爭議的焦點是牛奶中的insulin-like growth factor 1 (IGF-1，類胰島素生長因子-1) 。乳癌基金會的評論說：
“Elevated levels of IGF-1, in particular, have been associated with increased risk of breast cancer (Hankinson, 1998). Proponents of rBST argue that IGF-1 is harmless because it occurs naturally in humans, is contained in human saliva and is broken down during digestion. However, animal evidence indicates that digestion does not break down IGF-1 in milk because casein, the principal protein in cow’s milk, protects IGF-1 from the action of digestive enzymes (Xian, 1995).”
翻譯如下：
“尤其是（奶中）增加的IGF-1, 與乳腺癌的風險有關 (見：Hankinson, 1998)。使用牛生長激素的讚成者爭辯說，IGF-1 是無害的，因為它本來就天然地存在於人體，在人的唾液中就有，經過消化道時被降解。可是，有動物證據顯示，消化過程不能降解牛奶中的 IGF-1，原因在於奶中的酪蛋白（casein）。酪蛋白是牛奶中的主要蛋白成分，它能保護lGF-1 不受消化酶的作用 (見 Xian, 1995)。”
乳癌基金會評論鏈接：
http://www.breastcancerfund.org/clear-science/radiation-chemicals-and-breast-cancer/bovine-growth-hormone.html