Women vs. men: Orange colour indicates areas where men showed greater decreases in inhibition than women. These brain regions are involved in emotional regulation, conditioning and the motivation to eat. (Brookhaven National Laboratory)

Faced with their favourite foods, women are less able than men to suppress their hunger, a discovery that may help explain the higher obesity rate for females, a new U.S. study suggests.

Researchers trying to understand the brain's mechanisms for controlling food intake were surprised at the difference between the sexes in brain response.

Gene-Jack Wang of Brookhaven National Laboratory of Upton, N.Y., and colleagues were trying to figure out why some people overeat and gain weight while others don't.

They performed brain scans on 13 women and 10 men, who had fasted overnight, to determine how their brains responded to the sight of their favourite foods. They report their findings in Tuesday's edition of Proceedings of the National Academy of Sciences.

"There is something going on in the female," Wang said in a telephone interview. "The signal is so much different."

In the study, participants were quizzed about their favourite foods, which ranged from pizza to cinnamon buns and burgers to chocolate cake, and then were asked to fast overnight.

'Even though the women said they were less hungry when trying to inhibit their response to the food, their brains were still firing away in the regions that control the drive to eat.'— Gene-Jack Wang, researcher

The next day, they underwent brain scans while being presented with their favourite foods. In addition, they used a technique called cognitive inhibition, which they had been taught, to suppress thoughts of hunger and eating.

While both men and women said the inhibition technique decreased their hunger, the brain scans showed that men's brain activity actually decreased, while the part of women's brains that responds to food remained active.

"Even though the women said they were less hungry when trying to inhibit their response to the food, their brains were still firing away in the regions that control the drive to eat," Wang said.

Hard-wired to eat?

Nora Volkow, director of the National Institute on Drug Addiction and a co-author of the paper, said the gender difference was a surprise and may be because of different nutritional needs for men and women, although she stressed that idea is speculative.

Because the traditional role of the female is to provide nutrition to children, the female brain may be hard-wired to eat when foods are available, she said. The next step is to see if female hormones are reacting directly with those specific parts of the brain.

"In our society, we are being constantly being bombarded by food stimulus," she said in a telephone interview, so understanding the brain's response can help in developing ways to resist that stimulus.

Eric Stice, an expert on eating disorders at the Oregon Research Institute, called the findings provocative.

"I think it is very possible that the differences in hunger suppression may contribute to gender differences in eating disorders and that they are likely linked to gender differences in estrogen and related hormones," said Stice, who was not part of Wang's research team.

According to the U.S. Centers for Disease Control and Prevention, 35.3 per cent of American women and 33.3 per cent of men were considered obese in 2006.

Training treatment proposed

Rosalyn Weller, a professor of psychology at the University of Alabama-Birmingham, said she was surprised by the results and "thought the dissociation between subjective reports of hunger and brain activation in women but not men was very interesting."

The results suggest that training in reducing food desires or in reacting to food cues could be effective treatments to combat obesity, said Weller, who was not part of the research team.

Weller was a co-author of a recent paper in the journal NeuroImage that studied women's brains when participants were shown pictures of food. They found that obese women had a much stronger reaction than normal-weight women in brain regions related to reward.

Wang noted that behavioural studies have shown that women have a higher tendency than men to overeat when presented with tasty food or under emotional distress.

This may result from differences in sex hormones, he said, and further research is planned to see if that is the case.

"As we learn more about the different factors that go into making that decision we'll be better at helping people regulate" their eating, said Alice Lichtenstein, an expert in eating behaviour at Tufts University. She was not part of the research team.

The study was funded by the National Institute on Drug Abuse, the National Institute on Alcohol Abuse and Alcoholism and by the General Clinical Research Center of Stony Brook University.