A Moderate Serving of High-Quality Protein Maximally Stimulates Skeletal Muscle Protein Synthesis in Young and Elderly Subjects

Ingestion of sufficient dietary protein is a fundamental prerequisite for muscle protein synthesis and maintenance of muscle mass and function. Elderly people are often at increased risk for protein-energy malnutrition, sarcopenia, and a diminished quality of life. This study sought to compare changes in muscle protein synthesis and anabolic efficiency in response to a single moderate serving (113 g; 220 kcal; 30 g protein) or large serving (340 g; 660 kcal; 90 g protein) of 90% lean beef. Venous blood and vastus lateralis muscle biopsy samples were obtained during a primed, constant infusion (0.08 μmol/kg/min) of L-[ring-¹³C₆] phenylalanine in healthy young (n=17; 34±3 years) and elderly (n=17; 68±2 years) individuals. Mixed muscle fractional synthesis rate was calculated during a 3-hour postabsorptive period and for 5 hours after meal ingestion. Data were analyzed using a two-way repeated measures analysis of variance with Tukey's pairwise comparisons. A 113-g serving of lean beef increased muscle protein synthesis by approximately 50% in both young and older volunteers. Despite a threefold increase in protein and energy content, there was no further increase in protein synthesis after ingestion of 340 g lean beef in either age group. Ingestion of more than 30 g protein in a single meal does not further enhance the stimulation of muscle protein synthesis in young and elderly.     

Postprandial leucine deficiency failed to alter muscle protein synthesis in growing and adult rats

ObjectiveThis study examined the effect of a specific acute postprandial leucine deficiency on skeletal muscle protein synthesis in growing and adult rats. Because the anabolic action of dietary leucine supplementation is controversial, except during aging, we hypothesized that the maximum leucine effect might be already achieved for a normal postprandial rise of leucine. Preventing this rise during the 1- to 3-h period after feeding may reveal the leucine regulation.MethodsOn the day of the experiment, rats were fasted (postabsorptive, PA group) or fed for 1 h a control meal (postprandial, control, PP group) or a leucine-poor meal (postprandial, PP-Leu group). Muscle protein synthesis was assessed in vivo, over the 1- to 3-h period after meal distribution, using the flooding dose method (L-1-¹³C phenylalanine).ResultsAs expected, the postprandial increase in plasma free leucine was specifically abolished after feeding the leucine-poor meal, whereas all the other plasma free amino acids were roughly at normal postprandial levels. Plasma insulin increased after feeding in young rats but was constant in adult rats. Plasma insulin was similar whatever dietary leucine levels. Rates of muscle protein synthesis were stimulated by feeding in gastrocnemius and soleus muscles from young rats but only in gastrocnemius muscles from adult rats. The PP-Leu group did not differ from the control PP group regarding muscle protein synthesis.ConclusionThe rise in plasma free leucine is not required for the stimulation of muscle protein synthesis during the 1- to 3-h period after feeding young and adult rats, as previously observed in old rats.     

Leucine, glucose, and energy metabolism after 3 days of fasting in healthy human subjects

Adaptations of leucine and glucose metabolism to 3 d of fasting were examined in six healthy young men by use of L-[1-13C]leucine and D[6,6-2H2]glucose as tracers. Leucine flux increased 31% and leucine oxidation increased 46% after 3 d of fasting compared with leucine flux and oxidation after an overnight fast. Glucose production rate declined 38% and resting metabolic rate decreased 8% during fasting. Plasma concentrations of testosterone, insulin, and triiodothyronine were reduced by fasting whereas plasma glucagon concentrations were increased. We conclude that there is increased proteolysis and oxidation of leucine on short-term fasting even though glucose production and energy expenditure decreased.     

