Endocrine regulation of the hepatic fasting response: cues, cooperation and consequences.

Respiratory failure is a major cause of mortality during septic shock and is due in part to decreased ventilatory muscle contraction. Ventilatory muscles have high energy demands; fatty acid (FA) oxidation is an important source of ATP. FA oxidation is regulated by nuclear hormone receptors; studies have shown that the expression of these receptors is decreased in liver, heart, and kidney during sepsis. Here, we demonstrate that lipopolysaccharide (LPS) decreases FA oxidation and the expression of lipoprotein lipase (LPL), FA transport protein 1 (FATP-1), CD36, carnitine palmitoyltransferase beta, medium chain acyl-CoA dehydrogenase (MCAD), and acyl-CoA synthetase, key proteins required for FA uptake and oxidation, in the diaphragm. LPS also decreased mRNA levels of PPARalpha and beta/delta, RXRalpha, beta, and gamma, thyroid hormone receptor alpha and beta, and estrogen related receptor alpha (ERRalpha) and their coactivators PGC-1alpha, PGC-1beta, SRC1, SRC2, Lipin 1, and CBP. Zymosan resulted in similar changes in the diaphragm. Finally, in PPARalpha deficient mice, baseline CPT-1beta and FATP-1 levels were markedly decreased and were not further reduced by LPS suggesting that a decrease in the PPARalpha signaling pathway plays an important role in inducing some of these changes. The decrease in FA oxidation in the diaphragm may be detrimental, leading to decreased diaphragm contraction and an increased risk of respiratory failure during sepsis.

The accumulation of triglycerides (TG) in the liver, designated hepatic steatosis, is characteristically associated with obesity and insulin resistance, but it can also develop after fasting. Here, we show that fasting-induced hepatic steatosis is under genetic control in inbred mice. After a 24-h fast, C57BL/6J mice and SJL/J mice both lost more than 20% of body weight and approximately 60% of total body TG. In C57BL/6J mice, TG accumulated in liver, producing frank steatosis. In striking contrast, SJL/J mice failed to accumulate any hepatic TG even though they lost nearly as much adipose tissue mass as the C57BL/6J mice. Mice from five other inbred strains developed fasting-induced steatosis like the C57BL/6J mice. Measurements of the uptake of free fatty acids (FA) in vivo and in vitro demonstrated that SJL/J mice were protected from steatosis because their heart and skeletal muscle took up and oxidized twice as much FA as compared with C57BL/6J mice. As a result of this muscle diversion, serum-free FA and ketone bodies rose much less after fasting in SJL/J mice as compared with C57BL/6J mice. When livers of SJL/J and C57BL/6J mice were perfused with similar concentrations of FA, the livers took up and esterified similar amounts. We conclude that SJL/J mice express one or more variant genes that lead to enhanced FA uptake and oxidation in muscle, thereby sparing the liver from FA overload in the fasting state.

