I've fasted for years (not on water only, I use fruit juice) and can't imagine not brushing my teeth during a fast. That would give the bacteria in the mouth a heyday to create plaque and decay at a time when the body is expelling extra waste through the tongue and even the teeth.
I think that taking care of our teeth becomes even more important when we fast.
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I brush my teeth using a re-enamelizing toothpaste made with essential oils or simply dip my toothbrush into a shotglass of water with a drop of oregano in it before brushing. I use a Sonicare toothbrush.
I also scrape my tongue.
I've also started using an oral irrigator, which I love.
And I floss too. I put a drop of essential oil on the floss before using it.
For mouthwash, I use distilled water with several drops of oil of oregano / Thieves essential oil in it. Peppermint essential oil would be good too.