Women at Altitude: Sex-Related Physiological Responses to Exercise in Hypoxia.

Sex differences in physiological responses to various stressors, including exercise, have been well documented. However, the specific impact of these differences on exposure to hypoxia, both at rest and during exercise, has remained underexplored. Many studies on the physiological responses to hypoxia have either excluded women or included only a limited number without analyzing sex-related differences. To address this gap, this comprehensive review conducted an extensive literature search to examine changes in physiological functions related to oxygen transport and consumption in hypoxic conditions. The review encompasses various aspects, including ventilatory responses, cardiovascular adjustments, hematological alterations, muscle metabolism shifts, and autonomic function modifications. Furthermore, it delves into the influence of sex hormones, which evolve throughout life, encompassing considerations related to the menstrual cycle and menopause. Among these physiological functions, the ventilatory response to exercise emerges as one of the most sex-sensitive factors that may modify reactions to hypoxia. While no significant sex-based differences were observed in cardiac hemodynamic changes during hypoxia, there is evidence of greater vascular reactivity in women, particularly at rest or when combined with exercise. Consequently, a diffusive mechanism appears to be implicated in sex-related variations in responses to hypoxia. Despite well-established sex disparities in hematological parameters, both acute and chronic hematological responses to hypoxia do not seem to differ significantly between sexes. However, it is important to note that these responses are sensitive to fluctuations in sex hormones, and further investigation is needed to elucidate the impact of the menstrual cycle and menopause on physiological responses to hypoxia

Effects of a short-term high-nitrate diet on exercise performance

It has been reported that nitrate supplementation can improve exercise performance. Most of the studies have used either beetroot juice or sodium nitrate as a supplement; there is lack of data on the potential ergogenic benefits of an increased dietary nitrate intake from a diet based on fruits and vegetables. Our aim was to assess whether a high-nitrate diet increases nitric oxide bioavailability and to evaluate the effects of this nutritional intervention on exercise performance. Seven healthy male subjects participated in a randomized cross-over study. They were tested before and after 6 days of a high (HND) or control (CD) nitrate diet (~8.2 mmol 19day(-1) or ~2.9 mmol 19day(-1), respectively). Plasma nitrate and nitrite concentrations were significantly higher in HND (127 \ub1 64 \ub5M and 350 \ub1 120 nM, respectively) compared to CD (23 \ub1 10 \ub5M and 240 \ub1 100 nM, respectively). In HND (vs. CD) were observed: (a) a significant reduction of oxygen consumption during moderate-intensity constant work-rate cycling exercise (1.178 \ub1 0.141 vs. 1.269 \ub1 0.136 L\ub7min(-1)); (b) a significantly higher total muscle work during fatiguing, intermittent sub-maximal isometric knee extension (357.3 \ub1 176.1 vs. 253.6 \ub1 149.0 Nm\ub7s\ub7kg(-1)); (c) an improved performance in Repeated Sprint Ability test. These findings suggest that a high-nitrate diet could be a feasible and effective strategy to improve exercise performance

