Researchers in Australia explain why when obese people diet, they end up
losing less weight.
Imagine you are driving a car, and the harder you press on the accelerator,
the harder an invisible foot presses on the brake. That’s what happens when
obese people diet – the less food they eat, the less energy they burn, and the
less weight they lose.
While this is a known phenomenon, scientists at Sydney’s Garvan Institute
of Medical Research and the University of New South Wales (UNSW) have
pinpointed the exact brain circuitry behind it using various mouse models.
In the journal Cell Metabolism, Dr. Shu Lin, Dr. Yanchuan Shi, and
Professor Herbert Herzog and his team show that the neurotransmitter
Neuropeptide Y (NPY), known for stimulating appetite, also plays a major role
in controlling whether the body burns or conserves energy.
“This study is the first to identify the neurotransmitters and neural
pathways that carry signals generated by NPY in the brain to brown fat cells in
the body. It is also the first to show a direct connection between Arc NPY, the
sympathetic nervous system and the control of energy expenditure,” said Herzog.
The researchers found that NPY – produced in a particular region of the
brain called the arcuate nucleus (Arc) of the hypothalamus – inhibits the
activation of ‘brown fat,’ one of the primary tissues where the body generates
heat.
While NPY also influences other aspects of the sympathetic nervous system
such as heart rate and gut function, its control of heat generation through
brown fat seems to be the most critical factor in the control of energy
expenditure.
“When you don’t eat, or dramatically curtail your calorie intake, levels of
NPY rise sharply. High levels of NPY signal to the body that it is in
‘starvation mode’ and should try to replenish and conserve as much energy as
possible. As a result, the body reduces processes that are not absolutely
necessary for survival,” he said.
Until the twentieth century, people did not have ready access to foods high
in fat and sugar, said Herzog. So in evolutionary terms, the body had
mechanisms in place only to prevent weight loss, he said.
“Obesity is a modern epidemic, and the challenge will be to find ways of
tricking the body into losing weight – and that will mean somehow circumventing
or manipulating this NPY circuit, probably with drugs,” he said.
Source : Garvan Institute of Medical Research
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