Everyone knows that if you eat a plate of beans or a bowl of bran cereal, you're likely to get full pretty quickly. UC Davis nutrition researchers now have a better idea why.

A UC Davis study, published recently in the American Journal of Clinical Nutrition, indicates that increased fiber content in a meal boosts feelings of fullness in women and increases levels of a certain hormone associated with satiety.

Previous research has shown that the hormone cholecystokinin is released from the small intestine when a fat-containing food is eaten. It's thought that this hormone may be the chemical messenger that acts in response to fat to notify the brain that the body is getting full.

Now it appears that fiber can trigger the same signaling mechanism as fat.

In an effort to better understand cholecystokinin's role, the UC Davis researchers decided to test how levels of the hormone respond to increases in dietary fat and fiber, and how that hormonal response corresponds to feelings of satiety.

To do so they fed a test group, including equal numbers of men and women, three different breakfast meals. The test meals were either low-fiber, low-fat; high-fiber, low-fat; or low-fiber, high-fat.

Blood samples were drawn before, during and after the meals were eaten, to measure hormone levels. The release of the hormone cholecystokinin was correlated with the feelings of satiety reported by the subjects.

The researchers found that in women both the high-fat and high-fiber meals resulted in greater feelings of satiety and significantly higher levels of cholecystokinin, than did the low-fat, low-fiber meals.

In men, however, the two low-fat meals caused greater feelings of satiety, and there was not a significant difference in the hormonal increase between the various meals.

"These results indicate that the addition of fiber to a meal can increase a person's feeling of being full," said Barbara Schneeman, a UC Davis nutrition professor, who led the study. "It appears this is due not only to fiber creating a greater volume of food in the gastrointestinal tract, but also to fiber promoting the release of cholecystokinin."

She noted that further research is needed to help understand the long-term effects of fiber on controlling food consumption and energy balance, and to determine the role that gender plays in the mechanisms that control food intake.