Dietary fat and growth promotion of rat mammary tumors.

A hypothesis is presented that explains the mammary tumor-promoting effects of high fat diets on the basis of alteration in the hormonal milieu, namely the relative concentrations of circulating prolactin to estrogen. Evidence from in vivo and in vitro studies drawn from work in our laboratory and others is reviewed in light of this hypothesis. It is postulated that mammary tumor cell proliferation is stimulated when the prolactin:estrogen ratio is high and is inhibited when the ratio is low. Chronic high fat intake elevates serum prolactin levels, thus raises the prolactin:estrogen ratio, and thereby promotes mammary tumor cell growth.