Refeeding-Induced Brown Adipose Tissue Glycogen Hyper-Accumulation in Mice Is Mediated by Insulin and Catecholamines

Brown adipose tissue (BAT) generates heat during adaptive thermogenesis through a combination of oxidative metabolism and uncoupling protein 1-mediated electron transport chain uncoupling, using both free-fatty acids and glucose as substrate. Previous rat-based work in 1942 showed that prolonged partial fasting followed by refeeding led to a dramatic, transient increase in glycogen stores in multiple fat depots. In the present study, the protocol was replicated in male CD1 mice, resulting in a 2000-fold increase in interscapular BAT (IBAT) glycogen levels within 4–12 hours (hr) of refeeding, with IBAT glycogen stores reaching levels comparable to fed liver glycogen. Lesser effects occurred in white adipose tissues (WAT). Over the next 36 hr, glycogen levels dissipated and histological analysis revealed an over-accumulation of lipid droplets, suggesting a potential metabolic connection between glycogenolysis and lipid synthesis. 24 hr of total starvation followed by refeeding induced a robust and consistent glycogen over-accumulation similar in magnitude and time course to the prolonged partial fast. Experimentation demonstrated that hyperglycemia was not sufficient to drive glycogen accumulation in IBAT, but that elevated circulating insulin was sufficient. Additionally, pharmacological inhibition of catecholamine production reduced refeeding-induced IBAT glycogen storage, providing evidence of a contribution from the central nervous system. These findings highlight IBAT as a tissue that integrates both canonically-anabolic and catabolic stimulation for the promotion of glycogen storage during recovery from caloric deficit. The preservation of this robust response through many generations of animals not subjected to food deprivation suggests that the over-accumulation phenomenon plays a critical role in IBAT physiology.     

Fasting and refeeding in carp,Cyprinus carpio L.: the mobilization of reserves and plasma metabolite and hormone variations

Summary Carp, Cyprinus carpio, were subjected to a short term of fasting (2 months) and 12 days of refeeding. The early changes produced in plasma metabolites and hormones (insulin and glucagon) and their respective energy contribution in liver and muscle during fasting and refeeding was studied. Two phases of fasting were differentiated. The first phase (until day 8 of fasting) was characterized by a reduction in the hepatosomatic index mainly due to glycogen mobilization. A transitory increase in plasma glucose and lactate suggested an initial increase in energy demand. No changes were produced in the percentage of glycogen and protein in muscle, but musculosomatic index and the total body muscle protein decreased. Although the most depleted tissue in this phase was the liver, the loss of energy content of total muscle was higher. Stabilization of liver glycogen content, plasma glucose and lactate levels, decreased muscle protein levels and a reduction in the rate of body weight loss characterized the second phase (from day 8 of fasting). Protein content in whole muscle decreased by 22%, similar to the first phase. The energy expenditure of both liver and muscle was lower in this phase. Plasma insulin levels decreased two-fold and plasma glucagon three-fold in the first phase and remained low in the second phase of fasting. Twelve days of refeeding produced a greater increase in daily growth rate than in the control group and a recovery of plasma insulin, glucagon and glucose levels. Liver completely recovered. In contrast, musculosomatic index, protein and lipid content indicated that muscle did not completely recover from the 2 months of fasting, although and overshoot of muscle glycogen was observed.Abbreviations ANOVA analysis of variance - bw body weight - D1, D2, D5, D8, D19, D50 1, 2, 5, 8, 19 and 50 days of fasting, respectively - GSI gonadosomatic index - HSI hepatosomatic index - MSI musculosomatic index - P-DNA deoxyribonucleic acid phosphorus     

Changes in glucose,glycogen, thyroid activity and hypothalamic catecholamines in tench by starvation and refeeding

The effects of short-term food deprivation (7 days) and refeeding (2 days) on different biochemical and neuroendocrine parameters were studied in tench. A 7-days fast resulted in a significant reduction of plasma glucose and glycogen hepatic content, supporting the key role of liver glycogen as energy depot for being consumed during fasting. The rapid recovery of normal values of blood glucose and glycogen stores by refeeding indicates a rapid replenishment of liver glycogen stores. The short-term starvation decreased circulating thyroid hormones (both T3 and T4) and T4 release from thyroid, supporting an interaction between nutritional state and thyroid function in tench. All these metabolic and hormonal changes were partial or totally reversed under refeeding conditions. An increase in hypothalamic content of norepinephrine and dopamine was found in fasted fish. This result might be a consequence of stress induced by starvation.     

