Delicious Living’s medical editor, Robert Rountree, MD, has been doing some research with the Brassica Chemoprotection Laboratory at Johns Hopkins, the folks that originally discovered sulforaphane. Here’s his take:

Numerous published studies have shown that eating brassica vegetables can have a powerful effect in preventing chronic disease—especially cancer—an effect which is much more powerful than what is seen from consuming other classes of vegetables.

A compound called glucoraphanin that is unique to brassica is thought to be responsible for much of this benefit. When brassica vegetables are crushed or chewed, they release an enzyme called myrosinase that converts glucoraphanin—an inactive substance—into sulforaphane, which is the active compound that works in the body.

Broccoli sprouts are a very rich source of glucoraphanin; they also contain high levels of myrosinase. Broccoli seed extracts also contain high levels of glucoraphanin, but no myrosinase.
 
The assumption was that humans make sufficient quantities of myrosinase in the gut to convert all that glucoraphanin into sulforaphane. What these recent studies have shown (something we have also confirmed with our own research) is that myrosinase production is all over the map—some people make lots of it and others hardly make any. What this means is that some people greatly benefit from taking broccoli seed extracts and other people don’t benefit at all. The problem is that we have no way of predicting who is going to be a good myrosinase producer.

The other wrinkle: Most of the broccoli eaten around the world is thoroughly cooked, which destroys most of the myrosinase. But numerous studies show that people who eat a lot of broccoli have less cancer. We know people are benefitting from eating cooked broccoli, so they must be making a fair amount of myrosinase in their guts.