Dropping foods that are rich in water-soluble vitamins (like the Bs, C, folate) into cooking water leaches some of the vitamins. That’s fine for a soup or stew, less so if you’re draining the veggies. A Danish study found that boiled broccoli retained only 45% to 64% of its vitamin C after 5 minutes of boiling; steamed broccoli kept 83% to 100%.
What to do: Haul out that old steamer. Also good: microwaving.