 g/kg) immediately and 1 h after resistance exercisecan decrease myofibrillar protein breakdown and urinary urea excretion,resulting in a more positive body protein balance.

     

Effects of glucose, glucose plus branched-chain amino acids, or placebo on bike performance over 100km

Madsen, Klavs, Dave A. MacLean, Bente Kiens, and DirkChristensen. Effects of glucose, glucose plus branched-chain aminoacids, or placebo on bike performance over 100 km. J. Appl. Physiol. 81(6): 2644-2650, 1996.This studywas undertaken to determine the effects of ingesting either glucose(trial G) or glucose plusbranched-chain amino acids (BCAA; trialB), compared with placebo (trialP), during prolonged exercise. Nine well-trained cyclists with a maximal oxygen uptake of 63.1 ± 1.5 mlO₂ ^· min^{1 ·} kg¹performed three laboratory trials consisting of 100 km of cycling separated by 7 days between each trial. During these trials, the subjects were encouraged to complete the 100 km as fast as possible ontheir own bicycles connected to a magnetic brake. No differences inperformance times were observed between the three trials (160.1 ± 4.1, 157.2 ± 4.5, and 159.8 ± 3.7 min, respectively). Intrial B, plasma BCAA levels increased from339 ± 28 µM at rest to 1,026 ± 62 µM after exercise(P < 0.01). Plasma ammoniaconcentrations increased during the entire exercise period for allthree trials and were significantly higher intrial B compared withtrials G andP (P < 0.05). The respiratory exchange ratio was similar in the threetrials during the first 90 min of exercise; thereafter, it tended todrop more in trial P than intrials G andB. These data suggest that neitherglucose nor glucose plus BCAA ingestion during 100 km of cyclingenhance performance in well-trained cyclists.

     

Metabolic and performance responses during endurance exercise after high-fat and high-carbohydrate meals

We studied the effects of preexercise mealcomposition on metabolic and performance-related variables duringendurance exercise. Eight well-trained cyclists (maximal oxygen uptake65.0 to 83.5 ml · kg¹ · min¹)were studied on three occasions after an overnight fast. They weregiven isoenergetic meals containing carbohydrate (CHO), protein (P),and fat (F) in the following amounts (g/70 kg body wt):high-carbohydrate meal, 215 CHO, 26 P, 3 F; high-fat meal, 50 CHO, 14 P, 80 F. On the third occasion subjects were studied after an overnightfast. Four hours after consumption of the meal, subjects startedexercise for 90 min at 70% of their maximal oxygen uptake, followed by a 10-km time trial. The high-carbohydrate meal compared with the high-fat meal resulted in significant decreases(P < 0.05) in blood glucose, plasmanonesterified fatty acids, plasma glycerol, plasmachylomicron-triacylglycerol, and plasma 3-hydroxybutyrate concentrations during exercise. This was accompanied by anincrease in plasma insulin (P < 0.01 vs. no meal), plasma epinephrine, and plasma growth hormoneconcentrations (each P < 0.05 vs.either of the other conditions) during exercise. Despite these large differences in substrate and hormone concentrations in plasma, substrate oxidation during the 90-min exercise period was similar inthe three trials, and there were no differences in performance on thetime trial. These results suggest that, although the availability offatty acids and other substrates in plasma can be markedly altered bydietary means, the pattern of substrate oxidation during enduranceexercise is remarkably resistant to alteration.

     

Oxidation of an oral [13C]glucose load at rest and prolonged exercise in trained and sedentary subjects

The purpose of this study was to compare the oxidation of[¹³C]glucose (100 g)ingested at rest or during exercise in six trained (TS) and sixsedentary (SS) male subjects. The oxidation of plasma glucose was alsocomputed from the volume of¹³CO₂and¹³C/¹²Cin plasma glucose to compute the oxidation rate of glucose released from the liver and from glycogen stores in periphery (mainly muscle glycogen stores during exercise). At rest, oxidative disposal of bothexogenous (8.3 ± 0.3 vs. 6.6 ± 0.8 g/h) and liver glucose (4.4 ± 0.5 vs. 2.6 ± 0.4 g/h) was higher in TS than in SS.This could contribute to the better glucose tolerance observed at rest in TS. During exercise, for the same absolute workload [140 ± 5 W: TS = 47 ± 2.5; SS = 68 ± 3 %maximal oxygen uptake(O_{2 max})], [¹³C]glucose oxidationwas higher in TS than in SS (39.0 ± 2.6 vs. 33.6 ± 1.2 g/h),whereas both liver glucose (16.8 ± 2.4 vs. 24.0 ± 1.8 g/h) and muscle glycogen oxidation (36.0 ± 3.0 vs. 51.0 ± 5.4 g/h) were lower. For the same relative workload (68 ± 3% O_{2 max}:TS = 3.13 ± 0.96; SS = 2.34 ± 0.60 lO₂/min), exogenous glucose(44.4 ± 1.8 vs. 33.6 ± 1.2 g/h) and muscle glycogen oxidation (73.8 ± 7.2 vs. 51.0 ± 5.4 g/h) were higher in TS. However,despite a higher energy expenditure in TS, liver glucose oxidation was similar in both groups (22.2 ± 3.0 vs. 24.0 ± 1.8 g/h). Thus exogenous glucose oxidation was selectively favored in TSduring exercise, reducing both liver glucose and muscle glycogen oxidation.

