A new study suggests that brains of people who are obese may be wired differently for impulse control, such as resisting a doughnut when their blood sugar levels drop, compared with people who are not obese.
Researchers wanted to look at brain regions that regulate behaviour and impulsiveness, especially when sugar levels in the blood drop below normal, such as after fasting overnight.
Dr. Kathleen Page, an assistant professor of clinical medicine at the University of Southern California Keck School of Medicine in Los Angeles, and her colleagues used functional MRI to study nine non-obese and five obese subjects. Another seven people acted as controls to test the normal blood sugar conditions.
The participants watched pictures of high-fat or high-calorie food, low-calorie foods or non-food pictures such as utensils while the blood flow in their brain was recorded. Their insulin and glucose levels were tightly controlled during the experiment using IV infusions.
“We saw a clear difference between the obese and the non-obese groups,” said study co-author Rajita Sinha, chief of the psychology section in the psychiatry department at Yale University in New Haven, Conn.
When blood sugar levels were normal, there was an increase in prefrontal control activation, a region of the brain involved in decision making, regulating impulses and resisting desires. It is the region of the brain that tells us to stop eating when blood sugar levels are normal, Page explained. In contrast, when blood sugar levels were low or hypoglycemic, there was increased desire for high-calorie foods, the researchers found.
“For the obese group, this was more dramatic,” Sinha said. People who are obese may be particularly vulnerable, given that we live in an environment inundated with food images, she noted.
The researchers advise people who are obese to have regular meals that are small but healthy to keep their sugar levels up, and to see their doctor for a full checkup of glucose and insulin levels. They don't know if obesity itself may be causing the effect or if the brains of obese and lean people are wired differently, Page said.
The new findings show that “the regulatory role of glucose was missing in the obese,” says Elissa Epel of the University of California, San Francisco, an obesity researcher not involved with the new study. She says the data might “explain the drive to eat that some obese people feel despite how much they’ve eaten.”
Though small, this study was so well designed and controlled that it “lets us see some clean results in a relatively small cohort,” says obesity scientist Dianne Lattemann of the Veterans Administration Puget Sound Health Care System in Seattle. The research team’s ability to discern differences in the desire for foods relative to body weight — and based on a not-too-dramatic drop in blood sugar — “is extremely interesting,” she adds. The results pull together trends seen in animals and findings reported in more limited human trials.
The research, which appeared in Monday's issue of the Journal of Clinical Investigation, was funded by the U.S. National Institutes of Health, the Yale Center for Clinical Investigation and the Yale Stress Center.
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