A study of protein-munching rats shows that a low-carb
diet sparks a chain of biological events that ultimately
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
"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
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
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."
However, Lona Sandon, an assistant professor of clinical
nutrition at the University of Texas Southwestern Medical
Center, cautioned that the study might implicitly over-emphasize
the benefits of protein.
"There's definitely research out there that protein does
help us to feel satisfied with what we've eaten, on a
smaller amount of food," she said. "But it's not as simple
as that, because there's also certain carbohydrates --
particularly high-in-fiber-type carbohydrates, like whole
wheat, bran, fruits and vegetables -- that will do that
equally, if not better, than protein. So, there's this
misconception that we need to eat all this protein."
"Protein is just one piece of the puzzle," she advised.
"And the bottom line is we need to consider more than
protein when controlling appetite."