A study of protein-munching rats shows that a low-carb diet sparks a chain of
biological events that ultimately curbs hunger.
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.”
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.”
Check out the U.S. government’s Dietary Guidelines for Americans 2005.
SOURCES: Gilles Mithieux, Institut National de la Sante et de la
Recherche Medicale, Lyon, France; Lona Sandon, RD, assistant professor, clinical
nutrition, University of Texas Southwestern Medical Center, Dallas, and
spokeswoman, American Dietetic Association; Ken Fujioka, M.D., director,
nutition and metabolic research, Scripps Clinic, San Diego; November 2005
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