Effects of reciprocal changes of diets differing in fat content on pulmonary metastasis from the 13762 rat mammary tumor.

The effect of changing the amount of polyunsaturated fat in the diet of aged female Fischer 344 rats at the time of tumor implant on metastasis from the 13762 transplantable mammary tumor was studied. Three experiments were performed. (a) Retired breeders, maintained on standard commercial chows until 10 to 12 mo of age, were transferred to high fat (HF, 23% corn oil) or low fat (LF, 5% corn oil) diets for 4 wk; at tumor implant, half of each group were kept on their original diets, while half were changed to the other diet (i.e., HF----HF, HF----LF, LF----LF, LF----HF). (b) Aged virgins, 14 to 16 mo old, were fed HF and LF diets from weaning; at tumor implant, the LF group stayed on the LF diet, while half the HF group remained on the HF diet and half were changed to LF. (c) Retired breeders were fed Purina rodent chow (5% mainly saturated fat) until tumor implant when they were placed on either the HF or LF diets. Six wk after tumor implant, all rats were necropsied, and the extent of pulmonary metastasis was determined. Data were expressed as volume of pulmonary metastases. In Experiment 1, animals maintained on a HF diet or changed to a HF diet at implant had significantly more pulmonary metastases than those animals kept on a LF or changed to a LF diet. Likewise in Experiment 2, pulmonary metastasis was less in rats which were fed a HF diet from weaning and then changed to LF at tumor implant than in the animals maintained on a HF diet both before and after tumor implant. Finally, in Experiment 3, when rats were changed from Purina rodent chow to either the HF or LF diet at tumor implant, there was no significant difference in the extent of pulmonary metastasis between the two groups; in both, the extent of metastasis was comparable to that seen in animals maintained on the LF corn oil diet. Data on metastasis were also examined in light of body weight, growth of the primary tumor, and food disappearance. These results suggest that the amount of fat consumed by aged rats after tumor implant is an important determinant of the extent of pulmonary metastasis from the 13762 mammary tumor. However, a period of prefeeding the semipurified diets appears to be required in order for the HF corn oil diet to stimulate metastasis in this system.