This Is Your Brain on Diets

Jan. 20, 2009— -- A study that showed men and women's brains work differently when they're tempted with food has experts debating the reasons behind the gender gap in obesity rates.

Doctors asked 23 men and women of healthy weight to go on a three-day fast and then tempted them with morsels of their favorite foods and trips to buffets where study subjects were told they could not partake.

Brain scans of the test subjects showed men's brains looked calmer and worked less when they consciously faced temptation, but women's brains lit up in emotion and hunger desire areas, according to the study published in the Proceedings of the National Academy of Sciences.

The researchers behind this study suggest that the gender difference in the PET brain scans may explain the two percentage point lead women have over men in rising obesity rates.

However, other diet experts think environment and culture, not biology, explain the roots of obesity.

"When people move from the U.S. to other countries, they lose weight. When people move here to the U.S. from other countries, they gain weight," said Kelly Brownell, director of the Rudd Center for Food Policy and Obesity at Yale University.

"Is it that we can't control what we are eating that makes us obese, or is it that we are eating the wrong things and not moving around?" Brownell asked.

Dr. Gene-Jack Wang, lead author of the study, did his best to mimic a person's struggle with food in his lab to address the question.

Wang first asked each study subject to rate their favorite foods from a long list, and then start fasting for three days.

When the test subjects returned, he used their favorite foods to tempt them.

"We're going to show it to you, and you're going to put a small piece in your mouth for taste. And then we're going to talk about it with you," said Wang, who admitted he had to eat before each case to stop his own temptation.

"It's the actual food, and you're in the bed and you cannot escape," he said. After each temptation, which included a trip to a buffet at the end of the day, Wang and his colleagues did a PET (positron emission tomography) scan of each subject to see which parts of the brain are active during temptation.

Is Suppressing Hunger a Gender Issue?

Men and women's PET scans did not differ very much when Wang and his colleagues simply tempted them with food. It was only after the researchers asked their subjects to deliberately ignore the food and repress hunger that a gender difference appeared.

Both men and women reported success at repressing thoughts about food. "But in women, their brain and what they're saying is not necessarily the same," said Wang. "I hear them say they suppressed their appetite, but their brain is doing something else."

Neither the men nor women ever actually caved in and took the food, but Wang said the different PET scans could be a sign that women have a harder time suppressing their hunger in general, and that this could be a biological explanation for higher obesity rates among women.

According to the U.S. Centers for Disease Control, 33 percent of men and 35.3 percent of women were obese in the United States in 2006 -- a small, but consistent difference over of 2 percent over the years.

Elisabetta Politi, the nutrition director at the Duke Diet and Fitness Center in Durham N.C., thought the PET study was an interesting idea, but doubted it would change how people fight obesity.

"In my practice, we never worry about men or women, we just try to treat obesity -- for both their rate is alarming," said Politi. "The thing with obesity is that it is so multi-factorial and so complex that it's hard to pinpoint what makes a gender or ethnicity more prone."

Politi could name several possible explanations for the gender difference in obesity right off the bat -- the number of women who are around food preparing it, the natural biologic tendency for women to have higher body fat for pregnancy, the struggle to lose weight after pregnancy, and more stationary work in careers.

"I thought that the PET design was really, really interesting," said Politi. "But it's really a piece of the whole picture when you're trying to treat obesity."

For example, Politi would never recommend someone try to fast or suppress their appetite as the study subjects did.

"I'd say try to eat more frequent meals," said Politi. "To say that people who control their weight have more will power is simplifying. It's maintaining healthier behavioral changes over time."