A four-compartment compartmental model to assess net whole body protein breakdown using a pulse of phenylalanine and tyrosine stable isotopes in humans

Metabolic kinetics and muscle and brain health markers in older adults, and the role of age and presence of chronic morbidities: A large cross-sectional cohort study

BACKGROUND & AIMS

Older adults are at risk for muscle and cognitive function decline during advanced aging, but the underlying metabolic mechanisms and the role of aging-associated chronic morbidities remain unclear. In the present study, we examined whether protein and amino acid kinetics in older adults with and without chronic morbidities are different when 50-70 and 70-90 of age and related to markers of muscle and brain health declines.

METHODS

In a large cross-sectional observational study, 575 older adults from 12 trials (2014-2022) were stratified based on their age (50-70y vs. 70-95y) and the presence of chronic morbidities. The main outcomes were whole-body production (WBP) and interconversions of amino acids by stable amino acid tracers, body composition, and muscle and cognitive performance. Additionally, the association between metabolic markers and muscle and brain health was assessed.

RESULTS

Overall lower muscle strength, muscle and fat mass, and cognitive function (p < 0.03), but no mood disturbances, were found in 70-95y compared to 50-70y older adults. Presence of morbidities was associated with lower muscle strength and mass, and cognitive function, but higher visceral adipose tissue, and mood disturbances (p < 0.05). Aging was associated with suppressed WBP of most amino acids, de novo arginine production, and net protein breakdown, but higher myofibrillar protein breakdown (p < 0.007). Presence of morbidities was associated with lower WBP of glutamine, glutamate, histidine, isoleucine, phenylalanine, tyrosine, and net protein breakdown, and higher WBP of valine and taurine (p < 0.04). Age showed significant negative correlations with WBP of nearly all amino acids, de novo arginine production and net protein breakdown (r: [-0.407, -0.136], p < 0.01) but a positive correlation with WBP of myofibrillar protein breakdown (r = 0.133, p = 0.009). Lean mass showed positive correlations with de novo arginine production and net protein breakdown and WBP of all amino acids except for isoleucine (r: [0.16, 0.799], p < 0.005). MoCA showed a positive correlation with WBP of leucine and valine (r: [0.163, 0.2], p < 0.03). Worse cognitive performance was positively associated with WBP of tau-methylhistidine and taurine (r: [0.13, 0.141], p < 0.04), but negatively associated with WBP of glycine and valine, de novo arginine production, and net protein breakdown (r: [-0.222, -0.115], p < 0.05).

CONCLUSION

Comprehensive phenotyping of a large group of older adults revealed differences in metabolic health in response to advanced aging and chronic morbidities. Poor muscle health accompanied by advanced aging was associated with overall metabolic downregulation, except for enhanced myofibrillar (muscle) protein breakdown. Presence of chronic morbidities was further associated with disturbed muscle health, mood, arginine, and taurine pathways, and higher visceral adipose tissue. Therefore, different phenotypes among older adults need to be considered when evaluating therapeutic approaches to improve muscle and brain health.

Functional and metabolic effects of omega-3 polyunsaturated fatty acid supplementation and the role of β-hydroxy-β-methylbutyrate addition in chronic obstructive pulmonary disease: A randomized clinical trial

INTRODUCTION

Short-term (4 weeks) supplementation with n-3 polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) has recently been shown to improve protein metabolism in a dose dependent way in normal weight patients with Chronic Obstructive Pulmonary Disease (COPD). Furthermore, EPA/DHA supplementation was able to increase extremity lean soft tissue but not muscle function. No studies are available combining n-3 PUFAs and the leucine metabolite β-hydroxy-β-methylbutyrate (HMB) supplementation in chronic clinical conditions. Whether adding HMB to daily EPA/DHA supplementation for 10 weeks enhances muscle and brain health, daily functional performance, and quality of life of patients with COPD by further improving their protein and amino acid homeostasis remains unknown.

METHODS

Patients with COPD (GOLD: II-IV, n = 46) received daily for 10 weeks, according to a randomized double-blind placebo-controlled three-group design, EPA/DHA (n = 16), EPA/DHA to which HMB was added (n = 14), or placebo (n = 16). The daily dose of 2.0 g of EPA/DHA or soy + corn oil as the placebo was provided via gel capsules, and 3.0 g of Ca-HMB or maltodextrin as placebo as powders. At pre- and post-intervention, a pulse mixture of multiple amino acids was administered to measure postabsorptive net protein breakdown (netPB as primary endpoint) and whole body production (WBP) and conversion rates of the amino acids. As secondary endpoints, lean soft tissue and fat mass were assessed by dual-energy X-ray absorptiometry, upper and lower muscle function by handgrip and single leg isokinetic dynamometry, brain (cognitive, wellbeing) health by assessments, daily functional performance by measuring 6-min walk distance, 4-m gait speed, and postural balance, and quality of life by questionnaire. Plasma enrichments and concentrations were analyzed by LC-MS/MS, and systemic inflammatory profile and metabolic hormones by Luminex.

