The reason we pee so much has to do not only with what we drink but the fact that when we eat less carbohydrate than we need to provide glucose for our blood sugar our body releases stored carbohydrate from our livers. This stored form of carbohydrate, glycogen, turns into glucose and water when it is released. There is a lot more water than glucose in the glycogen, so it is that water that you are peeing away late on a fast day.
Were you to keep fasting (or to eat a very low carbohydrate diet) you would burn off all the stored glycogen that can be released and the peeing would stop. At that point, your body would switch over to burning first ketones and then, after a few weeks, free fatty acids. But since with intermittant fasting we mostly go back to eating carbs the day after a fast, our glycogen stores get recharged and we put back on the weight that is due to the glucose bonding with that extra water as glycogen is formed.
This cycling back and forth through glycogen burning and storing is the one thing I don't like about intermittant fasting. With low carb dieting you get past it for a few days and then that's that. No more wearing a path to the bathroom!
Were you to keep fasting (or to eat a very low carbohydrate diet) you would burn off all the stored glycogen that can be released and the peeing would stop. At that point, your body would switch over to burning first ketones and then, after a few weeks, free fatty acids. But since with intermittant fasting we mostly go back to eating carbs the day after a fast, our glycogen stores get recharged and we put back on the weight that is due to the glucose bonding with that extra water as glycogen is formed.
This cycling back and forth through glycogen burning and storing is the one thing I don't like about intermittant fasting. With low carb dieting you get past it for a few days and then that's that. No more wearing a path to the bathroom!