Human forearm muscle metabolism during exercise. 3. Uptake, release and oxidation of beta-hydroxybutyrate and observations on the beta-hydroxybutyrate/acetoacetate ratio

Reduced beta-hydroxybutyrate disposal after ketogenic diet feeding in mice

The ketogenic diet (KD) has garnered considerable attention due to its potential benefits in weight loss, health improvement, and performance enhancement. However, the phenotypic responses to KD vary widely between individuals. Skeletal muscle is a major contributor to ketone body (KB) catabolism, however, the regulation of ketolysis is not well understood. In this study, we evaluated how mTORC1 activation and a ketogenic diet modify ketone body disposal in muscle Tsc1 knockout (KO) mice, inbred A/J mice, and Diversity Outbred (DO) mice. Muscle Tsc1 KO mice demonstrated enhanced ketone body clearance. Contrary to expectations, KD feeding in A/J mice did not improve KB disposal, and in most strains disposal was reduced. Transcriptional analysis revealed reduced expression of important ketolytic genes in KD-fed A/J mice, suggesting impaired KB catabolism. Diversity Outbred (DO) mice displayed variable responses to KD, with most mice showing worsened KB disposal. Exploratory analysis on these data suggest potential correlations between KB disposal and cholesterol levels as well as weight gain on a KD. Our findings suggest that ketone body disposal may be regulated by both nutritional and genetic factors and these relationships may help explain interindividual variability in responses to ketogenic diets.

Nutritional Strategies for Endurance Cyclists - Periodized Nutrition, Ketogenic Diets, and Other Considerations

ABSTRACT

Cycling is a growing sport worldwide since the COVID-19 pandemic. With the growing availability and interest in long distance events, professional and amateur cyclists are pushing themselves further and harder than ever before. Training and nutrition should be understood by the sports medicine professional in order to guide counseling toward proper fueling to avoid health consequences. This article reviews macronutrients and micronutrients, periodized training and nutrition, and the relevance of the ketogenic diet for endurance cyclists riding greater than 90 min.

Biomarkers of Redox Balance Adjusted to Exercise Intensity as a Useful Tool to Identify Patients at Risk of Muscle Disease through Exercise Test

The screening of skeletal muscle diseases constitutes an unresolved challenge. Currently, exercise tests or plasmatic tests alone have shown limited performance in the screening of subjects with an increased risk of muscle oxidative metabolism impairment. Intensity-adjusted energy substrate levels of lactate (La), pyruvate (Pyr), β-hydroxybutyrate (BOH) and acetoacetate (AA) during a cardiopulmonary exercise test (CPET) could constitute alternative valid biomarkers to select “at-risk” patients, requiring the gold-standard diagnosis procedure through muscle biopsy. Thus, we aimed to test: (1) the validity of the V’O₂-adjusted La, Pyr, BOH and AA during a CPET for the assessment of the muscle oxidative metabolism (exercise and mitochondrial respiration parameters); and (2) the discriminative value of the V’O₂-adjusted energy and redox markers, as well as five other V’O₂-adjusted TCA cycle-related metabolites, between healthy subjects, subjects with muscle complaints and muscle disease patients. Two hundred and thirty subjects with muscle complaints without diagnosis, nine patients with a diagnosed muscle disease and ten healthy subjects performed a CPET with blood assessments at rest, at the estimated 1st ventilatory threshold and at the maximal intensity. Twelve subjects with muscle complaints presenting a severe alteration of their profile underwent a muscle biopsy. The V’O₂-adjusted plasma levels of La, Pyr, BOH and AA, and their respective ratios showed significant correlations with functional and muscle fiber mitochondrial respiration parameters. Differences in exercise V’O₂-adjusted La/Pyr, BOH, AA and BOH/AA were observed between healthy subjects, subjects with muscle complaints without diagnosis and muscle disease patients. The energy substrate and redox blood profile of complaining subjects with severe exercise intolerance matched the blood profile of muscle disease patients. Adding five tricarboxylic acid cycle intermediates did not improve the discriminative value of the intensity-adjusted energy and redox markers. The V’O₂-adjusted La, Pyr, BOH, AA and their respective ratios constitute valid muscle biomarkers that reveal similar blunted adaptations in muscle disease patients and in subjects with muscle complaints and severe exercise intolerance. A targeted metabolomic approach to improve the screening of “at-risk” patients is discussed.

Exogenous Ketone Supplements in Athletic Contexts: Past, Present, and Future

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.

In Vivo Estimation of Ketogenesis Using Metabolic Flux Analysis-Technical Aspects and Model Interpretation

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.

