Porcine somatotropin (pST) is a protein growth hormone found in swine. Injections of or implants containing this hormone cause pigs that weigh more than 100 pounds to gain more muscle and less fat (the overall fat reduction is sometimes as much as 80% in comparison with untreated animals). This results in meat that is much leaner than regular pork.
In past years, the hormone was very expensive because it could only be taken from the pituitary glands of slaughtered pigs, but in the early '90s researchers learned how to genetically engineer the bacterium E. coli to produce it. This hormone is a close chemical relative of bovine somatotropin.
This hormone is used in countries such as Australia, Mexico, Peru, Malaysia, and Vietnam and is sold under trade names like Reporcin. The hormone has not yet been approved for use in the U.S. by the USDA, though industry observers expect it will be approved soon. The WHO has rated the hormone as being safe.