1. Did People Once Believe Worms Caused Cavities?
Long ago, people believed small worms lived inside teeth and gnawed holes into them. This idea of "tooth worms" is said to be very old. In 1890, W. D. Miller presented an explanation that bacteria in the mouth ferment sugar to create acid, and that this acid dissolves the tooth [8]. The basic structure of this explanation is said to still be accepted today.
Current official explanations are almost the same. The phenomenon where bacteria in the mouth release acid to dissolve teeth is called "demineralization" [1]. So, where and how is this acid created?
2. How Bacteria in Plaque Turn Sugar into Acid
The main bacteria that cause cavities are Mutans streptococci and Lactobacilli. Both have the power to make mainly lactic acid from sugar [1]. Mutans streptococci have the ability to create plaque (a sticky film), which does not dissolve in water [1]. Inside plaque, acid tends to stay close to the tooth surface.
The enamel on the tooth surface is said to be the hardest tissue in the human body. It is made almost entirely of hydroxyapatite, a crystal of calcium phosphate [9]. The exact percentage varies by source, so remembering "almost all" is sufficient. These crystals dissolve when the acid becomes strong.
A rough guide is the acidity (pH) of the plaque. It is explained that tooth components dissolve when plaque pH drops below about 5.5, and return when it rises [3]. This is not a fixed boundary but a rough estimate. If there is a boundary for dissolving, there must also be times when it stays above that boundary. What happens to the tooth after it dissolves?
3. Remineralization: How Dissolved Tooth Parts Can Return Before a Hole Forms
In the early stages, it is known that dissolved parts can return to their original state. This phenomenon is called remineralization. Teeth constantly repeat demineralization and remineralization [1]. Cavities are not holes that appear suddenly. They are explained as the result of "melting" winning the competition for a long time.
Saliva plays a key role in returning the tooth. Saliva contains calcium, phosphate ions, and bicarbonate ions. It neutralizes acid and helps repair enamel through remineralization [4]. After eating or drinking ends, new saliva comes out, and repair progresses.
In early cavities, demineralization is said to occur not on the very outer surface of the enamel, but slightly below the surface layer (subsurface demineralization) [5]. Calcium and other substances from saliva enter and fill these spaces, which is remineralization. This means the surface may look safe, but the inside may already be dissolved. There is a stage where it can still return. However, once a hole has formed, the tooth does not return naturally on its own. If you are concerned, please see a dentist.
If there is a force to return, why do cavities still happen?
4. Why How Often You Eat Sugar Matters More Than How Much
If you put sugar in your mouth many times, or do not brush your teeth, demineralization progresses [1]. If you eat and drink continuously without set times, the mouth stays acidic for a long time. It is explained that remineralization cannot keep up.
A graph showing plaque pH measured over time is called the Stephan Curve. When sugar is taken into the mouth, pH drops, and it is said to return to normal after some time [3]. The specific time changes depending on the food, the person, and the conditions.
In this view, the problem is not just the "amount" of sugar, but the "frequency" and "time" of becoming acidic. If you finish eating in one go versus eating little by little continuously, the time your teeth spend on the "melting" side can change, even if the amount is the same.
5. How Fluoride Toothpaste Helps Teeth Repair Themselves
Fluoride-containing toothpaste is the most widely used method of fluoride application in the world. There are reports that it reduces cavities by about 24% [2]. Fluoride is explained to have effects such as increasing the acid resistance of tooth structure and promoting remineralization.
This 24% is an average value from multiple studies. It does not mean cavities will "always decrease by 24%." Please read the number as a rough guide.
A review in Nature Reviews Disease Primers states that the daily use of fluoride toothpaste is one of the main factors in countries where cavities have decreased over the past few decades [5]. If you think of this as help joining the "returning" side in the competition between "melting" and "returning," the result makes more sense.
6. What 4,000-Year-Old Teeth Reveal About Cavity Bacteria
In 2024, a research team including Trinity College Dublin reported that they assembled the ancient genome of Mutans streptococci for the first time from teeth found in an Irish cave. These teeth are from the early Bronze Age, about 4,000 years ago [6]. Note that this does not say "people 4,000 years ago had cavities." What was found was bacterial DNA.
The same study reported signs of an increase in this bacterium's lineage that overlaps with the period when sugar spread during the Industrial Revolution in the 18th century. The content of the genes also changed [7]. "Overlaps" means the timing matched. It does not assert that sugar was the only cause.
The story of cavities is made up of bacteria, food, and the competition of the tooth. What can you check in your own daily life?
7. How to Check How Often You Eat and Drink Each Day
Try writing down everything you put in your mouth today on paper, with the time. Include meals, snacks, sweet drinks, candy, and gum. Mark each entry as a chance for plaque to become acidic, and count them.
Compare a day with 3 meals to a day with continuous snacking. Differences in frequency and intervals can be seen. However, actual pH changes differ by food and person. This is a practice in thinking, not a measurement of pH change or demineralization.
The Ministry of Health, Labour and Welfare's e-Health Net has pages on how cavities form and fluoride toothpaste. The study on 4,000-year-old bacterial genomes was published in Molecular Biology and Evolution (2024). Tooth health varies from person to person. If you are concerned, please consult a dentist or dental hygienist.