Protein

At a glance:

• Protein is a macronutrient that people need for energy as well as growth and maintenance of the body.
• When dietary protein is consumed, it is metabolised into amino acids. Amino acids are the building blocks of protein.
• In the UK, average intakes of protein are above the Reference Nutrient Intake (RNI) including in vegetarians and vegans.
• Top contributors to protein intakes in the UK are meat and meat products, cereals and cereal products and milk and milk products.
• It is important for health to include a range of protein-containing foods in our diets.
• We are advised to eat more plant-derived protein foods for the benefit to our health and to improve environmental sustainability.
• People who do some physical activity, like going for a run or to an exercise class are unlikely to need any extra protein. For people exercising at a high level, having some protein soon after a training session can help muscles rebuild.

Functions of protein

Protein is a macronutrient that we need for growth, repair and maintenance in the body, especially for bones and muscles. It is also a source of energy. 

When dietary protein is consumed it is metabolised into amino acids. The body contains a pool of amino acids which are used to synthesise any proteins which are needed. Proteins are constantly being built and degraded in a dynamic process known as protein turnover.

Amino acids in protein

Proteins are molecules made up of long chains of amino acids. There are 20 different amino acids commonly found in plant and animal proteins. For adults, nine of these must be provided in the diet and are defined as ‘essential’ or ‘indispensable’ amino acids. These are:

1. Isoleucine
2. Leucine
3. Lysine
4. Methionine
5. Phenylalanine
6. Threonine
7. Tryptophan
8. Valine
9. Histidine

In children arginine, cysteine, glutamine, glycine, histidine, proline and tyrosine are also considered to be essential amino acids, because children cannot make enough of these to meet their needs. These are referred to as ‘conditionally’ essential because whether they are essential, depends on age.

Apart from the essential amino acids, the others do not have to be provided by the diet as the body can make them itself. These are known as ‘non-essential’ or ‘dispensable’ amino acids.

Protein quality

Looking at foods in isolation, animal proteins have a higher biological value than proteins from plant sources. Protein from animal sources contains the full range of essential amino acids required for an adult’s diet and are sometimes referred to as ‘complete’ proteins. Proteins can be ranked by ‘quality’, that is how many amino acids are present in a biologically available format.

As most plant foods do not provide all of the essential amino acids in significant quantities, it was once thought that for vegetarian and vegan diets, different foods containing complementary amino acids should be combined within each meal (such as cereals and pulses). However, it is now understood that protein from a variety of plant foods, eaten during a day, supplies enough of the essential amino acids when caloric requirements are met.

The nutritional benefits of protein

On average in the UK, protein provides 17% of dietary energy and is the second most abundant compound in the body, after water. A large proportion of the protein in the body is present within muscle (43% on average) with significant proportions being present in skin (15%) and blood (16%). The main types of protein in the body are collagen (connective tissue), haemoglobin (red blood cells), myosin and actin (muscle fibres).

Dietary proteins provide a source of nitrogen and energy (providing 4kcal per gram). Many protein-rich foods are important sources of micronutrients, such as iron and zinc in meat and calcium and iodine in dairy foods.

Importance of protein

• A core component of muscle and bone tissue
• Building blocks for important cellular structures
• Necessary for enzymatic activity, immunity, cell signalling, and muscle contraction
• Involved in body tissue repair and growth

Dietary requirements for protein

The Reference Nutrient Intake (RNI) is 0.75g of protein per kilogram body weight per day for adults, with additional requirements for growth and repair, including for children and in pregnancy. These are shown in the table below, based on average body weights at the time the recommendations were made.(note that this was in 1991 and that average body weights now are likely to be higher).

Table 1. Reference Nutrient Intakes for protein (Department of Health 1991)

When the RNIs were set there were some concerns about risks from high protein diets but in the absence of enough data to establish an upper limit, the Department of Health advised adults should avoid protein intakes which are more than twice the RNI. In 2012, the European Food Safety Authority (EFSA) also stated that data was still insufficient to establish a Tolerable Upper Intake Level (UL) for protein and that the potential detrimental effects of very high protein intake remain controversial. 

EFSA noted that intakes of up to twice the EU PRI (population reference intake, 0.83g protein/kg bodyweight for adults) are regularly consumed from mixed diets by some physically active and healthy adults in Europe and are considered safe. Furthermore, intakes of 3–4 times the PRI have been seen without apparent adverse effects, but acute adverse effects have been reported for protein intakes ≥45% of total energy intake. Any excess protein consumed is used in the body to provide energy. One gram of protein provides 17kJ (4kcal).

Current protein intakes in the UK

The average daily intakes of protein in the UK are 76.0g/day for adults aged 19-64 years, and 67.0g/day for adults aged 65 years and over. This is more than sufficient and exceeds the RNI for protein.  

Data from the NDNS show that the biggest contributors to protein intakes in the UK are meat and meat products (34%), cereals and cereal products (24%) and milk and milk products (13%). Chicken, turkey (and dishes made of these) alone contribute to 16% of intakes. 

The UK Government Eatwell Guide includes the following foods as examples of protein sources: beans, peas, lentils, meat, fish, nuts and eggs. It recommends choosing lower fat protein-rich foods, such as lean meats and reducing high intakes of red and processed meat and eating more fish (including oily fish), beans, pulses and lentils.

Protein on food labels

The protein content of a food is among the mandatory information to be provided in the nutrition declaration on food labels, according to Regulation (EU) No. 1169/2011. For labelling purposes, the reference intake for protein of an average adult (8400kJ/2000kcal) is 50g/day.

If they meet the conditions of use, products on the market in the EU can make claims on being either ‘a source of’ (12% of the energy value of the food from protein) or ‘high in’ protein (20% of the energy value of the food from protein). Health claims relating to protein and the growth and maintenance of muscle mass; the maintenance of normal bone and normal growth and development of bone in children are allowed, again with conditions of use.

