Proteins are made up of amino acids. These amino acids, which are made of carbon, hydrogen, oxygen, nitrogen and sometimes sulphur, can join together in thousands of combinations to give proteins with various functions. Our diet should provide the full range of amino acids in order for the body to utilise them to make the proteins it requires for normal functioning.
The word ‘protein’ is derived from the Greek word meaning ‘of first importance’. Second only to water, protein is the most abundant substance in the human body, comprising about 75% of the body’s dry weight.
In total, the human body manufactures over 100,000 different proteins, with functions as varied as providing structure (skin, bone, muscle), to preventing and fighting infection and disease (antibodies).
The amino acids derived from the digestion of dietary protein are re-assembled into the body’s own specific proteins. If an essential amino acid is deficient in the diet, the body does not make proteins that lack this amino acid, but instead simply makes less protein.
This means that the free amino acid ‘pool’ must always include sufficient amounts of essential amino acids if the body is to function at its optimum.
The nutritional value of a dietary protein, called its Protein Efficiency Ratio (PER), is a measure used to determine how adequately a protein supplies the essential amino acids. The PER is influenced by two factors:
The ratios of the essential amino acids in the protein.
The digestibility of the protein, i.e. how easily and to what degree the protein can be broken down to its component free amino acids by the enzymes of the digestive system.
In general, proteins derived from animal sources (eggs, meat, milk, fish, poultry) have the highest PERs and may be referred to as complete or ‘high biological value’ proteins. Proteins derived from vegetable sources tend to have lower PERs and must be mixed to complement each other’s strengths and weaknesses (soya is an exception to this).
Fortunately, natural balancing occurs in many commonly eaten vegetarian combinations such as beans on toast or rice and lentils (grains are short in lysine but rich in methionine, whereas pulses are rich in lysine but poor in methionine).
Proteins are important from another point of view in that they supply nitrogen to the body. Because nitrogen is involved in the synthesis of many crucial compounds, including the non-essential amino acids, a minimum daily intake of protein is vital to keep the body in ‘nitrogen balance’.
During illness or stress, the body often cannot make enough non-essential amino acids to meet its needs, so an adequate intake of sufficient good quality protein is critical at these times.