Liver uptake of free fatty acids in vivo in humans as determined with 14( R, S)-[18F]fluoro-6-thia-heptadecanoic acid and PET

New improved radiometabolite analysis method for [18F]FTHA from human plasma: a test-retest study with postprandial and fasting state

BACKGROUND

Fatty acid uptake can be measured using PET and 14-(R,S)-[18F]fluoro-6-thia-heptadecanoic acid ([18F]FTHA). However, the relatively rapid rate of [18F]FTHA metabolism significantly affects kinetic modeling of tissue uptake. Thus, there is a need for accurate chromatographic methods to analyze the unmetabolized [18F]FTHA (parent fraction). Here we present a new radiometabolite analysis (RMA) method, with comparison to a previous method for parent fraction analysis, and its use in a test-retest clinical study under fasting and postprandial conditions. We developed a new thin-layer chromatography (TLC) RMA method for analysis of [18F]FTHA parent fraction and its radiometabolites from plasma, by testing stationary phases and eluent combinations. Next, we analyzed [18F]FTHA, its radiometabolites, and plasma radioactivity from subjects participating in a clinical study. A total of 17 obese or overweight participants were dosed with [18F]FTHA twice under fasting, and twice under postprandial conditions and plasma samples were obtained between 14 min (mean of first sample) and 72 min (mean of last sample) post-injection. Aliquots of 70 plasma samples were analyzed using both methods, enabling head-to-head comparisons. We performed test-retest and group comparisons of the parent fraction and plasma radioactivity.

RESULTS

The new TLC method separated seven [18F]FTHA radiometabolite peaks, while the previous method separated three. The new method revealed at least one radiometabolite that was not previously separable from [18F]FTHA. From the plasma samples, the mean parent fraction value was on average 7.2 percentage points lower with the new method, compared to the previous method. Repeated [18F]FTHA investigations on the same subject revealed reproducible plasma SUV and parent fractions, with different kinetics between the fasted and postprandial conditions.

CONCLUSIONS

The newly developed improved radio-TLC method for [18F]FTHA RMA enables accurate parent fraction correction, which is required to obtain quantitative data for modelling [18F]FTHA PET data. Our test-retest study of fasted and postprandial conditions showed robust reproducibility, and revealed clear differences in the [18F]FTHA metabolic rate under different study settings.

TRIAL REGISTRATION

EudraCT No: 2020-005211-48, 04Feb2021; and Clinical Trials registry NCT05132335, 29Oct2021, URL: https://classic.

CLINICALTRIALS

gov/ct2/show/NCT05132335 .

Hepatic Positron Emission Tomography: Applications in Metabolism, Haemodynamics and Cancer

Evaluating in vivo the metabolic rates of the human liver has been a challenge due to its unique perfusion system. Positron emission tomography (PET) represents the current gold standard for assessing non-invasively tissue metabolic rates in vivo. Here, we review the existing literature on the assessment of hepatic metabolism, haemodynamics and cancer with PET. The tracer mainly used in metabolic studies has been [¹⁸F]2-fluoro-2-deoxy-D-glucose (¹⁸F-FDG). Its application not only enables the evaluation of hepatic glucose uptake in a variety of metabolic conditions and interventions, but based on the kinetics of ¹⁸F-FDG, endogenous glucose production can also be assessed. 14(R,S)-[¹⁸F]fluoro-6-thia-Heptadecanoic acid (¹⁸F-FTHA), ¹¹C-Palmitate and ¹¹C-Acetate have also been applied for the assessment of hepatic fatty acid uptake rates (¹⁸F-FTHA and ¹¹C-Palmitate) and blood flow and oxidation (¹¹C-Acetate). Oxygen-15 labelled water (¹⁵O-H₂O) has been used for the quantification of hepatic perfusion. ¹⁸F-FDG is also the most common tracer used for hepatic cancer diagnostics, whereas ¹¹C-Acetate has also shown some promising applications in imaging liver malignancies. The modelling approaches used to analyse PET data and also the challenges in utilizing PET in the assessment of hepatic metabolism are presented.

