Human fatty acid synthesis is stimulated by a eucaloric low fat, high carbohydrate diet

Fatty acid synthase and liver triglyceride metabolism: housekeeper or messenger?

Fatty acid synthase (FAS) catalyzes the de novo synthesis of fatty acids. In the liver, FAS has long been categorized as a housekeeping protein, producing fat for storage of energy when nutrients are present in excess. Most previous studies of FAS regulation have focused on the control of gene expression. However, recent findings suggest that hepatic FAS may also be involved in signaling processes that include activation of peroxisome proliferator-activated receptor α (PPARα). Moreover, reports of rapid alterations in FAS activity as well as findings of post-translational modifications of the FAS protein support the notion that dynamic events in addition to transcription impact FAS regulation. These results indicate that FAS enzyme activity can impact liver physiology through signaling as well as energy storage and that its regulation may be complex. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Fatty acids in the de novo lipogenesis pathway and risk of coronary heart disease: the Cardiovascular Health Study

BACKGROUND

De novo lipogenesis (DNL) is an endogenous pathway whereby carbohydrates and proteins are converted to fatty acids. DNL could affect coronary heart disease (CHD) or sudden cardiac arrest (SCA) via generation of specific fatty acids. Whether these fatty acids are prospectively associated with SCA or other CHD events is unknown.

OBJECTIVE

The objective was to investigate the relations of 4 fatty acids in the DNL pathway-palmitic acid (16:0), palmitoleic acid (16:1n-7), 7-hexadecenoic acid (16:1n-9), and cis-vaccenic acid (18:1n-7)-with incident CHD, including fatal CHD, nonfatal myocardial infarction (NFMI), and SCA.

DESIGN

A community-based prospective study was conducted in 2890 men and women aged ≥65 y, who were free of known CHD at baseline and who were followed from 1992 to 2006. Cardiovascular disease risk factors and plasma phospholipid fatty acids were measured at baseline by using standardized methods. Incident CHD was ascertained prospectively and was centrally adjudicated by using medical records. Risk was assessed by using multivariable-adjusted Cox proportional hazards.

RESULTS

During 29,835 person-years of follow-up, 631 CHD and 71 SCA events occurred. Both 18:1n-7 and 16:1n-9 were associated with a higher risk of SCA [multivariable-adjusted hazard ratio (95% CI) for the interquintile range: 7.63 (2.58, 22.6) for 18:1n-7 and 2.30 (1.16, 4.55) for 16:1n-9] but not of total CHD, fatal CHD, or NFMI. In secondary analyses censored to mid-follow-up (7 y) to minimize the effects of changes in concentrations over time, 16:1n-9 was also associated with a significantly higher risk of total CHD (2.11; 1.76, 2.54), including a higher risk of CHD death, NFMI, and SCA; 16:0 and 16:1n-7 were not associated with clinical CHD outcomes.

CONCLUSION

Higher plasma phospholipid 18:1n-7 and 16:1n-9 concentrations were prospectively associated with an elevated risk of SCA but not of other CHD events, except in secondary analyses.

Hepatic glucokinase expression is associated with lipogenesis and fatty liver in humans

BACKGROUND/AIMS

Glucokinase (GCK) phosphorylates glucose to form glucose 6-phosphate and thereby regulates hepatic glucose disposal and activates hepatic lipogenesis. Hepatic GCK activity is regulated on the level of GCK mRNA expression and by the inhibitory glucokinase regulatory protein. In this study, we aimed to investigate the relation between GCK mRNA expression and markers of lipogenesis as well as liver fat content in human liver biopsies. Additionally, we investigated whether genetic variation in the liver specific GCK promoter determines liver fat content in humans.

METHODS

Hepatic mRNA expression and liver triglyceride content was analyzed in 50 human liver biopsies. In a second cohort of 330 individuals, liver fat was precisely measured by 1H magnetic resonance spectroscopy.

