Carbonaceous chondrites contain various organic compounds, including amino acids (AAs), which may have contributed to the emergence of life on Earth. However, their origin remains debated. Previous studies have shown that amino acid precursors (AAPs) can form in ice mantles of interstellar dust particles within molecular clouds. These AAs and AAPs could have been incorporated into small celestial bodies during the formation of the solar system. It has been suggested that interstellar AAs and AAPs underwent aqueous alteration due to heat and radiation from the decay of radioactive nuclides such as 26Al. To test this hypothesis, we combined experiments simulating interstellar chemical reactions with those mimicking conditions inside meteorite parent bodies. We subjected AAs and interstellar AAP analogs to gamma irradiation in mixtures of formaldehyde (HCHO), methanol (CH3OH), ammonia (NH3) and water (H2O). The resulting products were analyzed by cation-exchange high-performance liquid chromatography (HPLC) and gas chromatography-quadrupole mass spectrometry (GC/MS). Our results demonstrated that interstellar AAP analogs were more resistant to gamma irradiation than free AAs. Among the free AAs, glycine exhibited the highest stability, while AAs lacking α-hydrogens were more stable than their isomeric counterparts with α-hydrogens. Additionally, gamma irradiation not only degraded AAs but also generated new ones. The yield and diversity of newly formed AAs depended on the specific AAPs or AAs present in the system. Notably, systems containing interstellar AAP analogs produced a greater variety and higher quantity of AAs than those containing free AAs or none at all. These findings suggest that interstellar organic matter, including AAPs, delivered to asteroids could have contributed to the formation of the diverse organic compounds observed in asteroids and meteorites.

Sepsis-related deaths remain prevalent in intensive care settings, with metabolic dysregulation as a key contributor. Although amino acid supplementation has shown promise, its clinical effectiveness in sepsis is unclear. This study evaluated the impact of intravenous amino acid administration on 28-d mortality in intensive care unit (ICU) sepsis patients using retrospective cohort analysis and Mendelian randomisation (MR). We analysed data from the Medical Information Mart for Intensive Care-IV database, matching 726 patients (363 per group) using propensity scores. The association between amino acid supplementation and mortality was assessed using logistic regression, Cox regression and targeted maximum likelihood estimation (TMLE). Two-sample MR was used to explore causal links between twenty common amino acids and sepsis mortality. In the cohort analysis, amino acid supplementation was consistently associated with significantly reduced 28-d mortality across all analytical methods (logistic regression: OR = 0·48, P < 0·01; Cox regression: HR = 0·48, P < 0·01; TMLE: average treatment effect = −0·102, P < 0·01). In contrast, the MR analysis did not find a significant causal association for any single amino acid after correction for multiple comparisons; although glycine showed a nominal protective signal, it did not remain significant after false discovery rate correction. This dual-method study demonstrates a strong association between compound amino acid infusions and reduced mortality in sepsis but did not identify any single amino acid as a robust causal mediator. These findings suggest the benefit may arise from a synergistic effect, highlighting the need for randomised controlled trials to validate these observational results and optimise nutritional strategies.

To determine values for the digestible indispensable amino acid score (DIAAS), it is recommended that ileal amino acid (AA) digestibility values obtained in growing pigs are used to characterise protein quality in different foods. Therefore, an experiment was conducted to determine the standardised ileal digestibility (SID) of AA in eight energy ingredients (barley, sorghum, wheat, brown rice, rice bran, wheat bran, cassava and paddy rice) fed to pigs, where SID values in pigs can be used to calculate approximate DIAAS values in humans. Among the data obtained for all energy ingredients, significant variations (P < 0·01) in CP and AA composition were observed. Rice bran and wheat bran had the highest CP (16·43 % and 18·16 %, respectively) and DIAAS scores of 81–88 for adult, qualifying as ‘good’ protein sources (> 75). Cassava, with the lowest CP (2·74 %), was limited by sulphur amino acid (54). Lysine (Lys) was the first-limiting AA in barley (74), sorghum (51) and wheat (49), with SID values lowest in wheat (71·04 %). Brown rice and paddy rice showed higher SIDLys (87·51 % and 78·13 %, respectively). These findings highlight the potential of bran-based ingredients and Lys fortification to improve protein quality in grain-dependent diets, providing the scientific basis to combat protein malnutrition.

