Integrative Physiology of Fasting

Rapid stimulation of protein synthesis in digesting snakes: Unveiling a novel gut-pancreas-muscle axis

AIM

Snakes exhibit remarkable physiological shifts when their large meals induce robust postprandial growth after prolonged fasting. To understand the regulatory mechanisms underlying this rapid metabolic transition, we examined the regulation of protein synthesis in pythons, focusing on processes driving early postprandial tissue remodeling and growth.

METHODS

Using the SUnSET method with puromycin labeling, we measured in vivo protein synthesis in fasting and digesting snakes at multiple post-feeding intervals. Pyloric ligation, pancreatectomy, and plasma transfusions were performed to explore the roles of gastrointestinal luminal signaling and pancreatic function across key tissues.

RESULTS

We observed profound and early stimulation of protein synthesis in gastrointestinal tissues and skeletal muscle already 3 h after ingestion, before any measurable rise in plasma amino acids from the meal. The gastrointestinal stimulation appears to be driven by luminal factors, while the stimulation of skeletal muscle protein synthesis is humoral with pancreatic insulin release as an integral mediator. The pre-absorptive anabolic activity is supported by the release of amino acids from the breakdown of endogenous proteins.

CONCLUSIONS

Our findings suggest that snakes initiate protein synthesis via distinct, tissue-specific pathways preceding nutrient absorption. This "pay before pumping" model shows how early protein synthesis prepares the digestive and muscular systems for later nutrient assimilation and growth. This intricate humoral regulation, involving a gut-pancreas-muscle axis, governs postprandial protein synthesis in snakes and provides insights into fundamental mechanisms driving metabolic adaptations and broader hyperplastic and hypertrophic responses.

Lipid metabolism and microbial regulation analyses provide insights into the energy-saving strategies of hibernating snakes

Hibernation is a necessary means for animals to maintain survival while coping with low temperatures and food shortages. While most studies have largely focused on mammalian hibernation, its reptilian equivalent has been less studied. In order to provide insights into the energy metabolism and potential microbial regulatory mechanisms in hibernating snakes, the serum, liver, gut content samples were measured by multi-omic methods. Here we show the active snakes have more vigorous lipid metabolism, whereas snakes in hibernation groups have higher sphingolipid metabolism. Furthermore, the results indicate that the potential energy supply pathway was gluconeogenesis. Microbial analysis reveals that Proteobacteria and Firmicutes showed dynamic changes with the transformation among active, pre-hibernation and hibernation periods. The correlation analysis reveals the potential role of Romboutsia, Providencia and Vagococcus in regulating above metabolism by producing certain metabolites. The results advance the understanding of the complex energy-saving strategy in hibernating poikilotherms.

Repeated fasting events sensitize enhancers, transcription factor activity and gene expression to support augmented ketogenesis

Mammals withstand frequent and prolonged fasting periods due to hepatic production of glucose and ketone bodies. Because the fasting response is transcriptionally regulated, we asked whether enhancer dynamics impose a transcriptional program during recurrent fasting and whether this generates effects distinct from a single fasting bout. We found that mice undergoing alternate-day fasting (ADF) respond profoundly differently to a following fasting bout compared to mice first experiencing fasting. Hundreds of genes enabling ketogenesis are 'sensitized' (i.e. induced more strongly by fasting following ADF). Liver enhancers regulating these genes are also sensitized and harbor increased binding of PPARα, the main ketogenic transcription factor. ADF leads to augmented ketogenesis compared to a single fasting bout in wild-type, but not hepatocyte-specific PPARα-deficient mice. Thus, we found that past fasting events are 'remembered' in hepatocytes, sensitizing their enhancers to the next fasting bout and augment ketogenesis. Our findings shed light on transcriptional regulation mediating adaptation to repeated signals.

Water deprivation induces a systemic procatabolic state that differentially affects oxidative and glycolytic skeletal muscles in male mice

Dehydration, characterized by the loss of total body water and/or electrolytes due to diseases or inadequate fluid intake, is prevalent globally but often underestimated. Its contribution to long-term chronic diseases and sarcopenia is recognized, yet the mechanisms involved in systemic and muscle protein metabolism during dehydration remain unclear. This study investigated metabolic adaptations in a 36-h water deprivation (WD) model of mice. Male C57BL/6 mice underwent 36-h WD or pair-feeding at rest, with assessments of motor skills along with biochemical and metabolic parameters. Dehydration was confirmed by hypernatremia, body mass loss, hyporexia, and increased activity of vasopressinergic and oxytocinergic neurons compared with controls. These results were associated with liver mass loss, decreased glycemia, and increased cholesterolemia. In addition, increased V̇o2 and a decreased respiratory exchange ratio indicated reduced carbohydrate consumption and potentially increased protein use during dehydration. Thus, skeletal muscle protein metabolism was evaluated due to its high protein content. In the oxidative muscles of the WD group, total and proteasomal proteolysis increased, which was associated with decreased Akt-mediated intracellular signaling. Interestingly, there was an increase in fiber cross-sectional area, likely due to higher muscle water content caused by increased intracellular osmolality induced by protein catabolism products. Conversely, no changes were observed in protein turnover or water content in glycolytic muscles. These findings suggest that short-term WD imposes a procatabolic state, depleting protein content in skeletal muscle. However, skeletal muscle may respond differently to dehydration based on its phenotype and might adapt for a limited time.NEW & NOTEWORTHY This study investigated the effects of WD on mouse homeostasis, focusing on energy substrates and skeletal muscle protein metabolism. Our findings revealed a shift toward reduced dependence on carbohydrate degradation and increased reliance on lipid oxidation, or even protein oxidation, as energy sources, since we observed increased proteolysis in one muscle phenotype. Despite body mass loss, soleus and EDL muscle masses were differently affected. These results indicate the procatabolic potential of short-term WD in mice.

FKBP5 as a key regulator of metabolic processes in birds: Insights from chicken pectoral muscle

FK506-binding protein 5 (FKBP5) is a negative regulator of the glucocorticoid response and may play an important role in regulating metabolic homeostasis in birds. However, limited information is available regarding its role in avian species. This study aimed to clarify the spatiotemporal characteristics of chicken FKBP5 and investigate the effects of exogenous stimuli on its expression. Real-time quantitative PCR (RT-qPCR) was utilized to analyze the spatiotemporal expression patterns of FKBP5 in chickens. Additionally, the impact of exogenous stimuli, including fasting, energy restriction, and insulin/pyruvate/glucose injections, on FKBP5 expression in the pectoral muscle was investigated. The results showed that FKBP5 is broadly expressed in various bird tissues, with dominant expression in insulin-sensitive tissues. The FKBP5 levels in birds are dramatically modulated during development in insulin-sensitive tissues, with the highest expression observed in striated muscle and liver at embryonic day 19 (E19) (P < 0.01). The basal level of FKBP5 in the pectoral muscle of broilers is higher than that in Silky chickens. Insulin administration leads to a rapid drop in blood glucose levels in both breeds, with a slower recovery in AA broilers (P < 0.01). However, FKBP5 expression in the pectoral muscle initially shows a slight drop, followed by a sharp increase over time after insulin administration (P < 0.01). Additionally, breed heterogeneity in FKBP5 expression was observed in the pectoral muscle upon insulin stimulation, with broilers showing stronger insulin sensitivity compared to Silky chickens. A 24-hour fasting period downregulated blood glucose levels (P < 0.01) and FKBP5 expression in the pectoral muscle of AA broilers (P < 0.01). A 30% energy restriction over three weeks slightly reduced blood glucose levels (P = 0.075) and significantly decreased FKBP5 levels in the pectoralis muscle of broilers. In addition, pyruvate increased blood glucose and pectoralis FKBP5 transcription levels at 60 min (P < 0.01), whereas glucose increased blood glucose levels but had no effect on FKBP5 expression. Overall analysis showed a negative correlation between FKBP5 expression in the pectoral muscle and blood glucose levels (ρ = -0.405, P < 0.001). In conclusion, FKBP5 is a key player in the metabolic regulation of birds, with its expression being highly dynamic and tissue-specific. Insulin regulates FKBP5 expression in pectoral muscles in a time-dependent manner, while pyruvate increases FKBP5 levels and fasting/energy restriction reduces FKBP5 mRNA expression in the pectoral muscles.

