Terry D. Etherton, PhD, and Dale E. Bauman, PhD, reported in their July 1998 paper "Biology of Somatotropin in Growth and Lactation of Domestic Animals," published in Physiological Reviews:

“The gross composition of milk (fat, protein, and lactose) is not altered by [supplemental] treatment with bST… Microconstituents of milk are also unchanged. For example, milk from bST-treated cows does not differ in vitamin content or in concentrations of nutritionally important mineral elements. Milk also contains many hormones and growth factors; two that have received substantial attention are ST [somatotropin] and IGF-1 [insulin-like growth factor-1]. At the bST doses that enhance milk yield, the concentration of bST in milk is unchanged…

In the case of IGF-1… it became evident that milk IGF-1 varied widely between cows and was affected by many factors (e.g., herd, stage of lactation, environment), and use of bST has minimal, if any, impact on the milk concentration of IGF-1. Overall, studies of the macro- and microcomponents of milk indicate that composition is unaltered by [supplemental] use of bST.”

July 1998