Decreased whole-body and splanchnic glutamate metabolism in healthy elderly men and patients with chronic obstructive pulmonary disease in the postabsorptive state and in response to feeding

Decreased plasma and muscle glutamate concentrations have been observed in patients with chronic obstructive pulmonary disease (COPD), suggesting disturbances in glutamate metabolism. The present study was conducted to further examine glutamate metabolism in 8 male COPD patients (68 +/- 4 y) by measurement of whole-body (WB) glutamate production and splanchnic glutamate extraction in the postabsorptive state as well as in response to feeding. Because COPD is particularly prevalent in the elderly and aging per se may also affect glutamate metabolism, 2 male control groups were included: 8 healthy elderly (63 +/- 3 y) and 8 young (22 +/- 1 y) subjects. On 2 test days, the stable isotope L-15N-glutamate was infused i.v. or enterally according to a primed constant and continuous infusion protocol. After 90 min of infusion, subjects ingested a carbohydrate-protein drink (28% milk protein, 72% maltodextrin) every 20 min for 2 h. Arterialized-venous blood samples were taken at the end of the postabsorptive and feeding periods. Postabsorptive WB glutamate production and splanchnic glutamate extraction were significantly lower in the elderly and COPD patients than in the young (P < 0.01). Feeding further decreased WB endogenous glutamate production in the elderly and COPD patients, with COPD patients tending (P = 0.07) to have a greater decrease. Splanchnic glutamate extraction increased during feeding in the elderly (P < 0.05) but did not change in COPD patients. In conclusion, aging reduces postabsorptive WB endogenous glutamate production and splanchnic glutamate extraction. COPD does not affect postabsorptive WB glutamate metabolism but may influence splanchnic glutamate metabolism during feeding.     

Co-ingestion of protein and leucine stimulates muscle protein synthesis rates to the same extent in young and elderly lean men

BACKGROUND: The progressive loss of skeletal muscle mass with aging is attributed to a disruption in the regulation of skeletal muscle protein turnover. OBJECTIVE: We investigated the effects on whole-body protein balance and mixed-muscle protein synthesis rates of the ingestion of carbohydrate with or without protein and free leucine after simulated activities of daily living. DESIGN: Eight elderly (75 +/- 1 y) and 8 young (20 +/- 1 y) lean men were randomly assigned to 2 crossover experiments in which they consumed either carbohydrate (CHO) or carbohydrate plus protein and free leucine (CHO+Pro+Leu) after performing 30 min of standardized activities of daily living. Primed, continuous infusions with L-[ring-13C6]phenylalanine and L-[ring-2H2]tyrosine were applied, and blood and muscle samples were collected to assess whole-body protein turnover and the protein fractional synthetic rate in the vastus lateralis muscle over a 6-h period. RESULTS: Whole-body phenylalanine and tyrosine flux were significantly higher in the young than in the elderly men (P < 0.01). Protein balance was negative in the CHO experiment but positive in the CHO+Pro+Leu experiment in both groups. Mixed-muscle protein synthesis rates were significantly greater in the CHO+Pro+Leu than in the CHO experiment in both the young (0.082 +/- 0.005%/h and 0.060 +/- 0.005%/h, respectively; P < 0.01) and the elderly (0.072 +/- 0.006%/h and 0.043 +/- 0.003%/h, respectively; P < 0.01) subjects, with no significant differences between groups. CONCLUSIONS: Co-ingestion of protein and leucine with carbohydrate after activities of daily living improves whole-body protein balance, and the increase in muscle protein synthesis rates is not significantly different between lean young and elderly men.     

Long-lasting improved amino acid bioavailability associated with protein pulse feeding in hospitalized elderly patients: A randomized controlled trial

ObjectiveAging is associated with a blunted anabolic response to dietary intake, possibly related to a decrease in systemically available amino acids (AAs), which in turn may stem from increased splanchnic AA metabolism. Splanchnic sequestration can be saturated by pulse feeding (80% of daily protein intake in a single meal), enabling increased protein synthesis. The aim of this study was to explore whether protein pulse feeding increased postprandial AA concentrations, and if so whether this increase persisted after 6 wk of dietary treatment.MethodsThis prospective randomized study enrolled 66 elderly malnourished or at-risk patients in an inpatient rehabilitation unit. All were given a controlled diet for 6 wk. In a spread diet (SD) group (n = 36), dietary protein was spread over the four daily meals. In a pulse diet (PD) group (n = 30), 72% of dietary protein (averaging 1.31 g/kg body weight daily) was consumed in one meal at noon. The patients were evaluated on day 1 and at 6 wk for plasma postprandial (five times from 0 to +180 min) AA concentrations (expressed as area under the curve above baseline).ResultsProtein pulse feeding was more efficient than protein spread feeding at increasing plasma postprandial AA concentrations, notably of essential AAs. This increased postprandial AA bioavailability was maintained after 6 wk.ConclusionsThis study demonstrates that increased postprandial AA bioavailability induced by protein pulse feeding persists after 6 wk (i.e., that there is no metabolic adaptation blunting AA bioavailability).     

