2013-03-21 Brain Awareness Night Dr. Boughter

2013 Brain Awareness Night: How Tricking the Brain's Taste System During Development May Lead to Obesity

Dr. John D. Boughter, Jr., Ph.D
Associate Professor, Department of Anatomy and Neurobiology
University of Tennessee, Health Science Center

“How Tricking the Brain’s Taste System During Development May Lead to Obesity”
Thursday, March 21, 2013

This talk will discuss how the sense of taste engages parts of the brain involved in eating and reward, and how dysfunction in these vital systems may occur.


Bill: Thank you, Gene. Thanks, everybody. I'm really happy to be doing this again. I was telling him that I've done it seven years, and I think it actually started eight years ago with David Smith, the former director of the Neuroscience Institute.
Before we introduce the speakers, I want to make the usual thanks. I have to thank [Gene Cashman] whom you just heard talk. Katy Spurlock, who has been our contact at Urban Child with this event for all these years. Susan and Bill Day who have helped with media events and organization as well at Urban Child. Our assistant at the Neuroscience Institute, Shannon Guyot.
And I have to give the ultimate thanks to Paul Herron. Paul, I'd wish you'd stand up for a second. Paul organizes this event every year, helps me determine speakers, interacts with the speakers and arranges all this good stuff, so I really appreciate that. He always does a great job.
Tonight's talks are going to feature aspects of obesity, and they're going to be a little different than the normal aspects. We're not going to talk much about the physiology of obesity, and as you know it's multifactorial. There are a million things ranging from genetic to all types of issues that would contribute to it. We're going to be focusing today on aspects of brain function and how that regulates eating.
If you talk to some people about obesity, even scientists, and say it's a brain function, they'll say no obesity is something else. They forget that the receptors in the brain that mediate your desire to eat or the feeling that you are satiated are targeted by pharma now. There are drugs out there now that you'll be able to talk that would say, for example, inhibit your eating. So the brain is ultimately involved and motivating you to eat and making sure you eat the right things.
I think what we're going to hear tonight is aspects of commercialism that are going to use that aspect of the brain's function to influence what you want to eat, and sometimes you'll see a negative direction. It's a little bit more unusual that we're going to be focusing a little bit on marketing tonight, and it's involved in the ingestion of foods that in large quantities in particular wouldn't be so good for you.
The first speaker is John Boughter who's at the University of Tennessee. I've known John since he came here in 2002; he's done a great job. He's been funded the whole time he's been here. Lots of publications, and really strong member of the department. He's a taste physiologist. He got his undergraduate degree from Binghamton University in New York, and his graduate degrees, both masters and PhD from Florida State, and used to be enemies with Memphis when we used to play Florida State when they were independent every year. Then he did a postdoc at Maryland, I think, with David Smith before he came down here. We're really happy to have John with us tonight for the first talk, which the title is "How Tricking the Brain's Taste System during Development May Lead to Obesity''.