Chemical Fingerprints of Emotional Body Odor

Chemical communication is common among animals. In humans, the chemical basis of social communication has remained a black box, despite psychological and neural research showing distinctive physiological, behavioral, and neural consequences of body odors emitted during emotional states like fear and happiness. We used a multidisciplinary approach to examine whether molecular cues could be associated with an emotional state in the emitter. Our research revealed that the volatile molecules transmitting different emotions to perceivers also have objectively different chemical properties. Chemical analysis of underarm sweat collected from the same donors in fearful, happy, and emotionally neutral states was conducted using untargeted two-dimensional (GC×GC) coupled with time of flight (ToF) MS-based profiling. Based on the multivariate statistical analyses, we find that the pattern of chemical volatiles (N = 1655 peaks) associated with fearful state is clearly different from that associated with (pleasant) neutral state. Happy sweat is also significantly different from the other states, chemically, but shows a bipolar pattern of overlap with fearful as well as neutral state. Candidate chemical classes associated with emotional and neutral sweat have been identified, specifically, linear aldehydes, ketones, esters, and cyclic molecules (5 rings). This research constitutes a first step toward identifying the chemical fingerprints of emotion.

Ketone Bodies and Exercise Performance: The Next Magic Bullet or Merely Hype?

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.

Metabolism of ketone bodies during exercise and training: physiological basis for exogenous supplementation

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.

Systemic, cerebral and skeletal muscle ketone body and energy metabolism during acute hyper-D-β-hydroxybutyratemia in post-absorptive healthy males

CONTEXT

Ketone bodies are substrates during fasting and when on a ketogenic diet not the least for the brain and implicated in the management of epileptic seizures and dementia. Moreover, D-β-hydroxybutyrate (HOB) is suggested to reduce blood glucose and fatty acid levels.

OBJECTIVES

The objectives of this study were to quantitate systemic, cerebral, and skeletal muscle HOB utilization and its effect on energy metabolism.

DESIGN

Single trial.

SETTING

Hospital.

PARTICIPANT

Healthy post-absorptive males (n = 6).

INTERVENTIONS

Subjects were studied under basal condition and three consecutive 1-hour periods with a 3-, 6-, and 12-fold increased HOB concentration via HOB infusion.

MAIN OUTCOME MEASURES

Systemic, cerebral, and skeletal muscle HOB kinetics, oxidation, glucose turnover, and lipolysis via arterial, jugular, and femoral venous differences in combination with stable isotopically labeled HOB, glucose, and glycerol, infusion.

RESULTS

An increase in HOB from the basal 160-450 μmol/L elicited 14 ± 2% reduction (P = .03) in glucose appearance and 37 ± 4% decrease (P = .03) in lipolytic rate while insulin and glucagon were unchanged. Endogenous HOB appearance was reduced in a dose-dependent manner with complete inhibition at the highest HOB concentration (1.7 mmol/L). Cerebral HOB uptake and subsequent oxidation was linearly related to the arterial HOB concentration. Resting skeletal muscle HOB uptake showed saturation kinetics.

CONCLUSION

A small increase in the HOB concentration decreases glucose production and lipolysis in post-absorptive healthy males. Moreover, cerebral HOB uptake and oxidation rates are linearly related to the arterial HOB concentration of importance for modifying brain energy utilization, potentially of relevance for patients with epileptic seizures and dementia.

Acute nutritional ketosis: implications for exercise performance and metabolism

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.

Adaptive reciprocity of lipid and glucose metabolism in human short-term starvation

The human organism has tools to cope with metabolic challenges like starvation that are crucial for survival. Lipolysis, lipid oxidation, ketone body synthesis, tailored endogenous glucose production and uptake, and decreased glucose oxidation serve to protect against excessive erosion of protein mass, which is the predominant supplier of carbon chains for synthesis of newly formed glucose. The starvation response shows that the adaptation to energy deficit is very effective and coordinated with different adaptations in different organs. From an evolutionary perspective, this lipid-induced effect on glucose oxidation and uptake is very strong and may therefore help to understand why insulin resistance in obesity and type 2 diabetes mellitus is difficult to treat. The importance of reciprocity in lipid and glucose metabolism during human starvation should be taken into account when studying lipid and glucose metabolism in general and in pathophysiological conditions in particular.

