Hepatic ketogenesis and gluconeogenesis in humans

The storage stability and concentration of acetoacetate differs between blood fractions

BACKGROUND

Plasma concentrations of 3-hydroxybutyrate (3HB) are measured more often than acetoacetate (AcAc) which may be due to the reported storage instability of AcAc. The aims of the study were to compare the storage stability of AcAc in different blood fractions over time (90days) when stored at -80°C and to determine the postprandial concentration of AcAc in whole blood, plasma and red blood cells.

METHODS

Blood was collected from fasting subjects (n=5): whole blood, plasma and red blood cells were isolated and deproteinised in perchloric acid, and supernatants were stored at -80°C until analysis. Postprandial concentrations of AcAc in whole blood, plasma and red blood cells were determined at regular intervals over 420min, after subjects (n=23) had consumed a mixed test meal.

RESULTS

Storing deproteinised plasma at -80°C resulted in no significant change in AcAc concentration over 60days. In contrast, whole blood AcAc concentrations significantly decreased by 51% (p=0.018) within 30days. The concentration of AcAc in fasting and postprandial plasma was notably higher than that of whole blood and red blood cells.

DISCUSSION

Our data demonstrates that plasma for AcAc analysis can be stored for longer than previously suggested provided that plasma is deproteinised and stored at -80°C.

Ketogenic diets for weight loss: A review of their principles, safety and efficacy

SUMMARY

Low-carbohydrate "ketogenic" diets have increased in popularity over recent years as a means of weight loss. Published studies of these diets have been highly heterogeneous, and it remains unclear to what degree dietary carbohydrate intake must be restricted in order to induce ketosis. Despite concern that they are often relatively high in fat, ketogenic low-carbohydrate diets have been generally shown to compare favourably with low-fat diets in terms of weight loss and improvements in triglyceride and high-density lipoprotein levels. This review includes a brief overview of ketone body metabolism, and summarises the literature regarding the safety and efficacy of ketogenic diets for weight loss.:

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.

Age-related effects of fasting on ketone body production during lipolysis in rats

OBJECTIVE

The age-related effects of fasting on lipolysis, the production of ketone bodies, and plasma insulin levels were studied in male 3-, 8-, and 32-week-old Sprague-Dawley rats.

METHODS

The rats were divided into fasting and control groups. The 3-, 8- and 32-week-old rats tolerated fasting for 2, 5, and 12 days, respectively.

RESULTS

Fasting markedly reduced the weights of perirenal and periepididymal white adipose tissues in rats in the three age groups. The mean rates of reduction in both these adipose tissue weights during fasting periods were higher in the order of 3 > 8 > 32-week-old rats. Fasting transiently increased plasma free fatty acid (FFA), total ketone body, β-hydroxybutyrate, and acetoacetate concentrations in the rats in the three age groups. However, plasma FFA, total ketone body, β-hydroxybutyrate, and acetoacetate concentrations in the 3-week-old rats reached maximal peak within 2 days after the onset of fasting, although these concentrations in the 8- and 32-week-old rats took more than 2 days to reach the maximal peak. By contrast, the augmentation of plasma FFA, total ketone body, β-hydroxybutyrate, and acetoacetate concentrations in the rats in the three age groups had declined at the end of each experimental period. Thus, the capacity for fat mobilization was associated with tolerance to fasting. Plasma insulin concentrations in the rats in the three age groups were dramatically reduced during fasting periods, although basal levels of insulin were higher in the order of 32 > 8 > 3 week-old rats.

CONCLUSION

These results suggest that differences in fat metabolism patterns among rats in the three age groups during prolonged fasting were partly reflected the metabolic turnover rates, plasma insulin levels, and amounts of fat storage.

Regional variations in the concentrations of ketone bodies in cirrhosis and hepatic encephalopathy: a study in patients with TIPSS

BACKGROUND

Little is known about the metabolism of acetoacetate and β-hydroxybutyrate in patients with cirrhosis and encephalopathy.

AIMS

We investigated the fate of ketone bodies in these conditions.

MATERIALS AND METHODS

We studied 18 cirrhotic patients with encephalopathy and 17 cirrhotics without. At the time of insertion of a transjugular intrahepatic portosystemic stent shunt (TIPSS) or at the time of portographical assessment of the shunt's patency, we collected blood from the internal jugular, the right atrium, the inferior vena cava, the hepatic, the portal, the splenic veins and the radial artery. We used nuclear magnetic resonance spectroscopy to measure the concentrations of acetoacetate and β-hydroxybutyrate.

RESULTS

There was no difference in the total ketone body concentrations between the two groups. The mitochondrial redox potential was significantly higher in the encephalopathics (142/54=2.63 vs 52/83=0.62) (P<0.01). β-hydroxybutyrate was significantly lower in the portal vein of encephalopathics (52 ± 4 vs 28 ± 3) (P<0.02) and in the splenic vein (48 ± 6 vs 32 ± 5) (P<0.04). Acetoacetate was significantly higher in encephalopathics in the internal jugular vein (134 ± 12 vs 92 ± 16) (P<0.03), the right atrium (112 ± 18 vs 68 ± 11) (P<0.03), the hepatic vein (162 ± 25 vs 115 ± 19) (P<0.05), the portal vein (133 ± 20 vs 81 ± 14) (P<0.02) and the splenic vein (167 ± 24 vs 122 ± 21) (P<0.04). All measurements are expressed in μmols/L.

CONCLUSIONS

There are significant variations in the regional concentrations of the ketone bodies in encephalopathy.

