BACKGROUND: Infant formulas (IFs), the only adequate substitute to human milk, are complex matrices that require numerous ingredients and processing steps that may impact protein digestion and subsequent amino acid (AA) absorption.
OBJECTIVE: The objective was to understand the impact of the protein ingredient quality within IFs on postprandial plasma AA profiles.
METHODS: Four isonitrogenous and isocaloric IFs were produced at a semi-industrial scale using whey proteins from different origins (cheese compared with ideal whey) and denaturation levels (IF-A, -B, -C), and caseins with different supramolecular organizations (IF-C, -D). Ten Yucatan minipiglets (12- to 27-d-old) were used as a human infant model and received each IF for 3 d according to a Williams Latin square followed by a 2-d wash-out period. Jugular plasma was regularly sampled from 10 min preprandial to 4 h postprandial on the third day to measure free AAs, urea, insulin, and glucose concentrations. Data were statistically analyzed using a mixed linear model with diet (IFs), time, and sex as fixed factors and piglet as random factor.
RESULTS: IFs made with cheese whey (IF-A and -B) elicited significantly higher plasma total and essential AA concentrations than IFs made with ideal whey (IF-C and -D), regardless of the pre- and postprandial times. Most of the differences observed postprandially were explained by AA homeostasis modifications. IFs based on cheese whey induced an increased plasma concentration of Thr due to both a higher Thr content in these IFs and a Thr-limiting degrading capability in piglets. The use of a nonmicellar casein ingredient led to reduced plasma content of AA catabolism markers (IF-D compared with IF-C).
CONCLUSIONS: Overall, our results highlight the importance of the protein ingredient quality (composition and structure) within IFs on neonatal plasma AA profiles, which may further impact infant protein metabolism.

Although the mechanisms underpinning short-term muscle disuse atrophy and associated insulin resistance remain to be elucidated, perturbed lipid metabolism might be involved. Our aim was to determine the impact of acipimox administration [i.e., pharmacologically lowering circulating nonesterified fatty acid (NEFA) availability] on muscle amino acid metabolism and insulin sensitivity during short-term disuse. Eighteen healthy individuals (age: 22 ± 1 years; body mass index: 24.0 ± 0.6 kg·m-2) underwent 2 days forearm immobilization with placebo (PLA; n = 9) or acipimox (ACI; 250 mg Olbetam; n = 9) ingestion four times daily. Before and after immobilization, whole body glucose disposal rate (GDR), forearm glucose uptake (FGU; i.e., muscle insulin sensitivity), and amino acid kinetics were measured under fasting and hyperinsulinemic-hyperaminoacidemic-euglycemic clamp conditions using forearm balance and l-[ring-2H5]-phenylalanine infusions. Immobilization did not affect GDR but decreased insulin-stimulated FGU in both groups, more so in ACI (from 53 ± 8 to 12 ± 5 µmol·min-1) than PLA (from 52 ± 8 to 38 ± 13 µmol·min-1; P < 0.05). In ACI only, and in contrast to our hypothesis, fasting arterialized NEFA concentrations were elevated to 1.3 ± 0.1 mmol·L-1 postimmobilization (P < 0.05), and fasting forearm NEFA balance increased approximately fourfold (P = 0.10). Forearm phenylalanine net balance decreased following immobilization (P < 0.10), driven by an increased rate of appearance [from 32 ± 5 (fasting) and 21 ± 4 (clamp) preimmobilization to 53 ± 8 and 31 ± 4 postimmobilization; P < 0.05] while the rate of disappearance was unaffected by disuse or acipimox. Disuse-induced insulin resistance is accompanied by early signs of negative net muscle amino acid balance, which is driven by accelerated muscle amino acid efflux. Acutely elevated NEFA availability worsened muscle insulin resistance without affecting amino acid kinetics, suggesting increased muscle NEFA uptake may contribute to inactivity-induced insulin resistance but does not cause anabolic resistance.NEW & NOTEWORTHY We demonstrate that 2 days of forearm cast immobilization in healthy young volunteers leads to the rapid development of insulin resistance, which is accompanied by accelerated muscle amino acid efflux in the absence of impaired muscle amino acid uptake. Acutely elevated fasting nonesterified fatty acid (NEFA) availability as a result of acipimox supplementation worsened muscle insulin resistance without affecting amino acid kinetics, suggesting increased muscle NEFA uptake may contribute to inactivity-induced insulin resistance but does not cause anabolic resistance.

