"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.
The researchers included Gilles Mithieux, Pierre Misery, Bruno Pillot, Amandine Gautier-Stein, Fabienne Rajas,and Carine Zitoun of the Institut National de la Sante et de la Recherche Medicale, the Institut National de la Recherche Agronomique, and the Universite Claude Bernard Lyon I, in Lyon, France; Christine Bernard, of Institut National de la Sante et de la Recherche Medicale and Universite Claude Bernard Lyon I, in Lyon, France; and Christophe Magnan of Centre National de la Recherche Scientifique and Universite Paris in Paris, France. The research was supported by INSERM, INRA and Universite Paris.
Mithieux et al.: "PORTAL SENSING OF INTESTINAL GLUCONEOGENESIS IS A MECHANISTIC LINK IN THE DIMINUTION OF FOOD INTAKE INDUCED BY DIET PROTEIN." Publishing in Cell Metabolism, Vol. 2; Issue: 5; November 005, pages 321-329. DOI 0.1016/j.cmet.2005.09.010 www.cellmetabolism.org