RESULTS

HMB + EPA/DHA but not EPA/DHA supplementation increased postabsorptive netPB (p = 0.028), and WBPs of glutamine (p = 0.024), taurine (p = 0.039), and tyrosine (p = 0.036). Both EPA/DHA and HMB + EPA/DHA supplementation resulted in increased WBP of phenylalanine (p < 0.05). EPA/DHA but not HMB + EPA/DHA was able to increase WBP of arginine (p = 0.030), citrulline (p = 0.008), valine (p = 0.038), and conversion of citrulline to arginine (p = 0.009). Whole body and extremity fat mass were reduced after HMB + EPA/DHA supplementation only, whereas lean soft tissue was increased after EPA/DHA (p = 0.049) and HMB + EPA/DHA (p = 0.073). No other significant findings were observed. Reductions in several proinflammatory cytokines were observed in the HMB + EPA/DHA group including IL-2, IL-17, IL-6, IL-12P40, and TNF-β (p < 0.05).

CONCLUSIONS

Ten weeks of supplementation with 2 g of EPA/DHA daily is sufficient to induce muscle gain in COPD but HMB is needed to induce fat loss. Whether HMB is solely responsible for the fat mass loss or has a synergistic effect with EPA/DHA remains unclear. The increase in net protein breakdown observed with HMB + EPA/DHA supplementation may indicate a beneficial enhanced protein turnover cycling associated with increased lean soft tissue.

CLINICAL TRIAL REGISTRY

ClinicalTrials.gov; NCT03796455.

Statin treatment reduces leucine turnover, but does not affect endogenous production of beta-hydroxy-beta-methylbutyrate (HMB)

BACKGROUND

Statins, or hydroxy-methyl-glutaryl coenzyme A (HMG-CoA) reductase inhibitors, are one of the most commonly prescribed medications for lowering cholesterol. Myopathic side-effects ranging from pain and soreness to critical rhabdomyolysis are commonly reported and often lead to discontinuation. The pathophysiological mechanism is, in general, ascribed to a downstream reduction of Coenzyme Q10 synthesis. HMG-CoA is a metabolite of leucine and its corresponding keto acid α-ketoisocaproic acid (KIC) and β-hydroxy-β-methylbutyrate (HMB), however, little is known about the changes in the metabolism of leucine and its metabolites in response to statins.

OBJECTIVE

We aimed to investigate if statin treatment has implications on the upstream metabolism of leucine to KIC and HMB, as well as on other branched chain amino acids (BCAA).

DESIGN

12 hyperlipidemic older adults under statin treatment were recruited. The study was conducted as a paired prospective study. Included participants discontinued their statin treatment for 4 weeks before they returned for baseline measurements (before). Statin treatment was then reintroduced, and the participants returned for a second study day 7 days after reintroduction (after statin). On study days, participants were injected with stable isotope pulses for measurement of the whole-body production (WBP) of all BCAA (leucine, isoleucine and valine), along with their respective keto acids and HMB.

RESULTS

We found a reduced leucine WBP (22 %, p = 0.0033), along with a reduction in valine WBP (13 %, p = 0.0224). All other WBP of BCAA and keto acids were unchanged. There were no changes in the WBP of HMB.

CONCLUSIONS

Our study shows that statin inhibition of HMG-CoA reductase has an upstream impact on the turnover of leucine and valine. Whether this impairment in WBP of leucine may contribute to the known pathophysiological side effects of statins on muscle remains to be further investigated.

Sex related differences in muscle health and metabolism in chronic obstructive pulmonary disease

BACKGROUND & AIMS

Sex differences in muscle function and mass, dyspnea, and clinical outcomes have been observed in patients with Chronic Obstructive Pulmonary Disease (COPD) despite a similar level of airflow obstruction. Protein and amino acid metabolism is altered in COPD, however, it remains unclear whether a difference in metabolic signature exists between males and females with COPD that may explain the observed differences in muscle health and clinical outcomes.

METHODS

In 234 moderate to severe COPD patients (males/females: 113/121) and 182 healthy controls (males/females: 77/105), we assessed, besides presence of comorbidities and clinical features, muscle function by handgrip and leg dynamometry, and body composition by dual-energy x-ray absorptiometry. In the postabsorptive state, a mixture of 18 stable isotopes of amino acids was administered by pulse and arterialized blood was sampled for 2 h. Amino acid concentrations and enrichments were analyzed by LC-MS/MS to calculate whole body (net) protein breakdown (WBnetPB) and whole body production (WBP) rates (μmol/hour) of the amino acids playing a known role in muscle health. Statistics was done by ANCOVA to examine the effects of sex, COPD, and sex-by-COPD interaction with as covariates age and lean mass. Significance was set as p < 0.05.

