Fundamentals
How cavities form
A cavity is not something you catch. It is a hole that forms slowly, over months, because acid dissolved more mineral out of your tooth than your saliva could put back.
Where does the acid come from?
Your mouth is full of bacteria. That is normal, and most of them are harmless. But when you eat something sugary or starchy, some of those bacteria feed on it and give off acid as waste. The acid sits against your teeth in a sticky film called plaque.
What the acid does to a tooth
Tooth enamel is almost entirely mineral. Acid pulls that mineral out. Saliva puts it back. This trade runs in both directions all day, every day. A cavity only appears when the losing side stays ahead for months.
That is why early damage can heal. Before there is a visible hole, the tooth has only lost mineral from just under the surface, and saliva can still refill it. Once the surface caves in and a real hole opens up, plaque hides inside where a brush cannot reach, and only a dentist can fix it.
Why snacking matters more than portion size
Every time you eat sugar, acid builds up within about 3 minutes. Your saliva then needs 30 to 60 minutes to clear it. That clock runs the same length whether you ate one piece of candy or twenty.
So a handful of candy eaten in one go costs your teeth roughly one acid attack. The same handful eaten one piece at a time across an afternoon costs you several. How often you eat sugar does more damage than how much.
Saliva is the repair system
Saliva neutralizes acid, rinses sugar away, and carries the same minerals your teeth are made of. It is doing the repair work in that trade. Saliva flow almost stops while you sleep, which is why a sugary drink right before bed is harder on teeth than the same drink at lunchtime.
What this means for your teeth
- Snack less often. Spacing sugar out through the day is worse than eating it all at once.
- Nothing sugary after you brush at night, when saliva is at its lowest.
- A chalky white spot on a tooth is an early warning, not permanent damage. At that stage the tooth can still repair itself.
- If your mouth is often dry, from medication or anything else, your risk goes up. It is worth mentioning to a dentist.
Read the full evidence review The mechanism in full, with 9 cited sources and a plain account of where the evidence is weak.
Dental caries is the most common chronic disease on earth, affecting more than two billion people in their permanent teeth.1 It is also widely misunderstood. A cavity is not something you catch, and it is not a hole that simply appears. It is the visible end point of a chemical balance that tips back and forth many times a day: mineral leaving the tooth when acid is present, mineral returning when it is not. A cavity forms when, over months, the leaving outpaces the returning.
Key points
- Enamel dissolves when the pH at the tooth surface falls below roughly 5.5, and rebuilds when it rises back above that threshold. A cavity is the cumulative result of that ledger running negative.
- Bacteria in dental plaque ferment dietary sugars into acid within two to three minutes of exposure, and plaque pH takes roughly thirty to sixty minutes to recover.
- How often you eat sugar matters more than how much, because each exposure buys a full acid episode regardless of size.
- The first stage of a cavity is a subsurface lesion, where mineral is lost beneath an intact outer layer. At that stage the process can still reverse.
- Caries is now understood as an ecological shift in a normal resident community rather than an infection by a foreign organism, which is why broad antibacterial approaches have performed poorly.
What a cavity actually is
Tooth enamel is about 96 percent mineral by weight, and that mineral is a calcium phosphate called hydroxyapatite. It is not inert. Sitting in saliva, it is in constant chemical exchange with the fluid around it, losing ions to solution and gaining them back.
Whether it loses or gains depends on whether the surrounding fluid is undersaturated or supersaturated with respect to that mineral, and the single variable that moves this most violently is pH. As acidity rises, the concentration of free phosphate and hydroxide ions in solution falls, the fluid becomes undersaturated, and mineral dissolves out of the crystal lattice to compensate. When acidity falls again, the fluid returns to supersaturation and mineral is redeposited.
The important consequence is that early caries is not a hole. Acid diffuses into the porosity of enamel and dissolves mineral from beneath the surface while the outermost layer stays relatively intact. The result is a subsurface lesion, visible clinically as a white spot. A cavitated hole appears only later, when so much subsurface mineral has been lost that the surface collapses into the void. Everything before that collapse is, in principle, reversible.
Critical pH, and why 5.5 is a useful lie
The figure usually quoted is that enamel begins to dissolve below pH 5.5. That number is a reasonable working threshold and a poor absolute one, and the distinction is worth understanding because a great deal of oral care marketing rests on it.
Critical pH is not a fixed property of enamel. It is the pH at which a particular fluid stops being supersaturated with respect to tooth mineral, and it therefore depends on how much calcium and phosphate that fluid already contains. Plaque fluid typically carries considerably more calcium and phosphate than saliva does, which pushes its critical pH lower, often to around 5.1 or below. The familiar 5.5 was derived from the solubility of enamel in saliva rather than in the plaque fluid actually sitting against the tooth.2
The practical reading: 5.5 is a good rule of thumb for enamel and a bad one for anything else. Root surfaces and dentin, which contain less mineral and more organic material, begin dissolving closer to pH 6.2. This is why gum recession changes the arithmetic of caries risk so sharply in older adults.
The Stephan curve: what happens after you eat
In 1944 Robert Stephan measured the pH of dental plaque before and after volunteers rinsed with glucose, and produced the curve that still carries his name. Plaque pH falls steeply within two to three minutes of a sugar exposure, reaches its minimum at roughly five to twenty minutes, and then climbs back toward baseline over the following thirty to sixty minutes.3
The recovery limb is the part that matters, and it is driven almost entirely by saliva: bicarbonate neutralizing the acid, and salivary flow physically clearing the substrate away. This is why the same sugar exposure is far more damaging at night, when unstimulated flow approaches zero, than in the middle of the day. It is also why the shape of the curve, not just its depth, determines risk.
