Diindoylymethane (DIM) is a component of Indole-3-carbinol (I3C) found in members of the Brassica family. Most notably broccoli, kale, and cauliflower.
It has potent effects on estrogen metabolism and is able to keep the body relatively balanced (by preventing either drastic increases or decreases in estrogen). In small amounts, it can both inhibit the aromatase enzyme (and prevent conversion of testosterone into estrogen) and it can act on more potent forms of estrogen and convert them into less potent forms; this conversion reduces the overall effects of estrogen in the body. However, taking too much DIM at once can actually induce the aromatase enzyme and act in the opposite manner and increase estrogen synthesis.
DIM also exerts numerous anti-carcinogenic (anti-cancer) effects in the body and is one of the reasons this vegetable family is seen as healthy.