This study investigates the accumulation of glycogen, amino acids, and fatty acids in male Mytilus coruscus during different stages of gonadal development and explores their relationships with reproductive processes. Glycogen levels were highest during the resting phase, decreasing progressively during the proliferation and maturation phases. A positive correlation was observed between glycogen and carbon content, indicating a close association between energy storage and metabolic processes. Amino acid content, particularly essential amino acids (EAAs), increased during gonadal development, reflecting the higher demand for protein synthesis and cellular metabolism. Branched-chain amino acids (BCAAs) such as isoleucine, leucine, and lysine were key in activating protein synthesis and supporting gametogenesis. Non-essential amino acids like aspartic acid, glutamic acid, and glycine also accumulate, supporting cellular function and reproductive regulation. Fatty acids, especially unsaturated fatty acids (UFAs) and polyunsaturated fatty acids (PUFAs), progressively accumulated in the testes, highlighting their role in energy supply and membrane integrity during gametogenesis. Phosphorus (P) accumulated in parallel with fatty acids, supporting DNA and RNA synthesis, energy metabolism, and cell membrane function. This study emphasizes the crucial role of these biochemical components in supporting gonadal development in male M. coruscus, providing insights into the metabolic pathways involved in marine bivalve reproduction.

Metabolomics of faecal samples offers a non-invasive method to monitor gastrointestinal (GI) development and microbial activity in dairy heifers during key physiological transitions. In this longitudinal study, faecal metabolites from 10 Holstein heifers were analyzed from birth to first calving using targeted metabolomics. Faecal samples were collected at 12 h post-birth, week 6 (pre-weaning), week 14 (weaning), 8 months (post-weaning), and at first calving (26 ± 2.3 months). Calves were fed 3.8 L of colostrum within 2 h of birth, followed by 6 L of maternal transition milk for 5 days, then 6 L of milk replacer twice daily. Group housing began at 14 days. Partial least squares discriminant analysis (PLS-DA) showed distinct temporal clustering of faecal metabolites. Heatmap analysis revealed significant metabolite alterations, particularly between pre- and post-weaning stages. A linear mixed-effects model identified significant stage effects for all 17 amino acids. Of the 55 biogenic amines and amino acid-related metabolites, 48 significantly differed across stages. Elevated amino acids and polyamines early in life reflected colostrum intake and immature digestion, decreasing post-weaning, indicating improved nutrient absorption and rumen functionality. Increased microbiota-derived compounds, including β-alanine, serotonin, and indole derivatives, reflected microbial colonization and co-regulation with the host. Elevated dopamine, homocysteine, and phenylethylamine in late gestation indicated neuroactive and redox adaptations. Overall, faecal metabolite profiles provide insights into metabolic remodelling related to nutrition, GI maturation, and reproductive development, highlighting faecal metabolomics as a useful non-invasive tool for monitoring heifer development.

My research on dietary protein and the regulation of proteostasis in muscle and the whole body during growth and adult maintenance is reviewed. Growth control involves both permissive and regulatory roles of protein acting with genetic determinants and functional demand, to mediate substrate flow into metabolic consumption, energy storage and growth. In 1995 a Protein-Stat hierarchical model for control of the fat-free mass was proposed and is updated here with special emphasis on the skeletal muscle mass. Control is exerted in large part through a central aminostatic appetite mechanism sensing changes in free amino-acid patterns in response to the balance between their supply in relation to their demand. This acts primarily to maintain skeletal muscle mass at a level set by the linear dimensions of the organism, which in turn is controlled by genetic programming and mediated by the developmental hormones acting together with an appropriate anabolic drive deriving from dietary protein. This, together with other important nutrients like zinc, calcium and vitamin D, provides the regulatory stimulus for growth and protein deposition in all tissues. The applicability of this model to childhood growth and development will be described as well as the maintenance of the adult phenotypic muscle mass within populations with protein intakes varying over a wide range, work which resulted in an adaptive metabolic demand model for protein and amino acid requirements and values for apparent adult protein and lysine requirements. Finally current understanding of aminostatic mechanisms of amino-acid sensing in the brain will be reviewed.