Computational Hypothesis: How Intra-Hepatic Functional Heterogeneity May Influence the Cascading Progression of Free Fatty Acid-Induced Non-Alcoholic Fatty Liver Disease (NAFLD)

Non-Alcoholic Fatty Liver Disease (NAFLD) is the most common type of chronic liver disease in developed nations, affecting around 25% of the population. Elucidating the factors causing NAFLD in individual patients to progress in different rates and to different degrees of severity, is a matter of active medical research. Here, we aim to provide evidence that the intra-hepatic heterogeneity of rheological, metabolic and tissue-regenerating capacities plays a central role in disease progression. We developed a generic mathematical model that constitutes the liver as ensemble of small liver units differing in their capacities to metabolize potentially cytotoxic free fatty acids (FFAs) and to repair FFA-induced cell damage. Transition from simple steatosis to more severe forms of NAFLD is described as self-amplifying process of cascading liver failure, which, to stop, depends essentially on the distribution of functional capacities across the liver. Model simulations provided the following insights: (1) A persistently high plasma level of FFAs is sufficient to drive the liver through different stages of NAFLD; (2) Presence of NAFLD amplifies the deleterious impact of additional tissue-damaging hits; and (3) Coexistence of non-steatotic and highly steatotic regions is indicative for the later occurrence of severe NAFLD stages.

Liver fat storage pathways: methodologies and dietary effects

PURPOSE OF REVIEW

Nonalcoholic fatty liver is the result of an imbalance between lipid storage [from meal, de novo lipogenesis (DNL) and fatty acid (FA) uptake] and disposal (oxidation and VLDL output). Knowledge on the contribution of each of these pathways to liver fat content in humans is essential to develop tailored strategies to prevent and treat nonalcoholic fatty liver. Here, we review the techniques available to study the different storage pathways and review dietary modulation of these pathways.

RECENT FINDINGS

The type of carbohydrate and fat could be of importance in modulating DNL, as complex carbohydrates and omega-3 FAs have been shown to reduce DNL. No effects were found on the other pathways, however studies investigating this are scarce.

SUMMARY

Techniques used to assess storage pathways are predominantly stable isotope techniques, which require specific expertise and are costly. Validated biomarkers are often lacking. These methodological limitations also translate into a limited number of studies investigating to what extent storage pathways can be modulated by diet. Further research is needed to elucidate in more detail the impact that fat and carbohydrate type can have on liver fat storage pathways and content.

Effects of short-term sprint interval and moderate-intensity continuous training on liver fat content, lipoprotein profile, and substrate uptake: a randomized trial

Type 2 diabetes (T2D) and increased liver fat content (LFC) alter lipoprotein profile and composition and impair liver substrate uptake. Exercise training mitigates T2D and reduces LFC, but the benefits of different training intensities in terms of lipoprotein classes and liver substrate uptake are unclear. The aim of this study was to evaluate the effects of moderate-intensity continuous training (MICT) or sprint interval training (SIT) on LFC, liver substrate uptake, and lipoprotein profile in subjects with normoglycemia or prediabetes/T2D. We randomized 54 subjects (normoglycemic group, n = 28; group with prediabetes/T2D, n = 26; age = 40-55 yr) to perform either MICT or SIT for 2 wk and measured LFC with magnetic resonance spectroscopy, lipoprotein composition with NMR, and liver glucose uptake (GU) and fatty acid uptake (FAU) using PET. At baseline, the group with prediabetes/T2D had higher LFC, impaired lipoprotein profile, and lower whole body insulin sensitivity and aerobic capacity compared with the normoglycemic group. Both training modes improved aerobic capacity (P < 0.001) and lipoprotein profile (reduced LDL and increased large HDL subclasses; all P < 0.05) with no training regimen (SIT vs. MICT) or group effect (normoglycemia vs. prediabetes/T2D). LFC tended to be reduced in the group with prediabetes/T2D compared with the normoglycemic group posttraining (P = 0.051). When subjects were divided according to LFC (high LFC, >5.6%; low LFC, <5.6%), training reduced LFC in subjects with high LFC (P = 0.009), and only MICT increased insulin-stimulated liver GU (P = 0.03). Short-term SIT and MICT are effective in reducing LFC in subjects with fatty liver and in improving lipoprotein profile regardless of baseline glucose tolerance. Short-term MICT is more efficient in improving liver insulin sensitivity compared with SIT. NEW & NOTEWORTHY In the short term, both sprint interval training and moderate-intensity continuous training (MICT) reduce liver fat content and improve lipoprotein profile; however, MICT seems to be preferable in improving liver insulin sensitivity.

