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.     

Fructose-maltodextrin ratio in a carbohydrate-electrolyte solution differentially affects exogenous carbohydrate oxidation rate, gut comfort, and performance

Solutions containing multiple carbohydrates utilizing different intestinal transporters (glucose and fructose) show enhanced absorption, oxidation, and performance compared with single-carbohydrate solutions, but the impact of the ratio of these carbohydrates on outcomes is unknown. In a randomized double-blind crossover, 10 cyclists rode 150 min at 50% peak power, then performed an incremental test to exhaustion, while ingesting artificially sweetened water or one of three carbohydrate-salt solutions comprising fructose and maltodextrin in the respective following concentrations: 4.5 and 9% (0.5-Ratio), 6 and 7.5% (0.8-Ratio), and 7.5 and 6% (1.25-Ratio). The carbohydrates were ingested at 1.8 g/min and naturally (13)C-enriched to permit evaluation of oxidation rate by mass spectrometry and indirect calorimetry. Mean exogenous carbohydrate oxidation rates were 1.04, 1.14, and 1.05 g/min (coefficient of variation 20%) in 0.5-, 0.8-, and 1.25-Ratios, respectively, representing likely small increases in 0.8-Ratio of 11% (90% confidence limits; ± 4%) and 10% (± 4%) relative to 0.5- and 1.25-Ratios, respectively. Comparisons of fat and total and endogenous carbohydrate oxidation rates between solutions were unclear. Relative to 0.5-Ratio, there were moderate improvements to peak power with 0.8- (3.6%; 99% confidence limits ± 3.5%) and 1.25-Ratio (3.0%; ± 3.7%) but unclear with water (0.4%; ± 4.4%). Increases in stomach fullness, abdominal cramping, and nausea were lowest with the 0.8- followed by the 1.25-Ratio solution. At high carbohydrate-ingestion rate, greater benefits to endurance performance may result from ingestion of 0.8- to 1.25-Ratio fructose-maltodextrin solutions. Small perceptible improvements in gut comfort favor the 0.8-Ratio and provide a clearer suggestion of mechanism than the relationship with exogenous carbohydrate oxidation.     

Exogenous carbohydrate oxidation rates are elevated after combined ingestion of glucose and fructose during exercise in the heat.

The first purpose of this study was to investigate whether a glucose (GLU)+fructose (FRUC) beverage would result in a higher exogenous carbohydrate (CHO) oxidation rate and a higher fluid availability during exercise in the heat compared with an isoenergetic GLU beverage. A second aim of the study was to examine whether ingestion of GLU at a rate of 1.5 g/min during exercise in the heat would lead to a reduced muscle glycogen oxidation rate compared with ingestion of water (WAT). Eight trained male cyclists (maximal oxygen uptake: 64+/-1 ml.kg-1.min-1) cycled on three different occasions for 120 min at 50% maximum power output at an ambient temperature of 31.9+/-0.1 degrees C. Subjects received, in random order, a solution providing either 1.5 g/min of GLU, 1.0 g/min of GLU+0.5 g/min of FRUC, or WAT. Exogenous CHO oxidation during the last hour of exercise was approximately 36% higher (P<0.05) in GLU+FRUC compared with GLU, and peak oxidation rates were 1.14+/-0.05 and 0.77+/-0.08 g/min, respectively. Endogenous CHO oxidation was significantly lower (P<0.05) in GLU+FRUC compared with WAT. Muscle glycogen oxidation was not different after ingestion of GLU or WAT. Plasma deuterium enrichments were significantly higher (P<0.05) in WAT and GLU+FRUC compared with GLU. Furthermore, at 60 and 75 min of exercise, plasma deuterium enrichments were higher (P<0.05) in WAT compared with GLU+FRUC. Ingestion of GLU+FRUC during exercise in the heat resulted in higher exogenous CHO oxidation rates and fluid availability compared with ingestion of GLU and reduced endogenous CHO oxidation compared with ingestion of WAT.     

