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.

All human tissues are in a constant state of remodelling, regulated by the balance between tissue protein synthesis and breakdown rates. It has been well-established that protein ingestion stimulates skeletal muscle and whole-body protein synthesis. Stable isotope-labelled amino acid methodologies are commonly applied to assess the various aspects of protein metabolism in vivo in human subjects. However, to achieve a more comprehensive assessment of post-prandial protein handling in vivo in human subjects, intravenous stable isotope-labelled amino acid infusions can be combined with the ingestion of intrinsically labelled protein and the collection of blood and muscle tissue samples. The combined application of ingesting intrinsically labelled protein with continuous intravenous stable isotope-labelled amino acid infusion allows the simultaneous assessment of protein digestion and amino acid absorption kinetics (e.g. release of dietary protein-derived amino acids into the circulation), whole-body protein metabolism (whole-body protein synthesis, breakdown and oxidation rates and net protein balance) and skeletal muscle metabolism (muscle protein fractional synthesis rates and dietary protein-derived amino acid incorporation into muscle protein). The purpose of this review is to provide an overview of the various aspects of post-prandial protein handling and metabolism with a focus on insights obtained from studies that have applied intrinsically labelled protein under a variety of conditions in different populations.

A progressive decrement in muscle mass and muscle function, sarcopoenia, accompanies ageing. The loss of skeletal muscle mass and function is the main feature of sarcopoenia. Preventing the loss of muscle mass is relevant since sarcopoenia can have a significant impact on mobility and the quality of life of older people. Dietary protein and physical activity have an essential role in slowing muscle mass loss and helping to maintain muscle function. However, the current recommendations for daily protein ingestion for older persons appear to be too low and are in need of adjustment. In this review, we discuss the skeletal muscle response to protein ingestion, and review the data examining current dietary protein recommendations in the older subjects. Furthermore, we review the concept of protein quality and the important role that nutrient-dense protein (NDP) sources play in meeting overall nutrient requirements and improving dietary quality. Overall, the current evidence endorses an increase in the daily ingestion of protein with emphasis on the ingestion of NDP choices by older adults.

An experiment was conducted to test the hypothesis that meat products have digestible indispensable amino acid scores (DIAAS) >100 and that various processing methods will increase standardised ileal digestibility (SID) of amino acids (AA) and DIAAS. Nine ileal-cannulated gilts were randomly allotted to a 9 × 8 Youden square design with nine diets and eight 7-d periods. Values for SID of AA and DIAAS for two reference patterns were calculated for salami, bologna, beef jerky, raw ground beef, cooked ground beef and ribeye roast heated to 56, 64 or 72°C. The SID of most AA was not different among salami, bologna, beef jerky and cooked ground beef, but was less (P < 0·05) than the values for raw ground beef. The SID of AA for 56°C ribeye roast was not different from the values for raw ground beef and 72°C ribeye roast, but greater (P < 0·05) than those for 64°C ribeye roast. For older children, adolescents and adults, the DIAAS for all proteins, except cooked ground beef, were >100 and bologna and 64°C ribeye roast had the greatest (P < 0·05) DIAAS. The limiting AA for this age group were sulphur AA (beef jerky), leucine (bologna, raw ground beef and cooked ground beef) and valine (salami and the three ribeye roasts). In conclusion, meat products generally provide high-quality protein with DIAAS >100 regardless of processing. However, overcooking meat may reduce AA digestibility and DIAAS.