Effects of short-term fasting on energy reserves of vampire bats (Desmodus rotundus)

Studies on metabolic responses to fasting in common vampire bats (Desmodus rotundus) have demonstrated the susceptibility of this species when subjected to long-term fasting. We investigated the effects of short-term fasting (12 h), a period similar to what they face in the field, on their energy reserves. Blood glucose (BG) levels in fed bats were similar to other mammals, but after 12 h without food, these levels were reduced. Plasma lactate and free fatty acids levels in fed bats were higher than in other mammals, although no changes in these levels were detected in response to fasting. Liver glycogen content decreased significantly following fasting. Muscle glycogen, as well as liver and muscle lipid and protein levels, remained unaltered for up to 12 h of fasting. Although BG levels decreased after short-term fasting, body energy reserves do not seem to play an important role for maintenance of glycemic homeostasis during fasting. Despite the decrease in liver glycogen, this small reserve seems insufficient to maintain adequate levels of BG, even during short periods of fasting. Because other reserves were not decreased after fasting, it is possible that the main source of glucose for common vampire bats might be the glucose content of their blood diet.     

Neuropeptide Y content in the hypothalamic paraventricular nucleus responds to fasting and refeeding in broiler chickens

To examine the neural mechanism by which hypothalamic neuropeptide Y (NPY) regulates energy homeostasis and feeding behavior in commercial broilers, we measured NPY content in several hypothalamic regions of birds that were fasted and then refed. After fasting for 48 and 72 h, body weight significantly decreased, and food intake significantly increased during the subsequent refeeding. The lost body weight was not restored to ad libitum feeding levels even after 3 days of refeeding. Plasma glucose concentration and body fat content significantly decreased and plasma non-esterified fatty acid (NEFA) concentration significantly increased after 48- and 72-h fasting. Refeeding for 24 h restored plasma metabolites and body fat content to pre-fasting levels. NPY content in the paraventricular nucleus (PVN) and infundibular nucleus significantly increased during fasting, and NPY content of the PVN was restored to pre-fasting levels after 24-h refeeding. However, there was no significant change in the NPY content of the lateral hypothalamic area during fasting or refeeding. The present results of changes in the hypothalamic NPY content during fasting and refeeding support the hypothesis that NPY plays a central role in regulation of energy homeostasis, with especially important effect on feeding behavior and body weight in broiler chickens.     

Some physiological and biochemical parameters of underyearling Laxmann’s shrews (Sorex cecutiens Laxmann) and even-toothed shrews (Sorex isodon Turov) under different population densities

Based on the results of a study conducted in the Buyunda River basin (tributary of the Kolyma River) in 2006–2010, influence of the population density of shrews (Sorex) on some physiological and biochemical parameters (glycogen and lipids in the liver, relative weight of the spleen, white and brown adipose tissue, cellularity of bone marrow) was investigated. The content of energy reserve substances was found to be closely connected with the number of animals (fat deposits had inverse relation; glycogen content in the liver had direct relation). Other physiological-biochemical parameters had no significant relation to the population density, although such tendency was observed for the content of brown fat and cellularity of bone marrow tissue in Sorex isodon, and for the relative weight of the spleen in both species of shrews. We suggest that the identified physiological changes indicate irregular feeding of animals in years with higher population densities.     

