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Exogenous Ketone Supplements in Athletic Contexts: Past, Present, and Future. Sports Med 2022; 52:25-67. [PMID: 36214993 PMCID: PMC9734240 DOI: 10.1007/s40279-022-01756-2] [Citation(s) in RCA: 17] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 07/23/2022] [Indexed: 12/15/2022]
Abstract
The ketone bodies acetoacetate (AcAc) and β-hydroxybutyrate (βHB) have pleiotropic effects in multiple organs including brain, heart, and skeletal muscle by serving as an alternative substrate for energy provision, and by modulating inflammation, oxidative stress, catabolic processes, and gene expression. Of particular relevance to athletes are the metabolic actions of ketone bodies to alter substrate utilisation through attenuating glucose utilisation in peripheral tissues, anti-lipolytic effects on adipose tissue, and attenuation of proteolysis in skeletal muscle. There has been long-standing interest in the development of ingestible forms of ketone bodies that has recently resulted in the commercial availability of exogenous ketone supplements (EKS). These supplements in the form of ketone salts and ketone esters, in addition to ketogenic compounds such as 1,3-butanediol and medium chain triglycerides, facilitate an acute transient increase in circulating AcAc and βHB concentrations, which has been termed 'acute nutritional ketosis' or 'intermittent exogenous ketosis'. Some studies have suggested beneficial effects of EKS to endurance performance, recovery, and overreaching, although many studies have failed to observe benefits of acute nutritional ketosis on performance or recovery. The present review explores the rationale and historical development of EKS, the mechanistic basis for their proposed effects, both positive and negative, and evidence to date for their effects on exercise performance and recovery outcomes before concluding with a discussion of methodological considerations and future directions in this field.
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In Vivo Estimation of Ketogenesis Using Metabolic Flux Analysis-Technical Aspects and Model Interpretation. Metabolites 2021; 11:metabo11050279. [PMID: 33924948 PMCID: PMC8146959 DOI: 10.3390/metabo11050279] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2021] [Revised: 04/21/2021] [Accepted: 04/23/2021] [Indexed: 01/26/2023] Open
Abstract
Ketogenesis occurs in liver mitochondria where acetyl-CoA molecules, derived from lipid oxidation, are condensed into acetoacetate (AcAc) and reduced to β-hydroxybutyrate (BHB). During carbohydrate scarcity, these two ketones are released into circulation at high rates and used as oxidative fuels in peripheral tissues. Despite their physiological relevance and emerging roles in a variety of diseases, endogenous ketone production is rarely measured in vivo using tracer approaches. Accurate determination of this flux requires a two-pool model, simultaneous BHB and AcAc tracers, and special consideration for the stability of the AcAc tracer and analyte. We describe the implementation of a two-pool model using a metabolic flux analysis (MFA) approach that simultaneously regresses liquid chromatography-tandem mass spectrometry (LC-MS/MS) ketone isotopologues and tracer infusion rates. Additionally, 1H NMR real-time reaction monitoring was used to evaluate AcAc tracer and analyte stability during infusion and sample analysis, which were critical for accurate flux calculations. The approach quantifies AcAc and BHB pool sizes and their rates of appearance, disposal, and exchange. Regression analysis provides confidence intervals and detects potential errors in experimental data. Complications for the physiological interpretation of individual ketone fluxes are discussed.
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Baur DA, Saunders MJ. Carbohydrate supplementation: a critical review of recent innovations. Eur J Appl Physiol 2020; 121:23-66. [PMID: 33106933 DOI: 10.1007/s00421-020-04534-y] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/17/2020] [Accepted: 10/12/2020] [Indexed: 12/29/2022]
Abstract
PURPOSE To critically examine the research on novel supplements and strategies designed to enhance carbohydrate delivery and/or availability. METHODS Narrative review. RESULTS Available data would suggest that there are varying levels of effectiveness based on the supplement/supplementation strategy in question and mechanism of action. Novel carbohydrate supplements including multiple transportable carbohydrate (MTC), modified carbohydrate (MC), and hydrogels (HGEL) have been generally effective at modifying gastric emptying and/or intestinal absorption. Moreover, these effects often correlate with altered fuel utilization patterns and/or glycogen storage. Nevertheless, performance effects differ widely based on supplement and study design. MTC consistently enhances performance, but the magnitude of the effect is yet to be fully elucidated. MC and HGEL seem unlikely to be beneficial when compared to supplementation strategies that align with current sport nutrition recommendations. Combining carbohydrate with other ergogenic substances may, in some cases, result in additive or synergistic effects on metabolism and/or performance; however, data are often lacking and results vary based on the quantity, timing, and inter-individual responses to different treatments. Altering dietary carbohydrate intake likely influences absorption, oxidation, and and/or storage of acutely ingested carbohydrate, but how this affects the ergogenicity of carbohydrate is still mostly unknown. CONCLUSIONS In conclusion, novel carbohydrate supplements and strategies alter carbohydrate delivery through various mechanisms. However, more research is needed to determine if/when interventions are ergogenic based on different contexts, populations, and applications.
