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Why do people become obese: how the gut talks to the brain.

October 25, 2019


Which part of the brain is responsible
for obesity? Many people think it’s our willpower and
the patients who are obese have a lack of moral fiber. However it’s actually the subcortical
areas of the brain such as the hypothalamus and nucleus tractus
solitarius that sit at the bottom of the brain that control our vital
functions including hunger and fullness. When we look at where the hypothalamus
is situated, we see that it does not have that much
contact with the areas of the brain we use for philosophy or mathematics, also
called the cortical areas of the brain. For that reason it is difficult for
somebody to think themselves thin. When we offered a choice of desert between a piece of broccoli or a lovely piece of cake most of us will select the second. Why do we behave like this when both of those foods may have similar nutrient value? To
understand this we have to go to functional magnetic resonance imaging or
fMRI studies. These studies use pictures to determine which part of the brain
receives the most blood flow when we look as a palatable food. Reward systems are activated when palatable food pictures are shown to normal and
overweight people we can then see which reward centers in which specific parts
of the brain receive more blood flow when a picture of palatable food is shown. A glucagon-like peptide-1 analog when injected into the periphery is taken up
by discrete areas of the brain related to hunger and fullness. This GLP-1 analog binds to the subcortical areas of the brain such as the arcuate nucleus where
our hunger and fullness signals are situated. However just because the analog binds it
doesn’t mean the GLP-1 activates the subcortical areas of the brain. By using
fMRI scans of the brain we can see that GLP-1 not only binds but
also activates these specific areas of the brain related to reward. Surgical
treatments for obesity include gastric banding and gastric bypass. These
patients have operations on their stomachs and lose a quarter of their weight but
often they come back to clinic and ask Dr. where did they operate? Did they
operate in my tummy or did they operate in my head because all the signals are
now receive are actually in my head. When we investigate patients after gastric bypass with fMRI and pictures of palatable foods, we see a reduction in
their reward centers. The operation on the stomach changes the way the brain is
responding to the visual cue of palatable food. This explains why all patients eat less
food and specifically less calorie dense food as the areas of the brain involved
with appetite and reward are now down regulated. In conclusion calorie dense
food activates brain reward centers. Therapies that attenuate appetite
centers also reduce reward areas in the brain to control appetite and food
intake therapies need to target these subcortical areas of the brain.

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