Metabolism of ketone bodies by skeletal muscle in starvation and uncontrolled diabetes

Previous studies have suggested that skeletal muscle may be responsible for as much as 25% of ketone body (KB) production in poorly controlled diabetes. In the present studies, acetoacetate (AcAc) and beta-hydroxybutyrate production was quantitated in the canine hindlimb from surgically placed arterial and venous catheters in conscious insulin-withdrawn diabetic (n = 5) and 4-day fasted (n = 7) dogs. A two-pool modeling technique, using simultaneous infusions of 13C acetoacetate and 14C beta-hydroxybutyrate (beta OHB) was employed to quantitate total body and hindlimb KB kinetics. Total KB production was 9.4 and 39.3 mumol.kg-1.min-1 in the fasted and diabetic animals, respectively. Hindlimb KB production was negligible in both groups. The two-pool model estimates of hindlimb KB utilization were similar to the values obtained by an arterial-venous difference calculation. In conclusion, the hindlimb does not contribute to de novo synthesis of KBs in either fasted or diabetic dogs. Since species differences in KB metabolism occur, it is possible that muscle may be a site for KB production in humans.

Pseudoketogenesis in hepatectomized dogs

Overestimation of ketone body turnover in vivo, measured by tracer kinetics, could occur if specific activity or molar percent enrichment is diluted in extrahepatic tissues by label exchange via reversal of 3-oxoacid-CoA transferase, a process we call pseudoketogenesis. To test this hypothesis, euglycemic hepatectomized dogs were injected with a bolus of acetoacetate (0.8 mmol/kg), 32% enriched in [3,4-13C2]acetoacetate. Concentrations and labeling patterns of blood acetoacetate and R-3-hydroxybutyrate were measured by selected ion-monitoring gas chromatography-mass spectrometry. During the 60 min after bolus injection of [3,4-13C2]acetoacetate, the molar percent enrichment of blood [3,4-13C2]acetoacetate decreased to 73 +/- 3% (n = 5) in controls and to 11.5 +/- 0.8% (n = 3) during infusion of dichloroacetate, an activator of pyruvate dehydrogenase. The enrichment of R-3-hydroxy-[3,4-13C2]butyrate followed closely that of [3,4-13C2]acetoacetate. These dilutions occurred despite a net uptake of ketone bodies. Concomitantly, 10.6 +/- 2.2 (n = 5) and 6.0 +/- 2.9% (n = 3) of [13C]acetoacetate molecules were labeled on all four carbons in control and dichloroacetate-treated dogs, respectively. This uniformly labeled acetoacetate arises from partial equilibration between [3,4-13C2]acetoacetate and [1,2-13C2]acetyl-CoA via the reactions catalyzed by 3-oxoacid-CoA transferase and acetoacetyl-CoA thiolase. Our data demonstrate the reversibility of the 3-oxoacid-CoA transferase in intact extrahepatic tissues and support the concept of pseudoketogenesis. This phenomenon has been quantitated by kinetic analysis of the data.

Ketone body production and disposal: effects of fasting, diabetes, and exercise

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)

Integrated metabolic regulation during acute rest states in man, similarity to fasting: a biochemical hypothesis

Many physiologic changes have been described in association with acute rest states in man. In particular, among metabolic changes, there occur marked decrease of forearm respiratory quotient, increased forearm venous beta-hydroxybutyric acid, decreased forearm lactate generation, and decreased red cell glycolysis. One primary hypothesis of metabolic control that is capable of interrelating these metabolic changes is that in peripheral tissue there is an overall shift of substrate away from glucose toward increased fatty acid beta-oxidation, such as occurs in fasting. Since regulation of lipid and carbohydrate metabolism is central to several major disorders, such a putative shift of metabolic control and its mechanism may have many basic and clinical implications, including, especially, slowing of biological aging.

Effect of insulin on ketone body clearance studied by a ketone body "clamp" technique in normal man

The effect of elevated plasma insulin concentration (55 +/- 2 mU/l) on peripheral clearance and production of total ketone bodies was determined using 3-14C-acetoacetate tracer infusions. Nine normal subjects were studied twice, once during insulin infusion (20 mU.m-2.min-1), once during basal plasma insulin concentrations (controls). Blood total ketone body concentrations (sum of acetone, acetoacetate and beta-hydroxybutyrate) were maintained in both studies at 2 mmol/l by feedback-controlled sodium acetoacetate infusions. The coefficient of variation of total ketone body concentrations during the two clamp studies was 10 and 11% respectively. The sodium acetoacetate infusion rate required during the clamp was 55 +/- 4% higher during hyperinsulinaemia than in controls (p less than 0.005). This was due to increased total ketone body clearance (8.4 +/- 0.7 vs 6.7 +/- 0.4 ml.kg-1.min-1, p less than 0.015), and to enhanced suppression of ketone body production (p less than 0.01). Hyperketonaemia alone decreased ketone body production by 42% and diminished ketone body clearance by 46%, the former being enhanced, the latter being in part antagonised by insulin. Since the plasma insulin concentrations were within those observed in patients treated for diabetic ketoacidosis, the data suggest that the antiketotic effect of insulin therapy results in part from an increase in peripheral ketone body disposal.

