Brain metabolism during fasting

Gene expression of calpains and their specific endogenous inhibitor, calpastatin, in skeletal muscle of fed and fasted rabbits

To investigate the role of calpains in myofibrillar protein degradation in skeletal muscle and the regulation of their activity in vivo, we studied the effects of fasting on gene expression of calpains and calpastatin in the skeletal muscle of rabbits. In response to fasting, myofibrillar protein degradation increased 2-fold and mRNA levels of calpain I, calpain II and calpastatin were also increased. However, calpain and calpastatin activities remained unchanged. To investigate this discrepancy, we analysed polysomal calpain mRNA. Results indicated that fasting caused a 2-fold increase in the loading of calpain I and II mRNAs on ribosomes. Thus transcription of genes encoding calpain may be increased during fasting to ensure adequate synthesis of the proteinases needed to mobilize muscle protein reserves. The effect of fasting on calpain and calpastatin mRNA expression is shared by cathepsin D and proteasome C2 but not by beta-actin, implying that fasting invokes control of several proteolytic systems in skeletal muscle and underscores the possibility that each proteolytic system plays a role in the adaptation of skeletal muscle to the fasted state.

Effect of starvation on glycogen and glucose metabolism in different areas of the rat brain

We have studied the changes in concentration of glycogen, glucose and the bisphosphorylated sugars, glucose 1,6-P2 and fructose 2,6-P2, in several rat brain regions during 72 h of starvation. The animals were killed by focused microwave irradiation. The activities of glycogen metabolizing enzymes in the different areas were measured. A large decrease in glycogen and glucose concentration was observed in all areas. The concentrations of bisphosphorylated sugars changed, suggesting that an increase in glycolysis could take place at the beginning of starvation, with blood glucose as a major energy source. Differences in metabolite concentration before starvation disappeared after 72 h. The activities of glycogen synthase, glycogen phosphorylase and glycogen phosphorylase kinase were similar in all areas, and they did not change during starvation.

Plasma carnitine in fasting neonatal and adult northern elephant seals

Maintenance of adequate body carnitine stores is a requisite for fasting mammals, whose energy is derived mainly from free fatty acid oxidation. The impact of longterm fasting on carnitine status is unclear, and there have been no reports of carnitine during naturally occurring fasts. Total (TC), free (FC), and acylated (AC) plasma carnitine levels were determined in 10 weaned and 11 adult northern elephant seals (Mirounga angustirostris) during natural fasts lasting from 1 to 3 mo. In pups, TC declined little and AC increased only slightly [P greater than 0.05, analysis of variance (ANOVA)] through 11 wk of fasting. Plasma FC dropped by 53 and 26% from week 1 values at 10 and 11 wk fasting, respectively (P = 0.014, ANOVA). The AC/FC ratio did not approach 1.0 until 7 wk of fasting. TC was 38.6 +/- 1.4 microM and 47.6 +/- 4.1 microM in adult females and males, respectively. Adult AC/FC ratios were 0.71 +/- 0.10 (females) and 0.08 +/- 0.04 (males). Plasma TC status is not negatively affected by extended fasting in adult and weaned northern elephant seals. These data support the hypothesis that fasting northern elephant seals defend plasma TC and maintain an attenuated AC/FC ratio well into their prolonged natural fast.

Regulation of cerebral glucose metabolism in normal and polycythemic newborn lambs

In contrast to previous investigations, a recent study of polycythemic lambs suggested that cerebral glucose delivery (concentration x blood flow), not arterial glucose concentration, determined cerebral glucose uptake. In the present study, the independent effects of arterial glucose concentration and delivery on cerebral glucose uptake were examined in two groups of chronically catheterized newborn lambs (control and polycythemic). Arterial glucose concentration was varied by an infusion of insulin. CBF was reduced in one group of lambs (polycythemic) by increasing the hematocrit. At all arterial glucose concentrations, the cerebral glucose delivery of the polycythemic group was 59.6% of the control group. At arterial glucose concentrations of greater than 1.6 mmol/L, cerebral glucose uptake was constant and similar in both groups. At arterial glucose concentrations of less than or equal to 1.6 mmol/L, cerebral glucose uptake was unchanged in the control group, but was significantly decreased in the polycythemic group. In contrast, the cerebral glucose uptake was similar in both groups over a broad range of cerebral glucose delivery values. At cerebral glucose delivery values less than or equal to 83 mumols/min/100 g, there was a significant decrease in cerebral glucose uptake in both groups. During periods of low cerebral glucose delivery and uptake, cerebral oxygen uptake fell in the control group but remained unchanged in the polycythemic group. Maintenance of cerebral oxygen uptake in the polycythemic group was associated with an increased extraction and uptake of lactate and beta-hydroxybutyrate. We conclude that cerebral glucose delivery, not arterial glucose concentration alone, determines cerebral glucose uptake.

Oxidative energy metabolism in Alzheimer brain. Studies in early-onset and late-onset cases

Reduction of the cerebral metabolic rate of glucose is one of the most predominant abnormalities generally found in the Alzheimer brain, whereas the cerebral metabolic rate of oxygen is only slightly diminished or not at all the beginning of this dementive disorder. This metabolic abnormality may induce severe functional disturbances, obviously preceding morphobiological changes. From the cerebral metabolic rates of oxidized glucose and oxygen, the cerebral ATP formation rate was calculated in incipient early-onset, incipient late-onset and stable advanced dementia of Alzheimer type. A reduction of ATP formation was found from at least 7% in incipient early-onset, to around 20% in incipient late-onset DAT, and from 35% to more than 50% in stable advanced dementia. This approximation was adjusted to findings demonstrating diminished activities of enzymes active in glucose metabolism and formation of oxidation equivalents for ATP production from substrates other than glucose. A reduction for energy formation to the same range was found, as was also recently reported, in vivo in Alzheimer patients. From this rather theoretical point of view, a permanent loss of energy by at least 7-20% in incipient and progressively advancing dementia of the Alzheimer type may be assumed, with an increasing tendency in stable advanced dementia to around 50% energy loss. This energy deficit may have drastic impacts on brain function.