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Affiliation(s)
- Daniel A Baur
- Department of Physical Education, Virginia Military Institute, 208 Cormack Hall, Lexington, VA, 24450, USA.
| | - Michael J Saunders
- Department of Kinesiology, James Madison University, Harrisonburg, VA, 22801, USA
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Abstract
Elite athletes and coaches are in a constant search for training methods and nutritional strategies to support training and recovery efforts that may ultimately maximize athletes’ performance. Recently, there has been a re-emerging interest in the role of ketone bodies in exercise metabolism, with considerable media speculation about ketone body supplements being routinely used by professional cyclists. Ketone bodies can serve as an important energy substrate under certain conditions, such as starvation, and can modulate carbohydrate and lipid metabolism. Dietary strategies to increase endogenous ketone body availability (i.e., a ketogenic diet) require a diet high in lipids and low in carbohydrates for ~4 days to induce nutritional ketosis. However, a high fat, low carbohydrate ketogenic diet may impair exercise performance via reducing the capacity to utilize carbohydrate, which forms a key fuel source for skeletal muscle during intense endurance-type exercise. Recently, ketone body supplements (ketone salts and esters) have emerged and may be used to rapidly increase ketone body availability, without the need to first adapt to a ketogenic diet. However, the extent to which ketone bodies regulate skeletal muscle bioenergetics and substrate metabolism during prolonged endurance-type exercise of varying intensity and duration remains unknown. Therefore, at present there are no data available to suggest that ingestion of ketone bodies during exercise improves athletes’ performance under conditions where evidence-based nutritional strategies are applied appropriately.
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Evans M, Cogan KE, Egan B. Metabolism of ketone bodies during exercise and training: physiological basis for exogenous supplementation. J Physiol 2017; 595:2857-2871. [PMID: 27861911 PMCID: PMC5407977 DOI: 10.1113/jp273185] [Citation(s) in RCA: 246] [Impact Index Per Article: 35.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2016] [Accepted: 10/24/2016] [Indexed: 01/04/2023] Open
Abstract
Optimising training and performance through nutrition strategies is central to supporting elite sportspeople, much of which has focused on manipulating the relative intake of carbohydrate and fat and their contributions as fuels for energy provision. The ketone bodies, namely acetoacetate, acetone and β-hydroxybutyrate (βHB), are produced in the liver during conditions of reduced carbohydrate availability and serve as an alternative fuel source for peripheral tissues including brain, heart and skeletal muscle. Ketone bodies are oxidised as a fuel source during exercise, are markedly elevated during the post-exercise recovery period, and the ability to utilise ketone bodies is higher in exercise-trained skeletal muscle. The metabolic actions of ketone bodies can alter fuel selection through attenuating glucose utilisation in peripheral tissues, anti-lipolytic effects on adipose tissue, and attenuation of proteolysis in skeletal muscle. Moreover, ketone bodies can act as signalling metabolites, with βHB acting as an inhibitor of histone deacetylases, an important regulator of the adaptive response to exercise in skeletal muscle. Recent development of ketone esters facilitates acute ingestion of βHB that results in nutritional ketosis without necessitating restrictive dietary practices. Initial reports suggest this strategy alters the metabolic response to exercise and improves exercise performance, while other lines of evidence suggest roles in recovery from exercise. The present review focuses on the physiology of ketone bodies during and after exercise and in response to training, with specific interest in exploring the physiological basis for exogenous ketone supplementation and potential benefits for performance and recovery in athletes.
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Affiliation(s)
- Mark Evans
- Institute for Sport and Health, School of Public Health, Physiotherapy and Sports ScienceUniversity College DublinBelfieldDublin4Ireland
| | - Karl E. Cogan
- Institute for Sport and Health, School of Public Health, Physiotherapy and Sports ScienceUniversity College DublinBelfieldDublin4Ireland
| | - Brendan Egan
- Institute for Sport and Health, School of Public Health, Physiotherapy and Sports ScienceUniversity College DublinBelfieldDublin4Ireland
- School of Health and Human PerformanceDublin City UniversityGlasnevinDublin9Ireland
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Cox PJ, Kirk T, Ashmore T, Willerton K, Evans R, Smith A, Murray AJ, Stubbs B, West J, McLure SW, King MT, Dodd MS, Holloway C, Neubauer S, Drawer S, Veech RL, Griffin JL, Clarke K. Nutritional Ketosis Alters Fuel Preference and Thereby Endurance Performance in Athletes. Cell Metab 2016; 24:256-68. [PMID: 27475046 DOI: 10.1016/j.cmet.2016.07.010] [Citation(s) in RCA: 340] [Impact Index Per Article: 42.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/07/2016] [Revised: 04/18/2016] [Accepted: 07/17/2016] [Indexed: 11/28/2022]
Abstract
Ketosis, the metabolic response to energy crisis, is a mechanism to sustain life by altering oxidative fuel selection. Often overlooked for its metabolic potential, ketosis is poorly understood outside of starvation or diabetic crisis. Thus, we studied the biochemical advantages of ketosis in humans using a ketone ester-based form of nutrition without the unwanted milieu of endogenous ketone body production by caloric or carbohydrate restriction. In five separate studies of 39 high-performance athletes, we show how this unique metabolic state improves physical endurance by altering fuel competition for oxidative respiration. Ketosis decreased muscle glycolysis and plasma lactate concentrations, while providing an alternative substrate for oxidative phosphorylation. Ketosis increased intramuscular triacylglycerol oxidation during exercise, even in the presence of normal muscle glycogen, co-ingested carbohydrate and elevated insulin. These findings may hold clues to greater human potential and a better understanding of fuel metabolism in health and disease.
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Affiliation(s)
- Pete J Cox
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK; Department of Cardiovascular Medicine, University of Oxford, Oxford OX3 9DU, UK.
