Dietary nitrate-induced increases in human muscle power: high versus low responders

Maximal neuromuscular power is an important determinant of athletic performance and also quality of life, independence, and perhaps even mortality in patient populations. We have shown that dietary nitrate (NO₃⁻), a source of nitric oxide (NO), improves muscle power in some, but not all, subjects. The present investigation was designed to identify factors contributing to this interindividual variability. Healthy men (n = 13) and women (n = 7) 22–79 year of age and weighing 52.1–114.9 kg were studied using a randomized, double‐blind, placebo‐controlled, crossover design. Subjects were tested 2 h after ingesting beetroot juice (BRJ) either containing or devoid of 12.3 ± 0.8 mmol of NO₃⁻. Plasma NO₃⁻ and nitrite (NO₂⁻) were measured as indicators of NO bioavailability and maximal knee extensor speed (V_max), power (P_max), and fatigability were determined via isokinetic dynamometry. On average, dietary NO₃⁻ increased (P < 0.05) P_max by 4.4 ± 8.1%. Individual changes, however, ranged from −9.6 to +26.8%. This interindividual variability was not significantly correlated with age, body mass (inverse of NO₃⁻ dose per kg), body mass index (surrogate for body composition) or placebo trial V_max or fatigue index (in vivo indicators of muscle fiber type distribution). In contrast, the relative increase in Pmax was significantly correlated (r = 0.60; P < 0.01) with the relative increase in plasma NO₂⁻ concentration. In multivariable analysis female sex also tended (P = 0.08) to be associated with a greater increase in Pmax. We conclude that the magnitude of the dietary NO₃⁻‐induced increase in muscle power is dependent upon the magnitude of the resulting increase in plasma NO₂⁻ and possibly female sex.

Measurement of nitrate and nitrite in biopsy-sized muscle samples using HPLC

Studies of rats have indicated that skeletal muscle plays a central role in whole-body nitrate ( NO3- )/nitrite ( NO2- )/nitric oxide (NO) metabolism. Extending these results to humans, however, is challenging due to the small size of needle biopsy samples. We therefore developed a method to precisely and accurately quantify NO3- and NO2- in biopsy-sized muscle samples. NO3- and NO2- were extracted from rat soleus samples using methanol combined with mechanical homogenization + ultrasound, bead beating, pulverization at liquid N₂ temperature or pulverization + 0.5% Triton X-100. After centrifugation to remove proteins, NO3- and NO2- were measured using HPLC. Mechanical homogenization + ultrasound resulted in the lowest NO3- content (62 ± 20 pmol/mg), with high variability [coefficient of variation (CV) >50%] across samples from the same muscle. The NO2- / NO3- ratio (0.019 ± 0.006) was also elevated, suggestive of NO3- reduction during tissue processing. Bead beating or pulverization yielded lower NO2- and slightly higher NO3- levels, but reproducibility was still poor. Pulverization + 0.5% Triton X-100 provided the highest NO3- content (124 ± 12 pmol/mg) and lowest NO2- / NO3- ratio (0.008 ± 0.001), with the least variability between duplicate samples (CV ~15%). These values are consistent with literature data from larger rat muscle samples analyzed using chemiluminescence. Samples were stable for at least 5 wk at -80°C, provided residual xanthine oxidoreductase activity was blocked using 0.1 mmol/l oxypurinol. We have developed a method capable of measuring NO3- and NO2- in <1 mg of muscle. This method should prove highly useful in investigating the role of skeletal muscle in NO3- / NO2- /NO metabolism in human health and disease. NEW & NOTEWORTHY Measurement of nitrate and especially nitrite in small, i.e., biopsy-sized, muscle samples is analytically challenging. We have developed a precise, accurate, and convenient method for doing so using an affordable commercial HPLC system.

Dietary Nitrate Enhances the Contractile Properties of Human Skeletal Muscle

We review recent studies of the effects of dietary nitrate on human muscle contractile function and discuss possible underlying mechanisms.

