Rapid intravenous sodium acetoacetate infusion in man. Metabolic and kinetic responses

Modeling digestion, absorption, and ketogenesis after administration of tricaprilin formulations to humans

At present, tricaprilin is used as a ketogenic source for the management of mild to moderate Alzheimer's disease. After administration of the medium-chain triglyceride, tricaprilin is hydrolyzed to octanoic acid and further metabolized to ketones, acting as an alternative energy substrate for the brain. In this investigation, we developed a physiologically-based biopharmaceutics model simulating in vivo processes following the peroral administration of tricaprilin. The model includes multiple data sources to establish a partially verified framework for the simulation of plasma profiles. The input parameters were identified based on existing literature data and in vitro digestion studies. Model validation was conducted using the data from a phase I clinical trial. A partial parameter sensitivity analysis elucidated various influences on the plasma ketone levels that are mainly responsible for the therapeutic effects of tricaprilin. Based on our findings, we concluded that dispersibility and lipolysis of tricaprilin together with the gastric emptying patterns are limiting ketogenesis, while other steps such as the conversion of octanoic acid to ketone bodies play a minor role only.

Does Neuroinflammation Underlie the Cognitive Changes Observed With Dietary Interventions?

Dietary interventions, such as calorie restriction and ketogenic diet, have been extensively studied in ageing research, including in cognitive decline. Epidemiological studies indicate beneficial effects of certain dietary regimes on mental health, including mood disorders and dementia. However, randomised-controlled trials (the gold-standard of evidence-based medicine) on calorie restriction diets and the ketogenic diet have yet to show clinically convincing effects in neuropsychiatric disorders. This review will examine the quality of studies and evidence base for the ketogenic and calorie restriction diets in common neuropsychiatric conditions, collating findings from preclinical experiments, case reports or small clinical studies, and randomised controlled clinical trials. The major cellular mechanisms that mediate the effects of these dietary interventions on brain health include neuroinflammation, neuroprotection, and neuromodulation. We will discuss the studies that have investigated the roles of these pathways and their interactions. Popularity of the ketogenic and calorie restriction diets has grown both in the public domain and in psychiatry research, allowing for informed review of the efficacy, the limitations, and the side effects of these diets in specific patient populations. In this review we will summarise the clinical evidence for these diets in neuropsychiatry and make suggestions to improve clinical translation of future research studies.

A Systematic Review of Intravenous β-Hydroxybutyrate Use in Humans - A Promising Future Therapy?

Therapeutic ketosis is traditionally induced with dietary modification. However, owing to the time delay involved, this is not a practical approach for treatment of acute conditions such as traumatic brain injury. Intravenous administration of ketones would obviate this problem by rapidly inducing ketosis. This has been confirmed in a number of small animal and human studies. Currently no such commercially available product exists. The aim of this systematic review is to review the safety and efficacy of intravenous beta-hydroxybutyrate. The Web of Science, PubMed and EMBASE databases were searched, and a systematic review undertaken. Thirty-five studies were included. The total beta-hydroxybutyrate dose ranged from 30 to 101 g administered over multiple doses as a short infusion, with most studies using the racemic form. Such dosing achieves a beta-hydroxybutyrate concentration >1 mmol/L within 15 min. Infusions were well tolerated with few adverse events. Blood glucose concentrations occasionally were reduced but remained within the normal reference range for all study participants. Few studies have examined the effect of intravenous beta-hydroxybutyrate in disease states. In patients with heart failure, intravenous beta-hydroxybutyrate increased cardiac output by up to 40%. No studies were conducted in patients with neurological disease. Intravenous beta-hydroxybutyrate has been shown to increase cerebral blood flow and reduce cerebral glucose oxidation. Moreover, beta-hydroxybutyrate reduces protein catabolism and attenuates the production of counter-regulatory hormones during induced hypoglycemia. An intravenous beta-hydroxybutyrate formulation is well tolerated and may provide an alternative treatment option worthy of further research in disease states.

Non-glucose modulators of insulin secretion in healthy humans: (dis)similarities between islet and in vivo studies

Optimal metabolic homeostasis requires precise temporal and quantitative control of insulin secretion. Both in vivo and in vitro studies have often focused on the regulation by glucose although many additional factors including other nutrients, neurotransmitters, hormones and drugs, modulate the secretory function of pancreatic β-cells. This review is based on the analysis of clinical investigations characterizing the effects of non-glucose modulators of insulin secretion in healthy subjects, and of experimental studies testing the same modulators in islets isolated from normal human donors. The aim was to determine whether the information gathered in vitro can reliably be translated to the in vivo situation. The comparison evidenced both convincing similarities and areas of discordance. The lack of coherence generally stems from the use of exceedingly high concentrations of test agents at too high or too low glucose concentrations in vitro, which casts doubts on the physiological relevance of a number of observations made in isolated islets. Future projects resorting to human islets should avoid extreme experimental conditions, such as oversized stimulations or inhibitions of β-cells, which are unlikely to throw light on normal insulin secretion and contribute to the elucidation of its defects.

Can ketone bodies inactivate coronavirus spike protein? The potential of biocidal agents against SARS-CoV-2

Biocidal agents such as formaldehyde and glutaraldehyde are able to inactivate several coronaviruses including SARS-CoV-2. In this article, an insight into one mechanism for the inactivation of these viruses by those two agents is presented, based on analysis of previous observations during electron microscopic examination of several members of the orthocoronavirinae subfamily, including the new virus SARS-CoV-2. This inactivation is proposed to occur through Schiff base reaction-induced conformational changes in the spike glycoprotein leading to its disruption or breakage, which can prevent binding of the virus to cellular receptors. Also, a new prophylactic and therapeutic measure against SARS-CoV-2 using acetoacetate is proposed, suggesting that it could similarly break the viral spike through Schiff base reaction with lysines of the spike protein. This measure needs to be confirmed experimentally before consideration. In addition, a new line of research is proposed to help find a broad-spectrum antivirus against several members of this subfamily.

Isolated Starvation Ketoacidosis: A Rare Cause of Severe Metabolic Acidosis Presenting with a pH Less than 7

Anion gap metabolic acidosis (AGMA) occurs when an anion gap exists along with metabolic acidosis, most commonly due to diabetic ketoacidosis (DKA) and lactic acidosis (LA). Isolated starvation ketoacidosis (ISK) is one of the rare causes of AGMA; however, it usually presents with a mild disturbance in pH. We report a rare case of a 45-year-old female with previously diagnosed squamous cell cancer (SCC) of the larynx. She presented to the emergency department complaining of difficulty in breathing following laryngectomy and tracheostomy for SCC. Her laboratory results on admission were consistent for isolated starvation ketoacidosis and the patient responded quickly to the appropriate treatment.