Minor role of ketone bodies in energy metabolism by skeletal muscle tissue during the postoperative course

To evaluate changes of peripheral ketone body (KB) metabolism after operation, muscle metabolism in postsurgical patients was studied at 3 hours (SI) and 24 hours (SII) after surgery by the forearm catheter technique. Data were compared to those of equivalent fasted controls (CI, CII). In a manner consistent with enhanced mobilization of endogenous substrate stores, arterial concentrations of free fatty acids (FFA), 3-hydroxybutyrate (3-HOB), and acetoacetate (AcAc) were markedly elevated immediately after surgery. This increase was accompanied by a rise in muscular utilization of AcAc (SI: 0.21 +/- 0.05 mumol/100 g/min; CI: 0.08 +/- 0.05, p less than 0.05) and 3-HOB (SI: 0.24 +/- 0.06 mumol/100 g/min; CI: 0.11 +/- 0.01, p less than 0.05). Surprisingly, on the first postoperative day, concentrations of AcAc and 3-HOB fell below those of fasting controls. Concomitantly, the utilization rate of AcAc by muscle (SII: 0.07 +/- 0.03 mumol/100 g/min; CII: 0.27 +/- 0.04, p less than 0.05) was significantly lower in patients than in controls. Reduction of the fractional extraction rate of AcAc (SI: 38.4 +/- 3.8%; SII: 24.0 +/- 6.1%, p less than 0.05), as well as a net production of 3-HOB by muscle (SII: -0.08 +/- 0.05 mumol/100 g/min; CII: 0.49 +/- 0.13, p less than 0.05) 24 hours after surgery indicated a reduced peripheral capacity for KB removal. Since this finding was related to a significantly higher rate of muscular glycerol production (SII: -0.13 +/- 0.03 mumol/100 g/min; CII: -0.06 +/- 0.02, p less than 0.05), one may suggest that increased intramuscular availability of FFA from triglyceride hydrolysis was responsible for the impairment of peripheral KB utilization. These results indicate that KBs contribute little to energy metabolism in skeletal muscle tissue in the late postoperative phase.

Hormonal and metabolic response to physical exercise, fasting and cold exposure in the rat. Effects on ketogenesis in isolated hepatocytes

Four groups of rats were subjected to the following conditions: (1) 48 h fasting, (2) 48 h of 4 degrees C cold exposure, (3) 5 h treadmill running, (4) 48 h fasting with 4 degrees C cold exposure. The groups were compared to fed control rats in order to study hormonal and metabolic responses in blood and tissue samples. Isolated hepatocytes were used to evaluate the rate of ketogenesis. Decreases in liver glycogen and increases in blood free fatty acids (FFA) confirmed that glycogenolysis and lipolysis occur in these situations of metabolic stress. Increases in the glucagon/insulin plasma ratio were also noted. Plasma catecholamine levels were only enhanced after running and after cold exposure. Production of blood ketone bodies was stimulated more by running and by fasting than by cold exposure. The low ketone body production observed after cold exposure seems to be linked to increases liver glycogen levels and decreased FFA availability. Liver cells isolated after cold exposure exhibited higher ketogenesis than these isolated after running. This difference in ketogenic capacity could result both from the longer hormonal stimulation by high glucagon/insulin plasma ratios and from the metabolic state of the liver.

Marked reduction of forearm carbon dioxide production during states of decreased metabolism

We studied oxygen consumption, carbon dioxide production and acid/base changes in 62 subjects during two hypometabolic states (35 during transcendental mediation and 27 during unstylized rest). The results indicate that, during these hypometabolic states, arterial-venous CO2 content difference declines, and that during transcendental meditation, arterial-venous CO2 content difference briefly disappears. This change is due to both an increase of arterial CO2 content and a decrease of venous CO2 content. Similar, but opposite and smaller, changes occurred in arterial and venous O2 content. Respiratory quotient was low at all times and decreased during the hypometabolic states. These results are consistent with the hypothesis that, during hypometabolic states significant ketogenesis is induced, and provision of energy from the normally predominant process of beta-oxidation of fat becomes sufficient to provide energy for greatly reduced activity without entry of two carbon fragments into the tricarboxylic acid cycle; under these conditions, biochemical feedback mechanisms inhibit the tricarboxylic acid cycle. The net result of these metabolic changes could explain the major observations: absent carbon dioxide production while oxygen consumption declines but continues and the venous effluent contains more acid.