EFFECTS OF ENHANCED NITRIC OXIDE BIOAVAILABILITY ON EXERCISE TOLERANCE IN DIFFERENT CONDITIONS

Nitric oxide (NO) is a signaling molecule that influences different aspects of the cellular homeostasis and regulates several physiological processes. It is normally produced endogenously starting from aminoacid L-arginine but recent evidences suggest that it can be also increased through the ingestion of food rich in inorganic nitrate (mainly green leafy vegetables and beetroot). Indeed, ingested inorganic nitrate (NO3-), after been absorbed, can be converted in the oral cavity to nitrite (NO2-) and be finally reduced into NO in the blood. This alternative NO3- - NO2- - NO pathway seems facilitated in condition of low O2 availability (ischemia and hypoxia) and low pH. In the last 10 years, several studies have been conducted to investigate the effects of dietary NO3- supplementation on skeletal muscle function, since NO regulates microvascular blood flow, muscle contractile proprieties, glucose homeostasis, intracellular calcium handling, and mitochondrial respiration. Dietary NO3- supplementation is likely to elicit a positive outcome when testing endurance exercise capacity in healthy subjects and enhance exercise tolerance in patients. However, there are still unresolved questions about the mechanisms utilized by inorganic NO3- to affect skeletal muscle functions, the possible potentiating effect of hypoxic conditions and the effectiveness of this intervention in other disease populations. My PhD projects tried to address some of these questions. The first study aimed to examine the effects of increased NO bioavailability on contraction economy. One of the most interesting effects of dietary NO3- supplementation is the reduction of muscle O2 consumption (V \u307O_2) at a given exercise intensity, since the V \u307O_2 requested to carry out a specific exercise is generally a fixed amount regardless of sex, age and training status of the subject involved in the exercise. A lower V \u307O_2 during moderate intensity exercise has been demonstrated both in healthy subjects and disease patients after dietary NO3- supplementation. The mechanistic basis is not clear yet, and this effect has been related to reduced ATP cost of contraction and/or an enhanced mitochondrial coupling efficiency. In our study, we evaluated skeletal muscle contraction economy following NO2- infusion in hypoxic condition. Contractions of the in-vivo isolated canine muscle were obtained by direct nerve stimulation. Muscle blood flow was kept constantly high by pump-perfusion. O2 consumption during exercise was assessed directly by Fick method. Mitochondrial respiration rates were evaluated by high-resolution respirometry from muscle biopsies. In hypoxic conditions, but in the presence of constant and normal convective O2 delivery, NO2- infusion did not affect canine skeletal muscle oxidative metabolism. These evidences suggest that the effects of increased NO availability on muscle contraction efficiency in hypoxia, if present, are likely not attributable to changes in mitochondrial respiratory efficiency. The second study aimed to investigate the effects of increased NO bioavailability on the physiological responses to exercise after prolonged permanence at altitude. Data from Tibetan population living at altitude from generations suggest that changes in NO bioavailability may contribute to hypoxia acclimatization and their enhanced exercise efficiency. Moreover, recent studies have demonstrated that NO3- supplementation can limit exercise impairment following exposure to acute hypoxia. However, it is not known if Caucasian subjects exposed to several days of hypobaric hypoxia can benefits from increased NO bioavailability. Thus, we investigated the ergogenic effects of dietary NO3- supplementation on exercise performance at different intensities during a prolonged permanence at altitude. Cycling and arm-cranking exercises were performed in order to test possible different effects of NO3- supplementation in relation to a different muscle fibers recruitment pattern. Dietary NO3- supplementation reduced O2 cost during moderate-intensity exercise both in cycling and arm-cranking. In cycle-ergometer exercise this effect was dependent from aerobic fitness level of the subjects, in accordance to previous results obtained in normoxia. Moreover, dietary NO3- supplementation enhanced severe-intensity exercise tolerance, suggesting that dietary NO3- supplementation can be a valid ergogenic aid to counteract exercise intolerance at altitude. Finally, in the third study we evaluated the possible ergogenic effects of dietary NO3- supplementation in obese subjects. Literature shows that NO3- can exert significant positive effects on exercise tolerance and, as a consequence, quality of life of patients with an impaired skeletal muscle oxidative metabolism such as chronic heart failure, chronic obstructive pulmonary diseases and peripheral arterial disease patients. Obese patients are characterized by a higher O2 cost of exercise, and therefore a reduced exercise tolerance during constant work-rate exercise compared with healthy subjects. We evaluated the effects of beetroot juice (BR, rich in NO3-) supplementation on the main physiological variables associated with exercise tolerance in obese adolescents. We observed a significant increase in plasma NO3- concentration after BR supplementation. The O2 cost of moderate-intensity exercise was not different in BR condition versus placebo, whereas, during severe-intensity exercise, signs of a reduced amplitude of the O2 uptake slow component were observed in BR, in association with a significantly longer time to exhaustion. Thus, exercise intolerance of obese adolescents, at least at severe-intensity, can be attenuated by short-term dietary NO3- supplementation. This intervention can be a useful aid to counteract early fatigue and reduced physical activity in this at-risk population. Overall, the studies carried out during my PhD extend the current knowledge about dietary NO3- supplementation on physiological responses to exercise, starting from a mechanistic investigation in isolated canine muscle up to the evaluation of the ergogenic benefits of this intervention on exercise tolerance in obese adolescents

Comparison between slow components of HR and VO2 kinetics: Functional significance

Purpose Aerobic exercise prescription is often based on a linear relationship between pulmonary oxygen consumption (VO2) and heart rate (HR). The aim of the present study was to test the hypothesis that during constant work rate (CWR) exercises at different intensities, the slow component of HR kinetics occurs at lower work rate and is more pronounced that the slow component of VO2 kinetics. Methods Seventeen male (age, 27 ± 4 yr) subjects performed on a cycle ergometer an incremental exercise to voluntary exhaustion and several CWR exercises: 1) moderate CWR exercises, below gas exchange threshold (GET); 2) heavy CWR exercise, at 45% of the difference between GET and VO2 peak (Δ); 3) severe CWR exercise, at 95% of Δ; 4) "HRCLAMPED" exercise in which work rate was continuously adjusted to maintain a constant HR, slightly higher than that determined at GET. Breath-by-breath VO2, HR, and other variables were determined. Results In moderate CWR exercises, no slow component of VO2 kinetics was observed, whereas a slow component with a relative amplitude (with respect to the total response) of 24.8 ± 11.0% was observed for HR kinetics. During heavy CWR exercise, the relative amplitude of the HR slow component was more pronounced than that for VO2 (31.6 ± 11.2% and 23.3 ± 9.0%, respectively). During HRCLAMPED, the decrease in work rate (14%) needed to maintain a constant HR was associated with a decreased VO2 (10%). Conclusions The HR slow component occurred at a lower work rate and was more pronounced than the VO2 slow component. Exercise prescriptions at specific HR values, when carried out for periods longer than a few minutes, could lead to premature fatigue