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.

Potential Synergies of β-Hydroxybutyrate and Butyrate on the Modulation of Metabolism, Inflammation, Cognition, and General Health

The low-carbohydrate high-fat diet (LCHFD), also known as the ketogenic diet, has cycled in and out of popularity for decades as a therapeutic program to treat metabolic syndrome, weight mismanagement, and drug-resistant disorders as complex as epilepsy, cancer, dementia, and depression. Despite the benefits of this diet, health care professionals still question its safety due to the elevated serum ketones it induces and the limited dietary fiber. To compound the controversy, patient compliance with the program is poor due to the restrictive nature of the diet and symptoms related to energy deficit and gastrointestinal adversity during the introductory and energy substrate transition phase of the diet. The studies presented here demonstrate safety and efficacy of the diet including the scientific support and rationale for the administration of exogenous ketone bodies and ketone sources as a complement to the restrictive dietary protocol or as an alternative to the diet. This review also highlights the synergy provided by exogenous ketone, β-hydroxybutyrate (BHB), accompanied by the short chain fatty acid, butyrate (BA) in the context of cellular and physiological outcomes. More work is needed to unveil the molecular mechanisms by which this program provides health benefits.

Can Ketones Help Rescue Brain Fuel Supply in Later Life? Implications for Cognitive Health during Aging and the Treatment of Alzheimer's Disease

We propose that brain energy deficit is an important pre-symptomatic feature of Alzheimer's disease (AD) that requires closer attention in the development of AD therapeutics. Our rationale is fourfold: (i) Glucose uptake is lower in the frontal cortex of people >65 years-old despite cognitive scores that are normal for age. (ii) The regional deficit in brain glucose uptake is present in adults <40 years-old who have genetic or lifestyle risk factors for AD but in whom cognitive decline has not yet started. Examples include young adult carriers of presenilin-1 or apolipoprotein E4, and young adults with mild insulin resistance or with a maternal family history of AD. (iii) Regional brain glucose uptake is impaired in AD and mild cognitive impairment (MCI), but brain uptake of ketones (beta-hydroxybutyrate and acetoacetate), remains the same in AD and MCI as in cognitively healthy age-matched controls. These observations point to a brain fuel deficit which appears to be specific to glucose, precedes cognitive decline associated with AD, and becomes more severe as MCI progresses toward AD. Since glucose is the brain's main fuel, we suggest that gradual brain glucose exhaustion is contributing significantly to the onset or progression of AD. (iv) Interventions that raise ketone availability to the brain improve cognitive outcomes in both MCI and AD as well as in acute experimental hypoglycemia. Ketones are the brain's main alternative fuel to glucose and brain ketone uptake is still normal in MCI and in early AD, which would help explain why ketogenic interventions improve some cognitive outcomes in MCI and AD. We suggest that the brain energy deficit needs to be overcome in order to successfully develop more effective therapeutics for AD. At present, oral ketogenic supplements are the most promising means of achieving this goal.

Association of ketone body levels with hyperglycemia and type 2 diabetes in 9,398 Finnish men

We investigated the association of the levels of ketone bodies (KBs) with hyperglycemia and with 62 genetic risk variants regulating glucose levels or type 2 diabetes in the population-based Metabolic Syndrome in Men (METSIM) study, including 9,398 Finnish men without diabetes or newly diagnosed type 2 diabetes. Increasing fasting and 2-h plasma glucose levels were associated with elevated levels of acetoacetate (AcAc) and β-hydroxybutyrate (BHB). AcAc and BHB predicted an increase in the glucose area under the curve in an oral glucose tolerance test, and AcAc predicted the conversion to type 2 diabetes in a 5-year follow-up of the METSIM cohort. Impaired insulin secretion, but not insulin resistance, explained these findings. Of the 62 single nucleotide polymorphisms associated with the risk of type 2 diabetes or hyperglycemia, the glucose-increasing C allele of GCKR significantly associated with elevated levels of fasting BHB levels. Adipose tissue mRNA expression levels of genes involved in ketolysis were significantly associated with insulin sensitivity (Matsuda index). In conclusion, high levels of KBs predicted subsequent worsening of hyperglycemia, and a common variant of GCKR was significantly associated with BHB levels.

Suppression of oxidative stress by β-hydroxybutyrate, an endogenous histone deacetylase inhibitor

Concentrations of acetyl-coenzyme A and nicotinamide adenine dinucleotide (NAD(+)) affect histone acetylation and thereby couple cellular metabolic status and transcriptional regulation. We report that the ketone body d-β-hydroxybutyrate (βOHB) is an endogenous and specific inhibitor of class I histone deacetylases (HDACs). Administration of exogenous βOHB, or fasting or calorie restriction, two conditions associated with increased βOHB abundance, all increased global histone acetylation in mouse tissues. Inhibition of HDAC by βOHB was correlated with global changes in transcription, including that of the genes encoding oxidative stress resistance factors FOXO3A and MT2. Treatment of cells with βOHB increased histone acetylation at the Foxo3a and Mt2 promoters, and both genes were activated by selective depletion of HDAC1 and HDAC2. Consistent with increased FOXO3A and MT2 activity, treatment of mice with βOHB conferred substantial protection against oxidative stress.

Glucokinase activation repairs defective bioenergetics of islets of Langerhans isolated from type 2 diabetics

It was reported previously that isolated human islets from individuals with type 2 diabetes mellitus (T2DM) show reduced glucose-stimulated insulin release. To assess the possibility that impaired bioenergetics may contribute to this defect, glucose-stimulated respiration (Vo(2)), glucose usage and oxidation, intracellular Ca(2+), and insulin secretion (IS) were measured in pancreatic islets isolated from three healthy and three type 2 diabetic organ donors. Isolated mouse and rat islets were studied for comparison. Islets were exposed to a "staircase" glucose stimulus, whereas IR and Vo(2) were measured. Vo(2) of human islets from normals and diabetics increased sigmoidally from equal baselines of 0.25 nmol/100 islets/min as a function of glucose concentration. Maximal Vo(2) of normal islets at 24 mM glucose was 0.40 ± 0.02 nmol·min(-1)·100 islets(-1), and the glucose S(0.5) was 4.39 ± 0.10 mM. The glucose stimulation of respiration of islets from diabetics was lower, V(max) of 0.32 ± 0.01 nmol·min(-1)·100 islets(-1), and the S(0.5) shifted to 5.43 ± 0.13 mM. Glucose-stimulated IS and the rise of intracellular Ca(2+) were also reduced in diabetic islets. A clinically effective glucokinase activator normalized the defective Vo(2), IR, and free calcium responses during glucose stimulation in islets from type 2 diabetics. The body of data shows that there is a clear relationship between the pancreatic islet energy (ATP) production rate and IS. This relationship was similar for normal human, mouse, and rat islets and the data for all species fitted a single sigmoidal curve. The shared threshold rate for IS was ∼13 pmol·min(-1)·islet(-1). Exendin-4, a GLP-1 analog, shifted the ATP production-IS curve to the left and greatly potentiated IS with an ATP production rate threshold of ∼10 pmol·min(-1)·islet(-1). Our data suggest that impaired β-cell bioenergetics resulting in greatly reduced ATP production is critical in the molecular pathogenesis of type 2 diabetes mellitus.