RESULTS

Hepatic GCK mRNA expression is associated with triglyceride content in human liver biopsies (r = 0.50, P = 0.0002). Furthermore, hepatic GCK mRNA expression is associated with lipogenic gene expression (fatty acid synthase, r = 0.49, P = 0.0003; acetyl-coenzyme A carboxylase-α, r = 0.44, P = 0.0015, and acetyl-coenzyme A carboxylase-β, r = 0.48, P = 0.0004) and the de novo lipogenesis index (r = 0.36, P = 0.01). In support of these findings, the single-nucleotide polymorphism rs2041547 in the liver-specific GCK promoter is associated with liver fat content in prediabetic individuals (P = 0.047).

CONCLUSIONS

In this study, we demonstrate for the first time that GCK mRNA expression is associated with markers of de novo lipogenesis and liver triglyceride content in humans. This suggests that increased GCK activity may induce fatty liver and its metabolic and hepatic consequences in humans. Thus, the widely used approach to nonspecifically activate β-cell and hepatic GCK to treat diabetes mellitus is therefore questionable and may cause serious side effects.

De novo lipogenesis in the differentiating human adipocyte can provide all fatty acids necessary for maturation

The primary products of de novo lipogenesis (DNL) are saturated fatty acids, which confer adverse cellular effects. Human adipocytes differentiated with no exogenous fat accumulated triacylglycerol (TG) in lipid droplets and differentiated normally. TG composition showed the products of DNL (saturated fatty acids from 12:0 to 18:0) together with unsaturated fatty acids (particularly 16:1n-7 and 18:1n-9) produced by elongation/desaturation. There was parallel upregulation of expression of genes involved in DNL and in fatty acid elongation and desaturation, suggesting coordinated control of expression. Enzyme products (desaturation ratios, elongation ratios, and total pathway flux) were also correlated with mRNA levels. We used (13)C-labeled substrates to study the pathway of DNL. Glucose (5 mM or 17.5 mM in the medium) provided less than half the carbon used for DNL (42% and 47%, respectively). Glutamine (2 mM) provided 9-10%, depending upon glucose concentration. In contrast, glucose provided most (72%) of the carbon of TG-glycerol. Pathway analysis using mass isotopomer distribution analysis (MIDA) revealed that the pathway for conversion of glucose to palmitate is complex. DNL in human fat cells is tightly coupled with further modification of fatty acids to produce a range of saturated and unsaturated fatty acids consistent with normal maturation.

A dual sugar challenge test for lipogenic sensitivity to dietary fructose

CONTEXT

Increased hepatic de novo lipogenesis (DNL) in response to dietary sugar is implicated in dyslipidemia, fatty liver, and insulin resistance.

OBJECTIVE

The aim of the study was to develop a simple outpatient tolerance test for lipogenic sensitivity to dietary sugar.

DESIGN AND SETTING

In inpatients given repeated doses of fructose, protocol 1 compared the acute increase in DNL determined from the percentage of palmitate ("new palmitate") and the percentage of isotopically labeled palmitate ("%DNL") in very low-density lipoprotein triglyceride (TG). Protocol 2 compared the increase in new palmitate in outpatients given three different sugar beverages in a randomized crossover design.

PARTICIPANTS

There were 15 lean and overweight volunteers in protocol 1 and 15 overweight volunteers in protocol 2.

INTERVENTIONS

In protocol 1, subjects received 1.4 g/kg fructose in divided oral doses over 6 h; in protocol 2, subjects received 0.5 g/kg fructose, 0.5 g/kg fructose plus 0.5 g/kg glucose, or 1 g/kg fructose plus 1 g/kg glucose each as a single oral bolus.

MAIN OUTCOME MEASURES

We measured the increase in DNL by two methods.

RESULTS

After repeated doses of fructose, new palmitate was significantly correlated with the increase in %DNL (Δ, r = 0.814; P < 0.001) and with fasting insulin levels (area under the curve, r = 0.754; P = 0.001). After a single sugar dose, new palmitate showed a dose effect and was greater after fructose plus glucose. Very low-density lipoprotein TG and total TG significantly increased in both protocols.