The neonatal rabbit brain glucose transporter

The Glut 1 (Hep G2/rat brain) isoform of glucose transporter is expressed in significant amounts in adult mammalian brain. The purpose of our present study was to determine the brain cellular localization of Glut 1 during the late newborn stage of development, when brain cellular proliferation and differentiation is highly active. Employing immunohistochemistry and in-situ hybridization in 10-day-old neonatal rabbit brain sections, we undertook cellular localization of Glut 1 expression. Glut 1 protein and mRNA were mainly noted in considerable amounts within the 10-day-old brain microvasculature. Lower concentrations of Glut 1 immunoreactivity were present in certain glial cells found within the deeper cortical layers of brain. Northern blot analysis of total RNA from isolated microvasculature-enriched preparation, isolated and cultured neuronal and glial cells, whole brain and whole brain with the exclusion of microvasculature obtained from the 10-day-old, revealed the universal presence of a approximately 2.8 kb Glut 1 mRNA with the exception of the neuron-enriched cultures. We conclude that during the neonatal period, when parenchymal cellular proliferation is at a peak, Glut 1 is localized not only to the microvasculature but also to certain cells which express glial morphological characteristics. The neuronal cells either do not express Glut 1 or express minute amounts.

Quantitative histochemical changes in enzymes involved in energy metabolism in the rat brain during postnatal development. II. Glucose-6-phosphate dehydrogenase and beta-hydroxybutyrate dehydrogenase

The postnatal maturation of glucose-6-phosphate and beta-hydroxybutyrate dehydrogenase activity was assessed by histochemistry in rats at eight postnatal stages, P0, P5, P10, P14, P17, P21, P35 and the adult stage. Enzyme activities were revealed on cryostat brain sections with nitroblue tetrazolium. Both enzyme activities were low and homogeneous at birth, and increased to reach a peak in all areas studied, at P17 for beta-hydroxybutyrate dehydrogenase and at P21 for glucose-6-phosphate dehydrogenase. Then, glucose-6-phosphate dehydrogenase activity decreased regularly by 20-49% from P21 to adult stage, except in cerebellar white matter where activity did not change after P21. beta-hydroxybutyrate dehydrogenase activity decreased regularly from P17 to adult stage in globus pallidus, hippocampus, thalamus, brainstem, genu of corpus callosum and cerebellar white matter. It sensorimotor cortex, medial geniculate body, caudate nucleus, hypothalamus and inferior colliculus, beta-hydroxybutyrate dehydrogenase activity stayed stable between P17 and P35 and decreased thereafter to adult levels. Finally, in parietal, auditory and cerebellar cortices, beta-hydroxybutyrate dehydrogenase activity either stayed stable or slightly increased after P17. The present study shows that there is a quite good correlation between postnatal changes in cerebral glucose-6-phosphate and beta-hydroxybutyrate dehydrogenase activities and the importance of pentose phosphate pathway and ketone body utilization in the developing brain. Our results also reflect the regional heterogeneity of beta-hydroxybutyrate utilization in the adult rat brain, translating into a remaining high activity of beta-hydroxybutyrate dehydrogenase in cerebral cortex.

Glucose, lactic acid, and perinatal hypoxic-ischemic brain damage

Investigations suggest that hyperglycemia, superimposed on hypoxia-ischemia or cerebral ischemia, accentuates brain damage in adult experimental animals and humans, but not in immature animals. Fundamental differences in the immature and adult brain, which account for the age-specific paradox, are discussed. Based on currently available data, we recommend that glucose supplementation not be curtailed during labor and delivery of asphyxiated human infants; on the contrary, glucose therapy may substantially reduce hypoxic-ischemic brain damage.

Substrate oxidation and the contribution of protein oxidation to energy expenditure after severe head injury

The 'flow' phase response to head injury is characterized by hypermetabolism and catabolism of lean body mass. In order to measure the contribution of protein oxidation (CPO) to resting metabolic expenditure (RME), 11 severely head injured patients (AIS 5) were studied. All patients had 24 h urine collections for at least 10 days after injury and RME was determined at intervals by indirect calorimetry. No patient received exogenous steroids. Peak urinary nitrogen excretion was 11.63 +/- 1.28 g/m2/day occurring between days 6 and 9 after injury. Fat oxidation was the greatest component of the RME at all times after head injury and the CPO to RME was 26.4 +/- 2.9 per cent during days 1-2, 31.8 +/- 3.3 per cent during days 3-5, 28.6 +/- 3.4 per cent during days 6-9 and 23.3 +/- 3.8 per cent during days 10-20 after injury. These figures are higher than those previously reported for burns, musculoskeletal injury or sepsis. The mechanism for the increased CPO is unclear. It may be related to such conditions of management as paralysis and fasting, but more likely it is an idiosyncratic feature of the metabolic response to head injury.