Morbid obesity and type 2 diabetes alter intestinal fatty acid uptake and blood flow

AIMS

Bariatric surgery is the most effective treatment to tackle morbid obesity and type 2 diabetes, but the mechanisms of action are still unclear. The objective of this study was to investigate the effects of bariatric surgery on intestinal fatty acid (FA) uptake and blood flow.

MATERIALS AND METHODS

We recruited 27 morbidly obese subjects, of whom 10 had type 2 diabetes and 15 were healthy age-matched controls. Intestinal blood flow and fatty acid uptake from circulation were measured during fasting state using positron emission tomography (PET). Obese subjects were re-studied 6 months after bariatric surgery. The mucosal location of intestinal FA retention was verified in insulin resistant mice with autoradiography.

RESULTS

Compared to lean subjects, morbidly obese subjects had higher duodenal and jejunal FA uptake (P < .001) but similar intestinal blood flow (NS). Within 6 months after bariatric surgery, obese subjects had lost 24% of their weight and 7/10 diabetic subjects were in remission. Jejunal FA uptake was further increased (P < .03). Conversely, bariatric surgery provoked a decrease in jejunal blood flow (P < .05) while duodenal blood flow was preserved. Animal studies showed that FAs were taken up into enterocytes, for the most part, but were also transferred, in part, into the lumen.

CONCLUSIONS

In the obese, the small intestine actively takes up FAs from circulation and FA uptake remains higher than in controls post-operatively. Intestinal blood flow was not enhanced before or after bariatric surgery, suggesting that enhanced intestinal FA metabolism is not driven by intestinal perfusion.

Brown adipose tissue lipid metabolism in morbid obesity: Effect of bariatric surgery-induced weight loss

OBJECTIVE

We aimed to investigate the effect of bariatric surgery on lipid metabolism in supraclavicular brown adipose tissue in morbidly obese women. We hypothesized that lipid metabolism improves after surgery-induced weight loss.

MATERIALS AND METHODS

A total of 23 morbidly obese women (BMI, 42.1 ± 4.2 kg/m² ; age, 43.8 ± 9.8 years) were assessed before and 6 months after bariatric surgery and 15 age- and sex-matched controls (22.6 ± 2.8 kg/m² ) were assessed once. In the supraclavicular fat depot, fractional (FUR) and NEFA uptake rates were measured with ¹⁸ F-FTHA-PET. We assessed tissue morphology (triglyceride content) using computed tomography (CT)-radiodensity (in Hounsfield Units[HU]) and the proportion of fat with high density (sBAT [%]) in the entire supraclavicular fat depot.

RESULTS

The supraclavicular fractional uptake rate was lower in obese women compared to controls (0.0055 ± 0.0035 vs 0.0161 ± 0.0177 1/min, P = .001). Both FUR (to 0.0074 ± 0.0035 1/min, P = .01) and NEFA uptake rates (to 0.50 ± 0.50 μmol/100 g/min, P = .001) increased after surgery. Compared to controls, obese women had lower CT-radiodensity (-101.2 ± 10.1 vs -82.5 ± 5.8 HU, P < .001) and sBAT (43.4 ± 8.4% vs 64.5 ± 12.4%, P < .001). After surgery, CT-radiodensity increased (to -82.5 ± 9.6 HU, P < .001), signifying decreased triglyceride content and sBAT improved (to 58.0 ± 10.7%, P < .001), indicating an increased proportion of brown fat. The change in tissue morphology, reflected as increase in CT-radiodensity and sBAT (%), was associated with a decrease in adiposity indices and an increase in whole-body insulin sensitivity.

CONCLUSIONS

A decrease in triglyceride content, coupled with the increased proportion of brown adipose tissue in the supraclavicular fat depot, may play a role in the improvement of whole-body insulin sensitivity observed in morbidly obese women after surgery-induced weight loss.

Increased Liver Fatty Acid Uptake Is Partly Reversed and Liver Fat Content Normalized After Bariatric Surgery

OBJECTIVE

Changes in liver fatty acid metabolism are important in understanding the mechanisms of diabetes remission and metabolic changes after bariatric surgery.