Response of ketone body metabolism to exercise during transition from postabsorptive to fasted state

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.

Ketone body metabolism in normal and diabetic human skeletal muscle

Although the liver is considered the major source of ketone bodies (KB) in humans, these compounds may also be formed by nonhepatic tissues. To study this aspect further, 3-[14C]hydroxybutyrate (BOH) or [3-14C]acetoacetate (AcAc) were constantly infused after a priming dose and contemporaneous arterial and venous samples were taken at splanchnic, heart, kidney, and leg sites in eight normal subjects (N) undergoing diagnostic catheterization and at the forearm site in five normal and six ketotic diabetic (D) subjects. After 70 min of infusion, tracer and tracee levels of AcAc and BOH reached a steady state in the artery and vein in both normal and diabetic subjects. The venous-arterial (V-A) difference at the forearm step for cold KB was negligible both in normal and diabetic subjects, whereas for labeled KB it was approximately 10-fold higher in diabetic subjects (V-A AcAc, -31 +/- 7 and -270 +/- 34 dpm/ml in N and D, respectively; V-A BOH, -38 +/- 6 and -344 +/- 126 dpm/ml in N and D, respectively). We assumed that the V-A difference in tracer concentration was consistent with dilution of the tracer by newly synthesized tracee inside the muscle and calculated that the forearm muscle produces KB at a rate of 16.2 +/- 3.3 mumol/min in D and 0.9 +/- 0.9 mumol/min in N. These findings can be accounted for by the hypothesis that the disappearance flux of KB from circulation was replaced by an equivalent flux of KB entering the vein at the muscle step in D but not in N. Moreover, in N KB were not only produced but also utilized by the splanchnic area (39 +/- 9 mumol/min).(ABSTRACT TRUNCATED AT 250 WORDS)

Turnover and splanchnic metabolism of free fatty acids and ketones in insulin-dependent diabetics at rest and in response to exercise

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.

Ketone body production in diabetic ketosis by other than liver

To determine if ketone bodies, synthesized from fatty acids by tissues other than the liver, enter the circulation, rats in diabetic ketosis were injected with sodium [6,13-14C]palmitate. Hydroxybutyrate was isolated from the urine excreted by each rat and from an aqueous extract of its carcass. The distribution of 14C in the four carbons of hydroxybutyrate in the extract was the same as in the urine. The ratio of 14C in carbon 1 to carbon 3 of the hydroxybutyrate averaged 1.80 and averaged 1.31 in carbon 2 to carbon 4. Hydroxybutyrate when formed by perfused liver has the same carbon 1-to-carbon 3 ratio as carbon 2-to-carbon 4 ratio. The results indicate that hydroxybutyrate synthesized by tissues other than the liver mixes in the circulation with that synthesized by the liver and a portion of the mix is then excreted in the urine. The difference between the carbon 1-to-3 carbon ratio 3 and carbon 2-to-carbon 4 ratio calculates to an estimated minimum of 15% to 17% of the hydroxybutyrate in the circulation of the ketotic diabetic rat having tissues other than the liver as its source. Assuming the liver and kidneys are the sources of the ketone bodies in diabetic ketosis, the ketone bodies produced by the kidneys are not excreted into the urine without first entering the circulation.

Absence of exercise-induced hypoglycaemia in type i (insulin-dependent) diabetic patients during maintenance of normoglycaemia by short-term, open-loop insulin infusion

To assess the risk and possible mechanisms of hypoglycaemia during moderate exercise in Type I (insulin-dependent) diabetic patients receiving constant insulin infusion, five insulin-dependent male diabetic patients were exercised 18 h after their last meal and 30 h after their last injection of intermediate acting insulin. Intravenous insulin was initially delivered via a closed-loop infusion system programmed to lower mean blood glucose from 11.3 +/- 1.8 to 4.8 +/- 0.4 mmol/l over approximately 3.5 h. Blood glucose was then maintained at this level for 4 h. At this time, the closed-loop infusion was discontinued and replaced by an open-loop system. The average amount of insulin infused per min during the 4 h normoglycaemic closed-loop period was calculated and this amount was infused at a constant rate during both a 30 min period of exercise on a bicycle ergometer (approximately 65% maximum oxygen uptake) and a 30 min rest period which followed. Five nondiabetic males served as control subjects. Despite significantly higher free insulin concentrations (p less than 0.05) and identical pre-exercise blood glucose concentrations, blood glucose rose during exercise only in the diabetic group (0.5 +/- 0.2 mmol/l; p less than 0.01). Changes in the serum concentrations of lactate, glycerol, glucagon, cortisol, non-esterified fatty acids and growth hormone were similar in the two groups and did not account for the increment of blood glucose in the diabetic patients. Beta-hydroxybutyrate concentrations were, however, higher in the diabetic patients at the onset of exercise (p less than 0.01) and decreased significantly more than the control subjects during exercise. We conclude that exercise under these conditions in diabetic patients is not attended by hypoglycaemia.

