Quoted from Our Stolen Future:
Hormone mimics lurk in parsley, sage, and garlic; in wheat, oats, rye, barley, rice, and soybeans; in potatoes, carrots, peas, beans, and alfalfa sprouts; in apples, cherries, plums, and pomegranates; and even in coffee and bourbon whiskey. Like DES and DDT, these plant compounds can fool the estrogen receptor.
If the clover in Australia were the only case of a natural hormone mimic in the annals of science, it might be shrugged off as an evolutionary fluke, but the presence of an estrogenic substance in so many diverse plant species suggests this is no accident.
So why are plants making estrogens?
“Plants are making oral contraceptives to defend themselves,” says Claude Hughes, a researcher exploring the effects of hormone-like compounds on the reproductive system.
The more Hughes explored the notion that plants might be making contraceptives, the more evidence he found consistent with the theory that this is indeed what plant are up to. By lacing their leaves with hormonally active substances, they suppress the fertility of the animals that feed on them. […] The plants that make estrogen mimics, he notes, are tasty ones sought out by animals an humans for food, not the unappetizing plant that contain foul-tasting compounds–an alternative defense strategy.
Humans have long used marijuana as a drug because the chemicals it contains act in the brain to alter mood and perception, creating a “high.” But as Hughes and others discovered, these chemicals do more than induce a pleasant mellowness; they interfere with reproduction in a variety of ways. The same compound that makes a pot smoker high also acts on the testicles to reduce the synthesis of testosterone and on the brain to suppress lutenizing hormone, a key hormone that cues ovulation in females and testosterone production in males. Studies have reported that marijuana feminized men who smoked it heavily.
Hughes’s work focused on the way that marijuana interferes with the hormone prolactin, which is produced in the brain and signals the breast to produce milk. Mother rats given marijuana produced no milk, and their pups died of starvation.
Our Stolen Future, by Theo Colborn et al., pg. 76-78.