Dr. Boughter: All right, good evening. Thanks, Bill, for that introduction. As Bill talked to you about, we're going to focus on different aspects of eating tonight. I'm going to talk to you about the taste system, and how it interacts with eating, and how it may be involved in the development of obesity. My lab studies the sense of taste. We study how taste is organized in the brain, especially how taste information is encoded and how it moves from one place to another in the brain. We're particularly interested in how taste areas of the brain engages other parts of your brain that are involved in homeostasis and reward.
I've got several objectives for this presentation tonight. First I want to talk a bit about the taste system itself so we can understand the importance and the organization of this crucial system, and then we'll talk about the contribution of taste to diet, and especially how that has to do with modern diets which really aren't necessarily the diets that our taste system has evolved to deal with.
Finally, I want to shed some light on mechanisms with diet formation during childhood which impacts obesity, and again that's going to be diet formation as it relates to taste choices.
Let's start with a little example of how taste interacts with eating and feeding. It's important to think about the process of feeding as having both excitatory as well as inhibitory factors. Let's take this example. Here's a young teenage man who has sat down to a nice meal.
What is the thing that's driven him to eat in this care? One of the important factors here would be hunger or appetite. That's an internal factor, and so we've really simplified that just by talking about hunger. Hunger is actually a very complex process, but it will stand for this example.
He's hungry which is what brings him to the table, and now we put down some food in front of him, and we can then talk about some other factors that are going to influence eating. We can think about some external sensory factors. What are those? Some external sensory factors could be the smell of the food. This food has a really nice smell to it.
Another external sensory factor would be the way the food looks. The food has to look good before you want to eat it. Those factors are coming into play here, and the net result is going to be an excitation of this system and so the young man will start to eat.
At this point, food has gone into the oral cavity and you're chewing the food and tasting it, and it's your sense of taste that now becomes the primary driver of eating behavior. In this care it's clearly appetitive taste. Things that taste good drive forward the eating behavior.
We think about the savory taste of the food, the saltiness of the food. There's also going to be fat in this food, so fat has a taste. You also think about the mouth feel of fat and how that's an important factor, as well as maybe even some sweet taste. There's a sweet taste for example probably in the pizza sauce. Most of our spaghetti sauces that we eat actually have high fructose corn syrup. All of these factors come into play. Now we take another piece and continue eating.
At some point we slow down a little bit and now we talk about some inhibitory factors that start to come into play. What's the major inhibitory factor involved in eating? It's going to be satiation. Satiety. Again, that's a complex concept. There are a lot of factors that go into satiety. We'll just call it one simple thing here, satiety. You start to feel full and that makes you want to stop eating.

I don't know about you but I often come upon this situation where I'm eating a food and I start to eat a whole bunch of it and start to get full of everything and I start to feel like I want to slow down and stop eating, but there's still something driving me to eat a little bit more. I still think about really good taste of the food that I'm eating.

That's what he's doing right here. He's wondering, "I feel full, but I just love the taste of this food. I just want a little bit more." So here we get to a point where we have a certain action that's going on, and that action is eating. Now we have sort of a battle between excitatory and inhibitory factors, and at this point one of the major excitatory factors is the sense of taste and that's kind of jousting with the inhibitory factor of satiety.
In this case he says "Ah, what the heck I'm going to go ahead and have another piece and finish off the meal'' and now he has overeaten. He doesn't feel so good anymore, so in this case he just ate too much of the food. The excitatory factors won out. This is just an example of how we can think about eating and overeating and the role taste plays in that.
Here's just sort of a schematic that goes over these concepts. It talks about excitatory factors, inhibitory factors. Notice that taste can not only be an excitatory factor, but also it can play a role as an inhibitory factor. You think about aversive tastes. You could be eating, but if you encounter something very sour or something bitter, you're going to stop eating, so it can play both roles.
So, what about the sense of taste? Why is it important besides for eating? It turns out that there is a very important clinical significance to taste. Sometimes we tend to think about the loss of taste or even the loss of smell is really more a quality of life issue rather than a serious health issue. It's true that a complete loss of taste, in other words what we call an ageusia, is going to be very rare, and the reason that a complete loss of taste is rare is because the taste buds in your oral cavity receive innervation from a bunch of different nerves, so it's really hard to damage all those nerves.
In one condition where you can get a complete loss of taste in your oral cavity is with radiation therapy, so when you have radiation therapy, for example for oral cancers, your taste buds will degenerate and then you can have a complete loss of taste. That's what this article up here is referring to, and then this article over here talks about a particular patient that had a certain surgical procedure and ended up losing all of his taste perception.
This subject basically stopped eating. He completely lost his appetite without the sense of taste. This goes to show you just how important the sense of taste is. If you don't have the sense of taste, you basically don't want to eat anymore.
Let's talk about how the sense of taste is organized a little bit. The picture on the right is a picture of a taste bud. A taste bud is a collection of about 50 or 60 taste receptor cells. These taste receptor cells extend the apical parts of their cells up into the oral cavity through the structure up here called a taste pore. This is where the taste receptors are located, and this is where the taste receptor cells will interact with taste stimuli that are dissolved in your saliva.
Notice the taste receptor cells are going to contact nerve fibers. These are peripheral nerve fibers that will carry the information from the taste cells into your brain.
Here's another picture showing a tongue, and here you can see the topography of taste buds on the tongue. You have these taste buds scattered all over the anterior tongue. Notice that you also have taste buds on your posterior tongue in a structure called the circumvallate papillae, as well as taste buds on the sides of your tongue in the foliate papillae.
You actually have taste buds in other places not shown here. You have taste buds all over your soft palate. You also have taste buds in your larynx, you have them on your epiglottis. Taste buds on your epiglottis are thought not to really participate in conscious taste, per se, but they play a role in protecting the airway from fluids.
In fact, you have taste receptor sells that extend all the way into your elementary canal, so you actually have taste receptor cells in your stomach and in your intestines, and here they're going to play a function that has to do with chemoreception; recognizing certain nutrients and toxins that you eat. Again, it's not a conscious taste reception, and not really part of your digestion, but more of a recognition of nutrients and toxins.
Let me just point out one more thing and that's that the taste map that you read about in old textbooks has shown to be incorrect. In other words, the idea that you only taste sweet things with the tip of your tongue, or sour things with the sides of your tongue. The truth is that taste buds all over your tongue and oral cavity can respond to all of the primary taste qualities, but there are some regional differences in sensitivity.
Another thing that I want to point out is you can think about taste receptors in the mouth, and then the gut system. The taste system and then your digestive system are playing parallel roles. One thing that's really interesting is that taste cells themselves have an endocrine function. They actually release particular feeding hormones, and can be stimulated by hormones.
That is what we call an anticipatory metabolic response. It's as if your taste system is priming the rest of your gut for the act of eating food and eventually absorbing nutrients. Another parallel which is what I just pointed out is the fact that you have taste receptors themselves lining your gut and the stomach and the small intestine where they play a role in nutrient detection and also this anticipatory response. You can think about these are parallel systems.

