Free Fatty Acid Turnover Research Articles

Lipid Storage, Lipolysis, and Lipotoxicity in Obesity.

The ratio of free fatty acid (FFA) turnover decreases significantly with the expansion of white adipose tissue. Adipose tissue and dietary saturated fatty acid levels significantly correlate with an increase in fat cell size and number. The G0/G1 switch gene 2 increases lipid content in adipocytes and promotes adipocyte hypertrophy through the restriction of triglyceride (triacylglycerol: TAG) turnover. Hypoxia in obese adipose tissue due to hypertrophic adipocytes results in excess deposition of extracellular matrix (ECM) components. Cluster of differentiation (CD) 44, as the main receptor of the extracellular matrix component regulates cell-cell and cell-matrix interactions including diet-induced insulin resistance. Excess TAGs, sterols, and sterol esters are surrounded by the phospholipid monolayer surface and form lipid droplets (LDs). Once LDs are formed, they grow up because of the excessive amount of intracellular FFA stored and reach a final size. The ratio of FFA turnover/lipolysis decreases significantly with increases in the degree of obesity. Dysfunctional adipose tissue is unable to expand further to store excess dietary lipids, increased fluxes of plasma FFAs lead to ectopic fatty acid deposition and lipotoxicity. Reduced neo-adipogenesis and dysfunctional lipid-overloaded adipocytes are hallmarks of hypertrophic obesity linked to insulin resistance. Obesity-associated adipocyte death exhibits feature of necrosis-like programmed cell death. Adipocyte death is a prerequisite for the transition from hypertrophic to hyperplastic obesity. Increased adipocyte number in obesity has life-long effects on white adipose tissue mass. The positive correlation between the adipose tissue volume and magnetic resonance imaging proton density fat fraction estimation is used for characterization of the obesity phenotype, as well as the risk stratification and selection of appropriate treatment strategies. In obese patients with type 2 diabetes, visceral adipocytes exposed to chronic/intermittent hyperglycemia develop a new microRNAs' (miRNAs') expression pattern. Visceral preadipocytes memorize the effect of hyperglycemia via changes in miRNAs' expression profile and contribute to the progression of diabetic phenotype. Nonsteroidal anti-inflammatory drugs, metformin, and statins can be beneficial in treating the local or systemic consequences of white adipose tissue inflammation. Rapamycin inhibits leptin-induced LD formation. Collectively, in this chapter, the concept of adipose tissue remodeling in response to adipocyte death or adipogenesis, and the complexity of LD interactions with the other cellular organelles are reviewed. Furthermore, clinical perspective of fat cell turnover in obesity is also debated.

Investigating Effects of Rice Bran on Obesity, Oxidative Stress, Inflammation, and Physical Performance: A Narrative Mini-review

Context: Regular physical activity reduces oxidative stress by increasing the capacity of antioxidant defense, reducing pro-inflammatory mediators, and increasing anti-inflammatory mediators in healthy and obese conditions with chronic diseases. The role of physical activities in oxidative stress, and inflammation is dual and depends on the mood, intensity, and duration of the activity. Evidence acquisition: One session of long-term and intense exercises can produce free radicals more than the capacity of antioxidant defense and cause oxidative stress. Medicinal plants also have anti-obesity, antioxidant, and anti-inflammatory properties due to their phytochemical compounds. Results: Rice bran contains nutrients and bioactive compounds, such as phytosterols, phenolic acids, flavonoids, anthocyanins, proanthocyanins, tocopherols, tocotrienols, phytic acid, gamma-aminobutyric acid, and gamma oryzanol. These compounds have antioxidant, anti-inflammatory, anti-obesity, and anti-diabetic effects. The examination of studies with human and animal subjects show that rice bran, gamma oryzanol, and anthocyanins improve endurance and resistance performance. Evidence shows that rice bran improves endurance performance by reducing lactate accumulation, and increasing muscle glycogen content, increasing free fatty acids turnover, and increasing muscle oxidative capacity and antioxidant enzymes. Conclusion: Anthocyanins improve physical performance by affecting blood flow, metabolic pathways, and muscle fatigue. Accordingly, rice bran can improve physical performance as an ergogenic factor and promote health development by reducing oxidative stress and inflammation. However, there is a need for more studies regarding the effect of rice bran on physical performance and health.