The mechanisms underpinning short-term muscle disuse atrophy remain to be elucidated, but perturbations in lipid metabolism may be involved. Specifically, positive muscle non-esterified fatty acid (NEFA) balance has been implicated in the development of disuse-induced insulin and anabolic resistance. Our aim was to determine the impact of acipimox administration (i.e. pharmacologically lowering circulating NEFA availability) on muscle amino acid metabolism and insulin sensitivity during short-term disuse. Eighteen healthy individuals (age 22±1 years, BMI 24.0±0.6 kg·m-2) underwent 2 days of forearm cast immobilization with placebo (PLA; n=9, 5M/4F) or acipimox (ACI; 250 mg Olbetam; n=9, 4M/5F) ingestion four times daily. Before and after immobilization, whole-body glucose disposal rate (GDR), forearm glucose uptake (FGU, i.e. muscle insulin sensitivity), and amino acid kinetics were measured under fasting and hyperinsulinaemic-hyperaminoacidaemic-euglycaemic clamp conditions using arteriovenous forearm balance and intravenous L-[ring-2H5]phenylalanine infusions. Immobilization did not affect GDR but decreased insulin-stimulated FGU in both groups, but to a greater degree in ACI (from 53±8 to 12±5 μmol·min-1) than in PLA (from 52±8 to 38±13 μmol·min-1; P<0.05). In ACI only, fasting arterialised NEFA concentrations were elevated to 1.3±0.1 mmol·L-1 post-immobilization (P<0.05), and fasting forearm NEFA balance increased ~4-fold (P=0.10). Forearm phenylalanine net balance tended to decrease following immobilization (P<0.10), driven by increases in phenylalanine rates of appearance (from 32±5 (fasting) and 21±4 (clamp) pre-immobilization to 53±8 and 31±4 post-immobilization; P<0.05) while rates of disappearance were unaffected and no effects of acipimox observed. Altogether, we show disuse-induced insulin resistance is accompanied by early signs of negative net muscle amino acid balance, which is driven by accelerated muscle amino acid efflux. Acutely elevated NEFA availability worsened muscle insulin resistance without affecting muscle amino acid kinetics, suggesting that disuse-associated increased muscle NEFA uptake may contribute to inactivity-induced insulin resistance but does not represent an early mechanism causing anabolic resistance.