Here's taste pathways in the brain. This is actually a section from a rodent, but it will do for this purpose. Information comes in from taste buds through these peripheral nerves. These nerves are going to synapse in brain stem nuclei. There are autonomic nuclei in the brain stem.
Then from these brain stem nuclei, you get two very interesting pathways in the brain. You get one pathway where taste information goes to a structure called the thalamus, and then to the cortex. This is really sort of a pathway that you can think about being involved in the conscious perception of taste.
You also have all these other interesting major projections from the taste brain stem into these sub cortical areas, these areas of the limbic forebrain including your amygdala and your lateral hypothalamus. The lateral hypothalamus, you may know, is a very important structure in terms of eating.
Here you have a way by which taste information can directly engage these parts of your brain that are involved in motivation and reward. The amygdale also plays a very important role in learning. You can see there is a lot of interconnectivity between these areas.
Here's a diagram showing you some taste qualities. Most of you think about the four primary taste qualities. Sweet, sour, salty and bitter. You can see there are a few others on this diagram, and one of these is called umami. Does everybody know what umami taste is? I think that's sort of made its way into the popular literature at this point, but umami is really interesting. It's a Japanese word that means delicious.
Basically, what umami is referring to is really like a savory taste. You might think about chicken broth, or the taste of mushrooms. There has always been a lot of debate between scientists in the West and scientists in Japan. Scientists in Japan said there aren't four taste qualities, there are five. One of these is umami.
Western scientists said maybe we can explain this savory taste by a mixture of the other tastes. Maybe what you're calling a distinct taste is really just a mixture of sweet and salty and sour for example. About 12 years ago when scientists actually cloned the various taste receptors, they actually cloned a taste receptor that responds specifically to L- type amino acids including glutamic acid, and we encounter glutamic acid in our diet most commonly as monosodium glutamate. That's the savory taste we talked about.
We actually do have a primary savory taste that is called umami, and we have a specific receptor for amino acids. So, you think about the kinds of foods that contain these amino acids. You think about certain vegetables like tomatoes or mushrooms, but also foods in which proteins have broken down so you have a lot of free amino acids. Those are things like cooked meat and also fermented foods which humans really like, so think about cheese for example, or breads which contain yeast.
Sweet taste is the taste of sugars and some carbohydrates. Sour taste is evoked by acids like citrus fruits, lemons. Bitter taste is equivalent to the taste of toxins. Just about everything that's toxic to a human actually tastes bitter. That's our sense of taste telling us that these are foods that we should not ingest.
Of course, we can come to prefer these foods which is interesting, so we can tolerate and even prefer them. Thinks like coffee or chocolate. But again, it's the sense of toxic taste.
We also have a sense of fat taste. There is some controversy about this and we're not really sure exactly what the receptor is. Fat is also a very effective somatosensory stimulus in the oral cavity, so that may be one of the ways that it works.
We know something now about almost all of the receptors for these primary tastes. Humans are really good at discriminating the primary tastes. We do a great job of recognizing and telling the difference between each of these.
Can anyone think of the reason why humans are such good tasters? It really has to do with our diet, and it has to do with the fact that humans are omnivores and they're opportunistic eaters. We can eat so many different types of foods that we really need a sensitive taste system to be able to recognize and discriminate among these foods.
That's not the case for the whole animal kingdom. You can think about certain mammals that have very limited diets, say certain mammals that only eat certain types of plants. Those mammals have very limited taste systems in terms of their ability to discriminate.
Another good example are obligate carnivores like cats. While a cat only eats meat, it turns out that a cat does not have a functional sense of sweet taste. They can't taste sweet things at all. They don't need to because it's not in their diet. They have a sweet taste receptor that's in their lineage, but that receptor is a pseudogene now. It doesn't do anything; it doesn't work.
Notice that we not only can talk about tastes in terms of taste qualities and how we can discriminate them, but we can also describe basic tastes according to their hedonic properties. In other words, certain things taste to us inherently good, while other things taste inherently bad.
We can think about things that taste sweet. Sweet things taste inherently good or pleasurable to us. On the other hand, something that's very bitter we immediately think of as being very bad. We have these inherent properties and this is apart from our ability to discriminate them.
There was a really famous experiment done by Jacob Steiner where he looked at the responses of newborns to basic tastes, and that's an important thing about this experiment, the fact that these babies had never encountered or experienced these particular tastes before. They could not have learned any particular reaction to them.
What he found is that these infants had innate reactions to different taste qualities. He took a little eyedropper and gave them a bit of a taste. All the way on the left you can see babies in an arresting state, and then you can see when he gave them water, there really wasn't that much of a reaction.