Clinical review: ketones and brain injury

Although much feared by clinicians, the ability to produce ketones has allowed humans to withstand prolonged periods of starvation. At such times, ketones can supply up to 50% of basal energy requirements. More interesting, however, is the fact that ketones can provide as much as 70% of the brain's energy needs, more efficiently than glucose. Studies suggest that during times of acute brain injury, cerebral uptake of ketones increases significantly. Researchers have thus attempted to attenuate the effects of cerebral injury by administering ketones exogenously. Hypertonic saline is commonly utilized for management of intracranial hypertension following cerebral injury. A solution containing both hypertonic saline and ketones may prove ideal for managing the dual problems of refractory intracranial hypertension and low cerebral energy levels. The purpose of the present review is to explore the physiology of ketone body utilization by the brain in health and in a variety of neurological conditions, and to discuss the potential for ketone supplementation as a therapeutic option in traumatic brain injury.

Interorgan coordination of the murine adaptive response to fasting

Starvation elicits a complex adaptive response in an organism. No information on transcriptional regulation of metabolic adaptations is available. We, therefore, studied the gene expression profiles of brain, small intestine, kidney, liver, and skeletal muscle in mice that were subjected to 0-72 h of fasting. Functional-category enrichment, text mining, and network analyses were employed to scrutinize the overall adaptation, aiming to identify responsive pathways, processes, and networks, and their regulation. The observed transcriptomics response did not follow the accepted "carbohydrate-lipid-protein" succession of expenditure of energy substrates. Instead, these processes were activated simultaneously in different organs during the entire period. The most prominent changes occurred in lipid and steroid metabolism, especially in the liver and kidney. They were accompanied by suppression of the immune response and cell turnover, particularly in the small intestine, and by increased proteolysis in the muscle. The brain was extremely well protected from the sequels of starvation. 60% of the identified overconnected transcription factors were organ-specific, 6% were common for 4 organs, with nuclear receptors as protagonists, accounting for almost 40% of all transcriptional regulators during fasting. The common transcription factors were PPARα, HNF4α, GCRα, AR (androgen receptor), SREBP1 and -2, FOXOs, EGR1, c-JUN, c-MYC, SP1, YY1, and ETS1. Our data strongly suggest that the control of metabolism in four metabolically active organs is exerted by transcription factors that are activated by nutrient signals and serves, at least partly, to prevent irreversible brain damage.

Pyruvate carboxylase deficiency: mechanisms, mimics and anaplerosis

Pyruvate carboxylase (PC) is a regulated mitochondrial enzyme that catalyzes the conversion of pyruvate to oxaloacetate, a critical transition that replenishes citric acid cycle intermediates and facilitates other biosynthetic reactions that drive anabolism. Its deficiency causes multiorgan metabolic imbalance that predominantly manifests with lactic acidemia and neurological dysfunction at an early age. Three clinical forms of PC deficiency have been identified: an infantile form (Type A), a severe neonatal form (Type B), and a benign form (Type C), all of which exhibit clinical or biochemical correlates of impaired anaplerosis. There is no effective treatment for these patients and most, except those affected by the benign form, die in early life. We review the physiology of this enzyme and dissect the major clinical, biochemical, and genetic aspects of its dysfunction, emphasizing features that distinguish PC deficiency from other causes of lactic acidemia that render PC deficiency potentially treatable using novel interventions capable of enhancing anaplerosis.

The human gastric pathogen Helicobacter pylori has a potential acetone carboxylase that enhances its ability to colonize mice

Background

Helicobacter pylori colonizes the human stomach and is the etiological agent of peptic ulcer disease. All three H. pylori strains that have been sequenced to date contain a potential operon whose products share homology with the subunits of acetone carboxylase (encoded by acxABC) from Xanthobacter autotrophicus strain Py2 and Rhodobacter capsulatus strain B10. Acetone carboxylase catalyzes the conversion of acetone to acetoacetate. Genes upstream of the putative acxABC operon encode enzymes that convert acetoacetate to acetoacetyl-CoA, which is metabolized further to generate two molecules of acetyl-CoA.

Results

To determine if the H. pylori acxABC operon has a role in host colonization the acxB homolog in the mouse-adapted H. pylori SS1 strain was inactivated with a chloramphenicol-resistance (cat) cassette. In mouse colonization studies the numbers of H. pylori recovered from mice inoculated with the acxB:cat mutant were generally one to two orders of magnitude lower than those recovered from mice inoculated with the parental strain. A statistical analysis of the data using a Wilcoxin Rank test indicated the differences in the numbers of H. pylori isolated from mice inoculated with the two strains were significant at the 99% confidence level. Levels of acetone associated with gastric tissue removed from uninfected mice were measured and found to range from 10–110 μmols per gram wet weight tissue.

Conclusion

The colonization defect of the acxB:cat mutant suggests a role for the acxABC operon in survival of the bacterium in the stomach. Products of the H. pylori acxABC operon may function primarily in acetone utilization or may catalyze a related reaction that is important for survival or growth in the host. H. pylori encounters significant levels of acetone in the stomach which it could use as a potential electron donor for microaerobic respiration.

Fuel metabolism in starvation

This article, which is partly biographical and partly scientific, summarizes a life in academic medicine. It relates my progress from benchside to bedside and then to academic and research administration, and concludes with the teaching of human biology to college undergraduates. My experience as an intern (anno 1953) treating a youngster in diabetic ketoacidosis underscored our ignorance of the controls in human fuel metabolism. Circulating free fatty acids were then unknown, insulin could not be measured in biologic fluids, and beta-hydroxybutyric acid, which was difficult to measure, was considered by many a metabolic poison. The central role of insulin and the metabolism of free fatty acids, glycerol, glucose, lactate, and pyruvate, combined with indirect calorimetry, needed characterization in a near-steady state, namely prolonged starvation. This is the main topic of this chapter. Due to its use by brain, D-beta-hydroxybutyric acid not only has permitted man to survive prolonged starvation, but also may have therapeutic potential owing to its greater efficiency in providing cellular energy in ischemic states such as stroke, myocardial insufficiency, neonatal stress, genetic mitochondrial problems, and physical fatigue.

