Amino acid oxidation by young pigs receiving diets with varying levels of sulphur amino acids

Post-prandial tracer studies of protein and amino acid utilisation: what can they tell us about human amino acid and protein requirements?

Nitrogen balance (NB), the principal methodology used to derive recommendations for human protein and amino acid requirements, has been widely criticised, and calls for increased protein and amino acid requirement recommendations have been made, often on the basis of post-prandial amino acid tracer kinetic studies of muscle protein synthesis, or of amino acid oxidation. This narrative review considers our knowledge of the homeostatic regulation of the FFM throughout the diurnal cycle of feeding and fasting and what can and has been learnt from post-prandial amino acid tracer studies, about amino acid and protein requirements. Within the FFM, muscle mass in well fed weight-stable adults with healthy lifestyles appears fixed at a phenotypic level within a wide range of habitual protein intakes. However homoeostatic regulation occurs in response to variation in habitual protein intake, with adaptive changes in amino acid oxidation which influence the magnitude of diurnal losses and gains of body protein. Post-prandial indicator amino acid oxidation (IAAO) studies have been introduced as an alternative to NB and to the logistically complex 24 h [13C-1] amino acid balance studies, for assessment of protein and amino acid requirements. However, a detailed examination of IAAO studies shows both a lack of concern for homeostatic regulation of amino acid oxidation and  major flaws in their design and analytical interpretation, which seriously constrain their ability to provide reliable values. New ideas and a much more critical approach to existing work is needed if real progress is to be made in the area.

Is It Time to Reconsider the U.S. Recommendations for Dietary Protein and Amino Acid Intake?

Since the U.S. Institute of Medicine's recommendations on protein and amino acid intake in 2005, new information supports the need to re-evaluate these recommendations. New lines of evidence include: (1) re-analysis/re-interpretation of nitrogen balance data; (2) results from indicator amino acid oxidation studies; (3) studies of positive functional outcomes associated with protein intakes higher than recommended; (4) dietary guidance and protein recommendations from some professional nutrition societies; and (5) recognition that the synthesis of certain dispensable amino acids may be insufficient to meet physiological requirements more often than previously understood. The empirical estimates, theoretical calculations and clinical functional outcomes converge on a similar theme, that recommendations for intake of protein and some amino acids may be too low in several populations, including for older adults (≥65 years), pregnant and lactating women, and healthy children older than 3 years. Additional influential factors that should be considered are protein quality that meets operational sufficiency (adequate intake to support healthy functional outcomes), interactions between protein and energy intake, and functional roles of amino acids which could impact the pool of available amino acids for use in protein synthesis. Going forward, the definition of "adequacy" as it pertains to protein and amino acid intake recommendations must take into consideration these critical factors.

Methionine and cysteine oxidation are regulated in a dose dependent manner by dietary Cys intake in neonatal piglets receiving enteral nutrition

Methionine (Met) is an indispensable amino acid (AA) in piglets. Met can synthesize cysteine (Cys), and Cys has the ability to reduce the Met requirement by 40% in piglets. However, whether this sparing effect on Met is facilitated by downregulation of Cys synthesis has not been shown. This study investigated the effects of graded levels of Cys on Met and Cys oxidation, and on plasma AA concentrations. Piglets (n = 32) received a complete elemental diet via gastric catheters prior to being randomly assigned to one of the eight dietary Cys levels (0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.40, 0.50 g kg-1d-1) with an adequate Met concentration (0.25g kg-1d-1). Constant infusion of L-[1-14C]-Met and L-[1-14C]-Cys were performed for 6 h on d 6 and d 8 to determine Met and Cys oxidation, respectively. Met oxidation decreased as Cys intake increased (P<0.05). At higher Cys intakes (0.15 to 0.5g kg-1d-1), Met oxidation decreased (P<0.05) at a slower rate. Cys oxidation was similar (P>0.05) among dietary Cys intakes; however, a significant polynomial relationship was observed between Cys oxidation and intake (P<0.05, R2 = 0.12). Plasma Met concentrations increased (P<0.05) linearly with increasing levels of dietary Cys, while plasma Cys concentrations changed (P<0.05) in a cubic manner and the highest concentrations occurred at the highest intake levels. Increasing dietary levels of Cys resulted in a reduction in Met oxidation until the requirement for the total sulfur AA was met, indicating the sparing capacity by Cys of Met occurs through inhibition of the transsulfuration pathway in neonatal piglets.

