A maternal high‐fat diet during pregnancy in rats results in a greater risk of carcinogen‐induced mammary tumors in the female offspring than exposure to a high‐fat diet in postnatal life

The association between a high‐fat diet and breast cancer risk is controversial. We hypothesized that the exposure of rats to a high‐fat diet in utero via the maternal diet would result in a greater risk of carcinogen‐induced mammary tumors than high‐fat diet exposure in postnatal life. Rats were exposed to a high‐fat diet with 40% of the energy source as safflower oil in utero (In utero group), at postnatal days 30–50 (Puberty group), postnatal days 150–170 (Adult group), postnatal days 1–230 (Postnatal group) or for their whole life from in utero (Whole group). Chow diet‐fed rats were used as the Nonexposure group. Mammary tumor incidence was significantly higher in the In utero (60%), Postnatal (61%) and Whole (91%) groups than in the Nonexposure group (32%), but there was no significant difference between the Puberty (44%), Adult (44%) and Nonexposure groups. Arachidonic acid levels were 10 times higher in mammary tumor tissue than in the normal mammary gland across all groups and were positively correlated with tumor weight. We conclude that the timing, but not the duration, of high‐fat diet exposure makes rats more susceptible to carcinogen‐induced mammary tumors and that exposure in utero to a maternal high‐fat diet during pregnancy is more important in increasing the risk of mammary tumors in the female offspring than exposure of the offspring to the same high‐fat diet later in life. © 2009 UICC