On the significance of the respiratory exchange ratio after different diets during exercise in man

Total respiratory exchange ratio (R) was compared to the respiratory exchange ratio over the legs (RQL) during exercise after different diets, to evaluate if R (which represents a mean for the whole body) can be used to estimate the relative proportions of fat and carbohydrate oxidation in exercising muscle. One important prerequisite for this is a steady state acid base balance, 7 subjects were studied at rest and during the later part of a 25 min exercise (65% of Vo2 max) on two occasions, the first preceded by a fat rich diet and the second by a carbohydrate rich diet. Oxygen uptake, R and arterial-femoral venous differences for [O2], [CO2], PCO2 and pH and arterial concentrations for lactate and beta-hydroxybutyrate were measured. Respiratory exchange ratio over the exercising legs (RQL) and ventilation/oxygen uptake were calculated. Arterial pH, PCO2, lactate and beta-hydroxybutyrate as well as specific ventilation attained steady levels during the later part of exercise after both diets. Although arterial lactate and beta-hydroxybutyrate differed between the diets, the arterial pH and specific ventilation were the same. Both R and RQL were higher after the carbohydrate than after the fat diet and there was no systematic difference between R and RQL. Therefore, it seems likely that R estimates the proportion of fat and carbohydrate oxidation in skeletal muscle during submaximal exercise after extreme diets.

Free fatty acid and ketone body metabolism during exercise in diabetes

In diabetic patients the metabolic response to physical exercise is to a large extent determined by the degree of insulin deficiency at the onset of exercise. Thus, in well-controlled insulin-treated patients with mild hyperglycaemia and no or minimal ketonemia the utilization of FFA, blood glucose and glycogen by working muscle is similar to that of healthy subjects, and exercise is accompanied by a fall in blood glucose levels. In contrast, patients in poor metabolic control with marked hyperglycaemia, elevated FFA levels and hyperketonemia may respond to exercise with a further rise in FFA, ketone levels and blood glucose, reflecting augmented rates of adipose tissue lipolysis and hepatic ketogenesis. The accelerated rate of ketogenesis seen during exercise continues unabated in diabetics but not in healthy subjects during the post-exercise recovery period, thereby contributing to the development of post-exercise ketosis. These considerations underscore the importance of adequate insulin administration in connection with exercise in diabetic patients.

Effect of continuously increasing concentrations of plasma ketone bodies on the uptake and oxidation of glucose by muscle in man

Muscle metabolism in man was studied by measuring the arterial and deepvenous concentrations of glucose, lactate, pyruvate, free fatty acids, beta-hydroxybutyrate and aceacetoacetate, and forearm blood flow. After the subjects had fasted overnight, their arterial free fatty acid and ketone levels rose continuously during a period of 90 min, leading to increased ketone body uptake by muscle. Hence, for each subject, a relation was obtained between arterial concentrations and arterial-deepvenous differences of beta-hydroxybutyrate and acetoacetate. As the ketone body utilization increased, the release of lactate rose as well. In spite of these alterations the uptake of glucose remained unchanged. These findings underline the current notion that accelerated ketone uptake reduces pyruvate oxidation but not glucose uptake by muscle.

Influence of acetate on the metabolism of beta-hydroxybutyrate in the perfused hind-quarter of the rat

The effect of acetate, at concentrations normally found during ethanol combustion in intact animals and man in vivo, on the uptake and oxidation of beta-hydroxybutyrate by the perfused rat hind-quarter was studied. The addition of acetate did not significantly affect the total uptake of beta-hydroxybutyrate, but caused a 40% decrease in the oxidation of beta-hydroxybutyrate to CO2. The oxidation of 14C-beta-hydroxybutyrate to 14CO2 accounted for about 10% of the total oxygen consumption by the perfused muscle in the absence of acetate. In the presence of acetate this figure was reduced to about 5%. The addition of beta-hydroxybutyrate did not significantly affect the metabolic fate of 14C-acetate. It is concluded that acetate is preferred as oxidative substrate to beta-hydroxybutyrate. The inhibited oxidation of beta-hydroxybutyrate by acetate did not affect the concentration ratio between beta-hydroxybutyrate and acetoacetate in the medium at the end of the perfusion, indicating that the ability of the muscle tissue to restore an increased redoxlevel, "exported" from the liver during ethanol combustion to extrahepatic tissues, was not impaired by acetate.

