I think av is lacking of leptin. 
LONDON (Reuters) - The hormone that tells us we are full also
regulates our desire for certain foods, researchers said on
Thursday, in a finding that sheds light on why people gain weight
and could lead to new treatments for obesity.
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The study showed that patients with a rare genetic disorder who
lacked the hormone called leptin ate less after receiving
injections of the hormone, said I.S. Farooqi, a researcher at
Cambridge University who led the study.
Previous research has shown the hormone does not help people with
normal leptin levels lose weight, but scientists still do not
completely understand how it works, Farooqi said.
"By studying patients who have no leptin and then treating them
with leptin, we can tell what it is doing," Farooqi said in a
telephone interview. "It gives a clear look at how leptin operates
in the brain."
In the study, published in the journal Science, researchers
searched for "circuits" in the brain that signal when a person is
hungry or full and found that they were linked to areas involved in
determining the enjoyment of food.
To see how the hormone worked, the researchers showed the patients
pictures of different types of food, ranging from tasty fare like
chocolate cake and pizza to blander choices such as cauliflower and
broccoli.
The patients with the genetic disorder -- of which there are about
a dozen known cases in the world -- liked all types of food, ate
excessively and were obese, the researchers said.
Using magnetic resonance imaging technology, the researchers
tracked the patients' brain activity as they responded to the
pictures and pinpointed several key areas that play an important
role when it comes to a desire for food.
After the patients received leptin injections, the areas that had
previously shown activity all the time at the sight of food were
only active if the people had not eaten the night before, which was
a normal response, Farooqi said.
It showed desire for food is driven by biology -- not greed --
which causes overeating and obesity, Farooqi said.
Knowing how leptin, which is produced by fat cells, triggers
different parts of the brain could lead to new drugs that target
obesity and help dangerously overweight people take pounds
off.
"If you find those molecules that leptin triggers then you can
manipulate or target them with drugs to treat obesity," Farooqi
said. "The first step is to work out what leptin does and how it
does it."
