| Scientists
have made significant strides to finding the reasons why it is so
difficult to lose weight. It was discovered that the mysterious hormone,
leptin, helps satisfy hunger and the sensation of feeling full. The
hormone, made by fat cells, plays a critical role in establishing
the functioning of the brain before birth and maintains the ability
to carefully rewire those neural connections throughout life.
Scientists believe the observations made in mice may also apply
to humans and could be used to answer how the impulses that tell
us to eat operate. They also discovered that despite efforts to
change their weight, many people contain a physical "set point"
that keeps them at their current weight.
One scientist said that the different wiring system that regulates
feeding might vary in people who are obese and those who are thin.
The hormone leptin was discovered 10 years ago and it was found
that when it entered the bloodstream, it helped suppress the appetite
and lack of it could lead to overeating and obesity. Despite these
findings, the notion of losing weight simply by taking leptin hasn’t
yet been tested by humans.
In the studies involving mice, researchers looked at the cells
in the area of the brain called the hypothalamus, which makes up
1 percent of the brain’s volume and contains some important
functions including body temperature, heart rate, blood pressure
and food intake. Each region contained two cell types, NPY cells
and POMC cells. When NPY cells were stimulated, it was shown that
feeding increased and fat accumulated. However, when POMC cells
were stimulated, the feeding decreased and ultimately the body mass
decreased also.
The actions of these cells were used to analyze how often and how
much an animal eats.
The study also revealed that when a mouse experienced leptin deficiency,
the NPY cells that induce feeding, were more active and POMC cells
became less active, compared to the normal mice whose NPY cells
became more repressed.
After the hormone-deficient mice were given leptin, there was a
significant reduction in the amount of connections to the NPY cells
that promote feeding and a near doubling of the satiety-inducing
connections. Researchers have used the results of this study to
help them determine the physical "set point." Experts
stated that when a person loses fat, leptin levels in the blood
drop, which may lead to changes in the hypothalamic wiring, causing
NPY cells to stimulate feeding and POMC cells to stop the feelings
of satiety.
Other scientists speculated that a person’s sensitivity to
leptin, based on how it was formed in their brain just after birth,
might affect their susceptibility to obesity in adulthood.
Washington
Post April 2, 2004
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