The primary objective of the study was to determine the relationship between the proportion of fat in the diet and the resultant enhancement of the formation of spontaneous mammary carcinoma. One experiment utilized C3H strain mice, with diets based on commercial components and four levels of dietary fat: 1.6, 5.7, 12, and 26 per cent; the second experiment utilized dba strain mice, with diets composed of semi-purified components and five levels of dietary fat: 2, 4, 8, 16, and 24 per cent. The diets were isocaloric, and the mean body weights were similar.

In both experiments, the rate of formation of mammary carcinoma, as measured both by incidence of tumors and by the average time of tumor appearance, tended to increase with increasing proportions of dietary fat. However, the effect was not arithmetically proportional to the level of fat: the enhancement of tumor formation resulting from increasing the dietary fat from 2 to 6 or 8 per cent was as great as that resulting from increasing the dietary fat from 6 or 8 per cent to 24 or 26 per cent. In contrast to the effect on tumor formation, phenomena related to the growth of the mammary carcinomas—increase in tumor size, survival time of the animal after appearance of the tumor, or the incidence of grossly visible metastases to the lungs—were not affected by the proportion of dietary fat in the range studied.

The decrease in energy expenditure of utilization of high-fat rations is too small to account entirely for the observed enhancement of tumor formation. Other suggestions are made, but it must be concluded that, at present, the mechanism through which fat-enriched diets accelerate the formation of spontaneous mammary carcinoma and carcinogen-induced skin tumors of the mouse is not known.

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This work was supported by a grant from Distillation Products, Inc., Rochester, N.Y.

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