Very small increases in estrogen levels can promote tumor growth. Researchers measured estrogen levels in milk purchased in several grocery stores; all of them contained estrogen and estrogen metabolites. Think about it, lactating cows are going to have a lot of estrogen in their milk. (Almost all cancers are estrogen receptor positive). Skim milk, although it had the smallest amount of free estrogens, contained 2-hydroxyestrone, known to be the most reactive and potentially cancer-promoting of the metabolites; amount second only to the concentration in buttermilk.