The objective was to quantify the effects of age and ractopamine (RAC) on whole body oxygen consumption and Leu flux, and oxygen flux and metabolism of nitrogenous compounds by the portal-drained viscera (PDV), liver, and hindquarters (HQ) of steers. Multicatheterized steers were fed a high energy diet every 2 h in 12 equal portions. Five younger steers (body weight, [BW] = 223 ± 10.1 kg) were 6 mo old and five older steers (BW = 464 ± 16.3 kg) were 14 mo old. Treatments were control (Cont) or 80 mg RAC per kg diet in a crossover design. Nitrogen (N) balance was measured on day 9 to 13. Whole body oxygen consumption and net flux were measured on day 11 and day 13, and net flux of N variables, Phe and Leu kinetics were measured on day 13. Whole body oxygen consumption increased (P < 0.05) in response to RAC in older but not younger steers. Retained N was greater (P = 0.009) for younger than older steers and increased (P = 0.010) with RAC in both ages of steers. Nitrogen retained as a percentage of N apparently absorbed increased (P < 0.05) in the older steers but not the younger steers in response to RAC. Oxygen uptake was greater (P < 0.05) in PDV, liver, and total splanchnic tissues in the younger steers and there was no response to RAC. In contrast, oxygen uptake in HQ increased (P < 0.05) with RAC in the older but not the younger steers. Concentration and net PDV release of α-amino N (AAN) were not affected by age or RAC. Uptake of AAN by liver decreased with RAC (P = 0.001). Splanchnic release of AAN was greater in younger steers (P = 0.020) and increased (P = 0.024) in response to RAC. For HQ tissues, uptake (P = 0.005) and extraction (P = 0.005) of AAN were lesser in older than younger steers and both increased (P = 0.001) in response to RAC. Based on Phe kinetics in HQ, RAC increased (P < 0.05) protein synthesis in older steers but not in younger steers. In contrast, protein breakdown decreased (P < 0.05) in response to RAC in younger steers. In response to RAC, protein degradation was less (P < 0.05) in younger than older steers. Based on Leu kinetics, whole body protein synthesis was greater in the younger steers (P = 0.022) but not altered in response to RAC. Ractopamine enhanced lean tissue growth by increasing supply of AAN to peripheral tissues and altering protein metabolism in HQ. These metabolic responses are consistent with established responses to RAC in production situations.
Ractopamine (RAC) is a feed additive that stimulates rate of gain, feed efficiency, and carcass leanness in finishing cattle. The objective of the present study was to quantify the effects of age and RAC on whole body metabolism and nutrient use by tissues of the portal-drained viscera, liver, and hindquarters of steers. Whole body oxygen consumption and oxygen uptake in hindquarters increased in response to RAC in older but not younger steers. Retained nitrogen was greater for younger than older steers and increased with RAC in both ages of steers. Uptake of α-amino nitrogen by liver decreased with RAC. For hindquarters tissues, uptake of α-amino nitrogen was lesser in older than younger steers and increased in both ages in response to RAC. Ractopamine increased protein synthesis in hindquarters of older steers but not in younger steers. In contrast, in response to RAC, protein breakdown decreased in younger steers and was less in younger than older steers. Ractopamine enhanced lean tissue growth by altering protein metabolism in hindquarters and increasing supply of α-amino nitrogen to peripheral tissues. The response to RAC was greater in older steers.

The physical and functional outcomes of lifelong treatment with a phenylalanine restricted diet for the management of Phenylketonuria (PKU) remain unknown. Given that the mainstay of dietary management consists of modifying the sources of ingested protein, various aspects of body protein status could be compromised.
To examine the existing evidence regarding the protein status of people with PKU and identify nutritional and lifestyle variables that influence protein status.
Studies reporting anthropometric, biochemical and/or functional measurements of body protein status in people with PKU were eligible.
MEDLINE (Ovid), Embase (Ovid), CENTRAL, Web of Science and Scopus, and conference abstracts.
Seventy studies were included in the review. The majority of studies assessing protein status based on anthropometric measurements observed no differences between people with PKU and controls, although deficits in muscle mass were reported within PKU cohorts. Findings for biochemical assessment of protein status were mixed and limited studies assessed protein status using functional measures. Factors such as participant age, sex, metabolic control, protein source, type of protein substitute, and pharmacological treatments were found to modulate protein status of people with PKU.
Findings were inconclusive regarding body protein status in people with PKU. The relationship between diet and protein status outcomes remains unclear and further research is warranted to determine the impact of dietary regimens on physical and functional outcomes, and to understand the best clinical assessments to reliably monitor the protein status in people with PKU.