Dietary Supplementation of dl-Methionine Potently Induces Sodium-Dependent l-Methionine Absorption in Porcine Jejunum Ex Vivo

BACKGROUND

Methionine is an essential amino acid (AA) with many fundamental roles. Humans often supplement l-Met, whereas dl-Met and dl-2-hydroxy-4-(methylthio)butanoic acid (dl-HMTBA) are more frequently used to supplement livestock.

OBJECTIVES

The study aimed to investigate whether dietary Met source alters the absorptive capacity for Met isomers in the small intestine of piglets.

METHODS

A total of 27 male 10-wk-old piglets in 3 feeding groups received a diet supplemented with 0.21% dl-Met, 0.21% l-Met, or 0.31% dl-HMTBA to meet the Met + cystine requirement. After ≥10 d, absorptive fluxes of d-Met or l-Met were measured at a physiological concentration of 50 μM and a high concentration of 5 mM in duodenum, middle jejunum, and ileum ex vivo. Data were compared by 2-factor ANOVA.

RESULTS

Across diets, fluxes of both Met isomers at both tested concentrations increased from duodenum to ileum by a factor of ∼2-5.5 (P < 0.05). Pigs supplemented with dl-Met had greater (P < 0.085) absorptive fluxes at 50 μM l-Met (0.50, 2.07, and 3.86 nmol · cm-2 · h-1) and d-Met (0.62, 1.41, and 1.19 nmol · cm-2 · h-1) than did pigs supplemented with dl-HMTBA (l-Met: 0.28, 0.76, and 1.08 nmol · cm-2 · h-1; d-Met: 0.34, 0.58, and 0.64 nmol · cm-2 · h-1) in duodenum, jejunum, and ileum, respectively. Only in jejunum of dl-Met-fed pigs, fluxes at 50 μM l-Met were reduced by the omission of luminal Na+ (from 3.27 to 0.86 nmol · cm-2 · h-1; P < 0.05) and by a cocktail of 22 luminal AAs (to 1.05 nmol · cm-2 · h-1; P < 0.05).

CONCLUSIONS

Dietary supplementation of dl-Met increases the efficiency of l-Met and d-Met absorption at physiologically relevant luminal Met concentrations along the small intestine of pigs, including a very prominent induction of an Na+-dependent transport system with preference for l-Met in the mid-jejunum. Dietary supplementation with dl-Met could be a promising tool to improve the absorption of Met and other AAs.

Short-term determination and long-term evaluation of the dietary methionine requirement in adult dogs

Methionine, an essential sulphur-containing amino acid (SAA), plays an integral role in many metabolic processes. Evidence for the methionine requirements of adult dogs is limited, and we employed the indicator amino acid oxidation (IAAO) method to estimate dietary methionine requirements in Labrador retrievers (n 21). Using semi-purified diets, the mean requirement was 0·55 (95 % CI 0·41, 0·71) g/4184 kJ. In a subsequent parallel design study, three groups of adult Labrador retrievers (n 52) were fed semi-purified diets with 0·55 g/4184 kJ (test diet 1), 0·71 g/4184 kJ (test diet 2) or 1·37 g/4184 kJ (control diet) of methionine for 32 weeks to assess the long-term consequences of feeding. The total SAA content (2·68 g/4184 kJ) was maintained through dietary supplementation of cystine. Plasma methionine did not decrease in test group and increased significantly on test diet 1 in weeks 8 and 16 compared with control. Reducing dietary methionine did not have a significant effect on whole blood, plasma or urinary taurine or plasma N-terminal pro B-type natriuretic peptide. Significant effects in both test diets were observed for cholesterol, betaine and dimethylglycine. In conclusion, feeding methionine at the IAAO-estimated mean was sufficient to maintain plasma methionine over 32 weeks when total SAA was maintained. However, choline oxidation may have increased to support plasma methionine and have additional consequences for lipid metabolism. While the IAAO can be employed to assess essential amino acid requirements, such as methionine in the dog using semi-purified diets, further work is required to establish safe levels for commercial diet formats.