Utilization of oxidizable substrates by the sheep hind limb: effects of starvation and exercise

Measurements of substrate uptake by the sheep hind limb show a pattern similar to human and other monogastric animals. Thus free fatty acids (FFA) are the principal substrates at rest and during exercise while beta-hydroxybutyrate and acetoacetate are major nutrients in starved animals. The hind limb has arteriovenous differences for glucose and lactate which indicate that glucose supplies about 27% of the fuel of respiration during exercise, but the hind limb in resting, fed, and starved animals returns essentially all of the glucose carbon to the blood in the form of lactate. This finding is consistent with a conservation of glucose in aminals which obtain very little dietary glucose. Although some acetate is extracted from the blood in fed sheep, the utilization of this nutrient can account for only 2% or less of the oxygen uptake in the hind limb of starved or exercising animals. Thus, while acetate is the major product of the sheep rumen it is not used directly as a major energy source. We propose that most of the actate is converted to FFA which can be stored as triglyceride or oxidized in muscle.

Intestinal metabolism of plasma free fatty acids. Intracellular compartmentation and mechanisms of control

Fatty acid metabolism in intestinal mucosa has been examined primarily in regard to lipid absorption. Since earlier studies suggested intestinal utilization of plasma free fatty acids (FFA), we investigated mucosal metabolism of plasma FFA in rats. Mucosal radioactivity (1 per cent of administered) was maximal 2 min after i.v. [14C]palmitate. Of mucosal 14C, 42 percent was in water-soluble metabolites, including CO2 and ketoacids, 28 percent in phospholipids, and only 16 per cent in triglycerides. The specific activity of mucosal triglyceride fatty acids (TGFA) was 11 times that of serum TGFA, confirming in situ synthesis. Double isotope experiments showed marked differences in the metabolism of fatty acids entering mucosa simultaneously from lumen and plasma. Whereas luminal fatty acids were chiefly esterified to triglyceride, plasma FFA were preferentially oxidized and incorporated into phospholipids. Crypts did not differ from villi, indicating that intestinal metabolism of plasma FFA is related to their site of entry into epithelial cells. Mucosal metabolism of i.v. [14C]palmitate was minimally affected by glucose administration. However, intraduodenal isocaloric ethanol inhibited mucosal oxidation of FFA by 60 per cent, and increased incorporation into triglycerides nearly twofold. During lipid absorption, mucosal uptake of plasma FFA doubled and incorporation into intestinal lymph triglycerides was increased sixfold. These studies demonstrate an intracellular compartmentation of fatty acids in the intestinal epithelium. In contrast to absorbed luminal fatty acids, plasma FFA in the fasting state are both an energy source and a substrate for the synthesis of tissue phospholipid. The fasting contribution of plasma FFA to mucosal and lymph triglyceride is minimal, but it increases during ethanol administration and fat absorption.

Ketone-body production and oxidation in fasting obese humans

Rates of plasma acetoacetate and total ketone-body production and oxidation to CO2 were determined by an isotope tracer technique in eight obese subjects undergoing progressive starvation. After a brief fast and under conditions of mild ketonemia and minimal ketonuria, rates of acetoacetate and total ketone-body production and oxidation were directly related to the increasing plasma concentration. After a longer fast and with severer ketonemia, acetoacetate and total ketone-body production and oxidation rates were higher but became constant and unrelated to the plasma concentrations. The maximum rates of total ketone-body production and oxidation were about 150 g/24 h and 129 g/24 h, respectively. Although an increased ketone-body production was the primary factor responsible for the hyperketonemia, an imbalance between production and removal of the ketone bodies cannot be excluded. Such an imbalance could account, at least in part, for the developing hyperketonemia and for the lack of relationship between production rates and plasma concentrations.

The interconversion and disposal of ketone bodies in untreated and injured post-absorptive rats

