Defining protein

Proteins are basic constituents in all living organisms. The building blocks of proteins are amino acids; in nature there are hundreds of amino acids, but only about twenty of these, referred to as proteinogenic, are used to build proteins in living organisms ( Present Knowledge in Nutrition 2012 ). Indispensable amino acids (IAAs) are those that cannot be synthesised by the human body to meet the body's needs and therefore must be provided in the diet ( EFSA 2012 ). Within the body, proteins are necessary for enzymatic activity, immunity, cell signalling, and muscle work. They are involved in repair and transport processes and are the building blocks for several cellular structural elements ( NNR 2012 ).

Dietary proteins are an important source of nitrogen (16% of protein weight is nitrogen ( FAO 2002 ) and when needed, also of energy (4 kcal/g of protein). The quantity of IAAs and the extent of their utilisation are used as criteria to determine protein quality. The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) is a method to assess protein quality that relies on these criteria. A PDCAAS ≥1 indicates that, after digestion, the protein can provide (per unit) 100% or more of the IAAs required. Recently, the Digestible Indispens­able Amino Acid Score (DIAAS) ( FAO 2013 , (pdf)) has been proposed to replace the PDCAAS.

In sensitive individuals, allergic reactions can be triggered by food proteins in gluten-containing cereals, crustaceans, fish, eggs, peanuts, soybean, milk, nuts, celery, mustard, sesame, lupin and molluscs. For example, gluten, a protein present in wheat, rye and barley and many other cereals, can trigger coeliac disease, a life-long systemic autoimmune disorder. For individuals with food allergies, the avoidance of allergenic foods, and gluten-free diet in those affected by coeliac disease, represent the conventional management of these conditions ( EFSA 2014 ).

Dietary sources of protein

Almost all foods of animal or plant origin contain proteins, although their protein content and amino acid composition differ broadly. Tables 1a and 1b show examples of protein content in some animal- and plant-derived raw foods.

Table 1a - Average protein content in and % of food energy in some animal-derived raw foods 

Table 1b - Average protein content and % of food energy in plant-derived raw foods 

Cereal proteins contain low amounts of some IAAs (i.e. lysine) and legume proteins contain low amounts of the sulphur-containing amino acids. Therefore, when considering mixed diets in which a variety of foods is consumed, a combination of different plant proteins can give a good distribution of IAAs, given also that most plant proteins are reasonably well digested (although those found in grains have slightly lower digestibility) ( NNR 2012 ). In some regions of the world where diets are based on single staple foods (e.g. millet, sorghum, cassava) and have low diversity, there may however be a risk of dietary protein inadequacy ( WHO/FAO/UNU 2007 ). Most Western diets, including European diets, have good protein quality; their PDCAAS score is equal to or higher than 1, because of the wide variety of plant food consumed and because of the considerable presence of animal foods.

Labelling of protein in the EU

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 ( EU 2011 , (pdf)). For labelling purposes, the reference intake for protein of an average adult (8400 kJ/2000 kcal) is 50 g/day. Reg. (EU) 1169/2011 also lays down rules for the mandatory declaration on labels of pre-packed foods and in non-prepacked foods of substances or products causing allergies or intolerances (discussed in Defining Protein above); the regulation applies also to foods served in restaurants and cafes. In addition, a Commission guidance document ( EC 2011 , (pdf)) has been adopted, aiming to assist consumers, businesses and national authorities to better understand the requirements of Regulation (EU) 1169/2011 regarding food allergies and intolerances.

Protein nutrition claims are permitted, according to Regulation (EC) No 1924/2006 ( EC 2006a , (pdf)), as follows:

• Source of protein: 'A claim that a food is a source of protein, and any claim likely to have the same meaning for the consumer may only be made where at least 12% of the energy value of the food is provided by protein'
• High protein: 'A claim that a food is high in protein, and any claim likely to have the same meaning for the consumer may only be made where at least 20% of the energy value of the food is provided by protein.'

Certain health claims for food products that are a source of protein are also permitted, mainly relating to the capacity to contribute to growth and maintenance of muscle mass and the capacity to maintain normal bones and normal growth and bone development in children. The list of all authorised and non-authorised nutrition and health claims is accessible at the relevant EU Register ( EU Registry on nutrition and health claims ).

Protein intake: effects on health

A number of national and international institutions have assessed the impact of protein intake on health and on the development of non-communicable diseases (NCDs). The statements or opinions from these institutions, as well as the strength of the supporting evidence, are shown in Table 2.

Table 2 - Health effects by food- and health-related organisations 

Recommended intakes of dietary protein

In the Scientific Opinion on Dietary Reference Values (DRV) for protein, EFSA concludes that 'an Average Requirement (AR) and a Population Reference Intake (PRI) for protein can be derived for adults, infants and children, and pregnant and lactating women based on nitrogen balance studies and on factorial estimates of the nitrogen needed for deposition of newly formed tissue and for milk output'.