Intermittent fasting shifts the diurnal transcriptome atlas of transcription factors

Intermittent fasting remains a safe and effective strategy to ameliorate various age-related diseases, but its specific mechanisms are not fully understood. Considering that transcription factors (TFs) determine the response to environmental signals, here, we profiled the diurnal expression of 600 samples across four metabolic tissues sampled every 4 over 24 h from mice placed on five different feeding regimens to provide an atlas of TFs in biological space, time, and feeding regimen. Results showed that 1218 TFs exhibited tissue-specific and temporal expression profiles in ad libitum mice, of which 974 displayed significant oscillations at least in one tissue. Intermittent fasting triggered more than 90% (1161 in 1234) of TFs to oscillate somewhere in the body and repartitioned their tissue-specific expression. A single round of fasting generally promoted TF expression, especially in skeletal muscle and adipose tissues, while intermittent fasting mainly suppressed TF expression. Intermittent fasting down-regulated aging pathway and upregulated the pathway responsible for the inhibition of mammalian target of rapamycin (mTOR). Intermittent fasting shifts the diurnal transcriptome atlas of TFs, and mTOR inhibition may orchestrate intermittent fasting-induced health improvements. This atlas offers a reference and resource to understand how TFs and intermittent fasting may contribute to diurnal rhythm oscillation and bring about specific health benefits.

Effects of Long-Term Fasting on Gut Microbiota, Serum Metabolome, and Their Association in Male Adults

BACKGROUND

Long-term fasting demonstrates greater therapeutic potential and broader application prospects in extreme environments than intermittent fasting.

METHOD

This pilot study of 10-day complete fasting (CF), with a small sample size of 13 volunteers, aimed to investigate the time-series impacts on gut microbiome, serum metabolome, and their interrelationships with biochemical indices.

RESULTS

The results show CF significantly affected gut microbiota diversity, composition, and interspecies interactions, characterized by an expansion of the Proteobacteria phylum (about six-fold) and a decrease in Bacteroidetes (about 50%) and Firmicutes (about 34%) populations. Notably, certain bacteria taxa exhibited complex interactions and strong correlations with serum metabolites implicated in energy and amino acid metabolism, with a particular focus on fatty acylcarnitines and tryptophan derivatives. A key focus of our study was the effect of Ruthenibacterium lactatiformans, which was highly increased during CF and exhibited a strong correlation with fat metabolic indicators. This bacterium was found to mitigate high-fat diet-induced obesity, glucose intolerance, dyslipidemia, and intestinal barrier dysfunction in animal experiments. These effects suggest its potential as a probiotic candidate for the amelioration of dyslipidemia and for mediating the benefits of fasting on fat metabolism.

CONCLUSIONS

Our pilot study suggests that alterations in gut microbiota during CF contribute to the shift of energy metabolic substrate and the establishment of a novel homeostatic state during prolonged fasting.

[Effects of intermittent fasting and sports performance: a narrative review]

Recently, fasted exercise has generated interest for its potential to stimulate metabolic and performance adaptations. The present study aims to analyze the effects of fasting and fasted training on performance and metabolism, acutely and chronically. The databases Medline (PubMed), Physiotherapy Evidence Database (PEDro), Cochrane, and Google Scholar were searched. In total, 767 studies were identified. Of those, 51 studies were finally included. Acutely, exercise on fasting promotes fat oxidation at low and moderate intensities, while protein catabolism is not increased. Performance is not affected in efforts lower than 1 hour. Chronically, fasting generates greater efficiency in fat metabolism and the ability to regulate blood glucose in the long term, although it has not been possible to determine whether these findings improve sports performance. More research is needed in elite athletes, with high training loads and with a periodized approach to fasting.

Fasting as a precursor to high-fat diet enhances mitochondrial resilience in Drosophila melanogaster

Changes in diet type and nutrient availability can impose significant environmental stress on organisms, potentially compromising physiological functions and reproductive success. In nature, dramatic fluctuations in dietary resources are often observed and adjustments to restore cellular homeostasis are crucial to survive this type of stress. In this study, we exposed male Drosophila melanogaster to two modulated dietary treatments: one without a fasting period before exposure to a high-fat diet and the other with a 24-h fasting period. We then investigated mitochondrial metabolism and molecular responses to these treatments. Exposure to a high-fat diet without a preceding fasting period resulted in disrupted mitochondrial respiration, notably at the level of complex I. On the other hand, a short fasting period before the high-fat diet maintained mitochondrial respiration. Generally, transcript abundance of genes associated with mitophagy, heat-shock proteins, mitochondrial biogenesis, and nutrient sensing pathways increased either slightly or significantly following a fasting period and remained stable when flies were subsequently put on a high-fat diet, whereas a drastic decrease of almost all transcript abundances was observed for all these pathways when flies were exposed directly to a high-fat diet. Moreover, mitochondrial enzymatic activities showed less variation after the fasting period than the treatment without a fasting period. Overall, our study sheds light on the mechanistic protective effects of fasting prior to a high-fat diet and highlights the metabolic flexibility of Drosophila mitochondria in response to abrupt dietary changes and have implication for adaptation of species to their changing environment.

Systemic and transcriptional response to intermittent fasting and fasting-mimicking diet in mice

BACKGROUND

Dietary restriction (DR) has multiple beneficial effects on health and longevity and can also improve the efficacy of certain therapies. Diets used to instigate DR are diverse and the corresponding response is not uniformly measured. We compared the systemic and liver-specific transcriptional response to intermittent fasting (IF) and commercially available fasting-mimicking diet (FMD) after short- and long-term use in C57BL/6 J mice.

RESULTS

We show that neither DR regimen causes observable adverse effects in mice. The weight loss was limited to 20% and was quickly compensated during refeeding days. The slightly higher weight loss upon FMD versus IF correlated with stronger fasting response assessed by lower glucose levels and higher ketone body, free fatty acids and especially FGF21 concentrations in blood. RNA sequencing demonstrated similar transcriptional programs in the liver after both regimens, with PPARα signalling as top enriched pathway, while on individual gene level FMD more potently increased gluconeogenesis-related, and PPARα and p53 target gene expression compared to IF. Repeated IF induced similar transcriptional responses as acute IF. However, repeated cycles of FMD resulted in blunted expression of genes involved in ketogenesis and fatty acid oxidation.

CONCLUSIONS

Short-term FMD causes more pronounced changes in blood parameters and slightly higher weight loss than IF, while both activate similar pathways (particularly PPARα signalling) in the liver. On individual gene level FMD induces a stronger transcriptional response, whereas cyclic application blunts transcriptional upregulation of fatty acid oxidation and ketogenesis only in FMD. Hence, our comparative characterization of IF and FMD protocols renders both as effective DR regimens and serves as resource in the fasting research field.

Transcriptional cascades during fasting amplify gluconeogenesis and instigate a secondary wave of ketogenic gene transcription

BACKGROUND AND AIMS

During fasting, bodily homeostasis is maintained due to hepatic production of glucose (gluconeogenesis) and ketone bodies (ketogenesis). The main hormones governing hepatic fuel production are glucagon and glucocorticoids that initiate transcriptional programs aimed at supporting gluconeogenesis and ketogenesis.

METHODS

Using primary mouse hepatocytes as an ex vivo model, we employed transcriptomic analysis (RNA-seq), genome-wide profiling of enhancer dynamics (ChIP-seq), perturbation experiments (inhibitors, shRNA), hepatic glucose production measurements and computational analyses.

RESULTS

We found that in addition to the known metabolic genes transcriptionally induced by glucagon and glucocorticoids, these hormones induce a set of genes encoding transcription factors (TFs) thereby initiating transcriptional cascades. Upon activation by glucocorticoids, the glucocorticoid receptor (GR) induced the genes encoding two TFs: CCAAT/enhancer-binding protein beta (C/EBPβ) and peroxisome proliferator-activated receptor alpha (PPARα). We found that the GR-C/EBPβ cascade mainly serves as a secondary amplifier of primary hormone-induced gene programs. C/EBPβ augmented gluconeogenic gene expression and hepatic glucose production. Conversely, the GR-PPARα cascade initiated a secondary transcriptional wave of genes supporting ketogenesis. The cascade led to synergistic induction of ketogenic genes which is dependent on protein synthesis. Genome-wide analysis of enhancer dynamics revealed numerous enhancers activated by the GR-PPARα cascade. These enhancers were proximal to ketogenic genes, enriched for the PPARα response element and showed increased PPARα binding.

CONCLUSION

This study reveals abundant transcriptional cascades occurring during fasting. These cascades serve two separated purposes: the amplification of the gluconeogenic transcriptional program and the induction of a gene program aimed at enhancing ketogenesis.

Single-cell resolution of intestinal regeneration in pythons without crypts illuminates conserved vertebrate regenerative mechanisms

Canonical models of intestinal regeneration emphasize the critical role of the crypt stem cell niche to generate enterocytes that migrate to villus ends. Burmese pythons possess extreme intestinal regenerative capacity yet lack crypts, thus providing opportunities to identify noncanonical but potentially conserved mechanisms that expand our understanding of regenerative capacity in vertebrates, including humans. Here, we leverage single-nucleus RNA sequencing of fasted and postprandial python small intestine to identify the signaling pathways and cell-cell interactions underlying the python's regenerative response. We find that python intestinal regeneration entails the activation of multiple conserved mechanisms of growth and stress response, including core lipid metabolism pathways and the unfolded protein response in intestinal enterocytes. Our single-cell resolution highlights extensive heterogeneity in mesenchymal cell population signaling and intercellular communication that directs major tissue restructuring and the shift out of a dormant fasted state by activating both embryonic developmental and wound healing pathways. We also identify distinct roles of BEST4+ enterocytes in coordinating key regenerative transitions via NOTCH signaling. Python intestinal regeneration shares key signaling features and molecules with mammalian gastric bypass, indicating that conserved regenerative programs are common to both. Our findings provide different insights into cooperative and conserved regenerative programs and intercellular interactions in vertebrates independent of crypts which have been otherwise obscured in model species where temporal phases of generative growth are limited to embryonic development or recovery from injury.

