I want to know how can I make sure my stomach extract as much of the food nutrition as it can? How can I improve my stomach's abilities to extract it?
:o

Could drinking water help?

Simply control the quality of your food intake. Eat appropriate portions, at least three times daily, square meals if possible. Yes, water is good. It's what the majority of your body is composed of. Decrease the amount of junk food you eat. Remember, the simpler the food is, usually the easier to digest. Your body will take what it needs from what you eat, and pass the rest through. You can't force your body to absorb more than that. Exercise (making the muscles call for more energy) will increase the body's demand for nutrition, and in effect increase your absorption. But it's on an as-needed basis. When you work your body hard, you have to replenish it afterwards.

Thank you. I read a lot of soldiers and war stories. And I read that, the poorer the people, the more their body adapt to the environment. Let say, the person has to cut wood everyday, so their muscles developed. But let say, the person has barely anything to eat. Does their stomach extract more nutrition, as a adaptation to less food?

In extreme circumstances, the body may revert to a mode of storing everything that it can process so that it can store it as fat to consume it later as energy. But in most normal situations, your body will only take what it can process on an as-needed basis and pass the rest as waste.

Are you trying to achieve something specific?