Protein

BACKGROUND

Protein occurs in all living cells and has both functional and structural properties. Amino acids, assembled in long chains, are the building blocks of protein. Of the 20 amino acids found in proteins, some can be made by the body while others are essential in the diet. Amino acids are used for the synthesis of body proteins and other metabolites and can also be used as a source of dietary energy. The proteins of the body are continually being broken down and resynthesised in a process called protein turnover.

Protein is the body's main source of nitrogen which accounts for about 16% the weight of protein. Non-protein nitrogenous compounds are usually present in the diet in minimal amounts. Thus, in assessing dietary protein sources, the total amount of protein, its digestibility and its content of essential amino acids need to be considered. Proteins also contain carbon, oxygen, hydrogen and, to a lesser extent, sulphur.

The nine indispensable or essential amino acids, defined as those that the body is unable to synthesise from simpler molecules, are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Cysteine and tyrosine can partly replace methionine and phenylalanine, respectively. Under certain extreme physiological conditions such as in prematurity or during some catabolic illnesses, the non-essential amino acids arginine, cysteine, glutamine, glycine, proline and tyrosine may be required in the diet. Under normal conditions, glutamine, glutamate or aspartate can supply arginine; methionine and serine can be converted to cysteine; glutaminic acid and ammonia can be converted to glutamine; serine or choline can supply glycine; glutamate can provide proline and phenylalanine can be converted to tyrosine. These amino acids are sometimes termed conditionally indispensable. Alanine, aspartic acid, asparagine, glutamic acid and serine are non-essential. The amino acids act as precursors for many coenzymes, hormones, nucleic acids and other molecules.

Proteins in the diet and the body are associated with a number of other vitamins and minerals and are more complex and variable than other energy sources such as fat and carbohydrate. The polypeptide chains that make up proteins are folded into three-dimensional structures that include helical regions and sheet-like structures due to interaction between the amino acids in the chain. The final shape of a mature protein often reflects its function and also interactions with other molecules. The protein's structure may influence its digestibility.

The body of a 76 kg man contains about 12 kg of protein. Nearly half of this protein is present as skeletal muscle, while other structural tissues such as blood and skin contain about 15% (Lentner 1981). Myosin, actin, collagen and haemoglobin account for almost half of the body's total protein content. Only 1% of the body's store is labile (Waterlow 1969, Young et al 1968), so its availability as a reserve energy store, compared to body fat, is limited. Unlike carbohydrate and fats, the body does not maintain an energy storage form of protein.

Proteins are found in both animal and plant foods. The amino acid profile of animal proteins is closer to that of humans but all of the necessary amino acids can be provided in the amounts needed from plant sources. The major sources in the Australian and New Zealand diet are meat, poultry and fish (about 33%), cereals and cereal-based foods (about 25%) and dairy foods (about 16%). Vegetables also provide about 8%. Certain proteins can cause allergic responses in some individuals notably milk, eggs, peanuts and soy in children and fish, shellfish, peanuts and tree nuts in adults.

The efficiency of dietary protein digestion is high. After ingestion, proteins are denatured by acid in the stomach and cleaved to smaller peptides. A number of gut enzymes including trypsin, chymotrypsin, elastase and carboxypeptidases, complete the process. The free amino acids and small peptides that result are absorbed into the mucosa by specific carrier systems. After intracellular hydrolysis of absorbed peptides, free amino acids are secreted to the portal blood where some of the amino acids are taken up and the remainder pass into systemic circulation for delivery to, and use by, peripheral tissues.

There is wide variation in dietary protein intake, to which the body is able to adapt over a few days. However, severe disease states or fasting can cause substantial body protein losses as energy needs take priority. The protein lost is, however, also necessary to the functioning of the body. A serious depletion in the body mass protein can be life threatening with muscle loss, including loss of heart muscle (Hansen et al 2000). Thus, not only must sufficient protein be provided for sustenance, but also sufficient non-protein energy so the carbon skeletons of amino acids are spared from providing energy. Similarly, unless amino acids are present in the right balance, protein utilisation will be compromised (Duffy et al 1981). Protein-energy malnutrition (PEM) is common on a worldwide basis in both children and adults (Stephenson et al 2000) causing the death of 6 million children a year (FAO 2000). In countries like Australia and New Zealand, PEM is seen most commonly associated with other diseases and in the elderly. Protein deficiency affects all organs including the developing brain (Pollitt 2000), as well as the immune system (Bistrian 1990) and gut mucosal function (Reynolds et al 1996).

There are two key methods for assessing protein requirements, factorial methods and nitrogen balance. For infants, the amount provided by the milk of healthy mothers is used to estimate the adequate intake.