Gender affects the brain's response to food. (His-and-Her Hunger Pangs).

Women have higher rates of obesity and eating disorders than men do, but scientists don't know why. New findings offer clues to the root of sex differences in eating behaviors. The study showed that men's and women's brains react differently to hunger, as well as to satiation.

This is the first research to document sex-specific brain activity related to eating, says study author Angelo Del Parigi of the Phoenix branch of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The report appears in the June American Journal of Clinical Nutrition.

Using positron emission tomography (PET), Del Parigi and his colleagues at NIDDK monitored the brains of 22 men and 22 women. A PET scan identifies areas where there are surges in blood flow that reflect activity. The scientists performed the scans after the participants endured a 36-hour fast and again after they drank a liquid meal to quench their hunger.

The scans revealed many similarities between the men's and women's brains during hunger and satiation. But they also found significant differences.

For example, when hungry, men had more activity than women did in the paralimbic region of the brain, an area involved in processing emotion. When sated, women had more activity than men did in the occipital cortex, the seat of vision, and men had more activity than women did in an area of the prefrontal cortex associated with feelings of satisfaction.

These and other results of the study hint at several specific ways that men and women may process hunger and satiation differently and how these differences could influence their food intake. For example, men may derive a more rewarding feeling from eating than women do, the researchers speculate.

"It will take many more experiments to find the neurophysiological explanations of sex differences in eating behavior," Del Parigi emphasizes. But he hopes his group's research will spur further studies.

"The area is under intense investigation right now," says neuroscientist Yijun Liu of the University of Florida in Gainesville. Liu notes that the NIDDK team turned up no gender differences in the hypothalamus, the brain area responsible for basic physiological responses to hunger and satiation. The researchers also found no gender differences in blood-chemical signals controlled by the hypothalamus.

Rather, he says, the newly observed brain patterns suggest that men and women differ in how they think and feel about what they eat rather than in the way they process food compounds.

"This is a very important finding for the future study of obesity," Liu concludes. He says the work should encourage scientists to recognize that gender is central to obesity and eating disorders and to develop sex-appropriate treatments.