Anesthetic management of a pediatric patient on a ketogenic diet

There are several specific considerations regarding seizure control during the perioperative period in patients who have been placed on a ketogenic diet (KD). A KD is high in fat and low in protein and carbohydrates and has a long history of use for the treatment of intractable seizures in children. Maintaining therapeutic ketosis and modifying the acid-base balance are particularly important for preventing seizures in patients on a KD. We report changes in the biochemical parameters of a patient with double cortex syndrome who was on a KD and who had been scheduled for the treatment of dental caries under sevoflurane anesthesia and acetate Ringer administration. Inhalation induction with a high concentration of sevoflurane should be reconsidered in view of recent reports describing the epileptogenic potential of sevoflurane.

Pathways and control of ketone body metabolism: on the fringe of lipid biochemistry

Ketone bodies become major body fuels during fasting and consumption of a high-fat, low-carbohydrate (ketogenic) diet. Hyperketonemia is associated with potential health benefits. Ketone body synthesis (ketogenesis) is the last recognizable step of lipid energy metabolism, a pathway that links dietary lipids and adipose triglycerides to the Krebs cycle and respiratory chain and has three highly regulated control points: (1) adipocyte lipolysis, (2) mitochondrial fatty acids entry, controlled by the inhibition of carnitine palmityl transferase I by malonyl coenzyme A (CoA) and (3) mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase, which catalyzes the irreversible first step of ketone body synthesis. Each step is suppressed by an elevated circulating insulin level or insulin/glucagon ratio. The utilization of ketone bodies (ketolysis) also determines circulating ketone body levels. Consideration of ketone body metabolism reveals the mechanisms underlying the extreme fragility of dietary ketosis to carbohydrate intake and highlights areas for further study.

Arterial ketone body ratio for the assessment of the severity of illness in pediatric patients following cardiac surgery

PURPOSE

The purpose of this study was to assess whether the arterial ketone body ratio (AKBR) can be effectively used to evaluate the severity of illness in children following cardiac surgery.

MATERIALS AND METHODS

AKBR was measured in 157 consecutive pediatric patients following heart surgery on the odd numbers of postoperative days. The relationship between AKBR and patient outcome was analyzed using the data of 141 patients with cardiopulmonary bypass.

RESULTS

Initial AKBR was frequently lower than 1.0, and this was associated with the increases in total ketone body counts. Insufficient glucose metabolism appeared to contribute to the low initial AKBR. As a result, the specificity of initial AKBR as a mortality predictor was lower than that of initial blood lactate. In the sequential analysis of AKBR for the 48 patients with PICU stay longer than 5 days, patients showing a sustained lower level <1.0 had significantly higher development of organ dysfunction (liver, heart) and greater mortality (56%).

CONCLUSIONS

Sustained postoperative decrease in AKBR <1.0 represents lethal outcome. The analysis of AKBR trend in combination with a measurement of blood lactate level in early postoperative period appears to be useful for the assessment of the severity of illness in pediatric patients following heart surgery.

Sick-day management in type 1 diabetes

Illness and stress are common occurrences. For the person with type 1 diabetes, these events can be triggers for counterregulation and [table: see text] subsequent metabolic deterioration if there is no attention to diabetes management tasks. Sick-day management requires increased monitoring of blood glucose and assessment for ketosis. Although urine testing for ketones has been the standard approach to sick-day management, new technology for self-monitoring of blood 3HB levels is now available. According to the American Diabetes Association, blood measurement of 3HB "may offer a useful alternative to urine ketone testing." This new technology may provide an opportunity to improve the management of uncontrolled diabetes and sick days in an attempt to reduce the human and economic burden of DKA.

Ketone bodies: a review of physiology, pathophysiology and application of monitoring to diabetes

Ketone bodies are produced by the liver and used peripherally as an energy source when glucose is not readily available. The two main ketone bodies are acetoacetate (AcAc) and 3-beta-hydroxybutyrate (3HB), while acetone is the third, and least abundant, ketone body. Ketones are always present in the blood and their levels increase during fasting and prolonged exercise. They are also found in the blood of neonates and pregnant women. Diabetes is the most common pathological cause of elevated blood ketones. In diabetic ketoacidosis (DKA), high levels of ketones are produced in response to low insulin levels and high levels of counterregulatory hormones. In acute DKA, the ketone body ratio (3HB:AcAc) rises from normal (1:1) to as high as 10:1. In response to insulin therapy, 3HB levels commonly decrease long before AcAc levels. The frequently employed nitroprusside test only detects AcAc in blood and urine. This test is inconvenient, does not assess the best indicator of ketone body levels (3HB), provides only a semiquantitative assessment of ketone levels and is associated with false-positive results. Recently, inexpensive quantitative tests of 3HB levels have become available for use with small blood samples (5-25 microl). These tests offer new options for monitoring and treating diabetes and other states characterized by the abnormal metabolism of ketone bodies.

