Metabolic Rate Research Articles

Influence of Gut Microbiota on Drug Metabolism and Therapeutic Efficacy

Background: The natural complex of microorganisms that inhabit the digestive system is estimated to be trillions of bacteria and is known as the gut microbiota. It is now being appreciated more for its involvement in metabolism of drugs, and effects exerted on efficacy and toxicity of drugs. Differences in the composition of the microbiota in the gut cause differences in drug response among people, and therapeutic processes.Objectives: To evaluate the modulation of gut microbiota on drug metabolism and outcome, and analyze the therapeutic outcomes influenced by the microbiome, and to also identify microbial indicators predictive of variable therapeutic outcomes.Study Design: A Cross-Sectional Study.Place and Duration of the Study: Department of Biochemistry, Saidu Medical College, Swat KP – Pakistan from January 2022 to January 2023.Methodology: The current study recruited a sample of 150 patients. The bacterial structure in the gut was investigated using 16S rRNA analysis, and drug metabolism was determined through the blood plasma samples taken at the required time points. Patients were categorized according to their drug efficacy performance, and statistical comparisons were made to evaluate the relationships, accompanied by standard deviations and p-values, respectively, for proper testing of the results’ significanceResults: Among the 150 patients, a higher number of bacterial strains was also associated with better drug outcomes in terms of SD (.35, p < 0.05). The comparison of the rates of metabolism of the drugs between both groups revealed a group difference (SD = 0.28, p = 0.01). They found that those patients who had longer and more complex bacterial signatures had better outcomes from their treatment than patient with less diverse levels of bacteria in their gut.Conclusion: The buy-in of the study is that a direct association between the gut microbiota profile and Drug metabolism efficiency and effectiveness exists. It may also help in identifying biomarkers of microbial origin that may help to improve treatment outcomes, improve compliance with individual treatment plans, and reduce side effects.

Demographic, morphological and coat factors in dogs after exercise at a fast course ability test (FCAT) trial.

Domestic dogs are a widely diverse species of endothermic mammals that show a positive correlation between body mass and whole-animal metabolic rate, but a negative correlation between body mass and lifespan, making them an interesting system for determining thermoregulatory patterns in relation to body mass, body morphology, and age within a single mammalian species. Though previous work has found differences in thermoregulation across seasons and with training in dogs of different sizes, we now seek to determine (1) whether sampling event-related temperature differences remained when dogs exercised intensely and acutely outdoors and (2) whether thermal differences were also expressed in short-term burst exercise in athletic dogs compared to long-term exercise in non-athletic dogs, as previously found. Here, we measured tympanic membrane temperature (Tear) as a correlate of core or internal body temperature (Tb). We also measured changes in body temperature across different body surfaces using thermal imaging (Teye, Tnose, and Tmouth) in dogs after exercise during Fast Course Agility Trial (FCAT) competitions between spring and summer months in Central New York State, USA (N = 20, July and August N = 26). We correlated these data to each dog's body mass (average(± standard error) = 29.97(± 0.24) lbs.), age (5.99(± 0.78) years), and various aspects of body part measurements and coat characteristics, such as length, type, and color. First, in our overall dataset, being sampled in May was the most significant predictor of temperature slope (p < 0.001), and we identified far more significant predictor variables in the May event dataset than in other datasets. Second, as we gave special attention to our study population, we found that running an outdoor, burst exercise course (FCAT trial) shows a different pattern of thermoregulation compared with previous work. Thus, our data may offer preliminary insights that thermoregulation in dogs varies with sampling event and exercise type, though additional research is needed to understand the complexity of these observed patterns. Our data also provides evidence that responses can be plastic depending on the dog's individual phenotype and that athleticism may affect thermoregulation in dogs, similar to humans.

Changes in the Basic Metabolic Rate of the Crew under conditions of Eight Months Isolation in a Hermetic Object with a Moderately Hypercapnic Artificial Gas Environment. Message 1

