>if we eat plants , then it has to be lost forever.

It is not lost "forever" the molecules just breakdown and get combined into different things which eventually breakdown themselves or converted into something else, eventually the moluces (hydrogen and oxygen) get back to being water and so the cycle continues.

Nothing is "lost forever" because it has to go somewhere but it'll come back eventually in some shape or form.

Water in plants consumed by organisms will be respired, excreted in waste, or ultimately also returned to the environment when the organism dies. There is always some amount of water locked up in the biosphere, but this water is not lost in a real sense.

When we eat plants we are eating lots of water. Consider a fresh apricot compared with a dried apricot.

The dried apricot is chewy and wrinkled because much of the water has been removed during the dehydration process. That water was turned to water vapour and went into the air.

If you eat a lot of watermelon you may need to pee more. Because there was a lot of water in it.

I forgot to water a plant of mine. The leaves went crispy and the whole pant shriveled up. Not only did it not get any new water to the point that it died… some of the water which was in the leaves and body of the plant was lost and is no longer in the plant anymore.

Dried herbs are dryer than fresh herbs. Because some of the water that was in the plant is no longer in the plant. It went somewhere else (likely into the air as water vapour).

Of course some of the water was used to build the plant itself as it didn’t come from nowhere.

The plant uses water and carbon dioxide to build sugars.

When you breathe in your breathe in oxygen (plus other gassed) When you breathe out you breathe out carbon dioxide (plus other gasses).

Where does the carbon come from? From the plants you eat.

The sugars in the plant are turned back into water and carbon dioxide.

When plants grow, they don't destroy the atoms that make up water (and carbon dioxide), they build new cells using them. Those cells break down when they die because the process that was supplying them with energy for homeostasis has ended. When the cells break down, those atoms are still there. If something eats them and there's water inside, they pee it out. It's always still there, somewhere.

When we eat the plants, the matter ultimately undergoes cellular respiration, which is the opposite reaction of photosynthesis, so all the water “captured” by the plants is converted back into water by consumers, and is eventually either excreted or released upon the organism’s death so the water cycle continues. The overall reaction is C6H12O6 + 6O2 -> 6H2O + 6CO2, and has water as a product.