RESEARCH DESIGN AND METHODS

Liver fatty acid uptake (LFU), blood flow, and fat content (LFC) were measured in 25 obese subjects before bariatric surgery and 6 months after using positron emission tomography/computed tomography and MRS; 14 lean individuals served as the control subjects.

RESULTS

The increased LFU in obese subjects was associated with body adiposity. LFU was reduced postoperatively but was still high compared with the control subjects. LFC was normalized. Liver blood flow (per unit volume) was higher in obese subjects than in the control subjects at baseline and was further increased postoperatively; however, the total organ blood flow was unchanged as the liver volume decreased.

CONCLUSIONS

The findings suggest that in a postoperative state, intrahepatic fatty acids are not stored in the liver but are used for oxidation to provide energy. Changes in perfusion may contribute to improved liver metabolism postoperatively.

Adipose tissue and liver

Adipose tissue and liver are central tissues in whole body energy metabolism. Their composition, structure, and function can be noninvasively imaged using a variety of measurement techniques that provide a safe alternative to an invasive biopsy. Imaging of adipose tissue is focused on quantitating the distribution of adipose tissue in subcutaneous and intra-abdominal (visceral) adipose tissue depots. Also, detailed subdivisions of adipose tissue can be distinguished with modern imaging techniques. Adipose tissue (or adipocyte) accumulation or infiltration of other organs can also be imaged, with intramuscular adipose tissue a common example. Although liver fat content is now accurately imaged using standard magnetic resonance imaging (MRI) techniques, inflammation and fibrosis are more difficult to determine noninvasively. Liver imaging efforts are therefore concerted on developing accurate imaging markers of liver fibrosis and inflammatory status. Magnetic resonance elastography (MRE) is presently the most reliable imaging technique for measuring liver fibrosis but requires an external device for introduction of shear waves to the liver. Methods using multiparametric diffusion, perfusion, relaxometry, and hepatocyte-specific MRI contrast agents may prove to be more easily implemented by clinicians, provided they reach similar accuracy as MRE. Adipose tissue imaging is experiencing a revolution with renewed interest in characterizing and identifying distinct adipose depots, among them brown adipose tissue. Magnetic resonance spectroscopy provides an interesting yet underutilized way of imaging adipose tissue metabolism through its fatty acid composition. Further studies may shed light on the role of fatty acid composition in different depots and why saturated fat in subcutaneous adipose tissue is a marker of high insulin sensitivity.

The effects of bariatric surgery on pancreatic lipid metabolism and blood flow

CONTEXT

Bariatric surgery leads to a rapid and sustained weight loss often accompanied with improvement in glucose homeostasis.

OBJECTIVE

The objective of this study was to investigate the effects of bariatric surgery on pancreatic lipid metabolism, blood flow, and glycemic control.

DESIGN

This was a longitudinal study.

SETTING

The study was conducted in a clinical research center.

PARTICIPANTS

This study included 27 morbidly obese and 15 healthy control subjects.

INTERVENTIONS

Measurements were performed using positron emission tomography with the palmitate analog 14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid and radiowater ([(15)O]H2O) and computed tomography. In morbidly obese subjects, positron emission tomography/computed tomography imaging studies were performed before and 6 months after bariatric surgery (either Roux-en-Y gastric bypass or sleeve gastrectomy).

MAIN OUTCOME MEASURES

Pancreatic fat and fat-free volume, fatty acid uptake and blood flow were measured as well as parameters of β-cell function, glucose tolerance, and insulin sensitivity.

RESULTS

Six months after bariatric surgery, 23% excess weight loss was observed (P < .0001), and diabetes remission was seen in 7 of 10 patients. When compared with preoperative values, after surgery, notable decreases in pancreatic fat volume (P < .01), fatty acid uptake, and blood flow (both P < .05) were seen, whereas no change was seen in pancreatic fat-free volume. The decrease in pancreatic fat volume and the preservation of blood flow were associated with favorable glucose homeostasis and β-cell function.

CONCLUSIONS

Bariatric surgery elicits marked alterations in pancreatic lipid metabolism and blood flow, which may contribute to the observed improvement in glucose homeostasis and remission of type 2 diabetes.

Pancreatic metabolism, blood flow, and β-cell function in obese humans

CONTEXT

Glucolipotoxicity is believed to induce pancreatic β-cell dysfunction in obesity. Previously, it has not been possible to study pancreatic metabolism and blood flow in humans.