| | - Tom Kirk
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK
| | - Tom Ashmore
- Department of Biochemistry & Cambridge Systems Biology Centre, University of Cambridge & MRC Human Nutrition Research, Cambridge CB1 9NL, UK
| | - Kristof Willerton
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK
| | - Rhys Evans
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK
| | - Alan Smith
- UK Sport, 40 Bernard Street, London WC1N 1ST, UK
| | - Andrew J Murray
- Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge CB2 3EG, UK
| | - Brianna Stubbs
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK
| | - James West
- Department of Biochemistry & Cambridge Systems Biology Centre, University of Cambridge & MRC Human Nutrition Research, Cambridge CB1 9NL, UK
| | - Stewart W McLure
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK
| | - M Todd King
- Laboratory of Metabolic Control, NIAAA/NIH, Rockville, MD 20852, USA
| | - Michael S Dodd
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK
| | - Cameron Holloway
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK; Department of Cardiovascular Medicine, University of Oxford, Oxford OX3 9DU, UK
| | - Stefan Neubauer
- Department of Cardiovascular Medicine, University of Oxford, Oxford OX3 9DU, UK
| | - Scott Drawer
- UK Sport, 40 Bernard Street, London WC1N 1ST, UK
| | - Richard L Veech
- Laboratory of Metabolic Control, NIAAA/NIH, Rockville, MD 20852, USA
| | - Julian L Griffin
- Department of Biochemistry & Cambridge Systems Biology Centre, University of Cambridge & MRC Human Nutrition Research, Cambridge CB1 9NL, UK
| | - Kieran Clarke
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK
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Cox PJ, Clarke K. Acute nutritional ketosis: implications for exercise performance and metabolism. EXTREME PHYSIOLOGY & MEDICINE 2014; 3:17. [PMID: 25379174 PMCID: PMC4212585 DOI: 10.1186/2046-7648-3-17] [Citation(s) in RCA: 62] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/05/2014] [Accepted: 09/29/2014] [Indexed: 01/13/2023]
Abstract
Ketone bodies acetoacetate (AcAc) and D-β-hydroxybutyrate (βHB) may provide an alternative carbon source to fuel exercise when delivered acutely in nutritional form. The metabolic actions of ketone bodies are based on sound evolutionary principles to prolong survival during caloric deprivation. By harnessing the potential of these metabolic actions during exercise, athletic performance could be influenced, providing a useful model for the application of ketosis in therapeutic conditions. This article examines the energetic implications of ketone body utilisation with particular reference to exercise metabolism and substrate energetics.
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Affiliation(s)
- Pete J Cox
- Department of Physiology, Anatomy and Genetics, University of Oxford, Sherrington Building, Parks Road, Oxford OX1 3PT, UK
- Department of Cardiovascular Medicine, University of Oxford, Oxford, UK
| | - Kieran Clarke
- Department of Physiology, Anatomy and Genetics, University of Oxford, Sherrington Building, Parks Road, Oxford OX1 3PT, UK
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Owen OE, Markus H, Sarshik S, Mozzoli M. Relationship between plasma and muscle concentrations of ketone bodies and free fatty acids in fed, starved and alloxan-diabetic states. Biochem J 2010; 134:499-506. [PMID: 16742810 PMCID: PMC1177836 DOI: 10.1042/bj1340499] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
1. Concentrations of ketone bodies, free fatty acids and chloride in fed, 24-120h-starved and alloxan-diabetic rats were determined in plasma and striated muscle. Plasma glucose concentrations were also measured in these groups of animals. 2. Intracellular metabolite concentrations were calculated by using chloride as an endogenous marker of extracellular space. 3. The mean intracellular ketone-body concentrations (+/-s.e.m.) were 0.17+/-0.02, 0.76+/-0.11 and 2.82+/-0.50mumol/ml of water in fed, 48h-starved and alloxan-diabetic rats, respectively. Mean (intracellular water concentration)/(plasma water concentration) ratios were 0.47, 0.30 and 0.32 in fed, 48h-starved and alloxan-diabetic rats respectively. The relationship between ketone-body concentrations in the plasma and intracellular compartments appeared to follow an asymptotic pattern. 4. Only intracellular 3-hydroxybutyrate concentrations rose during starvation whereas concentrations of both 3-hydroxybutyrate and acetoacetate were elevated in the alloxan-diabetic state. 5. During starvation plasma glucose concentrations were lowest at 48h, and increased with further starvation. 6. There was no significant difference in the muscle intracellular free fatty acid concentrations of fed, starved and alloxan-diabetic rats. Mean free fatty acid intramuscular concentrations (+/-s.e.m.) were 0.81+/-0.08, 0.98+/-0.21 and 0.91+/-0.10mumol/ml in fed, 48h-starved and alloxan-diabetic states. 7. The intracellular ketosis of starvation and the stability of free fatty acid intracellular concentrations suggests that neither muscle membrane permeability nor concentrations of free fatty acids per se are major factors in limiting ketone-body oxidation in these states.
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Affiliation(s)
- O E Owen
- Department of Medicine and Fels Research Institute, Temple University School of Medicine, Philadelphia, Pa. 19140, U.S.A
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9
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Elia M. The Inter-Organ Flux of Substrates in Fed and Fasted Man, as Indicated by Arterio-Venous Balance Studies. Nutr Res Rev 2007; 4:3-31. [DOI: 10.1079/nrr19910005] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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10
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Elia M, Stubbs RJ, Henry CJ. Differences in fat, carbohydrate, and protein metabolism between lean and obese subjects undergoing total starvation. OBESITY RESEARCH 1999; 7:597-604. [PMID: 10574520 DOI: 10.1002/j.1550-8528.1999.tb00720.x] [Citation(s) in RCA: 75] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Despite extensive experimental studies on total starvation, many of the findings relating to protein, fat (plus ketone body), and carbohydrate metabolism remain confusing, although they become more consistent when considered in relation to the degree of initial obesity. During prolonged starvation, protein loss and percent energy derived from protein oxidation are 2- to 3-fold less in the obese than in the lean; percent urine N excreted as urea is 2-fold less in the obese; and the contribution of protein to net glucose production is only about half in the obese compared to lean subjects. During short-term starvation (first few days) the following differences are reported: hyperketonaemia is typically 2-fold greater in lean subjects, but associated with a 2-fold lower uptake of ketone bodies by forearm muscle; glucose tolerance becomes impaired more in lean subjects; and both protein turnover and leucine oxidation increase in the lean, but may show no significant change in the obese. It is no longer acceptable to describe the metabolic response to starvation as a single typical response. The differences between lean and obese subjects have important physiological implications, some of which are of obvious relevance to survival.