Oxidation of exogenous glucose, sucrose, and maltose during prolonged cycling exercise.

The purpose of the present study was to investigate whether combined ingestion of two carbohydrates (CHO) that are absorbed by different intestinal transport mechanisms would lead to exogenous CHO oxidation rates of >1.0 g/min. Nine trained male cyclists (maximal O(2) consumption: 64 +/- 2 ml x kg body wt(-1) x min(-1)) performed four exercise trials, which were randomly assigned and separated by at least 1 wk. Each trial consisted of 150 min of cycling at 50% of maximal power output (60 +/- 1% maximal O(2) consumption), while subjects received a solution providing either 1.8 g/min of glucose (Glu), 1.2 g/min of glucose + 0.6 g/min of sucrose (Glu+Suc), 1.2 g/min of glucose + 0.6 g/min of maltose (Glu+Mal), or water. Peak exogenous CHO oxidation rates were significantly higher (P < 0.05) in the Glu+Suc trial (1.25 +/- 0.07 g/min) compared with the Glu and Glu+Mal trials (1.06 +/- 0.08 and 1.06 +/- 0.06 g/min, respectively). No difference was found in (peak) exogenous CHO oxidation rates between Glu and Glu+Mal. These results demonstrate that, when a mixture of glucose and sucrose is ingested at high rates (1.8 g/min) during cycling exercise, exogenous CHO oxidation rates reach peak values of approximately 1.25 g/min.     

Exogenous carbohydrate oxidation during ultraendurance exercise.

The purposes of this study were: 1) to obtain a measure of exogenous carbohydrate (CHO(Exo)) oxidation and plasma glucose kinetics during 5 h of exercise; and 2) to compare CHO(Exo) following the ingestion of a glucose solution (Glu) or a glucose + fructose solution (2:1 ratio, Glu+Fru) during ultraendurance exercise. Eight well-trained subjects exercised three times for 5 h at 58% maximum O2 consumption while ingesting either Glu or Glu+Fru (both delivering 1.5 g/min CHO) or water. The CHO used had a naturally high 13C enrichment, and five subjects received a primed continuous intravenous [6,6-2H2]glucose infusion. CHO(Exo) rates following the ingestion of Glu leveled off after 120 min and peaked at 1.24 +/- 0.04 g/min. The ingestion of Glu+Fru resulted in a significantly higher peak rate of CHO(Exo) (1.40 +/- 0.08 g/min), a faster rate of increase in CHO(Exo), and an increase in the percentage of CHO(Exo) oxidized (65-77%). However, the rate of appearance and disappearance of Glu continued to increase during exercise, with no differences between trials. These data suggest an important role for gluconeogenesis during the later stages of exercise. Following the ingestion of Glu+Fru, cadence (rpm) was maintained, and the perception of stomach fullness was reduced relative to Glu. The ingestion of Glu+Fru increases CHO(Exo) compared with the ingestion of Glu alone, potentially through the oxidation of CHO(Exo) in the liver or through the conversion to, and oxidation of, lactate.     

Reversal of fatigue during prolonged exercise by carbohydrate infusion or ingestion