RESULTS

Lung function was comparable between males and females with COPD. Being a female and presence of COPD were independently associated with lower appendicular lean mass, muscle strength, and WBnetPB (p < 0.05). Being a male was associated with higher visceral adipose tissue, C-reactive protein (CRP) (p < 0.05), and higher prevalence of heart failure and obstructive sleep apnea. Sex-by-COPD interactions were found indicating lower fat mass (p = 0.0005) and WBPs of phenylalanine (measure of whole body protein turnover) and essential amino acids (p < 0.05), particularly in COPD females. Higher visceral adipose tissue (p = 0.025), CRP (p < 0.0001), and WBP of tau-methylhistidine (p = 0.010) (reflecting enhanced myofibrillar protein breakdown) were observed in COPD males.

CONCLUSIONS

Presence of sex specific changes in protein and amino acid metabolism and cardiometabolic health in COPD need to be considered when designing treatment regimens to restore muscle health in males and females with COPD.

CLINICAL TRIAL REGISTRY

www.

CLINICALTRIALS

gov, NCT01787682, NCT01624792, NCT02157844, NCT02065141, NCT02770092, NCT02780219, NCT03327181, NCT03796455, NCT01173354, NCT01154400.

Females have a different metabolic response to critical illness, measured by comprehensive amino acid flux analysis

BACKGROUND

The trajectory from healthy to critical illness is influenced by numerous factors, including metabolism, which differs substantially between males and females. Whole body protein breakdown is substantially increased in critically ill patients, but it remains unclear whether there are sex differences that could explain the different health outcomes. Hence, we performed a secondary analysis of a study, where we used a novel pulse isotope method in critically ill and matched healthy males and females.

METHODS

In 51 critically ill ICU patients (26 males, 15 females) and 49 healthy controls (36 males and 27 females), we assessed their general and disease characteristics and collected arterial(ized) blood in the postabsorptive state after pulse administration of 8 ml of a solution containing 18 stable AA tracers. In contrast to the original study, we now fitted the decay curves and calculated non-compartmental whole body amino acid production (WBP) and compartmental measurements of metabolism, including intracellular amino acid production. We measured amino acid enrichments and concentrations by LC-MS/MS and derived statistics using AN(C)OVA.

RESULTS

Critically ill males and females showed an increase in the WBP of many amino acids, including those related to protein breakdown, but females showed greater elevations, or in the event of a reduction, attenuated reductions. Protein breakdown-independent WBP differences remained between males and females, notably increased glutamine and glutamate WBP. Only severely ill females showed a lower increase in WBP of many amino acids in comparison to moderately ill females, suggesting a suppressed metabolism. Compartmental analysis supported the observations.

CONCLUSIONS

The present study shows that females have a different response to critical illness in the production of several amino acids and changes in protein breakdown, observations made possible using our innovative stable tracer pulse approach.

CLINICAL TRIAL REGISTRY

Data are from the baseline measurements of study NCT02770092 (URL: https://clinicaltrials.gov/ct2/show/NCT02770092) and NCT03628365 (URL: https://clinicaltrials.gov/ct2/show/NCT03628365).

Short-chain fatty acid production in accessible and inaccessible body pools as assessed by novel stable tracer pulse approach is reduced by aging independent of presence of COPD

BACKGROUND

Production rates of the short-chain fatty acids (SCFA) acetate, propionate, and butyrate, which are beneficial metabolites of the intestinal microbiota, are difficult to measure in humans due to inaccessibility of the intestine to perform measurements, and the high first-pass metabolism of SCFAs in colonocytes and liver. We developed a stable tracer pulse approach to estimate SCFA whole-body production (WBP) in the accessible pool representing the systemic circulation and interstitial fluid. Compartmental modeling of plasma enrichment data allowed us to additionally calculate SCFA kinetics and pool sizes in the inaccessible pool likely representing the intestine with microbiota. We also studied the effects of aging and the presence of Chronic Obstructive Pulmonary Disease (COPD) on SCFA kinetics.

METHODS

In this observational study, we designed a two-compartmental model to determine SCFA kinetics in 31 young (20-29 y) and 71 older (55-87 y) adults, as well as in 33 clinically stable patients with moderate to very severe COPD (mean (SD) FEV₁, 46.5 (16.2)% of predicted). Participants received in the fasted state a pulse containing stable tracers of acetate, propionate, and butyrate intravenously and blood was sampled four times over a 30 min period. We measured tracer-tracee ratios by GC-MS and used parameters obtained from two-exponential curve fitting to calculate non-compartmental SCFA WBP and perform compartmental analysis. Statistics were done by ANCOVA.