Comparison of the anticatabolic effects of leucine and Ca-β-hydroxy-β-methylbutyrate in experimental models of cancer cachexia

ObjectiveLoss of skeletal muscle is the most debilitating feature of cancer cachexia, and there are few treatments available. The aim of this study was to compare the anticatabolic efficacy of L-leucine and the leucine metabolite β-hydroxy-β-methylbutyrate (Ca-HMB) on muscle protein metabolism, both in vitro and in vivo.MethodsStudies were conducted in mice bearing the cachexia-inducing murine adenocarcinoma 16 tumor, and in murine C₂ C₁₂ myotubes exposed to proteolysis-inducing factor, lipopolysaccharide, and angiotensin II.ResultsBoth leucine and HMB were found to attenuate the increase in protein degradation and the decrease in protein synthesis in murine myotubes induced by proteolysis-inducing factor, lipopolysaccharide, and angiotensin II. However, HMB was more potent than leucine, because HMB at 50 μM produced essentially the same effect as leucine at 1 mM. Both leucine and HMB reduced the activity of the ubiquitin-proteasome pathway as measured by the functional (chymotrypsin-like) enzyme activity of the proteasome in muscle lysates, as well as Western blot quantitation of protein levels of the structural/enzymatic proteasome subunits (20 S and 19 S) and the ubiquitin ligases (MuRF1 and MAFbx). In vivo studies in mice bearing the murine adenocarcinoma 16 tumor showed a low dose of Ca-HMB (0.25 g/kg) to be 60% more effective than leucine (1 g/kg) in attenuating loss of body weight over a 4-d period.ConclusionThese results favor the clinical feasibility of using Ca-HMB over high doses of leucine for the treatment of cancer cachexia.     

Determining the Influence of Habitual Dietary Protein Intake on Physiological Muscle Parameters in Youth and Older Age

Protein ingestion is a potent stimulator of skeletal muscle protein synthesis (MPS). However, older adults demonstrate resistance to anabolic stimuli. Some evidence has demonstrated that a larger acute protein dose is required in older compared to younger adults to elicit the same synthetic response, suggesting that older adults should be consuming higher habitual dietary protein to optimise muscle mass. However, limited research has explored dietary habits in different age groups or the relationship between habitual dietary intake and mechanistic physiological parameters associated with muscle mass and function. This work investigated the effect of habitual dietary intake in young (n = 10, 25.9 (3.2y)) and older (n = 16, 70.2 (3.2y)) community-dwelling adults (16:10 male: female) on physiological muscle parameters. Dietary intake was assessed using four-day diet diaries. Post-absorptive MPS and MPS responses to feeding (4.25x basal metabolic rate; 16% protein) were determined in muscle biopsies of the m. vastus lateralis via stable isotope tracer ([1, 2⁻¹³C₂]-leucine) infusions with mass-spectrometric analyses. Body composition was measured by dual-energy x-ray absorptiometry. Whole body strength was assessed via 1-repetition maximum assessments. No significant differences in habitual dietary intake (protein, fat, carbohydrate and leucine as g.kgWBLM⁻¹.day⁻¹) were observed between age groups. Whole-body lean mass (61.8 ± 9.9 vs. 49.8 ± 11.9 kg, p = 0.01) and knee-extensor strength (87.7 ± 28.3 vs. 56.8 ± 16.4 kg, p = 0.002) were significantly higher in young adults. Habitual protein intake (g.kg⁻¹.day⁻¹) was not associated with whole-body lean mass, upper-leg lean mass, whole-body strength, knee-extensor strength, basal MPS or fed-state MPS across both age groups. These findings suggest that differences in muscle mass and strength parameters between youth and older age are not explained by differences in habitual dietary protein intake. Further research with a larger sample size is needed to fully explore these relationships and inform on interventions to mitigate sarcopenia development.     

