THURSDAY, June 5 (HealthDay News) -- "You are what you eat" is a
frustrating truism familiar to the diet-conscious choosing between
carrots and carrot cake. But new research suggests that weight
control isn't just a matter of what you put in your mouth, but also
how the nervous system is genetically predisposed to process
fat.
The theory is based on research with worms that suggests that
the brain chemical serotonin -- long known for its appetite control
properties -- relies on independent but coordinated nerve pathways
to drive not only hunger, but also fat metabolism.
"I want to be clear that there is absolutely nothing in our
study that says that good nutrition and activity is not important
or not good for you," said study lead author Kaveh Ashrafi, an
assistant professor in the department of physiology and the
Diabetes Center at the University of California, San Francisco.
"But that said, I think that it is important to realize that there
is a major contributing factor to body weight that is genetic."
The new findings, based on research with so-called
C. elegans worms, is published in the June issue of the
journal
Cell Metabolism.
Ashrafi and his colleagues noted that, while the worms are
obviously smaller and simpler organisms than humans, they are
molecularly very close, having about 20,000 genes to a human's
25,000 genes. And both humans and worms have genetically evolved to
protect and conserve energy depending on food availability, the
researchers said.
Because of such similarities, the researchers attempted to
partially deconstruct the worm's nervous system by tracking cell
receptors that serotonin connects with when prompting feeding in
worms. Then they tracked only those pathways that serotonin
activates when regulating fat control.
The researchers were surprised to find that serotonin-directed
feeding and fat-burning pathways are, in fact, two separate
channels, each operating in a complementary but "molecularly
disassociated" fashion.
Also, the worms' feeding behavior appeared to be dependent on
their nervous system's shifting gauge -- both genetically and
environmentally triggered -- as to how much food was available for
consumption. When food was scarce, fat reserves piled up as the
worms' metabolic rate adjusted to conserve energy and save fat for
a rainy day.
Comparisons to humans are obviously limited by appreciable
systemic differences. For example, the authors noted that high
levels of serotonin appear to lead to fat loss in the worms, as is
the case with humans. But, while in worms, serotonin boosts lead to
increased eating
and fat loss, in humans, serotonin bumps are known to prompt
less eating, which in turn leads to a decrease in fat tissue.
Nevertheless, Ashrafi and his team concluded that their analysis
of worms offers clear cross-species indications that whether one is
fat or thin is not a pure calculation of caloric intake but rather
a complex result of multiple nervous system calculations based on
both genetic and environmental factors.
"We don't know the real answer yet," Ashrafi said. "We are just
beginning to scratch the surface. But I think chances are pretty
good that this is something likely to be operating in most
organisms. And, the fact is that the consumption of food is a very
dynamic process, with a strong genetic component that clearly goes
beyond the question of diet."
Alice H. Lichtenstein, director of the Cardiovascular Nutrition
Lab at Tufts University's USDA Human Nutrition Research Center in
Boston, cautioned that attributing weight and fat gain to
environmental or genetic sources can be tricky and complicated.
"We clearly know that the nervous system will impact on food
intake, and the exploration of genetic predispositions toward
weight gain and obesity is not new," she said. "In fact, there has
been an immense amount of effort over a long period of time devoted
toward sorting this out."
"This study probably adds a new dimension to the ongoing work,"
added Lichtenstein, immediate past chair of the American Heart
Association's nutrition committee. "But the issue is really very,
very complex. So, while I think it's important to bring up all the
different possibilities of what can impact on weight, it's
certainly going to take a long time to sort it all out."
More information
To learn more about the causes of obesity, visit the
U.S. Centers for Disease Control and
Prevention.
