Medium-chain Fatty Acid Oxidation Research Articles

The anaplerotic potential of pentanoate and B‐ketopentanoate in pig heart in vivo

We previously showed (Roe et al., J Clin Invest 110: 259, 2002) that the cardiovascular and muscle strength status of patients with long-chain fatty acid oxidation disorders was greatly improved by a diet therapy with triheptanoin. The benefit of the treatment was ascribed to the production of anaplerotic propionyl-CoA from heptanoate. We hypothesized that pentanoate (as tripentanoin) and β-ketopentanoate (as a glycerol ester) might be used for the acute treatment of patients with long-chain or medium-chain fatty acid oxidation defects. We investigated the metabolism and anaplerotic potential of these compounds in live pig hearts. [3,4,5-13C3]Pentanoate, [1-13C]pentanoate or β-keto-[3,4,5-13C3]pentanoate were infused at various concentrations (0 to 1.5 mM) in the coronary artery of anesthetized pigs. The uptake of pentanoate and β-ketopentanoate increased linearly with substrate concentration, and there was no impairment of cardiac function at all concentrations infused. From the mass isotopomer distribution of intermediates of the propionyl-CoA pathway and the citric acid cycle, we calculated the rate of anaplerosis from pentanoate and β-ketopentanoate (6–10% of the citric acid cycle). The infusion of pentanoate or β-ketopentanoate decreased the production of endogenous propionyl-CoA, and thus the oxidation of proteins in the heart. Supported by NIH grants DK069752 and P01HL074237.

Establishing a Reference Interval For Measurement of Flux through the Mitochondrial Fatty Acid Oxidation Pathway in Cultured Skin Fibroblasts

Mitochondrial fatty acid oxidation (FAO) represents a normal metabolic response to increased energy demands during periods of fasting, febrile illness, or muscular exertion. FAO is a complex pathway involving activation of free fatty acids to acyl-CoA species, transport into mitochondria, and cyclic oxidation to break the long-chain fatty acids into acetyl-CoA and a two-carbon chain-shortened acyl-CoA, which is then recycled. Acetyl-CoA is used in the Krebs cycle and in liver as a substrate for ketogenesis (1). Measurement of FAO flux in cultured cells provides valuable information about the integrity of the enzymes and transporters involved in FAO at all stages. Clinical indications for the measurement of FAO flux in skin fibroblasts include fatty acid oxidation enzyme defects, unexplained myopathies, cardiomyopathies, hypoglycemia, liver disease, sudden unexplained infant death, and hypoketotic hypoglycemia. FAO flux is typically measured by monitoring the conversion of radiolabeled fatty acids to carbon dioxide and water. Labeling can be with 14C with final analysis of the rate of production of 14CO2, or with 3H and analysis of production of 3H2O (2)(3). Palmitic acid (C16) is used to detect long-chain defects, including deficiencies in very long chain acyl-CoA dehydrogenase, long-chain l-3-hydroxy acyl-CoA dehydrogenase, long-chain 3-keto acyl-CoA thiolase, carnitine/acylcarnitine translocase, carnitine pamitoyltransferase 1, and carnitine pamitoyltransferase 2. Myristic acid (C14) is used to detect defects in medium-chain FAO, such as medium-chain acyl-CoA dehydrogenase deficiency. We use tritiated fatty acids labeled in the (9,10) position. The assay will not detect defects in carnitine palmitoyl transferase 1B; short-chain FAO defects, including short-chain acyl-CoA dehydrogenase and short-chain l-3-hydroxy acyl-CoA dehydrogenase; and short-chain-3-keto acyl-CoA thiolase deficiencies. Historically, patient samples have been compared with batch controls, and a reference interval was not established for this assay. In the present study we have assigned …

Oxidation of medium-chain and long-chain fatty acids by polka dot grouper Cromileptes altivelis