Amino acids are fundamental to sustaining life. They are crucial for intracellular processes, such as energy metabolism, biosynthesis of nucleotides, and maintenance of oxidative homeostasis. These processes ensure the proper functioning of cells (including immune cells) and organs. Many studies have demonstrated that immune cells, as key players in immune regulation, have distinct amino acid demands, and their rapid growth and activation are shaped by amino acid availability in their microenvironment. In particular, the proliferation, maturation, and functional responses of innate immune cells are closely linked to amino acid metabolism. The transport, sensing, and mobilization of amino acids drive metabolic reprogramming to support these processes. Therefore, this review focuses on the influence of amino acids on the fate and function of immune cells across development, homeostasis, activation, and effector phases, highlighting the underlying mechanisms. It provides a scientific basis for improving disease resistance and production efficiency in animals.

The objective of the trial was to evaluate the effects of arginine supplementation in the feed of gestating sows on the variability of piglet birth weight. The weight of the piglets was evaluated using descriptive analysis, correlation analysis and analysis of variance with a 2 × 3 factorial arrangement. This arrangement included no supplementation or supplementation with 1.0 % L-arginine, combined in three periods. Period 1: from days 25 to 53 of gestation, providing 23 g/day from days 25 to 28 and 18 g/day from days 29 to 53 of gestation; period 2: from days 30 to 60 of gestation and from day 80 of gestation to farrowing, providing 18 g/day in the first period and 45 g/day in the second period and period 3: from day 85 of gestation to delivery, with 24 g/day was provided from day 85 until farrowing and 28 g/day from days 85 to 107, increasing to 56 g/day from day 108 until farrowing. Supplementation with 1.0 % of L-arginine reduced the percentage of total piglets born and piglets born alive with less than 800 g by 2.26 and 2.05 percentage points, respectively; and increased the percentage of total piglets born and piglets born alive between 1601 and 1800g by 5.89 and 6.08 percentage points, respectively. Supplementing with 1.0 % of L-arginine improves litter uniformity, with an average reduction of 4.06 percentage points in the piglet population of less than 1180 g and an increase in the piglet population of 1180 to 1890 g by 4.70 percentage points.

The objectives of this experiment were to determine the digestible indispensable amino acid score (DIAAS) for eggs cooked in different forms and in traditional egg-bread or egg-hash brown combinations, and to test the hypothesis that DIAAS in eggs is greater than in breads or potatoes. Nine ileal cannulated gilts (average initial body weight: 51.1 ± 6.0 kg) were allotted to a 9 × 6 Youden square design with nine diets and six 7-day periods. Fried egg, boiled egg, scrambled egg, English muffin, Texas toast, and hash brown were included in the experiment. Six diets each contained one source of protein and three diets were combinations of fried eggs and English muffin, boiled eggs and Texas toast, or scrambled egg and hash brown. A nitrogen-free diet was also used and fed to all pigs in one period. The standardised ileal digestibility (SID) of crude protein (CP) and amino acids (AA) was calculated, and DIAAS was calculated for the individual ingredient and combined meals for children between 6 and 36 months and individuals older than 3 years. For both age groups, all cooked eggs had greater (P < 0.001) DIAAS compared with the other foods, and hash brown had greater (P < 0.001) DIAAS than both breads. All combined meals had DIAAS greater than 75 and there were no differences between measured and predicted DIAAS for the combined meals. In conclusion, eggs have ‘excellent’ protein quality for individuals older than 6 months and can compensate for the lower protein quality in plant-based foods, and DIAAS obtained from individual ingredients are additive in mixed meals.