[3-(14)C]Acetoacetate and beta-hydroxy[3-(14)C]butyrate were used to investigate the kinetics of ketone body metabolism in rats 3h after bilateral hind-limb ischaemia and in controls, both groups being in the post-absorptive state and in a 20 degrees C environment. Calculations were carried out as described by Heath & Barton (1973) and the following conclusions were reached. 1. In both injured and control rats, the rates of irreversible disposal (extrahepatic utilization) of beta-hydroxybutyrate and acetoacetate were proportional within experimental error to their blood concentrations up to at least 0.4mm (the maximum found in these rats), implying that they were determined, via these concentrations, by the rates of production by the liver. 2. Conversion of blood beta-hydroxybutyrate into blood acetoacetate took place mainly in the liver, but the reverse process occurred mainly in extrahepatic tissues. 3. The ;metabolic clearance rate' (the volume of blood which, if completely cleared of substrate in unit time, would give a disposal rate equal to that in the whole animal) was calculated for beta-hydroxybutyrate and acetoacetate. Comparison with the cardiac output showed that in control rats the proportion of circulating beta-hydroxybutyrate extracted was lower than that of acetoacetate, clearance of which appeared almost complete. After injury both metabolic clearance rates decreased, probably because of the lower cardiac output. 4. After injury, because the average blood concentrations of ketone bodies, especially acetoacetate, were higher, the mean total rate of disposal also increased. Assuming complete oxidation, the mean contribution of ketone bodies to the whole body O(2) consumption rose from 7 to 15%.

Human forearm metabolism during progressive starvation

Forearm muscle metabolism was studied in eight obese subjects after an overnight, 3 and 24 day fast. Arterio-deep-venous differences of oxygen, carbon dioxide, glucose, lactate, pyruvate, free fatty acids, acetoacetate, and beta-hydroxybutyrate with simultaneous forearm blood flow were measured. Rates of metabolite utilization and production were thus estimated. Oxygen consumption and lactate and pyruvate production remained relatively constant at each fasting period. Glucose, initially the major substrate consumed, showed decreased consumption after 3 and 24 days of fasting. Acetoacetate and beta-hydroxybutyrate consumption after an overnight fast was low. At 3 days of fasting with increased arterial concentrations of acetoactate and beta-hydroxybutyrate, consumption of these substrates rose dramatically. At 24 days of fasting, despite further elevation of arterial levels of acetoacetate and beta-hydroxybutyrate, the utilization of acetoacetate did not increase further and if anything decreased, while five out of eight subjects released beta-hydroxybutyrate across the forearm. Acetoacetate was preferentially extracted over beta-hydroxybutyrate. At 24 days of starvation, free fatty acids were the principal fuels extracted by forearm muscle; at this time there was a decreased glucose and also ketone-body consumption by skeletal muscle.

Evidence for an effect of inulin on the peripheral utilization of ketone bodies in dogs

The rates of transport and oxidation of acetoacetate have been measured in seven anesthetized, pancreatectomized, ketotic dogs using a constant infusion of acetoacetate-3-(14)C. Control experiments were performed in 14 normal dogs. In addition to the acetoacetate-(14)C, the latter were infused at a constant rate with varying amounts of unlabeled acetoacetate so as to obtain a range of ketone transport (26-65 mumoles/min.kg) comparable with that observed in the diabetic dogs (21-41 mumoles/min.kg). The specific activities of acetoacetate and beta-hydroxybutyrate in blood became equal during the infusion of labeled acetoacetate, indicating that the net transport of acetoacetate represents that of total ketones. In each group, the concentration of ketones was an exponential function of the rate of transport, but for any value below 30 mumoles/min.kg, ketone concentration in the diabetic dogs was about 3 times that in normal dogs, indicating an impairment of mechanisms for utilizing ketones in insulin deficient animals. Maximal capacity to utilize ketones in diabetic dogs was slightly more than half that of normal ones. A similar fraction (32-63%) of the infused (14)C appeared in respiratory CO(2) in the two groups and was independent of the rate of transport. In seven of the normal dogs, administration of insulin and glucose increased removal of the infused ketones and increased the fraction of (14)C appearing in respiratory CO(2). These results demonstrate that utilization of ketones in extrahepatic tissues is influenced by insulin; impaired utilization contributes to diabetic ketosis and is probably essential to the production of severe ketoacidosis.

Liver and kidney metabolism during prolonged starvation

This study quantifies the concentrations of circulating insulin, growth hormone, glucose, free fatty acids, glycerol, beta-hydroxybutyrate, acetoacetate, and alpha amino nitrogen in 11 obese subjects during prolonged starvation. The sites and estimated rates of gluconeogenesis and ketogenesis after 5-6 wk of fasting were investigated in five of the subjects. Blood glucose and insulin concentrations fell acutely during the 1st 3 days of fasting, and alpha amino nitrogen after 17 days. The concentration of free fatty acids, beta-hydroxybutyrate, and acetoacetate did not reach a plateau until after 17 days. Estimated glucose production at 5-6 wk of starvation is reduced to approximately 86 g/24 hr. Of this amount the liver contributes about one-half and the kidney the remainder. Approximately all of the lactate, pyruvate, glycerol, and amino acid carbons which are removed by liver and kidney are converted into glucose, as evidenced by substrate balances across these organs.