The French researchers explain it this way: Protein, the staple of such weight-loss regimens, appears to increase glucose production in the small intestine -- the rise of which is monitored by the liver and then registered by the brain. In turn, the brain sends out an "all full" message, cutting back on the drive to eat more.
"The current findings provide an answer to the question of how protein-enriched meals decrease hunger and reduce eating, unsolved up to now," the study authors, led by Gilles Mithieux of the Institut National de la Sante et de la Recherche Medicale in Lyon, France, said in a prepared statement.
"This novel understanding of the effect of diet protein will open new gates in the elaboration of future medical treatments of obesity," Mithieux said.
The researchers fed one group of rats a 50 percent-protein diet enriched with soya protein and casein. Another group ate a starch-enriched diet that contained just 17 percent protein.
Reporting in the November issue of Cell Metabolism, the French team found that by the end of just one week, rats on the protein-rich regimen had consumed 15 percent less food than those in the starch-diet group.
The protein-diet rats also gained significantly less weight over the course of the week than the starch-diet rats, the study found. And it wasn't that the rats on the protein-rich diet didn't like what they were eating, since the researchers had made sure to include foods the rodents loved.
A more complex explanation for the protein-linked weight loss was revealed through blood tests. They showed that two genes specifically involved in intestinal glucose production were much more active in the protein-diet group compared with the starch-diet group.
Even after food absorption had been completed, the small intestines of the protein-diet rats continued to deliver high levels of glucose into their portal vein -- a vessel that shuttles blood from the digestive system and other organs to the liver.
Glucose sensors in the liver of these protein-diet rats were found, in turn, to have signaled those areas of the brain responsible for appetite control -- bearing the message that liver glucose levels had risen. A quick and steady drop off in both hunger and eating ensued.
Based on these findings, Mithieux and his team believe they have unraveled -- at least in rats -- a connection between the digestive system and the central nervous system that may explain why protein so quickly curbs hunger.
Because the human intestine also produces glucose, the researchers believe this system might someday become key to treating weight disorders.
Dr. Ken Fujioka, director of nutrition and metabolic research at Scripps Clinic in San Diego, expressed enthusiasm for the researchers' work.
"The work is with rats, and in feeding it doesn't always translate to humans," he said. "But the way they've looked at this is novel, and it does seem to make sense."
"It's contrary to what many people think, which is that driving up glucose in the blood will drive up eating, but that's not necessarily true," added Fujioka. "And for a while now -- over the last five years -- we've really started to realize that protein is one of the best foods for satiating the brain. So, this paper shows that actually there's some biology behind this."