Effect of l-Arginine Infusion on Glucose Disposal during Exercise in Humans

Gut microbiota and metabolites exhibit different profiles after very-low-caloric restriction in patients with type 2 diabetes

Background and aims

To investigate the effect of short-term very-low-calorie restriction (VLCR) on metabolism in patients with type 2 diabetes (T2D), and elucidate the molecular mechanism through analyses on gut microbiota and small-molecule metabolites.

Methods

Fourteen T2D patients were hospitalized to receive VLCR (300-600 kcal/d) for 9 days. BMI, BP, and HR were taken before and after VLCR. Levels of blood lipids, fasting insulin, FBG, and 2h PBG were assessed. The microbial diversity in feces was detected by 16S rDNA high-throughput sequencing technology, and small-molecule metabolites in plasma and feces by untargeted metabolomics technology.

Results

After VLCR, BW, BMI, WC, BP, and levels of FBG and 2h PBG, insulin, HOMA-IR, and triglyceride decreased significantly in T2D patients (P<0.05). There was no significant change in the α-diversity of fecal microbiota, but the abundance of Bacteroidetes increased significantly, and the Firmicutes/Bacteroidetes ratio decreased significantly from 11.79 to 4.20. Parabacteroides distasonis showed an abundance having increased most prominently after VLCR treatment. Plasma level of amino acid metabolite L-arginine increased significantly. Plasma levels of three lipid metabolites, PC (14:0/20:4 [8Z, 11Z, 14Z, 17Z]), LysoPC (16:1 [9Z]) and LysoPC (18:1 [11Z]), were significantly reduced. Fecal levels of lipid metabolite LysoPC (18:1 [11Z]) and bile acid metabolite glycholic acid were significantly decreased.

Conclusion

In T2DM patients, VLCR can considerably reduce body weight and improve glucose and lipid metabolism without causing severe side effects. LysoPC (18:1 [11Z]) and Parabacteroides distasonis showed the most obvious difference after VLCR, which could be the indicators for VLCR in T2D.

Effect of arginine supplementation on the production of milk fat in dairy cows

Arginine, one of the conditionally essential AA, has been reported to affect fat synthesis and metabolism in nonruminant animals by influencing adenosine monophosphate activated protein kinase (AMPK) in some organs. In dairy cows, the effect of Arg on milk fat production is not clear, and any potential mechanism that underlies the effect is unknown. We tested the hypothesis that Arg infusion would improve the production of milk fat, and explored possible mechanism that might underlie any effect. We used 6 healthy lactating cows at 20 ± 2 d in milk, in fourth parity, with a body weight of 508 ± 14 kg, body condition score of 3.0 ± 0, and a milk yield of 30.6 ± 1.8 kg/d (mean ± standard deviation). The cows were blocked by days in milk and milk yield and each cow received 3 treatments in a replicated 3 × 3 Latin square design, with each of the experimental periods lasting 7 d with a 14-d washout between each period. The treatments, delivered in random order, were (1) infusion of saline (control); (2) infusion of 0.216 mol/d of l-Arg in saline (Arg); (3) infusion of 0.868 mol/d of l-Ala in saline (the Arg and Ala treatments were iso-nitrogenous) through a jugular vein. On the last day of each experimental period, blood was sampled to measure insulin, nitric oxide, glucose, and nonesterified fatty acid, and the liver and mammary gland were biopsied to measure the expression of genes. Milk yield was recorded, and milk fat percentage was measured daily during each of the experimental periods. The yield and composition of fatty acid (FA) in milk was measured daily on the last 3 d during each of the experimental periods. The data were analyzed using a mixed model with treatment as a fixed factor, and cow, period, and block as random factors. The daily milk yield and milk fat yield when the cows were infused with Arg were 2.2 kg and 76 g, respectively, higher than that in control, and 1.8 kg and 111 g, respectively, higher than that in Ala. When the cows were infused with Arg they had higher concentration and yield of de novo synthesized FA, than when they received the control or Ala infusions, although milk fat percentage, daily feed intake, and the digestibility of nutrients were not affected by treatment. The serum concentration of nitric oxide and insulin were higher during Arg than during control or Ala, with no difference between control and Ala. In the liver, the expression of the genes coding for AMPK (PRKAA1, PRKAB1, and PRKAG1) and genes related to the oxidation of FA were higher during Arg than during control or Ala, whereas in the mammary gland the expression PRKAB1 was lowest, and the expression of genes involved in the synthesis of milk fat were highest, during Arg infusion. The results suggest the intravenous infusion of Arg enhanced the production of milk fat by promoting the de novo synthesis of FA and increasing milk yield.