Within the framework of the SIRIUS international project, a study of the basic metabolism of a gender-mixed crew in a sealed object with a moderately high content of carbon dioxide in the artificial atmosphere was conducted. Using mathematical methods, we estimated the basic metabolic rate of a crew of 5 people (3 men and 2 women) at rest for 240 days of isolation when simulating a flight to the Moon in the “SIRIUS-21” experiment. The period of isolation lasted from 4.11.2021 to 3.07.2022. BMR studies were performed twice in the background (on –38–35, –6 days), 7 times during the isolation period (23–25, 50–52, 84–86, 110–112, 154–156, 181–183, 222–224 day) and twice during the aftereffect period (+1–2, +8–9 days). It was found that the basic metabolism in isolation decreased by an average of 6 kcal/kg of body weight per day compared with natural environmental conditions. The crew was isolated from the effects of seasonal lighting changes in a sealed facility, the Ground-Based Medical and Technical Facility (NEK) of the Scientific Research Center of the Russian Federation - Institute of Biomedical Problems of the Russian Academy of Sciences, which does not have portholes, and where artificial lighting was created without seasonal changes. Inside the NEK, the comfort temperature was constantly maintained at +21–23 degrees Celsius and an artificial gas environment was formed, in which the oxygen content was maintained at 21%, carbon dioxide no more than 0.35%. In conditions of isolation from the action of these geophysical environmental factors, seasonal fluctuations in basal metabolism with a wave span of an average of 4 kcal /kg of body weight per day were detected: in the spring calendar season, the level of basal metabolism increased relative to the winter season. Seasonal local maximums and minimums of the basic exchange level for 2 calendar seasons (winter 2021/2022 and in spring 2022) were determined for each of the volunteers. The results obtained in this work can be applied in the field of space physiology to clarify the calculated oxygen reserves and caloric content of the crew’s rations for a long-term space mission, as well as in the design and programming of life support systems and thermal management systems for inhabited hermetic objects.

Effect of voxelotor on cerebral perfusion and cerebral oxygen metabolism and cardiac stress in adult patients with sickle cell disease.

Sickle cell disease (SCD) is complicated by silent cerebral infarcts (SCIs), for which anemia is an important risk factor. Despite normal oxygen delivery (OD), cerebral vascular reserve (CVR), and cerebral metabolic rate of oxygen (CMRO2) are diminished in SCD, possibly causing the formation of SCIs. Voxelotor inhibits polymerization by increasing the hemoglobin oxygen binding, ameliorating hemolytic anemia. Furthermore, anemia is related to cardiac complications. Our aims were to assess the effect of voxelotor on markers of cerebral perfusion, cerebral oxygen metabolism, and markers of cardiac stress in SCD patients. Cerebral hemodynamics and oxygen metabolism were measured with MRI before and after 3 months of voxelotor treatment (1500 mg/day) in 18 adults with SCD (HbSS/HbSβ0-thalassemia). Hemoglobin levels significantly increased (p = .001) and markers of hemolysis decreased (p < .05). OD increased from 6.5 (IQR, 6.0-7.1) mL O2/100 g/min to 8.1 (IQR, 7.2-8.7) mL O2/100 g/min (p = .001). CBF and CVR did not change. CMRO2 decreased from 2.0 (IQR, 1.9-2.1) mL O2/100 g/min to 1.9 (IQR, 1.6-2.1) mL O2/100 g/min (p = .03). N-terminal pro-B type natriuretic peptide (NT-proBNP) levels decreased (p = .048) and maximum tricuspid regurgitation flow velocity (TRVmax) normalized in all but one patient with increased TRVmax. Voxelotor treatment in patients with severe SCD did not decrease CBF despite increased Hb levels. Cerebral oxygen metabolism slightly decreased, despite raised OD, most likely due to drug-induced increase in oxygen binding. Nonetheless, voxelotor improved clinically validated markers of cardiac stress.

Multiorgan ultrastructural changes in rats induced in synthetic torpor.

Torpor is a state used by several mammals to survive harsh winters and avoid predation, characterized by a drastic reduction in metabolic rate followed by a decrease in body temperature, heart rate, and many physiological variables. During torpor, all organs and systems must adapt to the new low-energy expenditure conditions to preserve physiological homeostasis. These adaptations may be exploited in a translational perspective in several fields. Recently, many features of torpor were shown to be mimicked in non-hibernators by the inhibition of neurons within the brainstem region of the Raphe Pallidus. The physiological resemblance of this artificial state, called synthetic torpor, with natural torpor has so far been described only in physiological terms, but no data have been shown regarding the induced morphological changes. Here, we show the first description of the ultrastructural changes in the liver, kidney, lung, skeletal muscle, and testis induced by a 6-hours inhibition of Raphe Pallidus neurons in a non-hibernating species, the rat.

Resting metabolic rate in obesity.