The adipogenic function and other biological effects of insulin

Studies on experimental animals with knockout of the insulin receptor gene (Insr) in the whole body or in certain tissues and/or related genes encoding proteins involved in realization of insulin signal transduction in target cells, have made an important contribution to the elucidation of insulin regulation of metabolism, particularly fat metabolism. Since the whole insulin secreted by β-cells, together with the products of gastrointestinal tract digestion of proteins, fats, and carbohydrates reaches in the liver, the latter is the first organ on which this hormone acts. The liver employs released amino acids for synthesis of proteins, including apo-proteins for various lipoproteins. Glucose is used for synthesis of glycogen, fatty acids, and triglycerides, which enter all the organs in very low density lipoproteins (VLDL). The LIRKO mice with knockout of the insr gene in the liver demonstrated inhibition of synthesis of macromolecular compounds from amino acids, glucose, and fatty acids. Low molecular weight substances demonstrated increased entry to circulation, and together with other disorders induced hyperglycemia. In LIRKO mice blood glucose levels and glucose tolerance demonstrated time-dependent normalization and at later stages the increase in glucose levels was replaced by hypoglycemia. These changes can be well explained if we take into consideration that one of the main functions of insulin consists in stimulation of energy accumulation by means of activation of triglyceride deposition in adipose tissue. FIRKO mice with selective knockout of adipose tissue Insr were characterized by decreased uptake of glucose in adipocytes, and its transformation into lipids. However, the level of body fat in animals remained normal, possibly due to preserved insulin receptor in the liver and insulin-induced activation of triglyceride production which maintained normal levels of body fat stores, the effective functioning of adipose tissue and secretion of leptin by adipocytes during inhibition of glucose transformation into triglyceride in adipose tissue. Knockout of the Insr gene in muscles blocked glucose uptake by myocytes, but it did not induce hyperglycemia, probably due to the increase in glucose uptake by other organs, which retained the insulin receptor, and induced some increase in fat resources in adipose tissue. Similar results were obtained in mice with knockout the glucose transporter 4 GLUT4 in muscle and/or adipose tissue. Insulin microinjections in the brain, in the cerebral ventricle 4 (CVI) and mediobasal hypothalamus (MBH) did not affect the insulin levels in the general circulation, but effectively activate lipogenesis and inhibited lipolysis in adipose tissue. They induced obesity, similar to conventional obesity when the insulin levels increased. These results may serve as an additional confirmation of the importance of the adipogenic insulin function in mechanisms of regulation of general metabolism.     

Rapid activation and inactivation of fatty acid synthesis from glucose in vivo

The flux of glucose carbon to total body fatty acids was measured in unanesthetized mice either after fasting or 50-80 min after they nibbled a small test meal containing 120 mg of glucose (fasted-refed). Flux was calculated from plasma [(14)C]glucose specific activity curves and from total body (14)C-labeled fatty acid 30 min after intravenous injection of tracer [(14)C]glucose. Mobilization of liver glycogen, changes in the body glucose pool size, and total flux of carbon through the glucose pool during periods of fasting and refeeding were defined. Liver glycogen was almost completely depleted 8 hr after food removal. Body glucose pool size fell during fasting and increased after refeeding the test meal. Irreversible disposal rate of glucose C varied directly with body glucose pool size; but flux of glucose C into fatty acids increased exponentially as body glucose concentration increased. Within an hour after nibbling a small test meal, the flux of glucose C into total body fatty acids increased 700% in mice previously starved for 24 hr. However, flux of glucose C into fatty acids in postabsorptive mice (food removed for 2 hr; livers rich in glycogen) was only about 2% of the value calculated from published studies in which the incorporation of an intubated [(14)C]glucose load into total body fatty acid was measured in mice. A possible explanation for this phenomenon is presented.     

Antidiabetic Activity of a Lotus Leaf Selenium (Se)-Polysaccharide in Rats with Gestational Diabetes Mellitus

A selenium (Se)-containing polysaccharide, lotus leaf selenium (Se)-polysaccharide (LLP), was isolated from a lotus leaf. The effects of LLP on antioxidant enzyme activities and insulin resistance in pregnant rats with gestational diabetes mellitus (GDM) were investigated. LLP administered orally at two doses (50 and 100 mg/kg) could significantly reverse the weight loss of pregnant rats before the delivery, fetal rats, and placentas in GDM rats (P < 0.05). Furthermore, LLP treatment induced a decrease of fasting blood glucose (FBG) and fasting blood insulin (FINS) levels in GDM rats, but an increase of hepatic glycogen content, when compared with those in GDM rats (P < 0.05). Also, oral administrations of LLP markedly improved the lipid profile of GDM rats, as evidenced by a reduction of total cholesterol (TC), triglyceride (TG), and low-density lipoprotein (LDL) cholesterol levels except for the high-density lipoprotein (HDL) cholesterol level. Additionally, antioxidant enzyme levels, such as superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx), and glutathione (GSH), in liver tissues of the GDM group were lower than those of the other groups, and following treatment of LLP, these indexes in liver tissues were equivalent to those of the control group (P > 0.05). All the data indicated that LLP may be a promising drug candidate or a healthcare food for GDM therapy or protection.     

