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The use of isotopes in the study of intermediary metabolism has given a picture of the state of living matter of contrast to the static theories formerly in vogue. The analogy of the combustion engine with its steady flow of fuel into a fixed system has been replaced by the concept that all constituents of living matter, structural or functional, single or complex, are in a steady state of rapid flux. Schoenheimer's pioneering efforts in the use of heavy hydrogen in labeling fatty acid molecules and his brilliant later work on amino acids and the formation of various excretory products are classics in this field. The three lectures in this short monograph were drafted by Dr. Schoenheimer and, after his untimely death, revised and delivered by Dr. Hans T. Clarke. They cannot be considered a review of the entire field of isotopic experimentation. Dr. Schoenheimer's theories are, however, well presented
The Dynamic State of Body Constituents. JAMA. 1942;120(8):654. doi:10.1001/jama.1942.02830430076032