Regional blood-brain barrier transport of ketone bodies in portacaval-shunted rats

The permeability of the blood-brain barrier to ketone bodies, substrates of the monocarboxylic acid carrier, was measured in individual brain structures of control and portacaval-shunted rats. The measurements were made 5-7 wk after the shunt or sham operation by quantitative autoradiography. Portacaval shunting caused the permeability to ketone bodies to decrease throughout the brain by approximately 70%. There was a striking change in the transport pattern in the cerebral cortex; deeper cortical layers were affected more than superficial layers. Ketone body consumption by brain is limited by the transport capacity of the monocarboxylic acid system. Therefore, in portacaval-shunted rats the very low activity of this system makes it unlikely that ketone bodies can make a substantial contribution during situations such as fasting. Likewise, other substrates of the monocarboxylic acid system, e.g., lactate and pyruvate, will have greatly restricted access to the brain after portacaval shunting. If the carrier is symmetrical, another consequence will be that exit of endogenously produced lactate will be retarded.

Metabolic substrate utilization by tumour and host tissues in cancer cachexia

Utilization of metabolic substrates in tumour and host tissues was determined in the presence or absence of two colonic tumours, the MAC16, which is capable of inducing cachexia in recipient animals, and the MAC13, which is of the same histological type, but without the effect on host body composition. Glucose utilization by different tissues was determined in vivo by the 2-deoxyglucose tracer technique. Glucose utilization by the MAC13 tumour was significantly higher than by the MAC16 tumour, and in animals bearing tumours of either type the tumour was the second major consumer of glucose after the brain. This extra demand for glucose was accompanied by a marked decrease in glucose utilization by the epididymal fat-pads, testes, colon, spleen, kidney and, in particular, the brain, in tumour-bearing animals irrespective of cachexia. The decrease in glucose consumption by the brain was at least as high as the metabolic demand by the tumour. This suggests that the tissues of tumour-bearing animals adapt to use substrates other than glucose and that alterations in glucose utilization are not responsible for the cachexia. Studies in vitro showed that brain metabolism in the tumour-bearing state was maintained by an increased use of lactate and 3-hydroxybutyrate, accompanied by a 50% increase in 3-oxoacid CoA-transferase. This was supported by studies in vivo which showed an increased metabolism of 3-hydroxybutyrate in tumour-bearing animals. Thus ketone bodies may be utilized as a metabolic fuel during the cancer-bearing state, even though the nutritional conditions mimic the fed state.

6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in rat brain

The concentration of fructose 2,6-bisphosphate in the brain remained stable during starvation and early stages of ischaemia, but decreased in diabetes or after lengthened ischaemia. 6-Phosphofructo-1-kinase activity was also decreased in diabetic and ischaemic animals, whereas 6-phosphofructo-2-kinase was not modified. The concentration of the bisphosphorylated metabolite seems to be remarkably constant under a wide variety of experimental conditions, suggesting that it plays an essential role in the basal activation of 6-phosphofructo-1-kinase. Purified 6-phosphofructo-2-kinase also showed fructose-2,6-bisphosphatase activity with an activity ratio similar to that of the purified heart isoenzyme. The brain enzyme also has a net charge similar to that of the heart isoenzyme. Its activity is not modified by sn-glycerol 3-phosphate, and it is more sensitive to citrate than the liver or muscle isoenzyme. Moreover, the enzyme from brain, similarly to that from heart and muscle, is not modified by the cyclic AMP-dependent protein kinase or protein kinase C. A near-full-length cDNA probe from liver hybridized with RNA from brain and heart. In both cases, a major band of 6.8 kb of RNA and a minor one of 4 kb of RNA were detected. All these properties support the hypothesis that brain contains a different isoenzymic form from that of liver and muscle, and it is probably related to the heart isoform.

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.

Metabolic Studies in Human Subjects

Details of the major pathways of metabolism have been elucidated, in large measure through work on experimental animals. It is unlikely that they differ qualitatively between mammalian species. The more important challenge facing researchers today is that of metabolic regulation, and of the integrated control of metabolic pathways in the whole organism. These aspects may well differ quantitatively or even qualitatively between species. In this review, methods for studying metabolism and metabolic regulation in humans are described. There are both scientific and practical advantages to performing studies of metabolic regulation in humans. The scientific advantages are clear from some fundamental differences in metabolic regulation between rats and humans, such as the importance of de novo lipogenesis to the deposition of body fat, and the metabolism of atherogenic triacylglycerol-rich lipoproteins. The practical advantages result mainly from the size of a human compared with that of most laboratory animals, enabling large blood samples to be obtained, and several measurement techniques to be applied at one time. Reasons for the persistence of animal experimentation as the norm, rather than the exception, among life science researchers are discussed.

Hypoglycemias

Low plasma glucose concentrations that may or may not be sufficiently low to result in symptoms can be observed as a concomitant of several diverse diseases. Treatment of the primary underlying disorder usually alleviates the hypoglycemia. For patients whose primary symptom is that of hypoglycemia, it is essential to confirm that the plasma glucose concentration is low during the occurrence of symptoms. Symptoms that occur after meals usually are mild and rarely signify serious disease. With rare exceptions, hypoglycemia resulting in major symptoms occurs in the food-deprived state. Lower concentrations of plasma insulin and C-peptide and a concomitant low plasma glucose are major clues to a correct diagnosis.