LXR-dependent enhancer activation regulates the temporal organization of the liver's response to refeeding leading to lipogenic gene overshoot

Transitions between the fed and fasted state are common in mammals. The liver orchestrates adaptive responses to feeding/fasting by transcriptionally regulating metabolic pathways of energy usage and storage. Transcriptional and enhancer dynamics following cessation of fasting (refeeding) have not been explored. We examined the transcriptional and chromatin events occurring upon refeeding in mice, including kinetic behavior and molecular drivers. We found that the refeeding response is temporally organized with the early response focused on ramping up protein translation while the later stages of refeeding drive a bifurcated lipid synthesis program. While both the cholesterol biosynthesis and lipogenesis pathways were inhibited during fasting, most cholesterol biosynthesis genes returned to their basal levels upon refeeding while most lipogenesis genes markedly overshoot above pre-fasting levels. Gene knockout, enhancer dynamics, and ChIP-seq analyses revealed that lipogenic gene overshoot is dictated by LXRα. These findings from unbiased analyses unravel the mechanism behind the long-known phenomenon of refeeding fat overshoot.

Long-term fasting induced basal thermogenesis flexibility in female Japanese quails

Male Japanese quails (Coturnix japonica) have been found to exhibit a three-phase metabolic change when subjected to prolonged fasting, during which basal thermogenesis is significantly reduced. A study had shown that there is a significant difference in the body temperature between male and female Japanese quails. However, whether female Japanese quails also show the same characteristic three-phase metabolic change during prolonged fasting and the underlying thermogenesis mechanisms associated with such changes are still unclear. In this study, female Japanese quails were subjected to prolonged starvation, and the body mass, basal metabolic rate (BMR), body temperature, mass of tissues and organs, body fat content, the state-4 respiration (S4R) and cytochrome c oxidase (CCO) activity in the muscle and liver of these birds were measured to determine the status of metabolic changes triggered by the starvation. In addition, the levels of glucose, triglyceride (TG) and uric acid, and thyroid hormones (T3 and T4) in the serum and the mRNA levels of myostatin (MSTN) and avian uncoupling protein (av-UCP) in the muscle were also measured. The results revealed the existence of a three-phase stage similar to that found in male Japanese quails undergoing prolonged starvation. Fasting resulted in significantly lower body mass, BMR, body temperature, tissues masses and most organs masses, as well as S4R and CCO activity in the muscle and liver. The mRNA level of av-UCP decreased during fasting, while that of MSTN increased but only during Phase I and II and decreased significantly during Phase III. Fasting also significantly lowered the T3 level and the ratio of T3/T4 in the serum. These results indicated that female Japanese quails showed an adaptive response in basal thermogenesis at multiple hierarchical levels, from organismal to biochemical, enzyme and cellular level, gene and endocrine levels and this integrated adjustment could be a part of the adaptation used by female quails to survive long-term fasting.

Intermittent fasting improves the oocyte quality of obese mice through the regulation of maternal mRNA storage and translation by LSM14B

Obesity has significant repercussions for female reproductive health, including adverse effects on oocyte quality, fertility, embryo development and offspring health. Here, we showed that intermittent fasting (IF) has several notable effects on follicular development, oocyte development and maturation and offspring health in obese mice. IF treatment prevents obesity-associated germline-soma communication defects, mitochondrial dysfunction, oxidative damage, apoptosis, and spindle/chromosomal disruption. RNA-sequencing analysis of oocytes from normal diet (ND), high-fat diet (HFD), and HFD + IF mice indicated that IF treatment improved mitochondrial oxidative phosphorylation function and mRNA storage and translation, which was potentially mediated by the Smith-like family member 14 B (LSM14B). Knockdown of LSM14B by siRNA injection in oocytes from ND mice recapitulates all the translation, mitochondrial dysfunction and meiotic defect phenotypes of oocytes from HFD mice. Remarkably, the injection of Lsm14b mRNA into oocytes from HFD mice rescued the translation, mitochondrial dysfunction and meiotic defect phenotypes. These results demonstrated that dysfunction in the oocyte translation program is associated with obesity-induced meiotic defects, while IF treatment increased LSM14B expression and maternal mRNA translation and restored oocyte quality. This research has important implications for understanding the effects of obesity on female reproductive health and offers a potential nonpharmacological intervention to improve oocyte quality and fertility in obese individuals.

Vitamin D3 Regulates Energy Homeostasis under Short-Term Fasting Condition in Zebrafish (Danio Rerio)

Vitamin D3 (VD3) is a steroid hormone that plays pivotal roles in pathophysiology, and 1,25(OH)2D3 is the most active form of VD3. In the current study, the crucial role of VD3 in maintaining energy homeostasis under short-term fasting conditions was investigated. Our results confirmed that glucose-depriving pathways were inhibited while glucose-producing pathways were strengthened in zebrafish after fasting for 24 or 48 h. Moreover, VD3 anabolism in zebrafish was significantly suppressed in a time-dependent manner under short-fasting conditions. After fasting for 24 or 48 h, zebrafish fed with VD3 displayed a higher gluconeogenesis level and lower glycolysis level in the liver, and the serum glucose was maintained at higher levels, compared to those fed without VD3. Additionally, VD3 augmented the expression of fatty acids (FAs) transporter cd36 and lipogenesis in the liver, while enhancing lipolysis in the dorsal muscle. Similar results were obtained in cyp2r1-/- zebrafish, in which VD3 metabolism is obstructed. Importantly, it was observed that VD3 induced the production of gut GLP-1, which is considered to possess a potent gluconeogenic function in zebrafish. Meanwhile, the gene expression of proprotein convertase subtilisin/kexin type 1 (pcsk1), a GLP-1 processing enzyme, was also induced in the intestine of short-term fasted zebrafish. Notably, gut microbiota and its metabolite acetate were involved in VD3-regulated pcsk1 expression and GLP-1 production under short-term fasting conditions. In summary, our study demonstrated that VD3 regulated GLP-1 production in zebrafish by influencing gut microbiota and its metabolite, contributing to energy homeostasis and ameliorating hypoglycemia under short-term fasting conditions.

Hoven's carp Leptobarbus hoevenii strategized metabolism needs to cope with changing environment

Current water warming and freshwater acidification undoubtedly affect the life of aquatic animals especially ammonotelic teleost by altering their physiological responses. The effect of temperature (28 °C vs 32 °C) and pH (7 vs. 5) on the metabolic compromising strategies of Hoven's carp (Leptobarbus hoevenii) was investigated in this study. Fishes were conditioned to (i) 28 °C + pH 7 (N28°C); (ii) 32 °C + pH 7 (N32°C); (iii) 28 °C + pH 5 (L28°C) and (iv) 32 °C + pH 5 (L32°C) for 20 days followed by osmorespiration assay. Results showed that feeding performance of Hoven's carp was significantly depressed when exposed to low pH conditions (L28°C and L32°C). However, by exposed Hoven's carp to L32°C induced high metabolic oxygen intake and ammonia excretion to about 2x-folds higher compared to the control group. As for energy mobilization, Hoven's carp mobilized liver and muscle protein under L28°C condition. Whereas under high temperature in both pH, Hoven's carp had the tendency to reserve energy in both of liver and muscle. The findings of this study revealed that Hoven's carp is sensitive to lower water pH and high temperature, thereby they remodeled their physiological needs to cope with the environmental changes condition.

Common and divergent molecular mechanisms of fasting and ketogenic diets

Intermittent short-term fasting (ISTF) and ketogenic diets (KDs) exert overlapping but not identical effects on cell metabolism, function, and resilience. Whereas health benefits of KD are largely mediated by the ketone bodies (KBs), ISTF engages additional adaptive physiological responses. KDs act mainly through inhibition of histone deacetylases (HDACs), reduction of oxidative stress, improvement of mitochondria efficiency, and control of inflammation. Mechanisms of action of ISTF include stimulation of autophagy, increased insulin and leptin sensitivity, activation of AMP-activated protein kinase (AMPK), inhibition of the mechanistic target of rapamycin (mTOR) pathway, bolstering mitochondrial resilience, and suppression of oxidative stress and inflammation. Frequent switching between ketogenic and nonketogenic states may optimize health by increasing stress resistance, while also enhancing cell plasticity and functionality.