In this study we tested the hypotheses that higher fat diets will promote greater fat oxidation, and that medium-chain fatty acids (MCFA) will be more readily oxidized than long-chain fatty acids (LCFA). The oxidation of dietary MCFA (coconut oil labelled with 14C-octanoic acid) and LCFA (olive oil labelled singly with 14C-palmitic acid and 14C-oleic acid) was measured in a metabolic chamber. MCFA and LCFA were included in the respective diets at 150 and 300 g kg−1. Polka dot grouper fed diets containing MCFA had a significantly higher (P < 0.05) respiration rate [2.4 mmol CO2 BW (kg)−0.79 h−1] and proportion of radioactivity in the respired CO2 (49.5%) than those fed diets containing LCFA [1.2 mmol CO2 BW (kg)−0.79 h−1 and 12.8%, respectively]. The amount of dietary fat did not significantly affect either of these two response attributes. The radioactivity in faeces and regurgitated material was low with diets containing MCFA, but was significantly higher with diets containing LCFA, and increased with increasing LCFA content, suggesting a decrease in digestibility. These results show that MCFA provide this species with a more readily utilizable source of energy than LCFA. Moreover, raising the amount of lipid in the diet above 150 g kg−1 did not increase the oxidation of fatty acids for energy.

PPAR alpha ligand protects during cisplatin-induced acute renal failure by preventing inhibition of renal FAO and PDC activity.

Previous studies demonstrated that during cisplatin-induced acute renal failure, there is a significant reduction in proximal tubule fatty acid oxidation. We now report on the effects of peroxisome proliferator-activated receptor-alpha (PPAR alpha) ligand Wy-14643 (WY) on the abnormalities of medium chain fatty acid oxidation and pyruvate dehydrogenase complex (PDC) activity in kidney tissue of cisplatin-treated mice. Cisplatin causes a significant reduction in mRNA levels and enzyme activity of mitochondrial medium chain acyl-CoA dehydrogenase (MCAD). PPAR alpha ligand WY ameliorated cisplatin-induced acute renal failure and prevented cisplatin-induced reduction of mRNA levels and enzyme activity of MCAD. In contrast, in cisplatin-treated PPAR alpha null mice, WY did not protect kidney function and did not reverse cisplatin-induced decreased expression of MCAD. Cisplatin inhibited renal PDC activity before the development of acute tubular necrosis, and PDC inhibition was reversed by pretreatment with PPAR alpha agonist WY. Cisplatin also induced increased mRNA and protein levels of pyruvate dehydrogenase kinase-4 (PDK4), and PPAR alpha ligand WY prevented cisplatin-induced increased expression of PDK4 protein levels in wild-type mice. We conclude that PPAR alpha agonists have therapeutic potential for cisplatin-induced acute renal failure. Use of PPAR alpha ligands prevents acute tubular necrosis by ameliorating cisplatin-induced inhibition of two distinct metabolic processes, MCAD-mediated fatty acid oxidation and PDC activity.

Malonyl coenzyme A and the regulation of functional carnitine palmitoyltransferase-1 activity and fat oxidation in human skeletal muscle.

Physiological hyperglycemia with hyperinsulinemia reduces fat oxidation in skeletal muscle. The mechanism responsible for this decrease in fat oxidation in human muscle is not known and may contribute to the development of insulin resistance. We hypothesized that the transfer of long-chain fatty acids (LCFAs) into the mitochondria via carnitine palmitoyltransferase-1 (CPT-1) is inhibited by increased malonyl coenzyme A (malonyl-CoA) (a known potent inhibitor of CPT-1) in human muscle during hyperglycemia with hyperinsulinemia. We studied six healthy subjects after an overnight fast and during an induced 5-hour period of hyperglycemia with hyperinsulinemia. Muscle fatty acid oxidation was calculated using stable isotope methodology combined with blood sampling from the femoral artery and vein of one leg. Muscle functional CPT-1 activity was assessed by concurrently infusing an LCFA tracer and a CPT-independent medium-chain fatty acid tracer. Muscle biopsies were obtained from the vastus lateralis after the periods of fasting and hyperglycemia with hyperinsulinemia. Hyperglycemia with hyperinsulinemia decreased LCFA oxidation, but had no effect on LCFA uptake or medium-chain fatty acid oxidation across the leg. Malonyl-CoA concentration significantly increased from 0.13 +/- 0.01 to 0.35 +/- 0.07 nmol/g during hyperglycemia with hyperinsulinemia. We conclude that hyperglycemia with hyperinsulinemia increases malonyl-CoA, inhibits functional CPT-1 activity, and shunts LCFA away from oxidation and toward storage in human muscle.