After completing this article, readers should be able to: Hypoglycemia is a common problem in neonates that has many causes. This review focuses on metabolic disorders that may be associated with hypoglycemia in the neonatal period.During intrauterine life, the fetus derives fuel, as glucose, from the mother via the placenta. After birth, the energy demands on the former fetus increase dramatically. The baby now must maintain its own body temperature and must undertake the work of breathing and other activities. Further, the maintenance of blood glucose levels requires glycogenolysis and gluconeogenesis. Postnatally, there are four sources of glucose: dietary glucose; glucose derived from the cleavage of more complex sugars in the gut (eg, lactose to glucose and galactose); glucose released from glycogen stores (primarily in the liver); and gluconeogenesis, in which glucose is synthesized from carbon skeletons derived from certain amino acids using energy derived from catabolism of fatty acids.Most term infants have sufficient glycogen stores to maintain blood glucose levels for several hours before gluconeogenesis is required. Infants who are breastfed in the United States typically are offered only water as a supplement to human milk during the first few postnatal days, a period when the mother's milk supply is not yet established. In contrast, formula-fed babies receive calories by mouth by the end of the first postnatal day. Consequently, the metabolic stress of prolonged fasting occurs more frequently in breastfed than in formula-fed babies. Although breastfed babies have higher levels of ketone bodies that appear to provide adequate energy during this high metabolic stress period, they may be more vulnerable to metabolic disorders that limit ketone production.Hypoglycemia is defined on the basis of symptoms and blood glucose levels. Most authorities regard a blood glucose level below 40 mg/dL (2.3 mmol/L) as low, regardless of the presence of clinical signs. (Blood glucose concentrations may be transiently lower in the first hours after birth.) Some infants exhibit clinical findings (eg, jitteriness, lethargy) at higher levels that respond immediately to glucose, suggesting that their blood glucose concentrations were too low for adequate function. Some of the symptoms of hypoglycemia (eg, lethargy) are due to lack of glucose; others (eg, jitteriness) are due to the hormonal response to hypoglycemia (especially increased catecholamine release). Apnea or seizures may occur, and there may be cardiac dysfunction.The disorders discussed in this article are presented in order based on the glucose source affected (ie, digestion and absorption, glycogenolysis, and gluconeogenesis).Disorders of absorption or digestion rarely are encountered in newborns; if they are present (as in lactose intolerance), they rarely are sufficiently severe to result in hypoglycemia. However, they typically cause significant diarrhea.Hepatocellular dysfunction from any cause may lead to hypoglycemia; the liver dysfunction should be obvious if there is jaundice. Infection and galactosemia are common causes. Galactosemia due to galactose-1-uridyl phosphate uridyltransferase deficiency commonly causes hepatic dysfunction, but may not cause marked hypoglycemia. Its diagnosis in the newborn period is critical because of the associated liver and renal dysfunction, cerebral edema, and cataracts and the risk of gram-negative sepsis. When suspected, all intake of galactose (human milk and cow milk formulas) must cease. The diagnosis can be suspected on the basis of positive reducing substances in the urine and confirmed by metabolite or enzyme assay. Urine obtained more than 1 day after cessation of galactose intake may be negative for reducing substances. Tyrosinemia may cause a similar picture of hepatocellular and renal dysfunction. Fructose intolerance appears similarly, but infants usually are not exposed to fructose or sucrose.The first glycogen storage disease discovered, von Gierke disease (GSD 1) is a defect of glucose release from hepatic cells due to abnormalities of glucose-6-phosphatase. Several subcategories exist. GSD 1 actually is a mixed disorder because glucose-6-phosphate, the substrate for the abnormal enzyme, is derived from both glycogen breakdown and gluconeogenesis. The result of the enzymatic defect is hypoglycemia as soon as intestinal sources of glucose are exhausted (typically 2 h after a feeding) and production of alternate fuels—lactate and ketone bodies—begins. It is not uncommon for an infant who has GSD 1 to have a blood glucose level of 20 mg/dL (1.1 mmol/L) and minimal symptoms due to the presence of alternate substrates.The other major defects of glycogenolysis are deficiencies of glycogen debrancher enzyme, liver phosphorylase, and the phosphorylase kinase system. These conditions usually are silent in the newborn period because prolonged fasting is rare. Fasting hypoglycemia without acidosis occurs after several hours. Hepatomegaly occurs within a few months and consists of both increased glycogen and fat. (Splenomegaly rarely is found in glycogen storage disorders, which is an important differential point.) Disorders due to impaired glycogen synthesis include brancher deficiency, which causes cirrhosis and may cause cardiomyopathy, and the very rare glycogen synthase deficiency (GSD type 0).The fasting that accompanies the first few days of breastfeeding is a major test of gluconeogenesis. Accordingly, defects that impair gluconeogenesis may result in significant and catastrophic decompensation. Disorders