Adaptation of body protein metabolism in adult and aging man

Aging causes a selective loss in lean body mass, muscle being most extensively eroded. This presentation is an analysis of the consequences of this age-related loss for protein and amino acid metabolism. The use of stable isotopes allows us to measure the dynamics of protein metabolism in the intact human subject, while the output of 3-methylhistidine permits estimates of muscle turnover to be made which show a smaller reduction in body protein turnover per kg body weight in elderly subjects. This is due to the reduced contribution by muscle protein turnover, which decreases from 30 per cent of total body protein turnover in the young adult to 20 per cent in the elderly, and can be entirely accounted for by the reduced mass of muscle, the rate of turnover per unit muscle weight being unchanged. The consequences of the changing proportions of major tissues with age has been examined for its impact on amino acid metabolism. The interaction of various organs in order to ensure metabolism of different amino acids is described. After a meal containing abundant protein, the liver selectively degraded the excess of most essential amino acids, while allowing the three branched-chain essential amino acids to pass through, so that in consequence they account for more than 50 per cent of the amino acid outflow from the splanchnic area. During this period of absorption muscle takes up amino acids, over 70 per cent being the branched-chain amino acids where they are catabolised, the nitrogen becoming available for formation of non-essential amino acids for muscle protein synthesis. The release of large amounts of glutamine from muscle is dependent on branched-chain amino acid degradation, whereas the release of large amounts of alanine is regulated by availability of pyruvate from glucose. The study of amino acid metabolism in elderly compared with young adults and its relationship to the declining mass of muscle with age is still in its initial stages. Metabolism of glycine at different levels of protein intake fails to demonstrate any significant effects of aging on metabolism of this amino acid. On the other hand, the flux of leucine shows a significant reduction in fasting elderly female subjects. This observation should be followed up by studying leucine kinetics following a meal of meat, in order to determine whether the reduced mass of muscle in the elderly will restrict the capacity to metabolise branched-chain amino acids. It has been concluded that the extensive age-related change in muscle mass should be explored for its impact on the nutritional requirements for branched-chain amino acids, their metabolism in health and disease, and their therapeutic uses at different ages.     

Limited Association between the Total Healthy Eating Index-2015 Score and Cardiovascular Risk Factors in Individuals with Long-Standing Spinal Cord Injury: An Exploratory Study: An Exploratory Study

BackgroundThe healthy eating index-2015 (HEI-2015) reflects diet quality in reference to the 2015-2020 Dietary Guidelines for Americans (DGA). Little is known regarding its application in individuals with chronic spinal cord injury (SCI).ObjectiveTo explore the relationship between diet quality as assessed by the HEI-2015 and cardiovascular risk factors among individuals with chronic SCI.DesignThis is a cross-sectional analysis of baseline data collected from August 2017 through November 2019 for an interventional study that evaluates the effects of a high-protein/low-carbohydrate diet on cardiovascular risk factors in individuals with chronic SCI at the University of Alabama at Birmingham.Participants/settingTwenty-four free-living adults with SCI (mean age, 45 ± 12 y; 8F/16M, level of injury: nine cervical, 15 thoracic; mean duration of injury: 20 ± 13 y) were included.Main outcome measuresParticipants underwent a 2-hour oral glucose tolerance test (OGTT) and a dual-energy x-ray absorptiometry scan. Dietary intake was assessed by three, 24-hour multiple-pass dietary recalls to calculate the HEI-2015 using the simple HEI scoring algorithm method.Data analysisMultiple linear regression analyses were performed to predict indices of lipid metabolism and glucose homeostasis and C-reactive protein (CRP) from the HEI-2015. Principal component analysis was used to reduce the number of covariates (level of injury, sex, and body fat percentage).ResultsOn average, participants’ diets were of low quality (HEI-2015, 47.2 ± 10.8). The regression models for fasting glucose (FG), cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and CRP had moderate to large effect sizes (adjusted R² ≥ 13%), suggesting good explanatory abilities of the predictors. Small or limited effect sizes were observed for glucose tolerance, fasting insulin, triglycerides, and Matsuda index (adjusted R² < 13%). The HEI-2015 accounted for a moderate amount of variation in FG (partial omega-squared, ω_P² = 13%). Each 10-point HEI-2015 score increase was associated with a 3.3-mg/dL decrease in FG concentrations. The HEI-2015 accounted for a limited amount of variation in other indices (ω_P² < 5%).ConclusionsAmong participants with SCI, higher conformance to the 2015-2020 DGA was 1) moderately associated with better FG homeostasis; and 2) trivially associated with other cardiovascular risk factors. Because of the small sample size, these conclusions cannot be extrapolated beyond the study sample. Future larger studies are warranted to better understand the relationship between diet quality and cardiovascular disease risks in this population.     

