Results
Glycogen-depleting exercise
Mean time to exhaustion for the sugar-free flavoured water (control), low molecular weight solution, and high molecular weight solution was 91 min (s = 12), 90 min (s = 15), and 88 min (s = 9), respectively, while cycling at an exercise intensity of 73.4% (s = 0.8), 72.5% (s = 1.9), and 73.1% (s = 1.3) of O2max, respectively.
Blood glucose
Blood glucose concentration was the same following the bout of exhaustive exercise during the sugar-free flavoured water (3.9 mmol ? l-1, s = 0.3), low molecular weight solution (3.7 mmol ? l-1, s = 0.2), and high molecular weight solution (3.9 mmol ? l-1, s = 0.4) trials ( Figure 1). After consumption of the sugar-free flavoured water, blood glucose concentration remained at 3.9 mmol ? l-1 for 2 h. Following consumption of the low and high molecular weight solutions, blood glucose concentration increased to a peak of 7.3 mmol ? l-1 (s = 0.8) and 8.1 mmol ? l-1 (s = 0.5) after 50 and 30 min, respectively, and then declined to similar values of 6.1 mmol ? l-1 (s = 0.5) and 6.1 mmol ? l-1 (s = 0.7), respectively, after 2 h. This increase in blood glucose concentration occurred at a faster rate following consumption of the high than the low molecular solution (0.14 vs. 0.07 mmol ? l-1 ? min-1), such that blood glucose concentration was higher at 10 (P < 0.05), 20 (P < 0.01), 30 (P < 0.001), and 40 (P < 0.01) min following ingestion.
Figure 1. Blood glucose concentration during a 2-h period of recovery from glycogen-depleting exercise and following the ingestion of a one-litre solution containing sugar-free flavoured water (CON), 100 g of a low molecular weight glucose polymer (LMW) or 100 g of a very high molecular weight glucose polymer (HMW). *P < 0.05, **P < 0.01, ***P < 0.01, HMW significantly greater than LMW. Values are means ? standard deviations.
Serum insulin
Serum insulin concentration was the same following the bout of exhaustive exercise during the sugar-free flavoured water (5.4 mU ? l-1, s = 2.9), low molecular weight solution (5.2 mU ? l-1, s = 3.0), and high molecular weight solution trials (6.1 mU ? l-1, s = 2.7), respectively ( Figure 2). Following consumption of the sugar-free flavoured water, serum insulin concentration remained around 6.0 mU ? l-1 for 2 h. Following consumption of the low and high molecular weight solutions, serum insulin concentration increased to a peak of 68.7 mU ? l-1 (s = 33.2) and 80.6 mU ? l-1 (s = 52.8) after 40 and 70 min, respectively, and then declined to similar values of 45.7 mU ? l-1 (s = 18.7) and 48.0 mU ? l-1 (s = 20.4), respectively, after 2 h. Serum insulin concentration was greater following consumption of the high than the low molecular weight solution at 20 (P < 0.05), 30 (P < 0.01), and 40 (P < 0.01) min following ingestion.
Figure 2. Serum insulin concentration during a 2-h period of recovery from glycogen-depleting exercise and following the ingestion of a one-litre solution containing sugar-free flavoured water (CON), 100 g of a low molecular weight glucose polymer (LMW) or 100 g of a very high molecular weight glucose polymer (HMW). *P < 0.05, **P < 0.01, HMW significantly greater than LMW. Values are means ? standard deviations.
Work output
Work output during the 15-min endurance performance time-trial test, performed 2 h after the ingestion of the sugar-free flavoured water, low molecular weight solution, and high molecular weight solution, was 137.5 kJ (s = 24.2), 149.4 kJ (s = 21.8), and 164.1 kJ (s = 21.1), respectively. Work output following the consumption of the low and high molecular weight solutions was greater than that following the consumption of the sugar-free flavoured water (P < 0.01 and P < 0.001, respectively). Furthermore, work output was 10% greater (P < 0.01) following ingestion of the high than the low molecular weight solution. Importantly, this increase in work output was observed in all participants studied (range 3.4 - 23.3%; Figure 3).