Identification of human PMP34 as a peroxisomal ATP transporter

In recent years much has been learned about the essential role of peroxisomes in cellular metabolism. Much less, however, is known about the permeability properties of peroxisomes although it is well established now that peroxisomes are impermeable to small molecules which implies the existence of transporters in the peroxisomal membrane. In this paper we report the identification of PMP34, a peroxisomal membrane protein belonging to the mitochondrial solute carrier family, as an adenine nucleotide transporter. This is concluded from different experimental findings including rescue of the defect in medium-chain fatty acid oxidation in Saccharomyces cerevisiae cells in which the ANT1 gene coding for Ant1p, the peroxisomal adenine nucleotide carrier, was disrupted. Furthermore, we have purified PMP34, reconstituted the protein in proteoliposomes, and provide direct proof that PMP34 is an adenine nucleotide transporter.

Physiological Effects of Medium-Chain Triglycerides: Potential Agents in the Prevention of Obesity

Medium chain fatty acids (MCFA) are readily oxidized in the liver. Animal and human studies have shown that the fast rate of oxidation of MCFA leads to greater energy expenditure (EE). Most animal studies have also demonstrated that the greater EE with MCFA relative to long-chain fatty acids (LCFA) results in less body weight gain and decreased size of fat depots after several months of consumption. Furthermore, both animal and human trials suggest a greater satiating effect of medium-chain triglycerides (MCT) compared with long-chain triglycerides (LCT). The aim of this review is to evaluate existing data describing the effects of MCT on EE and satiety and determine their potential efficacy as agents in the treatment of human obesity. Animal studies are summarized and human trials more systematically evaluated because the primary focus of this article is to examine the effects of MCT on human energy metabolism and satiety. Hormones including cholescytokinin, peptide YY, gastric inhibitory peptide, neurotensin and pancreatic polypeptide have been proposed to be involved in the mechanism by which MCT may induce satiety; however, the exact mechanisms have not been established. From the literature reviewed, we conclude that MCT increase energy expenditure, may result in faster satiety and facilitate weight control when included in the diet as a replacement for fats containing LCT.

In vivo absorption of medium-chain fatty acids by the rat colon exceeds that of short-chain fatty acids

Short-chain fatty acids (SCFAs) are main fuels of the colonic epithelium, and are avidly absorbed by the colon of animal and man. The current knowledge on colonic metabolism and absorption of medium-chain fatty acids (MCFAs) is limited. In some clinical situations, colonic absorption of high-energy substances could compensate for reduced absorptive capacity because of a shortened or malfunctioning small bowel. We evaluated and compared colonic absorption and metabolism of MCFAs (octanoate, decanoate, and dodecanoate), SCFAs (acetate and butyrate), and long-chain fatty acids (LCFAs) (oleate). Rats were surgically operated on to cannulate a 7-cm segment of proximal colon, isolate the vasculature, and cannulate the right colic vein draining this segment. The lumen was perfused with (14)C-labeled substrates for 100 minutes. Right colic vein blood was analyzed for total (14)C, (14)CO(2), and metabolites by scintillation counting and high-performance liquid chromatography. The transport from the colonic lumen to mesenteric blood of substrate carbon from MCFAs exceeded by 2-13-fold that of SCFAs and LCFAs. The CO(2) production from the oxidation of MCFAs was as high as or higher than that from SCFAs. CO(2) produced from the LCFA, oleate, was lower than from SCFAs or MCFAs. In addition to CO(2), ketone bodies were major metabolites of SCFAs and MCFAs. Ketogenesis from butyrate and the MCFAs was significantly higher than from acetate and oleate. A substantial proportion (50%-90%) of all substrates was absorbed without being metabolized. The colonic epithelium serves to absorb and partially metabolize MCFAs. For patients with a compromised small-bowel function, colonic absorption of MCFAs could represent an important way of receiving calories.

Pex11p plays a primary role in medium-chain fatty acid oxidation, a process that affects peroxisome number and size in Saccharomyces cerevisiae.

