During a recent discussion about cooking, somebody said to me, "You're wrong, you shouldn't cook vegetables." Are nutrients and enzymes destroyed during cooking? The easy answer is "yes," but like most things, the truth is more complicated.
Cooking can soften fiber, making cell walls less rigid. This not only makes food easier to digest, but it also can make certain vitamins in the cell wall more available for absorption. Carrots, noted for their beta carotene, have higher levels of the nutrient when cooked.
But with cooking comes compromise, too. Cooking carrots destroys many polyphenols. Antioxidants are used up during frying because the oil is constantly being oxidized. And although boiling can create more available beta-carotene in carrots, it also can leech water-soluble vitamins like vitamin C.
Another example is tomatoes: They have significantly less vitamin C when cooked, but also much higher levels of lycopene, which may be a more potent antioxidant than vitamin C. And since vitamin C is found in many other common foods, cooked tomatoes might be the better choice.
Similarly, cooking broccoli destroys myrosinase, but also increases indole levels. Both of these chemicals may prevent the growth of cancer cells.
There are benefits and drawbacks to any food, and any cooking method; there is no one way to eat, just whatever way works for you.