Reducing crude protein in amino acid-adequate diets for broiler chickens is effective in reducing nitrogenous emissions and competition for resources between the food and feed sectors. This review provides a comprehensive analysis of the literature on the relevance of nonessential amino acids in low protein diets for broiler chickens. Glycine and serine, owing to their interconvertibility summarised as glycine equivalents (Glyequi), limit growth when dietary crude protein is reduced below 19% in up to 3-week-old birds. Considering essential amino acids and the variable Glyequi requirements enables the reduction of dietary crude protein to ∼16% without compromising growth. Variation in Glyequi requirements likely occurs predominantly from the varying amounts of uric acid formed. Other influences seem to exert lower impacts on dietary Glyequi requirements. Asparagine or glutamine is probably the growth-limiting amino acid when crude protein is reduced below 16%. Alternatively, nonspecific amino-nitrogen may be lacking in such diets. The current potential to reduce dietary crude protein when using free essential and nonessential amino acids enables to increase the efficiency of nitrogen utilisation to a value above 80%. This coincides with reduced uric acid synthesis and energy expenditure for nitrogen excretion. The lower nitrogen excretion via the urine results in a lower energy expenditure. Hence, dietary energy may prospectively be reduced once the energy-sparing effect is quantified, thereby further reducing the competition for resources between food and feed.

Plant-based meat and dairy analogues contain less protein than their animal-based counterparts and rely on various plant protein sources, which frequently display incomplete amino acid (AA) profiles that do not reflect dietary requirements due to low quantities of one or more essential AA (EAA). There is little insight in the AA profiles of most of these plant-based analogues. We assessed the AA composition of forty plant-based meat and dairy analogues that were commercially available in The Netherlands in March 2023 and compared their EAA profile to dietary requirements and to the EAA profile of their meat and dairy counterparts. Total protein contents were lower in most analogues when compared with their animal-based counterparts (meat analogues, n 16 (80 %); lunch meats and cheese analogues, n 10 (100 %); milk and yoghurt analogues, n 9 (90 %)) and accompanied by lower EAA contents. In reference to dietary requirements, the sum of the total EAA contents was adequate in all but one of the analogues. Nevertheless, all analogues displayed deficiencies in one or more specific EAA. Methionine contents were most frequently low (n 39; 98 %), followed by lysine contents (n 11; 28 %). Essential AA compositions varied between analogues irrespective of the protein source(s) used. In conclusion, plant-based meat and dairy analogues exhibit incomplete EAA profiles, which may compromise adequate protein nutrition in plant-centred diets.

The RDA for dietary protein is likely insufficient for individuals with cystic fibrosis (CF). This study sought to characterise protein intake and diet quality in adults with cystic fibrosis (awCF), before and after elexacaftor/tezacaftor/ivacaftor (ETI) therapy, compared with healthy controls. Dietary intake was assessed by diet diary in awCF at baseline (BL, n 40) and at follow-up > 3 months post ETI therapy (follow-up (FUP), n 40) and in age-matched healthy controls (CON, n 80) free from known disease at a single time point. Protein intake dose and daily distribution, protein quality, protein source and overall diet quality were calculated for each participant. Both CON (1·39 (sd 0·47) g·kg–1·day–1) and CF (BL: 1·44 (sd 0·52) g·kg–1·day–1, FUP: 1·12 (sd 0·32) g·kg–1·day–1) had a higher mean daily protein intake than the protein RDA of 0·75g·kg–1·day–1. There was a significant reduction in daily protein intake in the CF group at FUP (P = 0·0003, d = 0·73), with levels below the alternative suggested dietary intake of ≥ 1·2 g·kg–1·day–1. There were no sex differences or noticeable effects on protein quality or source following the commencement of ETI therapy when compared with CON (all P > 0·05), although overall diet quality decreased between time points (P = 0·027, d = 0·57). The observed reduction in daily protein intake in the present cohort emphasises the importance of ensuring appropriate dietary protein intake to promote healthy ageing in adults with CF. More research is needed to evidence base dietary protein requirements in this at-risk population.

