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Janet Gray, Ph.D.
Janet Gray, Ph.D.

As author of our 2008 and 2010 State of the Evidence reports, Dr. Gray drives the science behind all our work.

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

CATEGORY*: Endocrine disruptor

FOUND IN: Dairy products

THE GIST: This hormone is injected into dairy cows to increase milk production and is found in milk, ice cream, buttermilk, cheese, yogurt and other dairy products. Though research outcomes are mixed, several human studies have linked dairy consumption with increased risk for breast cancer.

State of the Evidence on rBGH and rBST

Despite opposition from physicians, scientists and consumer advocacy groups, the U.S. 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 (Eaton, 2004). 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 potentially carcinogenic effects.


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Drinking any type of cow's milk noticeably raises body levels of insulin-like growth factor 1 (IGF-1), a hormone that occurs naturally in both cows and humans. Injecting cows with rBST leads to an increase in IGF-1 levels in milk (Daxenbeger, 1998; Chagas, 2012). A recent study has suggested that though plasma IGF-1 and total milk IGF-1 increased, increased milk output by treated animals diluted the output to within the normal range of concentrations (Collier, 2008). The content of IGF-1 in dairy milk is not altered by pasteurization or homogenization (Collier, 1991).

Although the data are complex, with studies reaching different conclusions, several epidemiological studies have indicated a relationship between dairy consumption and breast cancer risk in pre-menopausal women (Outwater, 1997). A new meta-analysis of relevant studies has just reported a direct correlation between high total dairy consumption and increased risk for breast cancer, with a stronger association for pre-menopausal breast cancer (Dong, 2011). But another study found that high total dairy intake reduced the risk for pre-menopausal breast cancer (Hjartaker, 2010), and a recent review of the relevant epidemiological data revealed no associations between consumption of cow milk and breast cancer (Parodi, 2012). Differences in results may reflect different kinds of dairy consumed and measured, with milk and especially low-fat milk being more likely to offer some protective effect than higher-fat (and higher-IGF-1-containing) milk or other dairy products (Chagas, 2012). Further, some results may be due to a potential protective effect of calcium, an important component of dairy products (Hjartaker, 2010; Zhang, 2011).

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).

*For chemicals that have been shown to be carcinogens, we provide classifications from two authoritative bodies: the International Agency for Research on Cancer (IARC, an international body) and the National Toxicology Program (NTP, a division of the U.S. Department of Health and Human Services). We have categorized endocrine-disrupting compounds where the body of peer-reviewed research indicates a strong foundation for doing so.