C6H12O6 + 6O2 = 6 CO2 + 6H2O

And that's aerobic respiration, what the plants and the animals do when they want some energy (which is all the time). New water molecules made, but it's from the same ol' hydrogen and oxygen atoms going round and round...

Worth remembering that living things don't use elemental hydrogen (of which there is very little anyway), and that all the oxygen in the atmosphere was put there by green plants.