Three studies- all conducted by Jeukendrup et all - all concerning whether mixed sources of carbs (specifically glucose + fructose) will increase endurance performance.
1) Superior endurance performance with ingestion of multiple transportable carbohydrates. Feb 0840(2):275-281, February 2008.
The aim of the present study was to investigate the effect of ingesting a glucose plus fructose drink compared with a glucose-only drink (both delivering carbohydrate at a rate of 1.8 g/min) and a water placebo on endurance performance.
Basically this study had the subjects exercise for 120 mins at 55% of Vo2 max while taking in either glucose only (G) or glucose and fructose (GF), and then complete a time trial and see which version spared more glucose. It was not clear whether the subjects were actually taking in the mixed carbs during the TT. The GF trial had the fastest time to completion on the TT and the following conclusion was reached:
Conclusion: Ingestion of GF led to an 8% improvement in cycling time-trial performance compared with ingestion of G.
2) Multiple transportable carbohydrates enhance gastric emptying and fluid delivery.
This study looked at whether the mixed carbs would increase gastric emptying. This time the subjects cycled at 61% of VO2 max for 120 mins.
with the result that the GLU+FRU resulted in faster rates of gastric emptying and that GLU+FRU also attenuated the rise in heart rate that occurred in GLU and WATER and resulted in lower ratings of perceived exertion. There was a greater loss in body weight with GLU corrected for fluid intake. These data suggest that ingestion of a combined GLU+FRU solution increases GE and "fluid delivery" compared with a glucose only solution.
3) Exogenous CHO oxidation with glucose plus fructose intake during exercise.
PURPOSE: The purpose of the present study was to determine whether combined ingestion of moderate amounts of glucose plus fructose would result in higher rates of exogenous CHO oxidation compared with an isocaloric amount of glucose alone.
METHODS: Seven endurance-trained male cyclists performed three experimental trials consisting of 150 min of cycling at 65% VO(2max). Subjects ingested a CHO solution providing glucose (GLU) at an average rate of 0.8 g min(-1), glucose (0.54 g min(-1)) plus fructose (0.26 g min(-1)) (GLU + FRU), or plain water (WAT) during exercise. CONCLUSION: The present study demonstrates that ingesting moderate amounts of glucose plus fructose does not increase exogenous CHO oxidation above that of an isocaloric amount of glucose alone.
My take on all 3 studies: What good is it to test trained cyclists at 55% and 61% of VO2 max for Gastric emptying or depletion of exogenous carbohydrates? Who in the world races at this low an intensity???!!!($*&_&%_(Q#&%_$)#Q***@%#$@!! :) Huh? Who? Tell me?
As the good Dr. Bill Eisner says, at that low an intensity I could eat pizza and probably not get an upset stomach and may indeed have fairly good rates of GE! Given that there are tales of gastric distress with fructose intake and that other studies have shown that typical rates of GE are around 60g of CHO (any kind) , don't we need to see these studies done at intensities that mimic race conditions? Even an ultradistance athlete will be going at around 75%VO2max. Let's see this study done.