Figure 3. Work output for each individual participant during a 15-min ?all-out? cycling time-trial that was performed 2 h after glycogen-depleting exercise and the ingestion of a one-litre solution containing sugar-free flavoured water (CON), 100 g of a low molecular weight glucose polymer (LMW) or 100 g of a very high molecular weight glucose polymer (HMW). **P < 0.01, ***P < 0.001, LMW and HMW significantly greater than CON, respectively. ??P < 0.01, HMW significantly greater than LMW.
Discussion
The main aim of the present study was to determine the effect of a unique, high molecular weight glucose polymer solution (known to increase the rate of gastric emptying and post-exercise muscle glycogen re-synthesis, compared with a low molecular weight glucose polymer solution) on performance during a subsequent cycling time-trial. In this respect, work output 2 h after glycogen-depleting exercise and the ingestion of the high molecular weight solution was 20% greater (P < 0.001) than with the sugar-free flavoured water and, more importantly, 10% greater (P < 0.01) than with the low molecular weight solution. Furthermore, this positive performance effect of the high molecular weight solution was observed in all eight participants ( Figure 3).
Previous studies have shown that the high molecular weight solution used in the present study emptied from the stomach twice as fast, and resulted in a 70% greater increase in muscle glycogen content 2 h after glycogen-depleting exercise, compared with a low molecular weight solution (Leiper et al., 2000; Piehl-Aulin et al., 2000). In accordance with this, peak blood glucose concentration following ingestion of the high molecular weight solution in the present study was 10% greater, and occurred 20 min earlier, than with the low molecular weight solution ( Figure 1). Furthermore, the rate of increase in blood glucose concentration was two-fold greater over the first 30 min following ingestion, which is in line with the two-fold greater rate of gastric emptying observed during the initial 10 min following ingestion of the high molecular weight solution by Leiper and colleagues (2000). The rapid increase in blood glucose concentration following ingestion of the high molecular weight solution also resulted in a significantly higher serum insulin concentration during the first hour of recovery (Figure 2). In the present study, there were no differences between visits in exercise time to exhaustion in the ?glycogen-depleting? phase of the study, suggesting that post-exercise muscle glycogen content was similar across treatments at exhaustion. Furthermore, blood glucose values before the cycling time-trial were the same for the high and low molecular weight trials. It is reasonable to speculate, therefore, that the improvement in exercise performance observed after ingestion of the high molecular weight solution in the present study was the result of greater re-synthesis of the skeletal muscle glycogen store during the 2 h of recovery following exhaustive exercise compared with after ingestion of the low molecular weight solution, particularly as the ingestion of carbohydrate per se resulted in an increase in performance compared with ingestion of the sugar-free flavoured water.
It should be noted, however, that in the study of Piehl-Aulin and colleagues (2000), there were no differences in blood glucose or serum insulin concentration between the high molecular weight and control drink. The reasons for the discrepancy between the findings of the present study and those of Piehl-Aulin et al. (2000) are unclear, but could be because a mixture of monomeric and short-chain oligomeric glucose, with a lower molecular weight than the low molecular weight drink used in the present study (∼500 vs. ∼900 g ? mol-1), was used in the control drink by Piehl-Aulin and colleagues. Additionally, any potential differences in blood glucose or serum insulin concentration during recovery in Piehl-Aulin and colleagues' study could have been missed because of the use of venous blood sampling (as opposed to arterialized-venous sampling in the present study), the relatively large time interval in blood sampling (30 min), the large inter-individual variation in blood glucose and serum insulin, or the repeated ingestion of carbohydrate every 30 min of recovery. Also, Piehl-Aulin et al. (2000) speculated that if a faster delivery of glucose to the intestine is combined with a faster glucose uptake by the muscle cell immediately after exercise, this may mask an increase in delivery of glucose to the blood from the intestine and result only in minor changes in blood glucose concentration.
