Proteins are highly complicated molecules. They are composed of hundreds of thousands of smaller units, known as amino acids. Amino acids are generally linked together in the form of long chains. All amino acids contain nitrogen, carbon, hydrogen and oxygen and some also contain other elements, such as sulphur. Every protein is made up of a specific combination and sequence of different amino acids that commonly occur in nature. The number of possible pat-terns of different amino acid sequences is virtually infinite, resulting in an enormous variety of different proteins, each one with its own particular properties. When you eat food containing proteins they are split into their constituent amino acids by enzymes (which are themselves proteins) in the stomach and small intestine. The amino acids are then absorbed through the intestine wall into the bloodstream and eventually carried in the blood to all the cells in the body, where they are rebuilt into whatever particular protein that cell requires.
Examples of proteins include collagen (found in skin, tendons and many other tissues), keratin (the major component of hair and nails), myosin (in muscles) and special molecules called enzymes that control the chemical reactions in cells. Antibodies, which combat infection, and most hormones are also proteins. The body needs about 20 different amino acids to produce all the different proteins it needs (which is several thousand). Roughly half of the 20 can be made in the body if enough proteins are present in the diet. But eight amino acids (ten in children) must be included in the diet, as the body itself cannot make them. For this reason these eight are called ‘essential amino acids’.
The number and type of proteins in food is exreemely variable. A protein source that breaks down to provide a good assortment of essential amino acids is said to be of ‘high quality’. Such proteins are found in animal foods such as meat, fish, milk, cheese and eggs. If none or only some of the essential amino acids are present then the food’s protein quality is ‘low’. Most sources of plant proteins, such as wheat, peas, beans and oats, are short of one or more of the essential amino acids, hence they are of a lower quality. But some high-quality vegetable proteins do exist (soya beans, for example). Luckily, few people eat just one protein-containing food; generally people eat several at the same meal and in this way one protein can compensate for the lack of another. For example, the essential amino acid called lysine is not present in cereals but there is plenty of lysine in milk. Pulse vegetables contain too little of the amino acid methionine, but eggs, wheat and sesame seeds (which are rich in methionine) can enhance the protein quality of a meal of pulses such as lentils or beans. Richest are cheese, lean meat and poultry such as chicken. Nuts, white (non-oily) fish, shellfish and crustaceans also contain much protein. The richest vegetable sources are nuts, followed by bread and pulses.
The important thing to remember is that you eat foods rather than protein, and you can juggle combinations of foods (including vegetarian ones) to give a diet that supplies all the essential amino acids. The following pairs of foods complement each other well in this way, providing between them protein that is high in quality: potatoes + milk cereals + milk, meat or fish wheat + peanuts pulses + cereals pulses + eggs.