RESULTS

Acetate, propionate, and butyrate WBP and fluxes between the accessible and inaccessible pools were lower in older than young adults (all q < 0.0001). Moreover, older participants had lower acetate (q < 0.0001) and propionate (q = 0.019) production rates in the inaccessible pool as well as smaller sizes of the accessible and inaccessible acetate pools (both q < 0.0001) than young participants. WBP, compartmental SCFA kinetics, and pool sizes did not differ between COPD patients and older adults (all q > 0.05). Overall and independent of the group studied, calculated production rates in the inaccessible pool were on average 7 (acetate), 11 (propionate), and 16 (butyrate) times higher than non-compartmental WBP, and sizes of inaccessible pools were 24 (acetate), 31 (propionate), and 55 (butyrate) times higher than sizes of accessible pools (all p < 0.0001).

CONCLUSION

Non-compartmental production measurements of SCFAs in the accessible pool (i.e. systemic circulation) substantially underestimate the SCFA production in the inaccessible pool, which likely represents the intestine with microbiota, as assessed by compartmental analysis.

A low postabsorptive whole body protein balance is associated with markers of poor daily physical functioning in Chronic Obstructive Pulmonary Disease

BACKGROUND & AIMS

Postabsorptive whole body protein kinetics are related to age, gender, body mass index (BMI), and habitual protein intake level. It is unclear how protein synthesis, breakdown, and postabsorptive protein balance rates are affected in Chronic Obstructive Pulmonary Disease (COPD)) and whether these relate to disease severity, lifestyle characteristics and poor daily functioning.

METHODS

We studied 91 COPD (GOLD 1-4) and 56 age matched control subjects without COPD or other chronic or acute health disease/condition in the postabsorptive state and measured body composition by Dual-energy X-ray Absorptiometry, and disease severity and comorbidities by medical screening, blood analysis and questionnaires. We assessed whole body production rates of phenylalanine and tyrosine by pulse stable isotope tracer infusion to calculate whole body protein breakdown (PB) and hydroxylation of phenylalanine to tyrosine, representative of postabsorptive protein balance. We measured muscle and cognitive function, and physical performance by isokinetic dynamometry, cognitive assessments, and 6-min walk test. We assessed physical activity level, mood and dietary protein intake by questionnaires. We measured plasma enrichments by LC-MS/MS and statistics by Fisher's exact test or analysis of covariance. Data are mean [95% CI].

RESULTS

The COPD patients had moderate to severe airflow obstruction, multiple comorbidities, and elevated values for plasma high sensitivity c-reactive protein (hs-CRP) and glucose. Although PB (3630 [3361, 3900] vs 3504 [3297, 3711] umol/h, p = 0.1649) was not different, postabsorptive protein balance was lower in COPD patients (274.2 [242.4, 306.1] vs 212.9 [194.7, 231.0] umol/h, p < 0.0001), both compared to control subjects. A lower postabsorptive protein balance was associated with age (p < 0.0001) and higher levels for systolic blood pressure (p = 0.0051) and hs-CRP (p = 0.0046) but not with lung function. Furthermore, a lower postabsorptive protein balance level was associated with a lower intake of total calories and protein (p < 0.0001) and lower muscle strength (p = 0.0248), while only in COPD with a lower physical performance (p = 0.0343). We found no association with cognitive function or mood. For all subjects, a cumulative model that included group, gender, age, BMI, systolic blood pressure, hs-CRP, caloric intake, protein intake, and leg strength was able to explain 55% of the variation in postabsorptive protein balance.

CONCLUSION

These data suggest that systemic inflammation, high blood pressure and low protein intake are risk factors of a lower postabsorptive protein balance in COPD patients. A lower postabsorptive protein balance is associated with markers of poor daily physical functioning.

Disturbances in branched-chain amino acid profile and poor daily functioning in mildly depressed chronic obstructive pulmonary disease patients

BACKGROUND

Depression is one of the most common and untreated comorbidities in chronic obstructive pulmonary disease (COPD), and is associated with poor health outcomes (e.g. increased hospitalization/exacerbation rates). Although metabolic disturbances have been suggested in depressed non-diseased conditions, comprehensive metabolic phenotyping has never been conducted in those with COPD. We examined whether depressed COPD patients have certain clinical/functional features and exhibit a specific amino acid phenotype which may guide the development of targeted (nutritional) therapies.