due to impaired fatty acid oxidation can result in hypoglycemia, with the added problem of the accumulation of toxic intermediates. The most common disorder of fatty acid oxidation is medium-chain acyl CoA dehydrogenase (MCAD) deficiency, which occurs in perhaps 1 in 10,000 people of northern European descent. A few percent of MCAD-deficient infants, especially breastfed ones, experience an episode of hypoglycemia in the first few postnatal days. However, most affected infants do not have symptoms until a few months of age or even later. Decompensations often are provoked by infection in conjunction with fasting and may be exacerbated by carnitine depletion. The response to intravenous administration of glucose may be slow, with the blood glucose concentration rising but the lethargy persisting, which reflects the toxicity of accumulated metabolic intermediates.Other disorders of fatty acid oxidation that may present in the newborn period are the defects of long-chain fatty acid oxidation. Cardiomyopathy, encephalopathy, and hepatic dysfunction may be prominent in deficiencies of very-long-chain acyl CoA dehydrogenase, long-chain hydroxyacyl CoA dehydrogenase (LCHAD), carnitine-acylcarnitine translocase, and carnitine palmitoyltransferases I and II. LCHAD deficiency in the fetus can provoke significant liver dysfunction (HELLP syndrome, acute fatty liver of pregnancy) in the heterozygous mother, although most cases of these maternal conditions are unrelated to LCHAD deficiency.Mild hypoglycemia certainly can occur in various organic acidurias (eg, propionic and methylmalonic acidemia, maple syrup urine disease), but the presenting urgent problems in these disorders most commonly are ketoacidosis, lactic acidosis, and hyperammonemia, with associated encephalopathy. Other causes of hepatocellular dysfunction also can lead to hypoglycemia, but the diagnosis in such cases generally is evident because of the presence of laboratory values suggestive of liver failure.Hyperinsulinism is a metabolic disorder that affects both glycogenolysis and gluconeogenesis and most commonly reflects the presence of maternal hyperglycemia. Other causes are fetal overgrowth syndromes, especially Beckwith-Wiedemann syndrome, and overgrowth of the pancreas, previously referred to as nesidioblastosis. (See Genetic and Nongenetic Forms of Hyperinsulinism in Neonates in this issue.)Other hormones involved in glucose regulation include glucagon, cortisol, growth hormone, thyroid hormone, and catecholamines. Deficiencies of any of these hormones from structural or functional defects may be associated with neonatal hypoglycemia. Growth hormone deficiency may be silent in the newborn period because insulin is a more important growth hormone in the fetus. Cortisol deficiency (as occurs in congenital adrenal hyperplasia) also may be cryptic initially, but it may present with subsequent complete metabolic collapse later in the first 2 postnatal weeks, with accompanying salt-wasting, hypoglycemia, and circulatory collapse.The congenital disorders of glycosylation (CDGs) (formerly carbohydrate-deficient glycoprotein disorders) form a new category of disorders that lead to impaired synthesis of many molecules, including hormone and lipid carriers. Hypoglycemia often occurs, and plasma cholesterol also may be very low in these conditions. Deficient steroid-binding protein leads to functional cortisol deficiency, while (pseudo)hypothyroidism may be detected by newborn screening because of low thyroid-binding globulin. CDG 1a, which is due to phosphomannomutase 2 deficiency, has accompanying malformations, including cerebellar hypoplasia. CDG 1b, with an associated defect in phosphomannose isomerase, may present with hypoglycemia followed by protein-losing enteropathy and hepatic fibrosis. This condition is treated successfully with oral mannose. Many other CDGs are known. All are recognized by isoelectric focusing of transferrin.The pregnancy and history of feeding and fasting can point to likely causes of hypoglycemia. Investigation of hypoglycemia includes the family history, pregnancy history (with particular reference to weight gain and glucose tolerance), peculiarities regarding labor and delivery, examination of the placenta (not always done), and examination of the infant. Common causes of neonatal hypoglycemia, such as sepsis, intrauterine growth restriction, and transient hyperinsulinism, must be ruled out before more unusual diagnoses are entertained. The feeding history and risk factors for infection are especially important. Overgrowth or intrauterine starvation is obvious. Prenatal infection can cause placental insufficiency, leading to intrauterine starvation with subsequent hypoglycemia and hepatic dysfunction, which can exacerbate abnormalities of glucose homeostasis.Essentially all of the metabolic disorders discussed in this article are inherited in an autosomal recessive pattern. The family history may be positive for similarly affected siblings or unexplained infant deaths. Consanguinity usually is not present, but can suggest the presence of a metabolic disorder that has a recessive inheritance when it is. Genital abnormalities (eg, virilization, hyperpigmentation) can point to adrenal hyperplasia. Midline facial defects may suggest abnormalities of pituitary function. MCAD deficiency is characterized by acute illness, not chronic problems. In contrast, infants who have organic acidurias may experience chronic feeding difficulties, but may not have complete metabolic collapse until after the first several days to a few weeks