The nutrition we take in, goes through several steps before it is incorporated into the body.
The food is broken down in the stomach and upper small intestines. Then, the small molecules are absorbed through the wall of the intestines, through the villi. They are picked up by the blood stream and taken to where they are needed.
The protein or other nutrients then go through several steps to be taken into the cell of the body, where they are going to be used.
So, it is up to you to decide when the nutrients become part of you - when absorbed into the blood stream or when incorporated into the cell.
I got it. The next question is that whether it can be said that the excretory waste that comes out of our body was a part of me or my body at some time? If yes then please explain the process when that waste was the part of my body and how it rejected from being the part of my body?