I also wanted to find this out and tried Oxypowder during a Water Fast (10 days). After the first day and a half (= the food that was still in my system) nothing at all came out. Unfortunately, this still leaves 3 possibilities:
- Oxypowder doesn't do what it claims
- mp doesn't exist
- I don't have any mp because of previous Water Fasts that I have done
I'm very interested to hear your results, but I think that you should fast for a bit longer than 2 days, otherwise it can still be the food that you have eaten before the fast that comes out.