Fasting doesn’t make your breath smell bad.
Burning fat through calorie restrictions does however.
Burning fat results in ketones that our cells use for energy. This is opposed to glucose, which is what our bodies use for energy when we’re eating regularly and not running a deficit. It turns out that some of those ketones get loose in our mouth as well and they kind of stink. That’s where it comes from.
Acetone is a ketone body that is exhaled through the mouth. Usually produced and exhaled when the body is not efficient at using ketones for fuel.
If you haven’t been in ketosis for 4-6 weeks, you’ll likely be inefficient at burning ketones for fuel. More experienced fasters and those who follow some version of keto are less prone to this side effect for the aforementioned reason.