The body is a tricky thing.
It adapts to exercise, the food consumed, and anything else it needs to get by.
While in the mean time, if stuff is done that it is not happy with, it will definitely let the person in question know about it.
And then there are the allergies... which is an extention of the above statement.
As for me...I don't know.
I'm trying to get fit and healthy but sometimes I gotta wonder.