after birth. An increasing number of metabolic disorders, including some discussed here, can be identified on the initial routine newborn screen.A blood sample obtained just before glucose is administered can provide invaluable information later, so it should be obtained if at all possible. This sample offers convincing information regarding insulin and other hormones, which changes rapidly after glucose is administered. The blood glucose test strip, based on glucose oxidase, is a rapid but not always reliable test at low levels, so abnormalities must be confirmed with a proper blood glucose determination. Samples for insulin, cortisol, growth and thyroid hormones, electrolytes, ammonia, amino acids, carnitine and acylcarnitines, blood culture, blood counts, and liver function/transaminases address most of the potential causes, but not all of these tests are needed in a given situation.Measurement of blood electrolytes, with calculation of the anion gap, can suggest acidosis and the presence of a missing anion (usually lactate or ketone bodies). The arterial pH may be normal, even in the presence of significant acidosis, because of respiratory compensation. If acidosis is suspected, lactate should be measured directly. Other blood tests should include measurement of insulin and other hormones (growth hormone, thyroid hormone, cortisol) and plasma amino acid analysis. Special attention should be paid to alanine (which reflects elevation of pyruvate and lactate) and the gluconeogenic amino acids. Analysis of blood spot acylcarnitines can reveal MCAD deficiency or other disorders of fatty acid oxidation rapidly.Urinalysis should be performed in all cases of suspected metabolic disorders, although the clinician should remember that the dipstick does not discriminate between the various reducing sugars. The dipstick also does not detect beta-hydroxybutyrate, a ketone body. Measurement of urine organic acids can reveal excessive lactate, ketone bodies, and the metabolites of organic acidurias and fatty acid oxidation defects. The acylcarnitine profile generally is abnormal in the presence of fatty acid oxidation defects, but infants who have organic acidurias may have normal organic acid levels between episodes of acute decompensation. A fasting stress test may be necessary to reveal a deficient hormonal response to hypoglycemia. Because such a test can be dangerous in MCAD deficiency and fatty acid oxidation defects, it should be undertaken only after these disorders have been ruled out by acylcarnitine analysis.The introduction of tandem mass spectrometry for newborn screening is leading to early diagnosis of many life-threatening disorders. At least 30 state newborn screening programs in the United States have adopted or are evaluating this technique, and two private laboratories also are offering it. The technology allows the separation of complex mixtures (extracts of dried blood spots) and identification of components of interest in about 2 minutes. (In comparison, urine organic acid analysis by gas chromatography-mass spectrometry can take 40 min per sample after sample preparation; quantitative amino acid analysis by column chromatography can take a few hours per sample.) The technique is conceptually simple: mass spectrometers "weigh" molecules (ie, determine their mass). Two mass spectrometers coupled in series, therefore, can determine the mass of a parent molecule and fragments derived from the parent.The addition of acylcarnitine profiling to newborn screening panels allows the identification of nearly all children who have the various fatty acid oxidation defects. Acylcarnitines share a common core and differ in their side chains, which have different masses. More than a dozen different disorders of fatty acid and organic acid metabolism can be distinguished rapidly by the different acylcarnitines that accumulate because of impaired enzyme activity (eg, various acyl-CoA dehydrogenase deficiencies). Many different amino acids also can be determined in the same instrument at the same time, leading to rapid diagnosis of more than a dozen aminoacidopathies, such as phenylketonuria, tyrosinemia, and maple syrup urine disease. The technique also can be used to identify infants who have many disorders of organic acid and urea cycle metabolism.Batched, semiautomated preparation of samples makes it possible to analyze several hundred samples per day on a single instrument. Testing usually is performed on a sample obtained between 48 and 72 hours of age. The sample must be dried, shipped to the screening laboratory, and analyzed. Thus, the infant may be nearly 1 week old before the results are known. During this interval, the disorders that are provoked by fasting in the newborn period, especially MCAD deficiency, already may have presented clinically.In some cases, the newborn screening test provides a definitive diagnosis; in others, the abnormality may be subtle, requiring repeat testing or different tests. A normal newborn screening result cannot rule out a particular disorder completely, so sick infants or children in whom metabolic disease is suspected should be evaluated as if newborn screening had not been performed.Although rare, metabolic disorders can lead to significant morbidity and mortality due to severe hypoglycemia and metabolic collapse. Breastfed infants may be at increased risk, particularly from disorders of ketogenesis, during the first 48 hours of postnatal life. Fortunately, improved techniques for newborn screening can help to identify many affected infants before they present clinically.

