The mammary tumor-promoting effects of a high-fat (HF) diet (23%, w/w) containing a 3:1 mixture of medium-chain triglycerides (MCT) and corn oil were compared with those of a low-fat (LF) corn oil diet (5%) and a HF:corn oil diet (23%, w/w). It was found that the ingestion of MCT in a HF diet resulted in no detectable tumor-promoting effects in animals initiated with the potent mammary carcinogen N-nitrosomethylurea. Total palpable mammary tumor incidence was 60% in the HF:corn oil plus MCT group, 66% in the LF:corn oil group, and 87% in the HF:corn oil group (p < 0.03 and p < 0.06, respectively). However, when palpable adenocarcinomas only were counted, differences in incidence between groups were not statistically significant, HF:MCT (57%) versus HF:corn oil (77%), p < 0.08. Mean time to first tumor (days) was 122 ± 40 (S.D.) in the MCT, 117 ± 36 in the LF:corn oil groups, and 86 ± 23 in the HF:corn oil group. The cumulative tumor incidence curves were similar for the MCT and LF:corn oil groups (p < 0.9); however, both curves were significantly different from that of the HF:corn oil group (p < 0.0099). No differences were found in tumor multiplicity, tumor size, or body weight gain in any of the treatment groups. Assay of serum total cholesterol and triglycerides showed that consumption of 23% corn oil diet significantly depressed serum cholesterol (but not triglyceride) levels compared to the LF:5% com oil- and the HF:MCT-containing diets. Analysis of serum fatty acid profiles indicated that animals fed 23% corn oil exhibited twice the amount of linoleic acid (C18:2) as did those fed either 5% corn oil or MCT. Differences in other fatty acids were of a much lesser magnitude.

These results indicate that the mammary tumor-promoting effect of a HF diet can be diminished by substituting saturated MCT for the more common longer-unsaturated-chain triglycerides. In addition, they suggest an association between promotion of mammary cancer and elevated levels of linoleic acid in serum lipids.

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This work was supported by Grant CA 29602 from the National Cancer Institute, Bethesda, MD.

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