The fat deposition and lipid composition directly influence meat quality and feed efficiency during pork production. Glutamate (GLU) is a major component of proteins and has been widely utilized in livestock production. However, the role of GLU in regulating lipid deposition and the lipo-nutritional quality of porcine fat remains unclear. This study aimed to investigate the effects of GLU on fatty acid composition and lipid profiles in subcutaneous fat (SF) and perirenal fat (PF) from Shaziling pigs. Forty-eight finishing pigs aged 150 days (31.56 ± 0.95 kg) were divided into the control (CON) group and the GLU-supplemented group (1% GLU), each consisting of 6 pens (4 pigs per pen). After 51 days, 6 pigs (1 pig/pen) from each group were slaughtered for analysis. Fatty acid analysis detected 46 species in SF and 40 in PF. In SF, 1% GLU significantly increased the content of C18:3n3 (P < 0.05), which was accompanied by an increase in n3 PUFA deposition (P < 0.05) and a decreased n6/n3 ratio (P = 0.06). In PF, GLU supplementation reduced the levels of C18:1n9t, C24:1, C22:6n3, and others (P < 0.05). The content of monounsaturated fatty acids (MUFAs) and n9 unsaturated fatty acids (UFAs) was significantly decreased in the GLU group (P < 0.05). Similarly, GLU significantly reduced the n6/n3 PUFA ratio in PF (P < 0.05). Lipidomics profiling identified 2264 unique lipids in fat tissues. GLU had minimal effects on lipid composition in SF but significantly reduced ceramides (Cer), phosphatidylserine (PS), and phosphatidylinositol (PIP) contents in PF (P < 0.05) compared to the CON group. Additionally, GLU influenced the acyl chain saturation degree, fatty acyl chain length, and individual acyl chain composition in glycerophospholipid (GP) pools of PF. These results demonstrate the regulatory role of GLU on lipid dynamics in porcine fat and provide insights into regulating fat deposition and liponutritional quality in indigenous Chinese pig breeds.

Protein fermentation in the human gut is often associated with adverse health effects. Hence, understanding the fermentation characteristics of dietary undigested proteins is important for a comprehensive nutritional value of foods. This study investigated the protein fermentation kinetics of diet-derived proteins from thirty-one different foods using an in vitro model and human faecal inoculum. The undigested diet-derived protein substrate originated from porcine ileal digesta obtained from assessment of the digestible indispensable amino acid score (DIAAS) of the foods. Significant variations in fermentation kinetic parameters, particularly in maximum gas production rate (Rmax) and time to reach cumulative gas production (GP) from the substrate (TGPs), were observed. The Rmax ranged from 15·5 (se 0·7) ml/h for wheat bran-derived to 24·5 (se 0·9) ml/h for oatmeal-derived proteins. Egg-derived proteins had the shortest TGPs (14·7 (se 0·7) h), while mushroom-derived proteins had the longest (27·6 (se 7·1) h). When foods were categorised into five groups (‘animal protein’, ‘grains’, ‘legumes’, ‘fungi, algae and microorganisms’ and ‘others’), no significant differences were found in fermentation kinetics parameters. Samples were additionally incubated with porcine inoculum to assess potential donor-species effects. Human inoculum showed significantly lower Rmax, cumulative GP and microbiota turnover than porcine inoculum, indicating reduced fermentative activity. Linear regression analysis revealed correlations between human and porcine-derived inoculum only for Rmax (R2 = 0·78, P < 0·01) and TGPs (R² = 0·17, P < 0·05). These findings underscore the importance of using human inoculum in in vitro studies to better predict health implications of foods with DIAAS values.

Glycine plays an essential role in a variety of biological and biochemical processes. As the smallest amino acid, glycine is especially important in studies of prebiotic chemistry and chemical evolution. The behaviour of glycine in aqueous solution under ionizing radiation fields is still not well understood. Understanding the reaction mechanism of glycine in an ionizing radiation environment may provide insights into the complex processes involved in prebiotic chemical synthesis. Such reaction conditions could provide clues about the environmental conditions that might favour the emergence of life. Numerical modelling based on reaction kinetics provides information on the feasibility of the reaction mechanisms. In this work, we developed a numerical model in Python that describes the behaviour of glycine, as prototype compound, in aqueous solution under gamma radiation. The model is based on a variety of reaction kinetics pathways that have been proposed to describe the principal reactions between glycine and the water radicals formed by ionizing radiation. The numerical results are consistent with the experiments of other researchers. We obtained similar numerical solutions from different reaction mechanisms that share the same initial reactions. The results suggest that the primary attack of water radicals on the glycine is the main factor that controls the general decay of the molar concentration of glycine and the secondary reactions do not have a strong influence, even at high doses of nearly 200 kGy. The numerical tests of the models indicate their stability with the changing initial condition of the molar concentration of glycine. This work contributes to the advancement of knowledge regarding the behaviour of glycine in aqueous solutions under ionizing radiation from a kinetic perspective. It also provides insights into their stability under conditions that are difficult to replicate in the laboratory. Finally, this work contributes to the evaluation of appropriate numerical methods for solving the system of stiff differential equations that describe the reaction mechanism of organic molecules under high radiation fields.