Fasting in the American marten (<Emphasis Type="Italic">Martes americana</Emphasis>): a physiological model of the adaptations of a lean-bodied animal

The American marten (Martes americana) is a boreal forest marten with low body adiposity throughout the year. The aim of this study was to investigate the adaptations of this lean-bodied species to fasting for an ecologically relevant duration (48 h) by exposing eight farm-bred animals to total food deprivation with seven control animals. Selected morphological and hematological parameters, plasma and serum biochemistry, endocrinological variables and liver and white adipose tissue (WAT) enzyme activities were determined. After 48 h without food, the marten were within phase II of fasting with depleted liver and muscle glycogen stores, but with active lipid mobilization indicated by the high lipase activities in several WAT depots. The plasma ghrelin concentrations were higher due to food deprivation, possibly increasing appetite and enhancing foraging behavior. The lower plasma insulin and higher cortisol concentrations could mediate augmented lipolysis and the lower triiodothyronine levels could suppress the metabolic rate. Fasting did not affect the plasma levels of stress-associated catecholamines or variables indicating tissue damage. In general, the adaptations to short-term fasting exhibited some differences compared to the related farm-bred American mink (Mustela vison), an example of which was the better ability of the marten to hydrolyze lipids despite its significantly lower initial fat mass.     

Adaptations of glycogen metabolism in rat epididymal adipose tissue during fasting and refeeding.

It is well documented that adipose tissue glycogen content decreases during fasting and increases above control during refeeding. We now present evidence that these fluctuations result from adaptations intrinsic to adipose tissue glycogen metabolism that persist in vitro: in response to insulin (1 milliunit/ml), [3H]glucose incorporation into rat fat pad glycogen was reduced to 10% of control after a 3-day fast; incorporation increased 6-fold over fed control on the 4th day of refeeding following a 3-day fast. We have characterized this adaptation with regard to alterations in glycogen synthase and phosphorylase activity. In addition, we found that incubation of fat pads from fasted rats with insulin (1 milliunit/ml) increased glucose-6-P content, indicating that glucose transport was not the rate-limiting step for glucose incorporation into glycogen in the presence of insulin. In contrast, feeding a fat-free diet resulted in dramatic increases in glycogen content of fat pads without a concomitant increase in glucose incorporation into glycogen in response to insulin (1 milliunit/ml). Thus, fasting and refeeding appeared to alter insulin action on adipose tissue glycogen metabolism more than this dietary manipulation.     

Mating behavior is controlled by acute changes in metabolic fuels

Mild food restriction for 48 h inhibits mating behavior in female musk shrews (Suncus murinus). However, mating behavior is restored after a 90-min feeding bout. In this series of experiments, we examined the role of metabolic fuels in this behavioral restoration. First, drugs reported to block glycolysis or fatty acid oxidation were given 2 h before mating. Both treatments inhibited mating in food-restricted females that were refed after treatment. Blood glucose levels were assessed in females that were fed ad libitum, food restricted, or food restricted and refed for 90 min. Food restriction significantly lowered blood glucose compared with ad libitum feeding or food restriction in combination with 90 min of refeeding. However, neither glucose nor fat alone could substitute for food and promote mating behavior in food-restricted females. In addition, analysis of ketone bodies and body composition in females demonstrated low or undetectable levels of these energy substrates. Our data suggest that musk shrews have relatively little stored energy. Therefore, female musk shrews rely on continuous food intake and monitor multiple cues acutely, including glucose availability and fatty acid oxidation. This ensures that mating does not occur when adequate energy is unavailable.     

The effects of fasting on some physiological parameters in the meadow vole, Microtus pennsylvanicus

1. The effect of fasting on respiratory quotient (RQ), metabolic rate, blood glucose, liver glycogen, carcass lipids, interscapular brown adipose tissue (IBAT), and body temperature was investigated in Microtus pennsylvanicus. 2. The utilization of carbohydrates during fasting leads to a severe hypoglycemia within 6 hr. 3. The hypoglycemia does not seem to stem from the inability to mobilize glycogen or fat reserves. 4. The hypoglycemic state may be responsible for the decreased SMR and body temperature. 5. The predominant use of carbohydrates may stem from a high metabolic rate coupled with a low calorie diet which forces the voles to feed frequently.     