     

A moderate glycemic meal before endurance exercise can enhance performance

Kirwan, John P., Donal O'Gorman, and William J. Evans.A moderate glycemic meal before endurance exercise can enhance performance. J. Appl. Physiol. 84(1):53-59, 1998.The purpose of this study was to determine whetherpresweetened breakfast cereals with various fiber contents and amoderate glycemic index optimize glucose availability and improveendurance exercise performance. Six recreationally active women ate 75 g of available carbohydrate in the form of breakfast cereals: sweetenedwhole-grain rolled oats (SRO, 7 g of dietary fiber) or sweetenedwhole-oat flour (SOF, 3 g of dietary fiber) and 300 ml of water orwater alone (Con). The meals were provided 45 min before semirecumbentcycle ergometer exercise to exhaustion at 60% of peakO₂ consumption (O_{2 peak}). Diet andphysical activity were controlled by having the subjects reside in theGeneral Clinical Research Center for 2 days before each trial. Bloodsamples were drawn from an antecubital vein for glucose, free fattyacid (FFA), glycerol, insulin, epinephrine, and norepinephrinedetermination. Breath samples were obtained at 15-min intervals aftermeal ingestion and at 30-min intervals during exercise. Muscle glycogenconcentration was determined from biopsies taken from the vastuslateralis muscle before the meal and immediately after exercise. PlasmaFFA concentrations were lower (P < 0.05) during the SRO and SOF trials for the first 60 and 90 min ofexercise, respectively, than during the Con trial. Respiratory exchangeratios were higher (P < 0.05) at 90 and 120 min of exercise for the SRO and SOF trials, respectively, than for the Con trial. At exhaustion, glucose, insulin, FFA, glycerol, epinephrine, and norepinephrine concentrations, respiratory exchange ratio, and muscle glycogen use in the vastus lateralis muscle weresimilar for all trials. Exercise time to exhaustion was 16% longer(P < 0.05) during the SRO thanduring the Con trial: 266.5 ± 13 and 225.1 ± 8 min,respectively. There was no difference in exercise time for the SOF(250.8 ± 12) and Con trials. We conclude that eating ameal with a high dietary fiber content and moderate glycemic index 45 min before prolonged moderately intense exercise significantly enhancesexercise capacity.

     

Influence of carbohydrate status on immune responses before and after endurance exercise

To determine the effect of carbohydrate (CHO)status on immune responses after long-duration exercise, on twooccasions, 10 men completed a glycogen-depleting bout of cycleergometry followed by 48 h of either a high-CHO diet (HiCHO; 8.0 gCHO/kg) or a low-CHO diet (LoCHO; 0.5 g CHO/kg). After the 48 h,subjects completed a 60-min ride at 75% maximalO₂ uptake (EX). Blood samples were taken predepletion, pre-EX, post-EX, and 2 and 24 h post-EX and wereassayed for leukocyte number and function, glucose, glutamine, andcortisol. The glucose responses were significantly higher in the HiCHO(4.62 ± 0.26 mM) vs. the LoCHO (3.19 ± 0.15 mM) condition post-EX, and glutamine was significantly higher in the HiCHO(0.472 ± 0.036 mM) vs. the LoCHO (0.410 ± 0.025 mM)condition throughout. Cortisol levels were significantly greater in theLoCHO (587 ± 50 nM) vs. the HiCHO (515 ± 62 nM) conditionthroughout the trial. Lymphocyte proliferation (phytohemagglutinin) wassignificantly depressed after exercise. However, there was nodifference between conditions, and the depression was not correlatedwith elevations in cortisol. Circulating numbers of leukocytes,neutrophils, lymphocytes, and lymphocyte subsets were significantlygreater in the LoCHO vs. the HiCHO condition at the post-EX and 2 hpost-EX time points. These data indicate that the exercise and dietmanipulation altered the number of circulating leukocytes but did notaffect the decrease in lymphocyte proliferation that occurred afterexercise.