METHODS

Seventy-eight outpatients with moderate to severe COPD (GOLD II-IV) were stratified based on presence of depression using a validated questionnaire. Lung function, disease history, habitual physical activity and protein intake, body composition, cognitive and physical performance, and quality of life were measured. Comprehensive metabolic flux analysis was conducted by pulse stable amino acid isotope administration. We obtained blood samples to measure postabsorptive kinetics (production and clearance rates) and plasma concentrations of amino acids by LC-MS/MS. Data are expressed as mean [95% CI]. Stats were done by graphpad Prism 9.1.0. ɑ < 0.05.

RESULTS

The COPD depressed (CD, n = 27) patients on average had mild depression, were obese (BMI: 31.7 [28.4, 34.9] kg/m2), and were characterized by shorter 6-min walk distance (P = 0.055), physical inactivity (P = 0.03), and poor quality of life (P = 0.01) compared to the non-depressed COPD (CN, n = 51) group. Lung function, disease history, body composition, cognitive performance, and daily protein intake were not different between the groups. In the CD group, plasma branched chain amino acid concentration (BCAA) was lower (P = 0.02), whereas leucine (P = 0.01) and phenylalanine (P = 0.003) clearance rates were higher. Reduced values were found for tyrosine plasma concentration (P = 0.005) even after adjustment for the large neutral amino acid concentration (= sum BCAA, tyrosine, phenylalanine and tryptophan) as a marker of dopamine synthesis (P = 0.048).

CONCLUSION

Mild depression in COPD is associated with poor daily performance and quality of life, and a set of metabolic changes in depressed COPD that include perturbation of large neutral amino acids, specifically the BCAAs. Trial registration clinicaltrials.gov: NCT01787682, 11 February 2013-Retrospectively registered; NCT02770092, 12 May 2016-Retrospectively registered; NCT02780219, 23 May 2016-Retrospectively registered; NCT03796455, 8 January 2019-Retrospectively registered.

Intestinal dysfunction in chronic disease

PURPOSE OF REVIEW

This review will discuss recent studies showing that patients with chronic wasting diseases suffer from a variety of small intestinal impairments which might negatively impact the colonic microbiota and overall well-being. New insights will be addressed as well as novel approaches to assess intestinal function.

RECENT FINDINGS

Small intestinal dysfunction can enhance the amount and alter the composition of undigested food reaching the colon. As a result of reduced protein digestion and absorption, a large amount of undigested protein might reach the colon promoting the presence of pathogenic colonic bacteria and a switch from bacterial fiber fermentation to protein fermentation. While microbial metabolites of fiber fermentation, such as short-chain fatty acids (SCFA), are mainly considered beneficial for overall health, metabolites of protein fermentation, i.e. ammonia, branched SCFAs, hydrogen sulfide, polyamines, phenols, and indoles, can exert beneficial or deleterious effects on overall health. Substantial advances have been made in the assessment of small intestinal dysfunction in chronic diseases, but studies investigating the connection to colonic microbial metabolism are needed. A promising new stable isotope approach can enable the measurement of metabolite production by the colonic microbiota.

SUMMARY

Several studies have been conducted to assess intestinal function in chronic diseases. Impairments in intestinal barrier function, sugar absorption, protein digestion, and absorption, as well as small intestinal bacterial overgrowth were observed and possibly might negatively impact colonic bacterial metabolism. We suggest that improving these perturbations will improve overall patient health.

Impact of β-hydroxy-β-methylbutyrate (HMB) on muscle loss and protein metabolism in critically ill patients: A RCT

PURPOSE

Muscle wasting deteriorates life quality after critical illness and increases mortality. Wasting starts upon admission to intensive care unit (ICU). We aimed to determine whether β-hydroxy-β-methylbutyrate (HMB), a metabolite of leucine, can attenuate this process.

METHODS

Prospective randomized, placebo-controlled double blind trial.

INCLUSION CRITERIA

ICU patients depending on mechanical ventilation on day 3 having a functional gastrointestinal tract. They were randomized to HMB (3 g/day) or placebo (maltodextrin) from day 4 on for 30 days.

PRIMARY OUTCOME

magnitude of loss of skeletal muscle area (SMA) of the quadriceps femoris measured by ultrasound at days 4 and 15.

SECONDARY OUTCOMES

body composition, change in protein metabolism assessed by amino acids tracer pulse, and global health at 60 days. Data are mean [95% CI]. Statistics by ANCOVA with correction for confounders sex, age and/or BMI.

RESULTS

Thirty patients completed the trial, aged 65 [59, 71] years, SAPS2 score 48 [43, 52] and SOFA 8.5 [7.4, 9.7]. The loss of total SMA was 11% between days 4 and 15 (p < 0.001), but not different between the groups (p = 0.86). In the HMB group, net protein breakdown (Δ Estimate HMB-Placebo: -153 [-242, -63]; p = 0.0021) and production of several amino acid was significantly reduced, while phase angle increased more (0.66 [0.09, 1.24]; p = 0.0247), and SF-12 global health improved more (Δ Estimate HMB-Placebo: 27.39 [1.594, 53.19], p = 0.04).