Factors Influencing AMPK Activation During Cycling Exercise: A Pooled Analysis and Meta-Regression

BACKGROUND

The 5' adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a cellular energy sensor that is activated by increases in the cellular AMP/adenosine diphosphate:adenosine triphosphate (ADP:ATP) ratios and plays a key role in metabolic adaptations to endurance training. The degree of AMPK activation during exercise can be influenced by many factors that impact on cellular energetics, including exercise intensity, exercise duration, muscle glycogen, fitness level, and nutrient availability. However, the relative importance of these factors for inducing AMPK activation remains unclear, and robust relationships between exercise-related variables and indices of AMPK activation have not been established.

OBJECTIVES

The purpose of this analysis was to (1) investigate correlations between factors influencing AMPK activation and the magnitude of change in AMPK activity during cycling exercise, (2) investigate correlations between commonly reported measures of AMPK activation (AMPK-α2 activity, phosphorylated (p)-AMPK, and p-acetyl coenzyme A carboxylase (p-ACC), and (3) formulate linear regression models to determine the most important factors for AMPK activation during exercise.

METHODS

Data were pooled from 89 studies, including 982 participants (93.8% male, maximal oxygen consumption [[Formula: see text]] 51.9 ± 7.8 mL kg^-1 min^-1). Pearson's correlation analysis was performed to determine relationships between effect sizes for each of the primary outcome markers (AMPK-α2 activity, p-AMPK, p-ACC) and factors purported to influence AMPK signaling (muscle glycogen, carbohydrate ingestion, exercise duration and intensity, fitness level, and muscle metabolites). General linear mixed-effect models were used to examine which factors influenced AMPK activation.

RESULTS

Significant correlations (r = 0.19-0.55, p < .05) with AMPK activity were found between end-exercise muscle glycogen, exercise intensity, and muscle metabolites phosphocreatine, creatine, and free ADP. All markers of AMPK activation were significantly correlated, with the strongest relationship between AMPK-α2 activity and p-AMPK (r = 0.56, p < 0.001). The most important predictors of AMPK activation were the muscle metabolites and exercise intensity.

CONCLUSION

Muscle glycogen, fitness level, exercise intensity, and exercise duration each influence AMPK activity during exercise when all other factors are held constant. However, disrupting cellular energy charge is the most influential factor for AMPK activation during endurance exercise.

Melatonin improves the antioxidant capacity in cardiac tissue of Wistar rats after exhaustive exercise

We investigated the effects of melatonin on the onset and resolution of the oxidative stress in the cardiac muscle in melatonin-treated and nontreated rats subjected to an exhaustive exercise session. Forty male rats were divided into: melatonin-treated (20 mg/kg supplemented for 10 d) and control. On the 10th day, each group was subdivided according to euthanasia moments: control or melatonin-treated not exercised (C0h and M0h); immediately after the exercise (CIA and MIA); and 2 h after exercise (C2h and M2h). The heart of animals was removed and the levels of oxidative stress index (OSI) and the formation of thiobarbituric acid reactive substances (TBARS), protein carbonyl, and the activities of aconitase, catalase (CAT), glutathione peroxidase (GPx), and superoxide dismutase (SOD) were evaluated. Total antioxidant status (TAS), total oxidant status (TOS), and the protein expression of CAT, GPx, and SOD was also measured. Our data revealed significant differences on: (i) OSI (p=.029), CAT activity (p=.016), CAT content (p<.001), GPx content (p=.014), reduced glutathione levels (p<.001), and aconitase activity (p<.001) for interaction of melatonin; (ii) GPx activity (p=.005), reduced glutathione (p=.004), protein carbonyl (p=.035), and TBARS levels (p=.028) between groups, and (iii) TBARS levels (p=.016) for significance between moments. Although the exhaustive exercise protocol imposed mild oxidative stress on the cardiac tissue of rats, melatonin induced antioxidant responses that rebalanced the redox status of the cardiac tissue, especially after exhaustive exercise.