But when he gave them something that was very sweet like sucrose, he did see a reaction. Here the infants basically lick their lips. They make suckling movements with their mouth. They really liked the sweet taste and they wanted to ingest it. There is an inherent quality that sweet tastes have.
On the other hand, for a sour taste they pucker their lips, didn't like it, bitter taste was awful. So here you have a baby howling and crying. Again, there's this innate value to the bitter taste. It turns out that these inherent properties of taste are actually things that are encoded way back in your brain stem. Experiments with animals have shown that you don't need to have a functional cortex to have these reactions.
So, the sweet taste is not only inherently good but it's also rewarding. Scientists who study reward talk about different properties of stimuli in turns of wanting or liking different stimuli. When something has a high hedonic value, you like that thing. When something has a high reward value, in other words when it's reinforcing, it's something that you want, you go out of your way to get it, and that's what we do with sweet tastes. We find sweet tastes very rewarding, something that we eat a lot of.
There's a reason for that, and that's because sweet tastes activate reward circuits in our brain. This is a picture from an article by [inaudible 20:42], it's a pretty well-known study. What they found is that sweet taste itself, in other words, in the absence of any kind of post-ingestive signal. Just the sweet taste itself was enough to cause a dopamine release in the reward system of the brain.
Here they're measuring dopamine release in this area of the brain called the nucleus accumbens, and after 20 minutes of stimulation with sucrose, they found a very large increase in dopamine in this area. They further showed that this was a property of the taste system by taking animals and lesioning the taste brain stem and showing that those animals, in other words, now they didn't have taste input to the reward system anymore, those animals did not have a very strong reward response. This experiment proves that taste directly activates reward systems.
Here's a diagram showing different reward areas in the brain, and you can see there is a complex intersection of not only taste processing, but also of reward activity in the brain, ultimately having an impact on feeding. There are all these different areas that play a role. Taste information gets to the reward system somehow. It wasn't exactly clear how that worked.
Notice also that the reward system is the same part of your brain that responds to drugs of abuse. Almost any drug that you can get addicted to, cocaine, alcohol, or amphetamines. All of these drugs have in common the fact that they cause dopamine release in this mesolimbic reward system.
We did some experiments in my own lab trying to understand the direct connection between the taste system and the reward system. What you're looking at here are nerve fibers stained in red. These are nerve fibers in terminals, and they're coming from the taste system. This is taste information.
Here we put a certain kind of neural tracer in the brain stem and we can trace out these neurons and see how they terminate in this part of the brain. This is a reward area of the brain called the ventral tegmental area. These neurons shown in green here are the dopamine-containing cells. These cells in turn will project to another structure called the nucleus accumbens where they'll release the dopamine and complete the reward circuit.
We found that these taste fibers had direct connections and made direct synapses with these dopaminergic cells in the ventral tegmental area. We described a short latency pathway by which taste information can engage this system.
Why should sweet tastes be rewarding? Why is it important that that's a sensory stimulus that evokes a reward response? It's very important for the human organism. Food sources, especially for children, it's very important that the intake of certain food sources gets rewarded.
You think for example right away about mother's milk. Mother's milk is high in sugars, so it's something that the infant needs to eat. You think about the good taste of fruits. These are foods that are good for you. It makes sense that your system would be set up in such a way to promote intake of these food sources.
The problem is that there is very little in nature that exclusively tastes sweet. That's why this reward system for sweet taste is so important because there are very few foods that purely taste sweet. You can think of maybe a couple. Honey. Very ripe fruits is another. But it's not a very common food source in nature.
It's actually very interesting. This is a very interesting video, The Botany of Desire, it's a PBS video that talks about apples and how apples have developed throughout human history. The original apples which were grown in central Asia were not very sweet. Most apples are not very sweet. Most of them are sour and bitter, they have some aspect of sweetness.
In fact, in times when colonists in the United States, when settlers were expanding across the country, they planted a lot of apple orchards at that time. You guys have heard about Johnny Appleseed. It turns out that the apples were predominantly used at that time to make hard cider, an alcoholic beverage. They really weren't eaten for their sweet taste, but then later, they became to be bred exclusively for sweet tastes.
So now you have a situation where you go into a grocery store, you don't have that many varieties. In nature, there are a lot of genetic-based varieties of apples, but in the store, you have your pick of many six or seven different common types, and what they have in common is that they're all very sweet. They've been bred to be very sweet.
Of course, we encounter foods all the time that are very sweet and are not representative at all of the types of foods that we would encounter in nature. You think about junk foods, sugary sodas, candies, snack cakes. In fact, we can think about these junk foods as super stimuli for the taste system.
