P R O T E I N S: All We Need To Know

Proteins are the main nitrogen-containing constituents of animal and plant tissues. They are essential for the synthesis of body tissues and regulatory proteins such as enzymes and hormones. Dietary protein usually accounts for about 10–20% of the energy in human diets. The

majority of people in developed populations eat far more protein than is required for the essential functions such as replacing body tissues, and much of the protein is broken down to produce energy.

Proteins are made of selected amino acids from the 20 different amino acids present in nature, and joined together through peptide links (amino group of one protein to acid group of the next) to form an almost infinite number of proteins with different structures and functions. Different combinations and sequences of the polypeptide chains allow them to take up different shapes

and carry out particular functions within the organism. Unlike the constituents of carbohydrates and fats, amino acids contain nitrogen as well as hydrogen, carbon, and oxygen, and some also contain phosphorus or sulphur.

Eight of the amino acids are essential in adults, i.e. they cannot be made in human tissues and must be obtained from the diet. These are phenylalanine, tryptophan, leucine, isoleucine, valine, threonine, methionine, and lysine. In addition, arginine and histidine are regarded as essential in infants as they cannot make enough for their requirements. The remaining amino acids are non essential as they can be made in the body.

When proteins in foods are ingested they are digested in the stomach and small intestine, and the constituent amino aids or short peptide chains are then absorbed into the blood, to be carried to tissues where they will be used to manufacture body proteins or non-protein products (e.g. nucleic acids), hormones (e.g. thyroxine), neuro transmitters (e.g. serotonin), or oxidized to provide energy.

Protein quality and intakes

Food proteins do not all have the same capacity to provide nitrogen and essential amino acids to the body. The use fulness or quality of a protein depends on the balance of amino acids and the digestibility of the protein. The body requires particular amounts of each essential amino acid,

and an ideal food protein would have an amino acid pattern as close to this as possible. Amino acids that are not part of the required pattern will be used for energy.

Animal proteins, especially egg and milk proteins, have amino acid patterns similar to the body’s requirements and are used as reference proteins. Plant proteins are relatively low in certain amino acids, and that which is present in the lowest amount relative to requirements is

called the limiting amino acid. Methionine and cysteine are the limiting amino acids in legumes, and lysine is the limiting amino acid in cereals. As most populations eat a mixture of proteins this is not usually important a meal such as beans on toast would correct the deficiencies of both the above food groups. However, if total food intake is too low to satisfy energy requirements, or consists largely of a particular food with a low protein content, protein deficiency may occur.

People in developed countries eating a range of foods that supply sufficient energy are unlikely to be short of protein. Most people eat more than enough, and any protein that is not used to build tissues is used to provide energy. There is therefore no advantage to eating a very high-protein diet. Some athletes take high-protein supplements or specific amino acid supplements.