Dihydrotestosterone (DHT) rapidly increase after maximal aerobic exercise in healthy males: the lowering effect of phosphodiesterase's type 5 inhibitors on DHT response to exercise-related stress

PURPOSE

Few data exist on dihydrotestosterone (DHT) adaptation to exercise-related stress. The aim of the study was to investigate on serum DHT and other androgens' responses to acute aerobic exercises, and to verify if a long-acting phosphodiesterase's type 5 inhibitors could influence these responses, as previously observed for salivary testosterone.

METHODS

In a double-blind cross over study, 12 healthy trained male volunteers were submitted to both an acute sub-maximal and maximal exercise tests on cycle ergometer, after randomly receiving a two days placebo or tadalafil administration (20 mg, Cialis^®, Ely-Lilly, Indianapolis, IN, USA). Blood sample collections were performed at different time points before and after exercise. Serum DHT, total testosterone (TT), dehydroepiandrosterone sulfate (DHEAS) and luteinizing hormone (LH), were assayed.

RESULTS

Serum DHT increase in placebo treatment immediately post maximal aerobic exercise and return to basal values at 60 min of recovery whereas tadalafil administration significantly reduced the DHT increase after exercise. The values of areas under curves showed the increase of TT after acute sub-maximal and maximal exercise and of DHEAS only after acute maximal aerobic exercise independently from treatment.

CONCLUSIONS

In addition to testosterone, also DHT plays an exercise-related adaptive role during high intensity aerobic exercise, but its rapid useful effects during exercise have to be determined. We hypothesized that the increased androgens secretion during exercise could be mainly related to steroidogenic enzymes modifications in peripheral tissues (i.e., muscles). Moreover, the blunting effect of tadalafil on DHT increase support a possible role of peripheral nitric oxide/GMPc related pathways in influencing physical-stress related DHT metabolism.

Nontargeted Metabolomic Analysis of Plasma Metabolite Changes in Patients with Adolescent Idiopathic Scoliosis

Objective

Adolescent idiopathic scoliosis (AIS) is a relatively common spinal rotation deformity, and the pathogenesis of AIS is accompanied by metabolic dysfunction and changes in biochemical factors. In this study, plasma metabolite changes in AIS patients were analyzed based on nontargeted metabolomics to provide new insights for clarifying functional metabolic abnormalities in AIS patients.

Methods

Clinical indexes and blood samples were collected from 12 healthy subjects and 16 AIS patients. Metabolomics was used to analyze the changes in metabolites in plasma samples. The correlation between plasma metabolites and clinical indexes was analyzed by the Spearman rank correlation coefficient.

Results

Analysis of clinical data showed that the body weight, body mass index (BMI), and bone mineral density (BMD) index of the AIS group significantly decreased, while the blood phosphorus and Cobb angles increased significantly. Metabolomic analysis showed significant changes in 72 differential metabolites in the plasma of the AIS group, mainly including organooxygen compounds, carboxylic acids and derivatives, fatty acyls, steroids and steroid derivatives, and keto acids and derivatives. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway showed that arginine biosynthesis, D-glutamine and D-glutamate metabolism, alanine, aspartate and glutamate metabolism, and citrate cycle (TCA cycle) were significantly enriched in the AIS and healthy groups. Spearman rank correlation coefficient analysis showed that the plasma metabolites C00026 (oxoglutarate), C00062 (L-arginine, arginine), C01042 (N-acetylaspartate), and C00158 (citrate) were significantly correlated with clinical indexes in AIS patients. In the healthy group, the plasma metabolites C00122 (fumarate), C00025 (glutamate and L-glutamic acid) and C00149 (malate, L-malic acid) were significantly correlated with clinical indexes, while C00624 (N-acetylglutamate) was not significantly correlated with the clinical indexes.