Volanesorsen, an Antisense Oligonucleotide to Apolipoprotein-CIII, Decreases Triglycerides and Increases Lipoprotein Lipase Activity in Partial Lipodystrophy

Partial lipodystrophy syndromes (PL) involve selective deficiency of adipose tissue, with regional deficiency of fat in the lower extremities and preservation or even excess fat in the face and neck. Clinical features typical of PL include severe insulin resistance, diabetes mellitus, hypertriglyceridemia and non-alcoholic fatty liver disease. Apolipoprotein CIII (Apo-CIII) is elevated in PL, and is thought to contribute to high TG by inhibiting lipoprotein lipase (LPL). However, prior studies of this drug in patients with LPL mutations demonstrated LPL-independent mechanisms of TG-lowering. We hypothesized that Volanesorsen, an antisense oligonucleotide (ASO) to apo-CIII, would decrease apo-CIII, increase LPL activity, and lower TG in PL. We further hypothesized that Volanesorsen would improve insulin resistance and glycemia by directing free fatty acids (FFA) into adipose tissue, rather than ectopic sites (e.g. liver) associated with insulin resistance. Five adults with PL and TG ≥500 mg/dL or TG≥200 with A1c >7.0% were enrolled in a 16-week placebo-controlled, randomized, double blind study of Volanesorsen, 300 mg SC weekly, followed by a 1-year open label extension. Here, we report within-subject effects of Volanesorsen lipids, glycemia and lipolysis, before and after 16 weeks of active drug. From week 0 to week 16, apoC-III decreased from 380 (246, 600) to 75 (26, 232) ng/mL, TG decreased from 503 (330, 1040) to 116 (86, 355) mg/dL; and LPL activity measured in post-heparin plasma utilizing the subject’s serum as activator increased from 22.0±3.0 to 35.5±5.9 nEq/ml/min. Free fatty acid turnover (measured by palmitate tracer studies) decreased from 0.41 (0.35, 0.45) to 0.25 (0.23, 0.29) mg/kg/min. There was no change in A1c (8.4±1.2 to 8.3±0.9%), however there was a decrease in HOMA-IR from 26 (20, 54) to 13 (9, 43) and an increase in peripheral insulin sensitivity (glucose infusion rate during euglycemic hyperinsulinemic clamp, 120 mU/m2/min) from 3.6±2.4 to 4.4±1.5 mg/kgFFM/min and in hepatic insulin sensitivity (% suppression of hepatic glucose production during clamp) from 78±19 to 90±13%. Adverse events include injection site reactions and decreased platelets. Volanesorsen decreased apo-CIII and triglycerides, at least in part through an LPL dependent mechanism, and may improve insulin resistance.

Triacylglycerols are preferentially oxidized over free fatty acids in heated soybean oil

In oil, free fatty acids (FFAs) are thought to be the preferred substrate for lipid oxidation although triacylglycerols (TAGs) are the predominant lipid class. We determined the preferential oxidation substrate (TAGs versus FFAs) in soybean oil heated at 100 °C for 24 h, after validating a method for quantifying esterified and free lipid oxidation products (i.e., oxylipins) with mass-spectrometry. Reaction velocities and turnover (velocity per unit substrate) of FFA, and free and TAG-bound (esterified) oxylipins were determined. FFA hydrolysis rate and turnover were orders of magnitude greater (16-4217 fold) than that of esterified and free oxylipin formation. The velocity and turnover of TAG-bound oxylipins was significantly greater than free oxylipins by 282- and 3-fold, respectively. The results suggest that during heating, TAGs are preferentially oxidized over FFAs, despite the rapid hydrolysis and availability of individual FFAs as substrates for oxidation. TAG-bound oxylipins may serve as better markers of lipid oxidation.

Indirect Regulation of Endogenous Glucose Production by Insulin: The Single Gateway Hypothesis Revisited.

On the basis of studies that investigated the intraportal versus systemic insulin infusion and transendothelial transport of insulin, we proposed the "single gateway hypothesis," which supposes an indirect regulation of hepatic glucose production by insulin; the rate-limiting transport of insulin across the adipose tissue capillaries is responsible for the slow suppression of free fatty acids (FFAs), which in turn is responsible for delayed suppression of hepatic endogenous glucose production (EGP) during insulin infusion. Preventing the fall in plasma FFAs during insulin infusion either by administering intralipids or by inhibiting adipose tissue lipolysis led to failure in EGP suppression, thus supporting our hypothesis. More recently, mice lacking hepatic Foxo1 in addition to Akt1 and Akt2 (L-AktFoxo1TKO), all required for insulin signaling, surprisingly showed normal glycemia. Inhibiting the fall of plasma FFAs in these mice prevented the suppression of EGP during a clamp, reaffirming that the site of insulin action to control EGP is extrahepatic. Measuring whole-body turnover rates of glucose and FFAs in L-AktFoxo1TKO mice also confirmed that hepatic EGP was regulated by insulin-mediated control of FFAs. The knockout mouse model in combination with sophisticated molecular techniques confirmed our physiological findings and the single gateway hypothesis.

