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. Some nutrition experts say this sweetener may pose special risks, but others and the industry reject that claim. And 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, 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.
“It implies that fructose, at least with regards to promoting food intake and weight gain, is a bad actor compared to glucose,” he said. He wrote a commentary that appears with the federally funded study in the Journal of the American Medical Association.
Researchers now are testing obese people to see if they react the same way to fructose and glucose as the normal-weight people in this study did.