The prevalence of obesity has continued to rise, and obesity and its attendant metabolic disorders are major global health threat factors. Among the current interventions for obesity, none have demonstrated sustained efficacy in achieving long-term outcomes. So, the identification of therapeutic targets is of paramount importance in the advancement and sustainability of obesity. Resting metabolic rate (RMR) constitutes 60%-75% of total energy expenditure and serves a crucial function in maintaining energy balance. Nevertheless, there exists considerable heterogeneity in RMR among individuals. Low RMR is associated with weight gain, elevating the susceptibility to obesity-related ailments. Hence, RMR will be the main focus of interest in the study of obesity treatment. In this review, we will elucidate the influence factors and mechanisms of action of RMR in obesity, with particular emphasis on the effects of obesity treatment on RMR and the alterations and influence factors of RMR in special types of populations with obesity.

Metabolic scaling, energy allocation tradeoffs, and the evolution of humans’ unique metabolism

All organisms use limited energy to grow, survive, and reproduce, necessitating energy allocation tradeoffs, but there is debate over how selection impacted metabolic budgets and tradeoffs in primates, including humans. Here, we develop a method to compare metabolic rates as quotients of observed relative to expected values for mammals corrected for size, body composition, environmental temperature, and phylogenetic relatedness. Contrary to previous analyses, these quotients reveal that nonhuman primates have total metabolic rates expected for similar-sized mammals in similar environments. In addition, data from several small-scale societies show that humans evolved exceptionally high resting, activity, and total metabolic rates apparently by overcoming tradeoffs between resting and active energy expenditures that constrain other primates. Enhanced metabolic rates help humans fuel expanded brains, faster reproductive rates, extended longevity, and high percentage of body fat.

Influence of Bladder Filling on Parameters of Body Composition by Bioimpedance Electrical Analysis: Observational Study.

Bioelectrical impedance analysis (BIA) is a widely used method for estimating body composition, and its accuracy may be influenced by various factors, including bladder filling. This study aims to investigate the impact of bladder filling on the accuracy of BIA measurements. An experimental crossover study was conducted with sedentary young adults. The influence of bladder filling on total body water (TBW), fat mass (FM), fat-free mass (FFM), and basal metabolic rate (BMR) was assessed. Participant in underwear followed an overnight fast. They were instructed to abstain from vigorous physical activity and alcohol for at least 24 h prior to the session. The results obtained from single-frequency and multi-frequency BIA devices were compared. The findings suggest that bladder filling does not affect measured impedance; however, changes in weight following bladder voiding influenced derived BIA results. Specifically, TBW, FM, and BMR values significantly reduced after voiding (p < 0.05). Furthermore, the study found poor agreement between single-frequency and multi-frequency BIA devices, indicating that they are not interchangeable. Bladder filling does affect BIA measurements, not clinically meaningful. Further research is needed to explore the implications of these findings for clinical practice and research protocols.

High energetic cost of color change in octopuses

For many animals, color change is a critical adaptive mechanism believed to carry a substantial energetic cost. Yet, no study to date has directly measured the energy expenditure associated with this process. We examined the metabolic cost of color change in octopuses by measuring oxygen consumption in samples of excised octopus skin during periods of chromatophore expansion and contraction and then modeled metabolic demand over the whole octopus as a function of octopus mass. The metabolic demand of the fully activated chromatophore system is nearly as great as an octopus's resting metabolic rate. This high metabolic cost carries ecological and evolutionary implications, including selective pressures in octopuses that may influence the adoption of nocturnal lifestyles, the use of dens, the reduction of the chromatophore system in deep-sea species, and metabolic trade-offs associated with foraging.

Analysis of physiological and biochemical changes and metabolic shifts during 21-Day fasting hypometabolism

This study aimed to evaluate the impact of prolonged fasting on the physiological and biochemical alterations and metabolic shifts in healthy adults and to provide experimental data and theoretical support for the hypometabolic state induced by prolonged fasting. Thirteen volunteers were selected through public recruitment to undergo a 21-day complete fasting experiment. The experimental period lasted 34 days, including a 3-day baseline, 21-day completing fasting, 5-day calorie restriction and 5-day full recovery diet. Physiological indicators such as body weight, blood pressure, blood glucose, blood ketones, and blood uric acid were evaluated along with resting metabolic rate, routine blood tests, liver function, and heart function indexes employing traditional approaches. During the 21-day complete fasting period, there was a significant decrease in body weight (average − 14.96 ± 1.55%), a reduction in blood glucose (average − 21.63 ± 0.058%), an increase in blood ketones (from baseline 0.1 ± 0.04 mmol/L to 6.61 ± 1.25 mmol/L) and blood uric acid (from baseline 385.38 ± 57.78 µmol/L to 866.31 ± 172.01 µmol/L), a continuous decline in resting energy expenditure (average − 20.3 ± 11.13%), and the respiratory quotient tending towards fat metabolism. Most of the items in the complete blood count and liver indicators remained stable and within the normal range. Heart function showed functional adaptive changes without structural damage. Prolonged fasting can reduce the body’s resting energy expenditure and adapt to body weight loss through physiological regulatory mechanisms without adverse effects on basic physiological functions or the structure of important organs. Under medical supervision, healthy adults can safely engage in prolonged fasting for up to 21 days with metabolic adaption and no damage to pivotal organ, which could provide potential technical support for human health and survival strategies in extreme conditions such as food shortages during long-duration manned spaceflight.

