This is usually a head gasket problem, meaning the engine will eventually fail without testing and/or repair.
The obvious answer would be that there would be a leak somewhere...
Gravity.
it is burning it!!
A possible leak
There could be any number of reasons, ranging from simple evaporation to slow leak.
Yes, a vacuum leak can affect the air-fuel mixture in the engine, causing the engine to run inefficiently and potentially burn more fuel. This can result in increased fuel consumption and make you lose gas quicker.
No. It is a diuretic. It causes you to lose water.
A leak on the return or supply lines and the automatic feeder is not working
A small amount of water, engine will idle poorly and lose power. A large amount of water, engine will cease to run.
your head could be cracked you will not notice the water lose when you car is running see if the water is coming out of the tale pipe
oil pump/ main bearings/ severe oil leaks/
The first place I would look is at the timing, a vacuum leak will also have this effect on an engine.