CONCLUSIONS

A single oral bolus of fructose and glucose rapidly increases serum TG and TG palmitate in overweight subjects. A dual sugar challenge test could prove useful to identify individuals at risk for carbohydrate-induced dyslipidemia and other adverse effects of increased DNL.

Brain insulin controls adipose tissue lipolysis and lipogenesis

White adipose tissue (WAT) dysfunction plays a key role in the pathogenesis of type 2 diabetes (DM2). Unrestrained WAT lipolysis results in increased fatty acid release, leading to insulin resistance and lipotoxicity, while impaired de novo lipogenesis in WAT decreases the synthesis of insulin-sensitizing fatty acid species like palmitoleate. Here, we show that insulin infused into the mediobasal hypothalamus (MBH) of Sprague-Dawley rats increases WAT lipogenic protein expression, inactivates hormone-sensitive lipase (Hsl), and suppresses lipolysis. Conversely, mice that lack the neuronal insulin receptor exhibit unrestrained lipolysis and decreased de novo lipogenesis in WAT. Thus, brain and, in particular, hypothalamic insulin action play a pivotal role in WAT functionality.

Relationships between hepatic stearoyl-CoA desaturase-1 activity and mRNA expression with liver fat content in humans

Stearoyl-CoA desaturase-1 (SCD1) has gained much interest as a future drug target to treat fatty liver and its consequences. However, there are few and inconsistent human data about expression and activity of this important enzyme. We investigated activity and expression of SCD1 and their relationships with liver fat (LF) content in human liver samples. Fifty subjects undergoing liver surgery were studied. SCD1 activity was estimated from the ratio of oleate (C18:1) to stearate (C18:0) within lipid subfractions. Furthermore, SCD1 mRNA expression and LF content were measured. Similarly to previous studies, we observed a strong positive correlation between LF content and the C18:1/C18:0 ratio in the combined fatty acid (FA) fractions (r = 0.96, P < 0.0001), which could be interpreted as higher SCD1 activity with increasing LF. However, hepatic SCD1 mRNA expression did not correlate with LF (r = 0.16, P = 0.13). To solve these conflicting data, we analyzed the FA composition of hepatic lipid subfractions. With increasing LF content the amount of FAs from the triglyceride (TG) fraction increased (r = 0.96, P < 0.0001), whereas the FAs from the phospholipid (PL) fraction remained unchanged (r = -0.17, P = 0.19). Of these two major lipid fractions, the C18:1/C18:0 ratio in TG was 16-fold higher than in PL. Supporting the SCD1 mRNA expression data, the C18:1/C18:0 ratio of the TG or PL fraction did not correlate with LF (r = 0.26, P = 0.12 and r = 0.08, P = 0.29). We provide novel information that SCD1 activity and mRNA expression appear not to be elevated in subjects with high LF content. We suggest that the FA composition of lipid subclasses, rather than of mixed lipids, should be analyzed to estimate SCD1 activity.

Trans-palmitoleic acid, metabolic risk factors, and new-onset diabetes in U.S. adults: a cohort study

BACKGROUND

Palmitoleic acid (cis-16:1n-7), which is produced by endogenous fat synthesis, has been linked to both beneficial and deleterious metabolic effects, potentially confounded by diverse determinants and tissue sources of endogenous production. Trans-palmitoleate (trans-16:1n-7) represents a distinctly exogenous source of 16:1n-7, unconfounded by endogenous synthesis or its determinants, that may be uniquely informative.

OBJECTIVE

To investigate whether circulating trans-palmitoleate is independently related to lower metabolic risk and incident type 2 diabetes.

DESIGN

Prospective cohort study from 1992 to 2006.

SETTING

Four U.S. communities.

PATIENTS

3736 adults in the Cardiovascular Health Study.