Amino acids (AA) are essential nutrients in human milk (HM) and critical for infant growth and development. Several maternal lifestyle factors have been suggested to influence HM AA composition, with possible consequences for the breastfed infant. Whether maternal dietary protein and AA intake is associated with AA concentrations in HM is still largely unknown. Therefore, the aim of this study was to investigate the association between maternal dietary AA intake and AA concentrations in HM over the first month postpartum. Data from the observational longitudinal Amsterdam Mother’s Milk study were used, consisting of 123 lactating women in their first postpartum month. HM samples were collected three times, on day 10, 17 and 24 postpartum. Maternal dietary protein and AA intake on these collection days was assessed using three 24-h recalls. HM protein-bound and free AA (BAA and FAA, respectively) were analysed by liquid chromatography. Associations between maternal AA intake and AA concentrations in HM were assessed using linear mixed models. Maternal intake was negatively associated with milk concentrations of free arginine (–0·0003; P = 0·01) and free lysine (–0·0004; P = 0·03) and was positively associated with free glutamine (0·002; P = 0·03) and free threonine (0·0008; P = 0·03). However, these associations were attenuated after correction for multiple testing. Both the quality and quantity of dietary protein intake in lactating women do not seem to influence the amino composition of their breast milk when living in an affluent environment.

This experiment aimed to investigate the impacts of tributyrin (TB) dietary supplementation on serum biochemical indices and meat quality characteristics of longissimus thoracis et lumborum (LTL) muscle of lambs after weaning. Thirty healthy Small-Tailed Han female lambs (27.5 ± 4.1 kg; mean ± standard deviation) were randomly assigned to five treatments: basal diet (1) without TB, (2) with 0.5 g/kg TB, (3) with 1.0 g/kg TB, (4) with 2.0 g/kg TB or (5) with 4.0 g/kg TB. Each treatment consisted of six lambs, and the lambs were weaned on d 90 and were raised until d 165. Results showed that supplementing TB significantly promoted serum immunoglobulin concentrations of lambs such as immunoglobulins G, A and M. Besides, TB significantly increased muscle ether extract content, intermuscular fat length, pH value and redness but decreased lightness, drip loss and shear force. In addition, TB significantly elevated inosine-5ʹ-phosphate content and upregulated the relative expressions of genes related to lipid metabolism such as SREBP-1C, SCD, PPARγ, FAS and LPL. The mostly important, TB significantly enhanced essential amino acids (EAAs) and conjugated linoleic acids contents of the LTL muscle, despite it decreased total unsaturated fatty acids level. In conclusion, supplementing TB not only could promote the healthy status of weaned lambs via promoting serum immunity but also may improve nutritional quality of LTL muscle by improving EAA and conjugated linoleic acid contents.

Many improvements have been made to bring infant formula (IF) closer to human milk (HM) regarding its nutritional and biological properties. Nevertheless, the protein components of HM and IF are still different, which may affect their digestibility. This study aimed to evaluate and compare the protein digestibility of HM and IF using the infant INFOGEST digestion method. Pooled HM and a commercial IF were subjected to the infant INFOGEST method, which simulates the physiological digestion conditions of infants, with multiple directions, i.e. the curd state, gel images of SDS-PAGE, molecular weight distribution, free amino acid concentrations and in vitro protein digestion rate. HM underwent proteolysis before digestion and tended to have a higher protein digestion rate with finer curds during gastric digestion, than the IF. However, multifaceted analyses showed that the protein digestibility of HM and IF was not significantly different after gastrointestinal digestion. In conclusion, the infant INFOGEST method showed that the digestibility of HM and IF proteins differed to some extent before digestion and after gastric digestion, but not at the end of gastrointestinal digestion. The findings of this study will contribute to the refinement of IF with better protein digestibility in infant stomach.