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.

Progressive adaptation of hepatic ketogenesis in mice fed a high-fat diet

Hepatic ketogenesis provides a vital systemic fuel during fasting because ketone bodies are oxidized by most peripheral tissues and, unlike glucose, can be synthesized from fatty acids via mitochondrial beta-oxidation. Since dysfunctional mitochondrial fat oxidation may be a cofactor in insulin-resistant tissue, the objective of this study was to determine whether diet-induced insulin resistance in mice results in impaired in vivo hepatic fat oxidation secondary to defects in ketogenesis. Ketone turnover (micromol/min) in the conscious and unrestrained mouse was responsive to induction and diminution of hepatic fat oxidation, as indicated by an eightfold rise during the fed (0.50+/-0.1)-to-fasted (3.8+/-0.2) transition and a dramatic blunting of fasting ketone turnover in PPARalpha(-/-) mice (1.0+/-0.1). C57BL/6 mice made obese and insulin resistant by high-fat feeding for 8 wk had normal expression of genes that regulate hepatic fat oxidation, whereas 16 wk on the diet induced expression of these genes and stimulated the function of hepatic mitochondrial fat oxidation, as indicated by a 40% induction of fasting ketogenesis and a twofold rise in short-chain acylcarnitines. Together, these findings indicate a progressive adaptation of hepatic ketogenesis during high-fat feeding, resulting in increased hepatic fat oxidation after 16 wk of a high-fat diet. We conclude that mitochondrial fat oxidation is stimulated rather than impaired during the initiation of hepatic insulin resistance in mice.

Pregnancy impairs ketone body disposal in late gestating ewes: Implications for onset of pregnancy toxaemia

The impact of pregnancy on ketone body disposal during a hyperketonaemic clamp was examined by tracer isotope dilution techniques in seven 12 h fasted sheep in three reproductive states, in the dry non-gestating period, late in gestation and during early lactation. After a sampling period of 60 min under basal conditions a DL-BHB racemate solution was continuously infused intravenously for 3 h at rates of 14.3-24.3 micromol/(kg min) to elevate the D-BHB concentration in blood plasma to values between 5 and 7 mmol/l. Two separate experiments were carried out with the same sheep in each of the three reproductive states. During pregnancy three ewes were pregnant with a single lamb and four ewes carried twins. Maximal D-BHB turnover rates fell significantly in late gestation by 26% relative to early lactation and by 22% when compared with the dry non-pregnant state. Reduction of maximal D-BHB disposal rate during late gestation was accompanied by a significant 297% (p<0.005) and a non-significant 49% increase in the basal BHB concentration in blood, a non-significant 10% and 4% decrease in the rate constant of D-BHB turnover and a non-significant 24% and 13% rise in the incremented increase of D-BHB concentration per unit D-BHB infusion, relative to the dry and the lactating period, respectively. Induction of hyperketonaemia significantly lowered NEFA and glycerol concentrations in blood by 67% and 57%, respectively, compared to the pre-infusional concentrations. The magnitude of this effect was the same in all reproductive states and was explained as a direct antilipolytic action of D-BHB on adipose tissue. It is concluded that the reduced ability of the late gestating ewe to utilize D-BHB promotes hyperketonaemia. Since hyperketonaemia exerts several adverse effects, e.g. on energy balance and glucose metabolism it appears that the impairment of ketone bodies disposal in late pregnancy facilitates development of pregnancy toxaemia, especially in ewes carrying twins.

Prolonged fasting significantly changes nutrient oxidation and glucose tolerance after a normal mixed meal

The aim of this study was to establish the experimental paradigm of fasting, followed by refeeding, to investigate individual differences in nutrient partitioning. Eight nonobese men were fed a normal meal (25% of daily energy requirements) on two occasions, after an overnight (13-h) fast and after a prolonged (72-h) fast. During the entire fasting period, subjects were resident in a whole room indirect calorimeter, and blood samples were drawn periodically. Because no other food was consumed over the 12 h after either meal, negative energy balance was observed after the overnight and prolonged fast. Postprandial carbohydrate oxidation was significantly reduced after the 72- vs. 13-h fast (P < 0.0001), whereas fat oxidation was significantly increased (P < 0.0001). Interestingly, carbohydrate balance was positive after the prolonged fast but negative after the overnight fast (24 +/- 17 vs. -57 +/- 16 g/12 h, respectively; P < 0.001), whereas fat balance was negative under both conditions (-78 +/- 7 vs. -47 +/- 8 g/12 h, respectively; P < 0.002). With 72 h of fasting, the glucose and insulin excursions in response to the mixed meal were significantly greater compared with the 13-h fast (P < 0.001). In conclusion, prolonged fasting resulted in a significant decrease in carbohydrate oxidation and an increase in fat oxidation, after a normal mixed meal, in healthy men. This was associated with a significant decrease in glucose tolerance. Because circulating free fatty acids were greatly elevated at all times after the prolonged fast, these may be mediating some of the changes in postprandial metabolism.