Exploring the impact of mitochondrial-targeting anthelmintic agents with GLUT1 inhibitor BAY-876 on breast cancer cell metabolism

BackgroundCancer cells alter their metabolic phenotypes with nutritional change. Single agent approaches targeting mitochondrial metabolism in cancer have failed due to either dose limiting off target toxicities, or lack of significant efficacy in vivo. To mitigate these clinical challenges, we investigated the potential utility of repurposing FDA approved mitochondrial targeting anthelmintic agents, niclosamide, IMD-0354 and pyrvinium pamoate, to be combined with GLUT1 inhibitor BAY-876 to enhance the inhibitory capacity of the major metabolic phenotypes exhibited by tumors.MethodsTo test this, we used breast cancer cell lines MDA-MB-231 and 4T1 which exhibit differing basal metabolic rates of glycolysis and mitochondrial respiration, respectively. Metabolic characterization was carried out using Seahorse XFe96 Bioanalyzer and statistical analysis was carried out via ANOVA.ResultsHere, we found that specific responses to mitochondrial and glycolysis targeting agents elicit responses that correlate with tested cell lines basal metabolic rates and fuel preference, highlighting the potential to cater metabolism targeting treatment regimens based on specific tumor nutrient handling. Inhibition of GLUT1 with BAY-876 potently inhibited glycolysis in both MDA-MB-231 and 4T1 cells, and niclosamide and pyrvinium pamoate perturbed mitochondrial respiration that resulted in potent compensatory glycolysis in the cell lines tested.ConclusionIn this regard, combination of BAY-876 with both mitochondrial targeting agents resulted in inhibition of compensatory glycolysis and subsequent metabolic crisis. These studies highlight targeting tumor metabolism as a combination treatment regimen that can be tailored by basal and compensatory metabolic phenotypes.

A Computational Pipeline for Identifying Gene Regulatory Networks: A Case Study of Response to Exercise.

Gene regulatory networks are foundational in the control of virtually all biological processes. These networks orchestrate a myriad of cell functions ranging from metabolic rate to the response to a drug or other intervention. The data required to accurately identify these control networks remains very cost and labor intensive typically leading to relatively sparse time course data that is largely incompatible with conventional data-driven model identification techniques. In this work, we combine empirical identification of gene-gene interactions with constraints describing the expected dynamic behavior of the network to infer regulatory dynamics from under-sampled data. We apply this to the identification of gene regulatory subnetworks recruited in groups of subjects participating in several different exercise interventions. Intervention-specific response networks are compared to one another and control actions driving differences are identified. We propose that this approach can extract statistically robust and biologically meaningful insights into gene regulatory dynamics from a dataset consisting of a small number of participants with very limited longitudinal sampling, for example pre- and post- intervention only.

The protective role of basal metabolic rate in cognitive decline: evidence from epidemiological and genetic studies.

This study aims to explore the relationship between basal metabolic rate (BMR) and cognitive impairment and assess the potential of BMR as a protective factor against cognitive decline. This investigation initially conducted a cross-sectional study of American adults from 2011 to 2014 using data from the National Health and Nutrition Examination Survey. It examined the correlation between participants' BMR and cognitive functions, exploring the association with cognitive impairment. Subsequently, publicly available genome-wide association study data was used to examine potential causal links between genetically determined BMR and specific cognitive disorders using Mendelian randomization. Cross-sectional findings revealed a significant positive correlation between higher BMR and cognitive scores. In Mendelian randomization analysis, BMR demonstrated an inverse causal relationship with Alzheimer's disease and Parkinson's dementia, suggesting BMR as a potential protective factor against these diseases. No causal links were found with vascular dementia, Lewy body dementia, and frontotemporal dementia. This study supports the role of BMR as a potential protective factor against Alzheimer's disease and Parkinson's dementia, suggesting that BMR may play an important role in preventing cognitive decline. However, due to the limitations of cross-sectional studies, further prospective studies and broader demographic samples are necessary to verify these results and explore underlying biological mechanisms. Key messages What is already known on this topic: Existing knowledge suggests a close relationship between BMR and health and cognitive functions, but detailed studies on its connection with specific cognitive impairments are still needed. What this study adds: This study found a significant positive correlation between higher BMR and cognitive improvement, potentially aiding in the prevention of Alzheimer's and Parkinson's dementia. How this study might affect research, practice, or policy: This finding guides public health strategies and personalized medicine, emphasizing the necessity for further research to validate BMR's protective effects.

