Almost all dairy products we consume come from cows, so there's a large demand for cows' milk. Since dairy farmers have often looked for ways to increase milk production from their existing herds, a bovine hormonal drug was approved by the United States Food and Drug Administration (FDA) for use in dairy cattle in 1993.
Recombinant bovine somatotropin (rBST) is a synthetic form of the naturally occurring bovine somatotropin (BST), which stimulates milk production, so it follows that giving cows injections of the additional synthetic hormone will increase the volume of milk produced.
Since the use of rBST became known, there have been questions about how it affects the milk composition and if it poses any safety risk. Currently, there is no evidence that milk from rBST-treated cows is unhealthier than milk from untreated cows. All milk has some level of bovine growth hormone (BGH) in it, and milk from rBST-treated cows doesn't have any more BGH than milk from untreated cows. It wouldn't matter if it did, because BGH is destroyed in the digestive tract so you don't absorb any of it.
There is some evidence that milk from rBST cows has a little more insulin-like growth factor-1 (IGF-1), which is a hormone produced in cows and in humans, and it isn't destroyed by digestion. In children, IGF-1 is necessary for growth, but epidemiology studies (large observational studies) suggested that adults who have elevated IGF-1 levels in their blood also had an increased risk of breast, prostate and colorectal cancer.
More recent research studies haven't found the same level of risk, but nonetheless, this is the usual area of concern for milk from rBST-treated cows. Drinking milk (whether from treated or untreated cows) appears to increase the levels of IGF-1 in your blood, but there aren't any studies that compare IGF-1 levels or cancer risk in people who drink milk from treated cows and milk from non-treated cows. Numerous organizations, including the American Cancer Society, have carefully reviewed the IGF-1 issue and concluded that the potential for meaningful exposure to humans from consumption of BGH treated milk is extraordinarily small if it is even true.
Canada and the European Union both voted to ban the use of rBST in dairy cattle. This wasn't because of human health risks, but because there were concerns about health risks for the cows that would be treated with rBST.
What about the cows? Cows treated with rBST produce more milk, so they are more likely to have udder infections, called mastitis, which must be treated with antibiotics. While there are concerns about overuse of antibiotics leading to antibiotic-resistant strains of bacteria, the antibiotics don't make it into our food systems because all bulk milk is tested for antibiotics when it is delivered to the dairies.
Milk from rBST-treated cows appears to be just as safe as milk from non-treated cows, but you can choose to avoid it if you prefer; most grocery stores sell milk produced by non-treated cows (the label will tell you), and of course, organic milk is also from untreated cows.
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American Cancer Society. "Recombinant Bovine Growth Hormone." Accessed April 26, 2012. http://www.cancer.org/cancer/cancercauses/othercarcinogens/athome/recombinant-bovine-growth-hormone
Ma J, Giovannucci E, Pollak M, Chan JM, Gaziano JM, Willett W, Stampfer MJ. "Milk intake, circulating levels of insulin-like growth factor-I, and risk of colorectal cancer in men." J Natl Cancer Inst. 2001 Sep 5;93(17):1330-6.
United States Food and Drug Administration. "Bovine Somatotropin (BST)." Accessed April 26, 2012. http://www.fda.gov/AnimalVeterinary/SafetyHealth/ProductSafetyInformation/ucm055435.htm
United States Food and Drug Administration. "Voluntary Labeling of Milk and Milk Products From Cows That Have Not Been Treated With Recombinant Bovine Somatotropin." Accessed April 26, 2012. http://www.fda.gov/Food/GuidanceComplianceRegulatoryInformation/GuidanceDocuments/FoodLabelingNutrition/ucm059036.htm