Since the pioneering work of Bergstrom and Hultman in the 1960s (Bergstrom & Hultman, 1966, 1967a, 1967b; Bergstrom et al., 1967), it has been recognized that a clear relationship exists between pre-exercise muscle glycogen concentration and prolonged exercise performance. In the present study, a 15-min high-intensity sub-maximal time-trial was used to measure exercise performance and it is unquestionable that, assuming a normal pre-exercise muscle glycogen content of 350 - 450 mmol ? kg dry muscle-1, muscle glycogen availability will not limit performance in this test. However, given that the participants performed prolonged exhaustive exercise before the time-trial, it is clear that muscle glycogen content would have been markedly reduced. Indeed, we have previously shown that this exercise model reduces muscle glycogen content to ∼25 mmol ? kg dry muscle-1 (Casey, Short, Hultman, & Greenhaff, 1995). Thus, we propose that muscle glycogen content would not have been restored during the 2 h of recovery that preceded the 15-min time-trial, particularly in the control condition where only water was ingested, such that its availability would have limited exercise performance during the 15-min time-trial (particularly in the control trial). By way of example, in the study by Piehl-Aulin et al. (2000), muscle glycogen content increased from 60 to 118 and 153 mmol ? kg dry muscle-1 during the 2 h of recovery following glycogen-depleting exercise in the low and high molecular weight trials, respectively. Furthermore, 300 g of carbohydrate was administered in Piehl-Aulin and colleagues' study compared with only 100 g in the present study. It is plausible, therefore, that post-exercise re-synthesis of muscle glycogen following the ingestion of the high molecular weight solution in the present study could well account for the increase in performance observed in all participants compared with the low molecular weight solution and certainly the sugar-free flavoured water. This is particularly the case when one considers that untrained individuals (normal pre-exercise muscle glycogen content of ∼300 mmol ? kg dry muscle-1) will utilize around 150 mmol ? kg dry muscle-1 of glycogen during 15 min of cycling exercise at 85%O2max (Dyck et al., 1993), which, based on heart rate responses, approximates the workload achieved in the present time-trial (170 - 180 beats ? min-1; data not shown).
The degree of glycogen re-synthesis in the liver may have also contributed to the difference in 15-min time-trial performance following post-exercise ingestion of the high molecular weight solution in the present study, particularly as liver-biopsy studies in healthy human volunteers have clearly demonstrated that the liver is extremely sensitive to changes in dietary carbohydrate intake (Nilsson & Hultman, 1973). Indeed, magnetic resonance spectroscopy studies have demonstrated that following exhaustive exercise liver glycogen is depleted to a considerable extent and, if the post-exercise carbohydrate load is inadequate, glycogen re-synthesis can impair glucose release from the liver and subsequent exercise capacity (Casey et al., 2000).
Conclusion
The ingestion of a unique, hign molecular weight glucose polymer solution (known to increase post-exercise muscle glucose delivery and glycogen re-synthesis compared with a standard, low molecular weight glucose polymer solution) increased work output during a subsequent highly reproducible, high-intensity sub-maximal time-trial cycling test. Furthermore, this effect was observed in all participants studied. These findings could be of practical importance for athletes who partake in training sessions, or indeed competitions, where rapid re-synthesis of the muscle glycogen store is required and performance must be maintained during a second period of exercise. It is noteworthy, however, that ingestion of this high molecular weight glucose polymer solution immediately before exercise does not appear to increase carbohydrate oxidation during exercise compared with a low molecular weight solution (Rowlands et al., 2005), most likely because carbohydrate oxidation during exercise is not limited by gastric emptying or muscle carbohydrate delivery following pre-exercise carbohydrate feeding. Thus, it would appear that the high molecular weight solution should be ingested immediately following exhaustive exercise when a significant period of recovery is anticipated before a subsequent bout of exercise.
References
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Fadi.