The nature of protein in the diet

Most foods contain either animal or plant cells and will therefore naturally contain protein. But the processing of foods may change the amounts and relative proportions of some amino acids; for example the Maillard reaction and the associated browning that occurs when foods are baked reduces the available lysine. The quality of the protein is also important and depends on the amino acids that are present. Proteins from animal sources have a higher biological value than proteins from plant sources. This is because the pattern of amino acids in animal cells is comparable to the pattern in human cells. Plant foods may have very different patterns of amino acids compared to animal proteins, and, in the past, this difference has lead to a concept of first-class and second-class proteins, for animal and plant foods respectively. However, diets are typically varied in the UK and rarely made up of single foods. A combination of plant proteins tends to have a complementary effect boosting their overall biological value.

Complementary action of proteins (plant protein)

Seeds are a good source of plant proteinIn most diets, different proteins tend to complement each other in their amino acid pattern, so when two foods providing vegetable protein are eaten at a meal, such as a cereal (e.g. bread) and pulses (e.g. baked beans), the amino acids of one protein may compensate for the limitations of the other, resulting in a combination of higher biological value. This is known as the complementary action of proteins. Thus if vegetarians and vegans eat a variety of vegetable proteins in combination, there is no reason why the quality of protein cannot be as good as in a diet comprising meat, milk, fish, eggs or other foods that contain animal protein. Good sources of plant protein include nuts, seeds, pulses, mycoprotein and soya products. There are also small amounts in grains.

In the UK, most people's diets contain plenty of protein and provide more than enough of the indispensable amino acids. However, in some countries where protein intakes are low, the complementary action of proteins plays an important role in helping individuals meet their nutritional requirements. For example, the tradition of combining lentils with rice in cuisines from the Indian sub continent.

Animal protein

Protein from animal sources contains the full range of essential amino acids required from an adult’s diet. Sources include meat, fish, eggs, milk and cheese. For most of us, low fat options of these foods are preferable as some can be high in saturated fat.