Oxidation of D(-)3-hydroxybutyrate administered to rats with extensive burns

Although the suppressive effect of 3-hydroxybutyrate (3-OHB) on post-traumatic protein catabolism in traumatized patients is well documented, the oxidation of exogenously administered 3-OHB during catabolic stress has not been investigated. The present study was designed to evaluate, using radioactive isotopes, total body oxidation in rats with and without burn stress to which 3-OHB had been exogenously administered, in comparison with total body oxidation in such rats that had received glucose. The rats were divided into four groups, based on whether or not a 30% full-thickness burn was inflicted, and the type of infusate they received after the burn, namely, 3-OHB or glucose. The total exhaled CO2 was collected for 6h after the infusion was commenced, and 14CO2 was assayed in a liquid scintillation spectrometer. Oxidation of the infusate was calculated from the percentage of exhaled 14CO2 derived from the infused substrates. The plasma concentration of 3-OHB was significantly increased after the infusion in both the burned and non-burned rats. The total exhaled 14CO2 from the rats infused with glucose decreased from 48.2 +/- 2.4% to 40.8 +/- 3.7% (means +/- SD, P < 0.001) after thermal injury. However, the total exhaled 14CO2 from the rats infused with 3-OHB appeared sooner, and there was no difference in the total expired 14CO2 derived from 3-OHB between the burned and non-burned rats, at 68.1 +/- 2.7% vs 66.4 +/- 3.4%, respectively. These findings suggest that even under conditions of burn stress, 3-OHB can be oxidized normally if the plasma concentration of 3-OHB is elevated by exogenous administration.

The effect of starvation on leucine, alanine and glucose metabolism in obese subjects

The relationship between changes in ketone concentrations and leucine metabolism (seven obese subjects), glucose and alanine metabolism (seven obese subjects) was investigated using radioisotopic techniques after 12 h, 60 h and 2 weeks starvation. Leucine metabolism was also measured in five lean subjects after 12 h and 60 h starvation. In the obese subjects leucine concentration increased after 60 h starvation and leucine metabolic clearance rate, glucose and alanine concentration decreased (P < 0.05). Glucose and alanine production rate (Ra) decreased after 2 weeks (P < 0.05) but there was no change in leucine Ra after 60 h or 2 weeks. In the lean subjects leucine concentration, production rate and oxidation rate were increased after 60 h (P < 0.005, P < 0.05, P < 0.05). Ketone concentration was inversely related to alanine Ra (r = -0.51, P < 0.02) but was not related to measurements of protein metabolism in the obese subjects. This study demonstrates that the effect of short-term starvation on protein metabolism differs in lean and obese subjects. The decrease in glucose Ra during long-term starvation may be in part due to a decreased supply of alanine for gluconeogenesis.

Hemodynamic, respiratory, and metabolic effects of medium-chain triglyceride-enriched lipid emulsions following valvular heart surgery

STUDY

A lipid emulsion containing 10 percent medium-chain triglycerides (MCT) and 10 percent long-chain triglycerides (LCT) was infused at a rate of 1 ml/kg/h (3.3 mg/kg/min) for 2 h, in 12 patients (2 males, 10 females; mean age, 54 +/- 3 (SEM) years; range, 34 to 67 years) 24 h after open-heart surgery (mitral valve replacement).

METHODS

Hemodynamic factors (pulmonary and radial artery indwelling catheters), oxygen and carbon dioxide partial pressures, oxygen saturation, oxygen delivery and consumption, and intrapulmonary shunt fraction were obtained before, during, and after lipid infusion (for 2 h), at 30-s intervals, along with some metabolic indexes (triglycerides, free fatty acids, glucose, insulin, lactate, acetoacetate).

RESULTS

No statistically significant changes in heart rate, cardiac index, systemic and pulmonary pressures and resistances, central venous and pulmonary capillary pressures, or arterial oxygen partial pressure were observed during infusion. Arterial carbon dioxide partial pressure values were constantly reduced throughout and after the end of lipid infusion, as compared with baseline values, while oxygen consumption was increased significantly without any change in oxygen delivery. No adverse effects on intrapulmonary shunt fraction were observed. Statistically significant increases of triglycerides, free fatty acids, acetoacetate and insulin (peak values at end of the lipid infusion) were found in comparison with baseline values. Plasma glucose increased significantly during lipid infusion and remained higher than baseline values until the end of the study. Lactate levels were unchanged except for a slight decrease at the end of the study, without any derangement of acid-base equilibrium. Neither arrhythmias nor adverse clinical reactions were observed as a consequence of lipid infusion.

CONCLUSIONS

Fat emulsions containing both MCT and LCT, when given at 3.3 mg/kg/min for 120 min following valvular heart surgery, do not exert negative cardiopulmonary effects, and could represent a source of rapidly metabolized substrates.

Substrate metabolism during modest hyperinsulinemia in response to isolated hyperketonemia in insulin-dependent diabetic subjects

To assess the metabolic effects of moderate hyperketonemia, six young male type 1 diabetic patients received a 200-minute intravenous (IV) infusion of (1) 0.9 mmol 3-hydroxybutyrate (3-OHB)/kg/h, and (2) saline. To ensure comparable metabolic conditions, a low-dose hyperinsulinemic euglycemic glucose clamp was performed from 5 hours before and throughout 3-OHB/saline infusions. The forearm technique was employed to estimate substrate fluxes in muscle. Infusion of 3-OHB caused: (1) increases (P less than .05) in circulating levels of 3-OHB (from 112 +/- 73 mumol/L to 825 +/- 111 mumol/L) and forearm arteriovenous differences of 3-OHB (from 19 +/- 10 mumol/L to 145 +/- 46 mumol/L), as well as an eightfold increase of plasma acetoacetate. (2) Decreased (P less than .05) levels of nonesterified fatty acids (NEFA; from 466 +/- 85 mumol/L to 201 +/- 14 mumol/L) and glycerol (from 39 +/- 7 mumol/L to 11 +/- 4 mumol/L) and decreased (P less than .05) arteriovenous differences of glycerol (from -16 +/- 8 mumol/L to -3 +/- 2 mumol/L). (3) Increased (P less than .05) levels of serum growth hormone (GH; from 4.1 +/- 1.5 micrograms/L to 15.9 +/- 8.0 micrograms/L). No change was recorded in circulating concentrations of free insulin, glucagon, glucose, lactate, or alanine. Nor were arteriovenous balances of these intermediary metabolites, isotopically determined glucose turnover or amounts of exogenously administered glucose affected. In conclusion, in type 1 diabetic man, the main regulatory effect of isolated hyperketonemia appears to be a direct negative feedback inhibition of lipolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ketone bodies on carbohydrate metabolism in non-insulin-dependent (type II) diabetes mellitus

