There is a lot of debating going on over microwaving vs. using a conventional oven. Some say microwaving is the devil because of nutrient loss and cancer causing properties. Some nutrients are broken down when they are exposed to heat. So, is microwaving better than cooking vegetables in a convention oven? Since both use heat to cook, and microwaving takes a shorter amount of time to cook food, it is a slightly better option than cooking in the oven.
Frying is probably one of the worst ways to cook your vegetables. The heat is so high that almost all nutrients are immediately destroyed. Frying also causes the release of free radicals, which are detrimental to your health.
Boiling is probably the second worst way to cook your vegetables, unless you plan on using the remaining broth. When you boil vegetables, all the nutrients leak out into the water. Since many people do not save the water from the vegetables, you are not getting the benefits of the cancer fighting properties they hold.
Steaming is the gold standard for cooking vegetables. Steaming helps to retain the nutritional value of your vegetables. Some key things to remember when steaming are: don't use too much water (1/2 cup is usually good) and don't overcook (5-7 minutes is usually good for most vegetables).
Just to clarify, I am only speaking about eating certain foods raw, like vegetables, not the raw food diet, where a person will only eat raw foods (no meats, fish, etc). Studies have shown that both raw and cooked vegetable consumption are inversely related to epithelial cancers, lung cancer, and breast cancer; however, these relationships may be stronger for raw vegetables than cooked vegetables.
Degradation of Vitamins & Minerals
Once vegetables are picked, their vitamin and mineral content begin to degrade. To slow this process, most vegetables should be refrigerated until they are used. Also, many vitamins are easily destroyed by oxygen. In order to minimize nutrient loss, cut vegetables should be stored in airtight containers in the refrigerator. Just to keep in mind, after about 24 hours in the fridge, cooked vegetables will lose 1/4 of their Vitamin C content. Vitamin C is the most likely vitamin to be lost in cooking. It is very sensitive to heat, air, and water. To minimize nutrient loss, cook in as little water as possible.
The bottom line is, whether you are eating your vegetables raw or cooked, at least you are consuming vegetables!