Two very similar sugars — fructose and glucose — cause quite opposite reactions in some regions of the brain, according to a new study. Glucose stimulates brain activity in those regions, while fructose decreases it.
The findings support the idea that an increase in Americans' fructose consumption over the past decades has helped fuel the nationâs obesity epidemic , the researchers said.
Although scientists don't fully understand the link between fructose and behaviors that lead to obesity, the fact that the brain scans show the sugars affect some regions differently supports the idea that such a link exists, said study researcher Dr. Jonathan Purnell, a professor of medicine at Oregon Health & Science University.
The areas of the brain affected by these sugars "have been implicated in our decision-making process about what foods we like and don't like," Purnell said.
While fruits and sweet beverages generally contain both sugars, sweetened beverages generally contain a higher ratio of fructose to glucose than other foods.
Your brain on sugar
No one had looked at the human brain before to observe the direct effects of these sugars, Purnell said. The findings will appear in the March issue of the journal Diabetes, Obesity and Metabolism.
The researchers used a functional magnetic resonance imaging (fMRI) scanner to observe the brains of nine people receiving an IV infusion of glucose, fructose or saline solution. The subjects were of normal body weight and had an average age of 29.
Two regions of the brain's cortex — the orbitofrontal cortex and the cingulate gyrus — were found to react much differently to the two sugars.
"Brain activity in these regions went up with glucose and went down with fructose" from baseline measurements, Purnell said. The magnitudes of the sugars' effects were equal — brain activity rose in response to glucose by the same extent that it fell in response to fructose.
"That's a 200 percent difference in brain signaling," Purnell told MyHealthNewsDaily.
Previous research on animals had shown that another brain region, the hypothalamus, reacted to these two sugars differently. The new study did not find such a difference in the human hypothalamus, Purnell said, but the methods used in the two studies differed, so there may be a reaction difference in people, too, that was not apparent in the brain images.
The study didn't measure changes in appetite or eating behaviors. Making a direct comparison to soda consumption is difficult, but the amount of sugar given by IV in the study was "certainly much more than you'd get in a can of soda," Purnell said.
The researchers used an IV instead of asking the study participants to drink sugar solutions because the act of drinking can further affect the brain. Drinking triggers a release of hormones by the intestines, and other changes in the body. To observe the effects of only the sugars, IV transfusions were needed.
What the findings mean
The study does not directly speak to a difference in the way the body processes regular sugar compared with the way it processes high-fructose corn syrup, Purnell said. Table sugar consists of sucrose, and sucrose molecules are basically one glucose molecule and one fructose molecule linked together.
"Other studies have shown, from an epidemiological standpoint, that a marked increase in processed foods, whether they're sweetened with fructose sweeteners or sucrose, which contains fructose, coincides with weight gain," he said.
Still other work has shown that among sugars, fructose has unique effects on the body, Purnell said. In a 2009 study reported in the Journal of Clinical Investigation, participants who consumed fructose had a greater increase in belly fat and triglyceride levels than those who consumed glucose.
The new study may explain why those effects were seen, Purnell said. "The question that needs to be answered is: Do these brain changes predict changes in food preference, and in food intake ?"
The answer is likely yes, he said. "I think these signal changes in the brain do represent what we're seeing, but we need to look at other areas, too. What we're picking up here is playing a role, but [we] need to get more specific with it, and see how the brain controls food-seeking behavior."
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