EFSA also considered several health outcomes that can be associated with protein intake, however 'data were found to be insufficient to establish DRVs'. Additionally, EFSA concluded that there is not sufficient data to establish a Tolerable Upper Intake Level (UL) for protein. In a joint meeting of the World Health Organisation (WHO), the Food and Agriculture Organization of the United Nations (FAO) and the United Nations University (UNU), the term protein requirement was defined as 'the lowest level of dietary protein intake that will balance the losses of nitrogen from the body, and thus maintain the body protein mass, in persons at energy balance with modest levels of physical activity, plus, in children or in pregnant or lactating women, the needs associated with the deposition of tissues or the secretion of milk at rates consistent with good health' ( WHO/FAO/UNU 2007 ).

Major nutrition and health-related organisations have issued recommendations regarding protein requirements for individuals in different life stages and physiological conditions. Such recommendations have mostly been derived from N balance studies and not on the basis of the optimal intake of protein for health, because as noted by WHO/FAO/UNU it is less quantifiable ( WHO/FAO/UNU 2007 ). Recommended intakes of protein - that are able to cover the needs of the majority of the population - have been calculated firstly by defining an average requirement (AR) of dietary protein able to maintain the body's nitrogen balance: in most cases this value corresponds to 0.66 g per kg body weight per day. Then, an extra amount of protein is added to allow for variability of protein needs among different individuals.

In adults, the recommended daily amount of protein ranges from 0.80 to 0.83 g per kilogram of body weight for both men and women with modest levels of physical activity. Recommended amounts for children and pregnant or lactating women are higher, to allow for the deposition of body tissues and the secretion of milk. Sedentary older adults have been identified as the population group most at risk for protein deficiencies; some health organizations have recommended higher amounts of protein while others have highlighted the need to provide the sedentary elderly population with more protein-dense diets rather than higher amounts of protein in absolute terms. Tables 3, 4 and 5 show, respectively the recommendations for protein intake for adults and older adults (including those for safe upper limit of intake), pregnant and lactating women, and infants, children and adolescents.

The recommended intakes are termed differently because different food- and health- related organisations used different criteria. Frequently used terms are: Average Requirement (AR), Population Reference Intake (PRI), Safe Level of Protein (SLP), Recommended Dietary Allowances (RDA); Dietary Reference Value (DRV).

Table 3 - Dietary recommendations for adults and older adults by food- and health-related organisations 

Table 4 - Dietary recommendations for pregnant and lactating women as by food- and health-related organisations 

Table 5 - Dietary recommendations for infants, children and adolescents by food- and health-related organisations 

Protein intake across European countries

In most European countries, the main contributor to the dietary protein intake in adults is meat and meat products, followed by grains and grain-based products, and milk and dairy products. According to EFSA, these three food groups contribute about 75% of the protein intake in the majority of EU Member States ( EFSA 2012 ).

Table 6 shows protein intake data extracted from recent food and nutrition surveys conducted in European countries. The data from Table 6 indicate that there is diversity in the methodology used for the assessment of food consumption in the surveys, as well as differences in categorisation of the age groups. These elements make comparison between countries difficult. However, it is possible to observe that many EU citizens have dietary protein intakes above the recommended intake levels.

Table 6 - Overview of protein intake in European countries 

Disease burden related to protein intake

In general, protein deficient diets are also deficient to varying degrees in a range of other nutrients. Protein rich foods contain a variety of micronutrients (vitamins, minerals and trace elements) and their suboptimal intake, together with suboptimal protein and energy intake, can determine different forms of the so-called protein-energy malnutrition. As noted by WHO/FAO/UNU ( WHO/FAO/UNU 2007 ), identifying a societal and economic burden strictly related to protein deficiency in populations is particularly challenging. The majority of protein-energy malnutrition occurs in individuals in low-income countries in the presence of a generalised low food intake and other adverse environmental factors (e.g. infection). In high-income countries, protein-energy malnutrition is predominantly diagnosed in hospitalised patients and is associated with disease or is often found in older adults. In this latter group, suboptimal protein intake has been related to sarcopenia, a condition characterised by loss of skeletal muscle mass and function ( Deer RR. 2015 , Bauer et al. 2013 ).

In high-income countries, the average protein intakes of populations consuming mixed diets usually exceed recommended intakes. Many food and health organisations have issued opinions/statements linking protein intake to the development of NCDs. However, in this case, health effects are not quantifiable and thus specific health and societal burden linked to protein intake has not been estimated.

Policies addressing protein intake

There are some policies that are specifically addressing, directly or indirectly, protein intake, protein rich-foods and protein quality (Table 7).

Table 7 - Policy recommendations or of implemented policies to address protein intake 

References

Overview of the references to this brief