Raw foods contain enzymes which act as catalysts to regulate the digestive process in the body. Heating (or freezing) food degrades or destroys these enzymes in food. Food without enzymes is thought to lead in the longer term to toxicity in the body, to excess consumption of food, and therefore to obesity. Living and raw foods is thought to usually have much higher nutrient values than foods which have been cooked. The main idea behind the raw food diet is that cooked food is supposedly toxic, because cooking destroys the enzymes contained in food.

Some research does not support the idea that enzymes in foods somehow survive the stomach (pepsin in the stomach quickly breaks down nearly all proteins, including enzymes). It is accepted that foods cooked at high heat contain toxins not found in raw or boiled foods, such as acrylamide, benzopyrene, and methylcholanthrene. There is no consensus as to whether these toxins introduced by high-heat cooking methods are cause for alarm. Raw foodists argue that since no other animals cook their foods, and (according to some) they don't get the extensive degenerative diseases that humans do, it's clearly logical to not cook or process food.