On the one hand cooking destroys the natural chemical composition of the food. The vitamins are altered and destroyed, the proteins are scrambled, the enzymes are torn to pieces. Even simple mixtures of just a few inorganic elements can often be totally changed in character through cooking over a Bunsen burner. Taking it from the immaterial angle, the great harm of cooking is that it destroys or drives off the life-force in the food. Just look at how strong and alive a fresh raw carrot looks, and compare that to the limp and decaying appearance of a cooked carrot.
Some people are able to dowse the life-force in foods via a pendulum.
The wider the circle the pendulum swings in, the greater the life-force. They find that the pendulum indicates much greater life-force in raw foods, as our common sense would expect. But the most convincing test is that of your own experience. You can make a comparison between a meal of raw vegetables, and a meal of the same types of vegetables cooked. The raw vegetables leave you feeling light, fresh and alert, whereas the same vegetables cooked will make you feel lethargic and sluggish.