CONCLUSION

HMB treatment did not significantly reduce muscle wasting over 10 days of observation (primary endpoint), but resulted in significantly improved amino acid metabolism, reduced net protein breakdown, a higher phase angle and better global health. CLINICALTRIALS.

GOV IDENTIFIER

NCT03628365.

Comprehensive metabolic amino acid flux analysis in critically ill patients

Amino acid (AA) metabolism is severely disturbed in critically ill ICU patients. To be able to make a more scientifically based decision on the type of protein or AA nutrition to deliver in ICU patients, comprehensive AA phenotyping with measurements of plasma concentrations and whole body production (WBP) is needed. Therefore, we studied ICU patients and matched control subjects using a novel pulse isotope method to obtain in-depth metabolic analysis. In 51 critically ill ICU patients (SOFA~6.6) and 49 healthy controls, we measured REE and body composition/phase-angle using BIA. In the postabsorptive state, we collected arterial (ized) blood for CRP and AA. Then, we administered an 8 mL solution containing 18 stable AA tracers as a pulse and calculated WBP. Enrichments: LC-MS/MS and statistics: t-test, ANCOVA. Compared to healthy, critically ill ICU patients had lower phase-angle (p < 0.00001), and higher CRP (p < 0.0001). Most AA concentrations were lower in ICU patients (p < 0.0001), except tau-methylhistidine and phenylalanine. WBP of most AA were significantly (p < 0.0001) higher with increases in glutamate (160%), glutamine (46%), and essential AA. Remarkably, net protein breakdown was lower. There were only weak relationships between AA concentrations and WBP. Critically ill ICU patients (SOFA 8-16) had lower values for phase angle (p = 0.0005) and small reductions of most plasma AA concentrations, but higher tau-methylhistidine (p = 0.0223) and hydroxyproline (p = 0.0028). Remarkably, the WBP of glutamate and glutamine were lower (p < 0.05), as was their clearance, but WBP of tau-methylhistidine (p = 0.0215) and hydroxyproline (p = 0.0028) were higher. Our study in critically ill ICU patients shows that comprehensive metabolic phenotyping was able to reveal severe disturbances in specific AA pathways, in a disease severity dependent way. This information may guide improving nutritional compositions to improve the health of the critically ill patient. CLINICAL TRIAL REGISTRY: Data are from the baseline measurements of study NCT02770092 (URL: https://clinicaltrials.gov/ct2/show/NCT02770092) and NCT03628365 (URL: https://clinicaltrials.gov/ct2/show/NCT03628365).

Nitrogen recycling buffers against ammonia toxicity from skeletal muscle breakdown in hibernating arctic ground squirrels

Hibernation is a state of extraordinary metabolic plasticity. The pathways of amino acid metabolism as they relate to nitrogen homeostasis in hibernating mammals in vivo are unknown. Here we show, using pulse isotopic tracing, evidence of increased myofibrillar (skeletal muscle) protein breakdown and suppressed whole-body production of metabolites in vivo throughout deep torpor. As whole-body production of metabolites is suppressed, amino acids with nitrogenous side chains accumulate during torpor, while urea cycle intermediates do not. Using ¹⁵N stable isotope methodology in arctic ground squirrels (Urocitellus parryii), we provide evidence that free nitrogen is buffered and recycled into essential amino acids, non-essential amino acids and the gamma-glutamyl system during the inter-bout arousal period of hibernation. In the absence of nutrient intake or physical activity, our data illustrate the orchestration of metabolic pathways that sustain the provision of essential and non-essential amino acids and prevent ammonia toxicity during hibernation.

