2 week fast after large bowel movement? lost electrolytes?

So I've planning to start a 14 day fast, but I'm a little worried about my electrolyte levels.

I was constipated for 3 days, then had a large bowel movemen on the third day (today.) I also feel another one coming, so I'm little concerned that I'm losing a lot of electrolytes the day before the fast.

I'm wondering, should I eat tomorrow and start fasting the next day to make sure I have high enough levels?