Proteins
Individual proteins are made up of various combinations of at least 50, and up to several thousand, amino acids strung together in a long chain. A complete protein depends upon this chain of amino acids gathering into a highly complex, multi-layered string, and then going on to twist itself into intricate
three-dimensional shapes. Proteins can be made by the body internally as well as imported ready-made via the diet. The important function of the dietary proteins is their supply of constituent amino acids, which the
body can resynthesise into new specialised proteins tailored for its various vital processes.
Amino acids come in 22 different forms and all are needed to build the many different kinds of proteins the body requires. Eight of these amino acids (ten for children) fall into the category termed essential amino acids – this means that they have to be supplied in food because the body is unable to manufacture them. Without them, the body cannot build the full range of proteins in amounts that are necessary for the growth and maintenance of the various tissues. The remaining 14 amino acids are called non-essential amino acids because the body is able to synthesise them itself. But despite their name, non-essential amino acids are just as important for health as the essential ones because all proteins are constructed from arrangements of both essential and non-essential amino acids.
HOW THE BODY USES PROTEIN
The enzymes in the digestive system break down the protein in food into its constituent amino acids, which then circulate in the bloodstream. The body cells stack up the amino acids like building blocks according to which amino acids the cell needs. Each amino acid has a unique role, and another one cannot be used in its place, so it is important to make sure your diet is varied enough to provide all the amino acids that your body needs.
COMPLETE AND INCOMPLETE PROTEINS
Proteins vary greatly in the number, type and arrangement of their amino acids, and so do their quality or ‘biological value’.
Animal foods are a major source of protein and the protein present in meat, fish, eggs, milk and milk products contains all the essential amino acids in the proportions the body requires. Animal protein, therefore, is known as ‘complete’ protein.
The protein in plant foods – cereals (grain products), nuts and pulses (peas, beans and lentils) – also contributes different amounts and types of essential amino acids. However, unlike animal sources, no single
plant source is able to provide a full complement of essential amino acids. Plant proteins, therefore, are ‘incomplete’ and have a low ‘biological value’.
In order for plant foods to supply complete proteins, those that are low in certain essential amino acids should be eaten with those that are relatively high in those amino acids. Wheat, corn and rice, for example, contain plenty of the essential amino acid methionine but not very much of another amino acid called lysine, whereas beans contain a good supply of lysine but not much methionine.
Without knowing the chemical and biological reasons, cooks around the world for centuries have combined grains and pulses to provide complete protein in a single dish – for example, baked beans on toast.
How much protein is enough?
The body gains and loses protein every day – for instance, simply cutting your nails or hair will cause a minor loss of protein, whereas every four days one-half of the intestinal lining, which has a vast surface area, is replaced. Therefore, it is important to have some protein
every day to replace the worn-out tissues that result from general bodily wear and tear and to manufacture new blood cells, hormones and enzymes.
