But even biological rhythms have their limits. In the normal unchallenged menstrual cycle, low amounts of estrogen and progesterone trigger hormones from the hypothalamus and pituitary to stimulate the ovaries to make more. When the estrogen rises, this stimulates a follicle to nurture a developing egg for ovulation. When ovulation occurs, the follicle remains to make progesterone, which maintains the richness of the uterine lining in preparation for a fertilized egg. And so on.
Obesity can affect the cycle by accidentally converting adrenal hormones into estrogen.
This steady stream of constant estrogen will muck up the works of the rising and falling ovarian cycle. This can result in irregular bleeding over long periods of time, or worse, overstimulation of the uterine lining which can become pre-cancerous. Extremes of exercise-running many miles a day or participating in the numerous extreme competitions, will decrease the hormonal levels, particularly progesterone by a complex scenario in which a woman won't ovulate regularly. The end result is much like that for obesity, in that there is either irregular bleeding or no bleeding at all. There seems to be a critical weight below which the hormones won't function normally.