The simple answer is yes—but not by much.
High heat can cause some vitamins to breakdown, including Vitamin C and the B Vitamins. It’s the way foods are cooked, however, that affects the amount of vitamins in a finished recipe. Simmering carrots in a beef stew results in a greater loss of these vitamins than when they are quickly sautéed. Cooking does not significantly change most other nutrients, such as fats, minerals and fiber, and many are made more digestible with cooking (as with proteins in meats or fiber in cooked oats).
Much more important is the freshness of the foods you are cooking. The “fresh” fruits and veggies that we eat are sometimes a week or more old before they make it into our refrigerators. In one study published in the Journal of Agricultural and Food Chemistry, it was shown that many of the cancer preventing nutrients (known as phytochemicals) are lost as vegetables were aged under conditions similar to those in stores and your refrigerator. Interestingly, canned or frozen vegetables are usually more nutritious because they are preserved soon after being picked. Read More "The Health of It All..." Articles