When vegetables are cooked in certain conditions this can have an impact on nutritional quality of the final product.  This is true of the B and C group vitamins which can be reduced or destroyed.  Vitamins B and C dissolve in water meaning if any vegetables are cooked in water by blanching or boiling; most of these vitamins will be lost when the water is thrown out.  It’s a different story if your final dish is soup where you drink the water.  You can retain more of the B and C vitamins in vegetables if you steam or microwave where possible as there is less contact with water.  Vitamin C is heat sensitive with microwaving seeming to be the best method in retaining more of it when cooking vegetables.  Vitamin C is also destroyed by ‘’Air’’ so any peeled fruit or vegetables shouldn’t be left sitting around without being covered for later use.  Cling film is a good idea here.  On the flip side of cooking effects on vitamins B and C, cooking vegetables can increase levels of vitamin E and K especially in the green ones.  Another important point to consider is that sautéing and oven baking can increase flavonol content in vegetables like onions with boiling having the opposite effect in say for example an onion soup.  When considering cooking your vegetables, you have to be strategic when thinking about which vitamins and/or nutrients you are looking to achieve in optimal amounts.