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, so-called
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 (4kcal/g of protein). The quantity of IAAs and the extent of their utilization 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 IAA required. Recently, the Digestible Indispensable Amino Acid Score (DIAAS) (FAO 2013) has been proposed to replace the PDCAAS.
In sensitive individuals,
allergic reactions can be triggered 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 and many other cereals like rye and barley, can trigger -in sensitive individuals- coeliac disease, a life-long systemic autoimmune disorder. For individuals suffering from food allergies, the avoidance of allergenic foods, and gluten free-diet in those affected by coeliac disease, represent the conventional management of coeliac disease (EFSA 2014).
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.
While cereal proteins contain low amounts of some IAAs (i.e. lysine) and legume proteins contain low amounts of the sulphur amino acids, when considering mixed diets in which a variety of foods is consumed, a combination of different plant proteins can give a good distribution of IAA, 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.
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). For labelling purposes, the reference intake for protein of an average adult (8400 kJ/2000 kcal) is 50g/day. Reg. (EU) 1169/2011 also lays down rules for 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 restaurant and cafes. In addition, a Commission guidance document (EC 2011) has been recently adopted, aiming to assist consumers, business and national authorities to better understand the requirements of Reg. (EU) 1169/2011 regarding food allergies or intolerances.
Protein nutrition claims are permitted, according to Regulation (EC) No 1924/2006
(EC 2006a), 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.'
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).
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 related to protein intake as described by food- and health-related organisations
Effect of protein intake on cardiovascular health Cardiovascular disease 'No consensus has been reached about whether such associations showing protective effects on protein for cardiovascular disease causal and no are convincing potential mechanisms have been proposed'. 'For cardiovascular diseases, the association between protein intake and coronary heart disease and stroke was statistically non-significant in six cohort studies, and the evidence was regarded as inconclusive'. Blood Pressure 'Overall, while it seems certain that protein intakes are not harmful for blood pressure, with cross-sectional population studies clearly showing benefit of increasing protein intakes, some caution is probably still justified since dietary associations can be confounded by highly correlated nutrients for which no adjustment has been made'. 'The evidence for an association between protein intake and blood pressure was assessed as inconclusive for total and animal protein, but an inverse association with vegetable protein intake was assessed as suggestive'. 'Evidence is suggestive for an inverse association between hypertension and intake of vegetable protein'. Blood lipids 'The evidence was assessed as probable to convincing in regard to the effect of soya protein on LDL-cholesterol concentration'.
Effect of protein intake on type 2 diabetes mellitus Evidence was assessed as suggestive regarding the relation of total and animal protein intake to increased risk of type-2 diabetes.
Effect of protein intake on cancer 'the evidence indicates that there is little effect of total protein intake on the incidence of cancer, but that specific foods, such as red or processed meat, might increase the risk relative to vegetable protein sources'. 'The evidence from the systematic literature reviews did not suggest that proteins specifically modify the risk of cancers of any sites'. 'The overall association between cancer and protein intake was assessed as inconclusive'.
Effect of protein intake on body weight/ energy intake 'Evidence for an association between the dietary macronutrient composition in prevention of weight gain after prior weight loss was inconclusive. The results suggested that the proportion of macronutrients in the diet was not important in predicting changes in weight or waist circumference'. Based on studies in individuals with healthy weight, evidence for an association between protein intake and energy intake was assessed as inconclusive. 'The evidence for an association between protein intake and body weight change was also assessed as inconclusive'. 'In shorter-term studies, low-calorie, high-protein diets may result in greater weight loss, but these differences are not sustained over time'.
'A moderate amount of evidence demonstrates that intake of dietary patterns with less than 45% calories from carbohydrate or more than 35% calories from protein are not more effective than other diets for weight loss or weight maintenance, are difficult to maintain over the long term, and may be less safe'.