Conclusion

The occurrence of AIS led to changes in clinical indexes and plasma metabolites. Plasma biomarkers and functional metabolic pathways were correlated with clinical indexes, which might provide new insights for the diagnosis and treatment of AIS.

Plasma metabolites, especially lipid metabolites, are altered in pregnant women with gestational diabetes mellitus

BACKGROUND AND AIMS

Gestational diabetes mellitus (GDM) is a pathological condition of glucose intolerance associated with adverse pregnancy outcomes and increased risk of developing maternal type 2 diabetes later in life. Metabolomics is finding increasing use in the study of GDM. To date, GDM-specific metabolomic changes have not been completely elucidated.

MATERIALS AND METHODS

In this pilot study, metabolomics fingerprinting data, obtained by ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometry (UHPLC/Q-TOF-MS), of 54 healthy pregnant women and 49 patients with GDM at the second and third gestational trimesters were analyzed. Multilevel statistical methods were used to process complex metabolomic data from the retrospective cohorts.

RESULTS

Using univariate analysis (p < 0.05), 41 metabolites were identified as having the most significant differences between these two groups. Lipid metabolites, particularly glycerophospholipids, were the most prevalent class of altered compounds. In addition, metabolites with previously unknown connection to GDM - such as monoacylglycerol, dihydrobiopterin, and 13S-hydroxyoctadecadienoic acid - were identified with strong discriminative power. The main metabolic pathways affected by GDM included glycerophospholipid metabolism, linoleic acid metabolism, and D-arginine and D-ornithine metabolism.

CONCLUSION

Our data provide a comprehensive overview of metabolite changes at different stages of pregnancy, which offers further insights into the pathogenesis of GDM.

Accumulation of 8-hydroxydeoxyguanosine, L-arginine and Glucose Metabolites by Liver Tumor Cells Are the Important Characteristic Features of Metabolic Syndrome and Non-Alcoholic Steatohepatitis-Associated Hepatocarcinogenesis

To uncover mechanisms and explore novel biomarkers of obesity, type 2 diabetes (T2DM) and nonalcoholic steatohepatitis (NASH)-associated hepatocarcinogenesis, cellular and molecular alterations in the liver, and hepatocellular carcinomas (HCCs) were investigated in NASH model 60-week-old Tsumura, Suzuki, Obese Diabetic (TSOD) mice and NASH HCC patients. Markedly elevated lipid deposition, inflammation, fibrosis, and peroxisome proliferation in the liver, preneoplastic lesions, and HCCs of TSOD mice were accompanied by accumulation of polysaccharides in the cellular cytoplasm and nuclei and increase of oxidative DNA damage marker, 8-hydroxydeoxyguanosine (8-OHdG) formation in the liver and altered foci. Metabolomics of TSOD mice HCCs demonstrated significant elevation of the concentration of amino acid L-arginine, phosphocreatine, S-adenosylmethionine/S-adenosylhomocysteine ratio, adenylate, and guanylate energy charges in coordination with tremendous rise of glucose metabolites, mostly fructose 1,6-diphosphate. L-arginine accumulation in HCCs was associated with significant under-expression of arginase 1 (ARG1), suppression of the urea cycle, methionine and putrescine degradation pathways, activation of Ser and Thr kinase Akt AKT, phosphoinositide 3-kinase (PI3K), extracellular signal-regulated kinase 1/2 (ERK1/2) kinases, β-catenin, mammalian target of rapamycin (mTOR), and cell proliferation. Furthermore, clinicopathological analysis in 20 metabolic syndrome/NASH and 80 HCV-positive HCC patients demonstrated significant correlation of negative ARG1 expression with poor tumor differentiation, higher pathological stage, and significant decrease of survival in metabolic syndrome/NASH-associated HCC patients, thus indicating that ARG1 could become a potential marker for NASH HCC. From these results, formation of oxidative stress and 8-OHdG in the DNA and elevation of glucose metabolites and L-arginine due to ARG1 suppression in mice liver cells are the important characteristics of T2DM/NASH-associated hepatocarcinogenesis, which may take part in activating oxidative stress resistance, synthesis of phosphocreatine, cell signaling, methylation, and proliferation.