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Affiliation(s)
- M Elia
- Addenbrooke's Hospital, Cambridge
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11
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Abstract
Early studies agree that fasting is detrimental to overall physical performance and to endurance performance in humans; however, a study in rats reported an ergogenic effect where time to exhaustion was increased by a glycogen-sparing effect of elevated free fatty acids in blood resulting from a 24-hour fast. Later studies on humans have also found a detrimental effect of fasting on exercise endurance, with the exception of 1 study which found no difference. The discrepancy between humans and rats could not be explained by level of glycogen sparing, mode of exercise, duration of the fast, physiological differences or level of training. The intensity of exercise, and a potential placebo effect of fasting, are possible reasons for the conflicting results. Despite reduced endurance performance, fasted humans are able to exercise and maintain blood glucose homeostasis; the specific cause of an earlier onset of fatigue during a single bout of exercise in the fasted state remains unclear.
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Avogaro A, Nosadini R, Doria A, Fioretto P, Velussi M, Vigorito C, Saccà L, Toffolo G, Cobelli C, Trevisan R. Myocardial metabolism in insulin-deficient diabetic humans without coronary artery disease. THE AMERICAN JOURNAL OF PHYSIOLOGY 1990; 258:E606-18. [PMID: 2333960 DOI: 10.1152/ajpendo.1990.258.4.e606] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Eleven insulin-dependent diabetes mellitus (IDDM) patients with angiographically normal coronary arteries and a normal hemodynamic response to an echocardiographic-dipyridamole test and 12 normal controls were studied at rest and after atrial pacing simultaneously sampling arterial and coronary sinus blood. In IDDM patients, despite hyperglycemia [10.0 +/- 2.0 (SE) mmol/l], myocardial glucose uptake was slightly lower than in controls. This process was significantly activated in both groups during atrial pacing. The isotopically calculated net flux of lactate across myocardium, in agreement with the net balance value based on unlabeled lactate-pyruvate arteriovenous differences, showed a net uptake in controls (3.5 +/- 0.6 mumol.min-1.1.73 m-2) and a net release in IDDM (12.4 +/- 2.6; P less than 0.01). Atrial pacing stimulated lactate uptake in both groups. Myocardial uptake of ketone bodies was significantly higher in IDDM (37.0 +/- 6.3 mumol.min-1.1.73 m-2) than in controls (10.1 +/- 3.4 mumol.min-1.1.73 m-2; P less than 0.01). Free fatty acid uptake was also significantly greater in IDDM than in controls (44.1 +/- 7.0 vs. 24.1 +/- 5.1 mumol.min-1.1.73 m-2; P less than 0.01). Alanine and branched amino acids were released by diabetic but not by control hearts at rest. The normalization of blood glucose concentrations restored normal patterns of lactate and ketone body kinetics across diabetic myocardium. In conclusion, 1) at rest, myocardial lactate and amino acid uptake is markedly impaired in IDDM without coronary artery disease, and 2) the metabolic abnormalities of the diabetic myocardium are not a primary phenomenon but rather a consequence of hypoinsulinemia and hyperglycemia because insulin administration, resulting in euglycemia, restored normal patterns of cardiac metabolism.
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Affiliation(s)
- A Avogaro
- Istituto di Medicina Interna, Policlinico Universitario, Padua, Italy
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Balasse EO, Féry F. Ketone body production and disposal: effects of fasting, diabetes, and exercise. DIABETES/METABOLISM REVIEWS 1989; 5:247-70. [PMID: 2656155 DOI: 10.1002/dmr.5610050304] [Citation(s) in RCA: 217] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
Turnover studies performed during progressive fasting in normal subjects indicate that the production rate and the concentration of KB rise markedly during the early phase of fasting and start reaching a plateau after about 5 days. In addition to increased production, a reduction in the metabolic clearance rate of KB contributes to the hyperketonemia. This reduced metabolic clearance rate reflects essentially the progressive saturation of muscular ketone uptake that occurs with increasing ketonemia. The hormonal and metabolic environment of fasting plays only a minor role in this process, since a fall in KB metabolic clearance similar to that observed during fasting is observed if hyperketonemia is artificially induced in the postabsorptive state by the infusion of exogenous ketones. As extraction of KB by muscle becomes limited during ongoing fasting, KB are preferentially taken up by the brain to serve as a substrate replacing glucose. The remarkable stability of ketonemia during prolonged fasting is maintained through the operation of a negative feedback mechanism whereby KB tend to restrain their own production rate. The antilipolytic and insulinotropic effects of KB are instrumental in this process. This homeostatic mechanism maintains ketogenesis only slightly above the maximal metabolic disposal rate, the difference corresponding to urinary excretion, which is always below 10% of total turnover under physiologic conditions. When type I insulin-deprived diabetic patients are compared at the same KB concentration with control subjects with fasting ketosis, the characteristics of KB kinetics are comparable in the two groups. The maximal KB removal capacity is identical in the two situations, and it is not possible to identify a ketone removal defect specific to diabetes. Thus, these data favor the concept that excessive production of KB represent the main factor leading to uncontrolled hyperketonemia. It should be realized that a production exceeding only slightly that prevailing during prolonged fasting is sufficient to cause a progressive build-up in concentration, leading to uncontrolled diabetic ketosis. In the overnight-fasted state, a prolonged exercise (2 h) performed at moderate intensity (50% VO2 max) stimulates the capacity of muscle to extract ketones from blood as evidenced by a stimulation of the metabolic clearance rate.(ABSTRACT TRUNCATED AT 400 WORDS)
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Affiliation(s)
- E O Balasse
- Laboratory of Experimental Medicine, University of Brussels, Belgium
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14
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Féry F, Balasse EO. Response of ketone body metabolism to exercise during transition from postabsorptive to fasted state. THE AMERICAN JOURNAL OF PHYSIOLOGY 1986; 250:E495-501. [PMID: 3518484 DOI: 10.1152/ajpendo.1986.250.5.e495] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
This study examines the effects of a 2-h exercise of moderate intensity (50% of VO2 max) on the tracer-determined turnover rate of ketone bodies (KB) in 21 normal subjects fasted for 16 h, 5 days, whose basal ketonemia ranged between 0.09 and 6.16 mM. The KB response observed at the end of exercise is a function of the initial degree of ketosis. When basal ketonemia is below 0.6 mM, exercise enhances ketogenesis (Ra), the amplitude of this process being positively correlated with KB level. There is a concomitant acceleration of the metabolic clearance rate (MCR) of KB attaining 40-50%. When ketonemia exceeds 2.5 mM, the stimulatory effects of exercise on Ra and on MCR become less marked as basal ketonemia rises and are completely abolished or even reversed when initial KB level is higher than 3-4 mM. The pattern of changes in the concentration and in the overall disposal rate of KB were similar to that of Ra. It is suggested that the parallel inhibition of the stimulatory effect of work on hepatic ketogenesis and on muscular extraction of ketones associated with increasing degrees of fasting hyperketonemia has two physiological implications: it maintains the preferential utilization of KB by nonmuscular tissues (presumably the brain) and prevents the development of uncontrolled hyperketonemia, despite the intense catabolic situation created by the combination of exercise and starvation.
