Leucine kinetics from [2H3]- and [13C]leucine infused simultaneously by gut and vein

Alterations in glutamine metabolism and its conversion to citrulline in sepsis

In enterocytes, glutamine serves as the major source of energy; another metabolic fate of glutamine is conversion to citrulline. Because sepsis can affect gut function and integrity, alterations in glutamine metabolism may exist and lead to decreased citrulline production. This study aimed to investigate how sepsis affects glutamine metabolism, including its conversion to citrulline, by measuring glutamine and citrulline flux, fractional splanchnic extraction of glutamine and leucine, and the contribution of glutamine nitrogen to citrulline in septic patients and healthy controls. Eight patients with severe sepsis and 10 healthy controls were given primed, constant intravenous infusion of [(2)H2]citrulline and sequential administration of intravenous and enteral [α-(15)N]glutamine and [(13)C]leucine in the postabsorptive state. The results showed that, compared with healthy controls, septic patients had a significantly lower whole body citrulline flux and plasma concentration, higher endogenous leucine flux, and higher glutamine clearance. Fractional splanchnic extraction of leucine was higher in septic patients than in controls, but fractional extraction of glutamine was not different. The majority of the (15)N label transferred from glutamine to citrulline was found at the α-position. These results demonstrate that lower glutamine plasma concentrations in sepsis were a result of increased glutamine clearance. Despite adequate splanchnic uptake of glutamine, there is decreased production of citrulline, suggesting a defect in the metabolic conversion of glutamine to citrulline, decreased uptake of glutamine by the enterocyte but increased uptake by the liver, and/or shunting of glutamine to other metabolic pathways.

Dietary proteins as determinants of metabolic and physiologic functions of the gastrointestinal tract

Dietary proteins elicit a wide range of nutritional and biological functions. Beyond their nutritional role as the source of amino acids for protein synthesis, they are instrumental in the regulation of food intake, glucose and lipid metabolism, blood pressure, bone metabolism and immune function. The interaction of dietary proteins and their products of digestion with the regulatory functions of the gastrointestinal (GI) tract plays a dominant role in determining the physiological properties of proteins. The site of interaction is widespread, from the oral cavity to the colon. The characteristics of proteins that influence their interaction with the GI tract in a source-dependent manner include their physico-chemical properties, their amino acid composition and sequence, their bioactive peptides, their digestion kinetics and also the non-protein bioactive components conjugated with them. Within the GI tract, these products affect several regulatory functions by interacting with receptors releasing hormones, affecting stomach emptying and GI transport and absorption, transmitting neural signals to the brain, and modifying the microflora. This review discusses the interaction of dietary proteins during digestion and absorption with the physiological and metabolic functions of the GI tract, and illustrates the importance of this interaction in the regulation of amino acid, glucose, lipid metabolism, and food intake.

Monosodium glutamate raises antral distension and plasma amino acid after a standard meal in humans

The consumption of monosodium glutamate (MSG) is advocated to elicit physiological and metabolic effects, yet these effects have been poorly investigated directly in humans and in particular in the postprandial phase. Thirteen healthy adults were supplemented for 6 days with a nutritional dose of MSG (2 g) or sodium chloride (NaCl) as control, following a crossover design. On the 7th day, they underwent a complete postprandial examination for the 6 h following the ingestion of the same liquid standard meal (700 kcal, 20% of energy as [(15)N]protein, 50% as carbohydrate, and 30% as fat) supplemented with MSG or NaCl. Real-ultrasound measures of antral area indicated a significant increased distension for the 2 h following the meal supplemented with MSG vs. NaCl. This early postprandial phase was also associated with significantly increased levels of circulating leucine, isoleucine, valine, lysine, cysteine, alanine, tyrosine, and tryptophan after MSG compared with NaCl. No changes to the postprandial glucose, insulin, glucagon-like peptide (GLP)-1, and ghrelin were noted between MSG- and NaCl-supplemented meals. Subjective assessments of hunger and fullness were neither affected by MSG supplementation. Finally, the postprandial fate of dietary N was identical between dietary conditions. Our findings indicate that nutritional dose of MSG promoted greater postprandial elevations of several indispensable amino acids in plasma and induced gastric distension. Further work to elucidate the possible sparing effect of MSG on indispensable amino acid first-pass uptake in humans is warranted. This trial was registered at clinicaltrials.gov as NCT00862017.