Environmental variation associated with overwintering elicits marked metabolic plasticity in a temperate salmonid, Salvelinus fontinalis

Poleward winters commonly expose animals, including fish, to frigid temperatures and low food availability. Fishes that remain active over winter must therefore balance trade-offs between conserving energy and maintaining physiological performance in the cold, yet the extent and underlying mechanisms of these trade-offs are not well understood. We investigated the metabolic plasticity of brook char (Salvelinus fontinalis), a temperate salmonid, from the biochemical to whole-animal level in response to cold and food deprivation. Acute cooling (1°C day-1) from 14°C to 2°C had no effect on food consumption but reduced activity by 77%. We then assessed metabolic performance and demand over 90 days with exposure to warm (8°C) or cold winter (2°C) temperatures while fish were fed or starved. Resting metabolic rate (RMR) decreased substantially during initial cooling from 8°C to 2°C (Q10=4.2-4.5) but brook char exhibited remarkable thermal compensation during acclimation (Q10=1.4-1.6). Conversely, RMR was substantially lower (40-48%) in starved fish, conserving energy. Thus, the absolute magnitude of thermal plasticity may be masked or modified under food restriction. This reduction in RMR was associated with atrophy and decreases in in vivo protein synthesis rates, primarily in non-essential tissues. Remarkably, food deprivation had no effect on maximum oxygen uptake rates and thus aerobic capacity, supporting the notion that metabolic capacity can be decoupled from RMR in certain contexts. Overall, our study highlights the multi-faceted energetic flexibility of Salvelinus spp. that likely contributes to their success in harsh and variable environments and may be emblematic of winter-active fishes more broadly.

Differential modulation of digestive enzymes and energy reserves at different times after feeding in juveniles of the marine estuarine-dependent flatfish Paralichthys orbignyanus (Valenciennes, 1839)

Integrative studies are lacking on the responses of digestive enzymes and energy reserves in conjunction with morphological traits at distinct postprandial times in marine estuarine-dependent flatfishes of ecological and economic importance, such as Paralichthys orbignyanus. We determined total weight (TW), hepato-somatic index (IH), activities of digestive enzymes in the intestine, and the concentration of energy reserves in the liver and the muscle at 0, 24, 72, and 360 h after feeding in juveniles of P. orbignyanus. Amylase activity decreased at 72 h (about 30%). Maltase, sucrose, and lipase activities reached peak at 24 h (67%, 600%, and 35%, respectively). Trypsin and aminopeptidase-N activities at 24 and 72 h, respectively, were lower than those at t = 0 (53% and 30%). A peak increase in the concentration of glycogen and triglycerides in the liver (24 h) (86% and 89%, respectively) occurred. In muscle, glycogen and triglyceride concentrations were unchanged at 24 h and higher at 72 and 360 h (100% and 60%). No changes were found in TW, IH, free glucose in the liver and muscle, and protein in the liver. The protein concentration in the muscle sharply increased at 24 and 360 h after feeding (60%). The results indicate a distinct and specific response of central components of carbohydrate, lipid, and protein metabolism that could be adjustments at the biochemical level upon periods of irregular feeding and even of long-term food deprivation inside coastal lagoons or estuaries. The distinct responses of digestive enzymes in the intestine and energy reserves in the liver and muscle suggest the differential modulation of tissue-specific anabolic and catabolic pathways that would allow the maintenance of physical conditions.

Three steps down: Metabolic depression in winter-acclimatized crucian carp (Carassius carassius L.)

Acclimatization of certain ectothermic vertebrates to winter conditions is associated with reduced energy consumption (winter dormancy). Principally, this may be achieved by reducing movement activity, depression of basal cellular functions, or by switching from aerobic to anaerobic energy production to sustain low energy consumption during anoxia. Therefore, we determined standard (SMR), routine (RMR) and anoxic (AMR) metabolic rates in summer- (SumA; 20 °C) and winter-acclimatized (WinA; 2 °C) crucian carp (Carassius carassius), an anoxia-tolerant teleost fish. At 20 °C, RMR was 39% lower in WinA than SumA fish (p < 0.05), indicating reduced movements in winter. SMR, measured for the first time in crucian carp, was 45% lower in WinA than SumA fish at 10 °C (p < 0.05), indicating significant reduction of energy consumption in vital cellular processes. At 2 °C, AMR - measured from ethanol production- was 78% and 97% of the SMR and RMR levels at the same temperature, respectively. The current study revealed that the winter dormancy in anoxia-tolerant crucian carp is achieved in 3 different steps: (1) by active reduction of SMR, possibly in anticipation of seasonal anoxia, (2) acute Arrhenius Q10 effect that slows SMR as well as RMR, and (3) direct response to the absence of oxygen (AMR). Furthermore, the anoxic energy production was strongly dependent on body mass with scaling exponents of -0.335 and - 0.421 for WinA and SumA fish, respectively.

Fasting durations of Steller sea lion pups vary among subpopulations-evidence from two plasma metabolites

Geographic differences in population growth trends are well-documented in Steller sea lions (Eumetopias jubatus), a species of North Pacific pinniped listed under the U.S. Endangered Species Act in 1990 following a marked decline in population abundance that began during the 1970s. As population growth is intrinsically linked to pup production and survival, examining factors related to pup physiological condition provides useful information to management authorities regarding potential drivers of regional differences. During dam foraging trips, pups predictably transition among three fasting phases, distinguished by the changes in the predominant metabolic byproduct. We used standardized ranges of two plasma metabolites (blood urea nitrogen and β-hydroxybutyrate) to assign pups to fasting categories (n = 1528, 1990-2016, 12 subpopulations): Recently Fed-Phase I (digestion/assimilation-expected hepatic/muscle glycogen usage), Phase II (expected lipid utilization), transitioning between Phases II-III (expected lipid utilization with increased protein reliance), or Phase III (expected protein catabolism). As anticipated, the majority of pups were classified as Recently Fed-Phase I (overall mean proportion = 0.72) and few pups as Phase III (overall mean proportion = 0.04). By further comparing pups in Short (Recently Fed-Phase II) and Long (all other pups) duration fasts, we identified three subpopulations with significantly (P < 0.03) greater proportions of pups dependent upon endogenous sources of energy for extended periods, during a life stage of somatic growth and development: the 1) central (0.27 ± 0.09) and 2) western (0.36 ± 0.13) Aleutian Island (declining population trend) and 3) southern Southeast Alaska (0.32 ± 0.06; increasing population trend) subpopulations had greater Long fast proportions than the eastern Aleutian Islands (0.10 ± 0.05; stabilized population). Due to contrasting population growth trends among these highlighted subpopulations over the past 50+ years, both density-independent and density-dependent factors likely influence the dam foraging trip duration, contributing to longer fasting durations for pups at some rookeries.

Aestivation in Nature: Physiological Strategies and Evolutionary Adaptations in Hypometabolic States

Aestivation is considered to be one of the "purest" hypometabolic states in nature, as it involves aerobic dormancy that can be induced and sustained without complex factors. Animals that undergo aestivation to protect themselves from environmental stressors such as high temperatures, droughts, and food shortages. However, this shift in body metabolism presents new challenges for survival, including oxidative stress upon awakening from aestivation, accumulation of toxic metabolites, changes in energy sources, adjustments to immune status, muscle atrophy due to prolonged immobility, and degeneration of internal organs due to prolonged food deprivation. In this review, we summarize the physiological and metabolic strategies, key regulatory factors, and networks utilized by aestivating animals to address the aforementioned components of aestivation. Furthermore, we present a comprehensive overview of the advancements made in aestivation research across major species, including amphibians, fish, reptiles, annelids, mollusks, and echinoderms, categorized according to their respective evolutionary positions. This approach offers a distinct perspective for comparative analysis, facilitating an understanding of the shared traits and unique features of aestivation across different groups of organisms.

Feeding and Amines Stimulate the Growth of the Salivary Gland following Short-Term Starvation in the Black Field Cricket, Teleogryllus commodus

The salivary gland of the black field cricket, Teleogryllus commodus Walker changed size between being starved and fed. Crickets without access to food for 72 h showed a reduction in both wet and dry mass of the glands compared with the glands from continuously fed animals at 72 h. Glands returned to size following ingestion within 10 min. Salivary glands of starved crickets (72 h) were incubated in saline containing either serotonin (5-HT) or dopamine (DA). Glands increased to pre-starvation size after 1 h incubation in situ with either 10^-4 moles L^-1 5-HT or 10^-4 moles L^-1 DA, although lower concentrations (10^-5 moles L^-1) did not affect gland size. From immunohistochemistry, amines appeared to shift from zymogen cells during starvation to parietal cells following feeding. High-performance liquid chromatography showed that serotonin concentration is higher than dopamine in the salivary gland removed from starved and fed crickets, but the quantity of these compounds was not dependent upon feeding state; the amine quantities increased as gland size increased. Further work is necessary to determine what might be the stimulus for gland growth and if dopamine and serotonin play a role in the stimulation of salivary gland growth after a period of starvation.

