In addition to recognizing sweet, sour, salty, savory, and bitter tastes, our tongues can also pick up on carbohydrates, the nutrients that break down into sugar and form our main source of energy.
Researchers say the ‘sense of carbs’ also triggers the brain’s pleasure centre – and could explain why diet foods are often unsatisfying.
Carbs activate brain regions that are not affected by artificial sweeteners, even when they’re only tasted but not swallowed, according to the small new study from New Zealand.
‘The mouth is a more capable sensory organ than we currently appreciate, able to distinguish carbohydrates from artificial sweeteners when both taste identical,’ said Dr. Nicholas Gant from the Sport and Exercise Science department at the University of Auckland’s Centre for Brain Research.
‘Carbohydrates are extremely powerful stimuli that have profound and immediate effects on the brain and the systems it controls.’
A sixth sense in the mouth for carbohydrates could explain why athletes respond immediately to carbs, as well as some aspects of uncontrolled eating disorders, Gant’s team writes in the journal Appetite.
It could also open avenues for food engineering Gant told Reuters Health.
Scientists already knew that carbohydrate mouth rinses increased activity in certain brain regions, and some studies have shown that swishing a carbohydrate solution in the mouth and spitting it out improves performance during strenuous exercise.
Gant and his team used special brain imaging, called functional MRI, to look at the effects of three different mouth rinses used before a simple exercise task performed by 10 study subjects.
They compared a sweet carbohydrate solution to a sweet solution that didn’t contain carbs and a third solution that was not sweet and did not contain carbs.
When the participants swished the sweet carbohydrate solution in their mouths, there was greater activation in brain regions associated with sensation and muscle performance than with either the sweet solution without carbs or the tasteless solution without carbs.
The sweet carbohydrate solution also produced greater activation in brain regions that control vision and in regions associated with reward.
The fact that the other solutions didn’t have the same effect indicates that we are able to detect carbohydrates in the mouth as a separate sense from sweetness, according to the researchers.
‘This ‘sixth taste sense’ for carbohydrate is likely one of many additional food qualities that are detectable by receptors in the mouth,’ Gant said.
‘It’s becoming evident that the brain knows far more about the foods we ingest that just our perception of taste.
‘Both test solutions used in our study were sweetened artificially, so the increased activation we observe is likely part of the ‘kick’ people complain is absent in diet beverages/products,” Dr. Gant said.
‘We may be able to use our experimental platform to help develop functional foods and artificial sweeteners that are almost as hedonistically rewarding as the real thing (sugar).’
It’s also been suggested, Gant pointed out, that a failure in signaling between the mouth and the brain is part of the problem in some eating disorders that cause frantic eating behavior.
Gant added that the study’s findings might have implications for athletes.
They could explain why athletes suddenly ‘perk up’ immediately after drinking a carbohydrate solution, even before the carbs have time to get absorbed by the body and converted to energy.
‘For endurance exercise these findings should be applied cautiously,’ Gant said.
‘The signals from the mouth send a message to the brain and body that energy is coming, indicating help is on the way, which may allow a depleted body to keep going,’ he said.
‘But if nutrients aren’t swallowed, and don’t arrive in the bloodstream,” Gant said, “the brain may be writing checks that the body can’t cash later in the race!’
Past studies have shown that some rodents can distinguish between sugars of different energy densities, while others can still tell carbohydrate and protein solutions apart even when their ability to taste sweetness is lost.
A similar ability has been proposed in humans, with research showing that merely having carbohydrates in your mouth can improve physical performance.
‘The presence of carbohydrate in the human mouth has been associated with the facilitation of motor output and improvements in physical performance,’ the researchers wrote in the new study, to be published in Appetite
‘Oral receptors have been identified as a potential mode of afferent transduction for this novel form of nutrient signalling that is distinct from taste.’