Contraction intensity and feeding affect collagen and myofibrillar protein synthesis rates differently in human skeletal muscle

Exercise stimulates muscle protein fractional synthesis rate (FSR), but the importance of contractile intensity and whether it interplays with feeding is not understood. This was investigated following two distinct resistance exercise (RE) contraction intensities using an intrasubject design in the fasted (n = 10) and fed (n = 10) states. RE consisted of 10 sets of knee extensions. One leg worked against light load (LL) at 16% of one-repetition maximum (1RM), the other leg against heavy load (HL) at 70% 1RM, with intensities equalized for total lifted load. Males were infused with [(13)C]leucine, and vastus lateralis biopsies were obtained bilaterally at rest as well as 0.5, 3, and 5.5 h after RE. Western blots were run on muscle lysates and phosphospecific antibodies used to detect phosphorylation status of targets involved in regulation of FSR. The intramuscular collagen FSR was evenly increased following LL- and HL-RE and was not affected by feeding. Myofibrillar FSR was unaffected by LL-RE, whereas HL-RE resulted in a delayed improvement (0.14 +/- 0.02%/h, P < 0.05). Myofibrillar FSR was increased at rest by feeding (P < 0.05) and remained elevated late in the postexercise period compared with the fasting condition. The Rp-s6k-4E-binding protein-1 (BP1) and the mitogen-activated protein kinase (MAPk) pathways were activated by the HL intensity and were suggested to be responsible for regulating myofibrillar FSR in response to adequate contractile activity. Feeding predominantly affected Rp-s6k and eukaryotic elongation factor 2 phosphorylations in correspondence with the observed changes in myofibrillar FSR, whereas 4E-BP1 remained to respond only to the HL contraction intensity. Thus the study design allows us to conclude that the MAPk- and mammalian target of rapamycin-dependent signaling responds to contractile activity, whereas elongation mainly was found to respond to feeding. Furthermore, although functionally linked, the contractile and the supportive matrix structures upregulate their protein synthesis rate quite differently in response to feeding and contractile activity and intensity.

Protein intake during hemodialysis maintains a positive whole body protein balance in chronic hemodialysis patients

Protein energy malnutrition is present in 18 to 56% of hemodialysis patients. Because hemodialysis has been regarded as a catabolic event, we studied whether consumption of a protein- and energy-enriched meal improves the whole body protein balance during dialysis in chronic hemodialysis (CHD) patients. Patients were studied on a single day between dialysis (HD- protocol) in the morning while fasting and in the afternoon while consuming six small test meals. Patients were also studied during two separate dialysis sessions (HD+ protocol). Patients were fasted during one and consumed the meals during the other. Whole body protein metabolism was studied by primed constant infusion of l-[1-(13)C]valine. During HD-, feeding changed the negative whole body protein balance observed during fasting to a positive protein balance. Dialysis deepened the negative balance during fasting, whereas feeding during dialysis induced a positive balance comparable to the HD- protocol while feeding. Plasma valine concentrations during the studies were correlated with whole body protein synthesis and inversely correlated with whole body protein breakdown. We conclude that the consumption of a protein- and energy-enriched meal by CHD patients while dialyzing can strongly improve whole body protein balance, probably because of the increased amino acid concentrations in blood.

Human apolipoprotein A-IV metabolism within triglyceride-rich lipoproteins and plasma

In order to investigate the metabolism of apo A-IV within TRL and plasma, we assessed TRL and plasma apo A-IV kinetics in 19 and 4 subjects, respectively, consuming an average US diet for a 6-week period. At the end of this diet study, each subject received a primed-constant infusion of deuterated leucine over a 15 h time period with hourly feeding, and blood samples were drawn at 10 time points. TRL was separated by ultracentrifugation. Apo A-IV was isolated by immunoprecipitation and/or SDS-PAGE. Apo A-IV concentrations were determined by immunoelectrophoresis. Stable isotope tracer/tracee ratios were measured by gas chromatography/mass spectrometry, and the data were analyzed by multicompartmental modeling. The mean concentrations of plasma and TRL apo A-IV during the isotope infusion period were 21.0+/-3.2 and 0.66+/-0.25 mg/dl, respectively, and these values were 11.5 and 30.5% higher than those of fasting samples. The mean TRL and plasma apo A-IV residence times (RT) were 1.97+/-0.57 and 2.71+/-0.65 days, and transport rates (TR) were 0.17+/-0.19 and 3.90+/-1.24 mg/kg per day, respectively. There were significant correlations between TRL apo A-IV concentrations and TR (r(2)=0.79, P<0.001), and between TRL apo A-IV pool size and TRL cholesterol levels (r(2)=0.29, P=0.02). Our data indicated that; (1) TRL apo A-IV has a RT of 1.97 days which is similar to that earlier reported for HDL apo A-IV; (2) Apo A-IV recirculates between TRL and other slowly turning over pools; (3) the primary determinant of TRL apo A-IV levels is its TR; and (4) there is no correlation between TRL apo A-IV and apo B48 fractional catabolism in TRL.

