Livestock in the U.S. are given hormones to increase production. Although the USDA and FDA(1) claim that these hormones are safe, there is concern that hormone residues in meat might be harmful to human health and the environment .

    There are 6 hormones used in beef production three which are naturally occurring—Estradiol, Progesterone and Testosterone—and three which are synthetic—Zeranol, Trenbolone, and Melengestrol. Two of these, estradiol, a type of estrogen, and progesterone–are considered probable carcinogens by the Toxicology Program at the National Institutes of Health. Estrogen has been linked with breast cancer in women and testosterone with prostate cancer in men.

    The question is whether hormone residues in the meat of "growth enhanced" animals and can disrupt human hormone balance, causing developmental problems, interfering with the reproductive system, and even leading to the development of breast, prostate or colon cancer.

CBS Article - Hormones and cancer

    Children , pregnant women and the unborn are thought to be most susceptible to these negative health effects. Children have relatively low natural levels of the sex hormones in their bodies so even small increases are cause for concern. "The younger you are [when exposed], the greater the risks," says Samuel Epstein, M.D., professor of occupational and environmental medicine at the University of Illinois Medical Center and founder of Cancer Prevention Coalition. Hormone residues in beef have been implicated in the early onset of puberty in girls, (the average age of menarche is now 12.5 down from 17) which could put them at greater risk of developing breast and other forms of cancer. The incidence of breast cancer has tripled in the last 50 years and this cannot be due to genetics.