Bioavailability of D-methionine relative to L-methionine for nursery pigs using the slope-ratio assay

This experiment was conducted to determine the bioavailability of D-methionine (Met) relative to L-Met for nursery pigs using the slope-ratio assay. A total of 50 crossbred barrows with an initial BW of 13.5 kg (SD = 1.0) were used in an N balance study. A Met-deficient basal diet (BD) was formulated to contain an adequate amount of all amino acids (AA) for 10–20 kg pigs except for Met. The two reference diets were prepared by supplementing the BD with 0.4 or 0.8 g L-Met/kg at the expense of corn starch, and an equivalent concentration of D-Met was added to the BD for the two test diets. The pigs were adapted to the experimental diets for 5 d and then total but separated collection of feces and urine was conducted for 4 d according to the marker-to-marker procedure. Nitrogen intakes were similar across the treatments. Fecal N output was not affected by Met supplementation regardless of source and consequently apparent N digestibility did not change. Conversely, there was a negative linear response (P < 0.01) to Met supplementation with both Met isomers in urinary N output, which resulted in increased retained N (g/4 d) and N retention (% of intake). No quadratic response was observed in any of the N balance criteria. The estimated bioavailability of D-Met relative to L-Met from urinary N output (g/4 d) and N retention (% of intake) as dependent variables using supplemental Met intake (g/4 d) as an independent variable were 87.6% and 89.6%, respectively; however, approximately 95% of the fiducial limits for the relative bioavailability estimates included 100%. In conclusion, with an absence of statistical significance, the present study indicated that the mean relative bioequivalence of D- to L-Met was 87.6% based on urinary N output or 89.6% based on N retention.

Effect of L- or DL-methionine Supplementation on Nitrogen Retention, Serum Amino Acid Concentrations and Blood Metabolites Profile in Starter Pigs

The objective of the current study was to evaluate the effect of supplementation of either L-methionine (L-Met) or DL-methionine (DL-Met) to diets of starter pigs on nitrogen (N) balance, metabolism, and serum amino acid profile. Eighteen crossbred (Duroc×Landrace×Yorkshire) barrows weighing 15.45±0.88 kg were randomly allotted to 1 of 3 diets with 6 pigs per treatment. The diets included a basal diet (Met-deficient diet) containing 0.24% standardized ileal digestibility Met with all other essential nutrients meeting the pig’s requirements. The other two diets were produced by supplementing the basal diet with 0.12% DL-Met or L-Met. The experiment lasted for 18 days, consisting of a 13-day adaptation period to the diets followed by a 5-day experimental period. Pigs were fed ad libitum and free access to water throughout the experiment. Results showed that the supplementation of either L-Met or DL-Met improved N retention, and serum methionine concentration, and decreased N excretion compared with basal diet (p<0.01). The N retention of pigs fed diets supplemented with the same inclusion levels of DL-Met or L-Met were not different (p>0.05). In conclusion, on equimolar basis DL-Met and L-Met are equally bioavailable as Met sources for starter pigs.

