Like a lot of folks, my favorite way to spontaneously fast is if I'm not hungry in the morning - just taking it a few more hours before eating.
But I am a very faithful morning toothbrusher and have started to wonder if my seemingly benign minty sweet toothpaste could be screwing with my physiology and thwarting my best intentions with fasting. So basically this spawned three questions:
No!!!!! To question #1, that is.
Brushing with even the sweetest of toothpastes has never interrupted ketosis for me, as per ketostix results.
Also, I don't believe a bit of insulin would push you out of ketosis (assuming that toothpaste would raise your insulin more than a microscopic amount). In fact, can't exogenous insulin put you INTO ketosis?
Brush to help prevent decay, that's why everyone should brush their teeth not just to have good breath.
There is no sugar or calories in toothpaste. And you're not swallowing so even if there was a few calories you wouldn't be consuming it.
Even if your body produced an insulin response to the presence of a sweet taste in the mouth (some clinical evidence suggests this is possible, but does not necessarily happen), there's no way you would interrupt the actual energy pathways that have been mobilized to supply your needs.
In other words... if you are fasting for no more than 16-18 hours, your energy needs are being met by stored glycogen (taken from the liver into the blood for the brain and used in the muscles for movement) and by fatty acids mobilized from the adipose tissue (for the muscles).
Toothpaste will not affect that.