Arginine and ornithine kinetics in severely burned patients: increased rate of arginine disposal

Arginine serves multiple roles in the pathophysiological response to burn injury. Our previous studies in burn patients demonstrated a limited net rate of arginine de novo synthesis despite a significantly increased arginine turnover (flux), suggesting that this amino acid is a conditionally indispensable amino acid after major burns. This study used [15N2-guanidino-5,5-2H2]arginine and [5-13C]ornithine as tracers to assess the rate of arginine disposal via its conversion to and subsequent oxidation of ornithine; [5,5-2H2]proline and [5,5,5-2H3]leucine were also used to assess proline and protein kinetics. Nine severely burned patients were studied during a protein-free fast ("basal" or fast) and total parenteral nutrition (TPN) feedings. Compared with values from healthy volunteers, burn injury significantly increased 1) fluxes of arginine, ornithine, leucine, and proline; 2) arginine-to-ornithine conversion; 3) ornithine oxidation; and 4) arginine oxidation. TPN increased arginine-to-ornithine conversion and proportionally increased irreversible arginine oxidation. The elevated arginine oxidation, with limited net de novo synthesis from its immediate precursors, further implies that arginine is a conditionally indispensable amino acid in severely burned patients receiving TPN.

The digestion rate of protein is an independent regulating factor of postprandial protein retention

To evaluate the importance of protein digestion rate on protein deposition, we characterized leucine kinetics after ingestion of "protein" meals of identical amino acid composition and nitrogen contents but of different digestion rates. Four groups of five or six young men received an L-[1-13C]leucine infusion and one of the following 30-g protein meals: a single meal of slowly digested casein (CAS), a single meal of free amino acid mimicking casein composition (AA), a single meal of rapidly digested whey proteins (WP), or repeated meals of whey proteins (RPT-WP) mimicking slow digestion rate. Comparisons were made between "fast" (AA, WP) and "slow" (CAS, RPT-WP) meals of identical amino acid composition (AA vs. CAS, and WP vs. RPT-WP). The fast meals induced a strong, rapid, and transient increase of aminoacidemia, leucine flux, and oxidation. After slow meals, these parameters increased moderately but durably. Postprandial leucine balance over 7 h was higher after the slow than after the fast meals (CAS: 38 +/- 13 vs. AA: -12 +/- 11, P < 0.01; RPT-WP: 87 +/- 25 vs. WP: 6 +/- 19 micromol/kg, P < 0.05). Protein digestion rate is an independent factor modulating postprandial protein deposition.

Duodenal vs. gastric administration of labeled leucine for the study of splanchnic metabolism in humans

Low-rate (6 ml/h) intragastric infusion of stable, isotope-labeled amino acids is commonly used to assess the splanchnic handling of amino acids in humans. However, when used in the postabsorptive state, this method yields unreliable plasma isotopic enrichments, with a coefficient of variation >10%. In this metabolic condition, we confirmed in six subjects that an intragastric infusion of L-[(2)H(3)]leucine at 6 ml/h yields an unreliable isotopic steady state in plasma amino acids with a coefficient of variation of 43 +/- 12% (mean +/- SD). In five additional subjects, we assessed the effects of 1) increasing the rate of delivery of a leucine tracer in an isotonic plasmalike solution at 240 ml/h into the gastric site, and 2) changing the site of infusion from gastric to duodenal with this same high rate of delivery. In contrast to the gastric route, and regardless of the rate of delivery, only the intraduodenal route allowed 1) isotopic plasma steady state (i.e., coefficients of variation were <10%: 5 +/- 3%), and 2) reproducible leucine extraction coefficients (22 +/- 5%). We conclude that an infusion site that bypasses the gastric emptying process, i.e., the duodenal route, along with delivery of a plasmalike solution, is necessary to reach isotopic steady state in plasma when labeled leucine is infused into the gastrointestinal tract in the postabsorptive state.

