The growth-promoting effect of dietary polyunsaturated fat is reported for six transplantable rat and mouse mammary adenocarcinomas which have different origins and characteristics. All six adenocarcinomas developed a significantly greater mass when implanted in animals receiving a 15% corn oil-containing diet than in animals receiving a fat-free diet. In the case of the two tumors implanted in BALB/c mice, addition of as little as 0.1% pure cis,cis-linoleic acid to a fat-free diet was as effective as 15% corn oil. However, in two tumors implanted in C3H mice, and in the rat tumor, 0.1% linoleic acid did not increase tumor weight. As little as 0.06% 5,8,11,14-eicosatetraynoic acid and 0.004% indomethacin, inhibitors of prostaglandin synthesis, in the diet prevented the tumor growth-promoting effect of polyunsaturated fat in BALB/c mice.

The observed difference in tumor mass due to dietary fat could not be accounted for by differences in the duration of the latent growth period following tumor implantation. The neoplasms taken from animals fed either fat-free or corn oil-containing diets were analyzed for DNA, protein, total lipids, phospholipids, and cholesterol. There were no differences in the chemical composition which could account for the difference in tumor weight observed.

The size of the proliferative cell population in tumors implanted in either BALB/c or C3H mice was not changed by dietary fat. Splenomegaly was observed in host BALB/c mice in direct proportion to the increase in tumor mass.

Our results suggest that the growth-enhancing effect of dietary fat on transplantable mammary tumors (a) is caused by cis,cis-linoleate, (b) varies quantitatively with species and strain of animals, (c) cannot be accounted for by an increased neoplastic cell proliferation, and, therefore, (d) may be mediated through the cellular immune system with the possible involvement of prostaglandins in this process.

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This work was supported by NIH Grants CA11736 and 5 SO7 RR05467.

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