In a series of experiments with male rat livers perfused with or without 5-tetradecyloxy-2-furoic acid (TOFA) in the presence and absence of oleate, the relationships between fatty acid synthesis, oxidation, and esterification from newly synthesized and exogenous fatty acid substrates have been examined. When livers from fed rats were perfused without exogenous fatty acid substrate, 20% of the triglyceride secreted was derived from de novo fatty acid synthesis. Addition of TOFA caused immediate and nearly complete inhibition of fatty acid synthesis, measured by incorporation of 3H2O into fatty acids. Concurrently, ketone body production increased 140% and triglyceride secretion decreased 84%. These marked reciprocal alterations in fatty acid synthesis and oxidation in the liver almost completely abolished the production of very low density lipoproteins (VLDL). Cholesterol synthesis was also depressed by TOFA, suggesting that this drug also inhibited lipid synthesis at a site other than acetyl-CoA carboxylase. When livers from fed rats were supplied with a continuous infusion of [1-14C]oleate as exogenous substrate, similar proportions, about 45-47%, of both ketone bodies and triglyceride in the perfusate were derived from the infused [1-14C]oleate. The production of ketone bodies was markedly increased by TOFA; the secretion of triglyceride and cholesterol were decreased. Altered conversion of [1-14C]oleate into these products occurred in parallel. While TOFA decreased esterification of oleate into triglyceride, incorporation of [1-14C]oleate into liver phospholipid was increased, indicating that TOFA also affected glycerolipid synthesis at the stage of diglyceride processing. The decreased secretion of triglyceride and cholesterol following TOFA treatment was localized almost exclusively in VLDL. The specific activities of 3H and of 14C fatty acids in triglyceride of the perfusate were greater than those of liver triglyceride, indicating preferential secretion of triglyceride produced from both de novo fatty acid synthesis and from infused free fatty acid substrate. These observations suggest the following chain of events in the liver following TOFA treatment: inhibition of fatty acid and cholesterol synthesis; increased fatty acid oxidation and ketogenesis; decreased triglyceride synthesis as a result of inhibition of fatty acid synthesis, stimulation of fatty acid oxidation, and altered partition of diglyceride between triglyceride and phospholipid synthesis; and decreased production of VLDL. These comparative rat liver perfusion experiments indicate that free fatty acids provide the major source of substrate for the hepatic production of triglyceri

