By Scott Robinson.
Supplementation of carbohydrate (CHO) sources (e.g. glucose or glucose polymers), has been widely observed to increase exercise capacity, most notably during prolonged exercise at moderate to high intensities (1). These effects are largely attributed to a prevention of hypoglycaemia and the maintenance of high rates of CHO oxidation towards the latter phases of exercise when endogenous stores are either low or depleted (2). On this basis, it appears somewhat intuitive for athletes to seek methods of maximising their rate of CHO oxidation in the hope that a greater contribution from exogenous sources will increase exercise capacity through the ‘sparing’ of endogenous sources.
Background: Whereas it was once thought that 1 g.min-1 was the absolute maximum rate of CHO oxidation, more recent advances demonstrate convincingly that this rate is, in fact, much higher (in excess of 1.5 g.min-1; 3). To understand maximum CHO oxidation rates, it is important to understand what limits this. In an eloquent series of studies from Asker Jeukendrup’s lab in Birmingham, it was found that limitations to CHO oxidation were in the absorptive process most likely because of a saturation of carbohydrate transporters (for review see 4). They demonstrated that glucose oxidation rate is limited by a sodium-dependent glucose transporter (SGLT1), which, once saturated, the additional feeding of this carbohydrate will not result in greater intestinal absorption and increased oxidation rate (5). HOWEVER, other sugars are limited by different transport mechanisms (GLUT5 in the case of fructose). Thus, it was proposed that the use of different transporters might increase total carbohydrate absorption (see Figure 1).
Figure 1. The oxidation rate of glucose plus fructose in a combined drink is higher than the oxidation rate of similar amounts of either glucose or fructose alone (4)
Research study 1: Wallis et al. (2005; 6) tested this hypothesis by investigating the oxidation of combined ingestion of maltodextrins and fructose during cycling exercise. In this study, eight trained cyclists performed three exercise trials, with each comprising of 150 min cycling at 55% maximum power output. During each trial, subjects received a solution providing either 1.8 g.min-1 of maltodextrin (MD), 1.2 g/min-1 of maltodextrin + 0.6 g/min-1 of fructose (MD+F), or plain water. Results revealed that peak exogenous carbohydrate oxidation (last 30 min of exercise) was ~40% higher with combined MD+F ingestion compared with MD only ingestion. The authors concluded that ingestion of large amounts of maltodextrin and fructose during cycling exercise enables exogenous carbohydrate oxidation rates to reach peak values of ~1.5 g.min-1, which is much higher than oxidation rates from ingesting maltodextrin alone.
Performance Implications: Since these studies, it has been demonstrated that the ingestion multiple carbohydrate transporters i.e. glucose+fructose, exerts favourable influences on subjects’ ratings of perceived exertion (7) and endurance capacity (time to fatigue; 8). What is more, increased CHO oxidation with multiple transporter carbohydrates is well-regarded to be accompanied by increased fluid delivery and improved oxidation efficiency, thus reducing the likelihood of gastrointestinal distress (9).
Research study 2: Currell and Jeukendrup (2005; 8) investigated the effect of ingesting a glucose+fructose drink compared with a glucose-only drink (both delivering CHO at a rate of 1.8 g∙min-1) and a water placebo on endurance performance. Eight male trained cyclists performed120 min of cycling exercise at 55% Wmax followed by a time trial of approximately 1h duration. 
Results revealed a staggering 8% quicker time to completion during the time trial in the GF condition when compared with the G condition (times, 4022 s compared with 3641 s for FG and G, respectively). Total CHO oxidation did not differ significantly between GF (2.54 +/- 0.25 g∙min-1)and G (2.50 g∙min-1), indicating that there was a sparing of endogenous CHO stores in the GF trial, because GF has been shown to have a greater exogenous CHO oxidation than G.
Take home message: In sharp contrast to the original guidelines, the new recommendations are dictated by the type and duration of exercise. Multiple transportable carbohydrates, ingested at high rates (1.8-2.4 g∙min-1), are likely to improve exercise performance during ultra-endurance events of duration 3 h or more by reducing rating of perceived exertion and increasing time to fatigue. Such feeding strategies are not necessary for shorter duration events seeing that saturation of gut glucose transporters would be unlikely, especially if access to additional CHO is limited (e.g. team sports where fluid breaks are limited to unscheduled breaks in play and half-time).
BIO: Scott is a First Class Honours Sports Science graduate from the Research Institute of Sports and Exercise Sciences, at Liverpool John Moores University. He acquires a range of experience in both playing and coaching sport having represented Stoke City Football Club at Youth level and coached football at the International Youth Games. Scott has been an Assistant Sports Scientist at Blackburn Rovers Football Club and Everton Football Club for the 2010/2011 and 2011/2012 seasons, respectively. He has also partaken in Sports Science related research for FIFA, where he travelled across Europe as part of a multi-national team of sports scientists and athletes. Scott is currently completing his Masters of Science in Sports Physiology, where his research focuses on creating the optimal sports drink for soccer performance, after which he is due to begin a PhD within the Exercise Metabolism Research Group at the University of Birmingham, in September. Contact info: scottr38@hotmail.co.uk and Twitter is @scottrobinson8
References
- Coyle EF, Coggan AR, Hemmert MK & Ivy JL. Muscle glycogen utilisation during prolonged strenuous exercise when fed carbohydrate. J Appl Physiol. 1986; 61: 165-172.
- Bosch AN, Dennis SC & Noakes TD. Influence of carbohydrate ingestion on fuel substrate turnover and oxidation during prolonged exercise. J Appl Physiol. 1994; 76: 2364-2372.
- Jentjens RL, et al. Oxidation of combined ingestion of glucose and fructose during exercise. J. Appl. Physiol. 2004; 96: 1277-1284.
- Jeukendrup, AE, Gleeson, M. Sport Nutrition, Leeds, UK, Human Kinetics. 2010.
- Jentjens RL, et al. Oxidation of exogenous glucose, sucrose and maltose during prolonged cycling exercise. J. Appl. Physiol. 2004; 96: 1285-1291.
- Wallis GA, et al. Oxidation of combined ingestion of maltodextrins and fructose during exercise. Med Sci Spo & Exer. 2005; 37: 426-432.
- Jeukendrup AE, et al.Exogenous carbohydrate oxidation during ultraendurance exercise. J. Appl. Physiol. 2006; 100: 1134-1141.
- Currell K, Jeukendrup AE. Superior endurance performance with ingestion of multiple transportable carbohydrates. Med Sci Sports Exerc. 2008; 40: 275-281.
- Jeukendrup AE. Carbohydrate feeding during exercise. Eur J Sport Sci; 8: 77-86.