Protein-rich animal foods are highly digestible, high-quality sources or protein, whereas the protein quality of plant-based foods can vary considerably. Given the growing interest in alternative non-animal-based sources of protein, it is important to establish the protein digestibility of these new foods and protein concentrates which have important health implications especially for vulnerable groups who don’t consume sufficient dietary protein. The human ileostomy model is ideal for measuring protein digestibility as it enables protein digestion to be quantified independent of protein degradation in the large intestine. The aim of this study was to determine the protein digestibility and quality of a wheat-based food containing legume flours. This randomised, double-blinded, controlled cross-over intervention was conducted in 4 proctocolectomised adults with conventional and well-functioning permanent ileostomies. The study was conducted over 2 weeks and on each testing day, the participant consumed 2 test muffins (125 g each) or 2 protein-free cookies in the morning (breakfast and morning tea) followed by a standardised low-protein lunch and afternoon tea. Test muffins were made using a standard muffin recipe using wheat flour and for 2 of the test muffins 50% of the flour was substituted with soy or lupin flour. An indigestible marker, titanium dioxide was added to the muffins so that the completeness of muffin recovered in ileal digesta could be calculated. The digestible indispensable amino acid score (DIAAS) was determined by comparing concentrations of true ileal digestible indispensable amino acids to recommended amino acid requirements(1). Data was reported as mean ± SD and repeated measures ANOVA was used to compare means between treatment groups with significance reported at P < 0.05. Substituting 50% of wheat flour in muffins with soy or lupin flour doubled the protein content of muffins (soy 11.8 g/100g and lupin 10.6 g/100g) compared to muffins that only contained wheat flour (wheat 5.1 g/100g). However, substituting wheat with legume flour did not affect protein digestibility which was similar for all muffin types; wheat (76.8 ± 7.0%), soy (77.9 ± 7.4%) and lupin (81.6 ± 6.9%) (P = 0.181). The DIAAS values for all muffins were below 75% which is classified as the cut off for a good quality protein food. In conclusion, substitution of wheat-based muffins with soy and lupin flour increased the protein content of wheat-based muffins but protein digestibility and overall protein quality was similar.

Optically active cationic complexes adsorbed on montmorillonite can be used for the resolution of racemic mixtures. Montmorillonite-Cu-lysine systems were used as a solid phase in high-pressure liquid chromatography for the resolution of the optical isomers of α-amino acids. Selectivity constants > 1.5 were measured for phenylalanine and tryptophan. The selectivity constants for the amino acids containing saturated-hydrocarbon side chains were in the range of 1.25–1.44. The montmorillonite-Cu-l-lysine complex displayed a stronger affinity for the l-isomers of α-amino acids than for the d-isomers at pHs near neutrality. Inasmuch as surface-catalyzed peptide formation on clays has been proposed as a step in chemical evolution, this stronger affinity between the clay-Cu-l-amino acid complex and l-amino acids might have been significant in prebiotic evolution. The mechanism of optical resolution probably involved ligand exchange. Optimizing the choice of the optically active ligands and of the chelating cation in the chiral agent may improve the resolution of the optical isomers.

Intracellular levels of glutathione, the major mammalian antioxidant, are reported to decline with age in several species. To understand whether ageing affects circulating glutathione levels in cats, blood was sampled from two age groups, < 3 years and > 9 years. Further, to determine whether dietary supplementation with glutathione precursor glycine (GLY) affects glutathione concentrations in senior cats (> 8 years), a series of free GLY inclusion level dry diets were fed. Subsequently, a 16-week GLY feeding study was conducted in senior cats (> 7 years), measuring glutathione, and markers of oxidative stress. Whole blood and erythrocyte total, oxidised and reduced glutathione levels were significantly decreased in senior cats, compared with their younger counterparts (P ≤ 0·02). The inclusion level study identified 1·5 % free GLY for the subsequent dry diet feeding study. Significant increases in erythrocyte total and reduced glutathione were observed between senior cats fed supplemented and control diets at 4 weeks (P ≤ 0·03; maximum difference of 1·23 µM). Oxidative stress markers were also significantly different between groups at 8 (P = 0·004; difference of 0·68 nG/ml in 8-hydroxy-2'-deoxyguanosine) and 12 weeks (P ≤ 0·049; maximum difference of 0·62 nG/mG Cr in F2-isoprostane PGF2α). Senior cats have lower circulating glutathione levels compared with younger cats. Feeding senior cats a complete and balanced dry diet supplemented with 1·5 % free GLY for 12 weeks elevated initial erythrocyte glutathione and altered markers of oxidative stress. Dietary supplementation with free GLY provides a potential opportunity to restore age-associated reduction in glutathione in cats.