Hypothermia is critical for survival during prolonged insulin-induced hypoglycemia in rats

Hypothermia is a well-known concomitant of hypoglycemia in mammals. We tested the hypothesis that this hypothermia is an important adaptive response to hypoglycemia in 11 normal Sprague-Dawley rats. Twelve-hour fasted, conscious animals received primed, continuous insulin infusions for up to 8 hours. Plasma glucose was clamped between 30 and 40 mg/dL and core body temperature was monitored continuously during the insulin infusions. Five of the animals were maintained in a room temperature environment (22 to 24 degrees C) during the hypoglycemia; all became hypothermic (mean +/- SE nadir core temperature, 31 +/- 0.5 degrees C). Spontaneous activity was reduced in these animals, but they remained conscious and responsive to external stimuli. All five returned to normal behavior after euglycemia was restored at the end of the insulin infusions. In the remaining six animals, hypothermia was prevented during hypoglycemia by warming of the air in their cages (mean of hourly core temperatures, 37 +/- 0.1 degrees C). None of these animals survived more than 7 hours. The severity of the hypoglycemia was no greater in the euthermic than in the hypothermic group, as judged by the mean of individual nadir plasma glucose levels (25 +/- 1 v 24 +/- 1 mg/dl, respectively) and by the mean number of glucose values per animal that were less than 30 mg/dL (2 +/- 1 v 7 +/- 1). Plasma osmolality did not change significantly in either group during the period of hypoglycemia, suggesting that dehydration was not the cause of death in the euthermic animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of early chronic phenobarbital treatment on cerebral arteriovenous differences of glucose and ketone bodies in the developing rat

The influence of an early chronic phenobarbital treatment on cerebral arteriovenous differences of glucose, lactate, pyruvate, beta-hydroxybutyrate and acetoacetate was studied in suckling rats. The animals were treated from day 2 to 21 after birth by a daily injection of 50 mg/kg phenobarbital or by saline and were studied at 10, 14 and 21 days. Phenobarbital treatment induced a decrease in cerebral arteriovenous difference of glucose at P14 and no change at P10 and P21. The barbiturate did not have any influence on cerebral arteriovenous difference of lactate and pyruvate at the three stages studied. Cerebral uptake of beta-hydroxybutyrate was unchanged at P10 and increased by two-fold at P14 and by threefold at P21 by phenobarbital. Cerebral arteriovenous difference of acetoacetate was low and did not change with the pharmacological treatment. At P14 and P21, the calculated amount of oxygen used by the brain for the oxidation of ketone bodies was twice as high in barbiturate- as in saline-treated rats and reached values of 47 and 16% respectively in phenobarbital-exposed animals. In addition, the barbiturate seemed to affect the carrier process of beta-hydroxybutyrate from blood to brain. The results of the present study are in good agreement with previous data from our laboratory showing that an early chronic phenobarbital treatment is able to induce a shift in the cerebral energy metabolism balance in favor of ketone bodies.

Triacetin: a potential parenteral nutrient

Triacetin, the water-soluble triglyceride of acetate, was infused in mongrel dogs at isocaloric (N = 6) or hypercaloric (approximately 1.5 REE, N = 7) rates in mongrel dogs for 3 hr. Ketone body and glucose production rates were quantified with [13C2] acetoacetate and [3H]glucose, respectively. Four additional animals were infused with glycerol to serve as controls for the hypercaloric triacetin infusion. Energy expenditure was determined in the isocaloric experiments.

RESULTS

no evidence of acute toxicity was observed during triacetin infusion at either rate. Plasma acetate concentrations increased from basal levels to approximately 1 and approximately 13 mmol/liter in the isocaloric and hypercaloric experiments, respectively. Plasma lactate and pyruvate concentrations decreased dramatically after 30 min of both isocaloric and hypercaloric triacetin infusions. Glucose production rates did not increase in either group, but glucose clearance decreased significantly in both groups (p less than 0.05) over the last hour of triacetin infusion. Plasma ketone body concentrations increased from 1.4 to 3.5 and 1.8 to 13.5 mumol/kg.min, respectively, during isocaloric and hypercaloric triacetin infusion. Resting energy expenditure increased from 3.0 +/- 0.3 to 4.0 +/- 0.5 kcal/kg.hr during isocaloric triacetin infusion (p less than 0.05). These studies indicate that triacetin can be administered to dogs at high rates without overt toxicity. The decrease in glucose clearance may represent competition between carbohydrate (glucose) and lipid (acetate). Triacetin infusion resulted in significant increases in ketone body production and concentration. These preliminary data indicate that triacetin may have a future role as a parenteral nutrient, and that further studies of its use are warranted.

The effect of glucose administration on carbohydrate metabolism after head injury

The role of intravenous infusion of glucose in limiting ketogenesis and the effect of glucose on cerebral metabolism following severe head injury were studied in 21 comatose patients. The patients were randomly assigned to alimentation with or without glucose. Systemic protein wasting, arterial concentrations of energy substrates, and cerebral metabolism of these energy substrates were monitored for 5 days postinjury. Both groups were in negative nitrogen balance, and had wasting of systemic proteins despite substantial protein intake. Blood and cerebrospinal fluid (CSF) glucose concentrations were highest on Day 1, but remained higher than normal fasting levels on all days of study, even in the patients who received no exogenous glucose. Although there were no differences in blood or CSF glucose concentrations in the two groups of patients, the glucose group had higher plasma insulin levels, with a mean +/- standard deviation of 14.8 +/- 7.3 microU/ml compared to 10.3 +/- 4.2 microU/ml in the saline group. The blood concentrations of beta-hydroxybutyrate, acetoacetate, pyruvate, glycerol, and the free fatty acids were higher in the saline group than in the glucose group. Cerebral oxygen consumption was similar in the two groups, while the cerebral metabolism of glucose and of the ketone bodies was dependent on whether glucose was administered. In the glucose group, glucose was the only energy substrate utilized by the brain. In the saline group, the ketone bodies beta-hydroxybutyrate and acetoacetate replaced glucose to the extent of 16% of the brain's total energy production. Cerebral lactate production and CSF lactate concentration were lower in the saline group. These studies suggest that administration of glucose during the early recovery period of severe head injury is a major cause of suppressed ketogenesis, and may increase production of lactic acid by the traumatized brain by limiting the availability of nonglycolytic energy substrates.

