For the first time, scientists used imaging tests to show fructose can trigger brain changes that may lead to overeating.

After drinking a fructose beverage, the brain doesn’t register the same feeling of being full as it does when simple glucose is consumed, researchers found.

While the study is small and does not prove that fructose or its relative, high-fructose corn syrup, can cause obesity, experts say it adds evidence that may play a role. These sugars often are added to processed foods and beverages, and consumption has risen dramatically since the ‘70s, along with obesity. A third of U.S. children and teens, as well as 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 metabolized differently in the body.  Table sugar is sucrose, which is half fructose, half glucose. High-fructose corn syrup is 55 percent fructose and 45 percent glucose. Some nutrition experts say this sweetener may pose special risks, but others in the industry reject that claim. And doctors say we eat too much sugar in all forms.

For the study, scientists used 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."

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.

What to do? Cook more at home and limit processed foods containing fructose and high-fructose corn syrup, suggested Dr. Jonathan Purnell, an endocrinologist at Oregon Health & Science University. "Try to avoid the sugar-sweetened beverages. It doesn't mean you can't ever have them," but control their size and how often they are consumed, he said.

A second study in the journal suggests that only severe obesity carries a high death risk–and that a few extra pounds might even provide a survival advantage. However, independent experts say the methods are too flawed to make those claims.

The study comes from a federal researcher who drew controversy in 2005 with a report that found thin and normal-weight people had a slightly higher risk of death than those who were overweight. Many experts criticized that work, saying the researcher–Katherine Flegal of the Centers for Disease Control and Prevention–painted a misleading picture by including smokers and people with health problems ranging from cancer to heart disease. Those people tend to weigh less and therefore make pudgy people look healthy by comparison.

Flegal defended her work. She noted that she used standard categories for weight classes. She said statistical adjustments were made for smokers, who were included to give a more real-world sample. She also said study participants were not in hospitals or hospices, making it unlikely that large numbers of sick people skewed the results.

"We still have to learn about obesity, including how best to measure it," Flegal's boss, CDC Director Dr. Thomas Frieden, said in a written statement.

"However, it's clear that being obese is not healthy - it increases the risk of diabetes, heart disease, cancer, and many other health problems. Small, sustainable increases in physical activity and improvements in nutrition can lead to significant health improvements."

Based on reporting by the Associated Press.

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