Skeletal muscle AMPK is not activated during 2 h of moderate intensity exercise at ∼65% V ̇ O 2 peak in endurance trained men

Key points

AMP‐activated protein kinase (AMPK) is considered a major regulator of skeletal muscle metabolism during exercise.

However, we previously showed that, although AMPK activity increases by 8–10‐fold during ∼120 min of exercise at ∼65% V˙O2peak in untrained individuals, there is no increase in these individuals after only 10 days of exercise training (longitudinal study).

In a cross‐sectional study, we show that there is also a lack of activation of skeletal muscle AMPK during 120 min of cycling exercise at 65% V˙O2peak in endurance‐trained individuals.

These findings indicate that AMPK is not an important regulator of exercise metabolism during 120 min of exercise at 65% V˙O2peak in endurance trained men.

It is important that more energy is directed towards examining other potential regulators of exercise metabolism.

Abstract

AMP‐activated protein kinase (AMPK) is considered a major regulator of skeletal muscle metabolism during exercise. Indeed, AMPK is activated during exercise and activation of AMPK by 5‐aminoimidazole‐4‐carboxyamide‐ribonucleoside (AICAR) increases skeletal muscle glucose uptake and fat oxidation. However, we have previously shown that, although AMPK activity increases by 8–10‐fold during ∼120 min of exercise at ∼65% V˙O2peak in untrained individuals, there is no increase in these individuals after only 10 days of exercise training (longitudinal study). In a cross‐sectional study, we examined whether there is also a lack of activation of skeletal muscle AMPK during 120 min of cycling exercise at 65% V˙O2peak in endurance‐trained individuals. Eleven untrained (UT; V˙O2peak = 37.9 ± 5.6 ml.kg⁻¹ min⁻¹) and seven endurance trained (ET; V˙O2peak = 61.8 ± 2.2 ml.kg⁻¹ min⁻¹) males completed 120 min of cycling exercise at 66 ± 4% V˙O2peak (UT: 100 ± 21 W; ET: 190 ± 15 W). Muscle biopsies were obtained at rest and following 30 and 120 min of exercise. Muscle glycogen was significantly (P < 0.05) higher before exercise in ET and decreased similarly during exercise in the ET and UT individuals. Exercise significantly increased calculated skeletal muscle free AMP content and more so in the UT individuals. Exercise significantly (P < 0.05) increased skeletal muscle AMPK α2 activity (4‐fold), AMPK αThr¹⁷² phosphorylation (2‐fold) and ACCβ Ser²²² phosphorylation (2‐fold) in the UT individuals but not in the ET individuals. These findings indicate that AMPK is not an important regulator of exercise metabolism during 120 min of exercise at 65% V˙O2peak in endurance trained men.

AMP‐activated protein kinase (AMPK) is considered a major regulator of skeletal muscle metabolism during exercise.

However, we previously showed that, although AMPK activity increases by 8–10‐fold during ∼120 min of exercise at ∼65% V˙O2peak in untrained individuals, there is no increase in these individuals after only 10 days of exercise training (longitudinal study).

In a cross‐sectional study, we show that there is also a lack of activation of skeletal muscle AMPK during 120 min of cycling exercise at 65% V˙O2peak in endurance‐trained individuals.

These findings indicate that AMPK is not an important regulator of exercise metabolism during 120 min of exercise at 65% V˙O2peak in endurance trained men.

It is important that more energy is directed towards examining other potential regulators of exercise metabolism.

