Great point.  Outside of and/or in addition to dosages, diet and hydration, genetics definitely play a role in the manifestation of side effects.
Funny you should suggest a fast, because that is what I did.  I decided to take 2 days off of MK677 and I lost water.  Muscle fullness deteriorated (which I missed, but my BP dropped, which was the ultimate goal). I then took the same dosage the night before last when I was 6 hours into a fast.  By mid-morning the next day, I could feel the water retention again, by hour 20, I could feel it even more.  Therefore, the water retention has little to do with my diet.  It must be genetic.
UPDATE:
After one full day back, on MK677 after taking two full days off, my BP was 120/80 this morning.  I hope it remains stable.  If so, I will stick with the 5 days on 2 days off dosing cycle.
I am going to start checking my blood glucose today.  More to come...