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16
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Wastney M, Hall S, Berman M. Ketone body kinetics in normal, diabetic, and obese humans. Math Biosci 1984. [DOI: 10.1016/0025-5564(84)90117-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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17
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Ketone body kinetics in humans: the effects of insulin-dependent diabetes, obesity, and starvation. J Lipid Res 1984. [DOI: 10.1016/s0022-2275(20)34462-x] [Citation(s) in RCA: 54] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
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18
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Wahren J, Sato Y, Ostman J, Hagenfeldt L, Felig P. Turnover and splanchnic metabolism of free fatty acids and ketones in insulin-dependent diabetics at rest and in response to exercise. J Clin Invest 1984; 73:1367-76. [PMID: 6715541 PMCID: PMC425159 DOI: 10.1172/jci111340] [Citation(s) in RCA: 95] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Nine insulin-dependent diabetics and six healthy controls were studied at rest, during, and after 60 min of bicycle exercise at a work load corresponding to 45% of their maximal oxygen intake. The catheter technique was employed to determine splanchnic and leg exchange of metabolites. FFA turnover and regional exchange was evaluated using [14C]oleate infusion. Basal glucose (13.8 +/- 1.1 mmol/l), ketone body (1.12 +/- 0.12 mmol/l), and FFA (967 +/- 110 mumol/l) concentrations were elevated in the diabetics in comparison with controls. In the resting state, splanchnic ketone acid production in the diabetics was 6-10-fold greater than in controls. Uptake of oleic acid by the splanchnic bed was increased 2-3-fold, and the proportion of splanchnic FFA uptake converted to ketones (61%) was threefold greater than in controls. In contrast, splanchnic fractional extraction of oleic acid was identical in diabetics and controls. A direct relationship was observed between splanchnic uptake and splanchnic inflow (plasma concentration X hepatic plasma flow) of oleic acid that could be described by the same regression line in the diabetic and control groups. During exercise, splanchnic ketone production rose in both groups. In the control group the increase in ketogenesis was associated with a rise in splanchnic inflow and in uptake of oleic acid, a rise in splanchnic fractional extraction of oleate, and an increase in the proportion of splanchnic FFA uptake converted to ketone acids from 20-40%. In the diabetic group, the increase in ketogenesis occurred in the absence of a rise in splanchnic inflow or uptake of oleic acid, but was associated with an increase in splanchnic fractional extraction of oleic acid and a marked increase in hepatic conversion of FFA to ketones, so that the entire uptake of FFA was accountable as ketone acid output. Splanchnic uptake of oleic acid correlated directly with splanchnic oleic acid inflow in both groups, but the slope of the regression line was steeper than in the resting state. Plasma glucagon levels were higher in the diabetic group at rest and during exercise, while plasma norepinephrine showed a twofold greater increment in response to exercise in the diabetic group (to 1,400-1,500 pg/ml). A net uptake of ketone acids by the leg was observed during exercise but could account for less than 5% of leg oxidative metabolism in the diabetics and less than 1% in controls. Despite the increase in ketogenesis during exercise, a rise in arterial ketone acid levels was not observed in the diabetics until postexercise recovery, during which sustained increments to values of 1.8-1.9 mmol/l and sustained increases in splanchnic ketone production were observed at 30-60 min. The largest increment in blood ketone acids and in splanchnic ketone production above values observed in controls thus occurred in the diabetics after 60 min of recovery from exercise. We concluded that: (a) In the resting state, increased ketogenesis in the diabetic is a consequence of augmented splanchnic inflow of FFA and increased intrahepatic conversion of FFA to ketones, but does not depend on augmented fractional extraction of circulating FFA by the splanchnic bed. (b) Exercise-induced increases in ketogenesis in normal subjects are due to augmented splanchnic inflow and fractional extraction of FFA as well as increased intrahepatic conversion of FFA to ketones. (c) When exercise and diabetes are combined, ketogenesis increases further despite the absence of a rise in splanchnic inflow of FFA. An increase in splanchnic fractional extraction of FFA and a marked increase intrahepatic conversion of FFA to ketones accounts for the exaggerated ketogenic response to exercise in the diabetic. (d) Elevated levels of plasma glucagon and/or norepinephrine may account for the increased hepatic ketogenic response to exercise in the diabetic. (e) Ketone utilization by muscle increases during exercise but constitutes a quantitatively minor oxidative fuel for muscle even in the diabetic. (f) The accelerated ketogenesis during exercise in the diabetic continues unabated during the recovery period, resulting in an exaggerated postexercise ketosis.