Dietary nitrate, a source of nitric oxide (NO), improves the contractile properties of human muscle. We present the hypothesis that this is due to nitrosylation of the ryanodine receptor and increased NO signaling via the soluble guanyl cyclase-cyclic guanosine monophosphate-protein kinase G pathway, which together increase the free intracellular Ca²⁺ concentration along with the Ca²⁺ sensitivity of the myofilaments themselves.

Dietary nitrate's effects on exercise performance in heart failure with reduced ejection fraction (HFrEF)

Heart failure with reduced ejection fraction (HFrEF) is a deadly and disabling disease. A key derangement contributing to impaired exercise performance in HFrEF is decreased nitric oxide (NO) bioavailability. Scientists recently discovered the inorganic nitrate pathway for increasing NO. This has advantages over organic nitrates and NO synthase production of NO. Small studies using beetroot juice as a source of inorganic nitrate demonstrate its power to improve exercise performance in HFrEF. A larger-scale trial is now underway to determine if inorganic nitrate may be a new arrow for physicians' quiver of HFrEF treatments.

Bariatric Surgery-Induced Cardiac and Lipidomic Changes in Obesity-Related Heart Failure with Preserved Ejection Fraction

OBJECTIVE

To determine the effects of gastric bypass on myocardial lipid deposition and function and the plasma lipidome in women with obesity and heart failure with preserved ejection fraction (HFpEF).

METHODS

A primary cohort (N = 12) with HFpEF and obesity underwent echocardiography and magnetic resonance spectroscopy both before and 3 months and 6 months after bariatric surgery. Plasma lipidomic analysis was performed before surgery and 3 months after surgery in the primary cohort and were confirmed in a validation cohort (N = 22).

RESULTS

After surgery-induced weight loss, Minnesota Living with Heart Failure questionnaire scores, cardiac mass, and liver fat decreased (P < 0.02, P < 0.001, and P = 0.007, respectively); echo-derived e' increased (P = 0.03), but cardiac fat was unchanged. Although weight loss was associated with decreases in many plasma ceramide and sphingolipid species, plasma lipid and cardiac function changes did not correlate.

CONCLUSIONS

Surgery-induced weight loss in women with HFpEF and obesity was associated with improved symptoms, reverse cardiac remodeling, and improved relaxation. Although weight loss was associated with plasma sphingolipidome changes, cardiac function improvement was not associated with lipidomic or myocardial triglyceride changes. The results of this study suggest that gastric bypass ameliorates obesity-related HFpEF and that cardiac fat deposition and lipidomic changes may not be critical to its pathogenesis.

Dietary Nitrate Increases VO2peak and Performance but Does Not Alter Ventilation or Efficiency in Patients With Heart Failure With Reduced Ejection Fraction

BACKGROUND

Patients with heart failure with reduced ejection fraction (HFrEF) exhibit lower efficiency, dyspnea, and diminished peak oxygen uptake (VO₂peak) during exercise. Dietary nitrate (NO₃^-), a source of nitric oxide (NO), has improved these measures in some studies of other populations. We determined the effects of acute NO₃^- ingestion on exercise responses in 8 patients with HFrEF using a randomized, double-blind, placebo-controlled, crossover design.

METHODS AND RESULTS

Plasma NO₃^-, nitrite (NO₂^-), and breath NO were measured at multiple time points and respiratory gas exchange was determined during exercise after ingestion of beetroot juice containing or devoid of 11.2 mmol of NO₃^-. NO₃^- intake increased (P < .05-0.001) plasma NO₃^- and NO₂^- and breath NO by 1469 ± 245%, 105 ± 34%, and 60 ± 18%, respectively. Efficiency and ventilation during exercise were unchanged. However, NO₃^- ingestion increased (P < .05) VO₂peak by 8 ± 2% (ie, from 21.4 ± 2.1 to 23.0 ± 2.3 mL^.min^-1.kg^-1). Time to fatigue improved (P < .05) by 7 ± 3 % (ie, from 582 ± 84 to 612 ± 81 seconds).

CONCLUSIONS

Acute dietary NO₃^- intake increases VO₂peak and performance in patients with HFrEF. These data, in conjunction with our recent data demonstrating that dietary NO₃^- also improves muscle contractile function, suggest that dietary NO₃^- supplementation may be a valuable means of enhancing exercise capacity in this population.