Co-ingestion of leucine with protein does not further augment post-exercise muscle protein synthesis rates in elderly men

Leucine has been suggested to have the potential to modulate muscle protein metabolism by increasing muscle protein synthesis. The objective of this study was to investigate the surplus value of the co-ingestion of free leucine with protein hydrolysate and carbohydrate following physical activity in elderly men. Eight elderly men (mean age 73 +/- 1 years) were randomly assigned to two cross-over treatments consuming either carbohydrate and protein hydrolysate (CHO+PRO) or carbohydrate, protein hydrolysate with additional leucine (CHO+PRO+leu) after performing 30 min of standardized physical activity. Primed, continuous infusions with L-[ring-(13)C(6)]phenylalanine and L-[ring-(2)H(2)]tyrosine were applied, and blood and muscle samples were collected to assess whole-body protein turnover as well as protein fractional synthetic rate in the vastus lateralis muscle over a 6 h period. Whole-body protein breakdown and synthesis rates were not different between treatments. Phenylalanine oxidation rates were significantly lower in the CHO+PRO+leu v. CHO+PRO treatment. As a result, whole-body protein balance was significantly greater in the CHO+PRO+leu compared to the CHO+PRO treatment (23.8 (SEM 0.3) v. 23.2 (SEM 0.3) micromol/kg per h, respectively; P < 0.05). Mixed muscle fractional synthetic rate averaged 0.081 (SEM 0.003) and 0.082 (SEM 0.006) %/h in the CHO+PRO+leu and CHO+PRO treatment, respectively (NS). Co-ingestion of leucine with carbohydrate and protein following physical activity does not further elevate muscle protein fractional synthetic rate in elderly men when ample protein is ingested.     

Preoperative Glucose and Protein Metabolism: The Influence of Diabetes Mellitus Type 2 in Patients With Colorectal Tumors

Hypermetabolism, abnormal plasma amino acid profiles, increased gluconeogenesis, and changes in liver and muscle protein turnover are well-described undesirable effects in patients with cancer and diabetes mellitus type 2 (DM2) The aim of the present study was to analyze the specific impact and interaction of these 2 disease patterns on patients’ preoperative glucose and protein metabolism. Eight nondiabetic and 8 diabetic patients devoid of cachexia underwent a stable isotope infusion study on the day before surgery for colorectal cancer or adenoma with high-grade dysplasia. Protein and glucose kinetics were assessed in a fasted state by L-[1-¹³C]leucine and [6,6²H₂]glucose. In diabetic patients, glucose metabolism was found to be elevated as the plasma glucose level increased (P = 0.013) and endogenous rate of appearance of glucose tended to be higher compared to nondiabetic patients (P = 0.083). Protein metabolism was not affected by the metabolic state of the 2 groups. Resting energy expenditure was higher in diabetic patients (P = 0.028). Under postabsorptive conditions, noncachectic patients with DM2 suffering from colorectal tumors showed an elevated turnover in glucose metabolism whereas the nondiabetic counterparts failed to demonstrate any metabolic changes due solely to malignancy.     

Muscle protein synthesis in cancer patients can be stimulated with a specially formulated medical food

Objective

Maintenance of muscle mass is crucial to improving outcome and quality of life in cancer patients. Stimulating muscle protein synthesis is the metabolic basis for maintaining muscle mass, but in cancer patients normal dietary intake has minimal effects on muscle protein synthesis. Adding leucine to high protein supplements stimulates muscle protein synthesis in healthy older subjects. The objective was to determine if a specially formulated medical food, high in leucine and protein, stimulates muscle protein synthesis acutely in individuals with cancer to a greater extent than a conventional medical food.

