Fructose May Affect Hunger Cues

Jan. 2, 2013 -- All sugars are not created equal, at least when it comes to the brain, a new study shows.

For the study, which is published in the Journal of the American Medical Association, researchers asked 20 healthy men and women to sip a cherry-flavored drink sweetened with either pure glucose or pure fructose.  

Both glucose and fructose are simple sugars. People rarely take in either one by itself. Instead, they’re usually added to foods and drinks as mixtures. Table sugar is about half glucose, half fructose, for example, while high-fructose corn syrup is about 55% fructose and 45% glucose.

Fructose is sweeter than glucose. It’s also less expensive. So over the years, the balance of calories from added sugars in the American diet has shifted to favor fructose.

“People consume a lot more fructose now than they used to, because it’s cheaper to put high-fructose corn syrup in the foods we eat,” says researcher Robert S. Sherwin, MD, an endocrinologist at the Yale University School of Medicine, in New Haven, Conn. 

Whether that shift may be contributing to our nation's growing obesity problem has been an open question.

Animal studies have shown that glucose and fructose can have different effects on appetite and metabolism. Sherwin and his team set out to see if that might also be true for humans.

Each person in the study completed the experiment twice with a gap of a few weeks to a few months between lab visits. They weren’t told which sugar they were given to drink.

Each time, they were given a scan that allowed researchers to watch what was happening to their brains in real time.

Study scientists were particularly interested in changes to a region called the hypothalamus, which helps to control appetite. They also took blood samples to check levels of hormones that control feelings of hunger and fullness, and asked the study participants how satisfied they felt after drinking the different sugar solutions.

As quickly as 15 minutes after people in the study finished the drinks, researchers began to see changes in brain blood flow and activity.

After the glucose drink, the body seemed to recognize and respond to the extra calories with an increase in glucose and insulin levels. That response, which blunts hunger, was significantly greater than fructose's. Brain activity also slowed in the hypothalamus, the region that stimulates appetite.

After the fructose drink, on the other hand, the hypothalamus continued to stay active. There was little increase in insulin, and study volunteers said they felt hungrier, even though they weren’t told which sugar they’d had.

Other hormones that are known to regulate hunger, such as ghrelin and leptin, were unchanged after ingestion of either type of sugar.

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