By adding the female sex hormones oestrogen and progesterone to cattle, scientists can stimulate the animals to produce extra muscle and fat. Adding the male sex hormone testosterone increases muscle growth, and decreases production of fat. Oestrogen has been linked to an increased risk of breast cancer and to reproductive disorders in men. Progesterone has been shown to increase the development of ovarian, breast and uterine tumours in laboratory animals. And testosterone has been linked to prostate cancer in men.
Scientists also use synthetic compounds to mimic the action of the naturally occurring hormones.
Trenbolone acetate mimics testosterone and zeranol mimics oestrogen. All these substances are implanted in tablet form under the skin of the animal, usually behind the ear or in an area that is unlikely to be eaten. Once implanted, the tablets can be active for 50-60 days, slowly circulating the hormones around the animal's system. Lingering concern Scientists argue that because these substances have no effect if swallowed, then they should not pose any health risk to humans eating animals whose flesh has been treated in this way. However, Professor Karl Linklater, a past president of the British Veterinary Association, said there is still concern over the safety of treating beef with hormones.