What is rBST in milk?

Recombinant bovine growth hormone or rBST is a hormone synthesized in a lab from genetically engineered bacteria. It is similar to the hormone bovine somatotropin, and is injected into cows to transiently increase their milk production.

Use of rBST is controversial, and is banned in many other countries such as Canada, Japan and Australia. Increased health problems of cows treated with rBST has also caused its use to come under closer examination by health authorities. Use of rBST is not permitted in foods that are labeled organic. Consumers of dairy products of rBST treated cows have been found to have higher levels of IGF-1, which is a factor in breast, colon, and prostate cancers.