Unusual increase of Scd1 and Elovl6 expression in rat inguinal adipose tissue

It is generally accepted that the location of body fat deposits may play an important role in the risk of developing some endocrine and metabolic diseases. We have studied the effect of food restriction and food restriction/refeeding, often practiced by individuals trying to lose body weight, on the expression of genes which are associated with obesity and certain metabolic disorders in inguinal, epididymal, and perirenal rat white adipose tissues. Gene expression was analyzed by real time semi-quantitative polymerase chain reaction and by Western blot. We found that prolonged food restriction caused a significant decrease of body and adipose tissue mass as well as the increase of Scd1 and Elovl6 gene expressions in all main rat adipose tissue deposits. Food restriction/refeeding caused increases of: a) Scd1 and Elovl6 mRNA levels in adipose tissue, b) Scd1 protein level and c) desaturation index in adipose tissue. The increased expression of both genes was unusually high in inguinal adipose tissue. The results suggest that the increase of Scd1 and Elovl6 gene expressions in white adipose tissue by prolonged food restriction and prolonged food restriction/refeeding may contribute to accelerated fat recovery that often occurs in individuals after food restriction/refeeding.     

Divergent molecular mechanisms for insulin-resistant glucose transport in muscle and adipose cells in vivo

Glucose homeostasis depends on regulated changes in glucose transport in insulin-responsive tissues (e.g. muscle and adipose cells). This transport is mediated by at least two distinct glucose transporters: "adipose-muscle" and "erythrocyte-brain." To understand the molecular basis for in vivo insulin resistance we investigated the effects of fasting and refeeding on the expression of these two glucose transporters in adipose cells and skeletal muscle. In vivo insulin resistance seen with fasting and hyperresponsiveness seen with refeeding influence glucose transporter expression in a transporter-specific and tissue-specific manner. In adipose cells only the adipose-muscle glucose transporter mRNA and protein decrease dramatically with fasting and increase above control levels with refeeding, changes that parallel effects on insulin-stimulated glucose transport. In contrast, in muscle expression of both glucose transporters increase with fasting and return to control levels with refeeding, also in accordance with changes in glucose uptake in vitro. Although expression of the adipose-muscle glucose transporter predicts the physiological response at the tissue level, factors in the hormonal/metabolic milieu appear to override its increased expression in muscle resulting in insulin-resistant glucose uptake in this tissue in vivo.     

The effects of starvation and force-feeding on the metabolism of the Northern pike, Esox lucius L.

The effects of starvation and force-feeding on certain tissue and blood constituents were studied in the Northern pike, Esox lucius L. Starvation resulted in a reduction of liver and muscle glycogen and liver lipid. Blood glucose concentration and haematocrit were reduced, total plasma cholesterol levels were increased, while the levels of plasma free fatty acids (FFA), amio acid nitrogen and protein remained unaltered. No significant changes were observed in either muscle protein, muscle water or the response to amino acid loading during the starvation period.
The force-feeding of pike starved for 3 months resulted in liver lipid and muscle glycogen being increased to levels higher than those observed in freshly-captured fish. Liver glycogen, however, increased to values only slightly higher than those of starved animals. Furthermore, while force-feeding had little effect on plasma FFA or protein concentrations, blood glucose, plasma cholesterol and haematocrit returned to the levels found in freshlycaptured fish and those of amino acid nitrogen were higher.
The results indicate that pike are well adapted for periods of prolonged starvation and that hepatic and extra-hepatic lipid and glycogen stores serve for metabolic needs during food shortage, while body protein is conserved. The endocrine basis for these changes in the tissue and blood constituents is discussed.     