The increasing demand for food and especially proteins leads to the search for alternative protein sources. Meat co-products, which are available but little used in human food, provide a potential solution to this challenge. The present study aimed to evaluate the nutritional quality of two beef protein ingredients (greasy greaves recovered proteins (GGRP) and water recovered proteins (WRP)), both co-products of the fat rendering process. Their true ileal digestibility (TID), digestible indispensable amino acid score (DIAAS) and kinetics of plasma amino acids (AA) were measured in ten growing pigs, each fed the two co-products and a protein-free diet. Titanium dioxide was used as an indigestible marker. Digesta samples were collected for 9 h after meal ingestion, and blood samples were collected at ten time points during the same period. Total nitrogen (N) and AA contents were determined. Data were statistically analysed using linear mixed models. The TID of total N was not different between WRP and GGRP (81–84 %, P > 0·05). The first-limiting AA was Trp for both ingredients, with a DIAAS much higher for GGRP than for WRP (74 and 10 % for adults, respectively; P < 0·001). Postprandial plasma AA concentration peaked earlier for WRP (3 h) than for GGRP (5 h). Plasma concentrations of total and essential AA were higher (P < 0·001) with GGRP diet than WRP diet. Overall, GGRP has a nutritional quality suitable to meet the needs of adults for AA, while WRP needs to be supplemented with other protein sources to fulfil the dietary requirements.

Postprandially, amino acids and di/tripeptides are thought to be primarily absorbed in the proximal small intestine. However, there have been no in vivo demonstrations of regional differences in amino acid transport dynamics between the proximal and distal small intestines. We monitored plasma amino acid responses in the jejunal and ileal mesenteric veins of rats after oral administration of a diet or an amino acid mixture (Expt 1) and in the portal vein after direct administration of the amino acid mixture into the lumen of the jejunum or ileum (Expt 2). In Expt 1, the total and some amino acid concentrations in the jejunal mesenteric vein were slightly higher than those in the ileal mesenteric vein after oral administration of the amino acid mixture, suggesting that the ileum actively transports luminal amino acids to the basolateral side, comparable to the jejunum. In Expt 2, portal amino acid concentrations were elevated to a greater extent after direct administration of the amino acid mixture into the ileal lumen than into the jejunal lumen. These results demonstrate regional differences in amino acid transport dynamics in vivo and suggest that the ileum has a higher capacity for transporting amino acids than the jejunum. Our findings highlight the importance of the ileum in postprandial amino acid absorption and metabolism.

Camelina cake (CAM) is a co-product proposed as an alternative protein source; however, piglet data are still limited. This study aimed to evaluate the effect of different doses of CAM in substitution of soyabean meal on the growth, health and gut health of weaned pigs. At 14 d post-weaning (d0), sixty-four piglets were assigned either to a standard diet or to a diet with 4 %, 8 % or 12 % of CAM. Piglets were weighed weekly. At d7 and d28, faeces were collected for microbiota and polyamine and blood for reactive oxygen metabolites (ROM) and thyroxine analysis. At d28, pigs were slaughtered, organs were weighed, pH was recorded on gut, colon was analysed for volatile fatty acids (VFA) and jejunum was used for morphological and gene expression analysis. Data analysis was carried out using a mixed model including diet, pen and litter as factors; linear and quadratic contrasts were tested. CAM linearly reduced the average daily gain from d0–d7, d0–d14, d0–d21 and d0–d28 (P ≤ 0·01). From d0–d7 increasing CAM linearly decreased feed intake (P = 0·04) and increased linearly the feed to gain (P = 0·004). CAM increased linearly the liver weight (P < 0·0001) and affected the cadaverine (P < 0·001). The diet did not affect the ROM, thyroxine, intestinal pH, VFA and morphology. All doses of CAM increased the α diversity indices at d28 (P < 0·05). CAM at 4 % promoted the abundance of Butyricicoccaceae_UCG-008. Feeding with CAM enhanced resilience in the gut microbiome and can be evaluated as a potential alternative protein source with dose-dependent limitations on piglet growth performance.