PO-305 An 8-week, low carbohydrate, high fat, ketogenic diet enhanced exercise capacity through improved ketolysis and lipolysis in mice

Interrelationship of free fatty acids and glucose metabolism in the dog.

Obesity affects the cardiovascular system at many different levels, including the heart muscle itself. Clinical and experimental studies have shown an accumulation of triglycerides and other lipid species in cardiomyocytes. Analogous to hepatic steatosis, investigators have introduced the term “cardiac steatosis”. The present review addresses the complex relationships between cardiac fuel homeostasis, insulin resistance, and proposed mechanisms of damage to cardiomyocytes in different models of obesity, insulin resistance, and lipotoxicity. Specifically, the review weighs the evidence whether there is a heart muscle disorder in human obesity. It discusses how adipokines can modulate cardiac metabolism, and it focuses on the metabolic remodeling accompanying increased fatty acid supply in the heart of rodent models of lipotoxicity, with special attention to the role played by mitochondrial uncoupling and futile cycling. We stress the notion that, in spite of the many proposed mechanisms, cardiac lipotoxicity is still a hypothesis rather than an established pathophysiologic principle. Although the concept of a “lipotoxic cardiomyopathy” seems attractive, we propose instead a series of steps on a path from adaptation to maladaptation of the heart in obesity. A case in point is insulin resistance of the heart which may be both adaptive (protecting the heart from excess fuel) or maladaptive (associated with reactive oxygen species formation and activation of signaling pathways of programmed cell death). The present literature reflects an extraordinary complexity of the heart’s metabolic, functional and structural changes in obesity.

Obese individuals are both insulin resistant and have high levels of circulating free fatty acids (FFAs). In cell culture, saturated but not unsaturated fatty acids induce endoplasmic reticulum (ER) stress. We hypothesized that chronic exposure to low dose fatty acids would significantly attenuate the acute stress response to a saturated fatty acid challenge and that unsaturated fatty acids (oleate) would be more protective than saturated fatty acids (palmitate). The ER stress response to palmitate was reduced after low dose fatty acid exposure in human hepatoma cells. Palmitate and oleate gave distinctive transcript responses, both acutely and after chronic low dose exposure. Differentially regulated pathways included lipid, cholesterol, fatty acid, and triglyceride metabolism, and IkappaB kinase and nuclear factor kappaB kinase inflammatory cascades. Oleate reduced palmitate-induced changes significantly more than low dose palmitate and completely blocked palmitate-induced phosphoinositide 3 kinase inhibitor (PIK3IP1) as well as induction of GADD45A and B. These changes are predicted to alter the PI3 kinase pathway and the pro-apoptotic p38 MAPK pathway. We recapitulated the oleate response by small interfering RNA-mediated block of PIK3IP1 stimulation with palmitate and significantly protected cells from palmitate-mediated ER stress. We show that transcriptional responses to oleate and palmitate are distinct, broad, and often discordant. We identified several potential candidates that may direct the transcriptional networks and demonstrate that PIK3IP1 partially accounts for the protective effects of oleate.

We previously showed that a fraction of the acetyls used to synthesize malonyl-CoA in rat heart derives from partial peroxisomal oxidation of very long and long-chain fatty acids. The 13C labeling ratio (malonyl-CoA)/(acetyl moiety of citrate) was >1.0 with 13C-fatty acids, which yields [13C]acetyl-CoA in both mitochondria and peroxisomes and < 1.0 with substrates, which yields [13C]acetyl-CoA only in mitochondria. In this study, we tested the influence of 13C-fatty acid concentration and chain length on the labeling of acetyl-CoA formed in mitochondria and/or peroxisomes. Hearts were perfused with increasing concentrations of labeled docosanoate, oleate, octanoate, hexanoate, butyrate, acetate, or dodecanedioate. In contrast to the liver, peroxisomal oxidation of 1-13C-fatty acids in heart does not form [1-13C]acetate. With [1-13C]docosanoate and [1,12-13C2]dodecanedioate, malonyl-CoA enrichment plateaued at 11 and 9%, respectively, with no detectable labeling of the acetyl moiety of citrate. Thus, in the intact rat heart, docosanoate and dodecanedioate appear to be oxidized only in peroxisomes. With [1-13C]oleate or [1-13C]octanoate, the labeling ratio >1 indicates the partial peroxisomal oxidation of oleate and octanoate. In contrast, with [3-13C]octanoate, [1-13C]hexanoate, [1-13C]butyrate, or [1,2-13C2]acetate, the labeling ratio was <0.7 at all concentrations. Therefore, in rat heart, (i) n-fatty acids shorter than 8 carbons do not undergo peroxisomal oxidation, (ii) octanoate undergoes only one cycle of peroxisomal beta-oxidation, (iii) there is no detectable transfer to the mitochondria of acetyl-CoA from the cytosol or the peroxisomes, and (iv) the capacity of C2-C18 fatty acids to generate mitochondrial acetyl-CoA decreases with chain length.