Passive stretch regulates skeletal muscle glucose uptake independent of nitric oxide synthase

Skeletal muscle contraction increases glucose uptake via an insulin-independent mechanism. Signaling pathways arising from mechanical strain are activated during muscle contractions, and mechanical strain in the form of passive stretching stimulates glucose uptake. However, the exact mechanisms regulating stretch-stimulated glucose uptake are not known. Since nitric oxide synthase (NOS) has been implicated in the regulation of glucose uptake during ex vivo and in situ muscle contractions and during exercise, and NO is increased with stretch, we examined whether the increase in muscle glucose uptake during stretching involves NOS. We passively stretched isolated extensor digitorum longus muscles (15 min at ~100-130 mN) from control mice and mice lacking either neuronal NOSµ (nNOSµ) or endothelial NOS (eNOS) isoforms, as well as used pharmacological inhibitors of NOS. Stretch significantly increased muscle glucose uptake appoximately twofold ( P < 0.05), and this was unaffected by the presence of the NOS inhibitors N^G-monomethyl-l-arginine (100 µM) or N^G-nitro-l-arginine methyl ester (100 µM). Similarly, stretch-stimulated glucose uptake was not attenuated by deletion of either eNOS or nNOSµ isoforms. Furthermore, stretching failed to increase skeletal muscle NOS enzymatic activity above resting levels. These data clearly demonstrate that stretch-stimulated skeletal muscle glucose uptake is not dependent on NOS. NEW & NOTEWORTHY Passive stretching is known to activate muscle glucose uptake through mechanisms that partially overlap with contraction. We report that genetic knockout of endothelial nitric oxide synthase (NOS) or neuronal NOS or pharmacological NOS inhibition does not affect stretch-stimulated glucose uptake. Passive stretch failed to increase NOS activity above resting levels. This information is important for the study of signaling pathways that regulate stretch-stimulated glucose uptake and indicate that NOS should be excluded as a potential signaling factor in this regard.

A serum nuclear magnetic resonance-based metabolomic signature of antiphospholipid syndrome

Antiphospholipid syndrome (APS) is a rheumatic inflammatory chronic autoimmune disease inducing hypercoagulable state associated with vascular thrombosis and pregnancy loss in women. Cardiac, cerebral and vascular strokes in these patients are responsible for reduction in life expectancy. Timely diagnosis and accurate monitoring of disease are decisive to improve the accuracy of therapy. In the present work, we present a NMR-based metabolomic study of blood sera of APS patients. Our data show that individuals suffering APS have a characteristic metabolomic profile with abnormalities associated to the metabolism of methyl group donors, ketone bodies and amino acids. We have identified for the first time the metabolomic fingerprint characterizing APS disease having potential application to improve APS timely diagnosis and appropriate therapeutic approaches.

Acute effects of an arginine-based supplement on neuromuscular, ventilatory, and metabolic fatigue thresholds during cycle ergometry

The purpose of the present study was to examine the effects of an acute dose of an arginine-based supplement on the physical working capacity at the fatigue threshold (PWCFT), lactate threshold (LT), ventilatory threshold (VT), and peak oxygen uptake during incremental cycle ergometry. This study used a double-blinded, placebo-controlled, within-subjects crossover design. Nineteen untrained men (mean age ± SD = 22.0 ± 1.7 years) were randomly assigned to ingest either the supplement (3.0 g of arginine, 300 mg of grape seed extract, and 300 mg of polyethylene glycol) or placebo (microcrystalline cellulose) and performed an incremental test on a cycle ergometer for determination of PWCFT, LT, VT, and peak oxygen uptake. Following a 1-week period, the subjects returned to the laboratory and ingested the opposite substance (either supplement or placebo) prior to completing another incremental test to be reassessed for PWCFT, LT, VT, and peak oxygen uptake. The paired-samples t tests indicated there were significant (P < 0.05) mean differences between the arginine and placebo conditions for the PWCFT (192 ± 42 vs. 168 ± 53 W, respectively) and VT (2546 ± 313 vs. 2452 ± 342 mL·min(-1)), but not the LT (135 ± 26 vs. 138 ± 22 W), absolute peak oxygen uptake (3663 ± 445 vs. 3645 ± 438 mL·min(-1)), or relative peak oxygen uptake (46.5 ± 6.0 vs. 46.2 ± 5.0 mL·kg(-1)·min(-1)). These findings suggested that the arginine-based supplement may be used on an acute basis for delaying the onset of neuromuscular fatigue (i.e., PWCFT) and improving the VT in untrained individuals.

