The science of protein

Key points

• Protein is a macronutrient that we need for growth, repair and maintenance in the body, especially for bones and muscles. Our protein needs change across the life course.
• Protein is an energy source and provides 4kcal per gram.
• In the UK, average intakes of protein are above the Reference Nutrient Intake (RNI) including in vegetarians and vegans.
• The RNI is 0.75g of protein per kg bodyweight per day for average-weight adults (~56g/day for men and ~45g/day for women depending on bodyweight).
• 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 and we are recommended to eat more plant-derived protein foods.
• 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.

Summary

Protein is needed for growth and repair of body tissues and is especially important for healthy muscles and bones, particularly for children. Good food sources of protein are meat, fish, dairy products, eggs, nuts and soya, beans, peas and lentils. Smaller amounts are also found in grains and cereals.

The Reference Nutrient Intake (RNI) for protein for adults is 0.75g protein per kg body weight per day; this equates to 56g/day and 45g/day for men and women of average body weights (75 and 60kg respectively). RNIs have also been set for children from birth to 10 years and for pregnancy and lactation. Average intakes of protein in the UK are above this recommendation in all age groups. 

What are proteins?

Proteins are molecules made up of long chains of amino acids. There are about 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, there used to be a school of 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 all the essential amino acids when caloric requirements are met.

In the UK, most people's diets contain plenty of protein from a variety of sources, which would likely provide all the amino acids needed. However, in some developing countries protein intakes are low, with diets heavily based on starchy staple foods such as rice, wheat, maize or cassava. Cereals tend to be low in the essential amino acid lysine, although lysine is present in higher amounts in soya and pulses. Evidence from observational studies has linked the protein quality within the food supply with stunting rates and lower blood levels of essential amino acids have been measured in stunted children.

Importance of protein

On average, protein provides 17% of dietary energy in the UK 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).

When protein is consumed it is metabolised into amino acids and 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.

• 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 proteins also provide an important 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.

Dietary requirements for protein

The Reference Nutrient Intake (RNI) is 0.75g of protein per kilogram bodyweight 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 bodyweights at the time the recommendations were made. Note that this was in 1991 and that average bodyweights 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 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 intakes and health

Protein is relatively ubiquitous in the food system so a diet which is deficient in protein is also likely to be deficient in other nutrients such as vitamins and minerals. In developing countries where access to a balanced dis challenging, protein-energy malnutrition can develop, which can lead to the conditions kwashiorkor (severe malnutrition characterised by oedema) or marasmus characterised by thinness. Protein deficiency in the body can occur in anyone at times of increased demand (such as infection or stress), increased losses (such as haemorrhage, burns or diarrhoea) or dysregulation in protein metabolism.

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 dependent 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.

Many protein products exist on the market such as protein shakes, bars, balls and powders; however, these are not normally necessary, although they can be useful if obtaining or consuming foods is likely to be difficult or if people do not feel hungry enough to eat food immediately after exercise. Despite their frequent use, there is little evidence of a benefit of branched chain amino acid (BCAA) supplementation. Nuts, lean chicken, tuna, eggs or milk are all examples of food-based protein snacks that could be consumed. There is some consensus that dairy protein (particularly whey; a component of milk) is advantageous over other types of dietary protein for stimulating muscle protein synthesis, due to its fast digestibility and high leucine content. However, more research is needed to assess the efficacy of other protein sources such as meat and vegetable proteins. There is also emerging evidence of a benefit of protein intake in close proximity to sleep to reduce overnight muscle protein breakdown, with the majority of research on casein, due its slower digestibility.

Evidence around consuming protein during exercise is currently unclear but there is a strong consensus that post-exercise protein intake is beneficial for muscle remodelling and recovery. It’s also important to be aware of the total calorie and carbohydrate content of the diet in supplying the athlete with enough energy to perform optimally.

Overall, based on consensus reports, including high quality protein at meals and snacks across the day is recommended to provide the amino acids needed to support muscle growth, development and repair. Furthermore, 15-25g of high-quality protein is suggested by sporting organisations and sports scientists as required within 2-3 hours after exercise to maximally stimulate muscle protein synthesis.

For more information read the American College of Sports Medicine Joint Position Statement on nutrition and athletic performance.

Protein and bone health

It has been suggested that high protein intakes could be detrimental for bone health by causing demineralisation due to increased urinary calcium excretion. This has not been demonstrated by RCTs though several systematic reviews note that the quality of evidence in this arena is low. A systematic review performed by the National Osteoporosis Foundation in 2017 reported that most observational studies support a positive association between protein consumption and bone health, one study that found a negative relationship could be explained by simultaneous low calcium intakes. A 2012 systematic review and meta-analysis including five studies found no difference in bone mineral density between lower protein diets and high protein diets providing up to 1.6g/kg bodyweight. There are health claims approved for use in the EU which state that protein contributes to the maintenance of normal bones and is needed for normal growth and development of bone in children.