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Food Reward: Approaching A Scientific Consensus

Review papers render a bird's-eye persuasion of a plain from the perspective of experts.  Recent review papers demo that many obesity researchers are converging on a model for the evolution of obesity that includes excessive nutrient reward*, inwards add-on to other factors such equally physical inactivity, behavioral traits, as well as alterations inwards the business office of the hypothalamus (a key encephalon share for the rule of trunk fatness).  Take for instance the 4 novel review papers I posted late past times obesity as well as vantage researchers:

Food as well as Drug Reward: Overlapping Circuits inwards Human Obesity as well as Addiction
Common Cellular as well as Molecular Mechanisms inwards Obesity as well as Drug Addiction
Is Fast Food Addictive?
Metabolic as well as Hedonic Drives inwards the Neural Control of Appetite: Who is the Boss?

The truth of the thing is, obesity researchers withdraw maintain been converging on the nutrient vantage hypothesis for years now-- as well as I'm a relative latecomer.  I late came across an outstanding 2008 review newspaper past times several leaders inwards the plain titled "Mechanisms of Leptin Action as well as Leptin resistance" (1).  I highly recommend this review to anyone amongst a background inwards the biological sciences who wants to empathize how leptin plant inwards both the lean as well as obese state.  Here's a passage from the destination of the paper:
Some testify exists for developmental alterations inwards neural as well as other systems that may underlie about propensity to obesity, but the laid upwardly availability of palatable, calorically dense nutrient (the terra firma for [diet-induced obesity] inwards experimental animals) clearly plays a dominant role. Indeed, the obesity as well as cellular leptin resistance of [diet-induced obese] animals are reversed past times replacing the palatable calorie-dense chow used to promote obesity amongst criterion chow. Although about of the obesogenic effects of tasty foods may last due to their nutrient content, the hedonic or rewarding properties of these foods equally good contribute...
How so is the activity of leptin to regulate the perception of nutrient vantage overwhelmed to promote obesity inwards the facial expression of plentiful tasty food? Leptin is exclusively 1 of many inputs into the mesolimbic [dopamine] organisation as well as other neural pathways that regulate the perception of nutrient reward, as well as physiological leptin levels may non last able to suppress the myriad other signals that compel us to eat tasty food. Although leptin may reasonably inhibit the drive to overeat foods amongst exclusively modestly rewarding properties, leptin may last insufficient to effectively compete amongst the rewarding properties of to a greater extent than palatable treats because these more-rewarding foods engage powerful neural responses that oppose leptin inside the mesolimbic [dopamine] organisation as well as elsewhere.
Basically, what they're maxim is pretty simple: the brain's hard-wired mechanisms for regulating nutrient intake as well as obese volume inwards a natural surroundings are non sufficient to protect against modern hyper-palatable as well as hyper-rewarding food.  These are researchers who primarily written report the mechanisms of leptin resistance inwards the brain.  If anything, they withdraw maintain an incentive to de-emphasize the nutrient vantage hypothesis equally it potentially takes the spotlight off their ain query (the same applies to me).  The fact that they nevertheless house such a strong emphasis on the thought speaks volumes. 

The magazine Cell exactly published a review newspaper past times Drs. Karen Ryan, Randy Seeley as well as Stephen Woods-- Drs. Seeley as well as Woods are leaders inwards obesity query (2).  Here's what they had to tell most nutrient reward:
When a [high-fat diet] is of a precipitous made available to an individual, sensory properties of the nutrient (e.g., odor, taste, oral cavity feel) are intrinsically pleasant as well as atomic number 82 to to a greater extent than nutrient beingness consumed. As this continues, excess calories accumulate equally increased adiposity, as well as increased circulating obese acids atomic number 82 to increased lipid flux into the brain, insulin/leptin resistance, an inflammatory condition, as well as so on. Stated about other way, the increased palatability of the diet initiates a fell bike inwards which hedonics elbow grease to a greater extent than nutrient to last eaten than is necessary to run across unloose energy needs, as well as the increased calories inwards plough initiate events that atomic number 82 to insulin/leptin resistance as well as a consequent style to eat fifty-fifty to a greater extent than food (Figure 4).
It's worth noting that they're referring specifically to insulin as well as leptin resistance inwards the brain inwards that passage.  I make differ amongst them somewhat on the usage of obese per se.

