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.
Sugars in the Brain
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.
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.
“When we eat, the body recognizes that food is coming in, and ultimately the brain is trying its best to regulate how many calories we need and how much fat we have in our bodies, and trying to maintain a balance so that we’re not overeating or under-eating,” Sherwin says.
Fructose seems to upset that balancing act, though researchers aren’t sure why.
One theory is that when humans evolved, the problem wasn’t overeating, but not getting enough calories. In that case, it wouldn’t have been a good idea for fructose, which is the primary sugar in fruit and fruit juices, to make us feel full since fruits may have been a primary food source.
That may have worked fine for cavemen, but it may not be so great for our sugar-saturated modern diets.
Advice to Dieters
So what does the study mean for health-conscious eaters?
That’s harder to say, says Jonathan Purnell, MD, an endocrinologist at Oregon Health & Science University in Portland. Purnell wrote an editorial on the study but was not involved in the research.
“This study didn’t prove that fructose causes weight gain,” Purnell says. “It doesn’t reflect real-world conditions.”
Industry representatives agree.
“When consumed together, as they almost always are, fructose and glucose balance each other out and would likely have no effect on normal hypothalamic blood flow,” says James Rippe, MD, professor of biomedical sciences at the University of Central Florida and a paid consultant for the Corn Refiners Association, the group that represents manufacturers of high-fructose corn syrup.
Ideally, Purnell says, the next phase of studies would test the kinds of sugar mixtures found in foods, like table sugars and high-fructose corn syrup, against a comparison condition like water or an artificial sweetener.
“This study shouldn’t, all by itself, lead people to cut back on this food or that food,” Purnell says.
It would be a mistake, he says, to give up fruit, which has naturally occurring fructose.
“We don’t recommend limiting fruit intake. Although there’s fructose there, it’s also present with water and fiber that alter the characteristics of straight fructose alone. We think that doesn’t make fruit as much of a bad actor,” Purnell says.
What’s probably more practical, says Rachel Begun, RD, a spokeswoman for the American Academy of Nutrition and Dietetics, is to try to limit added sugars.
Americans take in nearly 150 pounds of added sugars per year, according to the U.S. Department of Agriculture, an amount that breaks down to nearly 28 teaspoons or 440 calories every day.
“It's safe to say that Americans are consuming too much sugar in all forms and we need to significantly reduce our intake," says Begun, who wasn’t involved in the research.