Fat Cell and Fatty Acid Turnover in Obesity.

The ratio of free fatty acid (FFA) turnover decreases significantly with the expansion of white adipose tissue. Adipose tissue and dietary saturated fatty acid levels significantly correlate with an increase in fat cell size and number. Inhibition of adipose triglyceride lipase leads to an accumulation of triglyceride, whereas inhibition of hormone-sensitive lipase leads to the accumulation of diacylglycerol. The G0/G1 switch gene 2 increases lipid content in adipocytes and promotes adipocyte hypertrophy through the restriction of triglyceride turnover. Excess triacylglycerols (TAGs), sterols and sterol esters are surrounded by the phospholipid monolayer surface and form lipid droplets. Following the release of lipid droplets from endoplasmic reticulum, cytoplasmic lipid droplets increase their volume either by local TAG synthesis or by homotypic fusion. The number and the size of lipid droplet distribution is correlated with obesity. Obesity-associated adipocyte death exhibits feature of necrosis-like programmed cell death. NOD-like receptors family pyrin domain containing 3 (NLRP3) inflammasome-dependent caspase-1 activation in hypertrophic adipocytes induces obese adipocyte death by pyroptosis. Actually adipocyte death may be a prerequisite for the transition from hypertrophic to hyperplastic obesity. Major transcriptional factors, CCAAT/enhancer-binding proteins beta and delta, play a central role in the subsequent induction of critical regulators, peroxisome-proliferator-activated receptor gamma, CCAAT/enhancer-binding protein alpha and sterol regulatory element-binding protein 1, in the transcriptional control of adipogenesis in obesity.Collectively, in this chapter the concept of adipose tissue remodeling in response to adipocyte death or adipogenesis, and the complexity of lipid droplet interactions with the other cellular organelles are reviewed. Furthermore, in addition to lipid droplet growth, the functional link between the adipocyte-specific lipid droplet-associated protein and fatty acid turn-over is also debated.

Dose–response-time modelling: Second-generation turnover model with integral feedback control

This study presents a dose–response-time (DRT) analysis based on a large preclinical biomarker dataset on the interaction between nicotinic acid (NiAc) and free fatty acids (FFA). Data were collected from studies that examined different rates, routes, and modes of NiAc provocations on the FFA time course. All information regarding the exposure to NiAc was excluded in order to demonstrate the utility of a DRT model. Special emphasis was placed on the selection process of the biophase model. An inhibitory Imax-model, driven by the biophase amount, acted on the turnover rate of FFA. A second generation NiAc/FFA model, which encompasses integral (slow buildup of tolerance — an extension of the previously used NiAc/FFA turnover models) and moderator (rapid and oscillatory) feedback control, was simultaneously fitted to all time courses in normal rats. The integral feedback control managed to capture an observed 90% adaptation (i.e., almost a full return to baseline) when 10 days constant-rate infusion protocols of NiAc were used. The half-life of the adaptation process had a 90% prediction interval between 3.5–12 in the present population. The pharmacodynamic parameter estimates were highly consistent when compared to an exposure-driven analysis, partly validating the DRT modelling approach and suggesting the potential of DRT analysis in areas where exposure data are not attainable. Finally, new numerical algorithms, which rely on sensitivity equations to robustly and efficiently compute the gradients in the parameter optimization, were successfully used for the mixed-effects approach in the parameter estimation.

Adjusting glucose-stimulated insulin secretion for adipose insulin resistance: an index of β-cell function in obese adults.