Design

A randomized, controlled, double-blind, parallel-group design was used in 25 patients with radiographic evidence of cancer. Patients were studied before their cancer treatment was started or 4 weeks after their treatment was completed or halted. The fractional rate of muscle protein synthesis (FSR) was measured using the tracer incorporation technique with L-[ring-¹³C₆]-phenylalanine. The experimental group (n = 13) received a medical food containing 40 g protein, based on casein and whey protein and enriched with 10% free leucine and other specific components, while the control group (n = 12) was given a conventionally used medical food based on casein protein alone (24 g). Blood and muscle samples were collected in the basal state and 5h hours after ingestion of the medical foods.

Results

The cancer patients were in an inflammatory state, as reflected by high levels of C-reactive protein (CRP), IL-1β and TNF-α, but were not insulin resistant (HOMA). After ingestion of the experimental medical food, plasma leucine increased to about 400 μM as compared to the peak value of 200 μM, after the control medical food (p < 0.001). Ingestion of the experimental medical food increased muscle protein FSR from 0.073 (SD: 0.023) to 0.097 (SD: 0.033) %/h (p = 0.0269). In contrast, ingestion of the control medical food did not increase muscle FSR; 0.073 (SD: 0.022) and 0.065 (SD: 0.028) %/h.

Conclusions

In cancer patients, conventional nutritional supplementation is ineffective in stimulating muscle protein synthesis. This anabolic resistance can be overcome with a specially formulated nutritional supplement.     

Daily Leucine Intake Is Positively Associated with Lower Limb Skeletal Muscle Mass and Strength in the Elderly

Higher daily protein intake, with an emphasis on leucine content, is thought to mitigate age-related anabolic resistance, potentially counteracting age-related morphological and functional declines. The present study investigated potential associations between total daily leucine intake and dependent variables, including quadriceps muscle cross-sectional area (CSA) and maximum dynamic muscle strength (1-RM) in a cohort of healthy free-living older individuals of both sexes (n = 67; 34/33 men/women). Participants performed three 24 h dietary recalls and underwent a magnetic resonance imaging exam followed by 1-RM tests. Our results demonstrate moderate associations between total daily leucine and both quadriceps CSA (r = 0.42; p = 0.004) and 1-RM (r = 0.45; p = 0.001). Furthermore, our exploratory biphasic linear regression analyses, adjusted for sex, age, and protein intake relative to body weight, revealed a plateau for daily leucine intake and muscle mass and muscle strength (~7.6–8.0 g·day⁻¹) in older adults. In conclusion, we demonstrated that total daily leucine intake is associated with muscle mass and strength in healthy older individuals and this association remains after controlling for multiple factors, including overall protein intake. Furthermore, our breakpoint analysis revealed non-linearities and a potential threshold for habitual leucine intake, which may help guide future research on the effects of chronic leucine intake in age-related muscle loss.     

Oral citrulline does not affect whole body protein metabolism in healthy human volunteers: results of a prospective, randomized, double-blind, cross-over study

Background & aims

Citrulline increases protein synthesis during refeeding in rodents with short bowel syndrome, aging and malnutrition, and improves nitrogen balance in fed healthy humans. The aim of the current study therefore was to determine whether citrulline had affected protein metabolism in healthy volunteers.

Methods

In a randomized, double-blind, cross-over study, 12 healthy adults received a 5-h intravenous infusion of L-[1-¹³C]-leucine in the post-absorptive state, after a 7-day oral supplementation with 0.18 g/kg/day citrulline, or an iso-nitrogenous placebo. Treatment order was randomized, treatment periods were separated by 13-day wash out. Leucine appearance rate (Ra) was determined from plasma [1-¹³C]-keto-iso-caproate enrichment and leucine oxidation from expired ¹³CO₂, and nitrogen balance was estimated from 6-h urinary urea excretion.

Results

Compared with placebo, oral citrulline supplementation increased plasma citrulline, arginine and ornithine concentrations, but failed to affect albumin, transthyretin, free insulin and insulin-like growth factor (IGF)-1 plasma concentrations, urinary nitrate excretion, or nitrogen balance. Citrulline supplementation did not alter leucine Ra, leucine oxidation, nor whole-body protein synthesis.