Non-fasting TAG – postprandial lipaemia (PPL) – are to a higher degree associated with cardiovascular risk compared with fasting TAG. Dietary protein, especially whey proteins (WP), may lower PPL. We hypothesised that a WP pre-meal (17·6 g protein) consumed 15 v. 30 min before a fat-rich meal reduces the PPL response in subjects with the metabolic syndrome (MetS) and that a WP pre-meal has more potent effects than casein and gluten pre-meals. A total of sixteen subjects with the MetS completed an acute, randomised, crossover trial. WP pre-meals were consumed 15 and 30 min, and casein and gluten 15 min before a fat-rich meal. Blood samples were drawn 360 min postprandially to determine metabolite and hormone responses, S-paracetamol (for assessment of gastric emptying) and amino acids. Insulin and glucagon responses were affected by both timing and protein type (for all P <0·01), with significantly higher concentrations for WP given at –15 min than WP at –30 min and higher responses compared with gluten for the first 30 min after pre-meal consumption (for all P <0·05). The PPL responses changed neither by timing nor by protein type. Glucose-dependent insulinotropic peptide but not glucagon-like peptide 1 responses differed between the three protein types. S-paracetamol concentration was higher for WP (–30 min) than for WP (–15 min) 15 min after the main meal (P = 0·028), and higher for casein and gluten than for WP at time point 30 min (for all P <0·05). In conclusion, the PPL response was not changed by ingestion of a 17·6 g protein pre-meal, whereas both timing and protein quality affected hormone secretion (insulin and glucagon).

True ileal digestibility (TID) values of amino acid (AA) obtained using growing rats are often used for the characterisation of protein quality in different foods and acquisition of digestible indispensable amino acid scores (DIAAS) in adult humans. Here, we conducted an experiment to determine the TID values of AA obtained from nine cooked cereal grains (brown rice, polished rice, buckwheat, oats, proso millet, foxtail millet, tartary buckwheat, adlay and whole wheat) fed to growing Sprague–Dawley male rats. All rats were fed a standard basal diet for 7 d and then received each diet for 7 d. Ileal contents were collected from the terminal 20 cm of ileum. Among the TID values obtained, whole wheat had the highest values (P<0·05), and polished rice, proso millet and tartary buckwheat had relatively low values. The TID indispensable AA concentrations in whole wheat were greater than those of brown rice or polished rice (P<0·05), and polished rice was the lowest total TID concentrations among the other cereal grains. The DIAAS was 68 for buckwheat, 47 for tartary buckwheat, 43 for oats, 42 for brown rice, 37 for polished rice, 20 for whole wheat, 13 for adlay, 10 for foxtail millet and 7 for proso millet. In this study, the TID values of the nine cooked cereal grains commonly consumed in China were used for the creation of a DIAAS database and thus gained public health outcomes.

Corn and soyabean micronutrient-fortified-blended foods (FBF) are commonly used for food aid. Sorghum and cowpeas have been suggested as alternative commodities because they are drought tolerant, can be grown in many localities, and are not genetically modified. Change in formulation of blends may improve protein quality, vitamin A and Fe availability of FBF. The primary objective of this study was to compare protein efficiency, Fe and vitamin A availability of newly formulated extruded sorghum-, cowpea-, soya- and corn-based FBF, along with a current, non-extruded United States Agency for International Development (USAID) corn and soya blend FBF (CSB+). A second objective was to compare protein efficiency of whey protein concentrate (WPC) and soya protein isolate (SPI) containing FBF to determine whether WPC inclusion improved outcomes. Eight groups of growing rats (n 10) consumed two white and one red sorghum–cowpea (WSC1 + WPC, WSC2 + WPC, RSC + WPC), white sorghum–soya (WSS + WPC) and corn–soya (CSB14 + WPC) extruded WPC-containing FBF, an extruded white sorghum–cowpea with SPI (WSC1 + SPI), non-extruded CSB+, and American Institute of Nutrition (AIN)-93G, a weanling rat diet, for 4 weeks. There were no significant differences in protein efficiency, Fe or vitamin A outcomes between WPC FBF groups. The CSB+ group consumed significantly less food, gained significantly less weight, and had significantly lower energy efficiency, protein efficiency and length, compared with all other groups. Compared with WSC1 + WPC, the WSC1 + SPI FBF group had significantly lower energy efficiency, protein efficiency and weight gain. These results suggest that a variety of commodities can be used in the formulation of FBF, and that newly formulated extruded FBF are of better nutritional quality than non-extruded CSB+.