The ability of ketone bodies to suppress elevated hepatic glucose output was investigated in eight postabsorptive subjects with non-insulin-dependent diabetes mellitus (NIDDM). Infusion of sodium acetoacetate alone (20 mumols/kg/min) for 3 hours increased total serum ketones (beta-hydroxybutyrate and acetoacetate) to approximately 6 mmol/L, but did not reduce plasma glucose (14.0 +/- 0.8 to 12.3 +/- 0.9 mmol/L) or isotopically determined hepatic glucose output (17.5 +/- 1.4 to 12.7 +/- 1.0 mumols/kg/min) more than saline alone. Plasma C-peptide concentrations were unchanged, while serum glucagon increased from 131 +/- 13 to 169 +/- 24 ng/mL (P less than .015) and free fatty acids were suppressed by 43% (0.35 +/- 0.08 to 0.20 +/- 0.06 mmol/L, P less than .025). When sodium acetoacetate was infused with somatostatin (0.10 micrograms/kg/min) to suppress glucagon and insulin secretion, the decrease in both plasma glucose (13.3 +/- 0.9 to 10.2 +/- 0.7 mmol/L) and hepatic glucose output (17.2 +/- 1.6 to 9.4 +/- 0.6 mumols/kg/min) was greater than either acetoacetate or somatostatin infusion alone. Infusion of equimolar amounts of sodium bicarbonate had no effect on glucose metabolism. In conclusion, these results demonstrate that ketone bodies can directly suppress elevated hepatic glucose output in NIDDM independent of changes in insulin secretion, but only when the concomitant stimulation of glucagon secretion is prevented. Ketone bodies also suppress adipose tissue lipolysis in the absence of changes in plasma insulin and may serve to regulate their own production.

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.

Effect of beta-hydroxybutyrate and acetoacetate on insulin and glucagon secretion from perfused rat pancreas

To elucidate the physiological significance of ketone bodies on insulin and glucagon secretion, the direct effects of beta-hydroxybutyrate (BOHB) and acetoacetate (AcAc) infusion on insulin and glucagon release from perfused rat pancreas were investigated. The BOHB or AcAc was administered at concentrations of 10, 1, or 0.1 mM for 30 min at 4.0 ml/min. High-concentration infusions of BOHB and AcAc (10 mM) produced significant increases in insulin release in the presence of 4.4 mM glucose, but low-concentration infusions of BOHB and AcAc (1 and 0.1 mM) caused no significant changes in insulin secretion from perfused rat pancreas. BOHB (10, 1, and 0.1 mM) and AcAc (10 and 1 mM) infusion significantly inhibited glucagon secretion from perfused rat pancreas. These results suggest that physiological concentrations of ketone bodies have no direct effect on insulin release but have a direct inhibitory effect on glucagon secretion from perfused rat pancreas.

The effect of major thermal injury on plasma ketone body levels

Eleven patients with more than 30% total body surface burns were studied during 3 days of starvation and three more days of unrestricted feeding following their injury. All patients developed marked protein mobilization as demonstrated by 3rd day urine nitrogen excretion of 17.1 g daily compared to control excretion of 11.8 g N daily. As a group, the patients failed to mount the expected ketonemic response during their initial period of starvation. Whereas normal fasted controls achieved plasma ketone body levels of 727 +/- 81 mumol/liter, the burn patients responded with an average level of 385 +/- 77 mumol/liter (p less than 0.01).

Influence of beta-hydroxybutyrate infusion on glucose and free fatty acid metabolism in dogs

We have investigated the effect of infusion of DL-beta-hydroxybutyrate (BOHB) (30 mumol X kg-1 X min-1) on glucose and free fatty acid (FFA) metabolism by means of the primed constant infusion of [U-14C]glucose and [1,2-13C]palmitic acid. The role of the hormonal response to the ketone infusion was assessed by controlling the hormone levels pharmacologically. In one group hormones were not controlled, while in the other two groups insulin and glucagon were maintained at constant levels by infusion of somatostatin, insulin, and glucagon at constant rates. In one of these hormonally controlled groups, combined alpha- and beta-adrenergic blockade was also employed. BOHB infusion increased total ketone concentration approximately 10-fold and, when hormones were not controlled, induced a significant increase in glucagon concentration. Regardless of hormonal status, elevation of the ketone levels decreased the rate of glucose production and FFA appearance. Glucose oxidation decreased in proportion to the reduction in the rate of glucose uptake in all groups. When sympathetic activity was not blocked an increase in the percent of FFA uptake oxidized enabled the percent CO2 production from FFA oxidation to remain constant despite the decrease in FFA uptake. However, when sympathetic activity was blocked the increase in the percent of FFA uptake oxidized observed in the other groups was prevented. We conclude from these studies that an elevation in ketone levels directly affects glucose and FFA metabolism independent of changes in insulin and glucagon levels and sympathetic activity.

Effect of ketone bodies on glucose production and utilization in the miniature pig

The effect of ketone bodies on glucose production (Ra) and utilization (Rd) was investigated in the 24-h starved, conscious unrestrained miniature pig. Infusing Na-DL-beta-OH-butyrate (Na-DL-beta-OHB) and thus shifting the blood pH from 7.40 to 7.56 resulted in a decrease of Ra by 52% and of Rd by 45%, as determined by the isotope dilution technique. Simultaneously, the concentrations of arterial insulin and glucagon were slightly enhanced, whereas the plasma levels of glucose, lactate, pyruvate, alanine, alpha-amino-N, and free fatty acids (FFA) were all reduced. Infusion of Na-bicarbonate, which yielded a similar shift in blood pH, did not mimick these effects. Infusion of equimolar amounts of the ketoacid, yielding a blood pH of 7.35, induced similar metabolic alterations with respect to plasma glucose, Ra, Rd, and insulin; however, plasma alanine and alpha-amino-N increased. Infusing different amounts of Na-DL-beta-OHB resulting in plasma steady state levels of ketones from 0.25 to 1.5 mM had similar effects on arterial insulin and glucose kinetics. No dose dependency was observed. Prevention of the Na-DL-beta-OHB-induced hypoalaninemia by simultaneous infusion of alanine (1 mumol/kg X min) did not prevent hypoglycemia. Infusion of Na-DL-beta-OHB plus insulin (0.4 mU/kg X min) showed no additive effect on the inhibition of Ra. Ketones did not inhibit the insulin-stimulated metabolic clearance rate (MCR) for glucose. Infusion of somatostatin (0.2 micrograms/kg X min) initially decreased plasma glucose, Ra, and Rd, which was followed by an increase in plasma glucose and Ra; however, on infusion of somatostatin plus Na-DL-beta-OHB, hypoglycemia and the reduced Ra were maintained. In the anaesthetized 24-h starved miniature pig, Na-DL-beta-OHB infusion decreased the hepatic exchange for glucose, lactate, and FFA, whereas the exchange for glycerol, alanine, and alpha-amino-N as well as liver perfusion rate were unaffected. Simultaneously, portal glucagon and insulin as well as hepatic insulin extraction rate were elevated. Leg exchange for glucose, lactate, glycerol, alanine, alpha-amino-N, and FFA were decreased, while ketone body utilization increased. Repeated infusion of Na-DL-beta-OHB at the fourth, fifth, and sixth day of starvation in the conscious, unrestrained mini-pig resulted in a significant drop in urinary nitrogen (N)-excretion. However, this effect was mimicked by infusing equimolar amounts of Na-bicarbonate. In contrast, when only the ketoacid was given, urinary N-excretion accelerated. To summarize: (a) Ketone bodies decrease endogenous glucose production via an insulin-dependent mechanism; in addition, ketones probably exert a direct inhibitory action on gluconeogenesis. The ketone body-induced hypoalaninemia does not contribute to this effect. (b) The counterregulatory response to hypoglycemia is reduced by ketones. (c) As a consequence of the decrease in R(a), glucose utilization declines during ketone infusion. (d)The insulin-stimulated MCR for glucose is not affected by ketones. (e) Ketones in their physiological moiety do not show a protein-sparing effect.