The hyperbolic relationship between insulin secretion and sensitivity has been used to assess in vivo β-cell function (i.e., the disposition index). The disposition index emphasizes the importance of taking into account both skeletal muscle and hepatic insulin resistance to depict insulin secretion. However, we propose that adipose tissue insulin resistance also needs to be accounted for when characterizing glucose-stimulated insulin secretion (GSIS) because elevated plasma free fatty acids (FFAs) impair β-cell function. To characterize the adipose disposition index, we used [1-(14)C] palmitate infusion to determine basal FFA turnover rate/adipose insulin resistance and an oral glucose tolerance test to characterize the first (i.e., 0-30 min) and second phase (i.e., 60-120 min) of GSIS. We validated a simplified version of the tracer infusion calculation as the product of (1/plasma FFA concentration × plasma insulin concentration) × GSIS in 44 obese insulin-resistant subjects. The plasma FFA and palmitate tracer infusion calculations of the first- and second-phase disposition index were strongly correlated (r = 0.86, P < 0.000001 and r = 0.89, P < 0.000001, respectively). The first- and second-phase adipose disposition index derived from plasma FFA also was tightly associated with fasting hyperglycemia (r = -0.87, P < 0.00001 and r = -0.89, P < 0.00001, respectively) and 2-h glucose concentrations (r = -0.86, P < 0.00001 and r = -0.90, P < 0.00001). Adjusting GSIS for adipose insulin resistance provides an index of β-cell function in obese subjects across the glucose spectrum. Plasma FFA-derived calculations of β-cell function may provide additional insight into the role of adipose tissue in glucose regulation.

Effects of acute glucocorticoid blockade on metabolic dysfunction in patients with Type 2 diabetes with and without fatty liver.

To investigate the potential of therapies which reduce glucocorticoid action in patients with Type 2 diabetes we performed a randomized, double-blinded, placebo-controlled crossover study of acute glucocorticoid blockade, using the glucocorticoid receptor antagonist RU38486 (mifepristone) and cortisol biosynthesis inhibitor (metyrapone), in 14 men with Type 2 diabetes. Stable isotope dilution methodologies were used to measure the rates of appearance of glucose, glycerol, and free fatty acids (FFAs), including during a low-dose (10 mU·m −2·min−1) hyperinsulinemic clamp, and subgroup analysis was conducted in patients with high or low liver fat content measured by magnetic resonance spectroscopy (n = 7/group). Glucocorticoid blockade lowered fasting glucose and insulin levels and improved insulin sensitivity of FFA and glycerol turnover and hepatic glucose production. Among this population with Type 2 diabetes high liver fat was associated with hyperinsulinemia, higher fasting glucose levels, peripheral and hepatic insulin resistance, and impaired suppression of FFA oxidation and FFA and glycerol turnover during hyperinsulinemia. Glucocorticoid blockade had similar effects in those with and without high liver fat. Longer term treatments targeting glucocorticoid action may be useful in Type 2 diabetes with and without fatty liver.

Autophagy in COPD

Autophagy is an important cellular homeostatic process, where cell self-degrades long-lived proteins and damaged sub cellular organelles and proteins with the help of lysosomes in eukaryotic cells. In the past decade, remarkable advances have been made in describing autophagy as clinically relevant target to the treatment of pulmonary diseases. In this review I attempt to provide an overview of autophagy signaling pathway and also summarize the recent studies that suggest direct relevancy and function of autophagy pathway in the Chronic Obstructive Pulmonary Disease (COPD). However, further resolution of autophagic proteins and mechanisms is warranted in specifically designing autophagy-based therapeutic targets in COPD. Introduction Autophagy in COPD Mammalian macroautophagy (hereafter referred as autophagy) is evolutionary conserved degradative pathway that involves the fusion of double membrane autophagosomes and lysosomes for the turnover of free amino acids and fatty acids [1,2]. In addition to this, two other forms of autophagy mechanisms have been described. One type involves the invagination of lysosomal membrane to engulf cytosolic components into intralysosomal vesicles termed as microautophagy [3]. In chaperone-mediated autophagy, proteins with a particular consensus motif (KFERQ) are selectively delivered across the lysosomal membrane for degradation [4]. Autophagy can be upregulated in response to multiple forms of physiological stress including starvation [5], growth factor deprivation [6], reactive oxygen species [7], hypoxia [8], hyperoxia [9,10], endoplasmic reticulum stress [11], DNA damage [12], protein aggregates [13] or pathogens [14]. Therefore, autophagy can be predominantly viewed as a cellular response pathway to a variety of stress stimuli rather than a VRI Cell Signaling, Volume 1, Issue 2, October 2013 nutrient deprivation response pathway. Chronic or acute exposure of the aforementioned stimuli can result in dysregulated or deranged autophagy pathway with the development of several pathological conditions. In this regard stimulant cigarette smoke upon chronic exposure has recently demonstrated an increased expression of autophagy markers in the development of COPD [15,16]. A better understanding for the role of autophagy pathway in COPD will help support in the design of the therapies for the cure of COPD. eISSN 2330-0302 I S S N 2 3 3 0 0 3 0 2 ( O n l i i n e ) ; C O D E N : V R I C A O , V R I C e l l S i g n a l i n g , V e d i c Mini Review DOI: http://dx.doi.org/10.14259/cs.v1i2.68 Article Info: Received: September 25th, 2013; Accepted: October 1st, 2013

