Dear KB,
     As a general rule fresh foods will lose more nutrients the longer the time from field to plate. Cold can slow down this process, but it's heat that serves as the prime nutrient destroyer. The greater the heat and the longer the time of exposure, the greater the nutrient destruction.
     Minerals are the only nutrients that are unaffected by heat. Proteins, carbohydrates and fats have a reasonable degree of heat stability, but they too will succumb if the heat is high enough. Vitamins tend to be the vulnerable nutrients, with the water-soluble vitamins being the ones most effected. Vitamins C, thiamin and folic acid, for example, can be totally destroyed by heat, and the fat-soluble vitamins can be also destroyed with extended cooking times. There is less data on other plant chemicals (phytochemicals), but it is probable that they would be vulnerable to heat destruction as well.
     A list of cooking methods from shortest to longest might look as follows: microwave, sauteeing, stir-frying, pressure-cooking, steaming, deep-fat frying, roasting, broiling, boiling and baking. Cooking times, of course, can vary from food to food.
     Because it is the most rapid cooking method, a microwaved potato will have the greatest amount of vitamin C. A boiled potato will tend to have more vitamin C than a baked potato because a baked potato takes longer to cook.
     Nutrients can be lost in ways other than outright destruction. The use of water with boiling can represent a problem in that some nutrients will dissolve in water. If the water is then discarded, these nutrients are then lost in addition to those destroyed by the heat. Steaming and pressure cooking also use water, but they represent less of a problem because less water is used during the cooking process.