     

Oral [13C]glucose oxidation during prolonged exercise after high- and low-carbohydrate diets

The effect of a diet either high or low in carbohydrates (CHO)on exogenous ¹³C-labeled glucoseoxidation (200 g) during exercise (ergocycle: 120 min at 64.0 ± 0.5% maximal oxygen uptake) was studied in six subjects. Between 40 and 80 min, exogenous glucose oxidation was significantly higher afterthe diet low in CHO (0.63 ± 0.05 vs. 0.52 ± 0.04 g/min), butthis difference disappeared between 80 and 120 min (0.71 ± 0.03 vs.0.69 ± 0.04 g/min). The oxidation rate of plasma glucose, computedfrom the volume of¹³CO₂produced the¹³C-to-¹²Cratio in plasma glucose at 80 min, and of glucose released from theliver, computed from the difference between plasma glucose andexogenous glucose oxidation, was higher after the diet low in CHO (1.68 ± 0.26 vs. 1.41 ± 0.17 and 1.02 ± 0.20 vs. 0.81 ± 0.14 g/min, respectively). In contrast the oxidation rate ofglucose plus lactate from muscle glycogen (computed from the difference between total CHO oxidation and plasma glucose oxidation) was lower(0.31 ± 0.35 vs. 1.59 ± 0.20 g/min). After a diet low in CHO,the oxidation of exogenous glucose and of glucose released from theliver is increased and partly compensates for the reduction in muscleglycogen availability and oxidation.

     

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 beverage osmolality on intestinal fluid absorption during exercise

To determine howosmolality of an orally ingested fluid-replacement beverage would alterintestinal fluid absorption from the duodenum and/or jejunumduring 85 min of cycle exercise (63.3 ± 0.9% peakO₂ uptake) in a cool environment(22°C), seven subjects (5 men, 2 women, peakO₂ uptake = 54.5 ± 3.8 ml · kg¹ · min¹) participated infour experiments separated by 1 wk in which they ingested a waterplacebo (WP) or one of three 6% carbohydrate (CHO) beveragesformulated to give mean osmolalities of 197, 295, or 414 mosmol/kgH₂O. CHO solutions alsocontained 17-18 meq Na⁺ and3.2 meq K⁺. Nasogastric andmultilumen tubes were fluoroscopically positioned in the gastric antrumand duodenojejunum, respectively. Subjects ingested a total of 23 ml/kgbody mass of the test solution, 20% (370 ± 9 ml) of this volume 5 min before exercise and 10% (185 ± 4 ml) every 10 min thereafter.By using the rate of gastric emptying as the rate of intestinalperfusion (G. P. Lambert, R. T. Chang, D. Joensen, X. Shi, R. W. Summers, H. P. Schedl, and C. V. Gisolfi. Int. J. Sports Med. 17: 48-55, 1996), intestinal absorption was determined by segmental perfusion from the duodenum (0-25 cm) and jejunum (25-50 cm). There were no differences(P > 0.05) in gastric emptying (mean18.1 ± 1.3 ml/min) or total fluid absorption (802 ± 109, 650 ± 52, 674 ± 62, and 633 ± 74 ml · 50 cm¹ · h¹for WP, hypo-, iso-, and hypertonic solutions, respectively) amongbeverages; but WP was absorbed faster(P < 0.05) from the duodenum than inthe jejunum. Of the total volume of fluid ingested, 82 ± 14, 74 ± 6, 76 ± 5, and 68 ± 7% were absorbed forWP, hypo-, iso-, and hypertonic beverages, respectively. There were nodifferences in urine production or percent change in plasma volumeamong solutions. We conclude that total fluid absorption of 6%CHO-electrolyte beverages from the duodenojejunum during exercise,within the osmotic range studied, is not different from WP.

     

Effect of hypohydration on gastric emptying and intestinal absorption during exercise

Dehydration and hyperthermia may impair gastricemptying (GE) during exercise; the effect of these alterations onintestinal water flux (WF) is unknown. Thus the purpose of this studywas to determine the effect of hypohydration (~2.7% body weight) on GE and WF of a water placebo (WP) during cycling exercise (85 min, 65%maximal oxygen uptake) in a cool environment (22°C) and to alsocompare GE and WF of three carbohydrate-electrolyte solutions (CES)while the subjects were hypohydrated. GE and WF were determined simultaneously by a nasogastric tube placed in the gastric antrum andvia a multilumen tube that spanned the duodenum and the first 25 cm ofjejunum. Hypohydration was attained 12-16 h before experiments bylow-intensity exercise in a hot (45°C), humid (relative humidity 50%) environment. Seven healthy subjects (age 26.7 ± 1.7 yr,maximal oxygen uptake 55.9 ± 8.2 ml · kg¹ · min¹)ingested either WP or a 6% (330 mosmol), 8% (400 mosmol), or a 9%(590 mosmol) CES the morning following hypohydration. For comparison,subjects ingested WP after a euhydration protocol. Solutions (~2.0liters total) were ingested as a large bolus (4.6 ml/kg body wt) 5 minbefore exercise and as small serial feedings (2.3 ml/kg body wt) every10 min of exercise. Average GE rates were not different amongconditions (P > 0.05). Mean(±SE) values for WF were also similar(P > 0.05) for the euhydration (15.3 ± 1.7 ml · cm¹ · h¹)and hypohydration (18.3 ± 2.6 ml · cm¹ · h¹)experiments. During exercise after hypohydration, waterabsorption was greater (P < 0.05)with ingestion of WP (18.3 ± 2.6) and the 6% CES (16.5 ± 3.7),compared with the 8% CES (6.9 ± 1.5) and the 9% CES (1.8 ± 1.7). Mean values for final core temperature (38.6 ± 0.1°C),heart rate (152 ± 1 beats/min), and change in plasma volume(5.7 ± 0.7%) were similar among experimental trials. Weconclude that 1) hypohydration to~3% body weight does not impair GE or fluid absorption duringmoderate exercise when ingesting WP, and2) hyperosmolality (>400 mosmol)reduced WF in the proximal intestine.

