Researchers uncover how protein-rich diets curb hunger

10/11/05 Researchers have uncovered new evidence to explain the observation that diets rich in protein stunt the appetite, according to a report in the November Cell Metabolism. The findings suggest a novel link connecting macronutrients in the diet to hunger, the researchers said. The results also point to a potential new target for the treatment of eating disorders, they added.

"It is well known that protein feeding decreases hunger sensation and subsequent food intake in animals and humans," said study author Gilles Mithieux of INSERM and Universite Lyon in France. However, the mechanism by which proteins exert their control over appetite remained unclear, the researchers said. In fact, earlier studies have found that a rise in dietary protein shows little effect on the major hormones that regulate hunger, they added.

In a study of rats, Mithieux and colleagues made the surprising discovery that diets heavy in protein spark the production of glucose in the small intestine. That rise in glucose, sensed in the liver and relayed to the brain, led the animals to eat less, they reported.

"The current findings provide an answer to the question of how protein-enriched meals decrease hunger and reduce eating, unsolved up to now," according to the researchers. "Our data also bring to light a novel concept of control of food intake, involving the small intestine glucose metabolism as a key relay from the macronutrient composition of the diet to the amount of food ingested."

The group found that protein feeding of rats markedly increased the activity of genes involved in glucose production in the animals' small intestine. Those activities led to glucose synthesis and release into the portal vein--the vessel that conducts blood from digestive and other organs to the liver--a phenomenon lasting after the assimilation of glucose from the diet.

Furthermore, they found that the flux in glucose detected by the liver glucose sensor activated parts of the brain involved in the control of appetite, causing a decline in subsequent food consumption.

"In addition to protein's ability to diminish appetite, it had also been suggested that glucose appearance in the portal vein, as occurs during meal assimilation, may induce comparable consequences," Mithieux said. "Here, we connect these previous observations by reporting that intestinal synthesis of glucose is induced following food digestion in rats specifically fed a protein-enriched diet."

As in rats, diets high in protein suppress appetite in people, the researchers added. The human intestine also synthesizes glucose. "Therefore, glucose metabolism in the small intestine may be a new target in the treatment of food intake disorders," they said.