Intercalation of amino acids into 10.0-Å hydrated kaolinite was studied by powder X-ray diffraction (XRD), differential thermal analysis-thermal gravimetry (DTA-TG), and infrared (IR) spectroscopy. Intercalation was found to be dependent on the chain-length, pH, and the concentration of the amino acid zwitterion. Near the isoelectric point, fully intercalated phases were obtained in solutions of concentration >0.5–1 M for glycine (Gly), 2–3 M for β-alanine (β-Ala), and 12 M for both γ-aminobutyric acid (γ-Aba) and δ-aminovaleric acid (δ-Ava). ∊-aminocaproic acid (∊-Aca) with a long chain (C = 6) was only partially intercalated. Intercalated amino acid formed a mono-molecular arrangement with the alkyl chain tilting toward the layer at an angle related to H2O content. The compositions of the intercalates of the Gly and β-Ala are Al2Si2O5(OH)4·(Gly)0.67·0.24H2O and Al2Si2O5(OH)4·(β-Ala)0.63·0.25H2O, respectively, based on TG data. From IR data, Gly and β-Ala molecules are found intercalated as zwitterions and these molecules form hydrogen bonds with both the Al-OH and Si-O surfaces of kaolinite. Washing the intercalate with water produced a hydrated kaolinite, which may form a second amino-acid intercalate of high order. Thus, hydrated kaolinite intercalates or deintercalates amino acids depending on concentration and conditions.

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.

The transition towards more plant-based diets may pose risks for bone health such as low vitamin D and Ca intakes. Findings for the contribution of animal and plant proteins and their amino acids (AA) to bone health are contradictory. This 6-week clinical trial aimed to investigate whether partial replacement of red and processed meat (RPM) with non-soya legumes affects AA intakes and bone turnover and mineral metabolism in 102 healthy 20–65-year-old men. Participants were randomly assigned to diet groups controlled for RPM and legume intake (designed total protein intake (TPI) 18 E%): the meat group consumed 760 g RPM per week (25 % TPI) and the legume group consumed non-soya legume-based products (20 % TPI) and 200 g RPM per week, the upper limit of the Planetary Health Diet (5 % TPI). No differences in bone (bone-specific alkaline phosphatase; tartrate-resistant acid phosphatase 5b) or mineral metabolism (25-hydroxyvitamin D; parathyroid hormone; fibroblast growth factor 23; phosphate and Ca) markers or Ca and vitamin D intakes were observed between the groups (P > 0·05). Methionine and histidine intakes were higher in the meat group (P ≤ 0·042), whereas the legume group had higher intakes of arginine, asparagine and phenylalanine (P ≤ 0·013). Mean essential AA intakes in both groups met the requirements. Increasing the proportion of non-soya legumes by reducing the amount of RPM in the diet for 6 weeks did not compromise bone turnover and provided on average adequate amounts of AA in healthy men, indicating that this ecologically sustainable dietary change is safe and relatively easy to implement.