Sex affects myocardial blood flow and fatty acid substrate metabolism in humans with nonischemic heart failure

BACKGROUND

In animal models of heart failure (HF), myocardial metabolism shifts from high-energy fatty acid (FA) metabolism toward glucose. However, FA (vs glucose) metabolism generates more ATP/mole; thus, FA metabolism may be especially advantageous in HF. Sex modulates myocardial blood flow (MBF) and substrate metabolism in normal humans. Whether sex affects MBF and metabolism in patients with HF is unknown.

METHODS AND RESULTS

We studied 19 well-matched men and women with nonischemic HF (EF ≤ 35%). MBF and myocardial substrate metabolism were quantified using positron emission tomography. Women had higher MBF (mL/g/minute), FA uptake (mL/g/minute), and FA utilization (nmol/g/minute) (P < 0.005, P < 0.005, P < 0.05, respectively) and trended toward having higher FA oxidation than men (P = 0.09). These findings were independent of age, obesity, and insulin resistance. There were no sex-related differences in fasting myocardial glucose uptake or metabolism. Higher MBF was related to improved event-free survival (HR 0.31, P = 0.02).

CONCLUSIONS

In nonischemic HF, women have higher MBF and FA uptake and metabolism than men, irrespective of age, obesity, or insulin resistance. Moreover, higher MBF portends a better prognosis. These sex-related differences should be taken into account in the development and targeting of novel agents aimed at modulating MBF and metabolism in HF.

Oximetric angiosome imaging in diabetic feet

PURPOSE

To develop a noncontrast oximetric angiosome imaging approach to assess skeletal muscle oxygenation in diabetic feet.

MATERIALS AND METHODS

In four healthy and five subjects with diabetes, the feasibility of foot oximetry was examined using a 3T clinical magnetic resonance imaging (MRI) scanner. The subjects' feet were scanned at rest and during a toe-flexion isometric exercise. The oxygen extraction fraction of skeletal muscle was measured using a susceptibility-based MRI method. Our newly developed MR foot oximetric angiosome model was compared with the traditional angiosome model in the assessment of the distribution of oxygen extraction fraction.

RESULTS

Using the traditional angiosome during the toe-flexion exercise, the oxygen extraction fraction in the medial foot of healthy subjects increased (4.9 ± 3%) and decreased (-2.7 ± 4.4%) in subjects with diabetes (difference = 7.6%; 95% confidence interval = -13.7 ± 1.4; P = 0.02). Using the oximetric angiosome, the percent difference in the areas of oxygen extraction fraction within the 0.7-1.0 range (expected oxygen extraction fraction during exercise) between rest and exercise was higher in healthy subjects (8 ± 4%) than in subjects with diabetes (4 ± 4%; P = 0.02).

CONCLUSION

This study demonstrates the feasibility of measuring skeletal muscle oxygen extraction fraction in the foot muscle during a toe-flexion isometric exercise. Instead of assessing oxygen extraction fraction in a foot muscle region linked to a supplying artery (traditional angiosome), the foot oximetric angiosome model assesses oxygen extraction fraction by its different levels in all foot muscle regions and thus may be more appropriate for assessing local ischemia in ulcerated diabetic feet. J. Magn. Reson. Imaging 2016. J. MAGN. RESON. IMAGING 2016;44:940-946.

Dietary Nitrate and Skeletal Muscle Contractile Function in Heart Failure

Heart failure (HF) patients suffer from exercise intolerance that diminishes their ability to perform normal activities of daily living and hence compromises their quality of life. This is due largely to detrimental changes in skeletal muscle mass, structure, metabolism, and function. This includes an impairment of muscle contractile performance, i.e., a decline in the maximal force, speed, and power of muscle shortening. Although numerous mechanisms underlie this reduction in contractility, one contributing factor may be a decrease in nitric oxide (NO) bioavailability. Consistent with this, recent data demonstrate that acute ingestion of NO3 (-)-rich beetroot juice, a source of NO via the NO synthase-independent enterosalivary pathway, markedly increases maximal muscle speed and power in HF patients. This review discusses the role of muscle contractile dysfunction in the exercise intolerance characteristic of HF, and the evidence that dietary NO3 (-) supplementation may represent a novel and simple therapy for this currently underappreciated problem.

