Can Brushing Your Teeth be Bad for Them?

Believe it or not, this is a chemical question.

Last week on 6PR Millsy reported that a Dr Howard Gamble had said that brushing your teeth in the 20 minutes after drinking a soft drink could “drive the acid further into your teeth”, thus promoting decay.

In looking into this, I have been unable to get my hands on the original research, as the only thing that’s available online is the abstract, with its conclusions.

As a scientist myself, I would much rather see the original data and methodology, but in their absence we will just have it to work with what we have.

Firstly, we will accept as an observation that teeth that are brushed within 20 min of being exposed to a soft drink decay at a greater rate than teeth for which 30 mins elapse before being brushed.

The question now is how we explain it.

Let’s think about what is in the soft drink, and therefore what is now on your teeth.  Firstly, of course, there is sugar and various flavours.  Also, the bubbles, which are carbon dioxide, tell us that there is carbonic acid present.  The carbonic acid would account for the low pH of the soft drink (about 3).

If the drink was Coco Cola it would also contain phosphoric acid, but that appears not to be the case in this study, where they used a drink called Sprite.

The carbonic acid, we are told, is the source of the decay, as it is rubbed “deeper into the teeth.”

Unfortunately, there are at least two problems with this theory, and frankly I find it astonishing that a finding such as this can be published in a peer-reviewed journal.  I guess it only goes to show that dentists don’t know much about acid-base chemistry.

The first problem is that carbonic acid is a week acid that is in equilibrium with the carbon dioxide bubbles.  This means that as soon as the fizz dissipates, and there are no more bubbles, there is very little carbonic acid left.

The second problem is that acid-base reactions are very fast, and most toothpastes contain bicarbonate of soda (although since I haven’t seen the original research, I do not know what toothpastes day were using in this particular study).

Sodium bicarbonate is slightly alkaline and will instantly neutralise any carbonic acid that is in the mouth as soon as it is introduced. That’s simply the nature of acid-base chemistry – the entire technique of titrimetry is based on this principle. So when it comes to be actual brushing of the teeth, there will be no acid left to “rub in.”

So what is happening then?  Why do teeth decay quicker if they are brushed within 20 min of drinking soft drinks?

I have a theory on this.  Recently we saw that tooth decay is principally caused by lactic acid.  Sugar combines with proteins to form glycoproteins which form plaque.  The plaque is decomposed by anaerobic bacteria to form the lactic acid.

The key word here is “anaerobic” that I’ll come back to that in a minute.

We all know that sugar is very sticky.  If you have ever seen what happens when a bottle of orange cordial gets spilled on the supermarket floor, you will know what I mean.  To get it off requires copious mopping with a fair bit of water.  One thing you would not do is get down and scrub it with a scrubbing brush, and yet that is exactly what we do with our teeth.

The sugar in the soft drinks will adsorb readily to our teeth.  Over half and hour or so, our saliva will gradually dissolve iit.  But if we scrubbed it with a toothbrush it’s easy to see that it could be scrubbed into the porous tooth enamel.  Once embedded in our teeth it will readily form the plaque.

And here’s where the anaerobic nature of the lactic acid formation is relevant.  Our mouth off course contains a lot of oxygen at most times, but the levels of oxygen would rapidly drop as we go deeper into the enamel.  In other words, the only place in our mouth that would be anaerobic, which would allow the lactic acid to form, is in the enamel itself.

So that’s my explanation.  Sugar is adsorbed to our teeth, and rubbed into the enamel if we brush soon after drinking the soft drinks.  It then forms plaque, and due to the anaerobic conditions deep inside the enamel of our teeth, lactic acid is formed.  Because this acid actually forms in the teeth, it readily causes decay.


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