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The Wisdom of Psychopaths
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The obesity epidemic has led to increased scientific interest in how the brain controls human feeding behavior. Why do we get hungry? What biological mechanisms tell us what to eat and when to stop eating?
It’s long been assumed that two neurobiological mechanisms largely govern food intake: one that controls the need to eat and one that controls the desire to eat. The hypothalamus in the brain regulates the homeostatic control of food intake by receiving, coordinating and responding to metabolic cues and signals from the digestive system. By integrating these metabolic signals, the hypothalamus tells us when we need to eat to maintain a body weight “set point,” much like a thermostat set on a specific temperature. It is clear, however, that higher brain centers that control the desire to eat also substantially influence our food consumption. The dopamine reward system is one such brain center. (When you covet a bowl of chocolate ice cream after dinner, a food that you don’t need to eat for hunger but want to eat, it is your dopamine reward system that gets excited.) In many situations, this desire to eat can override the need to eat, leading people to consume tasty foods even when they’re not hungry. Our inability to forego these rewarding aspects of food intake override long-term homeostatic control, contributing to obesity.
Eating for Reward vs. Survival
Although the hypothalamus will direct intake based on the metabolic value of the food—when you’re very hungry, you seek out food with lots of calories—it remains to be determined whether the dopamine reward system can also sense a food’s energy content. In other words, does the dopamine system care about calories, or is it just concerned with taste and pleasure? Neuroscientist Ivan de Araujo and colleagues at Duke University (de Araujo is now at the John Pierce Laboratory, a research institute affiliated with Yale University) explored this question by using a line of mice genetically engineered to lack a functional receptor essential for detecting the taste of sweetness. In these mice, any change in reward behavior cannot be due to food palatability or the sensation of sweetness. If these mice prefer sweetness, thus, it is because sweeter foods have more calories, implying that there is something inherently rewarding about the consumption of calories.
In the first set of behavioral experiments, the authors showed that the genetically altered mice were completely insensitive to the “sweet” rewarding properties of sucrose (table sugar) and showed no preference for sucrose compared with water. In contrast, control mice without the genetic mutation strongly preferred the sucrose solution.
The scientists then exposed the different strains of mice to a “conditioning protocol” in which the rodents received alternating access to water or sucrose for six days. During these conditioning sessions, the genetically altered mice were able to associate the sweet solutions with caloric load post-ingestion, as the sugar water has more calories than plain water. Interestingly, both strains of mice now consumed significantly more sucrose. Although the genetically altered mice couldn’t taste the sweetness, they learned to prefer the sweeter water. This finding suggests that mice without functional sweet taste receptors were able to detect the reinforcing caloric properties of sucrose in the absence of sweet taste receptors. There seems to be something inherently pleasurable about ingesting food that contains calories.
As a critical control, the experiments were then repeated with sucralose (a.k.a. Splenda), an artificial sweetener that tastes sweet but contains no calories. Although normal mice consumed more sucralose than water during the conditioning period—they still preferred the sweet taste—the genetically altered mice did not.
Sweetness as Reward
These results indicate that sensing metabolic value can influence feeding behavior. It remained to be determined whether the dopamine reward system, known to respond to sweet taste, was also involved in calorie monitoring, however. To address this important question, the authors showed that calorie load increases dopamine levels in an area of the brain called the nucleus accumbens independent of taste in genetically altered mice using a technique known as in vivo microdialysis. Although both sucrose and sucralose increased dopamine above baseline in normal mice, genetically altered mice only showed an increase in dopamine with real sugar, indicating that caloric load (and not the taste of sweetness) was triggering their dopamine reward system. Even though these results undoubtedly show that calorie load affects the brain dopamine reward system independent of taste in the genetically altered mice, normal mice show no greater dopamine release for sucrose compared with sucralose. This discovery suggests that the presence of calories does not add more reinforcing power to the reward than taste alone. Future studies are needed to clarify whether this calorie load component can affect obesity independent of the taste of food.
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10 Comments
Add CommentCould it be that food corporations already knew this - and that is why SUGAR is packed into nearly every food in this country?
Reply | Report Abuse | Link to thisObesity will decline when we stop "processing" everything so much and when we stop adding sugar to everything.