The production of protein from animal sources is often criticized because of the low efficiency of converting plant protein from feeds into protein in the animal products. However, this critique does not consider the fact that large portions of the plant-based proteins fed to animals may be human-inedible and that the quality of animal proteins is usually superior as compared with plant proteins. The aim of the present study was therefore to assess changes in protein quality in the course of the transformation of potentially human-edible plant proteins into animal products via livestock production; data from 30 Austrian dairy farms were used as a case study. A second aim was to develop an approach for combining these changes with quantitative aspects (e.g. with the human-edible feed conversion efficiency (heFCE), defined as kilogram protein in the animal product divided by kilogram potentially human-edible protein in the feeds). Protein quality of potentially human-edible inputs and outputs was assessed using the protein digestibility-corrected amino acid score and the digestible indispensable amino acid score, two methods proposed by the Food and Agriculture Organization of the United Nations to describe the nutritional value of proteins for humans. Depending on the method used, protein scores were between 1.40 and 1.87 times higher for the animal products than for the potentially human-edible plant protein input on a barn-gate level (=protein quality ratio (PQR)). Combining the PQR of 1.87 with the heFCE for the same farms resulted in heFCE×PQR of 2.15. Thus, considering both quantity and quality, the value of the proteins in the animal products for human consumption (in this case in milk and beef) is 2.15 times higher than that of proteins in the potentially human-edible plant protein inputs. The results of this study emphasize the necessity of including protein quality changes resulting from the transformation of plant proteins to animal proteins when evaluating the net contribution of livestock to the human food supply. Furthermore, these differences in protein quality might also need to be considered when choosing a functional unit for the assessment of environmental impacts of the production of different proteins.

The role of dietary protein for the development of feline calcium oxalate (CaOx) uroliths has not been conclusively clarified. The present study evaluated the effects of a varying dietary protein concentration and quality on critical indices for the formation of CaOx uroliths. Three diets with a high protein quality (10–11 % greaves meal/diet) and a varying crude protein (CP) concentration (35, 44 and 57 % in DM) were compared. Additionally, the 57 % CP diet was compared with a fourth diet that had a similar CP concentration (55 % in DM), but a lower protein quality (34 % greaves meal/diet). The Ca and oxalate (Ox) concentrations were similar in all diets. A group of eight cats received the same diet at the same time. Each feeding period was divided into a 21 d adaptation period and a 7 d sampling period to collect urine. There were increases in urinary volume, urinary Ca concentrations, renal Ca and Ox excretion and urinary relative supersaturation (RSS) with CaOx with increasing dietary protein concentrations. Urinary pH ranged between 6·34 and 6·66 among all groups, with no unidirectional effect of dietary protein. Lower renal Ca excretion was observed when feeding the diet with the lower protein quality, however, the underlying mechanism needs further evaluation. In conclusion, although the observed higher urinary volume is beneficial, the increase in urinary Ca concentrations, renal Ca and Ox excretion and urinary RSS CaOx associated with a high-protein diet may be critical for the development of CaOx uroliths in cats.

PDCAAS is a widely used assay for evaluating protein quality. It is a chemical score, which is derived from the ratio between the first limiting amino acid in a test protein and the corresponding amino acid in a reference amino acid pattern and corrected for true faecal N digestibility. Chemical scores exceeding 100 % are truncated to 100 %. The advantages of the PDCAAS are its simplicity and direct relationship to human protein requirements. The limitations are as follows: the reference pattern is based on the minimum amino acid requirements for tissue growth and maintenance and does not necessarily reflect the optimum intake. Truncated PDCAAS of high-quality proteins do not give any information about the power of these proteins to compensate, as a supplement, for low levels of dietary essential amino acids in low-quality proteins. It is likely that faecal N digestibility does not take into account the loss from the colon of indispensable amino acids that were not absorbed in the ileum. Anti-nutritional factors, such as lectins and trypsin inhibitors, in several plant protein sources can cause heightened endogenous losses of amino acids, an issue which is particularly relevant in animal feedstuffs. The assumption that amino acid supplementation can completely restore biological efficiency of the protein source is incorrect since the kinetics of digestion and absorption between supplemented free amino acids and amino acids present in dietary proteins, are different.