As a consequence of treatments with glycine solutions, glycine molecules enter the interlayer of both Ca- and Cd-rich montmorillonite. Measurements of d value suggest that at low glycine concentration (0.01 and 0.1 M glycine solutions) a “flat” arrangement of the glycine molecules occurs in the interlayer. In contrast, intercalation of more than one monolayer of glycine molecules occurs for the montmorillonite treated with a higher concentration of glycine (1 M glycine solution).

Interlayer complexation of glycine occurs only for the Cd-rich form of montmorillonite, whereas no complexation is observed for Ca-rich montmorillonite. Both nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) results suggest that the adsorbed glycine, which fully protonates in the interlayer of montmorillonite to give the GlyH2− species, interacts with the interlayer Cd2+ to form the CdGlyx complex mainly through the carboxylate group. The interlayer cadmium, present as both Cd2+ and CdCl−, is complexed by the ligand glycine. In contrast, the cadmium adsorbed on the external surfaces of montmorillonite does not interact with the ligand. Complexation of CdCl+ only occurs for large amounts of adsorption of glycine (e.g., for samples treated with 1 M glycine solution).

Whole-body tissue protein turnover is regulated, in part, by the postprandial rise in plasma amino acid concentrations, although minimal data exist on the amino acid response following non-animal-derived protein consumption. We hypothesised that the ingestion of novel plant- and algae-derived dietary protein sources would elicit divergent plasma amino acid responses when compared with vegan- and animal-derived control proteins. Twelve healthy young (male (m)/female (f): 6/6; age: 22 ± 1 years) and 10 healthy older (m/f: 5/5; age: 69 ± 2 years) adults participated in a randomised, double-blind, cross-over trial. During each visit, volunteers consumed 30 g of protein from milk, mycoprotein, pea, lupin, spirulina or chlorella. Repeated arterialised venous blood samples were collected at baseline and over a 5-h postprandial period to assess circulating amino acid, glucose and insulin concentrations. Protein ingestion increased plasma total and essential amino acid concentrations (P < 0·001), to differing degrees between sources (P < 0·001), and the increase was further modulated by age (P < 0·001). Postprandial maximal plasma total and essential amino acid concentrations were highest for pea (2828 ± 106 and 1480 ± 51 µmol·l−1) and spirulina (2809 ± 99 and 1455 ± 49 µmol·l−1) and lowest for chlorella (2053 ± 83 and 983 ± 35 µmol·l−1) (P < 0·001), but were not affected by age (P > 0·05). Postprandial total and essential amino acid availabilities were highest for pea, spirulina and mycoprotein and lowest for chlorella (all P < 0·05), but no effect of age was observed (P > 0·05). The ingestion of a variety of novel non-animal-derived dietary protein sources elicits divergent plasma amino acid responses, which are further modulated by age.

Amino acids are ubiquitous in the Earth’s surface environments as reactive biological molecules produced by every living thing including bacteria. To evaluate the effects of amino acids on mineral dissolution and to reveal the mechanism by which they interact with the mineral surface, we performed dissolution experiments of X-ray amorphous silica in solution containing 0.1 mmol Na with 10.0 mmol amino acids such as cysteine, asparagine, serine, tryptophan, alanine, threonine, histidine, lysine and arginine in near-neutral solutions. Dissolution experiments in solutions of 0.1, 1.0 and 10.0 mmol NaCl without amino acids were also conducted as amino acid-free controls. The results of this study indicate that basic amino acids such as histidine, lysine and arginine can interact more strongly with the negatively charged surface of amorphous silica than other non-basic amino acids due to their greater dissociation, thus forming cationic species. This electrostatical interaction enhanced dissolution rates of amorphous silica by approximately one order of magnitude compared with amino acid-free controls. In contrast, no significant effect on the dissolution rates of amorphous silica was observed in solutions containing cysteine, asparagine, serine, tryptophan, alanine and threonine because of lesser interaction with the surface of amorphous silica.