Dr. Robert Lustig exactly published an editorial inwards the magazine Nature titled "Public Health: the Toxic Truth About Fructose" (3).  First of all, congratulations to Dr. Lustig for publishing his missive of the alphabet inwards such a high-impact journal.  He argues that carbohydrate consumption should last regulated past times the regime because inwards excess, it's a threat to world health.  One department of the article describes sugar's effects on the vantage as well as pleasance systems inwards the brain:
Sugar equally good has clear potential for abuse. Like tobacco as well as alcohol, it acts on the encephalon to encourage subsequent intake. There are at 1 time numerous studies examining the dependence-producing properties of carbohydrate inwards humans6. Specifically, carbohydrate dampens the suppression of the hormone ghrelin, which signals hunger to the brain. It equally good interferes amongst the normal carry as well as signalling of the hormone leptin, which helps to create the feeling of satiety. And it reduces dopamine signalling inwards the brain's vantage centre, thereby decreasing the pleasance derived from nutrient as well as compelling the private to eat more1, 6.
Another prissy nutrient vantage review newspaper (4), hot off the presses, had this to say:
Food reward, non hunger, is the primary driving strength behind eating inwards the modern obesogenic [obesity-promoting] environment. Palatable foods, to a greater extent than oftentimes than non calorie-dense as well as rich inwards sugar/fat, are thus readily overconsumed despite the resulting wellness consequences.
Last but non least, my ain recent review newspaper on obesity, published inwards the Journal of Clinical Endocrinology as well as Metabolism (5).  I don't claim to last a leader inwards the obesity query field, but my mentor Dr. Michael W. Schwartz, senior writer on the paper, is.  Here's what nosotros had to say:
These findings collectively advise that obesity tin dismiss arise when animals or humans are confronted amongst foods whose palatability/reward value greatly exceeds that to which they are genetically adapted, as well as thence that interventions that inhibit nutrient vantage tin dismiss forbid obese gain as well as promote obese loss.
These latter 4 review papers were all published inwards the final pair of months. 

Approaching a Consensus

There are many dissimilar query groups approaching obesity from many dissimilar angles, but most of the review papers beingness published today admit the fundamental usage of nutrient vantage inwards the evolution of obesity.  The ground is simple: the thought is strongly supported past times the scientific literature, but it's equally good exactly mutual sense.  In whatsoever scientific discipline, at that topographic point volition ever last about disagreement, but the testify at this indicate is strong plenty that the plain equally a whole seems to last reaching a consensus.

* Reward.  The encephalon contains a "reward" system, whose chore it is to guess the desirability of nutrient (among other things) as well as reinforce as well as motivate behaviors that favor the acquisition of desirable food.  For example, if yous eat a strong cheese for the commencement time, perhaps it won't gustatory modality really goodness to you.  As it's digested, your vantage organisation gets current of air that it's total of calories however, as well as the side past times side few times yous eat it, it tastes amend as well as amend until yous similar the flavor.  This is called an acquired taste, as well as the vantage organisation is what does the acquiring, motivating yous to obtain a nutrient it has deemed prophylactic as well as desirable.  Eventually, yous may move out of your means to buy the cheese or beer at the grocery shop because yous similar it so much, as well as perhaps you'll eat cheese or beer fifty-fifty if yous aren't hungry or thirsty.  This is an instance of the vantage organisation reinforcing as well as motivating behaviors related to foods that it considers desirable.  Processed "junk foods" such equally H2O ice cream, fast food, sweetened soda, cookies, cake, candy as well as deep fried foods are all archetypal hyper-rewarding foods. 

Palatability is a related concept-- it is merely the pleasantness of a food; how much a individual enjoys eating it.  Palatability is determined inwards component subdivision past times inborn preferences (e.g., a gustatory modality for carbohydrate as well as unloose energy dense foods), as well as inwards component subdivision past times the vantage organisation (acquired tastes).  Palatability is governed past times the hedonic organisation inwards the brain, which is closely integrated amongst the vantage system.

The vantage organisation is what motivates yous to larn nutrient as well as pose it to your lips, every fourth dimension yous eat.  When scientists closed it downwards inwards mice, they completely cease eating (6).  The hedonic organisation influences how much yous eat 1 time yous commence a meal-- highly palatable nutrient to a greater extent than oftentimes than non increases nutrient intake past times activating this organisation (7).  Together, vantage as well as hedonic circuitry inwards the encephalon decide inwards large component subdivision how oftentimes yous eat, what yous eat, as well as how much yous eat, as well as this is influenced past times the attributes of the nutrient that's available.



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