Basic metabolism: protein

Proteins serve a wide variety of functions. They are made by the sequential incorporation of amino acids in an order that is determined by genes and which in turn determines the final structure and thus the function of the protein. Proteins are constantly being synthesized and broken down at rates which exceed the amount of protein consumed in the diet and the equivalent amount of nitrogen that is excreted as urea in the urine. Some amino acids can be synthesized in the body (non-essential amino acids) but others (essential amino acids) must be supplied from dietary proteins in amounts that match the amounts that are oxidized. Hence the criterion of adequacy of a diet with respect to protein is its ability to maintain nitrogen balance in an adult and its ability to support normal growth in a child. During some critical illnesses there appears to be a constraint on the rate at which some non-essential amino acids can be synthesized, so that clinical benefits have been seen from supplementing the diet with these ‘conditionally-essential’ amino acids. Moreover, following surgery or during severe illness there is an increase in amino acid oxidation that can lead to muscle wasting. It is important to minimize the loss of lean tissue, but it may not be possible to prevent it completely by dietary means.