MEASUREMENTS

Anthropometric characteristics and levels of plasma phospholipid fatty acids, blood lipids, inflammatory markers, and glucose-insulin measured at baseline in 1992 and dietary habits measured 3 years earlier. Multivariate-adjusted models were used to investigate how demographic, clinical, and lifestyle factors independently related to plasma phospholipid trans-palmitoleate; how trans-palmitoleate related to major metabolic risk factors; and how trans-palmitoleate related to new-onset diabetes (304 incident cases). Findings were validated for metabolic risk factors in an independent cohort of 327 women.

RESULTS

In multivariate analyses, whole-fat dairy consumption was most strongly associated with higher trans-palmitoleate levels. Higher trans-palmitoleate levels were associated with slightly lower adiposity and, independently, with higher high-density lipoprotein cholesterol levels (1.9% across quintiles; P = 0.040), lower triglyceride levels (-19.0%; P < 0.001), a lower total cholesterol-HDL cholesterol ratio (-4.7%; P < 0.001), lower C-reactive protein levels (-13.8%; P = 0.05), and lower insulin resistance (-16.7%, P < 0.001). Trans-palmitoleate was also associated with a substantially lower incidence of diabetes, with multivariate hazard ratios of 0.41 (95% CI, 0.27 to 0.64) and 0.38 (CI, 0.24 to 0.62) in quintiles 4 and 5 versus quintile 1 (P for trend < 0.001). Findings were independent of estimated dairy consumption or other fatty acid dairy biomarkers. Protective associations with metabolic risk factors were confirmed in the validation cohort.

LIMITATION

Results could be affected by measurement error or residual confounding.

CONCLUSION

Circulating trans-palmitoleate is associated with lower insulin resistance, presence of atherogenic dyslipidemia, and incident diabetes. Our findings may explain previously observed metabolic benefits of dairy consumption and support the need for detailed further experimental and clinical investigation.

PRIMARY FUNDING SOURCE

National Heart, Lung, and Blood Institute and National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health.

Circulating palmitoleic acid and risk of metabolic abnormalities and new-onset diabetes

BACKGROUND

Animal experiments suggest that circulating palmitoleic acid (cis-16:1n-7) from adipocyte de novo fatty acid synthesis may directly regulate insulin resistance and metabolic dysregulation.

OBJECTIVE

We investigated the independent determinants of circulating palmitoleate in free-living humans and whether palmitoleate is related to lower metabolic risk and the incidence of diabetes.

DESIGN

In a prospective cohort of 3630 US men and women in the Cardiovascular Health Study, plasma phospholipid fatty acids, anthropometric variables, blood lipids, inflammatory markers, and glucose and insulin concentrations were measured between 1992 and 2006 by using standardized methods. Independent determinants of plasma phospholipid palmitoleate and relations of palmitoleate with metabolic risk factors were investigated by using multivariable-adjusted linear regression. Relations with incident diabetes (296 incident cases) were investigated by using Cox proportional hazards.

RESULTS

The mean (± SD) palmitoleate value was 0.49 ± 0.20% (range: 0.11-2.55%) of total fatty acids. Greater body mass index, carbohydrate intake, protein intake, and alcohol use were each independent lifestyle correlates of higher palmitoleate concentrations. In multivariable analyses that adjusted for these factors and other potential confounders, higher palmitoleate concentrations were independently associated with lower LDL cholesterol (P < 0.001), higher HDL cholesterol (P < 0.001), lower total:HDL-cholesterol ratio (P = 0.04), and lower fibrinogen (P < 0.001). However, palmitoleate was also associated with higher triglycerides (P < 0.001) and (in men only) with greater insulin resistance (P < 0.001). Palmitoleate was not significantly associated with incident diabetes.

CONCLUSIONS

Adiposity (energy imbalance), carbohydrate consumption, and alcohol use-even within typical ranges-are associated with higher circulating palmitoleate concentrations. Circulating palmitoleate is robustly associated with multiple metabolic risk factors but in mixed directions, perhaps related to divergent lifestyle determinants or endogenous sources (liver, adipose tissue) of fatty acid synthesis.

