Low‐fat milk promotes the development of 7,12‐dimethylbenz(A)anthracene (DMBA)‐induced mammary tumors in rats

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Abstract
Commercial cow milk contains considerable amounts of estrogens. Our study assessed the effect of commercial low‐fat milk on the development of 7,12‐dimethylbenz(a)anthracene (DMBA)‐induced mammary tumors in rats. Eighty 6‐week‐old female Sprague‐Dawley rats received a single oral dose of 5 mg DMBA. Twenty‐four hours later, the animals were divided into 4 groups of 20 animals each and given 1 of 4 test solutions for 20 weeks as their drinking liquid: low‐fat (1%) milk (M), artificial milk (A), estrone sulfate solution (0.1 μg/ml, E), or tap water (W). The artificial milk was formulated to supply essentially the same calories as the milk. The low‐fat milk contained 378 pg/ml estrone sulfate. Tumor incidence, the cumulative number of tumors and the sum of tumor diameters were higher in the M and E groups than in the A or W groups. Overall, the development of mammary tumors was in the order: M = E > A = W. Whereas the plasma 17β‐estradiol concentration in the M group was the 2nd highest after the E group, the plasma level of insulin‐like growth factor (IGF‐I) was significantly higher in the M group than in the other 3 groups. In conclusion, commercially available low‐fat milk promotes the development of DMBA‐induced mammary tumors in rats. The degree of the promotion is almost comparable to that of 0.1 μg/ml estrone sulfate. The high estrogen content in the milk may be responsible for the promotional effects, acting in concert with other hormones such as IGF‐I.