broccoli

Previous research on the cancer-preventing benefits of broccoli has found that
overcooking broccoli results in the elimination of up to 90 percent of
the vegetable's anti-cancer compounds. However, gently cooking broccoli
in a steamer, not in the microwave, can potentially unlock more of its
anti-cancer compounds than are present when eating the vegetable raw.

http://www.naturalnews.com/031498_broccoli_cancer.html#ixzz1EzWzdUUB