Role of nitric oxide in skeletal muscle glucose uptake during exercise

Nitric oxide is produced within skeletal muscle fibres and has various functions in skeletal muscle. There is evidence that NO may be essential for normal increases in skeletal muscle glucose uptake during contraction/exercise. Although there have been some discrepant results, it has been consistently demonstrated that inhibition of NO synthase (NOS) attenuates the increase in skeletal muscle glucose uptake during contraction in mouse and rat muscle ex vivo, during in situ contraction in rats and during exercise in humans. The NO-mediated increase in skeletal muscle glucose uptake during contraction/exercise is probably due to the modulation of intramuscular signalling that ultimately increases glucose transporter 4 (GLUT4) translocation and is, surprisingly, independent of blood flow. In this review, we discuss the evidence for and against a role of NO in regulating skeletal muscle glucose uptake during contraction/exercise and outline the possible mechanism(s) involved. Emerging findings regarding the role of neuronal NOS mu (nNOSμ) in this process are also discussed.

Sodium nitrate supplementation does not enhance performance of endurance athletes

PURPOSE

Supplementation with inorganic nitrate has been suggested to be an ergogenic aid for athletes as nitric oxide donor. The purpose of this study was to determine whether ingestion of inorganic sodium nitrate benefits well-trained athletes performing a 40-min exercise test in laboratory conditions. In addition, we investigated the effect of this supplement on plasma levels of endothelin-1 (ET-1) and in nitrated proteins.

METHODS

Thirteen trained athletes participated in this randomized, double-blind, crossover study. They performed a 40-min cycle ergometer distance-trial test after two 3-d periods of dietary supplementation with sodium nitrate (10 mg·kg of body mass) or placebo.

RESULTS

Concentration of plasma nitrate (256 ± 35 μM) and nitrite (334 ± 86 nM) increased significantly (P < 0.05) after nitrate supplementation compared with placebo (nitrate: 44 ± 11 μM; nitrite: 187 ± 43 nM). In terms of exercise performance, there were no differences in either the mean distance (nitrate: 26.4 ± 1.1 km; placebo: 26.3 ± 1.2 km; P = 0.61) or mean power output (nitrate: 258 ± 28 W; placebo: 257 ± 28 W; P = 0.89) between treatments. Plasma ET-1 increased significantly (P < 0.05) just after exercise in nitrate (4.0 ± 0.8 pg·mL) and placebo (2.4 ± 0.4 pg·mL) conditions. This increase was significantly greater (P < 0.05) in the nitrate group. Levels of nitrated proteins did not differ between treatments (nitrate: preexercise, 91% ± 23%; postexercise, 81% ± 23%; placebo: preexercise, 95% ± 20%; postexercise, 99% ± 19%).

CONCLUSION

Sodium nitrate supplementation did not improve a 40-min distance-trial performance in endurance athletes. In addition, concentration of plasma ET-1 increased significantly after exercise after supplementation with sodium nitrate.

