At Illinois, Rose focused his research work on amino acid metabolism and nutrition. He found that the 19 amino acids then known were not sufficient for growth,[5] and this led to his discovery in 1935 of the last of the common amino acids, α-amino-β-hydroxy-n-butyric acid, later named threonine.[2] His studies also distinguished the amino acids that are absolutely essential from those that are necessary only for optimal growth. His studies further led him to the point where it was "practicable to evaluate proteins in terms of their ability to meet human needs."