The Saccharomyces cerevisiae peroxisomal membrane protein Pex11p has previously been implicated in peroxisome proliferation based on morphological observations of PEX11 mutant cells. Pex11p-deficient cells fail to increase peroxisome number in response to growth on fatty acids and instead accumulate a few giant peroxisomes. We report that mutants deficient in genes required for medium-chain fatty acid (MCFA) beta-oxidation display the same phenotype as Pex11p-deficient cells. Upon closer inspection, we found that Pex11p is required for MCFA beta-oxidation. Disruption of the PEX11 gene results in impaired formation of MCFA-CoA esters as measured in intact cells, whereas their formation is normal in cell lysates. The sole S. cerevisiae MCFA-CoA synthetase (Faa2p) remains properly localized to the inner leaflet of the peroxisomal membrane in PEX11 mutant cells. Therefore, the in vivo latency of MCFA activation observed in Pex11p-deficient cells suggests that Pex11p provides Faa2p with substrate. When PEX11 mutant cells are shifted from glucose to oleate-containing medium, we observed an immediate deficiency in beta-oxidation of MCFAs whereas giant peroxisomes and a failure to increase peroxisome abundance only became apparent much later. Our observations suggest that the MCFA oxidation pathway regulates the level of a signaling molecule that modulates the number of peroxisomal structures in a cell.

Regulation of carbohydrate and fatty acid utilization by L-carnitine during cardiac development and hypoxia.

This study is designed to investigate whether substrate preference in the myocardium during the neonatal period and hypoxia-induced stress is controlled intracellularly or by extracellular substrate availability. To determine this, the effect of exogenous L-carnitine on the regulation of carbohydrate and fatty acid metabolism was determined during cardiac stress (hypoxia) and during the postnatal period. The effect of L-carnitine on long chain (palmitate) and medium chain (octonoate) fatty acid oxidation was studied in cardiac myocytes isolated from less than 24 h old (new born; NB), 2 week old (2 week) and hypoxic 4 week old (HY) piglets. Palmitate oxidation was severely decreased in NB cells compared to those from 2 week animals (0.456+/-0.04 vs. 1.207+/-0.52 nmol/mg protein/30 min); surprisingly, cells from even older hypoxic animals appeared shifted toward the new born state (0.695+/-0.038 nmol/mg protein/30 min). Addition of L-carnitine to the incubation medium, which stimulates carnitine palmitoyl-transferase I (CPTI) accelerated palmitate oxidation 3 fold in NB and approximately 2 fold in HY and 2 week cells. In contrast, octanoate oxidation which was greater in new born myocytes than in 2 week cells, was decreased by L-carnitine suggesting a compensatory response. Furthermore, oxidation of carbohydrates (glucose, pyruvate, and lactate) was greatly increased in new born myocytes compared to 2 week and HY cells and was accompanied by a parallel increase in pyruvate dehydrogenase (PDH) activity. The concentration of malonyl-CoA, a potent inhibitor of CPTI was significantly higher in new born heart than at 2 weeks. These metabolic data taken together suggest that intracellular metabolic signals interact to shift from carbohydrate to fatty acid utilization during development of the myocardium. The decreased oxidation of palmitate in NB hearts probably reflects decreased intracellular L-carnitine and increased malonyl-CoA concentrations. Interestingly, these data further suggest that the cells remain compliant so that under stressful conditions, such as hypoxia, they can revert toward the neonatal state of increased glucose utilization.

Troglitazone inhibits fatty acid oxidation and esterification, and gluconeogenesis in isolated hepatocytes from starved rats.

The effects of troglitazone and pioglitazone on glucose and fatty acid metabolism were studied in hepatocytes isolated from 24-h-starved rats. These thiazolidinediones inhibited long-chain fatty acid (oleate) oxidation and produced a very oxidized mitochondrial redox state. By contrast, thiazolidinediones did not affect the rate of medium-chain fatty acid (octanoate) oxidation or the activity of mitochondrial carnitine palmitoyltransferase (CPT) I. Thiazolidinediones inhibited selectively triglyceride synthesis but not phospholipid synthesis. The combined inhibition of oleate oxidation and esterification by troglitazone was due to a noncompetitive inhibition of mitochondrial and microsomal long-chain acyl-CoA synthetase (ACS) activities. It was suggested that troglitazone must be metabolized into its sulfo-conjugate derivative in liver cells to inhibit mitochondrial and microsomal ACS activities. Thiazolidinediones inhibited glucose production from lactate/pyruvate or from alanine. Analysis of gluconeogenic metabolite concentrations suggested that troglitazone would inhibit gluconeogenesis at the level of pyruvate carboxylase and glyceraldehyde-3-phosphate dehydrogenase reactions. It was concluded that 1) at a similar concentration, troglitazone was more efficient than pioglitazone to inhibit fatty acid metabolism and gluconeogenesis and 2) the inhibition of gluconeogenesis by troglitazone could be the result of the inhibition of long-chain fatty acid oxidation (decrease in acetyl-CoA, NADH-to-NAD+, and ATP-to-ADP ratios).