Effect of thiamin deficiency on energy metabolites in the turkey

The effects of thiamin deficiency on selected energy-related metabolites was investigated. A basal diet (B) was formulated to be 11% of NRC recommended level of 2 mg/kg of thiamin. Thiamin was added to this basal diet to generate the control diet (C). Twenty one-week-old female turkeys were fed either the B or C diet. On days four and five of the experiment, food intake was decreased significantly in B fed turkeys (P < 0.05). Plasma and brain samples were collected at this time. Brains were dissected and analyzed for ATP, ADP, uric acid, free fatty acids, glucose, and GABA. Adenosine triphosphate and the ATP/ADP ratio were decreased in the hindbrain (medulla-pons area) of thiamin deficient birds (P < 0.01). Uric acid was increased (P < 0.001) and free fatty acids were decreased (P < 0.0005) in the plasma of thiamin deficient birds. Based on the data, changes in ATP and ATP/ADP levels may be related to the anorectic behavior exhibited by the thiamin deficient bird.

Effect of starvation on human muscle protein metabolism and its response to insulin

Although starvation is known to impair insulin-stimulated glucose disposal, whether it also induces resistance to insulin's antiproteolytic action on muscle is unknown. To assess the effect of fasting on muscle protein turnover in the basal state and in response to insulin, we measured forearm amino acid kinetics, using [3H]phenylalanine (Phe) and [14C]leucine (Leu) infused systemically, in eight healthy subjects after 12 (postabsorptive) and 60 h of fasting. After a 150-min basal period, forearm local insulin concentration was selectively raised by approximately 25 muU/ml for 150 min by intra-arterial insulin infusion (0.02 mU.kg-1. min-1). The 60-h fast increased urine nitrogen loss and whole body Leu flux and oxidation (by 50-75%, all P less than 0.02). Post-absorptively, forearm muscle exhibited a net release of Phe and Leu, which increased two- to threefold after the 60-h fast (P less than 0.05); this effect was mediated exclusively by accelerated local rates of amino acid appearance (Ra), with no reduction in rates of disposal (Rd). Local hyperinsulinemia in the postabsorptive condition caused a twofold increase in forearm glucose uptake (P less than 0.01) and completely suppressed the net forearm output of Phe and Leu (P less than 0.02). After the 60-h fast, forearm glucose disposal was depressed basally and showed no response to insulin; in contrast, insulin totally abolished the accelerated net forearm release of Phe and Leu. The action of insulin to reverse the augmented net release of Phe and Leu was mediated exclusively by approximately 40% suppression of Ra (P less than 0.02) rather than a stimulation of Rd. We conclude that in short-term fasted humans 1) muscle amino acid output accelerates due to increased proteolysis rather than reduced protein synthesis, and 2) despite its catabolic state and a marked impairment in insulin-mediated glucose disposal, muscle remains sensitive to insulin's antiproteolytic action.

Beneficial effect of 1,3-butanediol on cerebral energy metabolism and edema following brain embolization in rats

We assessed the effect of 1,3-butanediol on cerebral energy metabolism and edema after inducing multifocal brain infarcts in 108 rats by the intracarotid injection of 50-microns carbonized microspheres. An ethanol dimer that induces systemic ketosis, 25 mmol/kg i.p. butanediol was injected every 3 hours to produce a sustained increase in the plasma level of beta-hydroxybutyrate. Treatment significantly attenuated ischemia-induced metabolic changes by increasing the concentrations of phosphocreatine, adenosine triphosphate, and glycogen and by reducing the concentrations of pyruvate and lactate. Lactate concentration 2, 6, and 12 hours after embolization decreased by 13%, 44%, and 46%, respectively. Brain water content increased from 78.63% in six unembolized rats to 80.93% in 12 saline-treated and 79.57% in seven butanediol-treated rats 12 hours after embolization. (p less than 0.05). The decrease in water content was associated with significant decreases in the concentrations of sodium and chloride. The antiedema effect of butanediol could not be explained by an osmotic mechanism since equimolar doses of urea or ethanol were ineffective. Our results support the hypothesis that the beneficial effect of butanediol is mediated through cerebral utilization of ketone bodies arising from butanediol metabolism, reducing the rate of glycolysis and the deleterious accumulation of lactic acid during ischemia.

Removal of an inorganic acid load in subjects with ketoacidosis of chronic fasting

When a large inorganic acid load is ingested by normals, the proton load is eliminated because the rate of excretion of ammonium can rise to 200 to 300 mmol/day. In subjects with ketoacidosis of chronic fasting, such a large increase in the rate of excretion of ammonium might not be possible because of ATP balance considerations in proximal cells. Subjects with ketoacidosis of chronic fasting excreted less net acid as defined in the conventional way when they consumed a large inorganic acid load (136 +/- 6 vs. 176 +/- 26 mmol/day in control fasted subjects). Nevertheless, the vast majority of this inorganic acid load was eliminated because they were in steady state and had only a slightly lower concentration of bicarbonate (13 +/- 0.6 vs. 15 +/- 0.5 mmol/liter) and ketoacid anions (3.3 +/- 0.2 vs. 5.5 +/- 0.2 mmol/liter) in their blood. Using a definition of net acid excretion where the component of bicarbonate loss was expanded to include "potential bicarbonate" (ketoacid anions) in the urine, the rate of excretion of net acid was higher in subjects who ingested the inorganic acid load, owing to a much lower rate of excretion of ketoacid anions (9 +/- 2 vs. 120 +/- 7 mmol/day). This lower rate of excretion was not only due to a lower filtered load, but also to a higher fractional reabsorption of ketoacid anions during acidosis (97 +/- 0.1 vs. 77 +/- 0.2%). This higher fractional reabsorption could not be explained by a lower filtered load of ketoacid anions or to a restricted intake of sodium.(ABSTRACT TRUNCATED AT 250 WORDS)