Aflatoxins in contaminated corn do not degrade in corn fermentation solution (CFS) during biofuel production; rather, they are enriched in the co-product, dried distillers grain. Aflatoxin B1 (AfB1) is the most toxic form of all aflatoxins. Removing AfB1 from CFS is desirable to minimize its toxicity to animals. Smectites can adsorb AfB1 from aqueous solutions and, therefore, inactivate the toxin, but proteins in CFS inhibit the adsorption of AfB1 by smectites. The current study aimed to minimize the interference by CFS in adsorption of AfB1 on smectite by modifying a calcium-smectite (Ca-3MS) with a small nutritive organic compound, e.g. carnitine, choline, arginine, histidine, or tryptophan. The organo-smectites were characterized by X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy, and adsorption of AfB1 in CFS by these composites was examined. Various degrees of intercalation of the organic nutrients into the smectites were observed with XRD and FTIR. After immersing the smectite and organo-smectites in the CFS, the d001 values of Ca-3MS expanded to ~1.82 nm due to protein interaction, but the organo-smectites were confined to ~1.39 nm, which indicated that the protein had limited access to the organo-smectite interlayers. The IR bands at ~1652, 1544, 1538, and 1454 cm–1 from the organo-smectites revealed, however, that complete protein inhibition was not achieved. The organo-smectites were capable of adsorbing AfB1 in simple aqueous solution with maximal adsorption capacity up to 0.55 mol kg–1. Significantly greater (p ≤ 0.05) AfB1 adsorption was achieved by choline- and carnitine-modified smectites compared with the original Ca-3MS in the presence of competing protein (pepsin) in simple aqueous solution. In real CFS, both AfB1 adsorption capacities (Qmax) and affinities (K) by all organo-smectites were greater (Qmax = up to 0.45 mol kg–1 and K = up to 0.165 μM–1) than those by Ca-3MS (Qmax = 0.22 mol kg–1 and K = 0.031 μM–1). The study suggested that using smectites modified with an organic nutritive compound could be an effective, economical, and safe strategy for removing mycotoxins, including aflatoxins, during biofuel production.

Amino acids are present in various geochemical environments and they interact with mineral surfaces. To evaluate the effects of amino acids on mineral dissolution at pH conditions less than their isoelectric points (pI), dissolution experiments of X-ray amorphous silica in solutions containing 10.0 mmol/L of various amino acids (cysteine, asparagine, serine, tryptophan, alanine, threonine, histidine, lysine, and arginine) at pH 4 were performed. The results confirmed that basic amino acids (histidine, lysine, and arginine) produce an 8- to 8.5-fold enhancement of the rate of dissolution of amorphous silica compared with an amino acid-free control. Neutral amino acids (cysteine, asparagine, serine, tryptophan, alanine, and threonine) enhanced rates of dissolution by a factor of ∼3 to 3.5. The rate-enhancement effects of amino acids are controlled by concentrations of the amino acid’s cationic species which interact with the negatively charged >SiO− sites at the surface of the amorphous silica.

