Cooking is a destructive process, not a constructive one. Nutrients are destroyed in various ways by applying fire to our food before we eat it. Certain nutrients are damaged or even completely oxidized by the cooking process. Vitamin C and all the B vitamins for example are easily destroyed by heat.
Unless you are boiling your food, there will of course be a loss of water content. Water is essential to our health and ideally we want to get most of it from the food we eat. Also, we don’t want to become dehydrated as a result of eating dry foods.
Proteins are denatured when exposed to heat, this means they lose their original shape and become enzyme resistant, meaning they are now more difficult, even impossible, to break down into amino acids by our digestive system. As a result, protein fragments called polypeptides now enter into our bloodstream, and our immune system reacts with an inflammatory response as if they were foreign invaders.
Cooking carbohydrates caramelizes the starch and creates neurotoxins like acrylamide, a very toxic chemical that is linked with cancer and disease. Fats are rendered toxic by the application of heat, and even release chemical compounds into the air that are dangerous to inhale.
In conclusion, the toxins created by cooking together with the negative impact heat has on the integrity of the nutrients themselves, makes cooked food something we should avoid.