Effect of Glucose and Fructose on the Brain

Regulating food and energy intake occurs in an area of the brain called hypothalamus. In the past, it was difficult to understand this mechanism due to technical limitations. Functional MRI (fMRI) is a new non-invasive technique available for studying brain activity in live animals and humans. Neuronal activity can be studied by brain oxygenation-level dependant fMRIs, or BOLD responses. The current study demonstrated that the effects of fructose and glucose on the activity of the brain cortex are opposite to each other.

Glucose and fructose are two types of sugars. The hypothalamus is a region of the brain involved in control of food intake and energy expenditure, due to the presence of glucose-sensing neurons in this area.  Non-invasive neuro-imaging technology like fMRI offers advanced facility to study the brain functions. In obese and diabetic patients, fMRI studies have shown that glucose-sensitive neurons in the hypothalamus do not function properly. Fructose is known to produce effects in the brain and is implicated in body weight regulation in animal models and humans. This study looked at the effects of glucose and fructose on brain response in participants with normal weight.

* Researchers recruited nine healthy subjects who underwent three fMRI studies on separate visits. At each visit, following an overnight fast, the subjects underwent a 60-minute fMRI scan to measure blood oxygen level dependant (BOLD) response.
* This scan consisted of an initial baseline 10-minute period, after which either an infusion of glucose (0.3 mg/kg), fructose (0.3 mg/kg) or saline was given intravenously.
* During the entire 60-minute period, blood was collected every five minutes. Measurements of glucose and insulin, lactate and fructose (when fructose was infused) were performed.
* Images were acquired using an MRI machine. The results were compared in all three groups.

* During the saline infusion, BOLD signals response in both the hypothalamus and brain cortical control regions did not vary significantly from the baseline.
* In response to the glucose infusion, BOLD signal in the control regions of the cortex increased significantly above the baseline and remained elevated for the next 20 minutes.
* In contrast, during the fructose infusion, BOLD signal decreased in the control regions of the brain cortex, and remained depressed for 20 minutes.
* As compared to baseline, there were no significant changes in signaling in the hypothalamic area during both glucose and fructose infusions.

Next steps/Shortcomings
The findings of this study are in contrast to other studies. In previous studies, hypothalamic signals were reduced during glucose infusion. This study failed to find such changes in the hypothalamus. It is possible that the glucose concentration used in the present study was lower than the levels used in the other studies, which might have been responsible for the discrepancy. This study shows opposite effects of glucose and fructose on cortical signals in fMRI. The exact significance of this finding in understanding energy and food regulation remains unclear.

High fructose consumption is linked with weight gain and obesity. The hypothalamus is a region situated near the brain ventricles; it has neurons that regulate energy metabolism. In rats and mice, fructose injection in the brain ventricles produced increase in the food intake but glucose similarly injected produced the opposite effect. FMRl studies of glucose and fructose injections in brain ventricles in rodents have also demonstrated opposite effects on the hypothalamus. This study found that intravenous injection of fructose decreases BOLD signals while intravenous glucose increases BOLD signals in another area of the brain called cortex. Food and energy regulation by the brain is achieved by complex pathways and further research is required in order to fully understand different roles played by different areas of the brain.

For More Information:
Brain Functional Magnetic Resonance Imaging Response to Glucose and Fructose Infusions in Humans
Publication Journal: Diabetes, Obesity and Metabolism, 2011
By J Q Purnell; B A Klopfenstein; Oregon Health & Science University, Portland, Oregon

*FYI Living Lab Reports Are Summaries of the Original Research.


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