Diet drinks and meals could cause people to put on weight and trigger diabetes even when they are low-calorie.
Modern diet products confuse the human body because there is nothing natural like them, scientists say.
Where a low-calorie drink is still as sweet as the normal version, the mismatch appears to send our metabolisms haywire.
Researchers at Yale University say in nature sweetness signals the presence of energy, so that the sweeter something tastes, the more calories it contains.
Boston University researchers found aspartame, a low-calorie sweetener, wreaks havoc on the arteries - as opposed to sugar-sweetened drinks - driving up one's risk of dementia
When a drink is too sweet for the calories it contains, the brain becomes confused.
It does not register that calories have been consumed, which could lead someone to end up eating more.
The perception of sweet taste
Co-author Professor Dana Small, from Yale University School of Medicine, said 'a calorie is not a calorie' in this situation, adding: 'The assumption that more calories trigger greater metabolic and brain response is wrong.
'Calories are only half of the equation - sweet taste perception is the other half.'
A sweet-tasting, lower-calorie drink can trigger a greater metabolic response than drinks with higher calories, explaining the association between artificial sweeteners and diabetes discovered in previous studies.
The results were discovered by giving 15 people drinks with varying calorie contents, then measuring their brain response in an MRI scanner.
Adults who have at least one diet drink a day are three times more at risk from a stroke or dementia,