It is healthy to stay fit and do regular exercise, but like everything else moderation is the key. Too much of anything, no matter how “healthy” it’s touted to be is bad for you.
“Strenuous exercise is known to reduce oestrogen levels in women and girls. This can delay the start of menstruation, and can lead to irregular periods in adult women. Low levels of oestrogen in premenopausal women have been linked to impaired mental function in later life.”