Activated whole-body arginine pathway in high-active mice

Our previous studies suggest that physical activity (PA) levels are potentially regulated by endogenous metabolic mechanisms such as the vasodilatory roles of nitric oxide (NO) production via the precursor arginine (ARG) and ARG-related pathways. We assessed ARG metabolism and its precursors [citrulline (CIT), glutamine (GLN), glutamate (GLU), ornithine (ORN), and phenylalanine (PHE)] by measuring plasma concentration, whole-body production (WBP), de novo ARG and NO production, and clearance rates in previously classified low-active (LA) or high-active (HA) mice. We assessed LA (n = 23) and HA (n = 20) male mice by administering a stable isotope tracer pulse via jugular catheterization. We measured plasma enrichments via liquid chromatography tandem mass spectrometry (LC-MS/MS) and body compostion by echo-MRI. WBP, clearance rates, and de novo ARG and NO were calculated. Compared to LA mice, HA mice had lower plasma concentrations of GLU (71.1%; 36.8 ± 2.9 vs. 17.5 ± 1.7μM; p<0.0001), CIT (21%; 57.3 ± 2.3 vs. 46.4 ± 1.5μM; p = 0.0003), and ORN (40.1%; 55.4 ± 7.3 vs. 36.9 ± 2.6μM; p = 0.0241), but no differences for GLN, PHE, and ARG. However, HA mice had higher estimated NO production ratio (0.64 ± 0.08; p = 0.0197), higher WBP for CIT (21.8%, 8.6 ± 0.2 vs. 10.7 ± 0.3 nmol/g-lbm/min; p<0.0001), ARG (21.4%, 35.0 ± 0.6 vs. 43.4 ± 0.7 nmol/g-lbm/min; p<0.0001), PHE (7.6%, 23.8 ± 0.5 vs. 25.6 ± 0.5 nmol/g-lbm/min; p<0.0100), and lower GLU (78.5%; 9.4 ± 1.1 vs. 4.1 ± 1.6 nmol/g lbm/min; p = 0.0161). We observed no significant differences in WBP for GLN, ORN, PHE, or de novo ARG. We concluded that HA mice have an activated whole-body ARG pathway, which may be associated with regulating PA levels via increased NO production.

Comprehensive metabolic flux analysis to explain skeletal muscle weakness in COPD

BACKGROUND & AIMS

Metabolic characterization of a well-defined group of patients could be a powerful tool in revealing metabolic signatures to explain limb muscle weakness in chronic diseases. Studies are currently limited in Chronic Obstructive Pulmonary Disease (COPD) to the identification of differential amino acid concentrations but lack comprehensive analysis of the flux through relevant muscle function related metabolic pathways.

METHODS

In 23 stable patients with moderate to very severe COPD and 19 healthy controls, a comprehensive metabolic flux analysis was conducted by administering an intravenous pulse and primed constant infusion of multiple stable tracers of amino acids known to play a role in muscle health. Blood samples were obtained to calculate production (WBP) and interconversion rates, and plasma concentrations of these amino acids. Lower and upper limb muscle strength, muscle mass, lung function, physical activity level, and disease history and characteristics were assessed.

RESULTS

The COPD group was characterized by lower and upper limb muscle weakness (P < 0.01) despite preserved muscle mass. Higher values were found in COPD for plasma glutamine, WBP of leucine (P < 0.001), 3-methylhistidine (P < 0.01) (marker of enhanced myofibrillar protein breakdown), citrulline (P < 0.05), and arginine to citrulline conversion (P < 0.05) (reflecting enhanced nitric oxide synthesis). Plasma concentration of β-hydroxy β-methylbutyrate (HMB with anticatabolic, anabolic and contractile properties), WBP of glycine (precursor of creatine and glutathione), and transcutaneous O₂ saturation explained up to 79% and 65% of the variation in strength of the lower and upper limb muscles, respectively, in COPD.

CONCLUSIONS

Comprehensive metabolic flux analysis revealed a homogenous metabolic signature in stable patients with COPD and a specific metabolic profile in those with skeletal muscle weakness.

CLINICAL TRIAL REGISTRY

ClinicalTrials.gov; No. NCT01787682; URL: www.clinicaltrials.gov.

New advances in stable tracer methods to assess whole-body protein and amino acid metabolism

PURPOSE OF REVIEW

Stable isotope methods have been used for many years to assess whole-body protein and amino acid kinetics in healthy conditions and in response to aging, exercise and (clinically stable) disease states.

RECENT FINDINGS

In recent years, tracer research expanded to the anabolic response to feeding in critical illness and its use during acute metabolic stressors. Furthermore, new isotope approaches and tracer insights have been obtained. In the postabsorptive state, the novel tracer pulse approach has several advantages above the established continuous tracer approach because of the metabolic information that can be obtained, easy applicability, and low tracer costs. The use of bolus versus sip-feeding approaches to assess the anabolic response to a meal is dependent on the research question and its feasibility. Promising new tracer approaches have been developed to measure the anabolic capacity, and protein digestibility and absorption. Advances have been made in the field of mass spectrometry in low enrichment analysis.

SUMMARY

Novel tracer approaches are available that can more readily be used in critical illness and during acute metabolic stressors. Besides the use of tracer application in various clinical conditions, more research is needed on how to incorporate isotopes on an individual level.

