Sure, Sugar’s Bad For Your Teeth, But Not How You Think; Ungar Explains Problem With Tooth Decay
Matt McGowan: This is Short Talks From the Hill, a podcast of the University of Arkansas. My name is Matt McGowan. I’m a science writer here at the university. Today I’m talking to Peter Ungar. Ungar is a Distinguished Professor of anthropology and director of the Environmental Dynamics Program at the University of Arkansas. For more than 30 years, Ungar has studied teeth and the diets of modern primates, early hominins and the mammals that coexisted with them. He has authored or co-authored more than 200 scientific papers, and he is the author of three books, Mammal Teeth: Origin, Evolution and Diversity; Teeth, a Very Short Introduction; and, most recently, Evolution’s Bite, a Story of Teeth, Diet and Human Origins. Welcome, Peter, thank you for being here.
Peter Ungar: Thank you.
MM: In this month’s issue of Scientific American, you published a story titled “The Trouble with Teeth.” You explain the evolution and composition of human teeth? So, I want to read a paragraph from the article if you don’t mind. How do I pronounce it? Caries? Which you’ve already defined as cavities?
PU: Well, caries is a process. It’s a disease. Cavities is how it manifests itself.
MM: Okay. All right, so what you wrote is this: “Dental caries is the most common and pervasive chronic disease in the world. It afflicts more than nine out of 10 Americans and billions of people across the globe. Yet over the past 30 years that I’ve studied hundreds of thousands of teeth of fossil species and living animals, I’ve seen hardly any tooth decay.” That’s pretty astounding. What is going on here?
PU: In a word, sugar. Not how your listeners might think that works, though. Sugar doesn’t rot teeth. Bacteria rot teeth. Basically what happens in your mouth, as in your gut, you have a microbiome. There are up to 700 species of bacteria that live in your mouth. Most of those are beneficial. They help you digest food. They fight disease and invaders, but some of them are harmful. And for your mouth that includes things like streptococcus mutans, lactobacillus, several different species, and what happens is they ferment carbohydrates, which essentially means that they use these for their own food, and the metabolic waste – their poop basically – is lactic acid. And that’s what really rots your teeth. So they do a really good job of fermenting sugars. And not only that, the sugars allow them to produce a side material that’s basically glue that binds these bacteria to the actual teeth. So when you combine binding bacteria to teeth, these are bacteria that produce acid. Plus the soft food that we’re eating is not wearing away the organic film that covers the teeth, so that keeps the bacteria on the teeth. When you combine all these things, it’s… it fuels the fire for the caries problem that we have.
Now, I will say that our ancestors and, in fact, all mammals today, as far as we know, have both beneficial and harmful bacteria. Your dentist will tell you that streptococcus mutans is sort of the satanic bacteria that rots your teeth. It’s true. But all mammals, to the best of our knowledge, have a acidogenic, that is, acid producing bacteria. The problem is when we put too much sugar into our mouths, those bacteria proliferate. And when they produce acid, they’re also very good at tolerating and surviving in an acidic environment, whereas the beneficial bacteria are not.
Now, in other animals and in our ancestors, the ratio of good to bad bacteria was much higher. There were many more good bacteria, and those were actually able to fight the bad bacteria. This is what we call the caries balance. When this balance gets disrupted, by consuming sugar, and allowing the bad bacteria to sort of overwhelm the good bacteria, that’s when we have a problem. And in fact, one might argue that that’s the reason that some people are susceptible and always seem to get cavities, whereas others don’t. It’s probably related to this bacterial load.
MM: One other thing that I read in the story that I thought was fascinating, also our diet… recent changes in our diet have also affected our palate. Could you talk a little bit about that?