Elongase of long chain fatty acids family 6 (ELOVL6) and stearoyl‐CoA desaturase ‐1 (SCD1) indices suggests potential metabolic benefits in patients with a range of liver fat contents

SCD1 and ELOVL6 enzyme activities and dietary fatty acid composition contribute to the balance of monounsaturated (MUFA), polyunsaturated (PUFA), and saturated fatty acids (SFA) in the plasma free fatty acid (FFA) pool. Given the potential lipotoxic effects of SFA, the present study investigated FFA composition in fasted and meal‐fed (42g fat, 100g carbohydrate) nondiabetic, African American and Hispanic adults (n = 14, BMI 35.6 ± 6.8 kg/m2, 47.7 ± 8.5 yrs) with low (&lt; 5.6%) or high (&gt; 5.6%) liver fat. FFA composition (mol %) was determined by GC, substrate oxidation by indirect calorimetry, and substrate metabolism using stable isotopes. Low ELOVL6 index (18:0/16:0) was associated with high postprandial insulin (r = − 0.622, P = 0.02) and lower in those with high liver fat (P = 0.01). Fasting FFA turnover correlated with FFA 18:1n9 (r = +0.694, P = 0.01) and 18:2n6 (r = +0.642, P = 0.01), and negatively with 18:0 (r = −0.731, P &lt; 0.01) and 16:0 (r = −0.731, P &lt; 0.01). Whole body fat oxidation correlated with 16:1n7 (r = +0.646, P = 0.02) and indices of SCD1 including 18:1n9/18:0 ratio (r = +0.558, P &lt; 0.05) and 16:1n7/16:0 (r = +0.633, P = 0.02). Fat oxidation correlated inversely with 18:0 (r = −0.592, P = 0.03). Thus, 18:1n9 may be a biomarker for both greater FFA turnover and disposal. These data support beneficial effects of MUFA over SFA in overweight subjects and a positive influence of ELOVL6 and SCD1 on insulin sensitivity and liver fat.

Enhanced fatty acid flux triggered by adiponectin overexpression.

Adiponectin overexpression in mice increases insulin sensitivity independent of adiposity. Here, we combined stable isotope infusion and in vivo measurements of lipid flux with transcriptomic analysis to characterize fatty acid metabolism in transgenic mice that overexpress adiponectin via the aP2-promoter (ADNTg). Compared with controls, fasted ADNTg mice demonstrated a 31% reduction in plasma free fatty acid concentrations (P = 0.008), a doubling of ketones (P = 0.028), and a 68% increase in free fatty acid turnover in plasma (15.1 ± 1.5 vs. 25.3 ± 6.8 mg/kg · min, P = 0.011). ADNTg mice had 2-fold more brown adipose tissue mass, and triglyceride synthesis and turnover were 5-fold greater in this organ (P = 0.046). Epididymal white adipose tissue was slightly reduced, possibly due to the approximately 1.5-fold increase in the expression of genes involved in oxidation (peroxisome proliferator-activated receptor α, peroxisome proliferator-activated receptor-γ coactivator 1α, and uncoupling protein 3). In ADNTg liver, lipogenic gene expression was reduced, but there was an unexpected increase in the expression of retinoid pathway genes (hepatic retinol binding protein 1 and retinoic acid receptor beta and adipose Cyp26A1) and liver retinyl ester content (64% higher, P < 0.02). Combined, these data support a physiological link between adiponectin signaling and increased efficiency of triglyceride synthesis and hydrolysis, a process that can be controlled by retinoids. Interactions between adiponectin and retinoids may underlie adiponectin's effects on intermediary metabolism.

