The 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, a new study suggests.

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.

To find out, 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.

The high-calorie food pictures included:

  • Hamburgers.
  • French fries.
  • Cookies.
  • Ice cream.
  • Chocolate.
  • Pizza.

Low-calorie foods pictured included:

  • Salads.
  • Broccoli.
  • Bean sprouts.
  • Tofu.
  • Fruit.

"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. 

Have regular meals

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 said.

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 study's authors acknowledged the study included few subjects, which is typical for fMRI studies. The normal-weight participants had an average age of 31 and the obese subjects had an average age of 30.

The brain likely plays a major role in appetite and willpower, said Ralf Brauckmann of Toronto, who lost 60 pounds this year. After a long battle to curb his cravings for ice cream, cookies and chocolate, he now weighs 199 pounds.

"You open a chocolate bar, you eat the darn whole thing," Brauckmann said of his vice. "I don't do that anymore."

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.

With files from CBC's Kelly Crowe and Melanie Glanz