The dietarily essential nitrogen-containing class of foods is called proteins. Like carbohydrates and lipids, proteins also contain carbon, hydrogen, and oxygen. All proteins are assembled from their basic units, the amino acids. The essential amino acid content of protein foods determines the quality of the protein. Most protein foods contain many different proteins, so the quality of a protein food reflects a composite of the amino acid content of several different proteins. In a research setting the protein status of humans is most commonly determined by nitrogen balance studies. Amino acids and their specific order in the primary protein structure provides the chemical specificity that dictates their function. Somatotrophin (or growth hormone), insulin, testosterone, and estrogen stimulate protein synthesis. Thyroxine increases the metabolic rate, and thus influences both the anabolism and catabolism of proteins. Both gluconeogenesis and ketogenesis from proteins are increased by glucocorticoids.