3-, 6- and 7-hydroxyoctanoic acids are metabolites of medium-chain triglycerides and excreted in urine as glucuronides.

Three new metabolites of medium-chain fatty acid oxidation, 3-, 6- and 7-hydroxyoctanoyl beta-D-glucuronide, were identified in the urine of six infants who were fed a diet enriched in medium-chain triglycerides (MCT). Glucuronides were extracted from the urine by organic solvent extraction with ethyl acetate and by solid-phase extraction on Sep-Pak C18 cartridges. The compounds of interest were also purified from the organic solvent extract by preparative one-dimensional thin-layer chromatography. Cleavage of the glucuronides was achieved by either alkaline hydrolysis or enzymatic hydrolysis with beta-D-glucuronidase. The analyses of the trimethylsilylated derivatives were performed both by gas chromatography with flame ionization detection (GC/FID) and by gas chromatography/mass spectrometry (GC/MS). The structure of the hydroxyoctanoic acids was proved by comparison of their mass spectra with those of reference substances. Authentic 6-hydroxyoctanoic acid was synthesized. The presence of 6-hydroxyoctanoyl glucuronide shows that in addition to beta-oxidation, omega-oxidation and (omega-1)-hydroxylation, medium-chain fatty acids can be oxidized at the omega-2 position. The conjugation of medium-chain hydroxy-monocarboxylic acids with glucuronic acid has not been described in humans before.

GLYCOLYTIC FLUX DURING EXERCISE REGULATES FATTY ACID OXIDATION 441

We tested the hypothesis that glycolytic flux during exercise regulates fat oxidation via carnitine palmitoyltransferase (CPT 1) inhibition of fatty acid(FA) transport into the mitochondria. This was accomplished by comparing the oxidation rates during constant rate intravenous infusion of trace amounts of a long chain FA (1-13C-palmitate; 0.16 μmol/kg/min; PAL)vs. a medium chain FA (1-13C-octanoate; 0.24 μmol/kg/min; OCT). OCT oxidation is independent of CPT activity. Six endurance-trained men cycled for 40 min at 50% VO2max under two conditions:. 1) after overnight fast (FAST); and 2) following glucose ingestion (2.8 g/kg total), 60 and 10 min before exercise (GLU). As intended, GLU caused hyperglycemia, hyperinsulinemia and a 28% increase in carbohydrate oxidation during exercise(all P<0.05). Increases in breath 13C/12C (above background and with 13C-bicarbonate prime) after 40 min of exercise, allowed calculation of exogenous PAL and OCT oxidation; expressed as a percentage of the 1-13CFA infusion rate. Oxidation of PAL was significantly reduced during GLU compared to FAST (i.e.; 64.9±4.0% vs. 77.4±1.7%; P<0.05). In contrast, oxidation of OCT was unaffected by GLU and identical to FAST (i.e.; 88.2±1.9% vs. 88.8±2.2%). Furthermore, the effect of GLU compared to FAST on endogenous metabolism was to reduce total fat oxidation (21±2vs. 32±2), the rate of endogenous plasma FFA disappearance(8±1 vs. 13±2) and plasma FFA oxidation (6±1vs. 11±1) (all P<0.05; expressed as μmol/kg/min). Intramuscular triglyceride oxidation, calculated as total fat oxidation minus plasma FFA oxidation, also was reduced during GLU compared to FAST(15±2 vs. 21±3 μmolFFA/kg/min; P<0.05). These findings indicate that fat oxidation during exercise is regulated both by FA oxidation by muscle and FA mobilization. The reduction in long-chain but not medium-chain FA oxidation with increased glycolytic flux suggests that carbohydrate metabolism regulates fat oxidation in muscle by influencing FA transport into the mitochondria.

Inhibition of oxidative phosphorylation by palmitoyl-CoA in digitonin permeabilized fibroblasts: implications for long-chain fatty acid β-oxidation disorders

Long-chain fatty acid oxidation deficient patients present early in life with more severe features than patients with a medium-chain fatty acid oxidation deficiency. This may be related to the more toxic effect of long-chain fatty acid derivatives. In this paper we have studied the effect of different acyl-CoA esters, and palmitoyl-CoA in particular, on succinate-driven oxidative phosphorylation, using digitonin permeabilized human fibroblasts. Palmitoyl-CoA was found to inhibit the succinate-driven oxidative phosphorylation in a concentration dependent manner. If the inhibition of the oxidative phosphorylation system is also expressed under in vivo conditions this might explain some of the abnormalities found in patients with defects in long-chain fatty acid β-oxidation.