OBJECTIVE

The objective of the study was to investigate whether pancreatic metabolism and blood flow are altered in obesity using positron emission tomography (PET). In the preclinical part, the method was validated in animals.

DESIGN

This was a cross-sectional study.

SETTING

The study was conducted in a clinical research center.

PARTICIPANTS

Human studies consisted of 52 morbidly obese and 25 healthy age-matched control subjects. Validation experiments were done with rodents and pigs.

INTERVENTIONS

PET and magnetic resonance imaging studies using a glucose analog ([(18)F]fluoro-2-deoxy-d-glucose), a palmitate analog [14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid], and radiowater ([(15)O]H2O) were performed. In animals, a comparison between ex vivo and in vivo data was performed.

MAIN OUTCOME MEASURES

Pancreatic glucose/fatty acid (FA) uptake, fat accumulation, and blood flow parameters of β-cell function were measured.

RESULTS

PET proved to be a feasible method to measure pancreatic metabolism. Compared with healthy participants, obese participants had elevated pancreatic FA uptake (P < .0001), more fat accumulation (P = .0001), lowered glucose uptake both during fasting and euglycemic hyperinsulinemia, and blunted blood flow (P < .01) in the pancreas. Blood flow, FA uptake, and fat accumulation were negatively associated with multiple markers of β-cell function.

CONCLUSIONS

Obesity leads to changes in pancreatic energy metabolism with a substrate shift from glucose to FAs. In morbidly obese humans, impaired pancreatic blood flow may contribute to β-cell dysfunction and in the pathogenesis of type 2 diabetes.

Clinical considerations and mechanistic determinants of postprandial lipemia in older adults

The typical diet of individuals in Western societies results in metabolic responses associated with fed-state fat metabolism for most of the daily life of the individual. This fat metabolism is characterized specifically by an increase in the concentration of plasma lipids, primarily triglycerides. Increased postprandial lipemia, which is typically observed in older individuals (i.e., >65 y old), has now emerged as an important correlate of cardiovascular disease risk. An understanding of the mechanisms contributing to the increased postprandial lipemia in older individuals becomes, therefore, of particular clinical importance in any effort to explain and address the well-documented increase in cardiovascular disease risk as individuals age. Current evidence points to an increase in the accumulation of ingested lipid in lipoprotein particles of hepatic origin, together with an overall accumulation of lipid in these lipoproteins during the postprandial period, as primary contributors to the postprandial lipemia in older persons. When this evidence is considered together with the evidence suggesting large atherogenic potential of lipoproteins of hepatic origin, this can, at least in part, explain the increased risk of cardiovascular disease in older individuals. Understanding changes in the metabolism of ingested fat in the immediate postprandial period with advancing age, and how lifestyle interventions such as diet and physical exercise can ameliorate the increase in postprandial lipemia in older individuals, is important in order to address the increased cardiovascular disease risk in this particularly affected and growing segment of the population.

Bringing physiology into PET of the liver

Several physiologic features make interpretation of PET studies of liver physiology an exciting challenge. As with other organs, hepatic tracer kinetics using PET is quantified by dynamic recording of the liver after the administration of a radioactive tracer, with measurements of time-activity curves in the blood supply. However, the liver receives blood from both the portal vein and the hepatic artery, with the peak of the portal vein time-activity curve being delayed and dispersed compared with that of the hepatic artery. The use of a flow-weighted dual-input time-activity curve is of importance for the estimation of hepatic blood perfusion through initial dynamic PET recording. The portal vein is inaccessible in humans, and methods of estimating the dual-input time-activity curve without portal vein measurements are being developed. Such methods are used to estimate regional hepatic blood perfusion, for example, by means of the initial part of a dynamic (18)F-FDG PET/CT recording. Later, steady-state hepatic metabolism can be assessed using only the arterial input, provided that neither the tracer nor its metabolites are irreversibly trapped in the prehepatic splanchnic area within the acquisition period. This is used in studies of regulation of hepatic metabolism of, for example, (18)F-FDG and (11)C-palmitate.

