Following a request from the European Commission, the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) was asked to deliver a scientific opinion on Population Reference Intakes for the European population, including protein.
Dietary proteins are the source of nitrogen and indispensable amino acids which the body requires for tissue growth and maintenance. The main pathway of amino acid metabolism is protein synthesis. In this opinion, “protein” is total nitrogen x 6.25 and protein requirements are based on nitrogen content. Protein digestion takes place in the stomach and in the small intestine. In healthy humans, the absorption and transport of amino acids is usually not limited by the availability of digestive enzymes or transport mechanisms, but some protein escapes digestion in the small intestine and is degraded in the colon through bacterial proteolysis and amino acid catabolism. By the time digesta are excreted as faeces, they consist largely of microbial protein. Therefore, when assessing protein digestibility, it is important to distinguish between faecal and ileal digestibility, as well as apparent and true nitrogen and amino acid digestibility.
The concept of protein requirement includes both total nitrogen and indispensable amino acid requirements. The quantity and utilisation of indispensable amino acids is considered to be an indicator of the dietary protein quality, which is usually assessed using the Protein Digestibility- Corrected Amino Acid Score (PD-CAAS). It is important to determine to what extent the nitrogen from dietary protein is retained in the body. Different values for the efficiency of protein utilisation have been observed for maintenance and for tissue deposition/growth; at maintenance, the efficiency of nitrogen utilisation for retention is about 47 % in healthy adults in nitrogen balance on mixed diets.
Foods of animal origin with a high protein content are meat, fish, eggs, milk and dairy products. Bread and other grain-based products, leguminous vegetables, and nuts are plant foods high in protein. Most of the animal sources are considered high quality protein having an optimal indispensable amino acid composition for human needs and a high digestibility, whereas the indispensable amino acid content of plant proteins and/or their digestibility is usually lower. In European countries the main contributors to dietary protein intake are meat and meat products, grains and grain-based products, and milk and dairy products.
Data from dietary surveys show that the average protein intakes in European countries vary between 67 to 114 g/d in adult men and 59 to 102 g/d in women, or about 12 to 20 % of total energy intake (E %) for both sexes. Few data are available for the mean protein intakes on a body weight basis, which vary from 0.8 to 1.25 g/kg body weight per day for adults.
In order to derive Dietary Reference Values (DRVs) for protein the Panel decided to use the nitrogen balance approach to determine protein requirements. Nitrogen balance is the difference between nitrogen intake and the amount lost in urine, faeces, via the skin and other routes. In healthy adults who are in energy balance the protein requirement (maintenance requirement) is defined as that amount of dietary protein sufficient to achieve zero nitrogen balance. The requirement for dietary protein is considered to be the amount needed to replace obligatory nitrogen losses, after adjustment for the efficiency of dietary protein utilisation and the quality of the dietary protein. The factorial method is used to calculate protein requirements for physiological conditions such as growth, pregnancy or lactation in which nitrogen is not only needed for maintenance but also for the deposition of protein in newly formed tissue or secretions (milk).
According to a meta-analysis of available nitrogen balance data as a function of nitrogen intake in healthy adults, the best estimate of average requirement for healthy adults was 105 mg N/kg body weight per day (0.66 g high quality protein/kg per day). The 97.5th percentile was estimated as 133 mg N/kg body weight per day (0.83 g high quality protein/kg per day) from the distribution of the logarithm of the requirement, with a coefficient of variation (CV) of about 12 %. The Panel considers that the value of 0.66 g/kg body weight per day can be accepted as the Average Requirement (AR) and the value of 0.83 g/kg body weight per day as the Population Reference Intake (PRI) derived for proteins with a PD-CAAS value of 1.0. This value can be applied to usual mixed diets in Europe which are unlikely to be limiting in their content of indispensable amino acids. For older adults, the protein requirement is considered to be equal to that for adults. The lower energy requirement of sedentary elderly people means that the protein to energy ratio of their requirement may be higher than for younger age groups.
For infants, children and adolescents, the Panel accepted the approach of WHO/FAO/UNU (2007) in which estimates of the protein requirements from six months to adulthood were derived factorially as the sum of requirements for maintenance and growth corrected for efficiency of protein utilisation. An average maintenance value of 0.66 g protein/kg body weight per day was applied. Average daily needs for dietary protein for growth were estimated from average daily rates of protein deposition, calculated from studies on whole-body potassium deposition, and from an efficiency of utilisation of dietary protein for growth of 58 %. The PRI was estimated based on the average requirement plus 1.96 SD using a combined SD for growth and maintenance.
For pregnant women, the Panel accepted the factorial method for deriving protein requirements during pregnancy which was based on the newly deposited protein in the foetus and maternal tissue, and on the maintenance requirement associated with the increased body weight. Because of the paucity of data in pregnant women and because it is unlikely that the efficiency of protein utilisation decreases during pregnancy, the efficiency of protein utilisation was taken to be 47 % as in non-pregnant women. Thus, for pregnant women a PRI for protein of 1, 9 and 28 g/d in the first, second and third trimesters, respectively, is proposed in addition to the PRI for non-pregnant women.
For lactation, the Panel accepted the factorial method which requires assessing milk volumes produced and the content of both protein nitrogen and non-protein nitrogen, as well as calculating the amount of dietary protein needed for milk protein production. As the efficiency of protein utilisation for milk protein production is unknown, the same efficiency as in the non-lactating adult (47 %) was assumed. The PRI was estimated by adding 1.96 SD to give an additional 19 g protein/d during the first six months of lactation (exclusive breastfeeding), and 13 g protein/d after six months (partial breastfeeding).
The Panel also considered several health outcomes that may be associated with protein intake. The available data on the effects of an additional dietary protein intake beyond the PRI on muscle mass and function, on body weight control and obesity (risk) in children and adults, and on insulin sensitivity and glucose homeostasis do not provide evidence that can be considered as a criterion for determining DRVs for protein. Likewise, the available evidence does not permit the conclusion that an additional protein intake might affect bone mineral density and could be used as a criterion for the setting of DRVs for protein.
Data from food consumption surveys show that actual mean protein intakes of adults in Europe are at, or more often above, the PRI of 0.83 g/kg body weight per day. In Europe, adult protein intakes at the upper end (90-97.5th percentile) of the intake distributions have been reported to be between 17 and 27 E%. The available data are not sufficient to establish a Tolerable Upper Intake Level (UL) for protein. In adults an intake of twice the PRI is considered safe.
DRVs have not been derived for indispensable amino acids since amino acids are not provided as individual nutrients but in the form of protein. In addition, the Panel notes that more data are needed to obtain sufficiently precise values for indispensable amino acid requirement.