Effect of Ambrisentan on Exercise Capacity in Adult Patients After the Fontan Procedure

The Fontan operation is a common end point for children born with a single functional ventricle. Fontan patients typically experience physiological deterioration leading to transplant or death in their third or fourth decades of life. This deterioration is partially attributable to progressive increases in pulmonary vascular resistance (PVR) and as such endothelin receptor antagonists, which are known to decrease pulmonary vascular resistance, have been proposed as potentially beneficial in this population. We conducted a single-center, randomized, double-blind, placebo-controlled, crossover study of 12 weeks of ambrisentan therapy (10 mg per day) versus placebo to test the hypothesis that endothelin receptor antagonism will improve cardiopulmonary exercise test parameters and 36-item short form (SF-36) assessed quality of life in adult Fontan patients. Twenty-eight patients entered the trial, 19 patients completed the protocol. Ambrisentan therapy improved peak oxygen consumption by 1.7 ml/kg/min in patients who achieved a respiratory exchange ratio of >0.95 (p = 0.05) and decreased the slope of the ventilatory equivalent ratio for oxygen (-2.8, p = 0.019) in all completers. It did not change SF-36 physical function score compared with placebo (p = 0.28). Ambrisentan therapy resulted in a decrease in (-1.4 g/dl, p <0.001) with no change in liver or renal function. Therapy was generally well tolerated, with no greater rate of side effects than placebo. In conclusion, ambrisentan is well tolerated and improves exercise capacity in adult Fontan patients.

A Diet Rich in Medium-Chain Fatty Acids Improves Systolic Function and Alters the Lipidomic Profile in Patients With Type 2 Diabetes: A Pilot Study

CONTEXT

Excessive cardiac long-chain fatty acid (LCFA) metabolism/storage causes cardiomyopathy in animal models of type 2 diabetes. Medium-chain fatty acids (MCFAs) are absorbed and oxidized efficiently. Data in animal models of diabetes suggest MCFAs may benefit the heart.

OBJECTIVE

Our objective was to test the effects of an MCFA-rich diet vs an LCFA-rich diet on plasma lipids, cardiac steatosis, and function in patients with type 2 diabetes.

DESIGN

This was a double-blind, randomized, 2-week matched-feeding study.

SETTING

The study included ambulatory patients in the general community.

PATIENTS

Sixteen patients, ages 37-65 years, with type 2 diabetes, an ejection fraction greater than 45%, and no other systemic disease were included.

INTERVENTION

Fourteen days of a diet rich in MCFAs or LCFAs, containing 38% as fat in total, was undertaken.

MAIN OUTCOME MEASURES

Cardiac steatosis and function were the main outcome measures, with lipidomic changes considered a secondary outcome.

RESULTS

The relatively load-independent measure of cardiac contractility, S', improved in the MCFA group (P < .05). Weight-adjusted stroke volume and cardiac output decreased in the LCFA group (both P < .05). The MCFA, but not the LCFA, diet decreased several plasma sphingolipids, ceramide, and acylcarnitines implicated in diabetic cardiomyopathy, and changes in several sphingolipids correlated with improved fasting insulins.

CONCLUSIONS

Although a diet high in MCFAs does not change cardiac steatosis, our findings suggest that the MCFA-rich diet alters the plasma lipidome and may benefit or at least not harm cardiac function and fasting insulin levels in humans with type 2 diabetes. Larger, long-term studies are needed to further evaluate these effects in less-controlled settings.

Acute Dietary Nitrate Intake Improves Muscle Contractile Function in Patients With Heart Failure: A Double-Blind, Placebo-Controlled, Randomized Trial

BACKGROUND

Skeletal muscle strength, velocity, and power are markedly reduced in patients with heart failure, which contributes to their impaired exercise capacity and lower quality of life. This muscle dysfunction may be partially because of decreased nitric oxide (NO) bioavailability. We therefore sought to determine whether ingestion of inorganic nitrate (NO3 (-)) would increase NO production and improve muscle function in patients with heart failure because of systolic dysfunction.