The WHO (2007) Technical Report on protein and amino acid requirements in human nutrition states that the best estimate for a population average requirement is 105 mg nitrogen/kg body weight per day, or 0·66 g protein/kg body weight per day. In many developing countries protein intake falls significantly short of these values. Apart from protein quantity, protein quality including bioavailability and digestibility, from different food sources, are currently on the global agenda. The 1st International Symposium on Dietary Protein for Human Health held in Auckland, in March 2011, and the consecutive Food and Agricultural Organization of the United Nations (FAO) Expert Consultation on Dietary Protein Quality, both highlighted the importance of assessing the quality of protein from different food sources through determination of amino acid content. Throughout the developed world, animal products and cereals are the two most important sources of protein; in developing countries this order is reversed. In low income countries only 3 % of total dietary energy, as an indicator of diet composition, is derived from meat and offal, 11 % from roots and tubers and 6 % from pulses, nuts and oilseeds. The remainder of the dietary energy is mainly derived from cereal-based staple food. Although the production of livestock has increased in developing countries, the consumption of protein in these countries with people consuming the most limited amounts of protein are continually decreasing. Undernutrition, including insufficient consumption of protein, remains a persistent problem in the developing world, and although many diets within these developing countries are deficient in the quantity of protein compared to recommendations, the quality of the protein also strongly comes into focus.

Cereals and legumes are staple foods in India and are limiting in lysine and sulphur amino acids, respectively. Available lysine loss, due to Maillard-type reactions that may occur during food preparation, exacerbates the problem of lysine deficiency particularly in cereals. Consequently, determining the contents of digestible essential amino acids, particularly lysine, is important. True ileal digestibilities of most amino acids (including total and reactive lysine) were determined for ten food ingredients and eleven foods commonly consumed in India. Semi-synthetic diets each containing either an ingredient or the prepared food as the sole protein source were formulated to contain 100 g kg− 1 protein (75 g kg− 1 for rice-based diets) and fed to growing rats. Titanium dioxide was included as an indigestible marker. Digesta were collected and the amino acid content (including reactive lysine) of diets and ileal digesta determined. Available (digestible reactive) lysine content ranged from 1·9–15·4 g kg− 1 and 1·8–12·7 g kg− 1 across the ingredients and prepared foods respectively. True ileal amino acid digestibility varied widely both across ingredients and prepared foods for each amino acid (on average 60–92 %) and across amino acids within each ingredient and prepared food (overall digestibility 31–96 %). Amino acid digestibility was low for many of the ingredients and prepared foods and consequently digestibility must be considered when assessing the protein quality of poorer quality foods. Given commonly encountered daily energy intakes for members of the Indian population, it is estimated that lysine is limiting for adults in many Indian diets.

Dietary protein and amino acid requirement recommendations for normal “healthy” children and adults have varied considerably with 2007 FAO/WHO protein requirement estimates for children lower, but dietary essential AA requirements for adults more than doubled. Requirement estimates as presented do not account for common living conditions, which are prevalent in developing countries such as energy deficit, infection burden and added functional demands for protein and AAs. This study examined the effect of adjusting total dietary protein for quality and digestibility (PDCAAS) and of correcting current protein and AA requirements for the effect of infection and a mild energy deficit to estimate utilizable protein (total protein corrected for biological value and digestibility) and the risk/prevalence of protein inadequacy. The relationship between utilizable protein/prevalence of protein inadequacy and stunting across regions and countries was examined. Data sources (n = 116 countries) included FAO FBS (food supply), UNICEF (stunting prevalence), UNDP (GDP) and UNSTATS (IMR) and USDA nutrient tables. Statistical analyses included Pearson correlations, paired-sample/non-parametric t-tests and linear regression. Statistically significant differences were observed in risk/prevalence estimates of protein inadequacy using total protein and the current protein requirements versus utilizable protein and the adjusted protein requirements for all regions (p < 0·05). Total protein, utilizable protein, GDP per capita and total energy were each highly correlated with the prevalence of stunting. Energy, protein and utilizable protein availability were independently and negatively associated with stunting (p < 0·001), explaining 41 %, 34 % and 40 % of variation respectively. Controlling for energy, total protein was not a statistically significant factor but utilizable protein remained significant explaining~45 % of the variance (p = 0·017). Dietary utilizable protein provides a better index of population impact of risk/prevalence of protein inadequacy than crude protein intake. We conclude that the increased demand for protein due to infections and mild to moderate energy deficits, should be appropriately considered in assessing needs of populations where those conditions still prevail.

