The growth hormone rBGH was approved to be used in cows for increased milk production since 1993. And while the hormone itself has not been shown to negatively affect your health, there is concern that growth hormone manipulation in cattle or other livestock may cause a chain reaction.
The manipulation of rBGH in livestock may increase an insulin-like growth factor called IGF, which may mimic human growth hormone. In fact, dairy from cows treated with rBGH have been shown to have 10% more IGF than milk from non-treated cattle.
Source aside, IGF has been associated with increased risk of various cancers including breast and prostate cancer. In a study conducted in 2004, people with higher than normal levels of IGF in their blood, had around a 50% increased risk of prostate cancer and a 65% increased risk of pre-menopausal breast cancer.
Of course, there are many other factors that can contribute to increased risk of these cancers including genes, smoking, and fat intake. However, scientists believe that there is at least some positive correlation between these cancers and IGF levels.
In addition to IGF, estrogen is also found in increased rates in our food supply. Many believe that increased rates of estrogen and other sex hormones may be the cause of early onset of puberty in children. On average, children are starting puberty earlier than those a few generations ago. However, it is unclear as to whether our food is the primary cause of this. While many advocates of genetically altered foods state that the amount of these sex hormones are miniscule and thus shouldn’t be cause for concern, critics say that for pre-pubescent children, even small amounts of hormones can be substantial.
In summary, there is research to suggest that manipulation of hormones in our animals may lead to negative health effects, but many of these conclusions are not definitive. Because of this, the best thing you can do is purchase organic or hormone-free foods as often as possible and do your research on the farms from which you are purchasing.