The body can tell energy is on the way as soon as you put something in your mouth, even before you swallow, according to research from the University of Auckland.
Scientists have shown for the first time that the mere presence of carbohydrate solution in the mouth immediately boosts muscle strength - suggesting a previously unknown neural pathway is activated when receptors in the mouth detect carbohydrates.
"There appears to be a pathway in the brain that tells our muscles when energy is on the way," said lead researcher Nicholas Gant from the Department of Sport and Exercise Science.
"We have shown that carbohydrate in the mouth produces an immediate increase in neural drive to both fresh and fatigued muscle and that the size of the effect is unrelated to the amount of glucose in the blood or the extent of fatigue."
The research has been published in the journal Brain Research.
Sixteen healthy young men, who had been doing biceps exercises for 11 minutes, were given a carbohydrate solution to drink or an identically flavoured energy-free placebo.
Around one second after swallowing the drink, neural activity increased by 30 percent and muscle strength 2 percent, with the effect lasting for around three minutes. The response was not related to the amount of glucose in the bloodstream or how fatigued the participants were.
"It might not sound like much, but a 2 percent increase in muscle strength is enormous, especially at the elite level. It's the difference between winning an Olympic medal or not," said co-author Cathy Stinear.
A second boost in muscle strength was observed after 10 minutes when carbohydrate reached the bloodstream and muscles through digestion, but no additional boost in neural activity.
The carbohydrate and placebo solutions were of identical flavour and sweetness, confirming that receptors in the mouth could process other sensory information aside from the basic taste qualities of food.
"The results suggest that detecting energy may be a sixth taste sense in humans," Dr Gant said.
In another experiment, 17 participants who had not been exercising and were not fatigued held one of the solutions in their mouths without swallowing.
Measurements of the muscle between the thumb and index finger showed a similar, though smaller effect as in the first experiment, indicating the response happened in both large powerful muscles and in smaller muscles responsible for fine hand movements.
It was a surprising result, which showed that carbohydrate in the mouth activates the neural pathway whether or not muscles are fatigued.
"We had expected that the response would be part of the brain's sophisticated system for monitoring energy levels during exercise," Dr Stinear said.
"Seeing the same effect in fresh muscle suggests that it's more of a simple reflex - part of our basic wiring - and it appears that very ancient parts of the brain such as the brainstem are involved.
"Reflexive movements in response to touch, vision and hearing are well known but this is the first time that a reflex linking taste and muscle activity has been described," she said.
Further research hopes to determine the precise mechanisms involved and to learn more about the size of the effect on fresh versus fatigued muscle.