Characteristics and a comparison of the gut microbiota in two frog species at the beginning and end of hibernation

Season has been suggested to contribute to variation in the gut microbiota of animals. The complicated relationships between amphibians and their gut microbiota and how they change throughout the year require more research. Short-term and long-term hypothermic fasting of amphibians may affect gut microbiota differently; however, these changes have not been explored. In this study, the composition and characteristics of the gut microbiota of Rana amurensis and Rana dybowskii during summer, autumn (short-term fasting) and winter (long-term fasting) were studied by high-throughput Illumina sequencing. Both frog species had higher gut microbiota alpha diversity in summer than autumn and winter, but no significant variations between autumn and spring. The summer, autumn, and spring gut microbiotas of both species differed, as did the autumn and winter microbiomes. In summer, autumn and winter, the dominant phyla in the gut microbiota of both species were Firmicutes, Proteobacteria, Bacteroidetes, and Actinobacteria. All animals have 10 OTUs (>90% of all 52 frogs). Both species had 23 OTUs (>90% of all 28 frogs) in winter, accounting for 47.49 ± 3.84% and 63.17 ± 3.69% of their relative abundance, respectively. PICRUSt2 analysis showed that the predominant functions of the gut microbiota in these two Rana were focused on carbohydrate metabolism, Global and overview maps, Glycan biosynthesis metabolism, membrane transport, and replication and repair, translation. The BugBase analysis estimated that among the seasons in the R. amurensis group, Facultatively_Anaerobic, Forms_Biofilms, Gram_Negative, Gram_Positive, Potentially_Pathogenic were significantly different. However, there was no difference for R. dybowskii. The research will reveal how the gut microbiota of amphibians adapts to environmental changes during hibernation, aid in the conservation of endangered amphibians, particularly those that hibernate, and advance microbiota research by elucidating the role of microbiota under various physiological states and environmental conditions.

Effects of Starvation and Refeeding on Growth, Digestion, Nonspecific Immunity and Lipid-Metabolism-Related Genes in Onychostoma macrolepis

The present research was conducted to assess the influences of starvation and refeeding on growth, nonspecific immunity and lipid metabolic adaptation in Onychostoma macrolepis. To date, there have been no similar reports in O. macrolepis. The fish were randomly assigned into two groups: control group (continuous feeding for six weeks) and starved–refed group (starvation for three weeks and then refeeding for three weeks). After three weeks of starvation, the results showed that the body weight (BW, 1.44 g), condition factor (CF, 1.17%), visceral index (VSI, 3.96%), hepatopancreas index (HSI, 0.93%) and intraperitoneal fat index (IPFI, 0.70%) of fish were significantly lower compared to the control group (BW, 5.72 g; CF, 1.85%; VSI, 6.35%; HSI, 2.04%; IPFI, 1.92%) (p < 0.05). After starvation, the serum triglyceride (TG, 0.83 mmol/L), total cholesterol (T-GHOL, 1.15 mmol/L), high-density lipoprotein (HDL, 1.13 mmol/L) and low-density lipoprotein (LDL, 0.46 mmol/L) concentrations were significantly lower than those in the control group (TG, 1.69 mmol/L; T-GHOL, 1.86 mmol/L; HDL, 1.62 mmol/L; LDL, 0.63 mmol/L) (p < 0.05). The activities of intestinal digestive enzymes (amylase, lipase and protease) in the starved-refed group were significantly lower than those in the control group after three weeks of starvation (p < 0.05). The highest activities of immune enzymes such as lysozyme (LZM), acid phosphate (ACP), alkaline phosphate (ALP), superoxide dismutase (SOD), glutathione peroxidase (GSH-PX) and catalase (CAT) in the hepatopancreas were presented in the starved–refed group at second week, and significantly higher than those in the control group (p < 0.05). Meanwhile, starvation significantly improved intestinal immune enzymes activities (p < 0.05). the lowest TG contents and the highest expression levels of lipolysis genes including hormone-sensitive lipase (HSL) and carnitine palmitoyl transferase 1 isoform A (CPT-1A) appeared in the hepatopancreas, muscle and intraperitoneal fat after starvation, indicating the mobilization of fat reserves in these tissues (p < 0.05). After refeeding, the recovery of TG content might be mediated by the upregulation of the expression levels of lipogenesis genes such as sterol regulatory element binding protein 1 (SREBP1) and fatty acid synthase (FAS). Understanding the duration of physiological and metabolic changes in O. macrolepis and their reversibility or irreversibility to supplementary feeding response could provide valuable reference for the adaptability of O. macrolepis in large-scale culturing, proliferation and release.

Adult male Xenopus laevis can tolerate months of fasting by catabolizing carbohydrates and lipids

The African clawed frog, Xenopus laevis, has been reported to tolerate long-term fasting without dormancy. However, the strategies for energy acquisition during fasting are unclear in this species. We performed 3- and 7-month fasting experiments to investigate how the metabolism of male X. laevis changes during long-term fasting. We found that the levels of several serum biochemical parameters, such as glucose, triglycerides, and free fatty acids, as well as liver glycogen were reduced after 3 months of fasting, whereas after 7 months of fasting, triglyceride levels were reduced, and fat body wet weight was lower than that of fed group indicating the onset of lipid catabolism. In addition, transcript levels of gluconeogenic genes, such as pck1, pck2, g6pc1.1, and g6pc1.2, were increased in the livers of animals fasted for 3 months, suggesting upregulation of gluconeogenesis. Our results raise the possibility that male X. laevis can tolerate much longer fasting than previously reported by utilizing several energy storage molecules. Further investigation of the effects of prolonged fasting on the metabolic switches from carbohydrates to lipids or amino acids in X. laevis is required.

The fire of evolution: energy expenditure and ecology in primates and other endotherms

Total energy expenditure (TEE) represents the total energy allocated to growth, reproduction and body maintenance, as well as the energy expended on physical activity. Early experimental work in animal energetics focused on the costs of specific tasks (basal metabolic rate, locomotion, reproduction), while determination of TEE was limited to estimates from activity budgets or measurements of subjects confined to metabolic chambers. Advances in recent decades have enabled measures of TEE in free-living animals, challenging traditional additive approaches to understanding animal energy budgets. Variation in lifestyle and activity level can impact individuals' TEE on short time scales, but interspecific differences in TEE are largely shaped by evolution. Here, we review work on energy expenditure across the animal kingdom, with a particular focus on endotherms, and examine recent advances in primate energetics. Relative to other placental mammals, primates have low TEE, which may drive their slow pace of life and be an evolved response to the challenges presented by their ecologies and environments. TEE variation among hominoid primates appears to reflect adaptive shifts in energy throughput and allocation in response to ecological pressures. As the taxonomic breadth and depth of TEE data expand, we will be able to test additional hypotheses about how energy budgets are shaped by environmental pressures and explore the more proximal mechanisms that drive intra-specific variation in energy expenditure.

Transcriptomic changes associated with maternal care in the brain of mouthbrooding cichlid Astatotilapia burtoni reflect adaptation to self-induced metabolic stress

Parental care in Astatotilapia burtoni entails females protecting eggs and developing fry in a specialized buccal cavity in the mouth. During this mouthbrooding behavior, which can last 2-3 weeks, mothers undergo voluntary fasting accompanied by loss of body mass and major metabolic changes. Following release of fry, females resume normal feeding behavior and quickly recover body mass as they become reproductively active once again. In order to investigate the molecular underpinnings of such dramatic behavioral and metabolic changes, we sequenced whole-brain transcriptomes from females at four time points throughout their reproductive cycle: 2 days after the start of mouthbrooding, 14 days after the start of mouthbrooding, 2 days after the release of fry and 14 days after the release of fry. Differential expression analysis and clustering of expression profiles revealed a number of neuropeptides and hormones, including the strong candidate gene neurotensin, suggesting that molecular mechanisms underlying parental behaviors may be common across vertebrates despite de novo evolution of parental care in these lineages. In addition, oxygen transport pathways were found to be dramatically downregulated, particularly later in the mouthbrooding stage, while certain neuroprotective pathways were upregulated, possibly to mitigate negative consequences of metabolic depression brought about by fasting. Our results offer new insights into the evolution of parental behavior as well as revealing candidate genes that would be of interest for the study of hypoxic ischemia and eating disorders.

