Excess Dietary Glucose Converted into Fructose in Human Brain

01 Mar 2017 --- Fructose, the form of sugar most commonly linked to obesity and diabetes, can be converted in the human brain from glucose, according to a new study from Yale University. The research raises questions about fructose's effects on the brain and eating behavior, as well as the protective effects of fructose-limiting diets, and the potential dangers of consuming too much glucose.

Fructose is a simple sugar found in fruits, vegetables, table sugar, and many processed foods. Excess consumption of fructose is known to contribute to high blood sugar and chronic diseases such as obesity.

The research team at Yale University had previously demonstrated that fructose and another simple sugar, glucose, had different effects on brain activity, but it was not known whether fructose was produced in the brain or crossed over from the bloodstream.

To investigate, the research team assessed sugar concentrations in the blood of eight healthy, lean individuals after giving them infusions of glucose over a four-hour period.

The researchers found cerebral fructose levels rose significantly in response to a glucose infusion, with minimal changes in fructose levels in the blood.

They concluded that the high concentration of fructose found in the brain of study participants was due to a metabolic pathway called the “polyol pathway” that converts glucose to fructose.

“Our new findings are very focused to the brain and suggest that the impact of fructose in the human brain may extend beyond that of dietary fructose,” Janice Hwang, M.D, first author of the study and assistant professor of medicine told NutritionInsight.

“I think we cannot make any extrapolations about the impact of dietary fructose consumption on human health, but what we can say is that the brain has a way to divert excess glucose into fructose and a lot of work still needs to be done to determine exactly what is the role of fructose in the brain.”

The finding raises several key research questions, which the research team plans to pursue.

“By showing that fructose in the brain is not simply due to dietary consumption of fructose, we've shown fructose can be generated from any sugar you eat,” said Hwang.

“It adds another dimension into understanding fructose's effects on the brain.”

“Glucose in the brain sends signals of fullness, but that is not the case with fructose,” she adds.

The conversion of glucose to fructose in the brain, known as the polyol pathway, also occurs in other parts of the body.

“This pathway may be one other mechanism by which high blood sugar can exert its adverse effects,” Hwang added.

Robert Sherwin, MD, the study’s second author, Yale Center of Clinical Investigation Director, and Director of the Diabetes Research Center, also commented on the study.

“Fructose comes into the body and goes to the liver. In the liver when there is too much in the diet, it has potentially bad effects.”

“In the liver, fructose is used in part as a carbohydrate, to create fat which is not good for it.”

Sherwin explains how excess glucose could be potentially harmful to the brain.

“The brain is designed to filter and consume glucose and very little fructose, however, when the brain has more glucose than it needs, the excess glucose can be converted in the brain to fructose, and that appears to be not good for brain cells.”

However, he adds that, “In short, the issue is complex, and as Janice points out, needs to be better understood, since our diet is now very different in the 21st century and contains much more fructose.”

“In reality, people rarely consume pure fructose or pure glucose. In fact, table sugar is sucrose, which is composed of glucose and fructose,” Hwang added.

“So, I personally think if you are talking about health, the main focus should be on decreasing total sugar consumption.”

By Hannah Gardiner

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