Caloric restriction (CR), or calorie restriction, is a dietary regimen that is based on low calorie intake. "Low" can be defined relative to the subject's previous intake before intentionally restricting calories, or relative to an average person of similar body type. Caloric restriction without malnutrition has been shown to work in a variety of species, among them yeast, fish, rodents and dogs to decelerate the biological aging process, resulting in longer maintenance of youthful health and an increase in both median and maximum lifespan.