 Early nutritional scientists such as the German Carl von Voit believed that protein was the most important nutrient for maintaining the structure of the body , because it was generally believed that " flesh makes flesh . " Karl Heinrich Ritthausen extended known protein forms with the identification of glutamic acid . At the Connecticut Agricultural Experiment Station a detailed review of the vegetable proteins was compiled by Thomas Burr Osborne . Working with Lafayette Mendel and applying Liebig 's law of the minimum in feeding laboratory rats , the nutritionally essential amino acids were established . The work was continued and communicated by William Cumming Rose . The understanding of proteins as polypeptides came through the work of Franz Hofmeister and Hermann Emil Fischer . The central role of proteins as enzymes in living organisms was not fully appreciated until 1926 , when James B. Sumner showed that the enzyme urease was in fact a protein . 