In 1989 the Joint FAO/WHO Expert Consultation on Protein Quality Evaluation recommended the use of the Protein Digestibility Corrected Amino Acid Score (PDCAAS) method for evaluating protein quality. In calculating PDCAAS, the limiting amino acid score (i.e., ratio of first limiting amino acid in a gram of target food to that in a reference protein or requirement) is multiplied by protein digestibility. The PDCAAS method has now been in use for 20 years. Research emerging during this time has provided useful data on various aspects of protein quality evaluation that has made a review of the current methods used in assessing protein quality necessary. This paper provides an overview of the use of the PDCAAS method as compared to other methods and addresses some of the key challenges that remain in regards to protein quality evaluation. Furthermore, specific factors influencing protein quality including the effects of processing conditions and preparation methods are presented. Protein quality evaluation methods and recommended protein intakes currently used in different countries vis-à-vis the WHO/FAO/UNU standards are further provided. As foods are frequently consumed in complement with other foods, the significance of the PDCAAS of single protein sources may not be evident, thus, protein quality of some key food groups and challenges surrounding the calculation of the amino acid score for dietary protein mixtures are further discussed. As results from new research emerge, recommendations may need to be updated or revised to maintain relevance of methods used in calculating protein quality.

Food regulation aims to protect public health through a safe and nutritious food supply produced by a compliant food industry. Food standards of developed countries generally do not regulate protein content or protein quality because the risk of dietary protein inadequacy in their national populations is very low. Protein is nevertheless regulated for reasons of product quality or protein labelling or to minimise assessed health risks associated with consumption of certain animal- and vegetable-protein foods; analogue products that extend or simulate commonly available animal-protein foods; and special purpose foods such as infant formula and foods, supplementary and medical foods, and foods for weight loss. The extent and approach to protein regulation varies greatly among jurisdictions but where it occurs, it is applied through minimum and sometimes maximum limits on protein content or quality measures or both using an inter-related approach. Protein quality measures range from amino acid profiles and digestibility corrected scores to protein rating, a rat bioassay and reference proteins not further described. Regulatory methods for protein quality determination are referenced to the published scientific literature or developed nationally. Internationally, the Codex Alimentarius regulates the protein content and quality of some foods. The Codex approach varies according to the food but is similar to the approaches used in national and regional food regulation. This paper provides a comparison of the regulation of protein in foods using examples from the food regulations of Australia New Zealand, Canada, the European Union, the United States of America and the Codex Alimentarius.