Effect of feed grade L-methionine on growth performance and gut health in nursery pigs compared with conventional DL-methionine

Two experiments were conducted to test if supplementation of LMET has beneficial effects on growth performance and gut health in nursery pigs compared with DL-Met. In Exp. 1, 168 pigs in 56 pens were randomly allotted to 7 dietary treatments for 20 d, including a basal diet (BD; 55% of the NRC requirement for Met), the BD+0.048% L-Met or DL-Met (70% of the NRC requirement), the BD+0.096% L-Met or DL-Met (85% of the NRC requirement), and the BD+0.144% L-Met or DL-Met (100% of the NRC requirement). Body weight and feed disappearance were recorded every 5 d for computation of growth performance. In Exp. 2, 20 individually housed nursery pigs were randomly allotted to 2 dietary treatments for 20 d: DML (0.16% Met from the BD+0.145% supplemental DL-Met) or LMET (0.16% Met from the BD+0.145% supplemental L-Met). Both diets had Met meeting 95% of the NRC requirement. Duodenum samples from all pigs were collected at the end of the trial to evaluate morphology and redox status. In Exp. 1, during the entire 20 d, pigs fed diets supplemented with L-Met tended to have greater (P=0.087) ADG and reduced (P<0.01) plasma urea nitrogen (PUN) than pigs fed diets supplemented with DL-Met. The relative bioavailability (RBA) of L-Met to DL-Met for ADG and G:F was 143.8 and 122.7%, respectively. In Exp. 2, pigs fed a diet supplemented with L-Met had duodenum tissue with greater (P<0.05) concentrations of glutathione (GSH) and greater villus height and width as well as lower (P<0.05) concentrations of protein carbonyl compared with pigs fed DL-Met. Overall, compared with DL-Met, the use of L-Met as a source of supplemental Met in nursery pig diets enhanced duodenum villus development in association with reduced oxidative stress and improved GSH. The beneficial effects of supplementing L-Met compared to DL-Met in gut of nursery pigs resulted in a potential enhancement of ADG and reduction of PUN.

Indicator amino acid oxidation: concept and application

The indicator amino acid oxidation (IAAO) method is based on the concept that when 1 indispensable amino acid (IDAA) is deficient for protein synthesis, then all other IDAA, including the indicator amino acid, will be oxidized. With increasing intakes of the limiting amino acid, IAAO will decrease, reflecting increasing incorporation into protein. Once the requirement for the limiting amino acid is met, there will be no further change in the indicator oxidation. Originally, the IAAO method was designed to determine amino acid requirements in growing pigs. The minimally invasive IAAO method developed in humans has been systematically applied to determine IDAA requirements in adults. Due to its noninvasive nature, the IAAO method has also been used to determine requirements for amino acids in neonates and children, and in disease. The IAAO model has recently been applied to determine the metabolic availability (MA) of amino acids from dietary proteins and to determine total protein requirements. The IAAO method is robust, rapid, and reliable; it has been used to determine amino acid requirements in different species, across the life cycle, and in diseased populations. The recent application of IAAO to determine MA of amino acids and protein requirements is also very novel.

An estimate of the methionine requirement and its variability in growing pigs using the indicator amino acid oxidation technique

Although AA requirements for the mean of a population of growing pigs have been established using traditional methods, there are no estimates of the variability within the population and whether this variation differs among AA. With the increased use of supplemental Lys in pig diets, there will be an increased need to supplement Met, commonly the second or third limiting AA in corn-soybean diets. The indicator AA oxidation method allows repeated measurements in a short period of time so that the AA requirement can be determined for individual pigs at a similar physiological stage. The objective of this study was to determine the mean Met requirement in individual gilts and to estimate the related variability. Six individually housed female pigs (initial BW = 8.8 kg, SD 1.5) each received diets providing 6 levels of dl-Met. The isonitrogenous and isoenergetic diets contained 0.187, 0.250, 0.290, 0.320, 0.350, and 0.377% Met (analyzed, as-fed basis). Cysteine (0.48%) and Lys (1.44%) concentrations were similar for all diets. Pigs were adapted for 6 d to the basal corn-soybean meal diet (0.187% Met), which was offered at 95 g/kg(0.75) of BW to ensure complete consumption of the test diets. During 4-h oxidation studies, 313.4 kBq, (SD 35.6) of L-[1-(14)C]Phe was mixed with each of 8 half-hourly meals, and expired CO(2) was collected. The breakpoint in Phe oxidation, representing the Met requirement, and its variability, was determined using 2-phase linear regression. Phenylalanine oxidation decreased as the Met content increased from 0.187 to 0.29%. Phenylalanine oxidation was not different (P > 0.2) for diets ranging from 0.320 to 0.377% Met. The dietary Met requirement varied from 0.320 to 0.373% for individual pigs. The mean Met requirement for individual pigs was determined to be 0.340% of diet (SD = 0.024%, CV= 7.1%), with 0.340, 0.364, and 0.388% covering the requirement of 50, 66, and 95% of the population, respectively. The present mean population estimate was similar to the recommended dietary Met concentration of 0.325% for pigs of this BW and feed intake. To maximize profitability, Met levels in starter pig diets should be determined, depending on the cost of crystalline Met and the fraction of the population whose requirement is to be met.