Energy expenditure and body composition in a hibernator, the alpine marmot

Visceral organs and tissues of 89 free-living alpine marmots (Marmota marmota) shot during a population control program in Switzerland, were collected. Between emergence from hibernation in April to July, the gastrointestinal tract (stomach to colon) gained 51% of mass and the liver mass increased by 24%. At the same time, the basal metabolic rate (BMR), determined with a portable oxygen analyzer, increased by 18%. The organ masses of the digestive system (stomach, small intestine, caecum, large intestine) were all significantly correlated with BMR. Interestingly, the mass of abdominal white adipose tissue (WAT) and of the remaining carcass (mainly skin and bones) were also significantly correlated with BMR. These results indicate that the gastrointestinal tract and organs involved in digestive function are metabolically expensive. They also show that it is costly to maintain even tissues with low metabolic rate such as WAT, especially if they are large. Heart and kidneys and especially brain and lungs did not explain a large proportion of the variance in BMR. Marmots increased the uptake of fat prior to hibernation, both by selective feeding and enhanced gastrointestinal capacity. Large fat reserves enable marmots to hibernate without food intake and to reproduce in spring, but at the cost of an elevated BMR. We predict that climate changes that disturb energy accumulation in summer, increase energy expenditure in winter, or delay the emergence from hibernation in spring, such as the occurrence of storms with increasing frequency, will increase mortality in alpine marmots.

Partial immune responses in Sichuan bream (Sinibrama taeniatus) after starvation

Background

Food deprivation is a severe stress across multiple fields and it might be a challenge to immune system.

Methods

In the present study, adult male Sinibrama taeniatus were deprived of feed for 7 to 28 days. We explored the effects of starvation on immunity in S. taeniatus through hematological analysis, antioxidant capacity analysis, detection of the content or activity of immune factors in plasma, and transcriptomic analysis.

Results

The results indicated that biometric indexes significantly decreased in the fish after starvation, the proportion of thrombocyte, neutrophil and monocyte increased and, conversely, the proportion of lymphocyte decreased. The antioxidant indexes (SOD and CAT) and innate immune parameters (LZM, C3) were upregulated in fish suffering from a short period of starvation, while adaptive immune parameter (IgM) conversely declined. The transcriptome analysis revealed the changes of various metabolic regulatory pathways involved in fatty acids and amino acids, as well as the immune responses and antioxidant capacity.

Conclusions

Taken together, this research in the present study suggested an induced innate immunity while a partly suppressed adaptive immunity under a short period starvation.

Pore-forming protein βγ-CAT promptly responses to fasting with capacity to deliver macromolecular nutrients

During animal fasting, the nutrient supply and metabolism switch from carbohydrates to a new reliance on the catabolism of energy-dense lipid stores. Assembled under tight regulation, βγ-CAT (a complex of non-lens βγ-crystallin and trefoil factor) is a pore-forming protein and trefoil factor complex identified in toad Bombina maxima. Here, we determined that this protein complex is a constitutive component in toad blood, that actively responds to the animal fasting. The protein complex was able to promote cellular albumin and albumin-bound fatty acid (FA) uptake in a variety of epithelial and endothelial cells, and the effects were attenuated by a macropinocytosis inhibitor. Endothelial cell-derived exosomes containing largely enriched albumin and FAs, called nutrisomes, were released in the presence of βγ-CAT. These specific nutrient vesicles were readily taken up by starved myoblast cells to support their survival. The results uncovered that pore-forming protein βγ-CAT is a fasting responsive element able to drive cell vesicular import and export of macromolecular nutrients.

Circadian rhythms in metabolic organs and the microbiota during acute fasting in mice

The circadian clock regulates metabolism in anticipation of regular changes in the environment. It is found throughout the body, including in key metabolic organs such as the liver, adipose tissues, and intestine, where the timing of the clock is set largely by nutrient signaling. However, the circadian clocks of these tissues during the fasted state have not been completely characterized. Moreover, the sufficiency of a functioning host clock to produce diurnal rhythms in the composition of the microbiome in fasted animals has not been explored. To this end, mice were fasted 24 h prior to collection of key metabolic tissues and fecal samples for the analysis of circadian clock gene expression and microbiome composition. Rhythm characteristics were determined using CircaCompare software. We identify tissue-specific changes to circadian clock rhythms upon fasting, particularly in the brown adipose tissue, and for the first time demonstrate the rhythmicity of the microbiome in fasted animals.

p53 Regulates a miRNA-Fructose Transporter Axis in Brown Adipose Tissue Under Fasting

Active thermogenic adipocytes avidly consume energy substrates like fatty acids and glucose to maintain body temperature upon cold exposure. Despite strong evidence for the involvement of brown adipose tissue (BAT) in controlling systemic energy homeostasis upon nutrient excess, it is unclear how the activity of brown adipocytes is regulated in times of nutrient scarcity. Therefore, this study aimed to scrutinize factors that modulate BAT activity to balance thermogenic and energetic needs upon simultaneous fasting and cold stress. For an unbiased view, we performed transcriptomic and miRNA sequencing analyses of BAT from acutely fasted (24 h) mice under mild cold exposure. Combining these data with in-depth bioinformatic analyses and in vitro gain-of-function experiments, we define a previously undescribed axis of p53 inducing miR-92a-1-5p transcription that is highly upregulated by fasting in thermogenic adipocytes. p53, a fasting-responsive transcription factor, was previously shown to control genes involved in the thermogenic program and miR-92a-1-5p was found to negatively correlate with human BAT activity. Here, we identify fructose transporter Slc2a5 as one direct downstream target of this axis and show that fructose can be taken up by and metabolized in brown adipocytes. In sum, this study delineates a fasting-induced pathway involving p53 that transactivates miR-92a-1-5p, which in turn decreases Slc2a5 expression, and suggests fructose as an energy substrate in thermogenic adipocytes.

Time-limited diets and the gut microbiota in cardiometabolic disease

In recent years, intermittent fasting (IF), including periodic fasting and time-restricted feeding (TRF), has been increasingly suggested to constitute a promising treatment for cardiometabolic diseases (CMD). A deliberate daily pause in food consumption influences the gut microbiome and the host circadian clock, resulting in improved cardiometabolic health. Understanding the molecular mechanisms by which circadian host-microbiome interactions affect host metabolism and immunity may add a potentially important dimension to effective implementation of IF diets. In this review, we discuss emerging evidence potentially linking compositional and functional alterations of the gut microbiome with IF impacts on mammalian metabolism and risk of development of hypertension, type 2 diabetes (T2D), obesity, and their long-term micro- and macrovascular complications. We highlight the challenges and unknowns in causally linking diurnal bacterial signals with dietary cues and downstream metabolic consequences and means of harnessing these signals toward future microbiome integration into precision medicine.

Inter-individual variation in mitochondrial phosphorylation efficiency predicts growth rates in ectotherms at high temperatures

There is increasing evidence that aquatic ectotherms are especially vulnerable to global warming since their metabolic demands increase with ambient temperature while water-oxygen content decreases. The possible role of shrinking aerobic scope in limiting performance has been much discussed; however, less attention has been given to whether tissue-level changes in the efficiency of oxygen usage occur at elevated temperatures. Here, we show that this varies widely among individuals, with consequences for performance. We examined the inter-individual variation in growth rate and mitochondrial function from white muscle and liver of brown trout (Salmo trutta) acclimated to either high (19.5°C) or near-optimal temperature (12°C). Liver (but not muscle) mitochondria showed a positive relationship between growth rate and maximal oxidative phosphorylation at both temperatures, and a negative relationship between growth rate and ROS release. There was a positive correlation in both tissues between individual mitochondrial phosphorylation efficiency and growth rate, but only at 19.5°C. In this representative of aquatic ectotherms, an individual's liver mitochondrial efficiency thus seems to dictate its capacity to grow at elevated temperatures. This suggests that individual heterogeneity in cellular function may cause variation in the thermal limits of aquatic ectotherms and could adversely affect wild populations in warming environments.

Daily and fasting variation in blood parameters and H/L ratio in house sparrows (Passer domesticus)

Fluctuations in food supply are frequent in nature during seasons, in different stages of annual cycle of animals and lately, climate change is a strong driving force that could affect food supply. An animal's capacity to resist fasting is, therefore, determined by its ability to store energy and control its allocation during periods of food restriction. Daily variations of metabolic substrates and their fluctuation during fasting are relevant to understand the acclimation to fasting. Since fasting is an ecological stressor, we investigated the daily variation of corticosterone levels in feeding and fasting in Passer domesticus Linnaeus, 1758 and its short-term effect on some biochemical parameters. We found daily variations in body mass, glucose, triglycerides, plasma total protein, uric acid and corticosterone. However, we did not find diurnal variations of hematocrit and the Heterophil:Lymphocyte ratio (H/L ratio). Moreover, we did not find sex-related differences in any parameter. Throughout fasting we found variations of glucose, triglycerides, total protein, uric acid, H/L ratio and corticosterone. During the photophase, biochemical and physiological responses to food deprivation contribute to the efficient use of resources in this small bird. In the course of fasting period corticosterone could be responsible of alterations of some blood parameters such as glucose.