Seven cyclists exercised at 70% of maximal O2 uptake (VO2max) until fatigue (170 +/- 9 min) on three occasions, 1 wk apart. During these trials, plasma glucose declined from 5.0 +/- 0.1 to 3.1 +/- 0.1 mM (P less than 0.001) and respiratory exchange ratio (R) fell from 0.87 +/- 0.01 to 0.81 +/- 0.01 (P less than 0.001). After resting 20 min the subjects attempted to continue exercise either 1) after ingesting a placebo, 2) after ingesting glucose polymers (3 g/kg), or 3) when glucose was infused intravenously ("euglycemic clamp"). Placebo ingestion did not restore euglycemia or R. Plasma glucose increased (P less than 0.001) initially to approximately 5 mM and R rose (P less than 0.001) to approximately 0.83 with glucose infusion or carbohydrate ingestion. Plasma glucose and R then fell gradually to 3.9 +/- 0.3 mM and 0.81 +/- 0.01, respectively, after carbohydrate ingestion but were maintained at 5.1 +/- 0.1 mM and 0.83 +/- 0.01, respectively, by glucose infusion. Time to fatigue during this second exercise bout was significantly longer during the carbohydrate ingestion (26 +/- 4 min; P less than 0.05) or glucose infusion (43 +/- 5 min; P less than 0.01) trials compared with the placebo trial (10 +/- 1 min). Plasma insulin (approximately 10 microU/ml) and vastus lateralis muscle glycogen (approximately 40 mmol glucosyl U/kg) did not change during glucose infusion, with three-fourths of total carbohydrate oxidation during the second exercise bout accounted for by the euglycemic glucose infusion rate (1.13 +/- 0.08 g/min).(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Remarkable metabolic availability of oral glucose during long-duration exercise in humans

It was reported previously that glucose ingestion prior to or at the beginning of muscular exercise was a readily available metabolic substrate. The aim of this study was to see what percentage of carbohydrate utilization can be covered by glucose ingested regularly during exercise. Male healthy volunteers exercised for 285 min at approximately 45% of their individual maximal O2 uptake on a 10% uphill treadmill. After 15 min adaptation to exercise they received either 200 g (group G 200) or 400 g (group G 400) glucose (0.25 g X ml H2O-1) orally in eight equal doses repeated every 30 min (G 200 = 8 X 25 g, n = 4; G 400 = 8 X 50 g, n = 4). Indirect calorimetry was used to evaluate carbohydrate and lipid oxidation. Naturally labeled [13C]glucose was used to follow the oxidation of the exogenous glucose. Total carbohydrate oxidation was 341 +/- 22 and 332 +/- 32 g, lipid oxidation was 119 +/- 8 and 105 +/- 5 g, and exogenous glucose oxidation was 137 +/- 4 and 227 +/- 13 g (P less than 0.005) in groups G 200 and G 400, respectively. Endogenous glucose oxidation was about half in G 400 of what it was in G 200: 106 +/- 27 vs. 204 +/- 24 g (P less than 0.02). During the last hour of exercise, exogenous oxidation represented 55.3 and 87.5% of total carbohydrate oxidation for groups G 200 and G 400, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Oral [(13)C]glucose and endogenous energy substrate oxidation during prolonged treadmill running.

Six male subjects were studied during running exercise (120 min, 69% maximal oxygen consumption) with ingestion of a placebo or 3.5 g/kg of [(13)C]glucose (approximately 2 g/min). Indirect respiratory calorimetry corrected for urea excretion in urine and sweat, production of (13)CO(2) at the mouth, and changes in plasma glucose (13)C/(12)C were used to compute energy substrate oxidation. The oxidation rate of exogenous glucose increased from 1.02 at minute 60 to 1.22 g/min at minute 120 providing approximately 24 and 33% of the energy yield (%En). Glucose ingestion did not modify protein oxidation, which provided approximately 4-5%En, but significantly increased glucose oxidation by approximately 7%, reduced lipid oxidation by approximately 16%, and markedly reduced endogenous glucose oxidation (1.25 vs. 2.21 g/min between minutes 80 and 120, respectively). The oxidation rate of glucose released from the liver (0.38 and 0.47 g/min, or 10-13%En at minutes 60 and 120, respectively), and of plasma glucose (1.30-1.69 g/min, or 34 and 45%En and 50 and 75% of glucose oxidation) significantly increased from minutes 60 to 120, whereas the oxidation of muscle glycogen significantly decreased (1.28 to 0.58 g of glucose/min, or 34 and 16%En and 50 and 25% of glucose oxidation). These results indicate that, during moderate prolonged running exercise, ingestion of a very large amount of glucose significantly reduces endogenous glucose oxidation, thus sparing muscle and/or liver glycogen stores.     