Dietary Cysteine Reduces the Methionine Requirement by an Equal Proportion in Both Parenterally and Enterally Fed Piglets

The sulfur amino acids (SAA), methionine and cysteine, are normally supplied in a 50:50 ratio in the oral diet of pigs. In contrast, cysteine is not included in any appreciable amounts in parenteral solutions due to its instability in solution. Cysteine can replace part of the methionine requirement, but is not required when methionine is supplied at a level that meets the entire SAA requirement. However, the role of the gut on cysteine sparing has not been investigated. In the present study, the enteral and parenteral methionine requirement was determined, with excess dietary cysteine, by using the indicator amino acid oxidation (IAAO) technique. Piglets [n = 28, 2 d, 1.65 +/- 0.014 kg (SE)] were fed elemental diets containing adequate energy, phenylalanine and excess tyrosine, with varied methionine concentrations and excess cysteine [0.55 g/(kg. d)]. Diets were infused continuously via intravenous (parenteral) or gastric (enteral) catheters. Phenylalanine oxidation was determined during a primed, constant infusion of L-[1-(14)C]-phenylalanine, by measuring expired (14)CO(2) and plasma specific radioactivity (SRA) of phenylalanine. For both the parenteral and enteral groups, phenylalanine oxidation (% of dose) decreased linearly (P < 0.01) as methionine intake increased and then became low and unchanging. Using breakpoint analysis, the methionine requirement was estimated to be 0.25 and 0.18 g/(kg. d) for enteral and parenteral feeding, respectively. These data show that the parenteral methionine requirement is approximately 70% of the enteral requirement when measured in the presence of excess dietary cysteine (P < 0.05). A comparison with our previous studies in which methionine was the only source of sulfur amino acids shows that the addition of dietary cysteine reduces the methionine requirement by approximately 40% in both enterally and parenterally fed neonatal piglets. Therefore, dietary cysteine is equally effective in sparing dietary methionine whether fed enterally or parenterally.

Lysine requirements of pre-lay broiler breeder pullets: determination by indicator amino acid oxidation

The indicator amino acid oxidation (IAAO) method allows the determination of amino acid requirements under conditions of low growth rate as found in pre-laying broiler breeder pullets. Cobb 500 breeder pullets (20 wk old; 2290 +/- 280 g, n = 4) were adapted (6 d) to a pelleted, purified control diet containing all nutrients at >or=110% of NRC recommendations. After recovery from surgery for implantation of a jugular catheter, each bird was fed, in random order, test diets containing one of nine levels of lysine (0.48, 0.96, 1.92, 2.88, 3.84, 4.80, 7.68, 9.60 and 14.40 g/kg of diet). Indicator oxidation was determined during 4-h primed (74 kBq/kg body), constant infusions (44 kBq x h(-1). kg body(-1)) of L-[1-(14)C]phenylalanine. Using the breakpoint of a one-slope broken-line model, the lysine requirement was determined to be 4.88 +/- 0.96 g/kg of diet or 366 +/- 72 mg x hen(-1) x d(-1) with an upper 95% CI of 6.40 g/kg of diet or 480 mg x hen(-1) x d(-1). IAAO allows determination of individual bird amino acid requirements for specific ages and types of birds over short periods of time and enables more accurate broiler breeder pullet diet formulation.