This study evaluated the importance of a correction for amino acids (AA) released into the hindgut on a measure of AA absorption kinetics and tested whether AA absorption kinetics are related to the extent of AA absorption using the growing pig as a model for humans. Thirty-six nine-week-old pigs (22·3 kg) received a diet containing whey protein as the sole protein source for 8 d. Pigs received their last meal containing the indigestible marker titanium dioxide before being euthanised at 1, 2, 3, 4, 6 and 12 h post-feeding. The entire content of each gastrointestinal tract (GIT) region was collected to determine AA released into the hindgut, and the kinetics and extent of AA absorption (uncorrected and corrected for AA entering the hindgut). Amounts of AA released into the hindgut increased over time (e.g. 33 and 180 mg of Glu for 4 and 6 h post-feeding). The corrected apparent amount of each AA absorbed from the GIT lumen after 4 h post-feeding was generally lower (P ≤ 0·05) than the uncorrected counterpart. Differences in both the kinetics and extent of AA absorption were observed across AA. For example, the time to reach half of the apparent AA absorption (T50) was 1·5 and 3·4 h for Met and Arg, respectively, whereas their extent of apparent absorption was 93 and 73 %. Negative correlations between parameters related to kinetics and the extent of apparent absorption were observed (e.g. for T50 r = −0·81; P < 0·001). The kinetics of AA absorption is related to the extent of AA absorption.

This review explores the evolution of dietary protein intake requirements and recommendations, with a focus on skeletal muscle remodelling to support healthy ageing based on presentations at the 2023 Nutrition Society summer conference. In this review, we describe the role of dietary protein for metabolic health and ageing muscle, explain the origins of protein and amino acid (AA) requirements and discuss current recommendations for dietary protein intake, which currently sits at about 0⋅8 g/kg/d. We also critique existing (e.g. nitrogen balance) and contemporary (e.g. indicator AA oxidation) methods to determine protein/AA intake requirements and suggest that existing methods may underestimate requirements, with more contemporary assessments indicating protein recommendations may need to be increased to >1⋅0 g/kg/d. One example of evolution in dietary protein guidance is the transition from protein requirements to recommendations. Hence, we discuss the refinement of protein/AA requirements for skeletal muscle maintenance with advanced age beyond simply the dose (e.g. source, type, quality, timing, pattern, nutrient co-ingestion) and explore the efficacy and sustainability of alternative protein sources beyond animal-based proteins to facilitate skeletal muscle remodelling in older age. We conclude that, whilst a growing body of research has demonstrated that animal-free protein sources can effectively stimulate and support muscle remodelling in a manner that is comparable to animal-based proteins, food systems need to sustainably provide a diversity of both plant and animal source foods, not least for their protein content but other vital nutrients. Finally, we propose some priority research directions for the field of protein nutrition and healthy ageing.

Reducing dietary crude protein (CP) concentration while maintaining adequate amino acid (AA) supply by free AA inclusion can contribute to attenuate the negative environmental effects of animal farming. This study investigated upper limits of dietary free AA inclusions without undesirable effects including the dependence on asparagine (Asn) and glutamine (Gln) supply. Ten broilers were allocated to sixty-three metabolism units each and offered nine experimental diets from day (d) 7–21 (n 7). One diet (167 g CP/kg) contained 80 g soya protein isolate (SPI)/kg. In the other diets, 25, 50, 75 and 100 % of the digestible AA from SPI were substituted with free AA. Digestible Asn+aspartic acid (Asp) and Gln+glutamic acid (Glu) were substituted with Asp/Glu or 50/50 mixes of Asp/Asn and Glu/Gln, respectively. Total excreta were collected from d 11–14 and from d 18–21. Growth and nitrogen accretion were unaffected by 25 and 50 % substitution without and with free Asn/Gln, respectively, but decreased at higher substitution (P ≤ 0·024). Circulating concentrations of Asp, Glu and Gln were unaffected by treatment, while Asn decreased at substitution higher than 50 % when Asn/Gln were not provided (P ≤ 0·005). Blood gas analysis on d 21 indicated a compensated metabolic acidosis at substitution higher than 50 and 75 % without and with free Asn/Gln, respectively (P ≤ 0·017). Results suggest that adding Asn/Gln increased an upper limit for proportion of dietary free AA from 10 to 19 % of dietary CP and enabled higher free AA inclusion without affecting the acid–base balance.