A single sugar exposure therefore costs roughly half an hour to an hour below or near the critical threshold. Two exposures an hour apart cost close to double. This is the mechanism behind the frequency finding below, and it is the entire reason that grazing is worse for teeth than eating the same quantity at once.
The full account of the recovery side, and why salivary flow rate governs it, is covered in our article on saliva and oral health.
Frequency versus quantity
The systematic review commissioned to inform World Health Organization guidance examined the relationship between free sugars intake and caries, and found consistent evidence of a dose-response relationship: caries is lower when free sugars account for less than 10 percent of energy intake, and lower still below 5 percent.4 A ten-year update reached broadly the same conclusion while noting the persistent limitations of the underlying evidence base.5
Quantity and frequency are correlated in real diets, which makes them statistically hard to separate, and the literature is honest about this. But the mechanism above gives frequency a clear theoretical primacy: because each exposure triggers a full acid episode whose duration is set by salivary clearance rather than by dose, ten small exposures produce far more total time below critical pH than one large one.
The bacteria, and why "the cavity germ" is the wrong model
For much of the twentieth century caries was explained by the specific plaque hypothesis: a particular organism, usually Streptococcus mutans, was held responsible, and the logical response was to eliminate it. That model has largely been replaced.
The ecological plaque hypothesis holds that the organisms implicated in caries are ordinary residents of a healthy mouth, present in small numbers, and that disease follows a shift in the conditions that favor them rather than their arrival.6 Frequent sugar produces frequent acid; frequent acid selects for organisms that tolerate acid and produce more of it; those organisms then drive pH lower still. The community does not get invaded. It gets pushed, and it reorganizes around the push.7
This distinction has real consequences. If caries were an infection, sterilizing the mouth would cure it. Because it is an ecological shift, broad antibacterial measures tend to suppress the resident community along with the acid producers, and the ecological pressure that caused the shift remains in place once treatment stops.
The caries balance
The framework that pulls this together is the caries balance, developed by John Featherstone and colleagues: caries progresses or reverses according to the running total of pathological factors against protective ones.8
| Pathological factors | Protective factors |
|---|---|
| Frequent fermentable carbohydrate | Salivary flow and its components |
| Acid-producing, acid-tolerant bacteria | Calcium and phosphate availability |
| Reduced salivary flow | Fluoride, where used |
| Plaque retention and poor clearance | Antibacterial components of saliva |
The clinically useful implication is that there are many routes to the same result. A person with abundant salivary flow tolerates a diet that would produce rampant caries in someone with a dry mouth. This is also why caries risk assessment has moved toward evaluating the whole balance for an individual rather than issuing identical advice to everyone.9
Why early lesions can reverse
Because the subsurface lesion sits beneath an intact surface, and because saliva is supersaturated with the ions the lesion has lost, mineral can diffuse back in and redeposit on the partially dissolved crystals. This is remineralization, and it is a continuous background process rather than an intervention.
Two things limit it. First, redeposition tends to concentrate near the surface, which can seal the outer layer and slow further diffusion inward, so deep lesions remineralize far less readily than shallow ones. Second, once the surface has cavitated, plaque colonizes a sheltered space that brushing cannot reach, and the process no longer reverses on its own. This is the point at which caries becomes a restorative problem rather than a chemical one.
Where the evidence is strong and where it is not
The chemistry in this article is not seriously disputed. Demineralization and remineralization kinetics, the Stephan curve, and the dependence of critical pH on the surrounding fluid are all well established and reproducible.
Several things are genuinely less settled. Isolating the independent effect of sugar frequency from sugar quantity in human populations remains methodologically difficult, and the reviews cited above are candid that much of the underlying evidence is of moderate to low quality. Predicting which individual will develop caries remains poor: risk assessment models perform reasonably at the group level and unreliably at the person level. And while the ecological model is now the mainstream view, the precise composition of a "healthy" oral community varies enough between people that no single target state has been defined.
References
- Li X, et al. Global burden of dental caries from 1990 to 2021 and future projections. International Dental Journal. 2025;75(5):100904. PMID 40714315.
- Dawes C. What is the critical pH and why does a tooth dissolve in acid? Journal of the Canadian Dental Association. 2003;69(11):722-724. PMID 14653937.
- Bowen WH. The Stephan Curve revisited. Odontology. 2013;101(1):2-8. PMID 23224410.
- Moynihan PJ, Kelly SA. Effect on caries of restricting sugars intake: systematic review to inform WHO guidelines. Journal of Dental Research. 2014;93(1):8-18. PMID 24323509.
- Moores CJ, Kelly SAM, Moynihan PJ. Systematic review of the effect on caries of sugars intake: ten-year update. Journal of Dental Research. 2022;101(9):1034-1045. PMID 35302414.
- Marsh PD. Microbial ecology of dental plaque and its significance in health and disease. Advances in Dental Research. 1994;8(2):263-271. PMID 7865085.
- Marsh PD. Are dental diseases examples of ecological catastrophes? Microbiology. 2003;149(Pt 2):279-294. PMID 12624191.
- Featherstone JD. Caries prevention and reversal based on the caries balance. Pediatric Dentistry. 2006;28(2):128-132. PMID 16708787.
- Featherstone JDB, Chaffee BW, et al. Evidence-based caries management for all ages: practical guidelines. Frontiers in Oral Health. 2021;2:657518. PMID 35048005.
This article is educational and describes published research on the chemistry and microbiology of dental caries. It is not medical or dental advice, not a diagnosis, and not a claim about any product. If you have a specific concern about your teeth or gums, ask a dentist.
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