There are things that are very, very effective in stimulating eating because they stimulate these reward pathways. They have a very strong hedonic value. These are not foods that our bodies were necessarily designed to eat, and so that's where the problem comes from. That's something that definitely drives overeating.
It's not only true just for sweet tastes, but it's also true for savory tastes. Umami taste, fat taste, salt taste, you think about French fries. All of these things are overwhelming your taste system.
In fact, food companies understand this very well and so their goal is to try to design foods that they can sell to you that optimally stimulate your taste system so as to promote as much intake as you can possibly give them so you'll eat more and more of their foods.
Here's a really interesting article from the New York Times that talks about this called The Extraordinary Science of Addictive Junk Food, and they talk about all the factors that go into making the perfect snack chip that you'll just eat as much as possible of, and so you'll think about the salt content, the fat content, the crunchiness, the mouth feel of the chip.
There's something that food scientists at these companies call the bliss point. They're always trying to achieve the bliss point. The food that will make you eat as much of it as you possibly can. They're not too shy about it, and you can take a look at this quote. They feel like they're doing what they have to do. "If that's what the consumer wants, we're not putting a gun to their head to eat it. That's what they want. If we give them less, they'll buy less and the competitor will get our market.'' So they're out to sell as many junk foods to you as possible.
Let's talk about a couple of other factors that play a role in taste and eating. One of these very important factors is taste- based learning. There are different types of taste learning. One of them that's a very powerful and interesting example is taste aversion learning.
You may have experienced this in your own lifetime before where you eat a particular food and then you get a food sickness, you get a stomach bug or food poisoning, and so you come back the next day and someone presents that food to you, you're not going to want to eat it. The whole thing just makes you sick, you want to keep it away from you.
In fact, studies in animals have shown that not only is that a visual and olfactory thing, but it's also a taste thing. The taste of the food itself actually changes from being something that was positive to something that is now negative. That's a very strong form of taste learning.
You can also talk about more gradual learning processes that take place over the lifetime. In other words, you can come to change your dietary habits a lot with experience.
A good example of that is how we sometimes consume foods that should normally be aversive. Certain bitter things like for example coffee. Everybody likes their coffee from Starbucks, and they like it really bitter. You really shouldn't like bitter tastes, but here's an example of where you've learned to like that taste and you've learned to actually prefer it.
Let's talk about taste-guided learning in children. We said earlier that children have an innate preference for sweet-tasting foods. We saw that in the pictures of the infants. We know this because sweet taste is hedonically positive; it activates the reward system.
If you give a baby a choice of sweet foods and non-sweet foods, the baby will always choose the sweet foods. They have this innate preference. But studies have shown that repeated exposure and experience with these types of foods is the best predictor of liking and food acceptance later in life.
You think about the things that you're feeding your kids at a very young age. Apple juice for example. Apple juice is something that tastes extremely sweet and in fact it's a beverage that's full of sugar. It doesn't have all that many health benefits to it. All the fiber in the fruit is gone when you get to the juice. It's really just a big sugary drink, yet this is something that give our kids to drink all the time.
Snack crackers. Goldfish crackers are high in salt, they're probably high in umami taste and so we're setting the stage at this age for our children to grow up and then prefer these types of food later on.
It's not only the fact that we can prefer individual tastes later on, or learn to prefer individual tastes, but there is also an interesting thing to consider with the overall food environment. In other words, the way we put different tastes together in particular meals.
We learn these different habits. We have a very peculiar habit in our Western diets where, for a lot of meals, we like to put together the taste of whatever primary food we're eating, like this sandwich here. The taste of meat, vegetables and bread, and for some reason we always have to put it together with something that's very sweet.
We can't have this sandwich without a big sugary drink, or a cookie afterwards, and this is another habit we teach our kids at a very young age, and it's clearly a habit that we maintain into adulthood.
So we go to a restaurant, we order a meal, and we just can't have that meal without a huge glass of sugary sweet tea. Maybe not just one glass, maybe two or three glasses. The waiter comes by with the pitcher and keeps refilling it. These types of habits are set very early in development.
A few conclusions, the taste system is organized as I said for optimal detection and intake of crucial food sources; however, we overwhelm this feature of our taste system with modern high-caloric and low-nutrient junk foods that have been specifically designed to take advantage of the fact that we like them.
Finally, as we've just looked at some examples of, early taste-based food selection can lead to long lasting poor dietary habits. I think that's it. Thank you.