Effect of protein intake on bone health 'Taken together, it does appear that dietary protein as part of a well-balanced diet is most likely to be beneficial for bone, possibly at dietary levels in excess of the recommended intake'. 'The fact that our current models of protein and energy requirements identify sedentary elderly people as most likely to be at risk from protein deficiency (…), together with the evidence of a beneficial effect of dietary protein on bone in elderly people, suggests that attention should be given to the provision of protein-dense foods to this particular population group'. 'Evidence for beneficial or adverse effects of higher protein intake in relation to bone health was assessed as inconclusive'. 'The association between bone health and protein intake was assessed as inconclusive. However, there seems to be an interaction with the intake level of calcium. Under conditions of low calcium intake an increased risk of fractures was found to be related to high animal-protein intake, but under conditions of high calcium intake (>800 mg) a decreased risk of fractures was related to high animal-protein intake'. 'Evidence for an association between vegetable protein intake and fracture risk was inconclusive'. In the elderly, evidence was assessed as suggestive 'in regard to a positive association between protein intake and bone mineral density'. 'Evidence was assessed as inconclusive regarding the relation of protein intake to bone loss and risk of fractures'. 'A favourable effect on bones seems to be likely: studies reveal a positive association between the level of protein intake and bone mineral density, without, however, indicating any reduction in the risk of fractures'.
Effect of protein intake on kidney health Kidney disease 'There is clear evidence that high intakes of protein by patients with renal disease contribute to the deterioration of kidney function'. 'Chronic protein intake is a determinant of glomerular filtration rate, but does not suggest a role for protein intake in the deterioration of kidney function'. 'Protein restriction on the grounds of renal function is justifiable and prudent only in subjects who are likely to develop kidney failure owing to diabetes, hypertension, or polycystic kidney disease'. 'The evidence for associations between protein intake and kidney function and kidney stones was regarded as inconclusive'. Kidney stones 'Although some studies suggest that high animal protein intake might increase the risk of kidney stones, particularly in those subjects who are classified as idiopathic calcium stone formers, as yet no clear conclusions can be drawn since dietary effects are apparent only in studies with very large differences in protein intakes (i.e. >185 g/day compared with 80 g/day)'. 'it is not yet clear whether there is a difference between proteins of animal versus plant origin'. 'in order to minimize the risk of kidney stones in patients who are at risk, the diet should ideally provide at least the safe level (0.83 g/kg per day), but not excessive amounts (i.e. less than 1.4 g/kg per day), preferably from vegetable sources'. 'The evidence for associations between protein intake and kidney function and kidney stones was regarded as inconclusive'.
Effect of protein intake on e xercise 'evidence was assessed as suggestive with regard to a positive relation between muscle mass and a total protein intake in the range of 13 E% to 20 E%'. 'The evidence was assessed as suggestive for the effect of training on whole-body protein retention'. Evidence on dietary protein intake for the optimal effect of physical exercise in older adults have been assessed as inconclusive. 'repetition of strength exercises leads to an increase in protein requirements, but, generally speaking, the protein requirements necessary for nitrogen balance equilibrium are covered by a balanced diet. The dietary habits of strength athletes mean that their protein intakes far exceed the quantities that can be recommended. It is training that explains the increase in muscle mass, and the protein intakes are justified by the necessary availability of amino acids to ensure increased synthesis of structural and functional proteins'. Muscle mass in elderly Based on studies on elderly populations, the evidence on the relationship between muscle mass and total protein intake (in the range of 13 E% to 20 E%) was assessed as limited-suggestive.
Effect of protein intake on mortality '…the evidence was assessed as suggestive regarding an increased risk of all-cause mortality' in relation to an low carbohydrate/high protein diet (LC/HP), with total protein intakes of at least 20–23 E%. 'For cardiovascular mortality, the evidence was assessed as suggestive for an inverse relation to vegetable protein intake based on three studies in which the protein intake was expressed in E% and on one study with an LC/HP diet score based on vegetable protein'. 'Generally, the use of an LC/HP score makes it uncertain whether the effects result from reduced carbohydrate or increased protein and/or fat, and thus the effect of protein per se cannot be assessed from LC/HP diets'. a The statements provided here come originally from the 2010 DGAC study; the 2015 update refers to them and states that 'The published literature since that review does not provide sufficient evidence to change these conclusions'.
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 0.66g/Kg/of 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 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.
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 11 EU Member States, plus Norway. 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 categorization 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.
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 generalized low food intake and other adverse environmental factors (e.g. infection). In high-income countries, protein-energy malnutrition is predominantly diagnosed in hospitalized patients and is associated with disease or is often found in older adults (ENHA 2005). In this latter group, suboptimal protein intake has been related to sarcopenia, a condition characterized 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.
There are some policies that are specifically addressing, directly or indirectly, protein intake, protein rich-foods and protein quality (Table 7).