Revised estimates of protein and amino acid requirements are under discussion by the Food and Agriculture Organization (FAO)/World Health Organizaion (WHO), and have been proposed in a recent report on Dietary Reference Intakes (DRIs) from the USA. The nature and magnitude of these requirements are not entirely resolved, and no consideration has been given to the potential influence of metabolic adaptation on dietary requirements. We have examined the implications of these new values, and of the conceptual metabolic framework in which they are used, for defining the nutritional adequacy of protein intakes in developed and developing countries. We have expressed proposed values for protein requirements in relation to energy requirements, predicted for physical activity levels of 1.5, 1.75 and 2.0 times basal metabolic rate, in order to generate reference ratios for protein energy/total energy (reference P/E ratio) as a function of age, body weight, gender and physical activity level. Proposed values for amino acid requirements have been used to adjust the available digestible P/E ratio of foods and diets for protein quality. Focusing on the diets of UK omnivores and vegetarians and on diets in India, the risk of protein deficiency is evaluated from a comparison of P/E ratios of metabolic requirements with protein-quality-adjusted P/E ratios of intakes. A qualitative and conservative estimate of risk of deficiency is made by comparing the adjusted P/E ratio of the intake with a reference P/E ratio calculated for age, body weight, gender and physical activity according to FAO/WHO/United Nations University. A semi-quantitative estimate of risk of deficiency has also been made by the cut point approach, calculated as the proportion of the intake distribution below the mean P/E ratio of the requirement. Values for the quality-adjusted P/E ratio of the diet range from 0.126 for the UK omnivore diet to 0.054 for a rice-based diet of adults in West Bengal, which is lysine-limited, falling to 0.050 for 1-year-old children. The reference P/E ratio for men and women increases with age, is higher for females than males, is higher for small compared with large adults at any age and decreases with physical activity. Thus if a particular diet is potentially limiting in protein, protein deficiency is most likely in large, elderly sedentary women followed by the adolescent female and least likely in moderately active young children, the opposite of what has usually been assumed. Within the currently accepted framework, the diets do not meet the protein needs of the entire population of the UK, have a significant risk of deficiency throughout India for all except extremely active small adults, and are grossly inadequate for all population groups, apart from physically active young children in West Bengal, regardless of body weight or level of food intake. The lysine limitation of the cereal-based Indian diets is dependent on the choice of lysine requirement values from the published range. We consider that the value selected is too high, because of uncertainties and inconsistencies in the approaches used. A more appropriate choice from the lower end of the range would remove the lysine limitation of cereal-based diets, and reduce some of the perceived risk of deficiency. However, diets remain limited by the amount of digestible protein for many population groups, especially in West Bengal. In the context of risk management, one option would be to accept the current values and the conceptual metabolic framework within which they have been derived. This would have major implications for the supplies of high-quality protein to the developing countries. An alternative option would be to re-evaluate the currently proposed values for the requirements for protein and amino acids. We conclude that the choice of values for the adult lysine requirement should be re-evaluated and that serious consideration should be given to the extent to which adaptive mechanisms might enable the metabolic requirement for protein to be met from current intakes. This will entail a better understanding of the relationships between dietary protein and health.

To determine the optimum indispensable (I) amino acid (AA) balance in Atlantic salmon (Salmo salar L.) fry, a single protocol established for the pig was adapted. The balance was calculated from the reduction in N gain after replacing about 45% of a single IAA by a mixture of dispensable AA in isonitrogenous diets. We confirmed that the mixture of AA simulating the AA pattern of cod-meal protein and gelatine (46:3, w/w) was used with the same efficiency as cod-meal protein and gelatine. From the deletion experiment an optimum balance between the IAA was derived. Expressed relative to lysine=100, the optimal balance was: arginine 76 (se 0·2), histidine 28 (se 2·2), methionine+cystine 64 (se 1·7), phenylalanine + tyrosine 105 (se 1·6), threonine 51 (se 2·4), tryptophan 14 (se 0·7), valine 59 (se 1·7). No estimates were made for isoleucine and leucine. Expressed as g/16g N, the optimal balance was: arginine 4·0 (se 0·0), histidine 1·5 (se 0·1), lysine 5·3 (se 0·2), methionine+cystine 3·4 (se 0·1), phenylaline+tyrosine 5·6 (se 0·1), threonine 2·7 (se 0·1), tryptophan 0·7 (se 0·0), valine 3·1 (se 0·1). This AA composition is close to that of the Atlantic salmon whole-body, but using it as an estimation of the IAA requirements may lead to an overestimation of the branched-chain AA requirements and an underestimation of aromatic and S-containing AA requirements. The results are discussed in accordance with the key assumptions associated with the model used (broken-line model, IAA efficiencies and maintenance requirements).

The nutritional quality of milk proteins, evaluated both in terms of digestibility and postprandial oxidation and retention in human subjects, was investigated in this study. Five healthy adult volunteers were given 480 ml 15N-labelled milk (i.e. 190 mmol N). 15N was subsequently determined at the ileal level, using a naso-intestinal intubation technique, as well as at the faecal level. Plasma and urine were sampled for 8 h after meal ingestion. Dietary exogenous N recovered at the terminal ileum after 8 h reached 8·6 (se 0·8) mmol while the amount collected in the faeces was 6·5 (se 0·7) mmol after 5 d. The true ileal and faecal digestibilities were 95·5 (se 0·4)% and 96·6 (se 0·4)% respectively. The appearance of [15N]amino acids in the plasma was rapid and prolonged. The measurement of 15N in the body urea pool and in the N excreted in the urine allowed us to calculate the deamination occurring after [15N]milk protein absorption. The net postprandial protein utilization (i.e. NPPU = (Nabsorbed - Ndeaminated)/Ningested), calculated as an index of protein quality 8 h after milk ingestion, was 81·0 (se 1·9)%. Our data confirm that milk protein has a high oro-ileal digestibility in man and demonstrate that milk protein has a high NPPU, an index corresponding to a period in which the dietary protein retention is maximal.