Nonoxidative Free Fatty Acid Disposal Is Greater in Young Women than Men

Large increases in systemic free fatty acid (FFA) availability in the absence of a corresponding increase in fatty acid oxidation can create a host of metabolic abnormalities. These adverse responses are thought to be the result of fatty acids being shunted into hepatic very low-density lipoprotein-triglyceride production and/or intracellular lipid storage and signaling pathways because tissues are forced to increase nonoxidative FFA disposal. The objective of the study was to examine whether variations in postabsorptive nonoxidative FFA disposal within the usual range predict insulin resistance and hypertriglyceridemia. We measured: systemic FFA turnover using a continuous iv infusion of [9-10, (3)H]palmitate; substrate oxidation with indirect calorimetry combined with urinary nitrogen excretion; whole-body and peripheral insulin sensitivity with the labeled iv glucose tolerance test minimal model. the study was conducted at the Mayo Clinic General Clinical Research Center. Participants included healthy, postabsorptive, nonobese adults (21 women and 21 men). There were no interventions. Nonoxidative FFA disposal (micromoles per minute), defined as the FFA disappearance rate minus fatty acid oxidation. Women had 64% greater nonoxidative FFA disposal rate than men but a better lipid profile and similar insulin sensitivity. There was no significant correlation between nonoxidative FFA disposal and whole-body sensitivity, peripheral insulin sensitivity, or fasting serum triglyceride concentrations in men or women. Healthy nonobese women have greater rates of nonoxidative FFA disposal than men, but this does not appear to relate to adverse health consequences. Understanding the sex-specific interaction between adipose tissue lipolysis and peripheral FFA removal will help to discover new approaches to treat FFA-induced abnormalities.

Relationship between postabsorptive respiratory exchange ratio and plasma free fatty acid concentrations

The relationship between overnight postabsorptive (fasting) respiratory exchange ratio (RER) and plasma FFA concentrations was addressed using data from three separate protocols, each of which involved careful control of the antecedent diet. Protocol 1 examined the relationship between fasting RER and the previous daytime RER. In Protocol 2 fasting, RER and plasma palmitate concentrations were measured in 29 women and 31 men (body mass index <30 kg.m(-2)). Protocol 3 analyzed data from Nielsen et al. (Nielsen, S., Z. K. Guo, J. B. Albu, S. Klein, P. C. O'Brien, M. D. Jensen. 2003. Energy expenditure, sex and endogenous fuel availability in humans. J. Clin. Invest. 111: 981-988.) to understand how fasting RER and palmitate concentrations relate within individuals during four consecutive measurements. The results were as follows: 1) Fasting RER was correlated (r = 0.74, P < 0.001) with the previous day's average RER, and less so with RER variability. 2) Fasting RER was correlated (r = -0.39, P = 0.007) with fasting plasma palmitate concentrations. 3) The pattern of the RER/palmitate relationship was similar within individuals and between individuals; a negative slope was observed significantly more often than a positive slope (chi(2) test; P < 0.001). Our findings suggest that, despite a fixed food quotient, the slight departures from energy equilibrium in a controlled General Clinical Research Center environment can effect plasma FFA concentrations. We suggest that including indirect calorimetry as part of FFA metabolism studies may aid in data interpretation.

Effects of exercise training and diet on lipid kinetics during free fatty acid-induced insulin resistance in older obese humans with impaired glucose tolerance

Elevated free fatty acids (FFA) are implicated with insulin resistance at the cellular level. However, the contribution of whole body lipid kinetics to FFA-induced insulin resistance is not well understood, and the effect of exercise and diet on this metabolic defect is not known. We investigated the effect of 12 wk of exercise training with and without caloric restriction on FFA turnover and oxidation (FFA(ox)) during acute FFA-induced insulin resistance. Sixteen obese subjects with impaired glucose tolerance were randomized to either a hypocaloric (n = 8; -598 +/- 125 kcal/day, 66 +/- 1 yr, 32.8 +/- 1.8 kg/m(2)) or a eucaloric (n = 8; 67 +/- 2 yr, 35.3 +/- 2.1 kg/m(2)) diet and aerobic exercise (1 h/day at 65% of maximal oxygen uptake) regimen. Lipid kinetics ([1-(14)C]palmitate) were assessed throughout a 7-h, 40 mU x m(-2) x min(-1) hyperinsulinemic euglycemic clamp, during which insulin resistance was induced in the last 5 h by a sustained elevation in plasma FFA (intralipid/heparin infusion). Despite greater weight loss in the hypocaloric group (-7.7 +/- 0.5 vs. -3.3 +/- 0.7%, P < 0.001), FFA-induced peripheral insulin resistance was improved equally in both groups. However, circulating FFA concentrations (2,123 +/- 261 vs. 1,764 +/- 194 micromol/l, P < 0.05) and FFA turnover (3.20 +/- 0.58 vs. 2.19 +/- 0.58 micromol x kg FFM(-1) x min(-1), P < 0.01) during hyperlipemia were suppressed only in the hypocaloric group. In contrast, whole body FFA(ox) was improved in both groups at rest and during hyperlipemia. These changes were driven by increases in intracellular lipid-derived FFA(ox) (12.3 +/- 7.7 and 14.7 +/- 7.8%, P < 0.05). We conclude that the exercise-induced improvement in FFA-induced insulin resistance is independent of the magnitude of weight loss and FFA turnover, yet it is linked to increased intracellular FFA utilization.

