Overcooking

Eating broccoli and other cruciferous vegetables (cauliflower, Brussels sprouts and cabbage, to name a few) may help ward off cancer. But how you prepare them makes all the difference.
The plants contain chemicals known as isothiocyanates that hunt down cancer cells and kill them off quickly. Overcook your veggies, though, and those seek-and-destroy compounds fail to materialize. So, instead, lightly steam your broccoli for two to four minutes; this will help preserve the cancer-fighting agents as well as other heat-sensitive nutrients, like vitamin C.
To get the most cancer protection, aim for three to five servings of cruciferous vegetables a week.