Low-Fat Milk Promotes the Development of 7,12-Dimethylbenz(a)anthracene (DMBA)-Induced Mammary Tumors in Rats International Journal of Cancer 2004;110:491-6.

Commercial cows ’ milk contains considerable amounts of estrogens. This 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 six-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 one of four 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 second 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 three 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