Fructose, the simple carb found in high-fructose corn syrup, which is itself found in almost everything Americans eat, has long been thought responsible for this nation's growing obesity epidemic . The average American weight began shooting upward at exactly the same time high-fructose corn syrup started getting substituted for regular sugar in processed food.

But the reason fructose makes you fatter than glucose was always a bit of a mystery--until now. Researchers at Oregon Health & Science University have discovered that it's a mental thing.

"What we've found ... is that the brain's response to fructose is very different to the response to glucose," Jonathan Purnell, lead author of the research, which was just published in the online edition of the journal Diabetes, Obesity, and Metabolism, told the press. In short, the brain's response to fructose is much more likely to promote weight gain.

Purnell and his colleagues did functional MRI scans on study subjects' brains after giving them infusions of fructose, glucose, and saline. While glucose caused the parts of the brain associated with food taste and smell to light up with activity, fructose had the opposite effect: It decreased activity in those cortical areas.

The researchers conclude that the opposite responses the brain makes to the two sugars reveals its inability to properly gauge the effects of fructose, or regulate its intake. In short, your confused brain doesn't know when to tell your stomach that it has had enough.

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