Carbohydrate-electrolyte (CE) feedings have been shown to improve endurance performance (more than 2 hours duration) at moderate intensities (60-75% VO2max). The effects of CE feedings during high intensity exercise (i.e. > or = 80% VO2 max.) of shorter duration (approximately 1 h) are less clear. Jeukendrup, A et al (Int. J Sports Med. 1997 Feb;18(2):125-9) investigated the effect of the ingestion of a 7.6% CE solution during exercise on time trial cycling performance of approximately 1 h. This type of performance testing has been shown to be more reproducible (coefficient of variation 3.35%) than the traditional exercise test to exhaustion. On two occasions and in random order nineteen endurance trained cyclists completed an exercise test requiring the accomplishment of a set amount of work as fast as possible (time trial) under strictly standardized conditions. As the start and during the trials they drank in total 14 ml/kg of either a 7.6% CE solution or artificially flavored and colored water (placebo). Time to complete the set amount of work was significantly reduced and thus performance was significantly increased (p < 0.001) with the CE drink by 2.3%. Time to complete the set amount of work was 58.74 +/- 0.52 min. with CE and 60.15 +/- 0.65 min. with placebo (p < 0.001). Average workload during the time trials was 297.5 +/- 1.4W and 291.0 +/- 10.3 W, respectively. Subjects exercised at 76.4 +/- 0.7% of their maximal work rate (Wmax) with CE and at 74.8% Wmax with placebo (p < 0.001). They concluded that in relative short term (1h), high intensity (75% Wmax) cycling exercise ingestion of a carbohydrate-electrolyte solution compared to placebo improved performance.
Below PR et al (Med. Sci Sports Exerc 1995 Feb;27(2):200-10) also studied the effect of fluid and carbohydrate ingestion on performance, core temperature, and cardiovascular responses during intense exercise lasting 1 h. On four occasions, eight men cycled at 80 +/- 1% (+/- SEM) of VO2max for 50 min. followed by a performance test. During exercise, they consumed either a large volume (1330 +/- 60 ml) of a 6% carbohydrate (79 +/- 4 g) solution or water or a small volume (200 +/- 10 ml) of a 40% maltodextrin (79 +/- 4 g) solution or water. These trials were pooled so the effects of fluid replacement (Large FR vs. Small FR) and carbohydrate ingestion (CHO vs. NO CHO) could be determined. Performance times were 6.5% faster during Large FR than small FR and 6.3% faster during CHO than NO CHO (P < 0.05). There results demonstrated that both fluids and carbohydrates were of benefit and showed that larger volumes of FR slightly attenuated the increase in heart rate and core temperature which occurred during small FR. They concluded that both fluid and carbohydrate ingestion equally improved 1 hour cycling performance and their effects were additive.
A similar beneficial effect can also be seen with glucose ingestion immediately before an short, high intensity ride. El-Sayed, MS et al (J Sports Sci 1997 Apr;15(2):223-30) examined the effect of an 8% carbohydrate or placebo solution carbohydrate ingestion on performance during a simulated 1 h cycling time trial. Eight trained male cyclists (VO2 peak = 66.5 ml kg-1 min.-1) rode their own bicycles mounted on a windload simulator to imitate real riding conditions. At a self-selected maximal pace, the cyclists performed two 1 h rides (separated by 7 days). The beverages were administered 25 min. before (4.5 ml kg-1) the ride. Mean power output was significantly (P < 0.05) greater during the carbohydrate compared with the placebo trial (mean +/- S.E.: 277 +/- 3 and 269 +/- 3 W, respectively). The greater distance covered in the carbohydrate compared with the placebo trial (41.5 +/- 1.06 and 41.0 +/- 1.06 km, respectively; P < 0.05) was equivalent to a 44 s improvement. They concluded that pre-exercise carbohydrate ingestion significantly increased endurance performance in trained cyclists during a 1 h simulated time trial. Although the mechanism for this enhancement in performance with carbohydrate ingestion could not be surmised from the results, it was speculated it might be related to a higher rate of carbohydrate oxidation, or to favorable effects of carbohydrate ingestion on the central component of fatigue.
And as might be expected, the benefits of carbohydrates becomes even more obvious in events of more than 90 to 120 minutes. In J Appl Physiol 2000 Jan;88(1):113-9, Angus DJ et al examined the effectiveness of ingesting a carbohydrate or carbohydrate + medium-chain triglycerides (MCT) on cycling performance. Eight endurance-trained men [peak O(2) uptake = 4.71 +/- 0.09 (SE) l/min.] completed 35 kJ/kg as quickly as possible [time trial (TT)] while consuming 250 ml/15 min. of either a 6% (wt/vol) carbohydrate solution (C), a 6% carbohydrate + 4.2% MCT solution C+M), or a sweet placebo (P). Time to complete the set amount of work was reduced in both C and C+M compared with P by 7 and 5%, respectively (C: 166 +/- 7 min.; C+M: 169 +/- 7 min.; P: 178 +/- 11 min.; P < 0.01). The estimated rate of carbohydrate oxidation was not different during the first 90 min. of exercise but thereafter was reduced (P < 0.05) in P and was maintained in both C and C+M. These data demonstrate that carbohydrate ingestion during exercise improves 100-km TT performance compared with a sweet placebo, but the addition of MCT does not provide any further performance enhancement.