According to Walter Gratzer, the study of nutrition probably began during the 6th century BC. In China, the concept of "qi" developed, a spirit or "wind" similar to what Western Europeans later called "pneuma". Food was classified into "hot" (for example, meats, blood, ginger, and hot spices) and "cold" (green vegetables) in China, India, Malaya, and Persia. "Humours" developed perhaps first in China alongside "qi". Ho the Physician concluded that diseases are caused by deficiencies of elements (Wu Xing: fire, water, earth, wood, and metal), and he classified diseases as well as prescribed diets. About the same time in Italy, Alcmaeon of Croton (a Greek) wrote of the importance of equilibrium between what goes in and what goes out, and warned that imbalance would result in disease marked by obesity or emaciation.