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Both honey and sugar are comprised of a combination of glucose and fructose. In sugar glucose and fructose are bound together to form sucrose while in honey glucose and fructose exist independently of each other.

Among the foods which cause tooth decay, Mayo Clinic counts the following:

Certain foods and drinks. Foods that cling to your teeth for a long time — such as milk, ice cream, honey, sugar..

On the other hand, an article on penndentist states that:

Research from Dr. Peter C. Molan of the University of Waikato, New Zealand, showed that certain types of honey are helpful in preventing cavities because of their antimocrobial potency.. The antibacterial properties of the honey will effect the flushing of the bad bacteria.

I found one study that compared the effect of consuming sugar vs. honey and found a smaller PH drop in the honey group than in the sugar group. The report also says that honey reduces the amount of harmful bacteria in our mouth. However, the study did not directly compare the amount of tooth decay in both groups - so it is difficult to draw a conclusion about the main question in this thread - if honey promotes tooth decay or prevents it.

So, without further ado: Does honey promote tooth decay or prevent it?

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It's important to begin by discussing the mechanism that drives tooth decay:

Chapter 99 – Microbiology of Dental Decay and Periodontal Disease. Loesche. Medical Microbiology. 1996.

Dental decay is due to the irreversible solubilization of tooth mineral by acid produced by certain bacteria that adhere to the tooth surface in bacterial communities known as dental plaque.

The tooth surface normally loses some tooth mineral from the action of the acid formed by plaque bacteria after ingestion of foods containing fermentable carbohydrates. This mineral is normally replenished by the saliva between meals. However, when fermentable foods are eaten frequently, the low pH in the plaque is sustained and a net loss of mineral from the tooth occurs. This low pH selects for aciduric organisms, such as S mutans and lactobacilli, which (especially S mutans) store polysaccharide and continue to secrete acid long after the food has been swallowed.

So, we know that pH plays a major role in tooth decay. Extended exposure to an acid environment has two main effects on the oral cavity. The first is mentioned in the excerpt above: "low pH selects for aciduric organisms... which store polysaccharide and continue to secrete acid long after the food has been swallowed." This holds true for people who have low pH in their mouth from a lack of saliva production (for example, from radiation therapy), who are at a higher risk for dental caries (cavities). The second effect of low pH is alluded to in the article you cited: "the pH in the honey group rapidly recovered 10–20 min after exposure and did not drop below the critical decalcification pH of 5.5." This critical decalcification value, as it turns out, is more variable than those authors might want you to think.

What Is the Critical pH and Why Does a Tooth Dissolve in Acid? Dawes. J Can Dent Assoc. 2003.

Therefore, a tooth will dissolve in saliva or plaque fluid only if the pH is reduced to less than the critical pH. In people with low salivary concentrations of calcium and phosphate, the critical pH may be 6.5, whereas in those with high salivary calcium and phosphate concentrations, it may be 5.5... Thus, the critical pH is not a constant, because the levels of calcium and phosphate in plaque fluid vary among individuals. The more calcium and phosphate that are present in a solution, the lower its critical pH.

Clinically, there are 2 situations in which dentists place acid in contact with enamel and in which enamel dissolves... The second situation is the use of acid fluorophosphate gels, which usually contain 0.1 mol/L phosphoric acid in addition to the sodium fluoride and which have a pH of 2.3. Again, these gels contain essentially no calcium and the Ip for HA is zero, which means that the tooth will begin to dissolve. Hence, to avoid excessive enamel loss, it is important not to exceed an exposure time of 4 min with these gels.

Patients, of course, may also introduce acid into their mouths, where it can cause erosion of enamel. The most devastating acid is gastric juice, which contains hydrochloric acid and low concentrations of calcium and phosphate and which has a pH of about 1. Other sources of acid include fruit juices and soft drinks, which have great potential to cause erosion because many of them have a pH of less than 3.

For reference, honey has an average pH of 3.9. Other people have studied honey-based mouth rinses since 2014 (when the paper you cited was published), and they report similar results:

Effectiveness of three mouthwashes – Manuka honey, Raw honey, and Chlorhexidine on plaque and gingival scores of 12–15‑year‑old school children: A randomized controlled field trial. Singhal et al. J Ind Soc Peridon. 2018.

Among the three types of mouthwash tested, 0.2% CHX was most effective in reduction of plaque and gingival scores. About 40% MH and 20% RH mouthwash also effectively decreased the amount of plaque and gingivitis from 0 day to 22nd day. Within the limits and scope of the study, it can be safely concluded that both RH and MH may be used as an adjunct to mechanical therapy for achieving a significant reduction in inflammatory periodontal changes.

