query: Who first identified metabolism?
passage: History of biochemistry
One of the most prolific of these modern biochemists was Hans Krebs who made huge contributions to the study of metabolism.[27] Krebs was a student of extremely important Otto Warburg, and wrote a biography of Warburg by that title in which he presents Warburg as being educated to do for biological chemistry what Fischer did for organic chemistry. Which he did. Krebs discovered the urea cycle and later, working with Hans Kornberg, the citric acid cycle and the glyoxylate cycle.[28][29][30] These discoveries led to Krebs being awarded the Nobel Prize in physiology in 1953,[31] which was shared with the German biochemist Fritz Albert Lipmann who also codiscovered the essential cofactor coenzyme A.
passage: Jens Nielsen
Using his systems biology toolbox developed for microorganisms, Jens Nielsen initiated work on human metabolism. In connection with this he developed a comprehensive genome-scale metabolic model for human cells and he was the first to use a human GEM to illustrate the metabolic heterogeneity of cancer metabolism. His work on human metabolism has involved studies of different diseases such as obesity, NAFLD and NASH, and hepatocellular carcinoma. Jens Nielsen further used human GEMs to identify that combined measurements of several glycosaminoglycans can be used as a very strong biomarker for clear cell renal cell carcinoma, probably the first systems biomarker.