It's common knowledge that sugar causes cavities, but new research provides evidence that - depending on your genetic makeup - starches could also be a contributing factor.
The study, published Feb. 19 in Microorganisms, explores the response of the oral microbiome to starch, finding that the number of copies of a particular gene, AMY1, in combination with starch, alters the complex composition of bacteria that play a role in oral health.
"Most people have been warned that if you eat a bunch of sugar, make sure you brush your teeth," said Angela Poole, senior author and assistant professor of molecular nutrition in the College of Agriculture and Life Sciences and the College of Human Ecology. "The takeaway finding here is that depending on your AMY1 copy number, you may want to be just as vigilant about brushing your teeth after eating those digestible starches."
Researchers, including first author Dorothy Superdock, Ph.D. '23, collected saliva samples from 31 subjects with a range of AMY1 copy numbers - copies of the AMY1 gene in the DNA - and added starch to the cultured samples, or biofilms, to see how the bacterial makeup changed. They found that, in general, the diversity of bacteria decreased when starch was added. For those samples with high numbers of AMY1, the starch significantly reduced the proportions of two bacteria, Atopobium and Veillonella, while Streptococcus appeared to increase.
All three bacteria are associated with tooth decay or gum disease, Poole said.
"Some increased and some decreased, so it's not so straightforward as saying, 'The whole thing is good or bad,'" Poole said. "It's an interaction, but it looks like the AMY1 copy number, as well as which species are present in people's mouths when they eat starch, is affecting the risk for developing these diseases."
AMY1 codes for the salivary amylase enzyme, which helps break down starch in the mouth. Previous studies have associated AMY1 with cavities and periodontal disease. Poole, in prior studies, found that a high AMY1 copy number is associated with higher levels of the species Porphyromonas endodontalis, which is strongly associated with periodontitis and gum disease.
But how the salivary amylase enzyme interacts with its main substrate, starch, to alter the oral microbiome and increase disease risk was unclear.
"That's what we wanted to know in this experiment," Poole said. "What's going on in the mouth if someone eats starch, and is the answer different if their copy number is high or if it's low? What we found was that there are other bacteria involved in these processes and that the changes depended on AMY1."
The researchers also found evidence that the oral microbiome has co-evolved in response to increasing copies of AMY1, which is found in higher numbers in populations where there's a long history of agriculture and starch consumption. In the pool of 31 samples, taken locally in Ithaca, the AMY1 number ranged from two to 20 copies.
"The populations that historically had greater access to starch tend to have more copies," Poole said, "which makes sense from a practical standpoint, because it would have given you a survival advantage when food is scarce, to be able to break down those starches more efficiently."
In saliva samples with a high AMY1 copy number, the researchers saw increased populations of bacteria, like Streptococcus, that feed off the starch's sugars.
"If someone has a high copy number, they break down starch efficiently, and bacteria that like those sugars are going to grow more in that person's mouth," Poole said. "So you can have species behave differently based on the different substrates. It's pretty incredible - how we adapt and these microbes turn around and adapt, too."
Co-authors of the study include Lynn M. Johnson, director of the Cornell Statistical Consulting Unit; doctoral student Megan Eno '22; former lab manager Jennifer Ren '19; and Alizeh Khan '22 and Shuai Man '23, M.A. '24.
Funding was provided by the Schwartz Research Fund and the National Institutes of Health.
