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.
The article can be found
at: Shi YC et al. (2013) Arcuate NPY
Controls Sympathetic Output and BAT Function via a Relay of Tyrosine
Hydroxylase Neurons in the PVN.
Source: Garvan Institute of Medical
Research;
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