The brain is quicker than the belly when it comes to sensing carbs, Kiwi researchers have found.
The study by Dr Nick Gant and his team at the Centre for Brain Research at the University of Auckland helps explain the "kick" people complain is absent in diet beverages or products.
The study used a unique brain imaging sequence to test the behavioural and neural response of ten participants, who performed arm exercises while their mouths were rinsed with carbohydrate, artificial sweetener or placebo solution.
The study found a 30 per cent increase in task-related brain activity when carbohydrate was present even though the liquid wasn't swallowed.
"Liquid solutions used in our study were sweetened artificially but when carbohydrate was present, we saw increased activation in the brain that we don't see when only sweetness is present," Dr Gant said.
"We may be able to use the experimental platform in this study to help develop functional foods and artificial sweeteners that are as hedonistically rewarding as the real thing."
Dr Gant said the study provided further evidence of a "sixth taste sense" for carbohydrates by receptors in the human mouth.
"The mouth signals that energy is on its way which in turn leads to increased activity in key regions of the brain including those that control movement and vision."
The findings could also explain why the "perk up" response noted in athletes after they have drunk carbohydrate is immediate, even though the body hasn't had time to absorb it and convert it to energy.
"Carbohydrates are extremely powerful oral stimuli that have profound and immediate effects on the brain and the mouth is a more capable sensory organ that we currently appreciate."
Brain networks that signal between the mouth and the brain are thought to break down in some eating disorders.
"Those responses are absent when nil by mouth patients are fed artificially and this help may explain why artificial nutrition therapy is less successful than ingesting food in the normal way."