Low birth weight (LBW) neonates show impaired growth compared with normal birth weight (NBW) neonates. Glutamine (Gln) supplementation benefits growth of weaning piglets, while the effect on neonates is not sufficiently clear. We examined the effect of neonatal Gln supplementation on piglet growth, milk intake and metabolic parameters. Sow-reared pairs of newborn LBW (0·8–1·2 kg) and NBW (1·4–1·8 kg) male piglets received Gln (1 g/kg body mass (BM)/d; Gln-LBW, Gln-NBW; n 24/group) or isonitrogenous alanine (1·22 g/kg BM/d; Ala-LBW; Ala-NBW; n 24/group) supplementation at 1–5 or 1–12 d of age (daily in three equal portions at 07:00, 12:00 and 17:00 by syringe feeding). We measured piglet BM, milk intake (1, 11–12 d), plasma metabolite, insulin, amino acid (AA) and liver TAG concentrations (5, 12 d). The Gln-LBW group had higher BM (+7·5%, 10 d, P = 0·066; 11–12 d, P < 0·05) and milk intake (+14·7%, P = 0·015) than Ala-LBW. At 5 d, Ala-LBW group had higher plasma TAG (+34·7%, P < 0·1) and lower carnosine (–22·5%, P < 0·05) than Ala-NBW and Gln-LBW, and higher liver TAG (+66·9%, P = 0·029) than Ala-NBW. At 12 d, plasma urea was higher (+37·5%, P < 0·05) with Gln than Ala supplementation. Several proteinogenic AA in plasma were lower (P < 0·05) in Ala-NBW v. Gln-NBW. Plasma arginine was higher (P < 0·05) in Gln-NBW v Ala-NBW piglets (5, 12 d). Supplemental Gln moderately improved growth and milk intake and affected lipid metabolism in LBW piglets and AA metabolism in NBW piglets, suggesting effects on intestinal and liver function.

Adequate protein intake is essential for the maintenance of whole-body protein mass. Different methodological approaches are used to substantiate the evidence for the current protein recommendations, and it is continuously debated whether older adults require more protein to counteract the age-dependent loss of muscle mass, sarcopenia. Thus, the purpose of this critical narrative review is to outline and discuss differences in the approaches and methodologies assessing the protein requirements and, hence, resulting in controversies in current protein recommendations for healthy older adults. Through a literature search, this narrative review first summarises the historical development of the Food and Agriculture Organization/World Health Organization/United Nations University setting of protein requirements and recommendations for healthy older adults. Hereafter, we describe the various types of studies (epidemiological studies and protein turnover kinetic measurements) and applied methodological approaches founding the basis and the different recommendations with focus on healthy older adults. Finally, we discuss important factors to be considered in future studies to obtain evidence for international agreement on protein requirements and recommendations for healthy older adults. We conclude by proposing future directions to determine ‘true’ protein requirements and recommendations for healthy older adults.

This study was designed to investigate the effects of dietary starch structure on muscle protein synthesis and gastrointestinal amino acid (AA) transport and metabolism of goats. Twenty-seven Xiangdong black female goats (average body weight = 9·00 ± 1·12 kg) were randomly assigned to three treatments, i.e., fed a T1 (normal maize 100 %, high amylose maize 0 %), T2 (normal maize 50 %, high amylose maize 50 %) and T3 (normal maize 0 %, high amylose maize 100 %) diet for 35 d. All AA in the ileal mucosa were decreased linearly as amylose:amylopectin increased in diets (P < 0·05). The plasma valine (linear, P = 0·03), leucine (linear, P = 0·04) and total AA content (linear, P = 0·03) increased linearly with the increase in the ratio of amylose in the diet. The relative mRNA levels of solute carrier family 38 member 1 (linear, P = 0·01), solute carrier family 3 member 2 (linear, P = 0·02) and solute carrier family 38 member 9 (linear, P = 0·02) in the ileum increased linearly with the increase in the ratio of amylose in the diet. With the increase in the ratio of amylose:amylopectin in the diet, the mRNA levels of acetyl-CoA dehydrogenase B (linear, P = 0·04), branched-chain amino acid transferase 1 (linear, P = 0·02) and branched-chain α-keto acid dehydrogenase complex B (linear, P = 0·01) in the ileum decreased linearly. Our results revealed that the protein abundances of phosphorylated mammalian target of rapamycin (p-mTOR) (P < 0·001), phosphorylated 4E-binding protein 1 (P < 0·001) and phosphorylated ribosomal protein S6 kinases 1 (P < 0·001) of T2 and T3 were significantly higher than that of T1. In general, a diet with a high amylose ratio could reduce the consumption of AA in the intestine, allowing more AA to enter the blood to maintain higher muscle protein synthesis through the mTOR pathway.