Whole-body synthesis-secretion rates of long-chain n-3 PUFAs from circulating unesterified α-linolenic acid in unanesthetized rats

Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), long-chain n-3 PUFAs important for brain and heart function, can be obtained from dietary fish products or by liver synthesis from alpha-linolenic acid (alpha-LNA). Their daily human dietary requirements are not clear, and their liver synthesis rates in humans and nonhumans are unknown. We estimated whole-body (presumably liver) synthesis rates in unanesthetized rats by infusing [U-(13)C]alpha-LNA intravenously for 2 h and measuring labeled and unlabeled n-3 PUFA in arterial plasma using negative chemical ionization GC-MS. Newly synthesized esterified [(13)C]DHA, [(13)C]EPA, and [(13)C]docosapentaenoic acid (DPA) appeared in arterial plasma after 60 min of infusion, then their concentrations rose in an S-shaped manner. Esterified concentration x plasma volume data were fit with a sigmoidal equation, whose peak first derivatives provided synthesis rates of unlabeled EPA, DPA, and DHA equal to 8.40, 6.27, and 9.84 mumol/day, respectively. The DHA synthesis rate exceeded the published daily rat brain DHA consumption rate by 30-fold, suggesting that liver synthesis from alpha-LNA could maintain brain DHA homeostasis were DHA absent from the diet. This stable isotope infusion method could be used to quantify whole-body DHA synthesis rates in human subjects.

Fat Metabolism During Exercise in Patients With Mitochondrial Disease

To determine whether patients with defects of the respiratory chain have metabolic adaptations that promote a preferential use of fats or carbohydrates, similar to what is observed in metabolic myopathies affecting glycolysis or fat oxidation. Causation and case-control study. Fat metabolism was determined by means of indirect calorimetry and stable isotope technique in patients and healthy subjects. Patients carried various types and loads (mean [SE], 72% [5%]) of mitochondrial DNA (mtDNA) mutations in skeletal muscle. All subjects exercised at the same absolute workload (mean [SE], 65 [10] W), corresponding to 72% (in patients) and 30% (in healthy subjects) of maximum oxygen consumption. Neuromuscular research unit. Ten patients with mtDNA mutations and 10 sex-matched healthy subjects. Fat turnover, plasma concentrations of palmitate and total free fatty acids, glucose mobilization, and total carbohydrate oxidation. Fat turnover and plasma concentrations of palmitate and total free fatty acids were similar in patients and healthy subjects at rest and during exercise. In line with the higher relative workload of the patients, glucose mobilization and total carbohydrate oxidation were higher in the patients compared with the healthy subjects. During moderate-intensity exercise, the balance between fat and carbohydrate use in patients with mtDNA mutations matches that seen in healthy subjects, indicating that manipulating dietary fat and carbohydrate content is not a feasible therapeutic option to improve exercise intolerance in these disorders.

Fasting decreases free fatty acid turnover in mice overexpressing skeletal muscle lipoprotein lipase

Skeletal muscle lipoprotein lipase (LPL) overexpression in mice results in whole-body insulin resistance and increased intramuscular triglyceride stores, but decreased plasma triglyceride concentration and unchanged plasma free fatty acid (FFA) concentration. The effects of skeletal muscle LPL overexpression and fasting duration on FFA kinetics are unknown. Transgenic mice with muscle-specific LPL overexpression (MCKhLPL) and control mice (Con) were studied at rest during a 50-minute constant infusion of [9,10- 3H]palmitate to determine FFA kinetics after both 4 and 16 hours of fasting. FFA concentration was not different between groups after the 4-hour (Con, 0.80 ± 0.06 mmol/L; MCKhLPL, 0.83 ± 0.07 mmol/L) and 16-hour (Con, 0.83 ± 0.04 mmol/L; MCKhLPL, 0.80 ± 0.07 mmol/L) fast. FFA turnover ( R a) was not significantly different between MCKhLPL and Con groups after the 4-hour fast (Con R a = 2.52 ± 0.36 μmol/min; MCKhLPL R a = 2.37 ± 0.27 μmol/min). However, FFA turnover was significantly decreased after the 16-hour fast in MCKhLPL mice vs controls (Con R a = 2.89 ± 0.52 μmol/min; MCKhLPL R a = 1.64 ± 0.17 μmol/min; P < .05). The significantly lower FFA R a in MCKhLPL vs control mice was due to a decrease in MCKhLPL FFA turnover from the 4- to 16-hour fast, whereas FFA turnover was unchanged in controls. The changes in FFA appearance after the 16-hour fast in MCKhLPL mice are most likely explained by increased reliance by skeletal muscle on plasma triglyceride as a fuel. These data suggest increased skeletal muscle LPL expression decreases dependence on plasma FFA during prolonged fasting in mice.