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Björkman O, Eriksson LS. Splanchnic glucose metabolism during leg exercise in 60-hour-fasted human subjects. THE AMERICAN JOURNAL OF PHYSIOLOGY 1983; 245:E443-8. [PMID: 6638171 DOI: 10.1152/ajpendo.1983.245.5.e443] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
An increased mobilization of the hepatic glycogen is necessary for the maintenance of glucose homeostasis during exercise. To examine the effect of exercise on glucose metabolism when the hepatic glycogen stores are depleted, five prolonged-fasted (60-h, PF) subjects were investigated. Arterial concentrations and splanchnic exchange of glucose and gluconeogenic precursors were studied at rest and during exercise (40 min, 60% of VO2max) using the hepatic venous catheter technique. Five overnight-fasted subjects (OF) served as controls. In the resting state, arterial glucose concentration (3.0 +/- 0.2 mmol/liter) and splanchnic glucose output (SGO) (0.3 +/- 0.1 mmol/min) were 30 and 55% lower, respectively, in the PF than in the OF subjects. During exercise SGO rose in both groups, but the increase was smaller in the PF subjects so that at the end of work SGO (0.9 +/- 0.2 mmol/min) was only one-third of that in the OF group (2.5 +/- 0.4 mmol/min). During exercise in the PF state the arterial lactate concentration (5.0 +/- 1.1 mol/liter) and the splanchnic lactate uptake (1.1 +/- 0.3 mmol/min) were threefold and twofold higher, respectively, than in the OF state. In the PF state, the splanchnic uptake of gluconeogenic precursors could account for more than 80% of the splanchnic glucose production both at rest and during exercise. Despite the lower SGO in the PF state, blood glucose concentrations rose during exercise, indicating a diminished peripheral glucose uptake.(ABSTRACT TRUNCATED AT 250 WORDS)
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Féry F, Balasse EO. Ketone body turnover during and after exercise in overnight-fasted and starved humans. THE AMERICAN JOURNAL OF PHYSIOLOGY 1983; 245:E318-25. [PMID: 6353933 DOI: 10.1152/ajpendo.1983.245.4.e318] [Citation(s) in RCA: 28] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
The concentration of ketone bodies and their rate of transport (estimated with an infusion of beta-[14C]-hydroxybutyrate) were determined before, during, and after exercise in overnight-fasted and 3- to 5-day-fasted subjects who walked on a treadmill for 2 h at approximately 50% of their VO2max. In overnight-fasted subjects, exercise increased the rate of turnover (+125% after 2 h) and the metabolic clearance rate of ketone bodies whose concentration rose from 0.20 to 0.39 mM. Discontinuation of exercise was associated with a marked increase in ketone levels (+0.73 mM after 30 min of recovery) that was related to a further stimulation of ketogenesis (+19%) and to a marked drop of the metabolic clearance rate to below preexercise values. In sharp contrast with overnight-fasted subjects, starved subjects (with a resting ketone level averaging 5.7 mM) responded to work by a decrease in the turnover rate and in the concentration of ketones, their metabolic clearance rate remaining unchanged. Thus, the response of ketogenesis and muscular ketone uptake to exercise are both markedly influenced by the initial degree of fasting ketosis.
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Phinney SD, Bistrian BR, Wolfe RR, Blackburn GL. The human metabolic response to chronic ketosis without caloric restriction: physical and biochemical adaptation. Metabolism 1983; 32:757-68. [PMID: 6865775 DOI: 10.1016/0026-0495(83)90105-1] [Citation(s) in RCA: 111] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
Abstract
To study the metabolic effects of ketosis without weight loss, nine lean men were fed a eucaloric balanced diet (EBD) for one week providing 35-50 kcal/kg/d, 1.75 g of protein per kilogram per day and the remaining kilocalories as two-thirds carbohydrate (CHO) and one-third fat. This was followed by four weeks of a eucaloric ketogenic diet (EKD)--isocaloric and isonitrogenous with the EBD but providing less than 20 g CHO daily. Both diets were appropriately supplemented with minerals and vitamins. Weight and whole-body potassium estimated by potassium-40 counting (40K) did not vary significantly during the five-week study. Nitrogen balance (N-Bal) was regained after one week of the EKD. The fasting blood glucose remained lower during the EKD than during the control diet (4.4 mmol/L at EBD, 4.1 mmol/L at EKD-4, P less than 0.01). The fasting whole-body glucose oxidation rate determined by a 13C-glucose primed constant infusion technique fell from 0.71 mg/kg/min during the control diet to 0.50 mg/kg/min (P less than 0.01) during the fourth week of the EKD. The mean serum cholesterol level rose (from 159 to 208 mg/dL) during the EKD, while triglycerides fell from 107 to 79 mg/dL. No disturbance of hepatic or renal function was noted at EKD-4. These findings indicate that the ketotic state induced by the EKD was well tolerated in lean subjects; nitrogen balance was regained after brief adaptation, serum lipids were not pathologically elevated, and blood glucose oxidation at rest was measurably reduced while the subjects remained euglycemic.