Rapid Diagnosis of Long Chain and Medium Chain Fatty Acid Oxidation Disorders Using Lymphocytes

A method based on the release of tritiated water from [9,10(n)-3H] palmitic and myristic acids previously described for fibroblasts, was adapted for lymphocytes for the rapid diagnosis of fatty acid oxidation disorders. Optimal concentrations for both substrates and linearity of the assay were established. Normal values were established in control subjects of different age groups (58 children and 117 adults) and 16 patients with known fatty acid oxidation disorders were tested. Tritiated water production from patients' lymphocytes was expressed as a ratio between residual oxidations of palmitate and myristate and the results show that this method allows good differentiation between long chain and medium chain fatty acid oxidation defects.

Medium-Chain Fatty Acid Oxidation in Colostrum-Deprived Newborn Piglets: Stimulative Effect of L-Carnitine Supplementation

To investigate the role of L-carnitine in medium-chain fatty acid (MCFA, fatty acids with 6-12 carbons) metabolism, 16 newborn pigs were fitted with umbilical arterial catheters. Pigs were placed in respiration chambers, and [1-14C]MCFA were infused for 9-12 h providing energy equivalent to 50-175% of the animals' metabolic rate. After 5-7 h (carnitine-free infusion period) of MCFA infusion, a primed (12.5, 25 or 50 mumol) co-infusion of L-carnitine [5, 10 or 20 mumol/(h.kg0.75), respectively] was started and maintained for 4-5 h (carnitine infusion period). The fatty acid oxidation rate (MCFA-derived CO2/total CO2 x 100) was calculated based on the specific radioactivity of expired CO2 (measured per 20-min interval) and the specific radioactivity of the MCFA infused. A single-pool exponential curve was fitted to the fatty acid oxidation rate of the carnitine-free infusion period and was extrapolated to the carnitine infusion period. For each piglet, the average difference between fatty acid oxidation rate during the carnitine infusion period and the extrapolated curve was calculated and tested for significance using a t test. Under steady state conditions, MCFA oxidation accounted for 40% of MCFA infused. Carnitine, independent of the level, increased the fatty acid oxidation rate by as much as 20% if the energy provided as MCFA exceeded 50% of the metabolic needs of the pig (P < 0.01), and the response above 50% was proportional to the relative rate of fatty acid infusion (increase in fatty acid oxidation rate = -3.9 + 0.07 x infusion rate, r0.76).(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of [1-14C]-Medium-Chain Fatty Acid Oxidation by Neonatal Piglets Using Continuous-Infusion Radiotracer Kinetic Methodology ,

In vivo oxidation rates of systemically infused medium-chain fatty acids were evaluated using neonatal pigs. Unanesthetized piglets (1.6 kg, n = 20) were housed in respiration chambers for total collection of expired CO2 and were continuously infused with [1-14C]-7:0, 8:0, 9:0 or 10:0 fatty acids via a central catheter. The fatty acids were administered at rates of 25, 50 or 100 μmol/min for 5 h and provided 74 kBq of 14C per h. Total expired CO2 was collected over consecutive 15-min intervals for determination of the amount and specific radioactivity of expired CO2. Portal blood samples were drawn from umbilical vein catheters for determination of 3-hydroxy-butyrate and plasma fatty acid concentrations. Infusion and oxidation rates (mmol/min) were multiplied by the molar ATP yield for each fatty acid (i.e., 52, 61, 69 and 78 mmol ATP/mmol fatty acid for 7:0 through 10:0, respectively) to adjust for differences in molar energy content of the various fatty acids. Expressed in this way, fatty acid oxidation rate was proportional to the rate of infusion and accounted for 63% of that infused. The various fatty acids were oxidized equally well, regardless of chain length, and satisfied up to 60 to 70% of the animals’ energy requirements, depending on the rate of infusion. These data are discussed in relation to previous work from our laboratory that has shown significant effects of fatty acid chain length on utilization of orally administered medium-chain triglyceride supplements.

