Gee, this is a really tough one. There is a lot of info on this, but it is so complex because of the intricacies of the sympathetic and parasympathetic nervous systems, microbiotica, and endocrine systems that all work together, along with the brain to affect mood and homeostasis.
If it were as simple as to say that if we lose a portion of our GI tract we suffer some sort of mental deficit, we would all suffer with one form or another of a mood disorder. But, that is not true at all.
I don’t know this for a fact, but the way I see it is that we all may be exposed to the same traumas of chronic illness and surgery, but we are equipped differently both innately and through our experiences and support systems. So, some of us have temporary situational depression/anxiety. Some have a chronic form. Some suffer none of it. Just like different people respond differently to a virus, we all respond and/or recover differently to the loss of an organ or the traumas associated with it.
I am one who never suffered with depression or anxiety. I do not consider myself special, just lucky.
Jan