No effect of acute L-arginine supplementation on O₂ cost or exercise tolerance

The extent to which dietary supplementation with the nitric oxide synthase (NOS) substrate, L-arginine (ARG), impacts on NO production and NO-mediated physiological responses is controversial. This randomised, double blinded, cross-over study investigated the effects of acute ARG supplementation on NO biomarkers, O₂ cost of exercise and exercise tolerance in humans. In one experiment, 15 subjects completed moderate- and severe-intensity running bouts after acute supplementation with 6 g ARG or placebo (PLA). In another experiment, eight subjects completed moderate- and severe-intensity cycling bouts after acute supplementation with 6 g ARG plus 25 g of carbohydrate (ARG + CHO) or an energy-matched dose of carbohydrate alone (CHO). The plasma nitrite concentration was not different after ARG (Pre: 204 ± 79; Post: 241 ± 114 nM; P > 0.05) or ARG + CHO consumption (Pre: 304 ± 57; Post: 335 ± 116 nM; P > 0.05). During moderate-intensity exercise, the steady-state pulmonary VO₂ was not different, relative to the respective placebo conditions, after ARG (PLA: 2,407 ± 318, ARG: 2,422 ± 333 mL min(-1)) or ARG + CHO (CHO: 1,695 ± 304, ARG + CHO: 1,712 ± 312 mL min(-1)) ingestion (P > 0.05). The tolerable duration of severe exercise was also not significantly different (P > 0.05) after ingesting ARG (PLA: 551 ± 140, ARG: 552 ± 150 s) or ARG + CHO (CHO: 457 ± 182, ARG + CHO: 441 ± 221 s). In conclusion, acute dietary supplementation with ARG or ARG + CHO did not alter biomarkers of NO synthesis, O₂ cost of exercise or exercise tolerance in healthy subjects.

Skeletal muscle nitric oxide signaling and exercise: a focus on glucose metabolism

Nitric oxide (NO) is an important vasodilator and regulator in the cardiovascular system, and this link was the subject of a Nobel prize in 1998. However, NO also plays many other regulatory roles, including thrombosis, immune function, neural activity, and gastrointestinal function. Low concentrations of NO are thought to have important signaling effects. In contrast, high concentrations of NO can interact with reactive oxygen species, causing damage to cells and cellular components. A less-recognized site of NO production is within skeletal muscle, where small increases are thought to have beneficial effects such as regulating glucose uptake and possibly blood flow, but higher levels of production are thought to lead to deleterious effects such as an association with insulin resistance. This review will discuss the role of NO in skeletal muscle during and following exercise, including in mitochondrial biogenesis, muscle efficiency, and blood flow with a particular focus on its potential role in regulating skeletal muscle glucose uptake during exercise.

L-Arginine but not L-glutamine likely increases exogenous carbohydrate oxidation during endurance exercise

The addition of L-arginine or L-glutamine to glucose-electrolyte solutions can increase intestinal water, glucose, and sodium absorption in rats and humans. We evaluated the utility of L-arginine and L-glutamine in energy-rehydration beverages through assessment of exogenous glucose oxidation and perceptions of exertion and gastrointestinal distress during endurance exercise. Eight cyclists rode 150 min at 50% of peak power on four occasions while ingesting solutions at a rate of 150 mL 15 min(-1) that contained (13)C-enriched glucose (266 mmol L(-1)) and sodium citrate ([Na(+)] 60 mmol L(-1)), and either: 4.25 mmol L(-1) L-arginine or 45 mmol L(-1) L-glutamine, and as controls glucose only or no glucose. Relative to glucose only, L-arginine invoked a likely 12% increase in exogenous glucose oxidation (90% confidence limits: ± 8%); however, the effect of L-glutamine was possibly trivial (4.5 ± 7.3%). L-Arginine also led to very likely small reductions in endogenous fat oxidation rate relative to glucose (12 ± 4%) and L-glutamine (14 ± 4%), and relative to no glucose, likely reductions in exercise oxygen consumption (2.6 ± 1.5%) and plasma lactate concentration (0.20 ± 0.16 mmol L(-1)). Effects on endogenous and total carbohydrate oxidation were inconsequential. Compared with glucose only, L-arginine and L-glutamine caused likely small-moderate effect size increases in perceptions of stomach fullness, abdominal cramp, exertion, and muscle tiredness during exercise. Addition of L-arginine to a glucose and electrolyte solution increases the oxidation of exogenous glucose and decreases the oxygen cost of exercise, although the mechanisms responsible and impact on endurance performance require further investigation. However, L-arginine also increases subjective feelings of gastrointestinal distress, which may attenuate its other benefits.