Glucogenesis in an insect, Manduca sexta L., estimated from the 13C isotopomer distribution in trehalose synthesized from [1,3-13C2]glycerol

Glucogenesis from [3-13C]alanine and [1,3-13C2]glycerol was demonstrated in the insect Manduca sexta by examining the 13C enrichment of trehalose, a non-reducing disaccharide of glucose synthesized in the insect fat body and released into the blood or hemolymph. In insects maintained on a low carbohydrate diet, trehalose synthesized from [3-13C]alanine was selectively enriched at C1 and C6, and C2 and C5. The 13C-labelling pattern indicated the carboxylation of [3-13C]pyruvate, formed by transamination of the [3-13C]alanine followed by randomization of the label at the fumarate step of the tricarboxylic acid cycle and glucose synthesis via the gluconeogenic pathway. 13C enrichment of trehalose was absent in similarly maintained insect larvae administered 3-mercaptopicolinic acid, an inhibitor of hepatic phosphoenolpyruvate carboxykinase. Insects on the low carbohydrate diet also synthesized trehalose from [1,3-13C2]glycerol. 13C multiplets were observed in trehalose C3 and C4 demonstrating the synthesis of three 13C enriched glucose isotopomers from the 13C-labelled glycerol. The relative contributions of 13C-labelled glycerol and unlabelled 3 carbon substrates to the synthesis of the 13C enriched trehalose isotopomers were determined from the multiplet structure at C3, and calculation of minimal rates of glucogenesis were based on the 13C enrichment of C4. The C4/C3 13C enrichment ratio in trehalose synthesized from [1,3-13C2]glycerol was close to unity, and total glucogenesis was calculated after estimation of the expected contribution of unlabelled trehalose synthesis from 3 carbon substrates by comparison of the ratio of unlabelled and labelled contributions to the 13C enriched trehalose isotopomers with the 13C enrichment of [1,3-13C2]glycerol-3-phosphate. The estimated total rates of glucogenesis varied from 0.33 to 2.80 micromol glucose/g fresh weight/h. The blood sugar level of M. sexta was also highly variable. Although the potential importance of glucogenesis from 3 carbon substrates to the maintenance of blood sugar was not established by the present investigation, insects maintained on the low carbohydrate diet had similar blood trehalose levels to those previously reported by others for insects maintained on a natural food.

Effects of chronic hypercortisolemia on carbohydrate metabolism during insulin deficiency

This study was undertaken to further investigate the effect of acute selective insulin deficiency on glycogenolysis and gluconeogenesis occurring during chronic physiological hypercortisolemia in conscious overnight fasted dogs. After an 80-min tracer and dye equilibration period and a 40-min basal period, selective insulin deficiency was created during the 180-min experimental period by infusing somatostatin peripherally (0.8 micrograms.kg-1.min-1) with basal replacement of glucagon intraportally (0.65 ng.kg-1.min-1). In the cortisol group (n = 5), a continuous infusion of hydrocortisone (3.5 micrograms.kg-1.min-1) was begun 5 days before the experiment. In the saline group (n = 5), there was no infusion of cortisol. [3-3H]glucose, [U-14C]alanine, and indocyanine green dye were used to assess glucose production and gluconeogenesis using tracer and arteriovenous difference techniques. During selective insulin deficiency in the saline group, the arterial plasma glucose level (Glc) increased from 109 +/- 2 to 285 +/- 19 mg/dl; glucose production increased from 2.7 +/- 0.2 to 4.5 +/- 0.3 mg.kg-1.min-1. Gluconeogenic efficiency and conversion of alanine to glucose (Conv) increased by 300 +/- 55 and 356 +/- 67%. During selective insulin deficiency in the cortisol group, Glc increased from 117 +/- 3 to 373 +/- 50 mg/dl; glucose production increased from 3.3 +/- 0.5 to 6.9 +/- 0.7 mg.kg-1.min-1. Gluconeogenic efficiency and Conv increased by 268 +/- 41 and 393 +/- 75%, respectively. The maximal glycogenolytic rate increased significantly more in the cortisol group than in the saline group, accounting for the difference in glucose production. These results suggest that, even during chronic hypercortisolemia, acute insulin deficiency has more pronounced effects on glycogenolysis than gluconeogenesis.

Evaluation of ketogenesis in seriously reduced hepatic mitochondrial redox state. An analysis of survivors and non-survivors in critically ill hepatectomized patients

Ketogenesis was evaluated in 33 critically ill hepatectomized patients in relation to the arterial ketone body ratio (acetoacetate to 3-hydroxybutyrate), which reflects hepatic mitochondrial redox state. In 15 patients whose arterial ketone body ratio decreased to below 0.4, blood ketone body levels were significantly increased concomitant with marked increase of blood lactate and plasma alanine levels. In the 6 survivors of these 15 patients, the arterial ketone body ratio was restored within the next 2 days, and blood ketone body levels were decreased. By contrast, in the nine non-survivors, the arterial ketone body ratio remained below 0.4, and blood ketone body levels were decreased, accompanied by significant increases in blood lactate and plasma alanine levels in the terminal stages. These results suggest that ketogenesis acts as an alternative process for ATP synthesis in the liver in critically ill patients. Death occurs when the liver falls into an energy crisis concomitant with the cessation of ketogenesis.

Glucose and ketone body turnover in fasting grey seal pups

Concentration and metabolic replacement (turnover) rate of glucose and ketone bodies were determined at intervals during a 52 day postweaning fast in five grey seal (Halichoerus grypus) pups, using bolus injections of radiotracers. Blood glucose was maintained at a high level throughout the fast, while beta-hydroxybutyrate increased 26 times from day 3 to day 37, whereafter it by and large was maintained. Glucose replacement rate decreased to 56% of the day 9 value at day 37 and dropped further to only 32% of the day 9 level at day 52 in two seals, while in another 2 seals it increased at this late stage. The average ketone body replacement rate ranged between 8.6 and 13.8 mumols min-1 kg-1, but did not change significantly (P greater than 0.05) during the fasting period. These results suggest a reduced gluconeogenesis from protein and increased production of ketone bodies, which may in part replace glucose as energy source during fasting.