Enteral nutrient intake level determines intestinal protein synthesis and accretion rates in neonatal pigs

Our objective was to determine the minimum enteral intake level necessary to increase the protein accretion rate (PAR) in the neonatal small intestine. Seven-day-old piglets received an equal total daily intake of an elemental diet, with different proportions given enterally (0, 10%, 20%, 40%, 60%, 80%, and 100%). After 7 days, piglets were infused intravenously with [(2)H(3)]leucine for 6 h, and the fractional protein synthesis rate (FSR) was measured in the proximal (PJ) and distal jejunum (DJ) and the proximal (PI) and distal ileum (DI). The jejunal FSR increased from 45%/day to 130%/day between 0 and 60% enteral intake, whereas the FSR in the ileum was less sensitive to enteral intake level. At 0% enteral intake, PAR was significantly negative in the PJ, DJ, and PI (range -70 to -43 mg/day) and positive in the DI (49 mg/day), whereas intestinal protein balance occurred at 20% enteral intake. At 100% enteral intake, the PAR was greatest in the DI, even though the rates of protein turnover were 50% lower than in the PJ. We conclude that there is net intestinal protein loss at 0% enteral intake, protein balance at 20% enteral intake, and maximal intestinal protein accretion at 60% enteral intake.

Kinetics of L-[1-(13)C]leucine when ingested with free amino acids, unlabeled or intrinsically labeled casein

In two groups of five adults, each adapted to two different dietary regimens for 6 days, the metabolic fate of dietary [1-(13)C]leucine was examined when ingested either together with a mixture of free amino acids simulating casein (extrinsically labeled; condition A), along with the intact casein (extrinsically labeled; condition B), or bound to casein (intrinsically labeled; condition C). Fed state leucine oxidation (Ox), nonoxidative leucine disposal (NOLD), protein breakdown, and splanchnic uptake have been compared using an 8-h oral [1-(13)C]leucine and intravenous [(2)H(3)]leucine tracer protocol while giving eight equal hourly mixed meals. Lower leucine Ox, increased NOLD, and net protein synthesis were found with condition C compared with condition A (19.3 vs. 24.9; 77 vs. 55.8; 18.9 vs. 12.3 micromol. kg(-1). 30 min(-1); P < 0.05). Ox and NOLD did not differ between conditions B and C. Splanchnic leucine uptake calculated from [1-(13)C]- and [(2)H(3)]leucine plasma enrichments was between 24 and 35%. These findings indicate that the form in which leucine is consumed affects its immediate metabolic fate and retention by the body; the implications of these findings for the tracer balance technique and estimation of amino acid requirements are discussed.

Mechanisms permitting nephrotic patients to achieve nitrogen equilibrium with a protein-restricted diet

Clinical experience suggests nephrotic patients are at risk for malnutrition. To determine if nephrotic patients can adapt successfully to a protein-restricted diet, nephrotic (glomerular filtration rate, 52+/-15 ml/min; urinary protein [Uprot.], 7.2+/-2.2 grams/d) and control subjects completed a crossover comparison of diets providing 0.8 or 1.6 grams protein (plus 1 gram protein/gram Uprot.) and 35 kcal per kg per day. Nitrogen balance (BN) was determined and whole body protein turnover measured during fasting and feeding using intravenous -[1-13C]leucine and intragastric -[5,5, 5- 2H3]leucine. BN was positive in both nephrotic and control subjects consuming either diet and rates of whole-body protein synthesis, protein degradation, and leucine oxidation did not differ between groups. In both nephrotic and control subjects anabolism was due to a suppression of whole-body protein degradation and stimulation of protein synthesis during feeding. The principal compensatory response to dietary protein restriction was a decrease in amino acid oxidation and this response was the same in both groups. With the low protein diet leucine oxidation rates during feeding correlated inversely with Uprot. losses (r = -0.83; P < 0. 05).

