Steaming is the cooking of food in hot vapor rather than in liquid. As the food never touches the liquid, the loss of vitamins is significantly reduced. And as steamed food is cooked without fat, it is more digestible and highly suitable for invalids and those on low-fat diets. You must, however, ensure that you always choose excellent quality ingredients for steaming - there is no help to be had from the flavor of butter or oil, nor does the food caramelize during cooking, so the raw materials must taste good without such assistance.
** Asian Cooking