The importance of carbohydrates as an energy source for exercise is well known: one of the first study to hypothesize and recognize their importance was the study of Krogh and Lindhardt at the beginning of the 20th century (1920); later, in the mid ‘60’s, Bergstrom and Hultman discovered the crucial role of muscle glycogen on endurance capacity.
Nowdays, the ergogenic effects of carbohydrate supplementation on endurance performance are well known; they are mediated by mechanisms such as:
- a sparing effect on liver glycogen;
- the maintenance of glycemia and rates of carbohydrate oxidation;
- the stimulation of glycogen synthesis during low-intensity exercise ;
- a possible stimulatory effect on the central nervous system.
However, their supplementation, immediately before and during exercise, has an improving effect also during exercise (running or cycling) of a shorter and more intense nature: >75% VO2max (maximal oxygen consumption) and ≤1 hour, during which euglycaemia is rarely challenged and adequate muscle glycogen store remains at the cessation of the exercise.
Hypothesis for carbohydrate mouth rinse
In the absence of a clear metabolic explanation it was speculated that ingesting carbohydrate solutions may have a ‘non-metabolic’ or ‘central effect’ on endurance performance. To explore this hypothesis many studies have investigated the performance responses of subjects when carbohydrate solutions (about 6% carbohydrate, often maltodextrins) are mouth rinsed during exercise, expectorating the solution before ingestion.
By functional magnetic resonance imaging and transcranial stimulation it was shown that carbohydrates in the mouth stimulate reward centers in the brain and increases corticomotor excitability, through oropharyngeal receptors which signal their presence to the brain.
Probably salivary amylase releases very few glucose units from maltodextrins which is probably what is needed in order to activate the purported carbohydrate receptors in the oropharynx (no glucose transporters in the oropharynx are known).
However, the performance response appears to be dependent upon the pre-exercise nutritional status of the subject: most part of the studies showing an improving effect on performance was conducted in a fasted states (3- to 15-h fasting).
Only one study has shown improvements of endurance capacity; in both fed and fasted states by carbohydrate mouth rinse, but in non-athletic subjects.
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Bergstrom J., Hultman E. A study of glycogen metabolism during exercise in man. Scand J Clin Invest 1967;19:218-228. doi:10.3109/00365516709090629
Bergstrom J., Hultman E. Muscle glycogen synthesis after exercise: an enhancing factor localized in muscle cells in man. Nature 1966;210:309-310. doi:10.1038/210309a0
de Salles Painelli V.S., Nicastro H., Lancha A. H.. Carbohydrate mouth rinse: does it improve endurance exercise performance? Nutrition Journal 2010;9:33. doi:10.1186/1475-2891-9-33
Fares E.J., Kayser B. Carbohydrate mouth rinse effects on exercise capacity in pre- and postprandial States. J Nutr Metab 2011, Article ID 385962. doi:10.1155/2011/385962
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Rollo I. Williams C. Effect of mouth-rinsing carbohydrate solutions on endurance performance. Sports Med. 2011;41(6):449-461. doi:10.2165/11588730-000000000-00000