Ingesting protein-containing supplements and foods provides essential amino acids (EAA) necessary to increase muscle and whole-body protein synthesis (WBPS). Large variations exist in the EAA composition of supplements and foods, ranging from free-form amino acids to whole protein foods. We sought to investigate how changes in peripheral EAA after ingesting various protein and free amino acid formats altered muscle and whole-body protein synthesis. Data were compiled from four previous studies that used primed, constant infusions of L-(ring-2H5)-phenylalanine and L-(3,3-2H2)-tyrosine to determine fractional synthetic rate of muscle protein (FSR), WBPS, and circulating EAA concentrations. Stepwise regression indicated that max EAA concentration (EAACmax; R2 = 0.524, p < 0.001), EAACmax (R2 = 0.341, p < 0.001), and change in EAA concentration (ΔEAA; R = 0.345, p < 0.001) were the strongest predictors for postprandial FSR, Δ (change from post absorptive to postprandial) FSR, and ΔWBPS, respectively. Within our dataset, the stepwise regression equation indicated that a 100% increase in peripheral EAA concentrations increases FSR by ~34%. Further, we observed significant (p < 0.05) positive (R = 0.420-0.724) correlations between the plasma EAA area under the curve above baseline, EAACmax, ΔEAA, and rate to EAACmax to postprandial FSR, ΔFSR, and ΔWBPS. Taken together our results indicate that across a large variety of EAA/protein-containing formats and food, large increases in peripheral EAA concentrations are required to drive a robust increase in muscle and whole-body protein synthesis.

Sex differences in plasma concentration of arginine and arginase activity of different tissues have been reported in mice. In addition, male but not female C57BL/6 mice have a dietary arginine requirement for growth.
The goal of this research was to test the hypothesis that arginase II is a key factor in the sexual dimorphism of arginine metabolism.
Young adult male and female wild type (WT), and heterozygous and arginase II knockout mice on a C57BL/6 background mice were infused with labeled citrulline, arginine, ornithine, phenylalanine, and tyrosine to determine the rates of appearance and interconversion of these amino acids. Tissue arginase activity was measured in the liver, heart, jejunum, kidney, pancreas, and spleen with an arginine radioisotope. The effect of genotype, sex, and their interaction was tested.
Female mice produced ∼36% more citrulline than their male littermates, which translated into a greater arginine endogenous synthesis, flux, and plasma concentration (42, 6, and 27%, respectively; P < 0.001). Female mice also had a greater phenylalanine flux (10%) indicating a greater rate of whole protein breakdown; however, they had a lower protein synthesis rate than males (18%; P < 0.001). The ablation of arginase II reduced the production of citrulline and the de novo synthesis of arginine in females and increased the rate of appearance of arginine and plasma arginine concentration in male mice (16 and 22%, respectively; P < 0.001). No effect of arginase II deletion, however, was observed for whole-body protein kinetics. Arginase II activity was present in the pancreas, kidney, jejunum, and spleen; WT females had a ∼2-fold greater renal arginase activity than their WT counterparts.
A clear sexual dimorphism exists in the endogenous synthesis of arginine and its disposal. Female mice have a greater arginine availability than their male littermates. The ablation of arginase II increases arginine availability in male mice.

Burns remain the fifth cause of non-fatal pediatric injuries globally, with muscle cachexia being a hallmark of the stress response to burns. Burn-induced muscle wasting is associated with morbidity, yet the determinants of muscle protein catabolism in response to burn trauma remains unclear. Our objective was to determine the effect of patient and injury characteristics on muscle protein kinetics in burn patients.
This retrospective, observational study was performed using protein kinetic data from pediatric patients who had severe burns (>30% of the total body surface area burned) and underwent cross-limb stable isotope infusions between 1999 and 2008 as part of prospective clinical trials. Mixed multiple regression models were used to assess associations between patient/injury characteristics and muscle protein fractional synthesis rate (FSR), net balance (NB), and rates of phenylalanine appearance (Ra; index of protein breakdown) and disappearance (Rd; index of protein synthesis) across the leg.
A total of 268 patients who underwent 499 studies were analyzed. Increasing time post injury was associated with greater FSR (p < 0.001) and NB (p = 0.01). Males were more catabolic than females (as indicated by lower NB, p = 0.04 and greater Ra, p = 0.008), a consequence of higher protein breakdown rather than lower synthesis. Increasing burn size was associated with higher protein synthesis rate (as indicated by higher FSR, p = 0.019) and higher protein breakdown rates (as indicated by greater Ra, p = 0.001). FSR was negatively associated with age (p < 0.001).
Data from this large patient cohort show that injury severity, sex, and time post injury influence skeletal muscle wasting in burned children. These findings suggest that individual patient characteristics should be considered when devising therapies to improve the acute care and rehabilitation of burn survivors.