Biomechanical and metabolic aspects of backward (and forward) running on uphill gradients: another clue towards an almost inelastic rebound

Purpose: On level, the metabolic cost (C) of backward running is higher than forward running probably due to a lower elastic energy recoil. On positive gradient, the ability to store and release elastic energy is impaired in forward running. We studied running on level and on gradient to test the hypothesis that the higher metabolic cost and lower efficiency in backward than forward running was due to the impairment in the elastic energy utilisation. Methods: Eight subjects ran forward and backward on a treadmill on level and on gradient (from 0 to + 25%, with 5% step). The mechanical work, computed from kinematic data, C and efficiency (the ratio between total mechanical work and C) were calculated in each condition. Results: Backward running C was higher than forward running at each condition (on average + 35%) and increased linearly with gradient. Total mechanical work was higher in forward running only at the steepest gradients, thus efficiency was lower in backward running at each gradient. Conclusion: Efficiency decreased by increasing gradient in both running modalities highlighting the impairment in the elastic contribution on positive gradient. The lower efficiency values calculated in backward running in all conditions pointed out that backward running was performed with an almost inelastic rebound; thus, muscles performed most of the mechanical work with a high metabolic cost. These new backward running C data permit, by applying the recently introduced ‘equivalent slope’ concept for running acceleration, to obtain the predictive equation of metabolic power during level backward running acceleration

Evaluation of skeletal muscle oxidative metabolism in Alzheimer's disease

Alzheimer disease (AD) is the most common form of dementia affecting the aging population and its hallmark is a progressive cognitive impairment. Beta-amyloid (\u3b2Ab) peptide deposits in the cerebral tissue are a histopathological feature of AD and cause impairment of central neurotransmission and synaptic plasticity primarily due to mitochondrial dysfunction. Recent studies have also described \u3b2Ab deposits in peripheral tissues of AD patients, including skeletal muscle cells. PURPOSE: Aim of this study was to evaluate whether in AD patients skeletal muscle oxidative metabolism is impaired. METHODS: Thirteen AD patients (73.0\ub14 years, mean\ub1SD) and twenty-nine healthy sex-matched control subjects (CTRL) (71.5\ub15 years) were investigated. During incremental cycle ergometer (CE) and one-leg knee extension (KE) exercise were determined: breath-by-breath pulmonary O2 uptake (VO2); cardiac output (CO); vastus lateralis muscle fractional O2 extraction by near-infrared spectroscopy (\u394[deoxy(Hb\ub1Mb)]). Blood lactate concentration ([La-]) was assessed at rest and after exercise. Maximal isometric knee extension test was performed in order to assess the maximal voluntary contraction (MVC) of the quadriceps muscle. Total daily energy expenditure (TEE) was also measured during three consecutive days. RESULTS: During CE, peak work-rate (94.1\ub17.1 vs. 128.3\ub18.5 watt) and VO2peak (22.0\ub10.8 vs. 26.4\ub11.1 mL*kg-1*min-1) were significantly lower in AD vs. CTRL. CO was reduced in AD (13.0\ub10.6 L*min-1) vs. CTRL (16.6\ub11.0 L*min-1) and \u394[deoxy(Hb\ub1Mb)] was significantly lower in AD (51.1\ub15.8 %) vs. CTRL (71.4\ub12.9 %). During KE, VO2peak (10.7\ub10.7 vs. 13.5\ub10.6 mL*kg-1*min-1) and \u394[deoxy(Hb\ub1Mb)] (40.0\ub15.8 vs. 61.0\ub14.7 %) were significantly lower in AD vs. CTRL. CO and [La-] were not significantly different between AD and CTRL. As for MVC, no significant difference was found between CTRL and AD (440.5\ub139.5 N vs. 438.3\ub186.7 N, respectively). TEE was similar in the two groups. CONCLUSIONS: Our findings show that AD patients have a reduced exercise capacity compared to CTRL likely due to a reduced muscle fractional O2 extraction capacity. Indeed, the impairment of muscle oxidative function was confirmed even reducing (by KE) constraints to oxidative function deriving from cardiovascular O2 delivery