An important element of these trials is that they were very short-term and do far more to show inhibited bacterial growth than inhibited tooth decay. Remember – the low pH-environment itself can erode enamel, and we can see that the purported pH of the honey rinses falls below the upper threshold named in the "Critical pH" paper, meaning that some individuals will experience dental erosion from >5-10 minutes of exposure to the honey rinse (I'm estimating this duration from the 4-minute acid fluorphosphate gel treatment at pH 2.3).

This isn't to say that the clinical trials are wrong. Honey definitely protects against bacterial growth (at least, as compared to other sugary mixtures):

Evaluation of Antimicrobial Action of Honey on Cariogenic Bacteria - An in vitro Study. Rakshanaa et al. J Pharm Sci & Res. 2017.

The present study concluded that honey has antimicrobial effect on streptococcus mutans after a definite time interval. Results can be due to production of hydrogen peroxide, inhibition of glucosyltransferase activity or presence of polyphenols in honey. All these factors are responsible for antimicrobial effect of honey on streptoccocus mutans.

However, to immediately assume that only bacterial inhibition equates to improved dental hygeine loses sight of the larger context – that an increasingly acidic oral environment might damage teeth directly or by selecting for more detrimental aciduric bacteria, which are features of sugary foods and drinks in general. It's true that honey is less cariogenic (cavity-causing) than other sugars, which might be attributed to its inhibitory effects on bacterial growth:

Effects of honey, glucose, and fructose on the enamel demineralization depth. Ahmadi-Motamayel et al. J Dent Sci. 2012.

In this study, we used fresh natural honey extracted from the honeycomb. However, different honeys from various sources may have different properties.

This study had a few limitations. First, the measurements were done by one observer rather than two observers, and the results might thus have been prone to a measurement bias. Second, the study was conducted in vitro. This may limit the generalizability of the results to clinical applications.

In conclusion, results of this study demonstrated that honey had lower caries activity than fructose and glucose. However, further evidence is required to demonstrate whether this food has any clinical application for preventing and reducing dental caries.

After this relatively comprehensive review, we know that honey is anti-bacterial and less cariogenic than other sugars (at least in vitro), but how cariogenic is honey itself? This was tested in desalivated rats to determine similar effects in human infants (breast/bottlefeeding recreates this low-saliva environment, but we can extrapolate these findings to an adult human's mouth, since the described enamel effects are consistent regardless of age).

Comparison of the Cariogenicity of Cola, Honey, Cow Milk, Human Milk, and Sucrose. Bowen et al. Pediatrics. 2005.

Results. Cola, sucrose, and honey were by far the most cariogenic. In addition, cola and honey induced considerable erosion. Human milk was significantly more cariogenic than cow milk probably because of its lower mineral content and higher level of lactose.

The big takeaway for me is that we love to perceive our behavior as being healthful, even when it's not. We should stick to fluorinated or distilled water or antibiotic rinses to improve our oral hygeine, but it's a lot of fun to imagine that a tasty, sweet honey rinse is just as good. As compared to other sweeteners, honey is less cariogenic, but it's still relatively low pH and full of fructose and glucose, none of which are "good" for your teeth. For example, hunter-gatherers who consume lots of honey instead of other sugars availabile in nearby cities present with more cavities:

In surprise, tooth decay afflicts hunter-gatherers. Gibbons. Science. 2017.

Hadza men in the bush hunt large and small game and gather honey and bee larvae. The women gather baobab fruit, berries, figs, drupes, and legumes, and they dig up fibrous tubers. In contrast, the Hadza who live in or near villages eat rice, maize, and beans, along with whatever meat and plants they can hunt and gather. They also drink homemade maize alcohol.

By comparing the teeth of 25 Hadza adults who live in the bush, 25 who live in villages, and another 25 who split their time between locations, Crittenden and Ungar found that 52% of the teeth of full-time male hunter-gatherers had cavities, compared with just 35% of the village men's teeth. The reason? “Men in the bush are consuming tremendous amounts of liquid honey,” Crittenden says.

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    Thanks for the comprehensive answer. In summary, I think it's fair to say that honey actually promotes tooth decay but not as much as sugar does. BTW, I tried looking at the sources you cited but some of the links redirect to page with Forbidden (403) - Access denied error, do I need to buy access? – Alex K Nov 27 '18 at 2:51
  • @AlexK I’ll go through and re-link them to their relevant NCBI pages. I posted this answer from campus so the links are likely protected by my institutional login. My mistake! I can send the full-texts to anyone interested in reading more. – Bruce Kirkpatrick Nov 27 '18 at 3:13
  • Yep! They were broken ProQuest links – didn't work for me (at home) either. I just checked every link after fixing those and everything is working on my apartment WiFi, so all of my references should be accessible now. – Bruce Kirkpatrick Nov 27 '18 at 3:29

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