Conclusion

In healthy, well nourished volunteers, oral citrulline increases plasma citrulline and arginine availability but does not affect whole-body protein kinetics in the post-absorptive state.     

Age-related changes in ribosomal profiles and in vitro protein synthesis in skeletal muscle during fasting and subsequent refeeding of rats

Age-dependent changes in ribosomal profiles and in in vitro protein synthesis in skeletal muscle (gastrocnemius muscle) were studied in rats subjected to fasting and subsequent refeeding. Male rats of 4, 7, and 21 weeks of age were used. Incorporation of 14C-leucine into protein in vitro using skeletal muscle ribosomes from fed rats decreased with the age of the animals. Sucrose density gradient analyses of ribosomal profiles revealed that the changes in protein synthesis were accompanied by progressive declines in the concentrations of polyribosomes and total RNA in skeletal muscle. After fasting for 1 or 2 days, protein synthesis in muscle of young rats dropped markedly and this change could be attributed to the fall in the concentrations of polyribosomes. Also, the concentration and total RNA in the muscles of the young rats decreased. During refeeding of rats after fasting for 2 days, the incorporation of 14C-amino acids into protein recovered more efficiently using ribosomes of skeletal muscle of young animals compared with using those of older animals. This rapid increase in the protein synthesizing activity of skeletal muscle may be related to the efficient recovery of the ribosomes toward heavier aggregates. The results indicate that young animals adapt more efficiently to fasting and refeeding than do older animals.     

Lung anabolic activity in patients with chronic heart failure: Potential implications for clinical practice

ObjectiveThe proteins in the lungs are in constant flux, undergoing degradation and resynthesis. We investigated pulmonary protein and amino acid metabolism, the biochemical basis of the remodeling process, in individuals with chronic heart failure receiving or not receiving β-blocker therapy with bisoprolol (BIS).MethodsClinically stable rehabilitative patients with chronic heart failure, without metabolic diseases or liver/renal failure, and with a stable weight over the preceding 3 mo underwent right heart catheterization, and radial artery cannulation. Mixed central venous and arterial blood samples were drawn simultaneously to calculate the venous-arterial difference of amino acids (pulmonary uptake and release).ResultsTwenty-two patients on BIS therapy and eight not receiving BIS were analyzed. The two groups showed a net pulmonary protein synthesis (i.e., a positive value of phenylalanine [venous-arterial difference] × cardiac index product) and amino acid extraction, the rates of which were significantly lower in patients on BIS therapy. The two groups had pulmonary hypertension (mean pulmonary artery pressure >19 mmHg). Pulmonary vascular resistance was 57% higher in patients not receiving BIS than in those on BIS therapy (6.65 ± 2.90 versus 4.23 ± 1.49 mmHg/L · min^?1 · m^?2, P < 0.05). Pulmonary vascular resistance correlated positively with the pulmonary extraction of total essential amino acids (r = +0.4576, P = 0.01) and leucine (r = +0.5083, P = 0.004), the most important amino acid for protein synthesis.ConclusionPatients with chronic heart failure have increased rates of amino acid extraction and pulmonary protein synthesis, suggesting, at least in part, an increased rate of lung remodeling. Therapy with BIS attenuates lung metabolic abnormalities.     

Aging is associated with diminished accretion of muscle proteins after the ingestion of a small bolus of essential amino acids