ChIP-seq and structural analyses delineating the regulatory mechanism of master regulator EsrB in Edwardsiella piscicida.

As a crucial virulence regulator, EsrB possesses a LuxR-like superfamily domain at the C-terminal, which is conserved within the canonical NarL family regulators. Due to its critically important role in virulence and pathogenicity in fish hosts, the DNA binding ability has been believed to allow EsrB to regulate genes associated with the invasion process of host cells and basal metabolism in response to environmental stimuli. The lack of EsrB's crystal structure has been a major obstacle in understanding the molecular mechanisms of EsrB-DNA interaction which choreographs EsrB-mediated pathogenic behavior. Here, we conducted ChIP-seq and solved the crystal structure of the C-terminal of EsrB (EsrBC) at 2.20-Å resolution, which revealed that EsrB preferred to bind to virulence-associated promoters with a distinct 7'-4-7' pseudopalindromic DNA motif and interacted with metabolic-related promoters with a high AT DNA motif in Dulbecco's Modified Eagle's Medium (DMEM) (mimicking in vivo environments). Our results facilitate a detailed understanding of EsrB's regulatory role in Edwardsiella piscicida pathogenesis and expand our knowledge of virulence regulators in the family Hafniaceae.

Measuring energy expenditure in narcolepsy using doubly-labelled water and respiration chamber calorimetry.

Hypocretin deficiency causes type 1 narcolepsy, a condition characterised by excessive daytime sleepiness, cataplexy, and fragmented nocturnal sleep. Two-thirds of people with narcolepsy are also overweight, of which half are obese. The pathophysiology behind weight gain in people with narcolepsy remains unknown. We assessed a possible decrease in energy expenditure as a cause for overweight in narcolepsy using respiration chamber calorimetry and doubly labelled water. Ten males with type I narcolepsy and nine matched (for age, sex, and BMI) healthy controls were enrolled. Subjects stayed in a respiration chamber for 24 hours. They subsequently received doubly labelled water and wore an accelerometer for two weeks to assess energy expenditure and physical activity under daily living conditions. Total daily energy expenditure, resting energy expenditure, overnight metabolic rate, physical activity level and activity-induced energy expenditure were measured. No significant differences were found in resting energy expenditure, mean 24-hour respiration chamber energy expenditure, overnight metabolic rate and activity-induced energy expenditure when comparing people with narcolepsy type 1 to controls. Physical activity was also comparable between groups. Energy expenditure in narcolepsy type 1 is similar to matched controls, suggesting comparable metabolism and physical activity rates. It remains possible that metabolic changes are most pronounced around disease onset. In addition, patients had to discontinue their medication which may have influenced the results. Still, our findings suggest that other factors may also play a role in weight gain in narcolepsy, such as differences in dietary behaviour.

Oxidative Stress in an African Ground Squirrel, a Case of Healthy Aging and Reproduction.

Oxidative stress plays a crucial role in mediating life-history processes, where it can compromise survival and reproduction through harmful alterations to DNA, lipids, and proteins. In this study, we investigated oxidative stress in Cape ground squirrels (Xerus inauris), a longer-lived African ground squirrel species with a high reproductive skew and unique life history strategies. We measured oxidative stress as total antioxidant capacity (TAC), total oxidant status (TOS), and an oxidative stress index (OSI) in blood plasma from individuals of approximately known ages. Our results reveal a distinct pattern of decreasing oxidative stress with age, consistent across both sexes. Females exhibited lower OSI and TOS levels than males. Males employing different life-history strategies, namely natal (staying at home), had significantly lower oxidative stress compared to the band (roaming male groups), likely due to variations in metabolic rate, activity, and feeding rates. However, both strategies exhibited reduced oxidative stress with age, though the underlying mechanisms require further investigation. We propose that selection pressures favoring survival contributed to the observed reduction in oxidative stress with age, potentially maximizing lifetime reproductive success in this species.