Organ-specific dietary fatty acid uptake in humans using positron emission tomography coupled to computed tomography

A noninvasive method to determine postprandial fatty acid tissue partition may elucidate the link between excess dietary fat and type 2 diabetes. We hypothesized that the positron-emitting fatty acid analog 14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid ((18)FTHA) administered orally during a meal would be incorporated into chylomicron triglycerides, allowing determination of interorgan dietary fatty acid uptake. We administered (18)FTHA orally at the beginning of a standard liquid meal ingested in nine healthy men. There was no significant (18)FTHA uptake in the portal vein and the liver during the 1st hour. Whole body PET/CT acquisition revealed early appearance of (18)FTHA in the distal thoracic duct, reaching a peak at time 240 min. (18)FTHA mean standard uptake value increased progressively in the liver, heart, quadriceps, and subcutaneous and visceral adipose tissues between time 60 and 240 min. Most circulating (18)F activity between time 0 and 360 min was recovered into chylomicron triglycerides. Using Triton WR-1339 treatment in rats that received (18)FTHA by gavage, we confirmed that >90% of this tracer reached the circulation as triglycerides. This novel noninvasive method to determine tissue dietary fatty acid distribution in humans should prove useful in the study of the mechanisms leading to lipotoxicity.

Pathophysiology of prediabetes

Prediabetes encompasses conventional diagnostic categories of impaired fasting glucose and impaired glucose tolerance but is a band of glucose concentrations and a temporal phase over a continuum extending from conventional normal glucose tolerance to overt type 2 diabetes. Insulin resistance and defective glucose sensing at the β-cell are the central pathophysiologic determinants that together cause hyperglycemia. Regardless of the cellular origin of insulin resistance, excessive tissue fat utilization is a consistent metabolic mechanism. Although genetic influences affect β-cell function, becoming overweight is the main acquired challenge to insulin action. The phenotype of prediabetes includes dyslipidemia and higher arterial blood pressure.

Liver fat content in type 2 diabetes: relationship with hepatic perfusion and substrate metabolism

OBJECTIVE

Hepatic steatosis is common in type 2 diabetes. It is causally linked to the features of the metabolic syndrome, liver cirrhosis, and cardiovascular disease. Experimental data have indicated that increased liver fat may impair hepatic perfusion and metabolism. The aim of the current study was to assess hepatic parenchymal perfusion, together with glucose and fatty acid metabolism, in relation to hepatic triglyceride content.

RESEARCH DESIGN AND METHODS

Fifty-nine men with well controlled type 2 diabetes and 18 age-matched healthy normoglycemic men were studied using positron emission tomography to assess hepatic tissue perfusion, insulin-stimulated glucose, and fasting fatty acid metabolism, respectively, in relation to hepatic triglyceride content, quantified by proton magnetic resonance spectroscopy. Patients were divided into two groups with hepatic triglyceride content below (type 2 diabetes-low) or above (type 2 diabetes-high) the median of 8.6%.

RESULTS

Type 2 diabetes-high patients had the highest BMI and A1C and lowest whole-body insulin sensitivity (ANOVA, all P < 0.001). Compared with control subjects and type 2 diabetes-low patients, type 2 diabetes-high patients had the lowest hepatic parenchymal perfusion (P = 0.004) and insulin-stimulated hepatic glucose uptake (P = 0.013). The observed decrease in hepatic fatty acid influx rate constant, however, only reached borderline significance (P = 0.088). In type 2 diabetic patients, hepatic parenchymal perfusion (r = -0.360, P = 0.007) and hepatic fatty acid influx rate constant (r = -0.407, P = 0.007) correlated inversely with hepatic triglyceride content. In a pooled analysis, hepatic fat correlated with hepatic glucose uptake (r = -0.329, P = 0.004).

CONCLUSIONS

In conclusion, type 2 diabetic patients with increased hepatic triglyceride content showed decreased hepatic parenchymal perfusion and hepatic insulin mediated glucose uptake, suggesting a potential modulating effect of hepatic fat on hepatic physiology.

Increased brain fatty acid uptake in metabolic syndrome

OBJECTIVE

To test whether brain fatty acid uptake is enhanced in obese subjects with metabolic syndrome (MS) and whether weight reduction modifies it.

RESEARCH DESIGN AND METHODS

We measured brain fatty acid uptake in a group of 23 patients with MS and 7 age-matched healthy control subjects during fasting conditions using positron emission tomography (PET) with [(11)C]-palmitate and [(18)F]fluoro-6-thia-heptadecanoic acid ([(18)F]-FTHA). Sixteen MS subjects were restudied after 6 weeks of very low calorie diet intervention.