Effects of carbohydrate ingestion before and during exercise on glucose kinetics and performance.

We investigated the effect of carbohydrate (CHO) ingestion before and during exercise and in combination on glucose kinetics, metabolism and performance in seven trained men, who cycled for 120 min (SS) at approximately 63% of peak power output, followed by a 7 kJ/kg body wt time trial (TT). On four separate occasions, subjects received either a placebo beverage before and during SS (PP); placebo 30 min before and 2 g/kg body wt of CHO in a 6.4% CHO solution throughout SS (PC); 2 g/kg body wt of CHO in a 25.7% CHO beverage 30 min before and placebo throughout SS (CP); or 2 g/kg body wt of CHO in a 25.7% CHO beverage 30 min before and 2 g/kg of CHO in a 6.4% CHO solution throughout SS (CC). Ingestion of CC and CP markedly (>8 mM) increased plasma glucose concentration ([glucose]) compared with PP and PC (5 mM). However, plasma [glucose] fell rapidly at the onset of SS so that after 80 min it was similar (6 mM) between all treatments. After this time, plasma [glucose] declined in both PP and CP (P < 0.05) but was well maintained in both CC and PC. Ingestion of CC and CP increased rates of glucose appearance (R(a)) and disappearance (R(d)) compared with PP and PC at the onset of, and early during, SS (P < 0.05). However, late in SS, both glucose R(a) and R(d) were higher in CC and PC compared with other trials (P < 0.05). Although calculated rates of glucose oxidation were different when comparing the four trials (P < 0.05), total CHO oxidation and total fat oxidation were similar. Despite this, TT was improved in CC and PC compared with PP (P < 0.05). We conclude that 1) preexercise ingestion of CHO improves performance only when CHO ingestion is maintained throughout exercise, and 2) ingestion of CHO during 120 min of cycling improves subsequent TT performance.     

Plasma glucose kinetics during prolonged exercise in trained humans when fed carbohydrate

Nine endurance-trained men exercised on a cycle ergometer at approximately 68% peak O2 uptake to the point of volitional fatigue [232 +/- 14 (SE) min] while ingesting an 8% carbohydrate solution to determine how high glucose disposal could increase under physiological conditions. Plasma glucose kinetics were measured using a primed, continuous infusion of [6,6-2H]glucose and the appearance of ingested glucose, assessed from [3-3H]glucose that had been added to the carbohydrate drink. Plasma glucose was increased (P < 0.05) after 30 min of exercise but thereafter remained at the preexercise level. Glucose appearance rate (R(a)) increased throughout exercise, reaching its peak value of 118 +/- 7 micromol. kg(-1). min(-1) at fatigue, whereas gut R(a) increased continuously during exercise, peaking at 105 +/- 10 micromol. kg(-1). min(-1) at the point of fatigue. In contrast, liver glucose output never rose above resting levels at any time during exercise. Glucose disposal (R(d)) increased throughout exercise, reaching a peak value of 118 +/- 7 micromol. kg(-1). min(-1) at fatigue. If we assume 95% oxidation of glucose R(d), estimated exogenous glucose oxidation at fatigue was 1.36 +/- 0.08 g/min. The results of this study demonstrate that glucose uptake increases continuously during prolonged, strenuous exercise when carbohydrate is ingested and does not appear to limit exercise performance.     

Oxidation of a glucose polymer during exercise: comparison with glucose and fructose