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.

Diabetic ketoacidosis--pathogenesis, prevention and therapy

Diabetic ketoacidosis is the principal cause of hospital admissions for diabetic patients under 20 years of age, and accounts for at least 4000 deaths per annum in the United States. Current mortality rates differ widely throughout the United States, ranging from 0-19 per cent, with an average of 10 per cent. The principal reason for this wide range in the percentage of mortality are the differing criteria for diagnosis and attributing deaths to diabetic ketoacidosis. There are many reported precipitating causes of diabetic ketoacidosis which may be reduced to four common pathways: insulin deficiency, stress hormone excess, dehydration and fasting. Infection is the most common precipitating cause in most reported series of diabetic ketoacidosis, but stress in any form can lead to metabolic decompensation. Omission of insulin is an unusual cause of ketoacidosis, and in approximately one-quarter of patients no cause can be identified. Each of the four common pathways through which these precipitating causes induce diabetic ketoacidosis results in a rise in ketone body and glucose production and/or concentration. Prevention of diabetic ketoacidosis has been underemphasized in the care of the ill diabetic patient. Prevention of metabolic decompensation in the stressed diabetic patient requires a knowledgeable physician and a cooperative patient. Appropriate physician management of insulin and suppression of stress hormones should prevent diabetic ketoacidosis in all patients who can ingest fluid. Recent studies suggest that if the mortality rate from diabetic ketoacidosis is to be significantly reduced, prevention of this complication is mandatory. Appropriate treatment of diabetic ketoacidosis is not difficult if the physician maintains an accurate flow chart and provides sufficient insulin, rehydration and potassium. We favour the use of low-dose insulin therapy, rehydration with isotonic saline, and aggressive potassium replacement. The administration of sodium bicarbonate is controversial and should be restricted to patients with an arterial pH of less than 7.0 and/or a patient in cardiogenic shock. The majority of complications encountered during the treatment of diabetic ketoacidosis are avoidable if proper care and attention is provided by the physician.

Effects of ketone bodies on insulin release and islet-cell metabolism in the rat

Ketone bodies promote insulin secretion from isolated rat pancreatic islets in the presence of 5 mM-glucose, but are ineffective in its absence. At concentrations of 10 mM or less, the relative abilities of the ketone bodies to potentiate release are in the order D-3-hydroxybutyrate greater than DL-3-hydroxybutyrate greater than acetoacetate. The response curve relating insulin release to D-3-hydroxybutyrate concentration displays a threshold at 1 mM and a maximum at 10 mM. D-3-Hydroxybutyrate (5 mM, but not 10 mM) promotes insulin secretion in the presence of 5 mM concentrations of both L-arginine and DL-glyceraldehyde, but not with L-leucine, L-alanine, L-glutamate or 4-methyl-2-oxopentanoate. The oxidation rates of the exogenous ketone bodies do not correlate well with their capacities to promote insulin release. Moreover, the oxidation of 5 mM-D-3-hydroxybutyrate can be inhibited by 25% with methylmalonate (10 mM) without any diminution of release. The potentiation with D-3-hydroxybutyrate occurs without an observable increase in total islet cyclic AMP. However, a small net efflux matches the relative abilities of the ketone bodies to promote insulin release. With islets from 48 h-starved animals the insulin response is both diminished and less sensitive than in fed animals, since insulin secretion is not significantly raised until a threshold of 5 mM-D-3-hydroxybutyrate is reached. These results suggest that, in the rat at least, there should be a reappraisal of the physiological role of ketone bodies in the promotion of insulin release.

Acid-base and metabolic responses to anion infusion in the anesthetized dog

This study evaluated the acid-base and metabolic effects of several organic anions which might have application in the correction of metabolic acidosis during hemodialysis. Anesthetized dogs were infused intravenously with the sodium salts of either chloride, bicarbonate, acetate, lactate, or pyruvate. Acetate perturbated metabolism more than any of the other organic anions infused. These perturbations included hypoxemia, reductions in serum potassium and phosphorus, a decrease in plasma, glucose, and increases in intermediary metabolites such as lactate, acetoacetate and beta-hydroxybutyrate. Transferance of our findings to hemodialysis suggests that acetate would compromise the ability of dialysis to reduce body burdens of potassium and phosphorus and provide proper base repletion. Pyruvate would appear superior to acetate as a base substitute by virtue of its lesser effects on oxygen consumption and electrolyte distribution and its capacity to produce glucose. However, the significant production of lactate with pyruvate infusion, coupled with diffusive losses of bicarbonate during dialysis and the possible instability of pyruvate in solution, would still hinder proper base repletion. Bicarbonate generation with lactate infusion was too slow to provide a practical alternative for base repletion in hemodialysis. Bicarbonate infusion caused minimal alterations in intermediary metabolism. This, in conjunction with obviating diffusive losses, suggests the use of bicarbonate would allow more appropriate base repletion during hemodialysis.

