A new Yale study has found that fructose is converted in the human brain from glucose, offering significant insight (but also raising questions) about our eating habits and cravings.

Fructose is found in fruits and veggies, but also in a lot of processed foods. It’s also been linked to obesity and cardiovascular issues. Image via Pixabay.

Fructose is a simple monosaccharide found in many fruits and vegetables, where it is often bonded to glucose. Pure, dry fructose is a very sweet, white, odorless, crystalline solid and is the most water-soluble of all the sugars. Because of its properties, it’s often added to processed and baked foods, to make them sweeter and tastier — but excess consumption contributes to high blood sugar and chronic diseases like obesity. A previous study had already shown that fructose and glucose have a significant effect on the brain, but it wasn’t clear if the fructose was produced in the brain or simply arrived there through the bloodstream.

To answer this question, researchers gave eight healthy participants infusions of fructose and glucose, while measuring sugar concentrations in their brains and bodies using a non-invasive technique called magnetic resonance spectroscopy. They found that when participants drank the glucose infusion, their fructose levels in the brain rose dramatically while levels in the brain remained relatively low.

“In this study, we show for the first time that fructose can be produced in the human brain,” said first author Dr. Janice Hwang, assistant professor of medicine.

“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,” Hwang added. “It adds another dimension into understanding fructose’s effects on the brain.”

This isn’t completely unexpected, as the same process had been observed in animals. However, it does bring a few interesting questions. It has been proven in rodents that fructose promotes feeding behavior while glucose doesn’t In other words, glucose makes you feel full, while fructose doesn’t. So what then is happening to our brain as it transforms glucose into fructose? Perhaps even more importantly, what does this mean for our health?

Dr. Kathleen Page, an endocrinologist and assistant professor of medicine at the University of Southern California Keck School of Medicine, who was not involved with the study, said that the findings are “intriguing” and, “if confirmed in future studies, could have important implications on the effects of sugar on brain function.”

Indeed, the study does come with a couple of limitations: for starters, there are only 8 participants, which is a pretty small sample size (though for the purpose, it seems relevant). Secondly, there’s only an indirect observation here, and the mechanism is still not properly understood. But we can still draw some conclusions.

The fact that fructose is increasingly used in the food industry is already worrying. Excess fructose consumption is already connected to insulin resistance, obesity, elevated LDL cholesterol and triglycerides, leading to metabolic syndrome. The substance is also associated with a greater incidence of hypertension and risk of cardiac disease.

Journal Reference: Janice J. Hwang et al — The human brain produces fructose from glucose.  doi:10.1172/jci.insight.90508.

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