Hormonal and metabolic response to operative stress in the neonate

It is evident from this review that newborns, even those born prematurely, are capable of mounting an endocrine and metabolic response to operative stress. Unfortunately, many of the areas for which a relatively well-characterized response exists in adults are poorly documented in neonates. As is the case in adults, the response seems to be primarily catabolic in nature because the combined hormonal changes include an increased release of catabolic hormones such as catecholamines, glucagon, and corticosteroids coupled with a suppression of and peripheral resistance to the effects of the primary anabolic hormone, insulin.

Decreased glucose oxidation during short-term starvation

Prolonged fasting (for days or weeks) decreases glucose production and oxidation. The effects of short-term starvation (ie, less than 24 hours) on glucose metabolism are not known. To evaluate this issue, glucose oxidation and glucose turnover were measured after 16-hour and subsequently after 22-hour fasting. Glucose oxidation was calculated by indirect calorimetry in 12 healthy men (age 22 to 44 years); glucose turnover was measured by primed, continuous infusion of 3-3H-glucose in eight of these 12 volunteers. After 16-hour fasting net glucose oxidation was 0.59 +/- 0.17 mg x kg-1 x min-1 and glucose tissue uptake 2.34 +/- 0.12 mg x kg-1 x min-1. No correlation was found between net glucose oxidation and glucose tissue uptake. Prolonging fasting with an additional 6 hours resulted in decreases of respiratory quotient (0.77 +/- 0.01 v 0.72 +/- 0.01) (P less than .005), plasma glucose concentration (4.7 +/- 0.1 v 4.6 +/- 0.1 mmol/L) (P less than .05), glucose tissue uptake (2.10 +/- 0.12 mg x kg-1 x min-1) (P less than .05), net glucose oxidation (0.09 +/- 0.04 mg x kg-1 x min-1) (P less than .005), and plasma insulin concentration (8 +/- 1 v6 +/- 1 mU/L) (P less than .005). Net glucose oxidation expressed as a percentage of glucose tissue uptake decreased from 22% +/- 8% to 2% +/- 1% (P less than .05). There was no net glucose oxidation in seven of 12 controls after 22-hour fasting.(ABSTRACT TRUNCATED AT 250 WORDS)

Validation of two-pool model for in vivo ketone body kinetics

Previous studies have indicated that simultaneous infusions of two ketone body tracers ([13C]acetoacetate and [14C]beta-hydroxybutyrate) provide accurate estimates of exogenous ketone body inflow when an open two-pool model is employed. In the present studies, net hepatic ketone body production was determined from surgically placed arterial, portal venous, and hepatic venous catheters in conscious diabetic (n = 6) and 4-day fasted (n = 7) dogs. [13C]acetoacetate and [14C]beta-hydroxybutyrate were infused simultaneously, and ketone body production was calculated from either acetoacetate (AcAc) single-isotope data, beta-hydroxybutyrate (beta-OHB) single-isotope data, the sum of individual fluxes, or the two-pool model. In fasted animals, both the AcAc single-isotope calculation and the sum of individual fluxes overestimated net hepatic production by approximately 50% (P less than 0.05), whereas the beta-OHB single-isotope calculation and the two-pool model gave accurate estimates. In the diabetic animals, the beta-OHB single-isotope calculation underestimated net hepatic production by approximately 30% (P less than 0.05). The sum of individual fluxes overestimated net hepatic production by approximately 46% (P less than 0.05), whereas both the AcAc single-isotope calculation and the two-pool model gave accurate estimates. In conclusion, single-isotope methods give erroneous estimates of net hepatic production of ketone bodies. In contrast, a two-pool model provided an accurate estimate of net hepatic production and thus appears to be suitable for determination of ketone body kinetics in humans.

The relationship between gluconeogenic substrate supply and glucose production in humans

The relationship between gluconeogenic precursor supply and glucose production has been investigated in 14-h and 86-h fasted humans. In protocols 1 and 2 [6,6-2H]glucose and [15N2]urea were infused to measure glucose and urea production rates (Ra) in response to infusions of glycerol and alanine. In protocol 3 first [15N]alanine, [3-13C]lactate, and [6,6-2H]glucose were infused before and during administration of dichloroacetate (DCA) to determine the response of glucose Ra to decreased fluxes of pyruvate, alanine, and lactate, then alanine was infused with DCA and glucose Ra measured. After a 14-h fast, neither alanine nor glycerol increased glucose Ra. Basal glucose Ra decreased by one-third after 86 h of fasting, yet glycerol and alanine infusions had no effect on glucose Ra. Glycerol always reduced urea Ra (P less than 0.05), suggesting that glycerol competitively inhibited gluconeogenesis from amino acids. DCA decreased the fluxes of pyruvate, alanine (P less than 0.01), and glucose Ra (P less than 0.01), which was prevented by alanine infusion. These findings suggest that 1) the reduction in glucose Ra after an 86-h fast is not because of a shortage of gluconeogenic substrate; 2) nonetheless, the importance of precursor supply to maintain basal glucose Ra is confirmed by the response to DCA; 3) an excess of one gluconeogenic substrate inhibits gluconeogenesis from others.