Bill: Are there any questions for John?

Dr. Boughter: Yes?

Audience question: Is it true that your taste preference changes?

Dr. Boughter: Your taste preference does change. I don't know if it's every seven years, but children tend to prefer sweet things, for example, more than adults do.

Audience question: Okay.

Dr. Boughter: So the question was, is it true that your taste preferences change as you grow older, and the answer is yes. Adults will tend to eat other sorts of things, more bitter-tasting things. Yes?

Audience question: Noticing the bitter food group, that pie-shaped chart had broccoli in it.

Dr. Boughter: Yeah.

Audience question: And coffee, and I'm sure that most of the people in this room ingest both of those.

Dr. Boughter: Right.

Audience question: Could you comment about if that's nature's way of saying this is not good for you? Is that what I understood? And if so, what are we to deduce from that?

Dr. Boughter: Cruciferous vegetables like broccoli do have a high level of bitterness, or a certain level of bitterness, but not so much that you can't eat them. Basically, every food you eat has some level of toxins in it. It just depends on how much there is to trigger a bitter taste response. There's always some trace level.

Audience question: Your recommendation is?

Dr. Boughter: My recommendation is you can eat those foods. Yeah.

Bill: Any other questions?

Audience question: So, by eating sweet foods, foods you really like or enjoy, do you enhance that pathway? Do you increase the number of taste buds for that food?

Dr. Boughter: You don't necessarily increase the number of taste buds, and there is some debate as to exactly what that mechanism is in terms of enhancing, for example, the reward pathway. It's not completely clear yet how that works, but that's a pretty good implication. Yes?