Urinary excretion of C4--C10-dicarboxylic acids and antiketogenic properties of adipic acid in ketogenic-stimulated rats due to diabetes, long-chain and short-chain monocarboxylic acids

The urinary excretion of C4--C10-dicarboxylic acids (succinic, adipic, suberic and sebacic acids) and the antiketogenicity of adipic acid have been studied in ketogenic-stimulated rats in three biochemically different states: diabetes, fat-feeding (long-chain monocarboxylic acids) and feeding of hexanoic acid (short-chain monocarboxylic acid). In diabetic rats urinary excretions of adipic and suberic acids were elevated before the rise in urinary excretions of 3-hydroxybutyric acid, i.e. before ketosis appeared. In severe diabetic ketosis sebacic acid was below normal values, whereas the excretion of succinic acid was unaltered. Rats, in which ketosis was provoked by hexanoic acid, had preketotic high urinary excretions of adipic and succinic acids. After ketosis the excretions of succinic acid declined again whereas the excretion of adipic acid rose further, together with that of suberic acid. Moreover, when rats which were ketotic due to treatment with long-chain triacylglycerol or hexanoic acid received 500 mg of adipic acid the urinary excretion of succinic acid rose significantly. However, no changes in succinic acid excretion were seen in diabetic ketotic rats treated with the same amount of adipic acid. Exogenously administered adipic acid was strongly antiketogenic towards ketosis caused by long-chain or short-chain monocarboxylic acids, but had no effect on diabetic ketosis.

C6--C10-dicarboxylic aciduria in starved, fat-fed and diabetic rats receiving decanoic acid or medium-chain triacylglycerol. An in vivo measure of the rate of beta-oxidation of fatty acids

Administration of decanoic acid to rats resulted not only in elevated urinary excretions of the C10-dicarboxylic acid (sebacic acid), but also in highly elevated excretions of the beta-oxidation products C8- and C6-dicarboxylic acids (suberic and adipic acids). Activation of the lipid metabolism by starvation, fat-feeding and experimental diabetes increased the excretions of adipic acid and decreased the excretions of sebacic acid, i.e. the rate of oxidation of fatty acids was correlated to the adipic : sebacic acid ratio in urine. Compared with nondiabetic unstarved rats the adipic : sebacic acid ratio was elevated 2--3-, 8--16-, 5--19-, and 22--88-times in rats which were, respectively, starved for 2 days, 4 days, on a fat-diet for 4 days, and ketotic due to streptozotocin-induced diabetes. All rats with ratios above 10 were ketotic (urinary excretions of 3-hydroxybutyric acid over 500 microgram/mg creatinine) and all rats with ratios below 4 were nonketotic, while ketosis was a variable finding in rats with intermediary ratios. Similar changes in the ratio of excreted dicarboxylic acids were found when medium-chain triacylglycerols were fed instead of decanoic acid.

Kinetics of ketone body metabolism in fasting humans

The rates of production of total ketone bodies (acetoacetate + beta-hydroxybutyrate) were determined using an isotope tracer technique in 23 obese subjects submitted to a fast of variable duration (15 hr--23 days). Constant infusions of 14C-acetoacetate were used in most studies, but similar results were obtained with pulse injections of this tracer or with constant infusions of 14C-D(-)-beta-hydroxybutyrate. Blood concentration, production rate, and urinary elimination of total ketones rose during approximately the first 3 days of fast and plateaued thereafter at values amounting, respectively, to 7.09 +/- 0.32 mumole/ml, 1908 +/- 80 mumole/min and 167 +/- 14 mumole/min. The rates of ketogenesis are significantly higher than those usually reported in the literature. Ketonemia was an exponential function of production rate suggesting that tissue uptake becomes progressively saturated as inflow rate rises. The same type of relationship between concentration and inflow rate was observed in nine control overnight fasted obese subjects rendered hyperketonemic with infusions of variable amounts of unlabeled acetoacetate. The comparison between the fasted and the control subjects at ketone concentrations of 3--10 mumole/ml showed that on an average, starvation is associated with a 35% decrease in the metabolic clearance rate of ketones. These data suggest that fasting is associated with an impairment of mechanisms for utilizing ketones, this defect contributing to the hyperketonemia of food deprivation.

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.

Human splanchnic metabolism during diabetic ketoacidosis

Splanchnic exchange rates of glucose, acetoacetate, beta-hydroxybutyrate, lactate, pyruvate, glycerol, alanine, glutamine, glutamate, free fatty acids, and triglycerides were measured in eight patients during moderate to severe diabetic ketoacidosis. Their arterial glucose concentration was 20.68 (9.80-52.79) mumole/liter and tic glucose release was 0.77 (0.09-2.44) mmole/min. Gluconeogenesis accounted for about one-half of net splanchnic glucose release, assuming quantitative conversion of net splanchnic extracted lactate, pyruvate, glycerol, alanine, and alpha-ketoglutarate equivalents to glucose. Net splanchnic free fatty acid extraction was 0.24 (0.09-0.52) mmole/min. There was a positive correlation between free fatty acid uptake and ketone-body release. Net splanchnic acetoacetate release was 0.50 (0.05-0.92) mmole/min and beta-hydroxybutyrate release was 0.35 (-0.16 to 0.84) mmole/min. Total ketone-body release was 0.84 (0.37-1.61) mmole/min. The wide ranges of net splanchnic glucose and ketone-body production rates show the heterogeneous characteristics of the diabetic patient in ketoacidosis. It is concluded that the hyperglycemia and hyperketonemia of diabetic ketoacidosis is due to the lack of reciprocity among rates of hepatic glycogenlysis, gluconeogenesis, and ketogenesis resulting in inappropriate net splanchnic release of glucose and ketone bodies.

Lipogenesis from ketone bodies in rat brain. Evidence for conversion of acetoacetate into acetyl-coenzyme A in the cytosol

The metabolism of acetoacetate via a proposed cytosolic pathway in brain of 1-week-old rats was investigated. (-)-Hydroxycitrate, an inhibitor of ATP citrate lyase, markedly inhibited the incorporation of carbon from labelled glucose and 3-hydroxybutyrate into cerebral lipids, but had no effect on the incorporation of labelled acetate and acetoacetate into brain lipids. Similarly, n-butylmalonate and benzene-1,2,3-tricarboxylate inhibited the incorporation of labelled 3-hydroxybutyrate but not of acetoacetate into cerebral lipids. These inhibitors had no effect on the oxidation to 14CO2 of the labelled substrates used. (-)-Hydroxycitrate decreased the incorporation of 3H from 3H2O into cerebral lipids by slices metabolizing either glucose or 3-hydroxybutyrate, but not in the presence of acetoacetate. (-)-Hydroxycitrate also differentially inhibited the incorporation of [2-14C]-leucine and [U-14C]leucine into cerebral lipids. The data show that, although the acetyl moiety of acetyl-CoA generated in brain mitochondria is largely translocated as citrate from these organelles to the cytosol, a cytosolic pathway exists by which acetoacetate is converted directly into acetyl-COA in this cellular compartment.

