bowel movement; regularity of bowel movements
Commonly used in medical and health contexts to refer to the act or regularity of defecation. Often appears in phrases like 便通がある/ない (to have/not have a bowel movement) or 便通を整える (to regulate bowel movements).
Lately, I've been troubled by irregular bowel movements.
Eating a lot of dietary fiber improves bowel regularity.