Bovine growth hormone
Recombinant bovine growth hormone, also known as recombinant bovine somatotropin (rBST) or rBGH, is a synthetic form of the growth hormone of cows. It can be produced using genetically modified bacteria. When injected into a dairy cow at regular intervals, it increases milk production by up to 10-15%  This means that more milk can be produced from the herd, or that fewer cows are required to produce the same amount.
There is no difference in the level of bovine growth hormone between the milk of cows injected with rBST and regular milk. Bovine growth hormone has no effect on humans. There is some difference in the level of insulin-like growth factor (IGF-1), a protein which controls adolescent growth and levels of which are related to cancer risk. However, the difference is considered to be within natural variation — the same concerns apply to regular milk. Even if 100% of IGF-1 was absorbed without degradation from the gut of an infant drinking 1.5 litres of milk daily, this would still constitute less than 1% of its body's normal production of this hormone.
There are adverse effects for cows treated with rBST, including increased risk of mastitis, foot problems and reactions at the injection site. This leads to increased use of antibiotics for management of those conditions, which can accelerate the development of super-bugs.
Environmental groups and organic food proponents claim that milk from rBST-treated cows causes cancer. As detailed above, this is implausible. Another mythconception is that there are "artificial hormones in milk" which interfere with child growth or accelerate the onset of menstruation and breast development in young girls. This is completely bogus on every level: not only is rBST inactive in humans and its level is the same in rBST milk, it is also a different kind of hormone: sexual development is controlled by gonadotropins, not somatotropins. There has been a rise in incidents of puberty beginning at a younger-than-normal age, but this is due to childhood obesity, not dietary hormones.
The valid reason to limit or prohibit the use of rBST is its adverse effect on cows, and this is the reason which was cited by most regulatory bodies which did so.
Milk manufacturers that use non-rBST-treated cows often tout this fact on their product labels.
In 2008, Ohio passed a law prohibiting manufacturers from advertising that their products were "Not from cows treated with rBST". However, on 29 September 2010, the Federal 6th Circuit ruled that such laws were unconstitutional, on the basis that milk from rBST-treated cows is not completely identical to milk from non-rBST treated cows. The FDA nevertheless requires such advertisements to carry the following disclaimer: "No significant difference has been shown between milk derived from rbST-treated and non-rbST-treated cows."
- AgBioForum: The Regulation Of rBST: The European Case
- Various: A review of bovine growth hormone
- American Cancer Society: Recombinant Bovine Growth Hormone
- Cancer Expert Counters Reckless Claims That Hormonal Milk Is Safe
- Genetically engineered hormones used by dairy industry promote cancer, Natural News
- Snopes.com: Wal-Mart Milk