Renal and hepatic aspects of ketoacidosis: a quantitative analysis based on energy turnover

The central theme explored is that the rate of ATP production cannot exceed its rate of use in any organ or compartment. Thus the rate of ATP turnover exerts an absolute control over the rates in pathways that synthesize it. This is manifested in two major ways: substrate competition for oxidation and the influence of changes in oxygen consumption rate on the rate of fuel oxidation. By direct measurement, the rate of ketogenesis in the liver is as high as 1500 mmol/day during chronic ketoacidosis of fasting. Given the limited ate of hepatic oxygen consumption, ketogenesis and glucose synthesis from amino acids compete as precursors for hepatic ATP synthesis. Thus There is little room to increase the rate of ketoacid production further in these subjects. Energy turnover considerations in the kidney during chronic fasting seem to limit renal NH4+ production. In this case, there is competition between glutamine and ketone bodies as ATP precursors. This aspect may be important in the regulation of lean body mass catabolism of fasting. There is a "trade-off" in maintaining high circulating ketone body concentrations during fasting. The benefit is primarily for the CNS, and the cost is small loss of lean body mass owing to the need for high rates of NH4+ excretion.

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)

Hypoglycemia: a pathophysiologic approach

An exploration of the factors that sustain glucose levels in the normal fasting subject reveals that the single major component is conservation of glucose rather than gluconeogenesis. Conservation is achieved by recycling of glucose carbon as lactate, pyruvate and alanine, and a profound decrease in the oxidation of glucose by the brain brought about by the provision and use of ketones. What glucose continues to be oxidized is for the most part formed from glycerol. Gluconeogenesis from protein plays little part in the process. Fasting hypoglycemia results from disorders affecting either one of the two critical sustaining factors--the recycling process or the availability and use of ketones. Individual hypoglycemic entities are examined against this background.

A dual-isotope technique for determination of in vivo ketone body kinetics

"Total ketone body specific activity" has been widely used in studies of ketone body metabolism to circumvent so-called "isotope disequilibrium" between the two major ketone body pools, acetoacetate and beta-hydroxybutyrate. Recently, this approach has been criticized on theoretical grounds. In the present studies, [13C]acetoacetate and beta-[14C]hydroxybutyrate were simultaneously infused in nine mongrel dogs before and during an infusion of either unlabeled sodium acetoacetate or unlabeled sodium beta-hydroxybutyrate. Ketone body turnover was determined using total ketone body specific activity, total ketone body moles % enrichment, and an open two-pool model, both before and during the exogenous infusion of unlabeled ketone bodies. Basal ketone body turnover rates were significantly higher using [13C]acetoacetate than with either beta-[14C]hydroxybutyrate alone or the dual-isotope model (3.6 +/- 0.5 vs. 2.2 +/- 0.2 and 2.7 +/- 0.2 mumol X kg-1 X min-1, respectively, P less than 0.05). During exogenous infusion of unlabeled sodium acetoacetate, the dual-isotope model provided the best estimate of ketone body inflow, whereas 14C specific activity underestimated the known rate of acetoacetate infusion by 55% (P less than 0.02). During sodium beta-hydroxybutyrate infusion, [13C]-acetoacetate overestimated ketone body inflow by 55% (P = NS), while better results were obtained with 14C beta-hydroxybutyrate alone and the two-pool model. Ketone body interconversion as estimated by the dual-isotope technique increased markedly during exogenous ketone body infusion. In conclusion, significant errors in estimation of ketone body inflow were made using single-isotope techniques, whereas a dual-isotope model provided reasonably accurate estimates of ketone body inflow during infusion of exogenous acetoacetate and beta-hydroxybutyrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical and laboratory observations in a child with hepatic phosphorylase kinase deficiency

A 3-year-old child with glycogenosis due to hepatic phosphorylase kinase deficiency is described. His clinical presentation was unusually severe. Biochemical studies revealed a lack of hypoglycemia, the presence of marked ketosis and hyperlipidemia, and a normal glycemic response to glucagon and to loading with galactose, fructose, and alanine. The ketosis was reversed by glucagon administration. Changes in plasma concentrations of lactate, pyruvate, beta-OH butyrate, and alanine in response to glucagon, galactose, fructose, and alanine administration are reported. The child responded poorly to a high protein diet. His condition improved markedly with a high carbohydrate diet. The significance of the findings is discussed.

Role of catabolic hormones in the hypoketonaemia of injury

Adaptation to fasting results in ketosis, protein conservation, and diminished energy requirements. To determine whether the attenuation of these responses following injury is due to an altered hormonal environment, we infused the catabolic hormones cortisol, glucagon, and adrenaline into seven fasting subjects for 72 h. Saline alone was administered during a comparable control period. Hormone infusion achieved blood levels typical of moderate injury and resulted in hypermetabolism, hyperglycaemia, hyperinsulinaemia, and marked suppression of fasting ketosis. The hypoketonaemia could not be accounted for by decreased precursor availability, accelerated ketone utilization or increased urinary excretion. The altered hormonal environment associated with critical illness attenuates fasting ketosis by limiting hepatic ketogenesis.

Splanchnic and leg exchange of free fatty acids in patients with liver cirrhosis

Splanchnic and leg exchange of free fatty acids (FFA), glycerol and ketone bodies, as well as FFA turnover, were determined in the post-absorptive state in 8 patients with liver cirrhosis and in 6 healthy control subjects. The catheter technique was used together with tracer ([14C]oleate) infusion. The arterial concentrations of FFA, glycerol and ketone bodies were 2-6-fold higher in the patients than in the controls. The FFA turnover was 230% greater in the patients, while the fractional turnover was similar in the two groups. In the splanchnic region as well as in the leg, both FFA uptake and release were increased 2-4-fold in the patients. The fractional uptake of FFA was reduced in both areas, indicating that the augmented uptake was due to the high circulating FFA levels alone. The splanchnic production of ketone bodies was four times higher in the patients than in the controls (295 +/- 30 vs 87 +/- 11 mumol/min). The fraction of FFA converted to ketone bodies was greater (42 +/- 6 vs 20 +/- 5%, P less than 0.05), indicating that the accelerated ketone body production was a combined effect of raised FFA uptake and altered intrahepatic metabolism of FFA. The splanchnic production of glucose was reduced by approximately 50% in the patients, while the uptake of glycerol was augmented. The leg uptake of 3-hydroxybutyrate was increased 300% and the release of glycerol was 200% greater in the patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic and renal metabolism before and after portasystemic shunts in patients with cirrhosis