the heart is an omnivore, able to oxidize a variety of substrates to support ATP synthesis: fatty acids, glucose, lactate, and even some amino acids ([Fig. 1][1]). Substrate selection is a highly dynamic process. For example, the uptake and utilization of substrates change from being primarily

Fasting readily induces hepatic steatosis. Hepatic steatosis is associated with hepatic insulin resistance. The purpose of the present study was to document the effects of 16 h of fasting in wild-type mice on insulin sensitivity in liver and skeletal muscle in relation to 1) tissue accumulation of triglycerides (TGs) and 2) changes in mRNA expression of metabolically relevant genes. Sixteen hours of fasting did not show an effect on hepatic insulin sensitivity in terms of glucose production in the presence of increased hepatic TG content. In muscle, however, fasting resulted in increased insulin sensitivity, with increased muscle glucose uptake without changes in muscle TG content. In liver, fasting resulted in increased mRNA expression of genes promoting gluconeogenesis and TG synthesis but in decreased mRNA expression of genes involved in glycogenolysis and fatty acid synthesis. In muscle, increased mRNA expression of genes promoting glucose uptake, as well as lipogenesis and beta-oxidation, was found. In conclusion, 16 h of fasting does not induce hepatic insulin resistance, although it causes liver steatosis, whereas muscle insulin sensitivity increases without changes in muscle TG content. Therefore, fasting induces differential changes in tissue-specific insulin sensitivity, and liver and muscle TG contents are unlikely to be involved in these changes.

A ketogenic diet (KD) characterized by very low carbohydrate intake and high fat consumption may simultaneously induce weight loss and be cardioprotective. The "thrifty substrate hypothesis" posits that ketone bodies are more energy efficient compared with other cardiac oxidative substrates such as fatty acids. This work aimed to study whether a KD with presumed increased myocardial ketone body utilization reduces cardiac fatty acid uptake and oxidation, resulting in decreased myocardial oxygen consumption (MVO2 ). This randomized controlled crossover trial examined 11 individuals with overweight or obesity on two occasions: (1) after a KD and (2) after a standard diet. Myocardial free fatty acid (FFA) oxidation, uptake, and esterification rate were measured using dynamic [11 C]palmitate positron emission tomography (PET)/computed tomography, whereas MVO2 and myocardial external efficiency (MEE) were measured using dynamic [11 C]acetate PET. The KD increased plasma β-hydroxybutyrate, reduced myocardial FFA oxidation (p < 0.01) and uptake (p = 0.03), and increased FFA esterification (p = 0.03). No changes were observed in MVO2 (p = 0.2) or MEE (p = 0.87). A KD significantly reduced myocardial FFA uptake and oxidation, presumably by increasing ketone body oxidation. However, this change in cardiac substrate utilization did not improve MVO2 , speaking against the thrifty substrate hypothesis.