Fasting prior to transient cerebral ischemia reduces delayed neuronal necrosis

A transient brain ischemia of 30-min duration was induced by the four-vessel occlusion technique in normally fed and in 48-hr-fasted rats. Evaluation of brain damage 72 hr after ischemia showed that fasting reduced neuronal necrosis in the striatum, the neocortex, and the lateral part of the CA1 sector of hippocampus. Signs of status spongiosis in the pars reticulata of the substantia nigra were seen in 75% of fed rats and in only 19% of fasted rats. The protective effect was associated with reduction in mortality and in postischemic seizure incidence. The metabolic changes induced by fasting were evaluated before and during ischemia. After 30 min of four-vessel occlusion, fasted rats showed a marked decrease in brain lactate level (14.7 vs 22.5 mumol/g in fed rats; P less than 0.001). The decrease in brain lactate concentration might explain the beneficial effect of fasting by minimizing the neuropathological consequences of lactic acidosis. Several factors may account for lower lactate production during ischemia in fasted rats: hypoglycemia, reduction in preischemic stores of glucose and glycogen, or increased utilization of ketone bodies aiming at reducing the glycolytic rate.

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)

In vivo utilization of D(-)-3-hydroxybutyrate by chick brain and spinal cord

The in vivo utilization of D-3-hydroxy[3-14C]butyrate for oxidation in the whole animal and for lipid and amino acid synthesis in brain and spinal cord of overnight-fasted 15-day-old chicks has been measured. Appreciable amounts of injected 3-hydroxy[3-14C]butyrate were expired as 14CO2 one hour after injection, the total amount of which increased with increasing dosages. Lipid synthesis was high in both brain and spinal cord. Free cholesterol and phospholipids were the main lipids labeled in both tissues, increasing with time after injection up to 120 min. The incorporation of radioactivity into triglycerides, esterified cholesterol and free fatty acids was not time-dependent. Increased concentrations of 3-hydroxybutyrate gave rise to higher synthetic rates both in brain and spinal cord. The rate of amino acid synthesis was slightly higher in brain than in spinal cord. Glutamate was always the major amino acid formed.

Regional distribution of ethanol-inducible cytochrome P450 IIE1 in the rat central nervous system

A specific form of cytochrome P450, P450 IIE1, active in ethanol oxidation, is known to be induced about 10-fold in rat liver following ethanol treatment. This isozyme of P450 participates effectively in the metabolic activation of precarcinogens, such as N-dimethylnitrosamines, and of solvents such as carbon tetrachloride and benzene. In the present investigation, two different polyclonal antisera against P450 IIE1 were used in order to map the regional distribution of this P450 form in the rat central nervous system. The presence of P450 IIE1 in various brain regions was confirmed by Western blot analysis. The P450 IIE1-immunoreactivity was heterogeneously distributed between brain areas. Neuronal cell bodies and glial cells of presumed astroglial as well as oligodendroglial identity contained immunoreactivity. All fiber tracts harbored P450 IIE1-immunoreactive glial cells as did the ependymal lining of the ventricular wall as well as small and large vessels throughout the brain. P450 IIE1-immunoreactive glial cells were present in all areas of the neocortex, in the olfactory bulb, in the piriform cortex and in several different thalamic nuclei. In the cerebellum, P450 IIE-immunoreactivity was found in all cell layers and was exclusively localized to glial cells and their processes. Staining of blood vessels was prominent in the white matter where P450 IIE1-immunoreactive glial cells were seen to have end-feet on the vessels. A subgroup of pyramidal cells of the frontal cortex showed strong P450 IIE1-immunoreactivity, as did a component of the olfactory nerve which innervates the accessory bulb. In the hippocampal region, the pyramidal cells of all subfields were P450 IIE1-immunoreactive. Some polymorphic cells of the hilus and subfield CA stained intensely with the P450 IIE1 antibodies. A high density of P450 IIE1-immunoreactivity was detected throughout the striatal complex. The immunoreactivity was localized to neuronal cell bodies as well as the neurophil. Fibers of the nigrostriatal system were strongly P450 IIE1-immunoreactive. Mechanical lesions of this pathway showed an accumulation of P450 IIE1-immunoreactivity proximal to the lesion relative to the striatum and a depletion in the reticular part of the substantia nigra, suggesting that the antigen may be transported from the striatum to the substantia nigra. In the brain stem a high density of P450 IIE-immunoreactive neurons was detected in the substantia nigra, the pontine nucleus, lateral superior olive and the nucleus of the trigeminal nerve and facial nucleus. A great number of large- to medium-sized immunoreactive neurons were situated in the central gray and in the reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Beta-hydroxybutyrate and response to hypoxia in the ground squirrel, Spermophilus tridecimlineatus

1. Previous studies have suggested that elevated ketone levels are associated with increased survival time in rodents exposed to hypoxia. In this study the association between whole blood BHB (beta-hydroxybutyrate) and hypoxic survival time was investigated in hibernating and non-hibernating ground squirrels and in rats. 2. Non-hibernating ground squirrels and rats were exposed to hypoxia (4.5% O2). One hundred per cent of ground squirrels survived 1 hr of hypoxia vs 20% of rats. 3. Ketone levels were significantly higher in ground squirrels than rats during hypoxia, and rats surviving the longest had the highest ketone levels. 4. When hibernation was induced in ground squirrels there was a significant increase in beta-hydroxy-butyrate from 0.45 to 1.6 mM (P = 0.0005). 5. Ground squirrel heart mitochondrial respiratory control ratios and ATP synthesis rates indicated no preferential ketone utilization which might suggest a possible extramitochondrial role of BHB during hypoxia. 6. We conclude that elevated blood BHB levels are associated with increased hypoxic survival and they may have evolved in response to life-threatening hypoxia as experienced during hibernation.