Metabolic and Neurologic Effects of an Intravenous Medium‐Chain Triglyceride Emulsion

These studies were undertaken to investigate the relationship between medium-chain fatty acid availability, medium-chain fatty acid oxidation, and central nervous system toxicity during infusion of medium-chain triglycerides in dogs. Six dogs received a sequential, stepwise infusion of trioctanoin at three different rates for 80 min each, providing calories below and equal to resting energy expenditure in the species. Ketone body production rates (using a 14C beta-hydroxybutyrate tracer) and plasma concentrations of lactate and octanoate were monitored. Three animals were infused with saline to serve as controls. Blood-brain barrier integrity was assessed with Evans blue dye, and brain samples were taken at the end of the study to quantify brain water. Three animals were studied under anesthesia to obtain good quality EEG and intracranial pressure measurements. Results were (1) plasma octanoate increased to 0.37 +/- 0.13, 0.78 +/- 0.2, and 1.44 +/- 0.41 mmol/liter during the three infusion intervals; (2) emesis, somnolence, and coma were observed at the two highest trioctanoin rates; (3) ketone body concentrations and production increased from 102 +/- 15 to 859 +/- 54 mumol/liter and 3.6 +/- 0.43 to 18.5 +/- 1.7 mumol/kg/min, respectively, at the highest trioctanoin infusion rate; and (4) plasma lactate also increased from 1.3 +/- 0.1 to 4.3 +/- 0.9 mmol/liter at the highest infusion rate. EEG changes were also observed, consisting of high amplitude slowing and reduction in amplitude of faster components. There was no extravasation of Evans blue dye, nor change in brain water or intracranial pressure. The conclusion--medium-chain triglycerides have significant dose-related central nervous system toxicity in dogs. Therefore, caution should be exercised in clinical studies with MCTs, including careful measurement of medium-chain fatty acid concentrations.

Ce:Role of carnitine in utilization of dietary medium-chain triglycerides by term infants

The role of carnitine in oxidation of dietary medium-chain fatty acids (as medium-chain triglycerides) was studied in term human infants. Infants were fed, alternately, formulas with fat content that was predominantly long-chain triglycerides, or 40% medium-chain triglycerides. Urinary acylcarnitine excretion was significantly higher and the ratio of free to total carnitine was significantly lower when infants were fed the formula with medium-chain triglycerides. Two groups of 10 infants were fed a commercial soy-protein-based formula modified to contain 40% of fat calories as medium-chain triglycerides and with or without added L-carnitine. By 56 d, infants fed the formula without added L-carnitine excreted significantly more medium-chain dicarboxylic acids than did the same infants at 28 d and significantly more than infants consuming the carnitine-supplemented formula at either 28 or 56 d. Results are consistent with a role for carnitine in metabolism of dietary medium-chain triglycerides in infants.

Urinary organic acid profiles in fatty Zucker rats: Indications for impaired oxidation of butyrate and hexanoate

The urinary excretion of 45 organic acids, monitored by gas-liquid chromatography, was compared in fatty ( fa fa ) and lean (Fa/?) Zucker rats maintained on a chemically simplified diet. At the age of 6, 16, and 22 weeks, fatty rats excreted more of the various organic acids than their lean counterparts. However, the greatest difference was in the excretion of ethylmalonate, even when excretion data were normalized to body weight. The next highest excretion difference was in adipate and an unknown compound, and the third highest in pyruvate. A second group of rats examined at 7 weeks also excreted an excess of these four acids, as well as glucuronate and indole-3-acetate. The excessive excretion of ethylmalonate and adipate, which is characteristic of human genetic defects in short- and medium-chain fatty acid oxidation, suggested that the oxidation of butyrate and hexanoate might be impaired in the fatty rat. Thus, as a test of their capacity to oxidize medium- and short-chain fatty acids, two groups of fatty and lean rats were transferred to diets enriched with either trioctanoylglyceride, a medium-chain triglyceride (MCT), or sodium butyrate, a short-chain fatty acid. Both lean and fatty rats on the MCT diet, but only the lean rats on the butyrate-enriched diet, increased their excretion of adipate. However, on both the MCT and butyrate diet, ethylmalonate excretion increased only in lean rats, almost reaching amounts found previously in fatty rats. These results suggest that the fatty rat has an impairment of the β-oxidation of butyrate and hexanoate, a defect that might increase intracellular concentrations of butyryl-CoA, the optimal primer for the synthesis of long-chain fatty acids.