The purpose of this study was to compare the oxidation of 13C-labeled glucose, fructose, and glucose polymer ingested (1.33 g.kg-1 in 19 ml.kg-1 water) during cycle exercise (120 min, 53 +/- 2% maximal O2 uptake) in six healthy male subjects. Oxidation of exogenous glucose and glucose polymer (72 +/- 15 and 65 +/- 18%, respectively, of the 98.9 +/- 4.7 g ingested) was similar and significantly greater than exogenous fructose oxidation (54 +/- 13%). A transient rise in plasma glucose concentration was observed with glucose ingestion only. However, plasma insulin levels were similar with glucose and glucose polymer ingestions and significantly higher than with water or fructose ingestion. Plasma free fatty acid and glycerol responses to exercise were blunted with carbohydrate ingestion. However, fat utilization was not significantly different with water (82 +/- 14 g), glucose (60 +/- 3 g), fructose (59 +/- 11 g), or glucose polymer ingestion (60 +/- 8 g). Endogenous carbohydrate utilization was significantly lower with glucose (184 +/- 22 g), glucose polymer (187 +/- 31 g), and fructose (211 +/- 18 g) than with water (239 +/- 30 g) ingestion. Plasma volume slightly increased with water ingestion (7.4 +/- 4.5%), but the decrease was similar with glucose (-7.6 +/- 5.1%) and glucose polymer (-8.2 +/- 4.6%), suggesting that the rate of water delivery to plasma was similar with the two carbohydrates.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of fluid, electrolyte and substrate ingestion on endurance capacity

The availability of carbohydrate (CHO) as a substrate for the exercising muscles is known to be a limiting factor in the performance of prolonged cycle exercise, and provision of exogenous CHO in the form of glucose can increase endurance capacity. The present study examined the effects of ingestion of fluids and of CHO in different forms on exercise performance. Six male volunteers exercised to exhaustion on a cycle ergometer at a workload which required approximately 70% of Vo2max. After one preliminary trial, subjects performed this exercise test on six occasions, one week apart. Immediately before exercise, and at 10-min intervals throughout, subjects ingested 100 ml of one of the following: control (no drink), water, glucose syrup, fructose syrup, glucose-fructose syrup or a dilute glucose-electrolyte solution. Each of the syrup solutions contained approximately 36 g CHO per 100 ml; the isotonic glucose-electrolyte solution contained 4 g glucose per 100 ml. A randomised Latin square order of administration of trials was employed. Expired air samples for determination of Vo2, respiratory exchange ratio and rate of CHO oxidation were collected at 15-min intervals. Venous blood samples were obtained before and after exercise. Subjects drinking the isotonic glucose-electrolyte solution exercised longer (90.8 (12.4) min, mean (SEM] than on the control test (70.2 (8.3) min; p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Metabolic response to carbohydrate ingestion during exercise in males and females

The present study investigated potential sex-related differences in the metabolic response to carbohydrate (CHO) ingestion during exercise. Moderately endurance-trained men and women (n = 8 for each sex) performed 2 h of cycling at approximately 67% Vo(2 max) with water (WAT) or CHO ingestion (1.5 g of glucose/min). Substrate oxidation and kinetics were quantified during exercise using indirect calorimetry and stable isotope techniques ([(13)C]glucose ingestion, [6,6-(2)H(2)]glucose, and [(2)H(5)]glycerol infusion). In both sexes, CHO ingestion significantly increased the rates of appearance (R(a)) and disappearance (R(d)) of glucose during exercise compared with WAT ingestion [males: WAT, approximately 28-29 micromol x kg lean body mass (LBM)(-1) x min(-1); CHO, approximately 53 micromol x kg LBM(-1) x min(-1); females: WAT, approximately 28-29 micromol x kg LBM(-1) x min(-1); CHO, approximately 61 micromol x kg LBM(-1) x min(-1); main effect of trial, P < 0.05]. The contribution of plasma glucose oxidation to the energy yield was significantly increased with CHO ingestion in both sexes (from approximately 10% to approximately 20% of energy expenditure; main effect of trial, P < 0.05). Liver-derived glucose oxidation was reduced, although the rate of muscle glycogen oxidation was unaffected with CHO ingestion (males: WAT, 108 +/- 12 micromol x kg LBM(-1) x min(-1); CHO, 108 +/- 11 micromol x kg LBM(-1) x min(-1); females: WAT, 89 +/- 10 micromol x kg LBM(-1) x min(-1); CHO, 93 +/- 11 micromol x kg LBM(-1) x min(-1)). CHO ingestion reduced fat oxidation and lipolytic rate (R(a) glycerol) to a similar extent in both sexes. Finally, ingested CHO was oxidized at similar rates in men and women during exercise (peak rates of 0.70 +/- 0.08 and 0.65 +/- 0.06 g/min, respectively). The present investigation suggests that the metabolic response to CHO ingestion during exercise is largely similar in men and women.     