Substrate availability other than glucose in the brain during euglycemia and insulin-induced hypoglycemia in dogs

Alternative substrates other than glucose could be used by the brain. In this study we hypothesized that lactate and ketone bodies can provide a significant portion of oxidative brain substrates in insulin-dependent diabetes mellitus (IDDM). Six control (C) and six insulin-treated streptozotocin diabetic (IDDM) dogs were studied during euglycemia (EU) and acute insulin induced hypoglycemia (HYPO). During EU for similar plasma glucose concentration (5.5 +/- 0.4 v 5.2 +/- 0.2 mmol/L in IDDM dogs showed a higher baseline lactate concentration (1.5 +/- 0.25 v 0.74 +/- 0.10 mmol/L; P less than .05). The ketone body concentrations were also increased in IDDM dogs but this increase was not statistically significant. The brain glucose uptake was 6.9 +/- 0.6 mumol/kg/min in C and 5.4 +/- 0.7 in IDDM. Lactate was released by the brain both in IDDM dogs (11.36 +/- 1.8 mumol/kg/min) and in C dogs (3.87 +/- 0.9; P less than .05). The brain ketones rate of disappearance (Rd) was 0.3 +/- 0.05 mumol/kg/min in IDDM dogs and 0.19 +/- 0.08 in C dogs. During HYPO the glucose uptake across the brain was 2.88 +/- 0.7 mumol/kg/min in IDDM and 3.12 +/- 0.5 in C dogs. We observed an overall brain lactate release (3.21 +/- 1.7 mol/kg/min) in C dogs and a net uptake (13.44 +/- 1.1; P less than .01) in IDDM (P less than .01). The brain ketones Rd was 0.1 +/- 0.2 mumol/kg/min in IDDM and 0.1 +/- 0.1 in C dogs.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein and amino acid metabolism during early starvation as reflected by excretion of urea and methylhistidines

Endogenous excretion of nitrogenous products was studied during early starvation in six healthy, nonobese subjects after six days on a well-defined diet, designed to achieve net protein balance and an adequate calorie supply. The diet contained 0.5 g myofibrillar-free protein and 35 kcal/kg body weight. The subjects then fasted for three days. Urine was collected for 24-hour periods and analyzed for urea, ammonia, 3-methylhistidine, and 1-methylhistidine. Blood glucose and serum urea levels were measured daily. In a second group of subjects, muscle biopsies for determination of free amino acid concentrations were taken in the overnight fasted state and after three days of fasting. During the period with a balanced diet, urea production fell initially and stabilized after two to three days at a level of 146 +/- 15 mmol/24 h. During the period of fasting, serum urea increased from 3.0 +/- 0.4 to a maximum value of 6.2 +/- 0.7 mmol/L and urea production rose markedly, to a peak of 293 +/- 16 mmol/24 h. Ammonia excretion was 24 +/- 2 mmol/24 h before and 71 +/- 13 mmol/24 h after three days of fasting. 3-Methylhistidine excretion was stable before fasting and then rose from 154 +/- 17 to 198 +/- 17 mumol/24 h. 1-Methylhistidine excretion was unchanged during fasting. Blood glucose levels were stable at 4.8 +/- 0.2 mmol/L before fasting and then fell to 3.7 +/- 0.3 mmol/L. Intracellular concentrations of amino acids in skeletal muscle decreased markedly during fasting; after three days of fasting the glutamine concentration had fallen by 34%.(ABSTRACT TRUNCATED AT 250 WORDS)

Fasting for 72 h decreases the responses of counterregulatory hormones to insulin-induced hypoglycaemia in normal man

We have evaluated the influence of fasting on the response of counterregulatory hormones to insulin-induced hypoglycaemia. Eight healthy, non-obese volunteers were studied after an overnight fast and again after a 72-h fast period. Levels of blood glucose were higher after overnight fasting (4.59 +/- 0.10 mmol/l) than after 72 h of fasting (3.38 +/- 0.12 mmol/l). Hypoglycaemia was induced by a constant insulin infusion (2.4 mU/kg/min) and clamped between 2.1 and 2.3 mmol/l of glucose by a variable glucose infusion. Hypoglycaemia evoked stimulation of glucagon release after the overnight fast but did not alter release after 72 h of fasting. The response of other counterregulatory hormones were also influenced by the longer fasting period: the normal rise in adrenaline levels during hypoglycaemia was delayed and attenuated and the normal rise in cortisol levels was absent; paradoxically, cortisol levels decreased during hypoglycaemia. Seventy-two hours of fasting, therefore, profoundly alters hormonal responses to hypoglycaemia.

