A calorie is a standard unit of energy when food is used as fuel. All calories have equal energy producing value when metabolized for general energy. But calories vary greatly in their total nutritional value. In order to differentiate between calories of high or low nutritional value, the word "calorie" is reserved for those of higher nutritional value, and the new word "killorie" used for those of low, little or negative nutritional value. Calories obtained from whole unprocessed or minimally processed foods have higher nutritional value than calories obtained from refined or highly processed foods. They contain a natural ratio of essential nutrients and fuel. In addition to providing fuel, calories contain variable amounts of essential nutrients and other substances necessary for optimal health. These include essential amino acids and lipids, inorganic elements called minerals, vitamins that serve as cofactors (catalysts) and vitamins that serve as preservatives and stabilizers. All whole foods contain a wide variety of all of these substances. Whole foods have high nutritional value per calorie. These foods consist of actual living tissue of plants and animals, the edible portions of plants and animals. Since these foods consist of whole cells of living things, they preserve the vital blend of nutrients found in living food and tissues.