Researchers studied the effects of boiling, steaming, microwave cooking and stir-fry cooking on fresh broccoli, Brussels sprouts, cauliflower and green cabbage. Boiling led to the biggest losses of cancer-fighting nutrients.n general, broccoli is safe to eat, and any side effects are not serious. The most common side effect is gas or bowel irritation, caused by broccoli's high amounts of fiber. All cruciferous vegetables can make you gassy, Jarzabkowski said.