Effects of free fatty acids and ketone bodies on in vivo non-insulin-mediated glucose utilization and production in humans

The current study was undertaken to examine the effect of an acute elevation of serum levels of free fatty acids (FFA) and ketone bodies (KB) on non-insulin-mediated glucose uptake (NIMGU) in humans. The study group consisted of 11 healthy men, mean age (+/- SD) 30 (+/- 7) years and mean weight (+/- SD) 72 (+/- 7) kg. To examine the effects of FFA levels on NIMGU and insulin-mediated glucose uptake (IMGU), glucose uptake was measured isotopically (3H-3-glucose) in six subjects on four separate days during saline infusion or lipid + heparin infusion with concomitant infusions of somatostatin (SHIF, 0.16 micrograms/kg/min) with or without insulin infusion (40 mU/m2/min) while the serum glucose level was clamped at approximately 11 mmol/L. To examine the effect of KB on NIMGU, saline or sodium acetoacetate (20 mumol/kg/min) was infused in five subjects on separate days during SRIF-induced insulinopenia while the serum glucose level was clamped sequentially at euglycemia and at approximately 11 mmol/L. During insulinopenia basal FFA levels rose twofold during saline infusion and sixfold during infusion of lipid + heparin. Rates of NIMGU were 2.49 +/- 0.27 v 2.41 +/- 0.14 mg/kg/min during saline and lipid infusion, respectively (P = NS). Rates of IMGU were decreased by 55% during lipid + heparin infusion. During insulinopenia basal beta-hydroxybutyrate (BOB) levels rose twofold during saline and approximately 11-fold during sodium acetoacetate infusion. Rates of NIMGU were unchanged by the sodium acetoacetate infusion at euglycemia and hyperglycemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of momentary stress on brain energy metabolism in weanling mice: apparent use of lactate as cerebral metabolic fuel concomitant with a decrease in brain glucose utilization

The hypothesis that the anxiety induced by repeated injections affects brain energy metabolism was tested. Normal 19- to 21-day-old mice were stressed by two sham intraperitoneal injections within 4 min, at which time they were decapitated. Noninjected, control littermates were quickly decapitated. Momentary stress increased plasma glucose (12%), glycerol (85%), beta-hydroxybutyrate (108%), and lactate (153%)--a reflection of elevated plasma cortisol (25%) and glucagon (45%). In brain, stress increased levels of glucose-6-P (15%) and fructose-6-P (17%). The brain pyruvate concentration increased 74%; lactate 76%. Citrate, alpha-ketoglutarate, and malate increased 15, 95, and 37%, respectively. Levels of glycogen, glucose, phosphocreatine, ATP, ADP, and AMP were unchanged. The brain lactate/pyruvate ratio was normal but the brain/plasma lactate ratio fell 32%. Metabolite changes in the stressed animals were compatible with a decrease in the glycolytic flux at the phosphofructokinase step and a paradoxical increased flux in the Krebs citric acid cycle. The decreased brain/plasma lactate ratio supported increased uptake of lactate from plasma and increased brain lactate oxidation. Metabolite changes similar to those described above occurred in unstressed mice injected with lactate. Findings confirm a positive effect of stress on brain metabolism, support a role for lactate as an oxidative fuel for brain, and caution that the rate of cerebral glucose utilization may not always reflect brain energy (oxidative) metabolism accurately.

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.

Generalized decrease in brain glucose metabolism during fasting in humans studied by PET

In prolonged fasting, the brain derives a large portion of its oxidative energy from the ketone bodies, beta-hydroxybutyrate and acetoacetate, thereby reducing whole body glucose consumption. Energy substrate utilization differs regionally in the brain of fasting rat, but comparable information has hitherto been unavailable in humans. We used positron emission tomography (PET) to study regional brain glucose and oxygen metabolism, blood flow, and blood volume in four obese subjects before and after a 3-wk total fast. Whole brain glucose utilization fell to 54% of control (postabsorptive) values (P less than 0.002). The whole brain rate constant for glucose tracer phosphorylation fell to 51% of control values (P less than 0.002). Both parameters decreased uniformly throughout the brain. The 2-fluoro-2-deoxy-D-glucose lumped constant decreased from a control value of 0.57 to 0.43 (P less than 0.01). Regional blood-brain barrier transfer coefficients for glucose tracer, regional oxygen utilization, blood flow, and blood volume were unchanged.

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)

Adaptation to mild hypoglycaemia in normal subjects despite sustained increases in counter-regulatory hormones

In diabetes, loss of awareness of and a defective hormonal response to hypoglycaemia have been associated with long disease duration, improved glycaemic control and possibly a change in insulin species. In contrast it is assumed that normal subjects always have symptoms when their blood glucose is low. We have tested this in 7 normal subjects at 3 levels of blood glucose (4.5, 3.5 and 3.0 mmol/l) using a hyperinsulinaemic glucose clamp with a euglycaemic (4.5 mmol/l) clamp as a control. After 60 min at a blood glucose of 3.5 mmol/l adrenaline and glucagon increased slightly but significantly, whereas cortisol, growth hormone and pancreatic polypeptide were unchanged. As soon as glucose was lowered to 3.0 mmol/l adrenaline increased to 1.10 nmol/l and rose further to 1.43 nmol/l after 60 min. Glucagon secretion increased similarly but other counter-regulatory hormones were significantly raised only after 60 min at 3.0 mmol/l. Awareness of hypoglycaemia (symptom score) increased after 40 min at a blood glucose of 3.0 mmol/l but after 60 min decreased to baseline levels with loss of awareness in 5 subjects. Reaction time improved in parallel with the change in symptom score. Thus, despite high levels of adrenaline, normal subjects lose awareness during sustained mild hypoglycaemia. Improved reaction time may reflect cerebral adaptation.

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.