ever try a salt tablet? As football players practicing two a days in the dog days of summer and when in basic training and instructing by being exercised almost to the point of simply passing out at basic training, SALT tablets have been used to help retain fluids.
Its the first thing that popped up in my pea sized brain when I read this.. Seriosuly, have you been told that before ? Might be a simple quick easy solution..
When comrades were over in the deserts of Iraq during operation desert storm and the foot grunts were out there, they drank a 16 oz. bottle of water every hour on the hour to keep hydrated. They also took a salt tablet..
Maybe you have a salt deficiency which is why your body would not retain water??? Just thinking out loud here big guy.. Something else for you and doc to consider...