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The Illustration For The Nutrient Vantage Hypothesis Of Obesity, Business Office I

Introduction

When y'all desire to investigate something using the scientific method, get-go y'all do a model that y'all promise describes a natural phenomenon-- this is called a hypothesis.  Then y'all become virtually testing that model against reality, nether controlled conditions, to come across if it has whatsoever predictive power.  There is rarely a unmarried experiment, or unmarried study, that tin demonstrate that a hypothesis is correct.  Most of import hypotheses demand many mutually buttressing lines of testify from multiple inquiry groups earlier they're widely accepted.  Although it's non necessary, agreement the machinery yesteryear which an resultant occurs, together with having that machinery hold out consistent alongside the hypothesis, adds substantially to the case.

With that inwards mind, this post service volition become into greater special on the testify supporting nutrient vantage together with palatability equally major factors inwards the rule of nutrient intake together with trunk fatness.  There is a large total of supportive testify at this point, which is chop-chop expanding due to the efforts of many bright researchers, silent for the sake of clarity together with brevity, thence far I've alone given a "tip of the iceberg" sentiment of it.  But at that spot are 2 types of people who desire to a greater extent than detail: (1) the skeptics, together with (2) scientifically inclined people who desire mechanism.  This post service is for them.  It volition larn technical at times, equally at that spot is no other agency to bring the cloth effectively.


First, to a greater extent than or less definitions.  One of the problems alongside nutrient vantage is it's defined differently yesteryear unlike people, fifty-fifty researchers, together with oft it isn't defined at all.  As defined inwards previous posts, I exercise the term nutrient vantage to refer specifically to the motivational value of food, i.e. its might to reinforce behavior.  For example, acquiring a sense of savor that causes a mortal to seek out the nutrient inwards enquiry to a greater extent than often.  This is how some, only non all, researchers define the term.  Others exercise the term "food reward" to refer to both the motivational together with the palatability value of food.  Palatability refers specifically to the enjoyment derived from a food, too called its hedonic value.  Palatability together with vantage typically go together, only non always.

The nutrient vantage hypothesis of obesity states that the vantage (reinforcing, motivational) together with hedonic (pleasure, palatability) value of nutrient influence nutrient intake together with trunk fatness, contributing to the evolution of obesity.  Quite a chip is known virtually the fundamental nervous organization (CNS, i.e. brain) circuitry that underlies vantage together with hedonic processing, together with how this circuitry influences nutrient intake together with trunk fatness.  Most of the inquiry on vantage together with hedonic processing began alongside the written report of drug addiction inwards animals together with humans, silent these circuits evolved to guide behaviors that heighten fitness inwards the natural environs such equally those relating to nutrient together with sex, together with inquiry too focuses on these processes.

Although nutrient vantage together with nutrient palatability typically occur together, they are non the same thing.  In the 1990s, Dr. Kent Berridge proposed that behavioral together with CNS responses to nutrient tin hold out divided into "wanting", corresponding to the reward/motivational aspects of food, together with "liking", corresponding to the hedonic/palatability aspects of food, together with this framework has been a practiced check for the testify since together with thence (1).  These 2 elements tin hold out separated from i to a greater extent than or less other experimentally, together with sometimes inwards daily life equally well.  One illustration is a mortal who is addicted to a drug despite no longer deriving pleasance from it.  This is a instance of rigid "wanting" without "liking".

Brain Circuits together with Neurotransmitters Underlying Food Reward together with Hedonic Processing


The fact that the vantage together with palatability value of nutrient tin hold out experimentally dissociated implies that they are mediated yesteryear distinct CNS circuits.  Indeed, although vantage together with hedonic circuits overlap to to a greater extent than or less score together with influence i another, each organization has its ain unique circuitry together with suite of chemic neurotransmitters.  CNS regions of import for vantage processing include the orbitofrontal cortex, amygdala, nucleus accumbens, dorsal striatum, ventral tegmental area, substantia nigra together with the lateral hypothalamus (2).  Dopamine signaling inwards these areas is a especially of import constituent of reward, only has piffling touching on on hedonic processes (3).  Dopamine isn't thence much a pleasance chemic equally it is a reward/motivation chemical. 

CNS regions of import for processing food-related hedonic data include the brainstem, pons, nucleus accumbens, ventral pallidum, amygdala, insula together with prefrontal cortex (4).  Note that these overlap somewhat alongside encephalon regions that procedure vantage information, only inwards to a greater extent than or less cases the subregions or neuron sub-populations that mediate these 2 processes are distinct fifty-fifty inside the same full general CNS region.  Opioid signaling inwards these regions is a major factor inwards hedonic processing, only has less touching on on vantage functions (5, 6).  Opioids, inwards for certain parts of the brain, are pleasance chemicals.

Another shape of chemic signals worth mentioning is the endocannabinoids.  The endocannabinoid receptor CB1 was identified equally the encephalon receptor for delta-9-THC, the primary psychoactive constituent of marijuana, together with and thence the endogenous ligands for the cannabinoid receptors were discovered (7).  We directly know that the principal 2 endogenous ligands are anandamide together with 2-arachidonylglycerol, together with the 2 receptors are CB1 together with CB2.  Endocannabinoid receptors are expressed inwards CNS regions related to vantage together with hedonic processing, together with they are known to hold out involved inwards both processes.

In add-on to these 3 chemic systems (dopamine, opioids together with endocannabinoids), at that spot are other neurotransmitters that are intimately involved such equally glutamate together with GABA, only these are not  specific to vantage together with hedonic processes since they are the primary excitatory together with inhibitory neurotransmitters of the brain, respectively.  So their interest inwards vantage together with hedonic processing depends strictly on which neurons they're acting on.

Testing the Food Reward Hypothesis


 If the nutrient vantage hypothesis is correct, nosotros should facial expression to discovery for certain things:

1. Increasing the reward/palatability value of the diet should elbow grease fatty arrive at inwards animals together with humans.

2. Decreasing the reward/palatability of the diet should elbow grease fatty loss inwards animals together with humans that comport excess fat.

3. Individual sensitivity to nutrient vantage should predict futurity fatty gain.

4. Brain circuitry that controls motivational together with hedonic processing should interact alongside circuits that command nutrient intake together with trunk fatness.

5. Manipulation of vantage together with hedonic circuits inwards the encephalon (e.g., yesteryear lesion, drugs or genetic manipulation) should touching on nutrient intake together with trunk fatness.

6. Genetic differences that influence vantage and/or hedonic circuits should correlate alongside differences inwards trunk fatness.

7. On a cultural level, obesity prevalence should rails alongside changes inwards the reward/palatability value of prevailing diet patterns.


In the adjacent post, I volition explore whether or non the testify is consistent alongside these predictions.


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