METHODS AND RESULTS

Using a double-blind, placebo-controlled, randomized crossover design, we determined the effects of dietary NO3 (-) in 9 patients with heart failure. After fasting overnight, subjects drank beetroot juice containing or devoid of 11.2 mmol of NO3 (-). Two hours later, muscle function was assessed using isokinetic dynamometry. Dietary NO3 (-) increased (P<0.05-0.001) breath NO by 35% to 50%. This was accompanied by 9% (P=0.07) and 11% (P<0.05) increases in peak knee extensor power at the 2 highest movement velocities tested (ie, 4.71 and 6.28 rad/s). Maximal power (calculated by fitting peak power data with a parabola) was therefore greater (ie, 4.74±0.41 versus 4.20±0.33 W/kg; P<0.05) after dietary NO3 (-) intake. Calculated maximal velocity of knee extension was also higher after NO3 (-) ingestion (ie, 12.48±0.95 versus 11.11±0.53 rad/s; P<0.05). Blood pressure was unchanged, and no adverse clinical events occurred.

CONCLUSIONS

In this pilot study, acute dietary NO3 (-) intake was well tolerated and enhanced NO bioavailability and muscle power in patients with systolic heart failure. Larger-scale studies should be conducted to determine whether the latter translates into an improved quality of life in this population.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT01682356.

Type 2 diabetes, obesity, and sex difference affect the fate of glucose in the human heart

Type 2 diabetes, obesity, and sex difference affect myocardial glucose uptake and utilization. However, their effect on the intramyocellular fate of glucose in humans has been unknown. How the heart uses glucose is important, because it affects energy production and oxygen efficiency, which in turn affect heart function and adaptability. We hypothesized that type 2 diabetes, sex difference, and obesity affect myocardial glucose oxidation, glycolysis, and glycogen production. In a first-in-human study, we measured intramyocardiocellular glucose metabolism from time-activity curves generated from previously obtained positron emission tomography scans of 110 subjects in 3 groups: nonobese, obese, and diabetes. Group and sex difference interacted in the prediction of all glucose uptake, utilization, and metabolism rates. Group independently predicted fractional glucose uptake and its components: glycolysis, glycogen deposition, and glucose oxidation rates. Sex difference predicted glycolysis rates. However, there were fewer differences in glucose metabolism between diabetic patients and others when plasma glucose levels were included in the modeling. The potentially detrimental effects of obesity and diabetes on myocardial glucose metabolism are more pronounced in men than women. This sex difference dimorphism needs to be taken into account in the design, trials, and application of metabolic modulator therapy. Slightly higher plasma glucose levels improve depressed glucose oxidation and glycogen deposition rates in diabetic patients.

In vivo creatine kinase reaction kinetics at rest and stress in type II diabetic rat heart

The effects of type II diabetes on cardiac creatine kinase (CK) enzyme activity and/or flux are unknown. We therefore measured steady‐state phosphocreatine (PCr) and adenosine triphosphate (ATP) content and forward CK reaction kinetic parameters in Zucker Diabetic Fatty (ZDF) rat hearts, a type II diabetes research model. At baseline the PCr to ATP ratio (PCr/ATP) was significantly lower in diabetic heart when compared with matched controls (1.71 ± 0.21 vs. 2.26 ± 0.24, P < 0.01). Furthermore, the forward CK reaction rate constant (k_f) was higher in diabetic animals (0.52 ± 0.09 s⁻¹ vs. 0.35 ± 0.06 s⁻¹, P < 0.01) and CK flux calculated as a product of PCr concentration ([PCr]) and k_f was similar between two groups (4.32 ± 1.05 μmol/g/s vs. 4.94 ± 1.23 μmol/g/s, P = 0.20). Dobutamine administration resulted in similar increases in heart rate (~38%) and k_f (~0.12 s⁻¹) in both groups. No significant change in PCr and ATP content was observed with dobutamine. In summary, our data showed reduced PCr/ATP in diabetic myocardium as an indicator of cardiac energy deficit. The forward CK reaction rate constant is elevated at baseline which might reflect a compensatory mechanics to support energy flux through the CK shuttle and maintain constant ATP supply. When hearts were stimulated similar increase in k_f was observed in both groups thus it seems that CK shuttle does not limit ATP supply for the range of workload studied.