Audience question: You know how you can damage your gums from brushing too hard? Is it possible to damage your taste buds?

Dr. Boughter: One great thing about your taste buds is that the cells in your taste buds turn over every seven to nine days, and so you're constantly regenerating a new set of taste buds. You can think about times when you burn your mouth, and you lose your sense of taste in that spot for a little while, but eventually it comes back. So long-term damage is probably not something you'll get there.
There are certain dental procedures that can affect your taste though, especially dental surgeries and things like that. Things that can cause permanent nerve damage.

Audience question: Can that turn over affect your preference?

Dr. Boughter: I don't know that it has any specific effect on your preference. The turnover is pretty rapid, so again, we're talking all the cells turn over about every seven to ten days.

Audience question: You said that there were drugs that were being used, are they targeting those reward systems?

Dr. Boughter: They are, yeah. They're targeting that reward system, so you think about the natural purpose of those reward systems. Eating and tasting is one of the natural purposes of that system. Absolutely.

Audience question: Evolutionarily, has there been change in any particular taste receptors just in humans alone? I know you said there are animals that have different distribution preference, but in humans alone have there been any evolutionary changes of diet?

Dr. Boughter: That's a really good question. I'm not really aware of any specific changes in taste receptors in humans.

Audience question: Also, genetically, I think one of the things was that Native Americans could not digest sugar, is that why there is an increased incidence of diabetes in this group? There's some data about that. Genetically they could not metabolize certain sugars.

Dr. Boughter: I don't know the answer to that.

Audience question: Thank you.

Audience question: [inaudible 35:59]. I grew up in Costa Rica, and it took my attention when you said that sweet foots rarely are in nature, because growing up in Costa Rica, we were so close to the plants and the trees of the fruits, and you actually get them from your backyard. I remember most of the fruits that I ate in my childhood were sweet. So I have a question if you have maybe some kind of study that proves that sweetness is nothing to your nature?

Dr. Boughter: Well it's certainly something that I've read and seen in different places before. Certainly ripe fruits is one category of foods that taste sweet. But the problem now is that so many foods taste sweet, almost everything you eat has some degree of sweetness. It's really interesting to think about.
I thought about another peculiar dietary habit for me is the fact that we come to associate sweet tastes with breakfast foods. I was sitting around eating some yogurt the other morning. That's a healthy thing to eat for breakfast, right? It's intensely sweet, high in sugar. If it's not yogurt we're eating, we're getting a sweet drink from Starbucks, or we're eating a muffin or a pastry, or doughnuts. We come to associate this with breakfast.

Audience question: There is probably a cultural difference here because I grew up in a culture where something salty is for breakfast, or at least in my family, so it may be that the culture is completely different.

Dr. Boughter: Yeah, definitely. In a lot of cultures in Europe the do not eat sweet things.

Audience question: It seemed like you were saying that sweets cause dopamine release, and you think of dopamine as one of the neurotransmitters that you need to get increased release, for example in kids with ADHD, but there are all these wives tales about not giving a low sugar diet to kids with ADHD, where this would imply that a high sugar diet would increase dopamine and better treat ADHD.

Dr. Boughter: That's interesting. I wasn't aware of that link between ADHD and dopamine.

Audience question: Most of the drugs for ADHD target dopamine.

Audience question: This deals with the dopamine release. Often when infants are just first born, in the hospital they will give sugar solution, sucrose, if there is a painful procedure. Is that why they do that?

Dr. Boughter: Actually, that has to do with opioid release. Sweet also activates opioid pathways, especially in an infant, and so there is actually a sweet taste projection--I was seeing if Dr. [Inis] was still here. This is actually something he studied. Sweet taste information projects to a pain related part of your brain called the periaqueductal gray, and so there is opioid release with sweet.

Audience question: Cool, thanks.

Bill: We'll go on and we'll leave time to ask John any other questions at the end after Amanda. So just go ahead now. I think we've all had the taste aversion experience.
A personal story is for years I couldn't look at a picture of Bacardi rum. I'm not going to tell you when I learned that association. There used to be a lot more liquor advertisements in magazines, so it was hard to look at Life magazine as a kid . . . oh, as an adult.