The methionine requirement is lower in neonatal piglets fed parenterally than in those fed enterally

The requirements for the sulfur amino acids (SAA), methionine (Met) and cysteine (Cys), have seldom been determined in neonates and to our knowledge have not previously been determined directly in parenterally fed neonates. The sulfur amino acids are catabolized largely in the liver and kidney, and their metabolism by the gut has been studied less frequently. In the present research, the enteral and parenteral Met requirement was determined, without dietary Cys, using the indicator amino acid oxidation (IAAO) technique. Piglets [n = 32, 2 d old, 1.66 +/- 0.13 kg (SD)] received elemental diets containing adequate energy, phenylalanine (Phe) and excess tyrosine, with varied Met concentrations and no Cys. Diets were infused continuously via intravenous or intragastric catheters. Phenylalanine oxidation was determined during a primed, constant infusion of L-[1-(14)C]-Phe, by measuring expired (14)CO(2) and plasma specific radioactivity of Phe. For both parenteral and enteral groups, Phe oxidation (% of dose) decreased linearly (P < 0.01) as Met intake increased, then became low and unchanging. Using breakpoint analysis, the Met requirement was estimated to be 0.42 and 0.29 g/(kg. d) for enteral and parenteral feeding, respectively. Breakpoint analysis using absolute phenylalanine oxidation [ micro mol/(kg. h)] resulted in an estimation of the Met requirement of 0.44 and 0.26 g/(kg. d) for enteral and parenteral feeding, respectively. These data show that the parenteral Met requirement is approximately 69% of the enteral requirement and suggest that extraction of SAA by first-pass splanchnic metabolism may be responsible for this difference.

Determination of the methionine requirement of male and female broiler chicks using an indirect amino acid oxidation method

The methionine requirement of 250-to-300-g broiler chicks was estimated from the oxidation of L-[1-14C] phenylalanine of chicks given meals containing graded levels of DL-methionine. L-[1-14C] phenylalanine was used as an indicator amino acid for amino acid oxidation and, indirectly, protein synthesis. Four experiments were conducted using an incomplete block design with three replications each. Chicks were crop intubated with semifluid diets at a ratio of 1 g of diet per 45 g of bird weight. Two feedings 2 h apart were used to reduce variability, and the sample collection period was 3 h after the second feeding. Regression analysis of 14CO2 release from L-[1-14C] phenylalanine was used to estimate the methionine requirement. The model was as follows: response = max + rc x (req - x) x I, where max = plateau, rc = rate constant, req = requirement, and I = 1 when x < req, otherwise I = 0. The methionine requirements of Ross x Ross chicks were 0.57 +/- 0.03% and 0.52 +/- 0.08% for male and female chicks, respectively in Experiment 1 and 0.55 +/- 0.05% and 0.52 +/- 0.04%, respectively, in Experiment 2. In the third experiment (Arbor Acre High-Yield), phenylalanine oxidation stabilized at a low rate when dietary methionine levels reached 0.54 +/- 0.03% and 0.53 +/- 0.04% for males and females, respectively. In a growth trial covering a longer period (Experiment 4), the methionine requirements of male and female Ross x Ross chicks, based on feed conversion, were 0.52 +/- 0.05% and 0.45 +/- 0.02%, respectively, and based on body gain were 0.54 +/0.09% and 0.48 +/- 0.04%, respectively. The results suggested that the methionine requirement of male chicks tended to be higher than that of females in both strains. However, differences were small and not significant.

