It is well documented that consuming carbohydrates (CHO) during exercise can enhance endurance performance8, at least in athletes consuming a typical western diet (i.e. predominantly CHO). Interestingly, this effect is also observed, albeit less consistently15, in bouts lasting an hour or less1,6. This is puzzling as, according to classic rationale, CHO supplementation should only benefit those events that:
- Are limited by muscle glycogen
- Are relatively reliant on blood glucose (e.g. > 2 hrs @ 50-70% VO2max)6
- Benefit from high rates of CHO oxidation
With the exception of the latter, performances of < 60 minutes do not fulfill those criteria13; so why do we see a benefit?
It was found in 2004 that the effects of CHO supplementation during high-intensity exercise (< 60 min) might be derived from “nonmetabolic” mechanisms4,10; that is, benefits seem to be “centrally” mediated.
Put simply, there are CHO receptors in the mouth that appear to be associated with motivational pathways in the brain. When these (presently obscure) receptors are triggered by a CHO solution, they reduce the perception of effort for a given pace/power—or, inversely, increase pace/power for a given RPE—hence the improved performance3,12. This performance enhancement is reportedly in the order of 2-3%5,11.
However, it is worth noting some research limitations in this area:
Firstly, benefits may have been exaggerated by the use of fasted subjects (i.e. those who hadn’t eaten for 3–15 hrs). Nonetheless, while the potency of mouth rinsing is less pronounced in fed subjects, its benefits are still practically meaningful9.
Secondly, in order to circumvent expectancy effects, it has been necessary to compare CHO mouth rinsing to a non-caloric placebo. This might also have overstated the benefits, as the act of mouth rinsing per se can be detrimental to performance. In other words, the benefits of a CHO mouth rinse are greater when compared to a non-CHO mouth rinse, than when compared to a non-rinsing condition5.
Finally, the majority of mouth-rinsing research has been conducted using cycling exercise. There is a dearth of running-based studies15, and those that do exist have been far less encouraging 16.
- Mouth rinsing is a strategy of interest to those competing in events of 45–75 mins2; events of shorter duration are unlikely to benefit7,12.
- Given that there is little value in actually swallowing a CHO solution, and indeed many will suffer from GI complaints as a result14, there seems cogent reason to advise against it (in events < 75 mins).
- Studies have generally involved swilling 25 mL of a ~6% carbohydrate solution in the mouth for ~5 sec every 5-10 mins. (NB: 6% = 6 g of carbohydrate per 100 mL)
- There is evidence to suggest a dose-response relationship between rinse duration and performance benefit14 (i.e. rinsing for longer is more effective).
- Given the practical limitations of the previous point, it has also been suggested that a more concentrated carbohydrate solution could deliver greater benefit9,14.
- There are of course drawbacks to mouth rinsing, perhaps most significant being the impairment of an athlete’s breathing “entrainment” (or “rhythm”)5. An alternative strategy, which has not yet been investigated, is placing a confectionary in the cheek cavity15.
- Below PR, Mora-Rodriguez R. Fluid and carbohydrate ingestion independently improve performance during 1 h of intense exercise. Med. Sci. Sports Exerc.27: 200–210, 1995.
- Burke LM, Hawley JA, Wong SHS, Jeukendrup AE. Carbohydrates for training and competition. Journal of Sports Sciences 29: S17–S27, 2011.
- Carter JM, Jeukendrup AE, Jones DA. The Effect of Carbohydrate Mouth Rinse on 1-h Cycle Time Trial Performance. Med. Sci. Sports Exerc. 36: 2107–2111, 2004.
- Carter JM, Jeukendrup AE, Mann CH, Jones DA. The Effect of Glucose Infusion on Glucose Kinetics during a 1-h Time Trial. Med. Sci. Sports Exerc. 36: 1543–1550, 2004.
- Gam S, Guelfi KJ, Fournier PA. Opposition of carbohydrate in a mouth-rinse solution to the detrimental effect of mouth rinsing during cycling time trials. Int. Journal Sport Nutr. Exerc. Metab. 23: 48–56, 2013.
- Jeukendrup AE, Brouns F. Carbohydrate-electrolyte feedings improve 1 h time trial cycling performance. Int. J. Sports Med. 18: 125–129, 1997.
- Jeukendrup AE, Hopkins S, Aragon-Vargas LF, Hulston C. No effect of carbohydrate feeding on 16 km cycling time trial performance. Eur. J. Appl. Physiol. 104: 831–837, 2008.
- Jeukendrup AE. Carbohydrate feeding during exercise. European Journal of Sport Science 8: 77–86, 2008.
- Lane SC, Bird SR, Burke LM, Hawley JA. Effect of a carbohydrate mouth rinse on simulated cycling time-trial performance commenced in a fed or fasted state. Appl. Physiol. Nutr. Metab. 38: 134–139, 2013.
- McConell GK, Canny BJ, Daddo MC. Effect of carbohydrate ingestion on glucose kinetics and muscle metabolism during intense endurance exercise. J. Appl. Physiol. 89: 1690–1698, 2000.
- Pottier A, Bouckaert J, Gilis W, Roels T, Derave W. Mouth rinse but not ingestion of a carbohydrate solution improves 1-h cycle time trial performance. Scand J Med Sci Sports 20: 105–111, 2010.
- Rollo I, Williams C, Gant N, Nute M. The influence of carbohydrate mouth rinse on self-selected speeds during a 30-min treadmill run. Int. Journal Sport Nutr. Exerc. Metab. 18: 585–600, 2008.
- Romijn JA, Coyle EF, Sidossis LS. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. Endocrinol. Metab. 28: E380–E391, 1993.
- Sinclair J, Bottoms L, Flynn C, Bradley E, Alexander G, McCullagh S, Finn T, Hurst HT. The effect of different durations of carbohydrate mouth rinse on cycling performance. European Journal of Sport Science 14: 259–264, 2014.
- Stellingwerff T, Cox GR. Systematic review: Carbohydrate supplementation on exercise performance or capacity of varying durations. Appl. Physiol. Nutr. Metab. 39: 998–1011, 2014.
- Whitham M, McKinney J. Effect of a carbohydrate mouthwash on running time-trial performance. Journal of Sports Sciences 25: 1385–1392, 2007.