Electrocardiographic parameters of chemically immobilized giant anteaters (Myrmecophaga tridactyla).

The giant anteater (Myrmecophaga tridactyla) is a vulnerable species that is threatened mostly due to anthropogenic pressure. The anteater is a highly specialized insectivore, challenging the species' ex situ maintenance and conservation efforts. Several dietary-associated health issues have been reported in captive anteaters, including heart conditions such as dilated cardiomyopathy. On the other hand, cardiopathy is mainly diagnosed only on necropsy, and lack of clinical reference is one of the constraints. This work describes electrocardiographic parameters in twelve zoo-kept giant anteaters (Myrmecophaga tridactyla). The giant anteaters were evaluated after chemical immobilization. Surface electrocardiography using a digital electrocardiograph was performed to acquire data on the six frontal plane leads. Four animals were placed in both left and right recumbencies to assess changes in waveforms. Nine anteaters were considered healthy and included in the statistics. The mean heart rate and electrical axis were 37.8bpm ± 3.45 and 75.6º ± 11.43, respectively. ECG parameters results were P wave duration (ms) 89.7 ± 9.2, P wave amplitude (mV) 0.14 ± 0.05, PR segment duration (ms) 148.6 ± 23, R wave amplitude (mV) 1.55 ± 0.56, QRS complex duration (ms) 88.6 ± 12.0, QT interval duration (ms) 529.6 ± 71.2, and T wave amplitude (mV) 0.76 ± 0.21. There was no difference between left or right recumbency. Heart parameters of giant anteaters are similar to other mammals and seem to be influenced both by size and metabolic rate when comparing with other species. This is the first description of ECG parameters in giant anteaters.

The mRNA N6-Methyladenosine Response to Dehydration in Xenopus laevis.

The African clawed frog, Xenopus laevis, exhibits remarkable adaptations to survive in its arid habitat, including behavioral and metabolic changes during periods of drought. During extreme dehydration, X. laevis undergoes estivation, a state characterized by increased urea and ammonia levels, depression of the metabolic rate, and tissue hypoxia. To understand the molecular mechanisms underlying these adaptations, we investigated the potential role of N6-methyladenosine (m6A), a widespread mRNA modification, in X. laevis during extreme dehydration. We analyzed the protein levels of key components in the m6A pathway, including writers (METTL3, METTL14, and WTAP), erasers (ALKBH5 and FTO), and readers (SRSF3, YTHDF1, YTHDF2, YTHDF3, and eIF3a), in the liver and kidneys of control frogs and frogs that had lost 35 ± 0.93% of their total body water. The relative protein levels generally decreased or remained unchanged, with the exception of YTHDF3, which depicted a protein level increase in the liver. Notable changes included eIF3a, which was downregulated by 26 ± 8% and 80 ± 8% in the dehydrated liver and kidney tissues, respectively. Additionally, the total m6A increased by 353 ± 30% and 177 ± 17% in dehydrated liver and kidney RNA samples, respectively. This study highlights the importance of epigenetic mechanisms in stress tolerance and provides a foundation for further exploration of the role of epigenetics in dehydration tolerance.

Commentary on Thoral et al. (2024) 'The relationship between mitochondrial respiration, resting metabolic rate and blood cell count in great tits'.

Commentary on Thoral et al. (2024) 'The relationship between mitochondrial respiration, resting metabolic rate and blood cell count in great tits'.

Near-maximally swimming schoolmaster snapper (Lutjanus apodus) have a greater metabolic capacity, and slightly lower thermal tolerance, than when tested at rest.

To assess the relationship among various measures of thermal tolerance and performance suggested for use in fish, we determined the critical thermal maximum (CTmax), critical swimming speed (Ucrit), maximum thermal tolerance while swimming [CTSmax] and realistic aerobic scope (ASR) of juvenile schoolmaster snapper (Lutjanus apodus). Their CTSmax (37.5±0.1°C) was only slightly lower than their CTmax (38.9±0.1°C) and this is probably because their maximum metabolic rate (MMR) and ASR during the former test were ∼42 and 65% higher, respectively. Furthermore, we did not observe a transition to unsteady (i.e. anaerobically fueled) swimming in the CTSmax test as we did in the Ucrit protocol. These data strongly suggest that thermal tolerance tests on fishes whose lifestyle involves schooling or sustained activity should be performed at ecologically relevant swimming speeds. Our results do not support the hypothesis that failure during a CTSmax test is due to a fish's inability to meet its tissue oxygen demands.