Development of procedures for determining the amino acid requirements of chickens by the indicator amino acid oxidation method

To better understand the amino acid requirements of chickens, a method is needed to determine a point estimate of the requirement. A method developed to determine the amino acid requirements of piglets and human infants by using the oxidation rate of an indicator amino acid as the test parameter was adapted for use in growing chickens. Polycarbonate chambers measuring 30 x 40 x 30 (L x W x H) were constructed to house one small mature chicken or several small immature chickens and to trap exhaled CO2. In the first experiment, 10-d-old chicks (250 to 300 g each) were feed deprived for 12 h and given 1 g of a methionine-deficient diet (gavage) with L-[1-C14]phenylalanine. Peak oxidation of L-[1-C14]phenylalanine occurred between 30 and 90 min (-200 dpm/g). A linear rate of oxidation (slope = -1.84; r2 = 0.96) was achieved by 120 min after feeding until after 180 min, when oxidation stabilized at a low rate. The second experiment tested different chambers and chicks with the same treatment to show repeatability. There were four chicks (250 to 300 g each) in each of three apparently identical chambers. Chicks were given two meals labeled with L-[1-C14]phenylalanine, 2 h apart. The CV for the 0- and 30-min collections were 17 and 10%, respectively. The CV for the remaining collections remained at <4%. These data supported a system of two feedings, 2 h apart to reduce variability and a collection period that included up to 3 h after the second feeding.

Effect of fasting and of methionine deficiency on L-methionine, DL-methionine and DL-2-hydroxy-4-methylthiobutanoic acid metabolism in broiler chicks

Metabolism of L-[1-14C]methionine, DL-[1-14C]methionine and DL-[1-14C]2-hydroxy-4-methylthiobutanoic acid (DL-HMB) by broiler chicks which had been fasted overnight or given a methionine-deficient diet was compared with fed (control) birds. The excretion of 14C-labelled material, total 14CO2 exhaled, 14C incorporation into tissue proteins and the 14C-labelled material in perchloric-acid-soluble tissue fractions were measured 6 h after injection of the 14C-labelled materials. The incorporation of 14C into tissue proteins and the relative rates of conversion of D-methionine and DL-HMB to L-methionine in tissues under different nutritional regimens were compared using protein-bound 14C:protein-free 14C values. Fasted birds exhaled more 14CO2 than control birds but excreted less 14C, while methionine-deficient birds behaved very similarly to the control animals in these respects. Fasted birds incorporated much less 14C into proteins of tissues other than liver and kidney from all three labelled tracers. The values for protein-bound 14C:protein-free 14C were lower in all tissues. Methionine-deficient birds had similar levels of 14C in tissue proteins but lower values for protein bound 14C:protein-free 14C. Examination of the values for protein-bound 14C:protein-free 14C suggest that brain and probably liver tissues from fasted and methionine-deficient birds showed improved rates of conversion of D-methionine and DL-HMB to L-methionine compared with control animals.

Influence of dietary protein concentration on the oxidation of phenylalanine by the young pig

1. Piglets were weaned at 3 d of age and reared to 2.5 kg on a liquid diet in which the protein was supplied by dried skim milk and a mixture of free amino acids. The oxidation of L-[1-14C]phenylalanine was measured as an indication of the partition of amino acids between retention and catabolism in pigs (2.5 kg) offered meals containing varied concentrations of crude protein (nitrogen x 6.25). 2. The dietary protein concentration was varied either by increasing the inclusion of a mixture of free amino acids in a series of diets containing 100 g protein/kg from skim milk, or by increasing the level of inclusion of the skim milk in a series of diets containing the equivalent of 100 g protein/kg from the free amino acid mixture. 3. The oxidation of phenylalanine was minimized by dietary protein concentrations of 240 and 258 g/kg for the diets containing increasing concentrations of free amino acids or skim milk respectively. 4. These results show that a mixture of free amino acids is used more effectively than intact protein for promoting retention of essential amino acids. 5. The recovery of radioactivity in expired carbon dioxide was inversely related to the recovery of radioactivity in liver tissue when the concentration of dietary crude protein was increased from deficient to adequate, demonstrating that the fractional oxidation of the indicator amino acid was inversely related to protein synthesis.

