MADRID: Study Identifies Molecule That Stimulates Muscle-Building In Humans

MADRID: In a randomized control study
of 10 healthy young men, a team of researchers compared how consuming the
single amino acid leucine or its two-molecule equivalent, dileucine, influenced
muscle-building and breakdown.

They
found that dileucine boosts the metabolic processes that drive muscle growth 42
per cent more than free leucine does.

The
findings were published in the Journal of Applied Physiology.

Leucine,
isoleucine, and valine all are branched-chain amino acids, famous among
bodybuilders and health enthusiasts for their purported muscle-enhancing
benefits. Like other amino acids, they are the building blocks of proteins. But
leucine also acts as a signaling molecule that triggers muscle-building
pathways in cells, said University of Illinois Urbana-Champaign kinesiology and
community health professor Nicholas Burd, who led the new research with
kinesiology graduate student Kevin Paulussen.

Digestion
breaks the chemical bonds between the amino acids that make up proteins,
resulting in a stew of shorter molecules, including free amino acids and
dipeptides. Previous studies have suggested that the small intestine absorbs
dipeptides like dileucine more rapidly than their single-molecule counterparts,
Burd said.

“But
few studies have examined whether dileucine in the diet makes it into the blood
as a dipeptide or is first broken down into two leucine molecules,” he
said. “And no studies have examined its effects on acute muscle-building
and breakdown.” Burd’s laboratory is one of a small number of research
facilities set up to study muscle protein metabolism in human participants.

For the
new study, participants came to the lab after a 12-hour fast and were infused
with stable isotopes, chemical probes that allow researchers to track the
process of muscle protein synthesis and breakdown in their muscles. Then
biopsies of muscle tissue were taken from the upper leg.

“After
that, we fed them either 2 grams of leucine or 2 grams of dileucine,” Burd
said. “And we studied their muscle-remodeling response for three
hours.” This was a double-blind study, meaning that the data were coded to
prevent participants and researchers from knowing who received leucine or
dileucine in the initial phases of the study. Three more muscle biopsies were
taken, at 30, 60 and 180 minutes after participants ingested the leucine or
dileucine.

“We
found that leucine got into the blood more quickly when participants consumed
dileucine than if they had just free leucine,” Burd said. “That means
that some of that dileucine is getting hydrolyzed, or cut up before it gets
into the bloodstream. But we also saw that dileucine was getting into the
bloodstream intact.”

The next
question was whether dileucine had any effect on muscle-building processes, he
said.

“So,
we looked at pathways that signal the muscle-building process, including
protein breakdown as part of the remodeling process. And we found no difference
in protein breakdown between the leucine alone and the dileucine
condition,” Burd said. “But on the protein synthesis side, we saw
that dileucine turns up the muscle-building process more than leucine
does.”

Those who
consumed dileucine had 42 percent more synthesis of new muscle proteins than
those who ingested only leucine.

“To
put that in perspective, exercise alone can cause a 100-150 per cent increase
in the muscle-building response,” Burd said.

The
researchers also showed that animal-based proteins are the best source of
dileucine in the diet. But Burd does not think people should start ingesting
large amounts of animal protein or taking dileucine supplements to enhance their
muscle metabolism.

The study
is only a first step toward understanding how the body uses dipeptides,
“and focusing on a single nutrient doesn’t provide a perspective on how
the overall diet and eating pattern impacts muscle growth,” he said.

“We
don’t yet know the mechanism by which dileucine works, this is just a first
attempt to understand how these types of peptides are playing a role in human
physiology,” Burd said.