CONCLUSIONS

(a) a diet providing 0.8 gram protein (plus 1 gram protein/gram Uprot.) and 35 kcal per kg per day maintains BN in nephrotic patients; (b) nephrotic patients activate normal anabolic responses to dietary protein restriction (suppression of amino acid oxidation) and feeding (stimulation of protein synthesis and inhibition of protein degradation); (c) the inverse correlation between leucine oxidation and Uprot. losses suggests that proteinuria is a stimulus to conserve dietary essential amino acids.

Liver transplantation in two cases of propionic acidaemia

Orthotopic liver transplantation (OLT) was performed in two patients with propionic acidaemia, a 7-year-old boy and a 9-year-old girl, diagnosed with a severe neonatal form with high risk of metabolic decompensation. In both cases the metabolic liver functions recovered within the 12 postoperative hours; no clinical symptoms of propionic acid toxicity, metabolic acidosis, severe hyperammonaemia, hyperglycinaemia or haematological abnormalities were observed. In both cases insulin-dependent diabetes mellitus occurred early after OLT (persisting in the boy's case). Severe post-transplantation complications were observed (acute rejection and CMV infection in both patients) which did not trigger metabolic decompensation. The boy developed chronic rejection and vanishing bile duct syndrome due to incomplete hepatic arterial thrombosis. He required permanent in-patient care with chronic hyperammonaemia and neurological sequelae involving the basal ganglia and died 15 months after OLT. The girl left hospital after 2 months and is presently leading a normal life with almost no dietary protein restriction (40 g protein per day). Urinary urea excretion and daily protein intake increased after liver transplantation. Propionyl- and tiglylglycine disappeared immediately after OLT. Urinary methylcitrate and 3-hydroxypropionate remained at concentrations corresponding to those before OLT. However, the total of all characteristic metabolites of organic acid analysis was reduced to 50-60% of the values before OLT in both patients. Propionylcarnitine was still detected at significant concentrations. Plasma odd-chain fatty acid concentrations decreased continuously after OLT only in the girl's case. Tissue of both transplanted livers showed increased odd-chain fatty acid concentrations 9 and 15 months after OLT, respectively, in both patients. We consider that at present OLT should only be performed in severe forms of propionic acidaemia.

Splanchnic metabolism of dietary arginine in relation to nitric oxide synthesis in normal adult man

Urinary nitrate (NO3) is the stable end product of nitric oxide, which is formed, in turn, from a guanidino nitrogen of arginine. We have conducted two experiments, each in four healthy adult men receiving a low nitrate diet for 7-10 days, to investigate the in vivo conversion of arginine to nitrate. In the first study [guanidino-15N2, 5,5-2H2]arginine was given on day 7 via a primed continuous intravenous infusion for 8 h. In the second study, the labeled arginine was given for 8 h by the intragastric route on day 7 and by the intravenous route on day 10. Measurement of 15NO3 output in urine collected for 24 h beginning at the time of the arginine tracer infusion revealed a more extensive transfer of 15N when the arginine tracer was given intragastricly. From the comparative labeling of 15NO3 after administration of the tracer arginine via the intragastric and intravenous routes, we estimate that 16% +/- 2% of the daily production of nitrate arises from the metabolism of dietary arginine that is taken up during its "first pass" in the splanchnic region. Hence, nitric oxide production occurs, to a measurable extent, in this area in healthy subjects, raising the question as to how various pathophysiological states might alter the relations between exogenous and endogenous sources of arginine as precursors of NO. and the relative contributions made by various organs to whole body (NO.) NO3 formation. These results also raise important questions about the use of nitric oxide synthase inhibitors in animal and human studies.

Protein and energy interactions throughout life. Metabolic basis and nutritional implications

We review selected aspects of the interactions between protein and energy in human metabolism and nutrition. Following a short account of the underlying metabolic basis for the effects of energy on protein metabolism, the contribution made by whole body protein turnover to the metabolic rate is discussed, including the relationship between protein turnover and energy metabolism at different phases of life. The effects of changes in energy metabolism and intake on the nitrogen economy of the host are also reviewed briefly and we explore the relationship between amino acid oxidation and requirements for indispensable amino acids. Interactions between energy and protein metabolism need to be investigated in greater detail and also they must be considered in relation to further attempts to establish more precisely energy and amino acid requirements of people under various circumstances.