Metabolic response to [13C]glucose and [13C]fructose ingestion during exercise

Seven healthy male volunteers exercised on a cycle ergometer at 50 +/- 5% VO2max for 180 min, on three occasions during which they ingested either water only (W), [13C]glucose (G), or [13C]fructose (F) (140 +/- 12 g, diluted at 7% in water, and evenly distributed over the exercise period). Blood glucose concentration (in mM) significantly decreased during exercise with W (5.1 +/- 0.4 to 4.2 +/- 0.1) but remained stable with G (5.0 +/- 0.4 to 5.3 +/- 0.6) or F ingestion (5.4 +/- 0.5 to 5.1 +/- 0.4). Decreases in plasma insulin concentration (microU/ml) were greater (P less than 0.05) with W (11 +/- 3 to 3 +/- 1) and F (12 +/- 4 to 5 +/- 1) than with G ingestion (11 +/- 2 to 9 +/- 5), and fat utilization was greater with F (103 +/- 11 g) than with G ingestion (82 +/- 9 g) and lower than with W ingestion (132 +/- 14 g). However F was less readily available for combustion than G; over the 3-h period 75% (106 +/- 11 g) of ingested G was oxidized, compared with 56% (79 +/- 8 g) of ingested fructose. As a consequence, carbohydrate store utilizations were similar in the two conditions (G, 174 +/- 20 g; F, 173 +/- 17 g; vs. W, 193 +/- 22 g). These observations suggest that, during prolonged moderate exercise, F ingestion maintains blood glucose as well as G ingestion, and increases fat utilization when compared to G ingestion. However, due to a slower rate of utilization of F, carbohydrate store sparing is similar with G and F ingestions.     

Glycogen depletion during prolonged exercise: influence of glucose, fructose, or placebo

We examined the influence of various carbohydrates of fuel homeostasis and glycogen utilization during prolonged exercise. Seventy-five grams of glucose, fructose, or placebo were given orally to eight healthy males 45 min before ergometer exercise performed for 2 h at 55% of maximal aerobic power (VO2max). After glucose ingestion, the rises in plasma glucose (P less than 0.01) and insulin (P less than 0.001) were 2.4- and 5.8-fold greater than when fructose was consumed. After 30 min of exercise following glucose ingestion, the plasma glucose concentration had declined to a nadir of 3.9 +/- 0.3 mmol/l, and plasma insulin had returned to basal levels. The fall in plasma glucose was closely related to the preexercise glucose (r = 0.98, P less than 0.001) and insulin (r = 0.66, P less than 0.05) levels. The rate of endogenous glucose production and utilization rose similarly by 2.8-fold during exercise in fructose group and were 10-15% higher than in placebo group (P less than 0.05). Serum free fatty acid levels were 1.5- to 2-fold higher (P less than 0.01) after placebo than carbohydrate ingestion. Muscle glycogen concentration in the quadriceps femoris fell in all three groups by 60-65% (P less than 0.001) during exercise. These data indicate that fructose ingestion, though causing smaller perturbations in plasma glucose, insulin, and gastrointestinal polypeptide (GIP) levels than glucose ingestion, was no more effective than glucose or placebo in sparing glycogen during a long-term exercise.     

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.     

Oxidation of exogenous carbohydrate during prolonged exercise: the effects of the carbohydrate type and its concentration.