Protein-containing foods

Data from the National Diet and Nutrition Survey (NDNS) show that the biggest contributors to protein intakes in the UK are meat and meat products (34%), cereals and cereal products (24%) and milk and milk products (13%). Chicken, turkey (and dishes made of these) alone contribute 16% to protein intakes.

The UK Government Eatwell Guide recommends eating more beans, pulses and lentils, as well as two portions (140g) of fish per week (one of which should be oily). In relation to red meat, current advice is that if you eat more than 90g of red or processed meat per day, try to cut down to no more than 70g per day (the current average intake of red and processed meat for adults aged 19-64 years in the UK is 56g/day).

People doing an intensive exercise programme may benefit from consuming protein after a workout, and spacing protein intakes throughout the day. Dietary guidelines set by the Department of Health in 1991 recommended that protein intakes do not exceed double the recommended intake, though insufficient data was available to set a safe upper limit.

Dietary sources of protein 

In terms of where we get protein from in the diet, the relative merits of plant versus animal derived protein are key for both nutrition science and environmental sustainability. Dietary guidelines around the world, including the UK, US and Canada promote a shift towards getting more protein from plant-based sources such as beans, peas, lentils and less from meat and meat products, particularly red and processed meats, as this tends to result in a dietary pattern that provides protein, with less saturated fat and salt and more fibre.

In an analysis performed as part of the revision of the UK Eatwell Guide in 2016, the total quantity of foods from the ‘Beans, pulses, fish, eggs, meat and other proteins’ food group was reduced in the scenario where dietary recommendations were met compared with the average UK diet. However, while meat and meat products were all reduced to meet dietary recommendations, daily intakes of both white and oily fish and of beans, pulses and other legumes increased. Table 2 below shows the calculations from the modelling carried out in terms of how daily intakes of different food groups would change in the ‘average diet’ versus the ‘Eatwell diet’.

Table 2. Mean daily intakes of selected foods from the ‘Beans, pulses, fish, eggs, meat and other proteins’ food group (Scarborough et al. 2016)

In relation to the impact of dietary protein sources on the environment as well as health, the EAT Lancet report, published in 2019, developed a diet based on their assessment of both health and sustainability considerations. This report emphasised a shift towards a more plant-based diet with red meat in particular being reduced by over 50% based on current global intakes. The suggested intakes of protein foods in the ‘planetary diet’ outlined in the report are shown in Table 3 below.

Table 3. Target intakes and possible ranges for dietary protein sources in the EAT Lancet planetary diet (Willet et al. 2019)

There are many ways to formulate dietary patterns in order to meet criteria for a healthy diet and the Eatwell and EAT Lancet examples above illustrate how this can be done in both a national and global context. However, there is no ‘perfect’ way to determine dietary patterns for populations and there will always need to be a degree of flexibility. Learn more about healthier and more sustainable diets by reading our pages on this topic.

Protein and physical activity

As outlined above; protein is needed for both building and repairing muscle and as such, is involved in how the body responds to exercise. Muscle protein synthesis and muscle protein breakdown occur simultaneously as a continuous process and the amount of muscle mass present at any one time is due to the net balance between these. 

Dietary protein and exercise impact on the rate of both muscle protein synthesis and breakdown. For instance, immediately after a protein-containing meal, muscle protein synthesis increases and as time passes, protein breakdown increases. This occurs regardless of whether physical activity has taken place. Exercise stimulates an increase in muscle protein synthesis, which can last for at least 24 hours. Weight training specifically causes muscle hypertrophy, which is an increase in the size of muscle cells, and therefore an increase in muscle mass.

However, it is known that excess protein consumed beyond physiological ‘need’ is used for energy and the excess nitrogen is lost in urine, due to increased levels of amino acid oxidation and ureagenesis, therefore, even in active people, very high protein intakes above normal requirements will not necessarily be converted to muscle.

Individuals undertaking vigorous physical activity or a strength-based training programme for longer durations (like athletes, bodybuilders, marathon runners) may need additional protein. The recommendations for protein intake from the American College of Sports Medicine (ACSM) are to have 1.2 to 2.0g/kg/day, met through consuming a regular spread of moderate amounts of high-quality protein across the day and following strenuous training sessions. This is a higher requirement than that for the general sedentary population (0.75g per kg bodyweight per day). These recommendations are the same for strength or endurance athletes. 

With relation to protein intake in those who are very active, it is also important to ensure athletes are consuming adequate energy and carbohydrates, so the protein can be used for muscle building and repair and not for energy. The daily recommendations differ slightly depending on the body that recommends them. Protein intake targets of 1.2-1.8g/kg body mass have been set by the International Olympic Committee for strength and endurance athletes with a slightly higher range of 1.4-2.0g/kg body mass has been suggested by the International Society of Sports Nutrition, who also highlighted that there is some novel evidence that higher intakes of protein – 3g/kg body mass or more – may help promote loss of fat mass in resistance-trained athletes. However, much of the research in this area has been for highly trained athletes, training their whole body over long periods, and therefore would not be recommended for the general population.

This is because protein ingestion immediately after exercise coincides with the time the body is ‘primed’ for muscle protein synthesis, so ensuring a sufficient dose of high-quality protein intake at this time would be optimal to promote muscle building and aid recovery. Although training elevates muscle protein synthesis for between 24 and 48 hours, it is beneficial to consume protein within the closest proximity to exercise. At this time, consumption of 20-25g of high-quality protein seems sufficient to maximise muscle protein synthesis rates, regardless of how well-trained people are. Despite this link between timing of protein intake and muscle protein building, total daily protein intake has been demonstrated to be a greater predictor of overall hypertrophy.