Prolonged fasting induces long-lasting metabolic consequences in mice

To endure prolonged fasting, animals undergo important acute physiological adjustments. However, whether severe fasting also leads to long-term metabolic adaptations is largely unknown. Forty-eight-hour fasting caused a pronounced weight loss in adult C57BL/6 male mice. Seven days of refeeding increased body adiposity to levels above baseline, whereas fasting-induced reductions in lean body mass and energy expenditure were not fully recovered. Respiratory exchange ratio and locomotor activity also remained altered. A fasting/refeeding cycle led to persistent suppression of Pomc mRNA levels and significant changes in the expression of histone deacetylases and DNA methyltransferases in the hypothalamus. Additionally, histone acetylation in the ventromedial nucleus of the hypothalamus was reduced by prolonged fasting and remained suppressed after refeeding. Mice subjected to 48-h fasting 30 days earlier exhibited higher body weight and fat mass compared to aged-matched animals that were never food-deprived. Furthermore, a previous fasting experience altered the changes in body weight, lean mass, energy expenditure and locomotor activity induced by a second cycle of fasting and refeeding. Notably, when acutely exposed to high-palatable/high-fat diet, mice that went through cumulative fasting episodes presented higher calorie intake and reduced energy expenditure and fat oxidation, compared to mice that had never been subjected to fasting. When chronically exposed to high-fat diet, mice that experienced cumulative fasting episodes showed higher gain of body and fat mass and reduced energy expenditure and calorie intake. In summary, cumulative episodes of prolonged fasting lead to hypothalamic epigenetic changes and long-lasting metabolic adaptations in mice.     

Effect of fasting on glycogen metabolism and activities of glycolytic and gluconeogenic enzymes in the mudskipper Boleophthalmus boddaerti

During starvation, muscle glycogen in Boleophthalmus boddaerti was utilized preferentially over liver glycogen. In the first 10 days of fasting, the ratio of the active‘a’form of glycogen phosphorylase to total phosphorylase present in the liver was small. During this period, the active‘I’form of glycogen synthetase increased in the same tissue. In the muscle, the phosphorylase‘a’activity declined during the first 7 days and increased thereafter while the total glycogen synthetase activity showed a drastic decline during the first 13 days of fasting. The glycogen level in the liver and muscle of mudskippers starved for 21 days increased after refeeding. After 6 and 12 h refeeding, liver glycogen level was 8·5 ± 2·3 and 6·9 ± 4·5 mg·g wet wt ¹, respectively, as compared to 5·8 ± l·6mg·g wet wt ¹ in unfed fish. Muscle glycogen level after 6 and 12 h refeeding was 0·96±0·76 and 0·82 ± 0·50 mg·g wet wt ¹, respectively, as opposed to 0·21 ± 0·12 mg·g wet wt ¹ in the 21-days fasted fish. At the same time, activities of glycogen phosphorylase in the muscle and liver increased while the active‘I’form of glycogen synthetase showed higher activity in the liver. Since glycogen was resynthesized upon refeeding, this eliminated the possibility that glycogen depletion during starvation was due to stress or physical exhaustion after handling by the investigator. Throughout the experimental starvation period, the body weight of the mudskipper decreased, with a maximum of 12% weight loss after 21 days. Liver lipid reserves were utilized at the onset of fasting but were thereafter resynthesized. Muscle proteins were also metabolized as the fish were visibly thinner. However, no apparent change in protein content expressed as per gram wet weight was detected as the tissue hydration state was maintained constant. The increased degradation of liver and muscle reserves was coupled to an increase in the activities of key gluconeogenic enzymes in the liver (G6Pase, FDPase, PEPCK, MDH and PC). The increase in glucose synthesis was possibly necessary to counteract hypoglycemia brought about by starvation in B. boddaerti.     

Effect of liver glycogen content on glucose production in running rats

The influence of supranormal compared with normal hepatic glycogen levels on hepatic glucose production (Ra) during exercise was investigated in chronically catheterized rats. Supranormal hepatic glycogen levels were obtained by a 24-h fast-24-h refeeding regimen. During treadmill running for 35 min at a speed of 21 m/min, Ra and plasma glucose increased more (P less than 0.05) and liver glucogen breakdown was larger in fasted-refed compared with control rats, although the stimuli for Ra were higher in control rats, the plasma concentrations of insulin and glucose being lower (P less than 0.05) in control compared with fasted-refed rats. Also, plasma concentrations of glucagon and both catecholamines tended to be higher and muscle glycogenolysis lower in control compared with fasted-refed rats. Lipid metabolism was similar in the two groups. The results indicate that hepatic glycogenolysis during exercise is directly related to hepatic glycogen content. The smaller endocrine glycogenolytic signal in face of higher plasma glucose concentrations in fasted-refed compared with control rats is indicative of metabolic feedback control of glucose mobilization during exercise. However, the higher exercise-induced increase in Ra, plasma glucose, and liver glycogen breakdown in fasted-refed compared with control rats indicates that metabolic feedback mechanisms are not able to accurately match Ra to the metabolic needs of working muscles.