A novel triple-tracer approach to assess postprandial protein turnover

Insulin and nutrients have profound effects on proteome homeostasis. Currently no reliable methods are available to measure postprandial protein turnover. A triple-tracer method was developed using phenylalanine stable isotope tracers to estimate appearance rates of ingested (R_{a meal}) and endogenous phenylalanine and the rate of phenylalanine disposal (R_d). This was compared with the "traditional" dual-tracer method, using one (1-CM)- and two (2-CM)-compartment models. For both methods, [¹³C₆]phenylalanine was given orally, and [¹⁵N]phenylalanine was constantly infused; the triple-tracer method added [²H₅]phenylalanine, infused at rates to mimic meal [¹³C₆]phenylalanine appearance. Additionally, incorporation of meal-derived phenylalanine into specific proteins was measured after purification by two-dimensional electrophoresis. The triple-tracer approach reduced modeling errors, allowing improved reconstruction of R_{a meal} with a tracer-to-tracee ratio that was more constant and better estimated R_d. The 2-CM better described phenylalanine kinetics and R_d than 1-CM. Thus, the triple-tracer approach using 2-CM is superior for measuring non-steady-state postprandial protein turnover. This novel approach also allows measurement of postprandial synthesis rates of specific plasma proteins. We offer a valid non-steady-state model to measure postprandial protein turnover and synthesis of plasma proteins that can safely be applied in adults, children, and pregnant women.

A critical evaluation of the anabolic response after bolus or continuous feeding in COPD and healthy older adults

After bolus and continuous enteral feeding of the same protein, different digestion and absorption kinetics and anabolic responses are observed. Establishing which mode of feeding has the highest anabolic potential in patients with chronic obstructive pulmonary disease (COPD) may aid in the prevention of muscle wasting, but an important confounding factor is the duration of assessments after bolus feeding. We hypothesized that the anabolic response to bolus and continuous feeding in COPD patients is comparable when methodological issues are addressed. Twenty-one older adults (12 patients with stage II-IV COPD and 9 healthy controls) were studied after intake of a fast-absorbing hydrolyzed casein protein-carbohydrate mixture either as a single bolus or as small sips (crossover design). Whole body protein synthesis (PS), breakdown (PB), net PS (PS - PB) protein efficiency (netPSPE), net protein balance (phenylalanine (PHE) intake - PHE hydroxylation) protein efficiency (netBalPE), and splanchnic PHE extraction (SPE_PHE) were assessed using stable isotope tracer methodology. Bolus feeding assessments were done at 90, 95, and 99% of the calculated duration of the anabolic response. At 99%, netBalPE was higher for sip feeding than bolus feeding in both groups (P<0.0001). Nevertheless, bolus feeding was associated with a lower SPE_PHE (P<0.0001) and higher netPSPE (P<0.0001). At 90% compared with 99%, PS and netBalPE after bolus feeding was significantly overestimated. In conclusion, several factors complicate a comparison of the anabolic capacity of bolus and continuous feeding in acute studies, including the critical role of SPE calculation and assumptions, and the duration of postprandial assessments after bolus feeding.

Metabolic phenotyping using kinetic measurements in young and older healthy adults

BACKGROUND

The aging process is often associated with the presence of sarcopenia. Although changes in the plasma concentration of several amino acids have been observed in older adults, it remains unclear whether these changes are related to disturbances in whole body production and/or interconversions.

METHODS

We studied 10 healthy young (~22.7y) and 17 older adults (~64.8y) by administering a mixture of stable amino acid tracers in a pulse and in a primed constant infusion. We calculated whole body production (WBP) and metabolite to metabolite interconversions. In addition, we measured body composition, muscle function, and provided questionnaires to assess daily dietary intake, physical activity, mood (anxiety, depression) and markers of cognitive function. Plasma enrichments and metabolite concentrations were measured by GC- and LC-MS/MS and statistics were performed by student t-test.

RESULTS

Older adults had a 11% higher body mass index (p=0.04) and 27% reduced peak leg extension force (p=0.02) than the younger group, but comparable values for muscle mass, mood and cognitive function. Although small differences in several plasma amino acid concentrations were observed, we found older adults had about 40% higher values of WBP for glutamine (221±27 vs. 305±21μmol/kgffm/h, p=0.03) and tau-methylhistidine (0.15±0.01 vs. 0.21±0.02μmol/kgffm/h, p=0.04), 26% lower WBP value for arginine (59±4 vs. 44±4μmol/kgffm/h, p=0.02) and a reduction in WBP (50%; 1.23±0.15 vs. 0.69±0.06μmol/kgffm/h, p=0.001) and concentration (25%; 3.5±0.3μmol/l vs. 2.6±0.2μmol/l, p=0.01) for β-Hydroxy β-Methylbutyrate. No differences were observed in protein catabolism. Clearance of arginine was decreased (27%, p=0.03) and clearance of glutamine (58%, p=0.01), leucine (67%, p=0.001) and KIC (76%, p=0.004) were increased in older adults.

CONCLUSIONS

Specific differences exist between young and older adults in amino acid metabolism.