Protein Quality Working Group
The Nevin Scrimshaw International Nutrition Foundation (INF) has recently reconstituted its Protein Quality and Amino Acid Working Group. The INF has a long and distinguished history of involvement and leadership in the exploration of the importance of protein requirements and protein quality dating all the way back to the founder, Nevin Scrimshaw's, work as Chairman of the Protein Advisory Group of the United Nations Agencies. In more recent years, the current President of the INF, Ricardo Uauy, has been very active in further defining protein requirements based on amino acid requirements. Part of our objective of this Working Group will be to bring together experts in the area of protein and amino acids to set the research agenda to better understand the core science around protein, protein quality and linear growth, weight gain, maternal nutrition and its effects on birth outcomes and subsequent linear growth.
The last WHO/FAO/UNU (2007) Consultation on Protein and Amino Acid Requirements of Man determined that the requirements of indispensable amino acids (IAA) were higher than previously thought. Conversely the new data on protein accretion based on K40 counting, suggested that the total protein needs of children should be slightly lowered. Thus the evidence supported a slightly lower need to total protein but a greater need for some of the essential amino acids (notably isoleucine, leucine, lysine, valine, threonine, phenylalanine + tyrosine). Thus protein quality became of greater relevance in meeting the IAA needs of children and adults alike. More recently, protein quality has been found to have an impact on broader issues such as reducing stunting and the successful development of human capital as marked by improved school performance associated to linear growth.
A landmark early life intervention study from Guatemala, comparing an energy supplement alone or a balanced protein and energy food provided during the first 2-3 years of life, has shown that the group that received the protein supplement did better in terms of their height as well as in their learning and earning capacity as adults (Hoddinott et al, Lancet 2008). Follow up of this cohort continues to reveal significant advances in both work productivity and health in terms of lower risk of diabetes and high blood pressure in those subjects that received animal protein in early life.
Defining the best way of meeting human protein aminoacid and protein needs requires we consider the impact on human health & well-being while remaining environmentally sensitive. We need to engage agriculturalists and food scientists emphasizing the production of high quality plant sources of protein, such as legumes. The immediate implication is that dietary protein quality matters to human health. Protein quality, in terms of IAA content has a greater importance in meeting the needs of populations across the developing world, as well as the nutritional quality of food commodities currently available to meet the human health needs through the human lifecycle. The quality of diet, specifically with reference to its protein quality, impacts a nation’s health, economy, agriculture and nutrient security.
UN agencies, FAO & IAEA have begun to address these issues releasing a Dietary Protein Quality Evaluation Report in 20113, which was a prelude the 2012 WHO technical note on foods for the management of moderate acute malnutrition in children4. The most recent meeting held by IAEA in Vienna (Expert Consultation in October 2013) discussed the use of stable isotope methods to measure protein quality in humans, this was followed by an FAO Expert Consultation in March 2014 held in Bangalore, to specifically define the most appropriate research methodologies to measure protein digestibility and utilization in humans.
Poor dietary quality has a marked negative impact on key sensitive periods for human development: pregnancy and the first two years of life (the first 1000 days). Dispensable and indispensable amino acids play key roles in the achieving healthy growth in early life. Faster growth rate leads to a greater the need for IAA, however, even non essential amino acids are key in promoting lean body mass gain and linear growth in particular by modulating the secretion of hormones (i.e arginine stimulates insulin secretion thus lowering serum glucose which in turn drives growth hormone and lean mass growth including height gain). These amino acids are also responsible for stimulating insulin like growth factors (IGFs)which promote length gain, specific proteins in milk and other animal foods are relevant in this process. Thus we need to characterize the best amino acid mix to promote healthy lean mass gain without inordinate fat mass gain.
This is not predicted by the amino acid need for accretion, but is best explained by the role of amino acids as regulators of hormones and growth factors responsible for tissue growth. Protein quality also impacts the quality of life at the later part of life. Women need to enter pregnancy in a well-nourished state, and eat appropriately during pregnancy. Fetal development, and child development during early life, is critically dependent on the appropriate amount of dietary proteins, for both appropriate growth and body composition, that may determine their human capacity and potential at adulthood. Protein synthesis rates in early gestation are associated with birth length (Jackson et al). Low intake of high quality protein (providing branched chain amino acids) is associated with sarcopenia, which is an important component of strength, mobility and quality of life at older ages.
Meeting population nutrient needs must also relate to the requirements of living under real life conditions such as repeated infections, poor environmental sanitation and psychosocial stress, all of which increase amino acid losses and prevent a strong anabolic response leading to recovery. Additionally, the effect of food processing and heating may alter absorption and utilization of some amino acids such as lysine, which may be already limiting protein quality of many predominantly cereal, based diets. In simple-stomached animals possessing a well-developed hind-gut (and this includes humans), a profuse and diverse microbiota acts on undigested material entering the large bowel, with a significant degree of metabolism of protein, peptides and amino acids. Ammonia, one of the products of the bacterial breakdown of protein and amino acids, is absorbed from the hindgut, but amino acids, as such, are not considered to be absorbed from the large intestine in nutritionally meaningful amounts (Wrong et al., 1981; Moughan, 2003; Moughan and Stevens, 2012).
Faecal protein is largely microbial protein, and compositionally bears no resemblance to the array of dietary amino acids remaining undigested at the end of the ileum. Estimates of amino acid digestibility based on analysis of faeces relative to foods ingested do not represent the fraction of amino acid absorbed. Accordingly, measurements of digestibility determined at the ileal level are critical in determining amino acid losses of both dietary and endogenous origin (Moughan, 2003; Fuller and Tomé, 2005). Faecal-ileal digestibility differences can be substantial and both amino acid and protein ileo-faecal digestibility differences have been shown across a wide range of simple-stomached animal species. Ideally, amino acid bioavailability should be measured from oro-ileal balances (the difference between ingested amino acids and the amino acids leaving the ileum), since absorption occurs only in the small intestine and colonic bacteria can recycle nitrogen. The measurement of intrinsic labelled stable isotope of plant protein offers a solution to this dilemma. This labelling approach using either 13CO2 or 2H2O. 2H2O, is economically feasible and appropriate for plant proteins. Indispensable amino acids (IAA) become labelled within the food matrix and following preparation and ingestion of the food, their appearance in the blood gives a unique measure of bioavailability. If a labelled test meal is accompanied by a trace quantity of a differently labelled reference protein, such as casein, or single cell proteins, comparative tracer appearance gives a direct measure of digestion/availability of the protein under test. New mass spectrometry instrumentation using newer sensitive methods (GC-C-IRMS, LCMS), allows for minimally invasive sampling, such as the use of saliva.