An experiment was carried out to study the effect of changes in either the quality or the quantity of dietary protein intake on the free essential amino acid profiles in plasma, muscle or liver of growing chickens. Following a randomized paired-feeding design based on metabolic body weight (kg M0·75), White Rock male broilers were allocated to one of three isoenergetic (14·5 kJ metabolizable energy per g dry matter (DM)) semisynthetic diets containing different levels of protein (60, 120, 180 or 240 g/kg DM). All diets were based on soya-bean meal, as the sole source of protein, either unsupplemented (diets S) or supplemented with 20 g/kg L-lysine (diets SL) or 2 g/kg DL-methionine (diets SM). Samples of blood, biceps muscle and liver were taken and amino acid analysis was determined by high performance liquid chromatography. Plasma was only adequate to detect the effect of the supplementation with methionine to balance the dietary amino acid (AA) profile. Plasma concentrations of free methionine significantly increased as a result of the supplementation with methionine (P < 0·001) and remained unaffected by the amount of protein ingested. When lysine was added to diets S to induce an AA imbalance, a significant increase in muscle concentrations of free valine, isoleucine and phenylalanine (P <0·05) and a significant decrease in liver concentrations of free arginine and phenylalanine were observed (P<0·05). The supplementation with DL-methionine gave rise to a significant fall in the concentrations of free histidine, glycine and threonine (P < 0·05) and increased those of isoleucine and methionine in the skeletal muscle (P < 0·05), while in liver it significantly lowered the concentrations of free arginine, valine, leucine, phenylalanine and methionine (P < 0·001 to P < 0·05) and raised that of free glycine (P < 0·001). The ratios lysine: phenylalanine, lysine: valine and methionine: glycine in muscle together with that of methionine: glycine in liver seem to be appropriate indexes of the adequacy of the dietary AA profile to requirements.

An experiment was carried out to study the effect of changes in either the quality or the quantity of dietary protein intake on the excretion of nitrogen (N) compounds in the chicken. Thirty-two White Rock male broilers (1 day old) were raised in batteries and fed a commercial starter diet for 9 days. Then they were randomly divided into eight groups each of four birds, of similar body weight (mean live weight: 178 (s.e. 1·9) g), and individually housed in metabolism cages. Following a paired-feeding design based on metabolic body weight (kg M0·75), each group of birds was given, for an experimental period of20 days, each of four levels of protein (60, 120, 180 or 240 g/kg: 5 days each) in two groups of isoenergetic (14·5 kj metabolizable energy per g dry matter) and semisynthetic diets based on soya-bean meal, known to be first limiting in methionine, either unsupplemented (diets S) or supplemented with 2 g/kg DL-methionine (diets SM). Excreta were collected every 3 days for 48 h, frozen and stored at −20°C. The excreta samples were subjected to chemical analysis for uric acid by a rapid high-performance liquid chromatographic method, for urea and ammonia by a colorimetric method, and for total N by the Kjeldahl procedure. In general, the excretion of major N compounds was markedly affected by either the quality or the content of dietary protein. Overall, the excretion of total N, uric acid-N, ammonia-N and urea-N significantly (P < 0·05) decreased with improvement in dietary protein quality and significantly (P < 0·05) increased with increase in protein intake. Regression equations were obtained relating the excretion of uric acid, urea, ammonia and total N on protein supply. For the partition ofN compounds output, the ratios of uric acid-N and ammonia-N to total N significantly (P < 0·05) decreased on improving dietary protein quality and increased or remained unchanged, respectively, with the increase in dietary protein content. The use of the ratio of ammonia-N to total N is recommended as a rapid, easy and accurate indicator of dietary protein adequacy, as an alternative to measures based on total N balance, without the need for separation of urine and faeces.