Aerobic fitness affects the exercise performance responses to nitrate supplementation

Purpose: Dietary nitrate supplementation has been shown to reduce O2 cost of submaximal exercise, improve exercise tolerance, and enhance performance in moderately trained individuals. In contrast, data have been provided that elite athletes do not benefit from nitrate supplementation. The aim of this study was to evaluate the effects of short-term nitrate supplementation on endurance performance in subjects with different levels of aerobic fitness. Methods: Twenty-one subjects (mean age, 22.7 ± 1.8 yr) with different aerobic fitness level (VO2peak value ranging from 28.2 to 81.7 mL·kg-1·min-1) participated in a crossover double-blind placebo-controlled study. Subjects were tested after 6 d of supplementation with either 0.5 l per day of nitrate (5.5 mmol)-containing water (NITR) or nitratefree water (PLA). Participants performed an incremental running test until exhaustion and four repetitions of 6-min submaximal (approximately 80% of gas exchange threshold) constant load exercise on a motorized treadmill. Moreover, subjects performed a 3-km running time trial on the field. Results: After NITR, a negative correlation between reduction of O2 cost of submaximal exercise and individual aerobic fitness level was observed (r2 = 0.80; P < 0.0001). A significant inverse correlation was also found between aerobic fitness level and improvement in performance for 3-km time trial after NITR (r2 = 0.76; P < 0.0001). Additionally, subjects responded differently to dietary nitrate supplementation according to aerobic fitness level with higher-fit subjects showing a lower increase in plasma [NO3-] (r2 = 0.86; P < 0.0001) and [NO2j] (r2 = 0.75; P G 0.0001). Conclusions: The results of the present study suggest that the individual aerobic fitness level affects the ergogenic benefits induced by dietary nitrate supplementation. The optimal nitrate loading regimen required to elevate plasma [NO2-] and to enhance performance in elite athletes is different from that of low-fit subjects and requires further studies

Effects of nitrate supplementation on aerobic performance in subjects with different fitness level

Introduction Dietary supplementation with either sodium nitrate or nitrate-rich beetroot juice has been consistently shown to reduce the oxygen demand of submaximal exercise (1) and improve time to exhaustion during high-intensity exercise (3,5). However, the ergogenic effect of nitrate supplementation in well-trained endurance athletes remains uncertain (2,4). Aim of this study was to evaluate the effects of short-term nitrate [NO3-] supplementation on aerobic performance in subjects with different fitness level. Material and Methods Twenty-one subjects (22.7\ub11.8 years, mean\ub1SD) with different fitness level were involved in a randomized double-blind crossover study. Subjects were tested after 6 days of supplementation with either 0.5 l per day of nitrate-containing (\uf07e5.0 mmol) water (NITR) or nitrate-free water (PLA). Participants performed an incremental running test in order to assess their peak oxygen uptake (V\u2019O2peak). Several repetitions of sub-maximal (about 60% V\u2019O2peak) constant load exercises on a motorized treadmill and a 3-km running time trial on the field were also performed. Results V\u2019O2peak value ranged from 34 to 63 ml*kg-1*min-1. Plasma [NO3-] was 13.4\ub15.7 \u3bcM and 83.5\ub137.7\u3bcM in PLA and NITR respectively. During constant-load exercise, V\u2019O2 at steady-state was significantly lower in NITR (1.90 \ub1 0.4 L*min-1) compared with PLA (2.05 \ub1 0.4 L*min-1). There was a significant negative correlation between the V\u2019O2peak value and the change in [NO3-] following NITR (R2 = 0.71, p<0.001) and between the V\u2019O2peak value and the change in VO2 at steady state (R2 = 0.69, p<0.001). As for 3-km Time Trial, no significant differences were observed between PLA (766.5\ub1140.8 sec) and NITR (766.4\ub1135.6 sec). However, if only subjects with a low fitness level (<50 ml*kg-1*min-1) were considered, the 3-km running performance significantly improved after NITR. Conclusion The results of the present study suggest that individual fitness level affects the ergogenic benefits induced by [NO3-] supplementation. Discrepancy results in literature may be explained by different source and/or duration of nitrate supplementation. Further studies are needed to clarify the molecular mechanism underlying this process. References 1. Bailey SJ, Winyard P, Vanhatalo A, et al. J Appl Physiol 2009 2. Besc\uf2s R, Ferrer-Roca V, Galilea PA, et al. Med Sci Sports Exerc 2012 3. Cermak NM, Gibala MJ, van Loon LJ. Int J Sport Nutr Exerc Metab 2012 4. Christensen PM, Nyberg M, Bangsbo J. Scand J Med Sci Sports 2013 5. Lansley KE, Winyard PG, Bailey SJ, et al. Med Sci Sports Exerc 201