Metabolic Inflexibility in Substrate Use Is Present in African-American But Not Caucasian Healthy, Premenopausal, Nondiabetic Women

There is an increased prevalence of obesity and insulin resistance in African-American compared with Caucasian females. Metabolic inflexibility (MI) is the inability to switch the use of lipids and carbohydrates in the peripheral tissue (i.e. muscle) based upon substrate availability. We examined whether MI exists in African-American females. MAIN OUTCOME MEASURES AND DESIGN: We measured substrate use differences during eucaloric, macronutrient-manipulated diets [high fat (50% fat, 35% carbohydrate, 15% protein) vs. low fat (30% fat, 55% carbohydrate, 15% protein)] between Caucasian and African-American women. We also compared differences in substrate use in response to insulin infusion during two-step pancreatic-euglycemic clamps and epinephrine infusion during lipolysis studies. In each study, similar groups of Caucasian and African-American women were compared. Caucasians had significantly higher fat oxidation (FO) (P = 0.01) and lower carbohydrate oxidation (P < 0.01) during the high-fat vs. low-fat diet, whereas no significant differences were observed in African-Americans. The African-American women also failed to significantly suppress FO during the second step of the pancreatic-euglycemic clamp despite a doubling of their fasting plasma insulin and failed to increase their FO or decrease their carbohydrate oxidation in response to epinephrine infusion as much as Caucasian women did. The response of free fatty acid turnover rates to insulin and epinephrine stimulation was similar between races. The impaired substrate use observed in African-American women during these three studies demonstrates the existence of MI and may contribute to their greater prevalence of obesity and insulin resistance.

From Obesity to Diabetes

The prevalence of obesity has been increasing dramatically in the last decades in the whole world, not only in industrialized countries but also in developing areas. A major complication of obesity is insulin resistance and type 2 diabetes. Diabetes is also rapidly increasing world-wide--reaching a prevalence in adults of approx. 5-6% in Central Europe and in the US, and more than 50% in specific, genetically prone populations. This article reviews pathogenetic mechanisms linking obesity and type 2 diabetes. Emphasis is placed on the observation that excessive amounts of adipocytes are associated with an impairment of insulin sensitivity, a key feature of the "metabolic syndrome". This is a cluster of metabolic abnormalities such as type 2 diabetes, hypertension and dyslipidemia; all of them are enhanced by the presence of visceral (abdominal) obesity and all contribute to the increased cardiovascular risk observed in these patients. Besides release of free fatty acids, adipocytes secrete substances that contribute to peripheral insulin resistance, including adiponectin, resistin, TNF-alpha and interleukin 6. Increased turnover of free fatty acids interferes with intracellular metabolism of glucose in the muscle, and they exert lipotoxic effect on pancreatic beta-cells. The pre-receptor metabolism of cortisol is enhanced in visceral adipose tissue by activation of 11 beta-hydroxysteroid dehydrogenase type 1. A new class of anti-diabetic drugs (thiazolidinediones, or glitazones) bind to peroxisome proliferator activated receptor (PPAR-gamma) and lower thereby plasma free fatty acids and cytokine production in adipocytes, in addition to a decrease of resistin and an increase in adiponectin observed in animals, resulting in an overall increase in insulin sensitivity and in an improvement of glucose homeostasis. However, the first step to avoid insulin resistance and prevent the development of diabetes should be a reduction in body weight in overweight subjects, and an increase in physical activity. There are now three published randomized controlled trials demonstrating that in high risk individuals, life style changes with modest weight lost, associated with diminished fat intake and an increase in fruit and vegetable consumption result in marked inhibition of the transition from the prediabetic state to manifest type 2 diabetes.