A Million Years of Cavities
Cavities are older than our species. Over a million years ago, early human relatives such as Paranthropus already had them. Decay grew in the Neolithic period, when people ate more plant foods rich in carbohydrates.
People also tried to fix it very early. In Pakistan, teeth from around 7000 BC to 5500 BC show nearly perfect holes made by simple dental drills. A Sumerian text from 5000 BC blamed a "tooth worm". During the European Age of Enlightenment, Pierre Fauchard, known as the father of modern dentistry, was one of the first to reject the worm idea. He also noted that sugar harmed the teeth.
Sugar kept changing the story. Decay jumped around 1000 AD, when sugar cane became easier to get in the Western world. It jumped again in the 19th century, when cheap refined sugar and flour turned many workers' meals into bread, jam and sweetened tea. Today 3.6 billion people have decay in their permanent teeth.
Tiny Acid Factories
Tooth decay, also called caries, needs four things: a tooth surface, the right bacteria, sugars they can ferment, and time. The bacteria live in dental plaque, a sticky, creamy-colored film that keeps forming on teeth. The leading culprit is a germ called Streptococcus mutans.
When you eat sugar, these bacteria turn it into acid, mostly lactic acid. That acid pulls minerals out of the tooth. About 96% of enamel is minerals, and it starts to dissolve at a pH of 5.5. Table sugar is especially bad. The germ uses it to make a very sticky substance called dextran, which helps it cling to the tooth.
The good news is that the damage does not only go one way. Saliva can neutralize acid, and minerals can return to the tooth in a process called remineralization. Decay only wins when minerals leave faster than they come back.
How Often Matters More Than You Think
After every meal or snack, mouth bacteria turn sugar into acid and the pH drops. Saliva slowly brings it back to normal. But during each acid attack, some minerals at the tooth surface dissolve and can stay dissolved for two hours. That is why how often you eat sugar matters so much.
The amount matters too. Cavities rise steeply as people eat more free sugars. In one population in Nigeria, where people ate about 2 grams of sugar a day, only two percent of people of any age had ever had a cavity.
Some foods are sneakier than others. Chewy and sticky foods such as candy, cookies, potato chips and crackers cling to teeth for longer. Raisins, apples and bananas leave the mouth quickly and do not seem to be a risk. People are also bad at guessing which foods stay behind. Letting a child fall asleep with a bottle of sweet drink is a classic cause of early childhood caries.
The Tooth Fights Back
The first sign of decay is often a chalky white spot on the tooth. At this stage the damage can still be stopped, and fluoride can help the surface take minerals back. Oddly, pressing such a spot with a sharp dental tool could create the very cavity it was meant to find, so some researchers warn against it.
Deeper inside, the tooth is not helpless. Enamel cannot regrow, because the cells that make it are gone once it is finished. But dentin, the layer underneath, is made throughout life. When decay arrives, the tiny channels in dentin narrow and fill with crystals that slow the bacteria down. The tooth can also build a fresh layer of dentin toward its center, at about 1.5 micrometres a day.
These defenses block the fluid thought to trigger pain signals, so early decay may not hurt at first. Sometimes the outer enamel even hardens again over hidden decay underneath, which only an X-ray can still reveal.
Fluoride, Sealants and a Clever Trap
Brushing and flossing remove plaque, but they cannot reach food forced deep into the grooves of back teeth. Those chewing-surface cavities make up 80 to 90% of cavities in children. Fluoride does better. It binds to the crystals in enamel, making them harder for acid to dissolve, and it cuts cavity rates by about 25%. Experts suggest brushing twice a day, including right before bed, then spitting out the extra paste instead of rinsing.
A dental sealant is a thin plastic-like coating put on the chewing surfaces of molars so food cannot get trapped there. For children, one clever method seals decay inside the tooth with a ready-made steel crown. Called the Hall Technique, it starves the bacteria of food and needs no local anesthetic injections.
The idea is like placing a glass over a candle. The flame burns out once the oxygen is used up, and sealed-in decay stalls in a similar way.