PU: Well, not only do nine out of 10 of us have a problem with caries, nine out of 10 of us have a problem with occlusion. That is, the interactions of our upper and lower teeth with one another. Right? I bet that most of your listeners have had either impacted or removed wisdom teeth. The upper teeth jut out in front of the lower teeth rather than coming in contact tip to tip and in fact, most dentists believe that that’s the normal type of occlusion, but it’s not. And most of your listeners probably have lower front teeth that are pushed together crowded and twisted. We don’t see that in most other animals. Okay, occasionally domesticated dogs But typically we don’t see that in other animals. We don’t see that very often in human ancestors, either. So yes, what’s going on here? It turns out that the problem is that there’s a mismatch between the sizes of our teeth and the length of our jaws. We used to think that our teeth were simply too large to fit into our jaws, because historically we wear our teeth down… and we eat soft, mushy food that’s clean now, so we don’t wear our teeth down. And so our teeth are too big to fit in our jaws. Well, it turns out that that’s probably not true. The reality is, we’re actually not growing long enough jaws to fit our normal teeth. And as a result, they’re crowded in the back, crowded in the front, because our jaws just aren’t long enough to accommodate them. Now, why aren’t our jaws long enough to accommodate our teeth? Well, again, it turns out that in order to grow bone, you need to stimulate it during development. You need to chew, and you need to chew vigorously. Right? So when you feed your kids mashed peas, or you cut their meat to into tiny little pieces, you’re not giving them the opportunity to chew enough. The cells that secrete bone in your jaw, the osteoblasts aren’t secreting the bone that they need to secrete to make your jaw long enough to fit your teeth. You cannot change the size of your teeth. Those are genetically programmed. Your jaw length is not. So this mismatch develops, and we see not enough room in the back for your third molars. You get impacted wisdom teeth. Not enough room in the front, the lower teeth get pushed together, the upper teeth shut out in front of the lowers. The traditional solution to this was based on this misunderstanding that our teeth are actually too large for our jaws rather than the other way around. And so, rather than wearing our teeth down, orthodontists used to pull out the front premolar teeth, and then sort of stick a wire around the remaining teeth and pull them back into line. Works, but that’s really bad. Because what it does is… it restricts your airway. There’s not enough room in your mouth for your tongue. And many have argued that this has actually led to the apnea epidemic. And so it’s a much better solution to actually grow your jaw longer. And orthodontists are actually starting to put spacers in to the jaw to stimulate growth, when you’re a child during the time that that that bone is actually developing. So this is a way that we can actually use evolution and human evolution to gain practical insights that have clinical significance.
MM: And this can actually happen and within one’s lifetime, especially when a human being is young, during development. If one chews more, you can actually grow the bone, extend the bone during development.
PU: That’s right. It has to be before you’re about 18 when the jaw growth stops.
MM: Back in February, you and several co-authors published a paper about the teeth of very old dogs. This research has contributed to a debate about when humans domesticated dogs. I’d like for you to talk about… I want you to talk about the research and the study and what you all found. But first, can you talk about this debate? It’s somewhat controversial, just to provide a little bit of context.
PU: Sure. There’s basically two models for the domestication of dogs. Dogs are the very first animals that we domesticated, and we love them. So they’re important to us. According to the first model, dogs became domesticated with the origins of agriculture, when people settled down. People settled down, wolves sort of come into the peripheries of the settlements, and slowly but surely they’re welcomed in and they become domesticated and our pets. The second model is that it happened way before then. So, according to the first model was about 10,000 years ago, according to the second model, it was 30,000 years ago, three times earlier in time. According to the second model, dogs were initially domesticated to help with hunting, to help corral animals to help identify or find animals. And this has been a subject of a huge debate. There are geneticists who’ve worked on this for a long time, there are questions about how many times dogs were domesticated, where they were domesticated, on and on and on, people seem to care about this. And actually, there was a book recently written that even argued that the domestication of dogs was critical to our evolution and the extinction of the Neanderthals, because it gave our ancestors a competitive advantage in hunting over the Neanderthals. So, this is a meaningful debate, and it’s one that people seem to care about. So basically, what I looked at was a site that’s about 28,500 years old called Predmosti in the Czech Republic and Predmosti is one of these sites that sort of highlighted in this debate, because it’s among the oldest places where we have what have been called doglike canids. Canids are the group that includes wolves and coyotes and dogs and so forth. But there’s some speculation as to whether these really are proto dogs or not. Basically what you’ve got is two different types of canids from this one site. And there are a lot of them. Some of them have really long and thin jaws and their teeth are well spaced out, which is what wolves look like today. Others have short, compressed jaws, much thicker jaws, and their teeth are pushed together, which is more what dogs look like today. The basic idea has been that the more dogs-like ones had evolved for crushing hard foods.You want a thick, narrow jaw or short, thick jaw, if you’re going to generate forces for crushing hard foods like bone, whereas a wolf will take more soft tissue.
MM: And why would the dog-like canids had access to bones?
PU: Well, basically, if you are helping somebody hunt, the hunter is going to get the meat, going to get the soft stuff, the good stuff, and it’s just going to be the bony scraps that end up in the dog’s mouth. So the hunter will help kill the animal. The dog will help the hunter get the meat, and in return the hunter will give the scraps to the dog. And that’s the basic model for the evolution of dogs, if you accept the early model. So essentially one thing that… I study something called dental microwear, and dental microwear, if it’s good at anything, it’s good at telling the difference between foods that are eaten that are hard and brittle versus foods that are eaten that are soft and tough, which would correspond to bone and meat. So I figured… my hypothesis would be that the dog-like forms, if they were in fact eating bone, would have larger, deeper pit-like features, and the wolf-like forms would have wispier, scratchier features, and that’s pretty much what we found. I’m not going so far as to say that that’s evidence for domestication. But I do believe that that’s evidence for two… at least two different populations of canids, one with a more dog-like adaptation, the other with a more wolf-like adaptation.
MM: Peter, thank you so much for being here.
PU: Thank you, Matt.
MM: Music for Short Talks From the Hill was written and performed by local musician Ben Harris. For more information and additional podcasts, visit researchfrontiers.uark.edu, the home of research news at the University of Arkansas.