during sleep, which he cannot prevent. But if he induces vomiting, it expels the food and drink transformed in the stomach from which he derives nourishment. Similarly, masturbation, besides involving sexual pleasure, expels semen, which is formed in the stomach through the transformation of blood. It thus expels the blood that nourishes him. This is why excessive discharge of semen harms a person, and it comes out red.
The blood discharged during menstruation likewise entails a loss of blood. A menstruating woman can fast at other times, when she is not bleeding. Her fasting then is balanced: she does not lose the blood that strengthens her body and sustains it. Fasting during menstruation, however, entails losing the blood that sustains her, depleting and weakening her body, and taking her fasting beyond moderation. She was therefore commanded to fast outside the times of menstruation.
This differs from the case of a woman with irregular non-menstrual bleeding, for such bleeding is not confined to particular times, leaving her no particular time at which she can be instructed to fast. Nor can she avoid it. It is like being overcome by vomiting, bleeding from wounds and boils, wet dreams, and similar occurrences that have no fixed time allowing a person to avoid them. These were therefore not deemed incompatible with fasting, unlike menstrual bleeding.