RESULTS

At baseline, brain global fatty acid uptake derived from [(18)F]-FTHA was 50% higher in patients with MS compared with control subjects. The mean percentage increment was 130% in the white matter, 47% in the gray matter, and uniform across brain regions. In the MS group, the nonoxidized fraction measured using [(11)C]-palmitate was 86% higher. Brain fatty acid uptake measured with [(18)F]-FTHA-PET was associated with age, fasting serum insulin, and homeostasis model assessment (HOMA) index. Both total and nonoxidized fractions of fatty acid uptake were associated with BMI. Rapid weight reduction decreased brain fatty acid uptake by 17%.

CONCLUSIONS

To our knowledge, this is the first study on humans to observe enhanced brain fatty acid uptake in patients with MS. Both fatty acid uptake and accumulation appear to be increased in MS patients and reversed by weight reduction.

Effect of weight loss on liver free fatty acid uptake and hepatic insulin resistance

OBJECTIVE

Weight loss has been shown to decrease liver fat content and whole-body insulin resistance. The current study was conducted to investigate the simultaneous effects of rapid weight reduction with a very-low-calorie diet on liver glucose and fatty acid metabolism and liver adiposity.

HYPOTHESIS

We hypothesized that liver insulin resistance and free fatty acid uptake would decrease after weight loss and that they are associated with reduction of liver fat content.

DESIGN

Thirty-four healthy obese subjects (body mass index, 33.7 +/- 8.0 kg/m(2)) were studied before and after a very-low-calorie diet for 6 wk. Hepatic glucose uptake and endogenous glucose production were measured with [(18)F]fluorodeoxyglucose during hyperinsulinemic euglycemia and fasting hepatic fatty acid uptake with [(18)F]fluoro-6-thia-heptadecanoic acid and positron emission tomography. Liver volume and fat content were measured using magnetic resonance imaging and spectroscopy.

RESULTS

Subjects lost weight (11.2 +/- 2.9 kg; P < 0.0001). Liver volume decreased by 11% (P < 0.002), which was partly explained by decreased liver fat content (P < 0.0001). Liver free fatty acid uptake was 26% lower after weight loss (P < 0.003) and correlated with the decrement in liver fat content (r = 0.54; P < 0.03). Hepatic glucose uptake during insulin stimulation was unchanged, but the endogenous glucose production decreased by 40% (P < 0.04), and hepatic insulin resistance by 40% (P < 0.05).

CONCLUSIONS

The liver responds to a 6-wk period of calorie restriction with a parallel reduction in lipid uptake and storage, accompanied by enhancement of hepatic insulin sensitivity and clearance.

The Effect of Insulin on the Intracellular Distribution of 14(R,S)-[18F]Fluoro-6-thia-heptadecanoic Acid in Rats

PURPOSE

The aim of this study was to determine the effect of hyperinsulinemia on myocardial and hepatic distribution and metabolism of 14(R,S)-[18F]fluoro-6-thia-heptadecanoic acid ([18F]FTHA).

PROCEDURES

Mitochondrial retention and intracellular lipid incorporation of [18F]FTHA were compared to that of [14C]-2-bromopalmitate or [14C]palmitate during hyperinsulinemic clamp vs. saline infusion in male Wistar rats.

RESULTS

Mitochondrial 18F activity was increased in the heart (1.7 +/- 0.4 vs. 0.5 +/- 0.1% ID/g, P < 0.05), whereas it was reduced in the liver (1.1 +/- 0.3 vs. 1.8 +/- 0.4% ID/g, P < 0.05) during insulin vs. saline infusion, respectively. Mitochondrial [14C]-2-bromopalmitate activity was affected by insulin in a similar way in both tissues. The fractional esterification of [18F]FTHA into triglycerides was impaired compared to [14C]palmitate in both tissues, and [18F]FTHA was insensitive to the shift of esterification of fatty acids into complex lipids in response to insulin.

CONCLUSIONS

[18F]FTHA is sensitive to insulin-induced modifications of free fatty acid oxidative metabolism in rats but is insensitive to changes in nonoxidative fatty acid metabolism.