After drinking a fructose beverage, the brain does not register the feeling of being full as it does when simple glucose is consumed, researchers found.
The small study does not prove that fructose or its relative, high-fructose corn syrup, can cause obesity, but experts say it adds evidence that they may play a role.
These sugars are often added to processed foods and beverages and consumption has risen dramatically since the 1970s, along with obesity. A third of US children and teens and more than two-thirds of adults are obese or overweight.
All sugars are not equal — even though they contain the same amount of calories — because they are metabolised differently in the body.
Table sugar is sucrose, which is half fructose, half glucose. High-fructose corn syrup is 55% fructose and 45% glucose.
Doctors say we eat too much sugar in all forms.
For the study, scientists used 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, Yale University endocrinologist Dr 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 is convincing, said Dr Jonathan Purnell, an endocrinologist at Oregon Health & Science University, is that the imaging results mirrored how hungry the people said they felt, as well as what earlier studies found in animals.
The solution is to cook more at home and limit processed foods containing fructose and high-fructose corn syrup, Dr Purnell said.
A second study suggests that only severe obesity carries a high death risk — and that a few extra pounds might even provide a survival advantage.