Key Performance Indicators of Common Carp (Cyprinus carpio L.) Wintering in a Pond and RAS under Different Feeding Schemes

Overwintering impacts common carp performance, yet the nature of changes is not known. The aim of the study was to compare the zootechnical and key performance indicators (KPI) of Cyprinus carpio wintering in a pond with no supplementary feeding (MCF), in a Recirculating Aquaculture System (RAS) fed typical (30% of protein and 8% of fat) carp diet (AFC), and in a RAS fed high protein (42%) and fat (12%) diet (ABF). The analysis showed that ABF fish had the highest final body weight and the Fulton’s condition factor, as well as the lowest food conversion rate compared with AFC and MCF fish. Histomorphological assessment revealed that MCF fish had thinner skin layers, a depleted population of mucous cells in skin, an excessive interlamellar mass in the gills, and no supranuclear vacuoles in the intestine compared to fish from RAS. At the molecular level, higher transcript levels of il-1β and il-6 transcripts were found in the gills of MCF than in fish from RAS. The transcript level of the intestinal muc5b was the highest in ABF fish. Relative expression of il-1β and il-6 in gills were presumably the highest due to lamellar fusions in MCF fish. Described KPIs may assist carp production to ensure sustainability and food security in the European Union.

The neuroprotective effects of intermittent fasting on brain aging and neurodegenerative diseases via regulating mitochondrial function

Intermittent fasting (IF) has been studied for its effects on lifespan and the prevention or delay of age-related diseases upon the regulation of metabolic pathways. Mitochondria participate in key metabolic pathways and play important roles in maintaining intracellular signaling networks that modulate various cellular functions. Mitochondrial dysfunction has been described as an early feature of brain aging and neurodegeneration. Although IF has been shown to prevent brain aging and neurodegeneration, the mechanism is still unclear. This review focuses on the mechanisms by which IF improves mitochondrial function, which plays a central role in brain aging and neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and Huntington's disease. The cellular and molecular mechanisms of IF in brain aging and neurodegeneration involve activation of adaptive cellular stress responses and signaling- and transcriptional pathways, thereby enhancing mitochondrial function, by promoting energy metabolism and reducing oxidant production.

Intermittent Fasting: Potential Bridge of Obesity and Diabetes to Health?

Obesity has been an escalating worldwide health problem for decades, and it is likely a risk factor of prediabetes and diabetes. Correlated with obesity, the number of diabetic patients is also remarkable. A modest weight loss (5–10%) is critical to alleviate the risk of any other metabolic disease. Reduced energy intake has been an essential factor for weight loss reduction. As a new behavior intervention to lose weight, intermittent fasting (IF) attracts considerable attention and has become a popular strategy among young people. IF is a diet pattern that cycles between periods of fasting and eating on a regular schedule, involving various types, mainly Intermittent Energy Restriction and Time-Restricted Fasting. Accumulating evidence shows that short-term IF has a greatly positive effect in animal studies and contributes favorable benefits in human trials as well. Nevertheless, as an emerging, diverse, and relatively premature behavior intervention, there are still limited studies considering patients with obesity and type 2 diabetes mellitus. It is also a controversial intervention for the treatment of metabolic disease and cancer. The risks and challenges appear consequently. Additionally, whether intermittent fasting can be applied to long-term clinical treatment, and whether it has side effects during the long-term period or not, demands more large-scale and long-term experiments.

Changes in apolipoprotein abundance dominate proteome responses to prolonged fasting in elephant seals

Unlike many animals that reduce activity during fasting, northern elephant seals (NES) undergo prolonged fasting during energy-intensive life-history stages such as reproduction and molting, fueling fasting energy needs by mobilizing fat stores accrued during foraging. NES display several unique metabolic features such as high fasting metabolic rates, elevated blood lipid and high-density lipoprotein (HDL) cholesterol levels, efficient protein sparing and resistance to oxidative stress during fasting. However, the cellular mechanisms that regulate these adaptations are still not fully understood. To examine how metabolic coordination is achieved during prolonged fasting, we profiled changes in blubber, skeletal muscle and plasma proteomes of adult female NES over a 5 week fast associated with molting. We found that while blubber and muscle proteomes were remarkably stable over fasting, over 50 proteins changed in abundance in plasma, including those associated with lipid storage, mobilization, oxidation and transport. Apolipoproteins dominated the blubber, plasma and muscle proteome responses to fasting. APOA4, APOE and APOC3, which are associated with lipogenesis and triglyceride accumulation, decreased, while APOA1, APOA2 and APOM, which are associated with lipid mobilization and HDL function, increased over fasting. Our findings suggest that changes in apolipoprotein composition may underlie the maintenance of high HDL levels and, together with adipokines and hepatokines that facilitate lipid catabolism, may mediate the metabolic transitions between feeding and fasting in NES. Many of these proteins have not been previously studied in this species and provide intriguing hypotheses about metabolic regulation during prolonged fasting in mammals.

N1-Methylnicotinamide: Is It Time to Consider as a Dietary Supplement for Athletes?

Abstract:

Exercise is considered to be a “medicine” due to its modulatory roles in metabolic disorders such as diabetes and obesity. The intensity and duration of exercise determine the mechanism of energy production by various tissues of the body, especially by muscles, in which the requirement for adenosine triphosphate (ATP) increases by as much as 100-fold. Naturally, athletes try to improve their exercise performance by dietary supplementation with, e.g., vitamins, metabolites, and amino acids. MNAM, as a vitamin B3 metabolite, reduces serum levels and liver contents of triglycerides, and cholesterol and induces lipolysis. It stimulates gluconeogenesis and prohibits liver cholesterol and fatty acid synthesis through the expression of sirtuin1 (SIRT1). It seems that MNAM is not responsible for the actions of NNMT in the adipose tissues as MNAM inhibits the activity of NNMT in the adipose tissue and acts like inhibitors of its activity. NNMT-MNAM axis is more activated in the muscles of participants who were undergoing the high-volume-low-intensity exercise and caloric restriction. Therefore, MNAM could be an important myokine during exercise and fasting where it provides the required energy for muscles through the induction of lipolysis and gluconeogenesis in the liver and adipose tissues, respectively. Increased levels of MNAM in exercise and fasting led us to propose that the consumption of MNAM during training especially endurance training could boost exercise capacity and improves performance. Therefore, in this review, we shed light on the potential of MNAM as a dietary supplement in sports medicine.

Traditional and Medical Applications of Fasting

Fasting has been practiced for millennia, for religious, ethical, or health reasons. It is also commonplace among different species, from humans, to animals, to lower eukaryotes. Research on fasting is gaining traction based on recent studies that show its role in many adaptive cellular responses such as the reduction of oxidative damage and inflammation, increase of energy metabolism, and in boosting cellular protection. In this expert review, we recount the historical evolution of fasting and we critically analyze its current medical applications, including benefits and caveats. Based on the available data, we conclude that the manipulation of dietary intake, in the form of calorie restriction, intermittent fasting, dietary restriction with the exclusion of some nutrients, prolonged fasting, and so forth, is anthropologically engraved in human culture possibly because of its positive health effects. Indeed, many studies show that fasting ameliorates many biochemical parameters related to cardiovascular and cancer risk, and neurodegeneration. Mechanistic studies are plentiful, but largely limited to cell cultures or laboratory animals. Understandably, there are no controlled trials of any form of fasting that gauge the effects on [any cause] mortality. Physicians should be aware that misinformation is pervasive and that their patients often adopt dietary regimens that are far from being clinically validated. Moreover, doctors are often unaware of their patients’ religious or traditional fasting and of its potential health effects. Based on current evidence, no long-term fasting should be undertaken without medical supervision until future research will hopefully help shed further light on fasting and its effects on human health.

Gut microbiota-a positive contributor in the process of intermittent fasting-mediated obesity control

Historically, intermittent fasting (IF) has been considered as an effective strategy for controlling the weight of athletes before competition. Along with excellent insight into its application in various spaces by numerous studies, increasing IF-mediated positive effects have been reported, including anti-aging, neuroprotection, especially obesity control. Recently, the gut microbiota has been considered as an essential manipulator for host energy metabolism and its structure has been reported to be sensitive to dietary structure and habits, indicating that there is a potential and strong association between IF and gut microbiota. In this paper, we focus on the crosstalk between these symbionts and energy metabolism during IF which hold the promise to optimize host energy metabolism at various physical positions, including adipose tissue, liver and intestines, and further improve milieu internal homeostasis. Moreover, this paper also discusses the positive function of a potential recommendatory strain (Akkermansia muciniphila) based on the observational data for IF-mediated alternated pattern of gut microbiota and a hopefully regulatory pathway (circadian rhythm) for gut microbiota in IF-involved improvement on host energy metabolism. Finally, this review addresses the limitation and perspective originating from these studies, such as the association with tissue-specific bio-clock and single strain research, which may continuously reveal novel viewpoints and mechanisms to understand the energy metabolism and develop new strategies for treating obesity, diabetes, and metabolic disorders.

