Although many vegetables provide maximum nutrition in raw form, others are more nutritious when they're cooked. Heating a carrot, for example, releases extra beta-carotene from within the vegetable's cell walls, allowing your body to absorb a larger amount than if you ate it raw. Bottled, canned or homemade tomatoes are better sources of lycopene, a carotenoid that lowers cancer risk, when they're heated.
Cooking also releases slightly more calcium, iron and zinc in many vegetables.
You don't want to heat all your produce, though. Certain nutrients, such as vitamin C and folic acid, are easily lost during cooking or when produce is left on a store shelf, in a refrigerator or on a buffet table. Boiling results in a great loss of nutrients. Choose vegetables that have deep, rich colors. Bright orange carrots, dark green broccoli and vivid red tomatoes have higher nutrient contents than paler produce.