In late Renaissance medicine, the example of digestion was frequently invoked to prove the elemental composition of the human body. Food was considered as being decomposed in its first elements by the stomach, and digested into a thick juice which is assimilated by the liver and the body parts. Such a process points to the structure of the human body into four elements that are transformed into different types of humors during several stages of “concoction”. This chapter examines the Galenic interpretation of digestion expounded by the French physician Jean Fernel (1497-1558) in his Physiologia (1567). In this treatise, Fernel states the body composition into elemental portions, while stressing the role of the “innate heat” as the physiological counterpart of the body’s essence or “substantial form”. He applies this view in his account of digestion, where he states that the conversion of food follows the rule of “mixture”. This chapter aims to explore how Fernel applies his interpretation of elements and innate heat to the process of digestion, as well as his sources in Galen’s De facultatibus naturalibus, Avicenna’s Canon, and Aristotle’s Meteorologica. It first examines the role of the natural soul and its “nourishing” faculties in nutrition as a physiological function. It then considers the role of elements, humors and innate heat during the “concoction” of food in the stomach, liver and veins.