Evolution has trained our brain to expect calories or energy from sweet-tasting foods and that is why low-calorie cookies or diet drinks do not satisfy us, suggests a research.
The results could explain why dieting may backfire, leading us to eat more until the body meets its energy needs.
We may even have hormones that help the brian distinguish real sugar from artificial sweetener, the study published the journal Neuron, said.
"We knew that the human brain could tell the difference between real and fake sugar, we just did not know how," said first author Monica Dus, assistant professor at the University of Michigan in the US.
For the study, researchers deprived fruit flies of food for several hours and then gave them a choice between diet, non-nutritive sweeteners and real sugar.
When the flies licked the real sugar, it activated a group of six neurons that released a hormone with receptors in the gut and brain.
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The hormone fuelled digestion and allowed the fly to lick more of the nutritious food.
On the other hand, when the fly licked the diet sweetener, it never produced this hormone/digestive reaction because zero-calorie sweetener has no nutritional or energy value.
In every case, the flies abandoned the artificial sweetener and chose the regular sugar because the starved flies needed the energy provided by the calories in the real sugar.
From an evolutionary perspective, sweet taste means sugar (traditionally from fruit or high concentrate carbohydrates) and a subsequent big energy boost.
"Fruit flies can not call out for pizza -- their brains expect calories if they eat something sweet, and that is why they chose the regular sugar," said Dus.
"Fruit flies and humans share about 75 percent of the same disease-causing genes."
If our brains work the same way, this helps explain why diet foods do not satiate or satisfy us, and we gain weight while dieting, Dus said.
It is analogous to a person eating that entire sleeve of low-calorie cookies and the body telling her she is still hungry. She keeps snacking until she eats something with nutritional value that meets her energy needs, the study said.