Hepatic cirrhosis with portal hypertension and gastroesophageal hemorrhage is a disease complex that continues to be treated by surgical portasystemic shunts. Whether or not a reduction or diversion of portal blood flow to the liver adversely affects the ability of the liver to maintain fuel homeostasis via gluconeogenesis, glycogenolysis, and ketogenesis is unknown. 11 patients with biopsy-proven severe hepatic cirrhosis were studied before and after distal splenorenal or mesocaval shunts. Hepatic, portal, and renal blood flow rates and glucose, lactate, pyruvate, glycerol, amino acids, ketone bodies, free fatty acids, and triglyceride arteriovenous concentration differences were determined to calculate net precursor-product exchange rates across the liver, gut, and kidney. The study showed that hepatic contribution of glucose and ketone bodies and the caloric equivalents of these fuels delivered to the blood was not adversely affected by either a distal splenorenal or mesocaval shunt. In addition to these general observations, isolated findings emerged. Mesocaval shunts reversed portal venous blood and functionally converted this venous avenue into hepatic venous blood. The ability of the kidney to make a substantial net contribution of ketone bodies to the blood was also observed.

Influence of a 60-hour fast on insulin-mediated splanchnic and peripheral glucose metabolism in humans

A brief period of starvation (2-3) depletes the hepatic glycogen stores but results in only a limited reduction of the muscle glycogen depots. In this situation insulin resistance contributes to the glucose intolerance, but it is not known which tissue or tissues are responsible for the decreased insulin sensitivity. The present study was therefore undertaken to examine the influence of a 60-h fast on insulin sensitivity in splanchnic and peripheral tissues in normal humans. Euglycemic (95 mg/dl) 1-mU insulin and hyperglycemic (215-225 mg/dl) glucose clamp studies were conducted for 2 h in overnight (12 h) and prolonged (60 h) fasted nonobese subjects. Splanchnic exchange of glucose and gluconeogenic precursors was measured using the hepatic vein catheter technique. During the euglycemic clamp, insulin infusion resulted in similar steady state insulin levels in 60-h and 12-h fasted subjects (73 +/- 7 vs. 74 +/- 5 microU/ml). Total glucose disposal was reduced by 45% after 60 h of fasting (4.0 +/- 0.3 vs. 7.6 +/- 1.1 mg/kg per min, P less than 0.05) and the splanchnic glucose balance reverted from a net release in the basal state (12 h fast, -1.7 +/- 0.2, and 60-h fast, -0.9 +/- 0.1 mg/kg per min, P less than 0.01) to a net uptake during the clamps that was similar after 60 h and 12 h of fasting (0.6 +/- 0.1 vs. 0.6 +/- 0.2 mg/kg per min). During the hyperglycemic clamp, insulin levels rose rapidly in all subjects. In the 12-h fasted group this rise was followed by a further gradual one, reaching significantly higher values than in 60-h fasted subjects during the second hour (67 +/- 15 vs. 25 +/- 2 microU/ml, P less than 0.05). Total glucose disposal was lower, though not significantly so, after the 60-h fast (2.6 +/- 0.4 vs. 5.4 +/- 1.3 mg/kg per min, 0.05 less than P less than 0.10), and as with the euglycemic clamp, the splanchnic glucose balance was altered from a basal net release to a net uptake during the clamp (1.3 +/- 0.2 vs. 1.1 +/- 0.2 mg/kg per min). After an overnight fast, splanchnic lactate uptake fell and the arterial lactate concentration rose in response to both hyperglycemia and hyperinsulinemia, whereas these variables were unchanged in the 60-h fasted subjects during both types of clamp studies.

Stimulatory effect of norepinephrine on ketogenesis in normal and insulin-deficient humans

Elevation of plasma norepinephrine concentrations to stress levels (1,800 pg/ml) resulted in normal subjects in a significant increase in ketone body production by 155% (determined by use of [14C]acetoacetate infusions), in a decrease of the metabolic clearance rate by 38%, hyperketonemia, and in increased plasma free fatty acid (FFA) levels by 57% after 75 min. Norepinephrine infusion during somatostatin-induced insulin deficiency resulted in an augmented and sustained increase in ketone body concentrations due to increased production and decreased peripheral clearance of ketone bodies. Norepinephrine's stimulatory effect on lipolysis waned with time, and its effect on ketogenesis in normal subjects was greater than its influence on plasma FFA levels, and thus presumably on hepatic FFA uptake, suggesting a direct stimulatory effect on hepatic ketogenesis. The data demonstrate that in normal humans the hyperketonemic effect of elevated plasma norepinephrine concentrations results from a combination of three factors: increased ketone body production from augmented FFA supply to the liver; accelerated hepatic ketogenesis; and modestly decreased metabolic clearance of ketone bodies. Acute insulin deficiency augments all these effects and results in progressive ketosis.

