Scientists have used imaging tests to show for the first time that fructose, a sugar that saturates diets, can trigger brain changes that may lead to overeating.
After drinking a fructose beverage, the brain doesn't register the feeling of being full as it does when simple glucose is consumed, researchers found.
It's a small study and does not prove that fructose or its relative, high-fructose corn syrup, can cause obesity, but experts say it adds to evidence that they may play a role.
The sugars often are added to processed foods and beverages and consumption has risen dramatically since the 1970s along with obesity.
A third of children and teens in the United States and more than two-thirds of adults are obese or overweight. In Australia in 2011-12, 63 per cent of adults were overweight or obese, as were a quarter of children aged five to 17, the Australian Bureau of Statistics reported.
All sugars are not equal – even though they contain the same amount of kilojoules – because they are metabolised differently in the body.
Table sugar is sucrose, which is half fructose, half glucose, while high-fructose corn syrup is 55 per cent fructose and 45 per cent glucose. Some nutrition experts say this sweetener may pose special risks, but others and the industry reject that claim.
Doctors say we eat too much sugar in all forms.
For the study, scientists used magnetic resonance imaging, or MRI, scans to track blood flow in the brain in 20 young, normal-weight people before and after they had drinks containing glucose or fructose in two sessions several weeks apart.
Scans showed that drinking glucose "turns off or suppresses the activity of areas of the brain that are critical for reward and desire for food", said one study leader, the Yale University endocrinologist Professor Robert Sherwin.
With fructose, "we don't see those changes", he said.
"As a result, the desire to eat continues – it isn't turned off."
What's convincing, said Jonathan Purnell, an endocrinologist at Oregon Health and Science University, is that the imaging results mirrored how hungry the people said they felt, as well as what earlier studies found in animals.
"It implies that fructose, at least with regards to promoting food intake and weight gain, is a bad actor compared to glucose," Dr Purnell said.
He wrote a commentary that appears with the federally funded study in Wednesday's Journal of the American Medical Asso-ciation.
Researchers are now testing obese people to determine if they react the same way to fructose and glucose as the normal-weight people in the study.