The debate over which is healthier, raw vegetables or cooked vegetables is on-going. The answer is-it depends on the vegetable and how you cook it. Sometimes raw is better, sometimes cooked is better. "Unfortunately, the pros and cons of eating raw vs. cooked vegetables are not clear cut," according to UAB Medicine, the largest academic medical center in Alabama. (See Reference 1)
EAT A COMBINATION OF BOTH
One serving of vegetables may contain 100 different phytochemicals, which are chemical compounds that have protective and disease-preventive properties. Cooking can destroy some of these phytochemicals, but it can also enhance others, so it is important to eat a mix of raw and cooked vegetables.
Broccoli and other cruciferous vegetables, such as cauliflower, cabbage, Brussels sprouts and kale, are good examples of vegetables that should be eaten both raw and cooked. Raw broccoli contains an enzyme called myrosinase that breaks down into sulforaphane, a compound thought to help prevent cancer and stomach ulcers. Cooking broccoli damages myrosinase. On the other hand, cooking broccoli and other cruciferous vegetables forms the compound indole, a phytonutrient that fights precancerous cells before they cause damage and turn malignant.
Raw carrots supply polyphenols, a group of chemicals with antioxidant properties thought to reduce the risk of heart disease and cancer. Boiling carrots destroys all the polyphenols. Cooking also breaks down the vitamin C found in carrots, which easily degrades when exposed to water and heat. That said, cooked carrots supply more of the antioxidant beta-carotene---which your body converts to vitamin A---than raw carrots. The same holds true for spinach, asparagus, cabbage, peppers and many other vegetables.
Tomatoes contain lycopene, an antioxidant compound linked to the prevention of cancer and other chronic diseases. Ohio State University researchers have found that "lycopene molecules in tomatoes change their shape, which makes them more usable by the body, when combined with a small amount of fat and subjected to intense heat during processing." The amount of lycopene in tomatoes is intensified and it is better absorbed by the body after cooking, particularly with a little oil, and processing, rather than eaten raw or fresh.
QUICKLY STEAMING IS BEST COOKING METHOD
The cooking method matters as well. Steaming, rather than boiling, helps retain water-soluble B and C vitamins. High cooking temperatures and long cooking times destroy heat-sensitive nutrients such as B and C vitamins and folate, so it's best to keep cooking times to a minimum. The Journal of Food Science studied 20 vegetables, with results that indicated microwaving and baking led to the lowest losses of antioxidants, while pressure-cooking and boiling led to the greatest losses, according to Ohio State University researchers in August 2009.