Cooking methods like steaming or saut from a brief period helps retain vitamins and phytonutrients like carotenoids and overcooking destroys both vitamins and phytonutrients. If the cooking method is steaming toss in a little healthy fat (olive oil, sunflower oil) before serving to ensure carotenoid absorption. Example lycopene in tomatoes is absorbed in highest amounts from cooked tomato products that contain a little olive oil.

Source: http://www.gud2eat.com