'A long-term high fat diet may actually promote short-term overconsumption of highly palatable foods high in dietary fat by reducing sensitivity to at least one important feedback signal which would ordinarily limit eating.'

One group of rats was maintained on a high-fat diet and another given low-fat food for the study, which is published in the Journal of Nutrition.

Then both groups of animals were offered a rat snack high in fat and calories. Those accustomed to the high-fat menu were less sensitive to a hormonal signal to stop eating than the rats who had been kept on the low-fat diet, the researchers found.

'Stop Eating' Hormone

"When we gave the rats doses of a 'stop eating' hormone, the rats on the low-fat diet significantly suppressed their intake of the snack -- but not the rats on the high-fat diet," says Dr. Mihai Covasa, assistant professor of nutritional sciences and a member of the Penn State Neuroscience Institute, who led the study.

"These results suggest that a long-term high fat diet may actually promote short-term overconsumption of highly palatable foods high in dietary fat by reducing sensitivity to at least one important feedback signal which would ordinarily limit eating," Covasa adds.

From Small Intestine to Brain

Cells in the small intestine release the "stop eating" hormone -- cholecystokinin, or CCK -- when fat or protein is present. Its release activates nerves that connect the intestine with the brain, where the decision to stop eating is made.

In earlier studies, human subjects on a high-fat diet had more CCK in their bloodstream but were less responsive to it, researchers found. They typically reported feeling increased hunger and ate more.

No human study of snacking and CCK has been reported prior to the Penn State study with rats, the researchers note. It is the first to link diminished sensitivity to CCK following exposure to a high-fat diet with overconsumption of a high-calorie, high fat snack.

40 Percent More

In the current study, the rats were given access to the high-calorie, high fat snack only for three hours a day. The rest of the time they received either low-fat or high-fat rat chow. The high- and low-fat chows were regulated so that they were equivalent in calories, and both groups of rats gained weight at the same rate.

Even though the rats on the high-fat diet ate, on average, 40 percent more of the high-calorie, high fat snack than the rats on the low-fat diet, they didn't gain extra weight. Unlike humans, rats cut back on their usual chow when they snack.

"Rats are notorious in compensating for food to maintain a constant body weight," Covasa says.

"Although adaptation to the high-fat diet led to overconsumption of the high-calorie, high fat snack, there was no difference in weight gain between the two groups of rats during the 20 days of testing because the rats compensated by eating less of their maintenance diet," he explains.