BACKGROUND: Previous evidence suggests that aging in healthy persons does not result in decreased incorporation of muscle proteins after a bolus ingestion of 15 g essential amino acids (EAAs). OBJECTIVE: We sought to examine whether ingestion of a smaller bolus of EAAs is associated with diminished accretion of muscle proteins in the elderly when compared with the young. DESIGN: Eleven elderly subjects (mean +/- SEM: 68 +/- 2 y) and 8 young control subjects (mean +/- SEM: 31 +/- 2 y) were studied in the postabsorptive state and for 3.5 h after a bolus ingestion of approximately 7 g EAAs. Muscle protein accretion and synthesis were measured with the femoral arteriovenous phenylalanine net balance technique during a constant infusion of L-[ring-(2)H5]phenylalanine. RESULTS: Similar to previous observations, no significant differences in the postabsorptive phenylalanine net balance were observed between the groups. However, the mean (+/-SEM) net phenylalanine uptake after EAA ingestion was significantly less in the elderly (9.9 +/- 3.7 mg/leg) than in the young (25.1 +/- 3.7 mg/leg; P < 0.05). The mean (+/-SEM) rate of disappearance of phenylalanine during the same period significantly increased above basal rates in the young (36 +/- 3 compared with 30 +/- 3 nmol x min(-1) x 100 mL leg volume(-1); P < 0.05) but not in the elderly (30 +/- 3 compared with 28 +/- 5 nmol x min(-1) x 100 mL leg volume(-1); P > 0.05). CONCLUSIONS: These data indicate that aging results in a diminished accretion of muscle proteins after ingestion of a small dose of EAAs. These findings may have practical implications with respect to the amount of protein contained in supplements given to the elderly for enhancing the stimulation of muscle protein synthesis.     

Free amino acid and glutathione concentrations in muscle during short-term starvation and refeeding

AIM: The effects of short-term starvation and refeeding on the free amino acids and glutathione in skeletal muscle in healthy man are not known. This is necessary baseline knowledge when studying the effects of nutrition, trauma and sepsis on protein, amino acid and glutathione metabolism. METHODS: Concentrations of free amino acids and glutathione in muscle and plasma from young healthy male volunteers (n = 8) were measured before and after a 3-day fast and then again after 2 days refeeding. Nitrogen balance was determined during the study period. RESULTS: The cumulated nitrogen loss was 36.9+/-5.4 g during the fasting period indicating a condition of protein catabolism. During the fasting period decreases were seen in muscle glutamate by 48 +/- 20% and in glutamine by 38 +/- 12%. These changes were returned back to normal levels during the refeeding period. The changes seen in other muscle amino acids during the study period were reflected by similar changes in plasma amino acids, again with normalisation after the refeeding period. Muscle glutathione concentration and the redox status of glutathione remained unaffected of short-term starvation and refeeding. CONCLUSION: A short-term fasting followed by a refeeding period induced changes in the concentrations of concentrations of glutamate, glutamine, branched chained and basic amino acids in muscle and plasma. Despite this, no changes were seen regarding the glutathione levels in muscle and plasma or its redox status, indicating that the glutathione system is of priority.     

Enhanced Glutamine Availability Exerts Different Effects on Protein and Amino Acid Metabolism in Skeletal Muscle From Healthy and Septic Rats

Background: Enhanced glutamine (GLN) intake may affect the catabolism of branched‐chain amino acids (BCAAs; valine, leucine, and isoleucine), which play a regulatory role in protein turnover. We examined the effects of enhanced GLN availability on leucine oxidation, amino acid concentrations, and protein metabolism in muscles from healthy and septic rats. Methods: Cecal ligation and puncture were used as a model of sepsis. Twenty‐four hours after surgery, the soleus (SOL, red muscle) and the extensor digitorum longus (EDL, white muscle) were incubated in medium containing 0.5 or 2.0 mM GLN. Protein breakdown, protein synthesis, and leucine oxidation were determined via 3‐methylhistidine release, muscle L‐[1‐¹⁴C]leucine radioactivity, and the radioactivity of released ¹⁴CO₂, respectively. Results: In muscles from septic animals, increased proteolysis and leucine oxidation and decreased protein synthesis were detected. These effects were more pronounced in the EDL. In septic muscles, the addition of GLN decreased leucine oxidation in both muscles and increased protein synthesis in the EDL. In muscles from untreated animals, decreased leucine oxidation after the addition of GLN to the medium was associated with decreased protein synthesis in the SOL and decreased concentrations of serine, glycine, histidine, alanine, arginine, proline, and lysine in both muscles. Conclusions: White muscle fibers are more sensitive to septic stimuli than red fibers are. In sepsis, enhanced GLN intake may ameliorate GLN deficiency, inhibit BCAA catabolism, and stimulate protein synthesis. In the healthy state, surplus of GLN may lead to severe alterations in the intramuscular concentration of several amino acids and impair protein synthesis.