We studied rates of exogenous carbohydrate (CHO) oxidation during 90 min of cycling exercise in trained cyclists exercising at 70% of maximal oxygen consumption (VO2max) when they ingested glucose, sucrose, or glucose polymer solutions at concentrations of 7.5%, 10% or 15%. Drinks were labelled with [U-14C]glucose or sucrose and were ingested at a rate of 100 ml.10 min-1. Rates of oxidation of the ingested CHO were calculated from the specific radio-activity of the labelled CHO, expired 14CO2 and carbon dioxide output (VCO2). Total CHO oxidation, determined from oxygen consumption and VCO2 was not influenced by CHO type or concentration. Gastric emptying (P = 0.01) and the rate of exogenous CHO oxidation (P = 0.028) was greatest for the glucose polymer solutions, and least for glucose. Although gastric emptying (P = 0.006) decreased with increasing CHO concentration, CHO delivery to the intestine and exogenous CHO oxidation increased linearly with increasing CHO concentration. The percentage of the CHO delivered to the intestine that was oxidized ranged from 30.0% for 7.5% CHO to 38.1% for 15% CHO. Our results indicated that the rate of gastric emptying for CHO was not controlled to provide a constant rate of energy delivery as is commonly believed and that factors subsequent to gastric emptying limit the rate of exogenous CHO oxidation from the ingested solution.     

Effect of osmolality on availability of glucose ingested during prolonged exercise in humans

The aim of this study was to investigate whether the osmolality of a glucose solution, ingested at the beginning of a prolonged exercise bout, affects exogenous glucose disposal. We investigated the hormonal and metabolic response to a 50-g glucose load dissolved in either 200 (protocol A), 400 (protocol B), or 600 (protocol C) ml of water and given orally 15 min after adaptation to exercise in five healthy male volunteers. Naturally labeled [13C]glucose was used to follow the conversion of the ingested glucose to expired-air CO2. Total carbohydrate oxidation (indirect calorimetry) was similar in the three protocols (A, 237 +/- 20; B, 258 +/- 17; C, 276 +/- 20 g/4 h), as was lipid oxidation (A, 128 +/- 4; B, 132 +/- 15; C, 124 +/- 12 g/4 h). Exogenous glucose oxidation rates were similar under the three experimental conditions, and the total amount of exogenous glucose utilized was slightly, but not significantly, increased with the more diluted solution (A, 42.6 +/- 4.4; B, 43.4 +/- 4.1; C, 48.7 +/- 7.2 g/4 h). The blood glucose response was similar in the three protocols. Thus, within the range investigated, the osmolality of the glucose solution ingested had no significant influence either on its oxidation (which was 86-98% of the load ingested) or on the utilization of endogenous carbohydrate, lipid, or protein stores.     

Exogenous carbohydrate oxidation from maltose and glucose ingested during prolonged exercise

Intestinal perfusion studies have shown that glucose absorption from maltose occurs faster than from isocaloric glucose. To determine whether ingested maltose might be a superior source of carbohydrate (CHO) for endurance athletes, we compared the rates of gastric emptying, absorption and oxidation of 15 g.100 ml-1 solutions of maltose and glucose. Six endurance-trained cyclists drank 1200 ml of either U-14C maltose or U-14C glucose as a 400-ml loading bolus immediately before exercise, and as 8 x 100-ml drinks at 10-min intervals during a 90-min ride at 70% of maximal oxygen consumption. The rates of gastric emptying [maltose 690 (SD 119) ml.90 min-1; glucose 655 (SD 93) ml.90 min-1], the appearance of U-14C label in the plasma, and the peak rates of exogenous CHO oxidation [maltose 1.0 (SD 0.09) g.min-1; glucose 0.9 (SD 0.09) g.min-1] were not significantly different. Further, the 51 (SD 8) g of maltose and the 49 (SD 9) g of glucose oxidised during exercise were similar. Each accounted for approximately 20% of the total CHO oxidised during the 90 min of exercise. Since only half of the CHO delivered to the intestine was oxidised in the 90-min ride (maltose 49%; glucose 50%), we conclude that neither the rate of gastric emptying, nor digestion limited the rate of ingested CHO utilisation during the early stages of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)