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Nesher R, Karl IE, Kaiser KE, Kipnis DM. Epitrochlearis muscle. I. Mechanical performance, energetics, and fiber composition. THE AMERICAN JOURNAL OF PHYSIOLOGY 1980; 239:E454-60. [PMID: 6449876 DOI: 10.1152/ajpendo.1980.239.6.e454] [Citation(s) in RCA: 40] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
An in vitro rat muscle preparation is described that can contract at rates of 12-240 twitches/min. Maximum dF/dt paralleled maximum twitch tension, their ratio being constant at approximately 8 ms for contraction rates of 12-120 twitches/min. Time to peak tension was 8-13 ms, time to peak dF/dt 5-8 ms, and half-relaxation time 4 ms. These parameters were unaffected by rate of contraction or duration of isometric work. Differential ATPase staining demonstrated that 60-65% of the fibers were fast-twitch white, 20% fast-twitch red, and 15% slow-twitch red. The preponderance of fast-twitch fibers correlated with the observed mechanical performance of the muscle. Muscles contracting for 60 min at rates up to 48 twitches/min maintained total adenine nucleotide content (ATP, ADP, AMP) at near resting levels. At higher twitch rates (72-240 twitches/min), total adenine nucleotide content decreased 40%, reflecting exclusively a fall in ATP in the presence of adequate phosphocreatine stores. Adequate oxygenation was reflected by lactate-to-pyruvate ratios in the range of 11-15 at all rates of contraction.
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Phinney SD, Horton ES, Sims EA, Hanson JS, Danforth E, LaGrange BM. Capacity for moderate exercise in obese subjects after adaptation to a hypocaloric, ketogenic diet. J Clin Invest 1980; 66:1152-61. [PMID: 7000826 PMCID: PMC371554 DOI: 10.1172/jci109945] [Citation(s) in RCA: 94] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023] Open
Abstract
To study the capacity for moderate endurance exercise and change in metabolic fuel utilization during adaptation to a ketogenic diet, six moderately obese, untrained subjects were fed a eucaloric, balanced diet (base line) for 2 wk, followed by 6 wk of a protein-supplemented fast (PSF), which provided 1.2 g of protein/kg ideal body wt, supplemented with minerals and vitamins. The mean weight loss was 10.6 kg. The duration of treadmill exercise to subjective exhaustion was 80% of base line after 1 wk of the PSF, but increased to 155% after 6 wk. Despite adjusting up to base line, with a backpack, the subjects' exercise weight after 6 wk of dieting, the final exercise test was performed at a mean of 60% of maximum aerobic capacity, whereas the base-line level was 76%. Resting vastus lateralis glycogen content fell to 57% of base line after 1 wk of the PSF, but rose to 69% after 6 wk, at which time no decrement in muscle glycogen was measured after >4 h of uphill walking. The respiratory quotient (RQ) during steady-state exercise was 0.76 during base line, and fell progressively to 0.66 after 6 wk of the PSF. Blood glucose was well maintained during exercise in ketosis. The sum of acetoacetate and beta hydroxybutyrate rose from 3.28 to 5.03 mM during exercise after 6 wk of the PSF, explaining in part the low exercise RQ. The low RQ and the fact that blood glucose and muscle glycogen were maintained during exhausting exercise after 6 wk of a PSF suggest that prolonged ketosis results in an adaptation, after which lipid becomes the major metabolic fuel, and net carbohydrate utilization is markedly reduced during moderate but ultimately exhausting exercise.
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Féry F, Balasse EO. Differential effects of sodium acetoacetate and acetoacetic acid infusions on alanine and glutamine metabolism in man. J Clin Invest 1980; 66:323-31. [PMID: 7400318 PMCID: PMC371714 DOI: 10.1172/jci109860] [Citation(s) in RCA: 40] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
It has been suggested that ketone bodies might participate in the nitrogen-sparing process occurring during prolonged starvation by inhibiting the muscular production of alanine and glutamine, which are the main gluconeogenic amino acids. The results of the ketone infusion studies on which this theory is based have been reevaluated in this study by following the plasma levels of ketone bodies, alanine, glutamine, and other substrates during 11.5 h in five groups of normal overnight-fasted subjects. Subjects of groups I, II, and III were infused for 3 h, respectively, with Na acetoacetate, Na bicarbonate, or free acetoacetic acid administered in comparable amounts (about 20 mumol/kg per min), whereas group IV was infused with hydrochloric acid (7.0 mumol/kg per min). A control group (V) received no infusion. Na acetoacetate induced a rise in blood pH (+0.1+/-0.003) and a fall in the plasma levels of alanine (-41.8+/-4.6%) and glutamine (-10.6+/-1.4%), whereas free acetoacetic acid had a barely detectable lowering effect on blood pH and induced a rise in alanine (+22.5+/-8.0%) and glutamine (+14.6+/-3.2%) levels. Both infusions were associated with a lowering of plasma glucose, which therefore seems independent of the changes in alanine and glutamine concentrations. Sodium bicarbonate reproduced the alkalinizing effect and the hypoalaninemic action of Na acetoacetate, which seems thus unrelated to hyperketonemia. On the other hand, acidification of blood with hydrochloric acid did not mimic the effects of acetoacetic acid. If the hyperalaninemic and hyperglutaminemic effects of ketone bodies infused in their physiological form (free acids) reflect a stimulation of the muscular output of these amino acids, the participation of ketone bodies in the nitrogen-sparing process of prolonged fasting seems very unlikely. On the other hand, during brief starvation, when both ketogenesis and gluconeogenesis are markedly stimulated, ketone bodies might indirectly contribute in supplying the liver and the kidney with gluconeogenic substrates.