Role of basal glucagon levels in the regulation of splanchnic glucose output and ketogenesis in insulin-deficient humans

The aim of the present study was to investigate the influence of hepatic glycogen depletion and increased lipolysis on the response of splanchnic glucose output and ketogenesis to combined glucagon and insulin deficiency in normal man. Healthy subjects were studied after a 60-h fast and compared with a control group studied after an overnight fast. Net splanchnic exchange of glucose, gluconeogenic precursors, free fatty acids (FFA) and ketone acids were measured in the basal state and during intravenous infusion of somatostatin (9 micrograms/min) for 90-140 min (overnight fasted subjects) or for 5 h (60-h fasted subjects). During the infusion of somatostatin, euglycemia was maintained by a variable intravenous infusion of glucose. Prior to somatostatin infusion, after an overnight (12-14 h) fast, splanchnic uptake of glucose precursors (alanine, lactate, pyruvate, glycerol) could account for 26% of splanchnic glucose output (SGO) indicating primarily glycogenolysis. Somatostatin infusion resulted in a 50% reduction in both insulin and glucagon concentrations and a transient decline in SGO which returned to baseline values by 86 +/- 11 min at which point the glucose infusion was no longer necessary to maintain euglycemia. Arterial concentrations of FFA and beta-OH-butyrate and splanchnic beta-OH-butyrate production rose 2.5-fold, 6-fold and 7.5-fold, respectively, in response to somatostatin infusion. In the 60-h fasted state, basal SGO (0.29 +/- 0.03 mmol/min) was 60% lower than after an overnight fast and basal splanchnic uptake of glucose precursors could account for 85% of SGO, indicating primarily gluconeogenesis. Somatostatin administration suppressed the arterial glucagon and insulin concentrations to values comparable to those observed during the infusion in the overnight fasted state. SGO fell promptly in response to the somatostatin infusion and in contrast to the overnight fasted state, remained inhibited by 50-100% for 5 h. Infusion of glucose was consequently necessary to maintain euglycemia throughout the 5-h infusion of somatostatin. Splanchnic uptake of gluconeogenic precursors was unchanged during somatostatin despite the sustained suppression of SGO. Basal arterial concentration and splanchnic exchange of beta-OH-butyrate were respectively 22-fold and 6- to 7-fold elevated and basal FFA concentration was 70% increased as compared to the corresponding values in the overnight fasted state.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of long-term starvation on acetate and ketone body metabolism in obese patients

The turnover of ketone bodies and acetate was evaluated as well from the disappearance rate of (3-14C)acetoacetate or (1-14C)acetate respectively as from the conversion of FFA into these metabolites in normal weight and obese overnight-fasted and in obese long-term starved patients. The disappearance rate of (1-14C)oleate was the same in all three groups. Long-term starvation enhanced ketone body turn-over almost 10-fold, whereas the disappearance rate for ketone bodies decreased from 0.035 to 0.015 min-1. Under the same circumstances the turnover of acetate was about 1 mumol g-1 min-1 accounting for about 5% of FFA turnover. Long-term starvation decreased the conversion of (1-14C)oleate into triglycerides by almost 50% and increased the (2-C)-(4-C)/(1-C) ratio of radioactivity in ketone bodies. The reincorporation of radioactivity from the (1-C)position of (1-14C)oleate into the ( (2-C)-(n-C) ) position of FFA, which is a measure of the reutilization of acetyl-CoA for FFA synthesis decreased significantly during long-term starvation.

Nature and quantity of fuels consumed in patients with alcoholic cirrhosis

Although alcoholism is a leading cause of morbidity and mortality of middle-aged Americans, there are no data available pertaining to the consequences of Laennec's cirrhosis on total body energy requirements or mechanisms for maintaining fuel homeostasis in this patient population. Therefore, we simultaneously used the techniques of indirect calorimetry and tracer analyses of [14C]palmitate to measure the nature and quantity of fuels oxidized by patients with biopsy-proven alcoholic cirrhosis and compared the results with values obtained from health volunteers. Cirrhotic patients were studied after an overnight fast (10-12 h). Normal volunteers were studied after an overnight fast (12 h) or after a longer period of starvation (36-72 h). Total basal metabolic requirements were similar in overnight fasted cirrhotic patients (1.05 +/- 0.06 kcal/min per 1.73 m2), overnight fasted normal subjects (1.00 +/- 0.05 kcal/min per 1.73 m2), and 36-72-h fasted normal volunteers (1.10 +/- 0.06 kcal/min per 1.73 m2). Indirect calorimetry revealed that in cirrhotic patients the percentages of total calories derived from fat (69 +/- 3%), carbohydrate (13 +/- 2%), and protein (17 +/- 4%) were comparable to those found in 36-72-h fasted subjects, but were clearly different from those of overnight fasted normal individuals who derived 40 +/- 6, 39 +/- 4, and 21 +/- 2% from fat, carbohydrate, and protein, respectively. These data are strikingly similar to data obtained through tracer analyses of [14C]palmitate, which showed that in overnight fasted patients with alcoholic cirrhosis, 63 +/- 4% of their total CO2 production was derived from oxidation of 287 +/- 28 mumol free fatty acids (FFA)/min per 1.73 m2. In contrast, normal overnight fasted humans derived 34 +/- 6% of their total CO2 production from the oxidation of 147 +/- 25 mumol FFA/min per 1.73 m2. On the other hand, values obtained from the normal volunteers fasted 36-72 h were similar to the overnight fasted cirrhotic patients. These results show that after an overnight fast the caloric requirements of patients with alcoholic cirrhosis are normal, but the nature of fuels oxidized are similar to normal humans undergoing 2-3 d of total starvation. Thus, patients with alcoholic cirrhosis develop the catabolic state of starvation more rapidly than do normal humans. This disturbed but compensated pattern for maintaining fuel homeostasis may be partly responsible for the cachexia observed in some patients with alcoholic cirrhosis. This study also showed remarkably good agreement between the results obtained with indirect calorimetry and those obtained with 14C tracer analyses.

Splanchnic glucose metabolism during leg exercise in 60-hour-fasted human subjects

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)