ABSTRACT
Protein nutrition and metabolism The need for protein in the diet was demonstrated early in the 19th century, when it was shown that animals fed only on fats, carbohydrates, and mineral salts were unable to maintain their body weight and showed severe wasting of muscle and other tissues. It was known that proteins contain nitrogen (mainly in the amino groups of their constituent amino acids, Section 4.4.1), and methods for measuring the total amount of nitrogenous compounds in foods and excreta were soon developed. It is obvious that a growing child or animal requires a dietary intake of protein to increase the total protein content of the body with growth. However, it was not until the middle of the 20th century that it became clear why an adult also requires a daily intake of protein. There is continual catabolism and replacement of tissue proteins. In the fasting state between meals, proteins are catabolised to provide amino acids for gluconeogenesis (Section 5.7), and in the fed state, amino acids in excess of immediate requirements for protein synthesis are not stored, but are catabolised and used as metabolic fuel.