Chronic alcohol consumption is associated with fatty liver disease in mammals. The object of this study was to gain an understanding of dysregulated lipid metabolism in alcohol-fed C57BL/6 mice using a targeted lipidomic approach. Liquid chromatography tandem mass spectrometry was used to analyze several lipid classes, including free fatty acids, fatty acyl-CoAs, fatty acid ethyl esters, sphingolipids, ceramides, and endocannabinoids, in plasma and liver samples from control and alcohol-fed mice. The interpretation of lipidomic data was augmented by gene expression analyses for important metabolic enzymes in the lipid pathways studied. Alcohol feeding was associated with i) increased hepatic free fatty acid levels and decreased fatty acyl-CoA levels associated with decreased mitochondrial fatty acid oxidation and decreased fatty acyl-CoA synthesis, respectively; ii) increased hepatic ceramide levels associated with higher levels of the precursor molecules sphingosine and sphinganine; and iii) increased hepatic levels of the endocannabinoid anandamide associated with decreased expression of its catabolic enzyme fatty acid amide hydrolase. The unique combination of lipidomic and gene expression analyses allows for a better mechanistic understanding of dysregulated lipid metabolism in the development of alcoholic fatty liver disease.

The structural and metabolic integrity of isolated rat liver cells was verified by the high percentage of trypan blue exclusion, a low degree of lactate dehydrogenase release into the medium, a constant rate of gluconeogenesis from l(+)-lactate, and increased oxygen consumption following the addition of 2,4-dinitrophenol. Palmitic acid, incubated in an albumin-bound form with isolated liver cells, was esterified to form phospholipids, triglycerides, diglycerides, and cholesterol esters and was oxidized to CO2 and ketone bodies. In liver cells from fed rats, the major portion of palmitate was esterified, an intermediate quantity was oxidized to ketone bodies, and a smaller amount was oxidized to CO2. The partition of palmitic acid between esterification and ketogenesis was inversed by fasting, whereas oxidation to CO2 and the total rate of palmitate utilization were unaltered. Greater esterification of [14C]palmitate in cells from fed rats was not a result of carbon recycling via chain elongation or de novo synthesis. Liver cells from fasted rats derived more energy from fatty acid oxidation than cells from fed rats. The results indicate that citric acid cycle flux and endogenous lipolysis were unaffected by fasting. These observations signify that altered partition of free fatty acids between the pathways of oxidation and esterification in the liver is a major causative factor in the increased ketogenesis in the fasting state. An increase in [1-14C]palmitate concentration augmented palmitate uptake, ketogenesis, and esterification, whereas 14CO2 production was only slightly affected. The estimated citric acid cycle flux and the acetoacetate to β-hydroxybutyrate ratio were diminished. Increased ketogenesis in response to sequential elevation of the palmitate concentration could not be accounted for by diminished citric acid cycle flux and therefore resulted from increased β oxidation. Ketone body specific activity approached a constant value at v of 3 to 4. Results indicate intracellular mixing of free fatty acids derived from endogenous lipolysis and from the medium, prior to β oxidation. Phospholipid was the predominant esterification product at low concentrations of added palmitate, but, as phospholipid formation approached saturation, a sigmoid increase in diglyceride and triglyceride formation occurred. Fructose and glycerol each decreased ketogenesis from added palmitate. Fructose, glycerol, and, to a lesser extent, glucose increased palmitate esterification in liver cells isolated from fasting rats. This effect was characterized by increased conversion of added palmitate to diglycerides, triglycerides, and phospholipids and decreased conversion to cholesterol esters. These substrates did not alter the rate of fatty acid uptake. Fructose, glycerol, and, to a lesser extent, glucose elevated 14CO2 production from [1-14C]palmitate. At higher fructose concentrations the elevated 14CO2 production was reversed. Results suggest that substrates which enter glycolysis beyond fructose 1,6-diphosphate decrease ketogenesis by competition with fatty acid oxidation and enhance esterification by the resulting increased availability of long chain free fatty acids and by a separate preferential stimulation of glycerolipid formation. Results indicate that decreased availability of non-fatty acid substrates and, thereby, decreased competitive oxidation of these substrates is a participating causative factor in the increased oxidation, and the decreased esterification, of long chain fatty acids in the liver in the fasting state.

How to avoid running on empty.