If you are really worried about it, buy a blood glucose meter and measure yourself multiple times when fasting. This can be considered a proxy of sorts for Insulin in the blood.

I can tell you that I don’t avoid fluoride. People with bad teeth die younger from many causes, but often related to infections. Plenty of studies point to that.

Anyway, if you don’t have Insulin you die. If you don’t have enough Insulin you die. If you have way too much Insulin you die, but likely slowly. So you need some Insulin and if something triggers some, it probably isn’t a bad thing.

Now the definition of fasting is not eating. I have no clue how people claim that drinking zero calorie water breaks a fast even if your Insulin levels increase. That would trigger blood glucose to drop and in turn insulin levels will decline.

Hard exercise can cause sharp increases in blood glucose due to the Cori cycle. The clearly would cause an Insulin spike. So do you want to say a hard workout breaks a fast? It doesn’t make sense.

The body is very complex and there are many things that could impact insulin levels besides food. However blood insulin isn’t easy to measure, so I don’t think it is really a valid tool to use to define a fast. In fact even blood glucose is invalid because the body stores glucose and can create glucose. In fact that is what you want while fasting for the body to use up stored glucose or to just produce more glucose. Doing so causes the body to also consume fat. Seems to be actually a major benefit of fasting.

So unless your tap water is killing you, I wouldn’t worry about it.