Of course they know this. In the 19th century, the nutrition experts decided that Americans were too thin, and needed to put on weight. Their solution - add a little sugar to everything. The Fannie Farmer cookbooks exemplify this. American food is full of sugar, compared to the European countries that many of us count as our ancestors. Europeans don't add sugar to bread, yoghurt, beets, sausage, soup, etc. The low fat diets just make this worse, since taking the fat out removes the taste, so they add sugar to make the food palatable.
Reply | Report Abuse | Link to thisWhy are corporations and processing always vilified? They have both done so much for us that gets constantly ignored every time someone wants to duck personal responsibility.
Reply | Report Abuse | Link to thisEvery thing's always a conspiracy
There's no conspiracy. Sugar makes things taste good, and people prefer sweet foods. Thus, sweet foods sell better. It's a business move. The consequence of which is making people fat. However, if people could exercise a bit of personal responsibility and put down the Twinkies they might be able to keep the weight off. Fruit is pretty sweet, eat fruit instead of that chocolate ice cream.
Reply | Report Abuse | Link to thisPeople need to quit pointing the finger everywhere else and point it at themselves. That being said, I think this article does a good job illuminating the process. Perhaps if none of us could taste sugar, we would be better off. People eat too much and get fat because they enjoy eating. I think that's perfectly reasonable. Now, people just need to learn to override this "enjoyment" and actually work to maintain their health.
I suppose it's just too convenient to eat whatever we want and do nothing to maintain our health... since it takes little work to shovel food into our mouths and an extra hour or two of hard work every day to exercise, not to mention all the thought involved in eating foods that are good for us, checking calories, balancing the diet, etc...
Thanks for rational people! Those conspiracies will eventually get you candide! It could be the fluoridated water, or perhaps vaccines, maybe digital televisions... but with all those people plotting against you, and it is all of us plotting against you, I don't know how you can take it!
Reply | Report Abuse | Link to thisAny person in America that can read should know what calories are, how many they actually use a day, and how many they take in with every meal and what kinds of calories they take in. If you don't know, it's your own fault. You should know how much you burn through different activities. But I don't blame everyone for not knowing, most Americans live in caves with no contact no contact with the outside and without access to huge informational databases through the internet. right? Corporate America is probably hiding that information as we speak.
What you put in your mouth is your business, nobody is putting it there for you. If you must become obese through your own inability to regulate your diet, then the best we can hope for is that your body gives out before you reproduce. A far better solution than limiting what the rest of us can eat.
Thanks for reminding me I'm not alone!
Reply | Report Abuse | Link to thisI have to admit it's hard to accept personal responsibility in this case. there's millions of years of evolution that has chosen those that eat everything in sight. Hopefully science will provide us with the nutritional equivalent of birth control so we can live with our evolutionary traits.
I generally agree, except on one point. The article states that mice preferred the sucrose-laced water over the sucralose-water, indicating that the calories themselves are pleasurable for those mice (which can't taste sweetness.)
Reply | Report Abuse | Link to thisSo it is possible that even if we couldn't taste sweetness, we would still crave calories, and still become obese.
Most Americans don't care enough about losing weight or else they would be doing more about it. It's kind of the same principle as why most Americans have credit card debt. We need to burn more calories so we can lose more lbs and decrease health care costs. Alas, we'll see what happens in the future.
Reply | Report Abuse | Link to thisI think its time to replace the rewarding or reward prediction theory of dopamine with incentive salience theory of dopamine(berridge et al). These results are compatible with berridge's tema's data that showed a dissociation between liking and wanting food/sweetness. As dopamine just marks the incentive salience of a stimulus, t would be activated even if the food was unpalatable as long as it was salient (provided calories in food-deprivation) states. I dont know what additional information, than what has already been done in th incentive-salience framework, does this study shed light on.
Reply | Report Abuse | Link to thisI don't think they need to know it, Candide.. we all already know that sweet is delicious, so they were probably just responding to that.
Reply | Report Abuse | Link to thisCarlchwe: I don't know about that. If there was no reward, we probably wouldn't eat so much. But we'd still eat the same type of things we do now.