Noninvasive ³¹P MRS was used to measure PCr concentration ([PCr]) and creatine kinase (CK) reaction flux in type II diabetic rat hearts. [PCr] was reduced in diabetic myocardium as compared to controls, indicative of impairment in mitochondrial ATP production. The forward CK reaction rate constant was elevated, possibly reflecting a compensatory mechanism to support increased flux through the CK shuttle required to support cardiac work. CK reaction velocity increased in both diabetic and control hearts to maintain constant ATP content at higher work.

Effect of acute dietary nitrate intake on maximal knee extensor speed and power in healthy men and women

Nitric oxide (NO) has been demonstrated to enhance the maximal shortening velocity and maximal power of rodent muscle. Dietary nitrate (NO3(-)) intake has been demonstrated to increase NO bioavailability in humans. We therefore hypothesized that acute dietary NO3(-) intake (in the form of a concentrated beetroot juice (BRJ) supplement) would improve muscle speed and power in humans. To test this hypothesis, healthy men and women (n = 12; age = 22-50 y) were studied using a randomized, double-blind, placebo-controlled crossover design. After an overnight fast, subjects ingested 140 mL of BRJ either containing or devoid of 11.2 mmol of NO3(-). After 2 h, knee extensor contractile function was assessed using a Biodex 4 isokinetic dynamometer. Breath NO levels were also measured periodically using a Niox Mino analyzer as a biomarker of whole-body NO production. No significant changes in breath NO were observed in the placebo trial, whereas breath NO rose by 61% (P < 0.001; effect size = 1.19) after dietary NO3(-) intake. This was accompanied by a 4% (P < 0.01; effect size = 0.74) increase in peak knee extensor power at the highest angular velocity tested (i.e., 6.28 rad/s). Calculated maximal knee extensor power was therefore greater (i.e., 7.90 ± 0.59 vs. 7.44 ± 0.53 W/kg; P < 0.05; effect size = 0.63) after dietary NO3(-) intake, as was the calculated maximal velocity (i.e., 14.5 ± 0.9 vs. 13.1 ± 0.8 rad/s; P < 0.05; effect size = 0.67). No differences in muscle function were observed during 50 consecutive knee extensions performed at 3.14 rad/s. We conclude that acute dietary NO3(-) intake increases whole-body NO production and muscle speed and power in healthy men and women.

A "PET" area of interest: myocardial metabolism in human systolic heart failure

Myocardial substrate metabolism provides the energy needed for cardiac contraction and relaxation. The normal adult heart uses predominantly fatty acids (FAs) as its primary fuel source. However, the heart can switch and use glucose (and to a lesser extent, ketones, lactate, as well as endogenous triglycerides and glycogen), depending on the metabolic milieu and superimposed conditions. FAs are not a wholly better fuel than glucose, but they do provide more energy per mole than glucose. Conversely, glucose is the more oxygen-efficient fuel. Studies in animal models of heart failure (HF) fairly consistently demonstrate a shift away from myocardial fatty acid metabolism and toward glucose metabolism. Studies in humans are less consistent. Some show the same metabolic switch away from FA metabolism but not all. This may be due to differences in the etiology of HF, sex-related differences, or other mitigating factors. For example, obesity, insulin resistance, and diabetes are all related to an increased risk of HF and may complicate or contribute to its development. However, these conditions are associated with increased FA metabolism. This review will discuss aspects of human heart metabolism in systolic dysfunction as measured by the noninvasive, quantitative method-positron emission tomography. Continued research in this area is vital if we are to ameliorate HF by manipulating heart metabolism with the aim of increasing energy production and/or efficiency.