I highly recommend Marion Nestle’s Food Politics blog as an excellent source of accurate news and information.
This post on the loss or retention of vitamins after foods have been steamed, stored, boiled, microwaved, or just eaten raw is refreshingly clarifying.
Bottom line: it’s a bit more complicated than you thought:
Cooking also destroys vitamin C. While I was looking for that article I came across this one, which describes experiments looking at the effects of common cooking practices (boiling, microwaving, and steaming) on beneficial antioxidants and phytochemicals in Brussels sprouts.
Steaming increased phytochemicals in fresh and frozen sprouts. Boiling did too, but only in the fresh vegetables. Cooking reduced phytochemical content in frozen samples. Microwaving was the best cooking method for retaining color and vitamin activity. As expected, all cooking methods destroyed vitamin C.
So what to make of this? Eat a mixture of cooked and uncooked vegetables and the vitamins will take care of themselves. If you do cook, steaming is great and microwaving is better for preserving vitamin activity. For vitamin C, raw wins every time.