Immune and hormonal regulation of the Boa constrictor (Serpentes; Boidae) in response to feeding

Feeding upregulates immune function and the systemic and local (gastrointestinal tract) concentrations of some immunoregulatory hormones, as corticosterone (CORT) and melatonin (MEL), in mammals and anurans. However, little is known about the immune and hormonal regulation in response to feeding in other ectothermic vertebrates, especially snakes, in which the postprandial metabolic changes are pronounced. Here, we investigated the effects feeding have on hormonal and innate immune responses in the snake, Boa constrictor. We divided juvenile males into two groups: fasting and fed with mice (30% of body mass). We measured the rates of oxygen consumption, plasma CORT levels, heterophil/lymphocyte ratio (HL ratio), plasma bacterial killing ability (BKA), and stomach and intestine MEL in fasting snakes and 48 h after meal intake. We observed increased rates of oxygen consumption, plasma CORT levels, and HL ratio, along with a tendency of decreased stomach and intestine MEL in fed snakes compared to fasting ones. BKA was not affected by feeding. Overall, we found that feeding modulates metabolic rates, CORT levels, and immune cell distribution in boas. Increased baseline CORT may be important to mobilize energy to support the metabolic increment during the postprandial period. Increased HL ratio might be an immunoregulatory effect of increased CORT, which has been shown in different physiological situations such as in response to immune challenge. Our results suggest that feeding activates the hypothalamic-pituitary-adrenal axis and modulates immune cell redistribution, possibly contributing to fighting potential injuries and infections derived from predation and from pathogens present in ingested food.

Immune and hormonal modulation in the postprandial period of bullfrogs (Lithobates catesbeianus)

Mammals show immune up-regulation and increased plasma and local (gastrointestinal tract) concentrations of some immunoregulatory hormones, such as corticosterone and melatonin, after feeding. However, little is known about the endocrine and immune modulation in the postprandial period of ectothermic animals. This study investigated the effects of feeding on endocrine and immune responses in the bullfrog (Lithobates catesbeianus). Frogs were fasted for 10 days and divided into two groups: fasted and fed with fish feed (5% of body mass). Blood and gastrointestinal tract tissues (stomach and intestine) were collected at 6, 24, 48, 96 and 168 h to measure neutrophil/lymphocyte ratio, plasma bacterial killing ability, phagocytosis of blood leukocytes, plasma corticosterone and melatonin, and stomach and intestine melatonin. Feeding increased plasma corticosterone at 24 h and decreased it at 168 h, and increased neutrophil/lymphocyte ratio at 6, 24 and 96 h. We also observed decreased bacterial killing ability 48 h after feeding. Stomach melatonin increased after 17 days of fasting. We show that feeding activates the hypothalamic-pituitary-interrenal axis and promotes transient immunosuppression, without stimulating an inflammatory response. Increased corticosterone may mobilize energy to support digestive processes and melatonin may protect the stomach during fasting. We conclude that feeding modulates secretion of immunoregulatory hormones, initially increasing plasma corticosterone levels, followed by a decrease at the end of meal digestion, and causes systemic immune cell redistribution, increasing neutrophil/lymphocyte ratio for almost the entire period of meal digestion in bullfrogs. Also, fasting modulates secretion of melatonin in the stomach.

Meander reshaping – the formation process of a wintering riverbed depression of fish

The study of the spatiotemporal distribution of fish is an important and poorly studied aspect of the ecology of aquatic organisms. The research work was performed using the modern hydroacoustic method and geographic information systems. A section in the lower reaches of the Irtysh, a large transboundary Siberian river (in Western Siberia, Russian Federation), was studied. It has a strong development of meandering. The merging (i.e., reshaping of closely spaced meanders and erosion-accumulating channel processes) results in development of wintering riverbed depression, which is a critical “temporal bottleneck” during the winter period of the fish life cycle. The average density of fish in the study area in summer and autumn was 8,031 and 9,194 individuals per ha, respectively. Analysis of the distribution of fish showed that the distribution in the horizontal aspect had a more aggregated character in the autumn. In the vertical aspect, it had a more surface (pelagic) character. The ichthyofauna in the water area of the riverbed depression is mainly represented by cyprinids. The loop-shaped evolution of the channel formed a section of the river with multidirectional, circulating, and counter-current flows. It also created zones with depths exceeding 20 m and aggregations of fish. These features characterize the studied water area as a wintering riverbed depression of the fish of the Lower Irtysh. This section of the river should be included in the list of protected wintering biotopes of fish in the West Siberian fishery basin, which will ensure the conservation of fish at the critical stage of the life cycle.

Meander reshaping – the formation process of a wintering riverbed depression of fish

The study of the spatiotemporal distribution of fish is an important and poorly studied aspect of the ecology of aquatic organisms. The research work was performed using the modern hydroacoustic method and geographic information systems. A section in the lower reaches of the Irtysh, a large transboundary Siberian river (in Western Siberia, Russian Federation), was studied. It has a strong development of meandering. The merging (i.e., reshaping of closely spaced meanders and erosion-accumulating channel processes) results in development of wintering riverbed depression, which is a critical “temporal bottleneck” during the winter period of the fish life cycle. The average density of fish in the study area in summer and autumn was 8,031 and 9,194 individuals per ha, respectively. Analysis of the distribution of fish showed that the distribution in the horizontal aspect had a more aggregated character in the autumn. In the vertical aspect, it had a more surface (pelagic) character. The ichthyofauna in the water area of the riverbed depression is mainly represented by cyprinids. The loop-shaped evolution of the channel formed a section of the river with multidirectional, circulating, and counter-current flows. It also created zones with depths exceeding 20 m and aggregations of fish. These features characterize the studied water area as a wintering riverbed depression of the fish of the Lower Irtysh. This section of the river should be included in the list of protected wintering biotopes of fish in the West Siberian fishery basin, which will ensure the conservation of fish at the critical stage of the life cycle.

Feeding effects on liver mitochondrial bioenergetics of Boa constrictor (Serpentes: Boidae)

Snakes are interesting examples of taxa that can overcome energy metabolism challenges, as many species can endure long periods without feeding, and their eventual meals are of reasonably large sizes, thus exhibiting dual extreme adaptations. Consequently, metabolic rate increases considerably to attend to the energetic demand of digestion, absorption and protein synthesis. These animals should be adapted to transition from these two opposite states of energy fairly quickly, and therefore we investigated mitochondrial function plasticity in these states. Herein, we compared liver mitochondrial bioenergetics of the boid snake Boa constrictor during fasting and after meal intake. We fasted the snakes for 60 days, and then we fed a subgroup with 30% of their body size and evaluated their maximum postprandial response. We measured liver respiration rates from permeabilized tissue and isolated mitochondria. From isolated mitochondria, we also measured Ca2+ retention capacity and redox status. Mitochondrial respiration rates were maximized after feeding, reaching an approximately 60% increase from fasting levels when energized with complex I-linked substrates. Interestingly, fasting and fed snakes exhibited similar respiratory control ratios and citrate synthase activity. Furthermore, we found no differences in Ca2+ retention capacity, indicating no increase in susceptibility to mitochondrial permeability transition, and no changes in mitochondrial redox state, although fed animals exhibited increases in the release of H2O2. Thus, we conclude that liver mitochondria from B. constrictor snakes increase respiration rates during the postprandial period and quickly improve the bioenergetic capacity without compromising redox balance.

Metabolic fasting stress is ameliorated in Kinin B1 receptor-deficient mice

The liver has an essential role in responding to metabolic demands under stress conditions. The organ stores, releases, and recycles metabolism-related substrates. However, it is not clear how the Kallikrein-Kinin System modulates metabolic flexibility shift between energetic sources.

AIMS

To analyze the hepatic metabolism in kinin B1 receptor deficient mice (B1KO mice) under fasting conditions.

MAIN METHODS

WT and B1KO male mice were allocated in a calorimetric cage for 7 days and 48 h before the euthanasia, half of the animals of both groups were under fasting conditions. Biochemical parameters, ketone bodies (KB), and gene expression involving the liver energetic metabolism genes were evaluated.

KEY FINDINGS

Kinin B1 receptor (B1R) modulates the metabolic shift under fasting conditions, reducing the VO₂ expenditure. A preference for carbohydrates as an energetic source is suggested, as the B1KO group did not display an increase in KB in the serum. Moreover, the B1KO animals displayed higher serum triglycerides concentration compared to WT fasting mice. Interestingly, the lack of B1R induces the increase expression of enzymes from the glycolysis and lipolysis pathways under the fed. However, under fasting, the enzymatic expression of gluconeogenesis, glyceroneogenesis, and ketogenesis of these pathways does not occur, suggesting an absence of the shift metabolism responsivity, and this condition is modulated by PDK4 under FOXO1 control.

SIGNIFICANCE

B1R has an important role in the hepatic glucose metabolism, which in turn influences the energetic metabolism, and in long-term outcomes, such as in the decrease in hepatic glycogen stores and in the enhancement of hepatic metabolism.