Protein

• Protein is essential for growth and repair of the body and maintenance of good health.
• Protein also provides energy; 1 gram provides 17 kJ (4 kcal).
• The Reference Nutrient Intake (RNI) is set at 0.75g of protein per kilogram body weight per day for adults.
• The amount of protein we need changes during a lifetime.
• Different foods contain different amounts and different combinations of amino acids (the building blocks of proteins). Protein from animal sources (e.g. meat, fish, eggs and dairy products) contains the full range of essential amino acids needed by the body. However, vegans and vegetarians can get all the amino acids they need by combining different plant sources of protein, e.g. pulses and cereals.

Importance of protein

Proteins are fundamental structural and functional elements within every cell of the body and are involved in a wide range of metabolic interactions. All cells and tissues contain protein, therefore protein is essential for growth and repair and the maintenance of good health. Protein provides the body with approximately 10 to 15% of its dietary energy and it is the second most abundant compound in the body, following water. A large proportion of this will be muscle (43% on average) with significant proportions being present in skin (15%) and blood (16%).

Amino acids

Proteins are large molecules made up of long chains of amino acids. Amino acids are the building blocks of proteins. The biochemical activity of proteins is characterised by their individual structure, size and shape. These factors are determined by the sequence and characteristics of the constituent amino acids.

There are about 20 different amino acids commonly found in plant and animal proteins. For adults, 8 of these, have to be provided in the diet and are therefore defined as ‘essential’ or ‘indispensable’ amino acids. These are:

Leucine
Isoleucine
Valine
Threonine
Methionine
Phenylalanine
Tryptophan
Lysine.

In children, arginine, histidine, cysteine, glycine, tyrosine, glutamine and proline are also considered to be essential (indispensable) amino acids, because children are unable to make enough to meet their needs. These are referred to as ‘conditionally’ essential. There may also be certain disease states during adult life when a particular amino acid becomes conditionally essential.

The other amino acids do not have to be provided by the diet. This is because the amino group of these amino acids can be transferred to another amino acid with a different amino group by a process called transamination. In this way the body is able to make some amino acids itself. These are known as ‘non-essential’ or ‘dispensable’ amino acids.

How much protein should we eat?

The Dietary Reference Values for protein are based on estimates of need. For adults, an average requirement of 0.6g of protein per kilogram bodyweight per day is estimated. The Reference Nutrient Intake (RNI) is set at 0.75g of protein per kilogram bodyweight per day in adults. This equates to approximately 56g/day and 45g/day for men and women aged 19-50 years respectively. There is an extra requirement for growth in infants and children and for pregnant and breast feeding women.

Any excess protein can be used to provide energy. 1g of protein provides 17kJ (4 kcal) but carbohydrate, and to a lesser extent fat, should be the main sources of dietary energy. At present, protein provides around 16% of energy on average in the British diet.

Current protein intakes

The average daily intake of protein in the UK is 88g for men and 64g for women, which is more than sufficient: it exceeds the Dietary Reference Values for protein. Foods vary in the amount of protein they provide but the main sources include meat, fish, eggs, milk, cheese, cereals and cereal products (e.g. bread), nuts and pulses (beans and lentils). The most common sources of protein in the British diet are shown in figure 1.

 

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)

In 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.

Good sources of protein

Table 1: Protein content of some common foods found in the diet

Adults and children should consume two to three servings of protein every day. If plant sources dominate, it is important to make sure that different types are consumed.

One typical portion size equates to:

•100gofleanbonelessmeat(redandpoultry)
•140goffish
•2mediumeggs
• 3 tablespoons of seeds or nuts.

It is important to choose lower fat protein-rich foods, such as lean meats or reduced fat dairy products as some high protein foods can also be high in saturated fat. This will help minimise the risk of developing cardiovascular disease.

 

Protein and weight management

Protein-rich foods tend to make people feel fuller than foods rich in carbohydrates or fat. So including a lean source of protein with a meal can help to minimise feelings of hunger and decrease overall energy intake.

Some studies have shown protein-rich diets with a low carbohydrate content to be associated with slightly greater initial weight loss compared with widely recommended low fat eating programmes. However, no differences have generally been reported when comparisons have been made over a longer duration (e.g. after 12 months). Long-term studies are needed to compare the effects of protein-rich, low carbohydrate diets versus low fat diets on nutritional status and body composition and to assess their effects on disease risk and nutritional status.

Some popular high protein, low carbohydrate diets claim to be effective at producing weight loss despite a high fat intake from fatty meats and full fat dairy products. Such a diet remains contradictory to current healthy eating messages. Regardless of the composition of the diet, weight loss will only occur if a deficit in energy intake compared to output through activity is created to achieve a negative energy balance. There are also safety concerns about very-high-protein diets that involved cutting out other food groups and caution should therefore be exercised in promoting them.

When reducing energy intake in order to lose weight, it will be necessary to reduce intake of some foods but it is a good idea to maintain intake of low-fat, protein rich foods. In the context of a lower energy intake, this will result in a relatively high proportion of energy coming from protein in the diet, but will not represent an increase in the absolute amount of protein consumed or require the other food groups to be cut out of the diet.