     

Effect of caffeine on metabolism, exercise endurance, and catecholamine responses after withdrawal

In this studythe effects of acute caffeine ingestion on exercise performance,hormonal (epinephrine, norepinephrine, insulin), and metabolic (freefatty acids, glycerol, glucose, lactate, expired gases) parametersduring short-term withdrawal from dietary caffeine were investigated.Recreational athletes who were habitual caffeine users(n = 6) (maximum oxygen uptake 54.5 ± 3.3 ml · kg¹ · min¹and daily caffeine intake 761.3 ± 11.8 mg/day) were tested under conditions of no withdrawal and 2-day and 4-day withdrawal from dietarycaffeine. There were seven trials in total with a minimum of 10 daysbetween trials. On the day of the exercise trial, subjects ingestedeither dextrose placebo or 6 mg/kg caffeine in capsule form 1 h beforecycle ergometry to exhaustion at 80-85% of maximum oxygen uptake.Test substances were assigned in a random, double-blind manner. A finalplacebo control trial completed the experiment. There was nosignificant difference in any measured parameters among days ofwithdrawal after ingestion of placebo. At exhaustion in the 2- and4-day withdrawal trials, there were significant increases in plasmanorepinephrine in response to caffeine ingestion. Caffeine-inducedincreases in serum free fatty acids occurred after 4 days and only atrest. Subjects responded to caffeine with increases in plasmaepinephrine (P < 0.05) atexhaustion and prolonged exercise time in all caffeine trials comparedwith placebo, regardless of withdrawal from caffeine. It is concluded that increased endurance is unrelated to hormonal or metabolic changesand that it is not related to prior caffeine habituation inrecreational athletes.

     

Comparative effects of a low-carbohydrate diet and exercise plus a low-carbohydrate diet on muscle sarcoplasmic reticulum responses in males

We employed a glycogen-depleting session of exercise followed by a low-carbohydrate (CHO) diet to investigate modifications that occur in muscle sarcoplasmic reticulum (SR) Ca²⁺-cycling properties compared with low-CHO diet alone. SR properties were assessed in nine untrained males [peak aerobic power (O_{2 peak}) = 43.6 ± 2.6 (SE) ml·kg^–1·min^–1] during prolonged cycle exercise to fatigue performed at 58% O_{2 peak} after 4 days of low-CHO diet (Lo CHO) and after glycogen-depleting exercise plus 4 days of low-CHO (Ex+Lo CHO). Compared with Lo CHO, Ex+Lo CHO resulted in 12% lower (P < 0.05) resting maximal Ca²⁺-ATPase activity (V_max = 174 ± 12 vs. 153 ± 10 µmol·g protein^–1·min^–1) and smaller reduction in V_max induced during exercise. A similar effect was observed for Ca²⁺ uptake. The Hill coefficient, defined as slope of the relationship between cytosolic free Ca²⁺ concentration and Ca²⁺-ATPase activity, was higher (P < 0.05) at rest (2.07 ± 0.15 vs. 1.90 ± 0.10) with Ex+Lo CHO, an effect that persisted throughout the exercise. The coupling ratio, defined as the ratio of Ca²⁺ uptake to V_max, was 23–30% elevated (P < 0.05) at rest and during the first 60 min of exercise with Ex+Lo CHO. The 27 and 34% reductions (P < 0.05) in phase 1 and phase 2 Ca²⁺ release, respectively, observed during exercise with Lo CHO were not altered by Ex+Lo CHO. These results indicate that when prolonged exercise precedes a short-term Lo CHO diet, Ca²⁺ sequestration properties and efficiency are improved compared with those during Lo CHO alone. calcium cycling; vastus lateralis; contractile activity; glycogen; phosphorylation potential