In female rats subjected to energy restriction, the combination of endurance exercise and dietary cholesterol intake results in a disproportionate increase in estradiol relative to progesterone, indicating selective modulation of estrogen synthesis or metabolism.

When an animal eats less food and exercises a lot, its body makes less of a hormone called leptin. This lets an enzyme called aromatase work more freely, turning other hormones into estradiol. If the animal also eats more cholesterol, that cholesterol gives the enzyme more raw material to make even more estradiol. Progesterone does not increase because the enzyme that makes it is not affected the same way.