Effect of hyperphenylalaninaemia on lipid synthesis from ketone bodies by rat brain

The effect of hyperphenylalaninaemia on the metabolism of ketone bodies in vivo and in vitro by developing rat brain was investigated. The incorporation in vivo of [14C]acetoacetate into cerebral lipids was decreased by both chronic (for 3 days) and acute (for 6h) hyperphenylalaninaemia induced by injecting phenylalanine into 1-week-old rats. In studies in vitro it was observed that the incorporation of the radioactivity from [14C]acetoacetate and 3-hydroxy[14C]butyrate into cerebral lipids was inhibited by phenyl-pyruvate, but not by phenylalanine. Phenylpyruvate also inhibited the incorporation of 3H from 3H2O into lipids by brain slices metabolizing either 3-hydroxybutyrate or acetoacetate in the presence of glucose. These findings suggest that the decrease in the incorporation in vivo of [14C]acetoacetate into cerebral lipids in hyperphenylalaninaemic rats is most likely caused by phenylpyruvate and not by phenylalanine. Phenylpyruvate as well as phenylalanine had no inhibitory effects on ketone-body-catabolizing enzymes, namely 3-hydroxybutyrate dehydrogenase, 3-oxo acid CoA-transferase and acetoacetyl-CoA thiolase, in rat brain. Phenylpyruvate but not phenylalanine inhibited the activity of the 2-oxoglutarate dehydrogenase complex from rat and human brain. These findings suggest that the metabolism of ketone bodies is impaired in brains of untreated phenylketonuric patients, and in turn may contribute to the diminution of mental development and function associated with phenylketonuria.

The contribution of endogenous insulin secretion to the ketogenic response to glucagon in man

The magnitude and direction of the lipolytic and ketogenic responses following exogenous glucagon administration is controversial and consideration of the possible role of endogenous insulin secretion upon these events has not been clarified. The present study examines the role of endogenous insulin secretion in modulating the net lipolytic and ketogenic activity of glucagon. Three groups characterized by different levels of endogenous insulin secretory capacity were studied. In all three groups, the responses in plasma insulin, betahydroxybutyrate, and free fatty acids were observed following bolus administration of 1.0 mug/kg glucagon. In the obese subjects with increased endogenous insulin secretion, glucagon administration resulted in a decline below basal levels of both free fatty acid and betahydroxybutyrate. In the diabetic subjects with no demonstrable endogenous insulin secretion, glucagon administration was followed by a rise in plasma free fatty acids and an exaggerated rise in plasma betahydroxybutyrate. The normal control group exhibited a response in betahydroxybutyrate midway between the obese and diabetic groups. These obwervations support the thesis that the magnitude of endogenous insulin secretion modulates the lipolytic and ketogenic actions of glucagon in man.

Inhibition of ketogenesis by ketone bodies in fasting humans

Although there exists some indirect evidence that circulating ketone bodies might inhibit their own production rate, the direct demonstration of this homeostatic feed-back phenomenon is still lacking. The present work aims at demonstrating the operation of this control mechanism in human fasting ketosis. Six obese subjects, who fasted 2-23 days, were given a primed constant i.v. infusion of 3- 14C-acetoacetate for 4 hr. After a control period of 2 hr, unlabeled sodium acetoacetate was administered as a primed constant i.v. infusion at the rate of 0.688-1.960 mmol/min until the end of the study. During both periods, the rates of inflow of ketones were estimated from the specific activity of total ketones measured under near isotopic steady state conditions. During the control period, total ketone concentration amounted to 3.98-9.65 mumol/ml and production rates of total ketones ranged between 1.450 and 2.053 mmol/min. The levels of free fatty acids, glycerol, glucose, and insulin averaged respecitvely 1.30 mumol/ml, 0.11 mumol/ml, 74 mg/100 ml, and 5.2 muU/ml. The administration of exogenous ketones during the second phase of the study induced a 47%-92% increase in total ketone levels. During this period, the endogenous production of ketones (calculated as the difference between total inflow rate and acetoacetate infusion rate) amounted only to 67%-90% of control values. Among other factors, this inhibition of ketogenesis was probably partially related to the direct antilipolytic effect of infused ketones. Indeed, there was a concomitant fall in FFA and in glycerol levels averaging respectively 13.5% and 17.3%, without significant changes in peripheral insulin concentrations. Our results demonstrate that during fasting, circulating ketone bodies exert an inhibitory influence on the rate of ketogenesis. This mechanism might play an important role in preventing the development of uncontrolled hyperketonemia during starvation.

Effect of ketone infusions on amino acid and nitrogen metabolism in man

To evaluate the role of hyperketonemia in the hypoalaninemia and decreased protein catabolism of prolonged starvation, Na dl-beta-hydroxybutyrate was administered as a primed continuous 3-6-h infusion in nonobese subjects and in obese subjects in the postabsorptive state and after 3 days and 3-5 1/2 wk of starvation. An additional obese group received 12-h ketone infusions on 2 consecutive days after 5-10 wk of fasting. The ketone infusion in nonobese and obese subjects studied in the postabsorptive state resulted in total blood ketone acid levels of 1.1-1.2 mM, a 5-15 mg/100 ml decrease in plasma glucose, and unchanged levels of insulin, glucagon, lactate, and pyruvate. Plasma alanine fell by 21% (P smaller than 0.001) in 3 h. In contrast, other amino acids were stable or varied by less than 10%. Infusions lasting 6 h reduced plasma alanine by 37%, reaching levels comparable to those observed in prolonged starvation. Equimolar infusions of NaC1 and/or administration of NaHCO3 failed to alter plasma alanine levels. During prolonged fasting, plasma alanine, which had fallen by 40% below prefast levels, fell an additional 30% in response to the ketone infusion. In association with repeated prolonged (12 h) infusions in subjects fasted 5-10 wk, urinary nitrogen excretion fell by 30%, returning to base line after cessation of theinfusions and paralleling the changes in plasma alanine. Ketone infusins resulted in two- to fourfold greater increments in blood ketone acids in fasted as compared to postabsorptive subjects. It is concluded that increased blood ketone acid levels induced by infusions of Na DL-beta-hydroxybutyrate result in hypoalaninemia and in nitrogen conservation in starvation. These data suggest that hyperketonemia may be a contributory factor in the decreased availability or circulating alanine and reduction in protein catabolism characteristic of prolonged fastings9