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Minuk HL, Hanna AK, Marliss EB, Vranic M, Zinman B. Metabolic response to moderate exercise in obese man during prolonged fasting. THE AMERICAN JOURNAL OF PHYSIOLOGY 1980; 238:E322-9. [PMID: 6990774 DOI: 10.1152/ajpendo.1980.238.4.e322] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
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Owen OE, Reichard GA, Patel MS, Boden G. Energy metabolism in feasting and fasting. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 1979; 111:169-88. [PMID: 371355 DOI: 10.1007/978-1-4757-0734-2_8] [Citation(s) in RCA: 67] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
During feasting on a balanced carbohydrate, fat, and protein meal resting metabolic rate, body temperature and respiratory quotient all increase. The dietary components are utilized to replenish and augment glycogen and fat stores in the body. Excessive carbohydrate is also converted to lipid in the liver and stored along with the excessive lipids of dietary origin as triglycerides in adipose tissue, the major fuel storage depot. Amino acids in excess of those needed for protein synthesis are preferentially catabolized over glucose and fat for energy production. This occurs because there are no significant storage sites for amino acids or proteins, and the accumulation of nitrogenous compounds is ill tolerated. During fasting, adipose tissue, muscle, liver, and kidneys work in concert to supply, to convert, and to conserve fuels for the body. During the brief postabsorptive period, blood fuel homeostasis is maintained primarily by hepatic glycogenolysis and adipose tissue lipolysis. As fasting progresses, muscle proteolysis supplies glycogenic amino acids for heightened hepatic gluconeogenesis for a short period of time. After about three days of starvation, the metabolic profile is set to conserve protein and to supply greater quantities of alternate fuels. In particular, free fatty acids and ketone bodies are utilized to maintain energy needs. The ability of the kidney to conserve ketone bodies prevents the loss of large quantities of these valuable fuels in the urine. This delicate interplay among liver, muscle, kidney, and adipose tissue maintains blood fuel homeostasis and allows humans to survive caloric deprivation for extended periods.
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Simonelli C, Eaton RP. Cardiovascular and metabolic effects of exercise: the strong case for conditioning. Postgrad Med 1978; 63:71-7. [PMID: 628638 DOI: 10.1080/00325481.1978.11714751] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
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Garber AJ, Menzel PH, Boden G, Owen OE. Hepatic ketogenesis and gluconeogenesis in humans. J Clin Invest 1974; 54:981-9. [PMID: 4430728 PMCID: PMC301639 DOI: 10.1172/jci107839] [Citation(s) in RCA: 167] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023] Open
Abstract
Splanchnic arterio-hepatic venous differences for a variety of substrates associated with carbohydrate and lipid metabolism were determined simultaneously with hepatic blood flow in five patients after 3 days of starvation. Despite the relative predominance of circulating beta-hydroxybutyrate, the splanchnic productions of both beta-hydroxybutyrate and acetoacetate were approximately equal, totaling 115 g/24 h. This rate of hepatic ketogenesis was as great as that noted previously after 5-6 wk of starvation. Since the degree of hyperketonemia was about threefold greater after 5-6 wk of starvation, it seems likely that the rate of ketone-body removal by peripheral tissues is as important in the development of the increased ketone-body concentrations observed after prolonged starvation as increased hepatic ketone-body production rate. Splanchnic glucose release in this study was 123 g/24 h, which was less than that noted previously after an overnight fast, but was considerably more than that noted during prolonged starvation. Hepatic gluconeogenesis was estimated to be 99 g/24 h, calculated as the sum of lactate, pyruvate, glycerol, and amino acid uptake. This was greater than that observed either after an overnight fast or after prolonged starvation. In addition, a direct relationship between the processes of hepatic ketogenesis and gluconeogenesis was observed.
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Ahlborg G, Felig P, Hagenfeldt L, Hendler R, Wahren J. Substrate turnover during prolonged exercise in man. Splanchnic and leg metabolism of glucose, free fatty acids, and amino acids. J Clin Invest 1974; 53:1080-90. [PMID: 4815076 PMCID: PMC333093 DOI: 10.1172/jci107645] [Citation(s) in RCA: 512] [Impact Index Per Article: 10.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023] Open
Abstract
Arterial concentrations and substrate exchange across the leg and splanchnic vascular beds were determined for glucose, lactate, pyruvate, glycerol, individual acidic and neutral amino acids, and free fatty acids (FFA) in six subjects at rest and during 4 h of exercise at approximately 30% of maximal oxygen uptake. FFA turnover and regional exchange were evaluated using (14)C-labeled oleic acid. The arterial glucose concentration was constant for the first 40 min of exercise, but fell progressively thereafter to levels 30% below basal. The arterial insulin level decreased continuously, while the arterial glucagon concentration had risen fivefold after 4 h of exercise. Uptake of glucose and FFA by the legs was markedly augmented during exercise, the increase in FFA uptake being a consequence of augmented arterial levels rather than increased fractional extraction. As exercise was continued beyond 40 min, the relative contribution of FFA to total oxygen metabolism rose progressively to 62%. In contrast, the contribution from glucose fell from 40% to 30% between 90 and 240 min. Leg output of alanine increased as exercise progressed. Splanchnic glucose production, which rose 100% above basal levels and remained so throughout exercise, exceeded glucose uptake by the legs for the first 40 min but thereafter failed to keep pace with peripheral glucose utilization. Total estimated splanchnic glucose output was 75 g in 4 h, sufficient to deplete approximately 75% of liver glycogen stores. Splanchnic uptake of gluconeogenic precursors (lactate, pyruvate, glycerol, alanine) had increased 2- to 10-fold after 4 h of exercise, and was sufficient to account for 45% of glucose release at 4 h as compared to 20-25% at rest and at 40 min of exercise. In the case of alanine and lactate, the increase in precursor uptake was a consequence of a rise in splanchnic fractional extraction. It is concluded that during prolonged exercise at a low work intensity (a) blood glucose levels fall because hepatic glucose output fails to keep up with augmented glucose utilization by the exercising legs; (b) a large portion of hepatic glycogen stores is mobilized and an increasing fraction of the splanchnic glucose output is derived from gluconeogenesis; (c) blood-borne substrates in the form of glucose and FFA account for a major part of leg muscle metabolism, the relative contribution from FFA increasing progressively; and (d) augmented secretion of glucagon may play an important role in the metabolic adaptation to prolonged exercise by its stimulatory influence on hepatic glycogenolysis and gluconeogenesis.
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