Proline as an essential amino acid for the young pig

1. The catabolism of L-[1-14C]phenylalanine was used to indicate the effects of single amino acid supplementation of an inadequate protein diet (200 g crude protein (nitrogen x 6.25)/kg) on the utilization of dietary amino acids in pigs of 2.5 kg body-weight reared on an adequate protein diet (240 g crude protein/kg) containing skim milk and a mixture of free amino acids. 2. The oxidation of phenylalanine was decreased by the addition of proline or arginine to the inadequate protein diet but not by the addition of threonine, methionine, lysine or a mixture of essential amino acids, indicating that proline and arginine were limiting the utilization of dietary amino acids in the inadequate protein diet. 3. Dietary proline concentrations of 13.9 and 14.2 g/kg minimized phenylalanine oxidation in diets containing 200 or 260 g protein/kg. This indicates a dietary proline requirement of 14 g/kg. 4. Increasing the dietary arginine concentration in a diet containing 240 g protein/kg showed that an arginine concentration of 5.1 g/kg minimized phenylalanine oxidation. However, increasing the arginine concentration in a diet containing 200 g protein/kg increased phenylalanine oxidation, suggesting an amino acid imbalance involving arginine at this lower level of protein.

Ontogenetical aspects of nutritional requirements in fish

Changes in morphology i.e. "metamorphosis", mode of respiration, allometric growth of organs, mode of swimming and efficiency of biochemical pathways are briefly reviewed. It is suggested that these processes form the basis for progressive changes in nutrient requirements involving formation and development of several organs, systems and function. Digestive tract morphology changes during ontogenesis and aspects of fish metabolism, protein synthesis rate and body growth rate are interconnected and an attempt is made to explain these processes so as to understand the specificity of larval and juvenile fish nutrient requirements as compared to subadults. Protein and amino acid requirements given the body mass perspective and the generalization of the protein maintenance requirement in protein requirements for maximum growth was estimated to amount to 5-20%. Several cases of amino acid deficiency symptoms showed strong dependence on fish weight (age), but even most numerous studies on salmonids are lacking complete research throughout the life history of one species in defined nutritional and environmental conditions. Larval and juvenile fish have reduced capacity of catabolic adaptability and this fact links them to strictly carnivorous mammals. An attempt is made, for the first time, to relate amino acid needs of fish to young and/or carnivorous mammals. Vitamin requirements of fish are reviewed, taking into account the relationship between body size and time of the first appearance of deficiency symptoms. These are virtually no studies on vitamin requirements in larval warm-water fishes and very few on first feeding salmonids. The same applies to the vitamin need in reproductory fish. Fatty acid deficiencies manifest themselves faster in juvenile fish, but larval fish might require separate classes of lipids, phosphatidylcholine, in the diet to develop and grow at all. It seems that the studies on nutrient requirements have so far not used an ontogenetical perspective, but evidence given throughout this work argues that it would be worthwhile.

Oxidation of an indicator amino acid by young pigs receiving diets with varying levels of lysine or threonine, and an assessment of amino acid requirements

The catabolism of [14C]phenylalanine was used to indicate the effects of varying the dietary level of lysine and threonine on the retention of dietary amino acids by 2-week-old pigs receiving diets containing skim milk and a mixture of free amino acids. Reducing the dietary level of lysine from 16 to 12 g/kg had no influence on phenylalanine oxidation, reducing the lysine level from 12 to 11 then to 10 g/kg caused an almost linear increase in phenylalanine oxidation whereas further reduction to 9 or 8 g/kg resulted in a less-marked increase in phenylalanine oxidation. This showed that 12 g lysine/kg was required to maximize amino acid retention and indicated that lysine was conserved more effectively at low dietary concentrations than at dietary concentrations approaching the requirement. Reducing the dietary level of threonine from 8 to 6 g/kg had no influence on phenylalanine oxidation, whereas further reduction to 4 g/kg caused a linear increase in phenylalanine catabolism showing that 6 g threonine/kg was required to maximize amino acid retention. Reduction of the levels of lysine, threonine and methionine from the generous levels characteristic of a diet containing 240 g protein from skim milk/kg, to the requirement levels determined separately in the presence of the generous levels of all the other amino acids, resulted in a twofold increase in phenylalanine catabolism. This shows that the pig seems able to conserve limiting intakes of a single amino acid, but not if the intakes of two or three amino acids are limiting.