Bovine Growth Hormone (rBGH)/ Recombinant Bovine Somatotropin (rBST)

Despite opposition from physicians, scientists and consumer advocacy groups, the Food and Drug Administration in 1993 approved Monsanto’s genetically engineered hormone product, recombinant bovine growth hormone (rBGH), for injection in dairy cows to increase milk production. This hormone quickly found its way (without labeling) into the U.S. milk supply, and from there into ice cream, buttermilk, cheese, yogurt and other dairy products. Since its introduction, rBGH (subsequently renamed recombinant bovine somatotrophin, rBST) has proven controversial because of its potential carcinogenic effects.

Although the data are complex with some studies reaching different conclusions, several epidemiological studies have indicated a relationship between dairy consumption and breast cancer risk in pre-menopausal women. In many of these studies the risks associated with dairy consumption was much higher than that found for meat consumption.

Drinking any type of cow’s milk noticeably raises body levels of insulin growth factor 1 (IGF-1), a naturally occurring hormone in both cows and humans. Elevated levels of IGF-1 have been associated with increased risk of breast cancer. Notably, rBGH has been shown to raise insulin-like growth factor 1(IGF-1) levels in the body as well. A prospective study of American women found that pre-menopausal women with the highest levels of IGF-1 in their blood (drawn before cancer developed) were seven times as likely to develop breast cancer as women with the lowest levels. No increased risk was noted in post-menopausal women. Three studies reported in 2005 by scientists in Sweden, the United Kingdom and the United States also showed an association between circulating levels of IGF-1 and the risk of breast cancer in pre-menopausal women. These studies confirm earlier research linking elevated levels of IGF-1 with increased breast cancer risk.

Laboratory studies have shown that IGF-1 can regulate the growth and increase the proliferation of breast cancer cells (MCF-7) grown in vitro and decrease the death of mammary tumor cells in laboratory animals.

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.

Learn more about policy and research recommendations to reduce exposure to hormones in meat and milk »

For references, see State of the Evidence 2008.