Genetic control of de novo lipogenesis: role in diet-induced obesity

De novo lipogenesis (DNL) is a complex yet highly regulated metabolic pathway, and transcription factors such as liver X receptor (LXR), sterol regulatory element-binding protein-1c (SREBP-1c), and carbohydrate response element binding protein (ChREBP) exert significant control over the de novo synthesis of fatty acids. An increase in de novo lipogenesis (DNL) is an important contributor to increased fat mass, while a reduction in lipogenesis may be protective against the development of obesity. In this review, we explore fatty acid synthesis in the context of new insights gleaned from global and tissue-specific gene knockout mouse models of enzymes involved in fatty acid synthesis, namely acetyl-CoA carboxylase, fatty acid synthase, fatty acid elongase 6, and stearoyl-CoA desaturase 1. A disruption in fatty acid synthesis, induced by the deficiency of any one of these enzymes, affects lipid metabolism and in some cases may protect against obesity in a tissue and gene-specific manner, as discussed in detail in this review.

Endogenous red blood cell membrane fatty acids and sudden cardiac arrest

Little is known of the associations of endogenous fatty acids with sudden cardiac arrest (SCA). We investigated the associations of SCA with red blood cell membrane fatty acids that are end products of de novo fatty acid synthesis: myristic acid (14:0), palmitic acid (16:0), palmitoleic acid (16:1 n7), vaccenic acid (18:1 n7), stearic acid (18:0), oleic acid (18:1 n9), and a related fatty acid, cis-7 hexadecenoic acid (16:1 n9). We used data from a population-based case-control study where cases, aged 25 to 74 years, were out-of-hospital SCA patients attended by paramedics in Seattle, WA (n = 265). Controls, matched to cases by age, sex, and calendar year, were randomly identified from the community (n = 415). All participants were free of prior clinically diagnosed heart disease. We observed associations of higher red blood cell membrane levels of 16:0, 16:1n-7, 18:1n-7, and 16:1n-9 with higher risk of SCA. In analyses adjusted for traditional SCA risk factors and trans- and n-3 fatty acids, a 1-SD-higher level of 16:0 was associated with 38% higher risk of SCA (odds ratio, 1.38; 95% confidence interval, 1.12-1.70) and a 1-SD-higher level of 16:1n-9 with 88% higher risk (odds ratio, 1.88; 95% confidence interval, 1.27-2.78). Several fatty acids that are end products of fatty acid synthesis are associated with SCA risk. Further work is needed to investigate if conditions that favor de novo fatty acid synthesis, such as high-carbohydrate/low-fat diets, might also increase the risk of SCA.

Respostas metabólicas à suplementação com frutose em exercício de força de membros inferiores

A frutose, por seu metabolismo independente da insulina, realiza significativas alterações no metabolismo hepático, promovendo um entorno metabólico favorável ao metabolismo tanto da glicose como dos lipídios, durante o exercício. Essa condição tem sido bastante estudada em exercício de endurance; no entanto, nenhum estudo sobre a suplementação com frutose no exercício de força (EF) foi encontrado. O objetivo do presente estudo foi avaliar os efeitos agudos da adição de frutose a um suplemento de glicose sobre o metabolismo de lipídios em EF. Vinte homens treinados ingeriram suplemento de glicose (G) ou glicose mais frutose (G+F), 15 minutos antes de realizar exercício de força (10 séries de 10 repetições). Os sujeitos foram testados em ordem randômica em um desenho cruzado e com uma semana de intervalo em duas condições experimentais: EF+(G) e EF+(G+F). A análise dos resultados mostrou que os valores de triglicérides durante o exercício foram maiores (p < 0,05) quando os sujeitos foram suplementados com G+F do que quando suplementados apenas com G. Ao final do exercício, os valores de ácidos graxos livres foram maiores quando os sujeitos foram suplementados G+F (p < 0,05). A glicemia foi menor durante o exercício e maior na recuperação (p < 0,05) para essa condição. O comportamento da insulina não diferiu entre os experimentos durante o exercício de força (p > 0,05), mas foi maior em G+F que em G (p < 0,05) durante a recuperação. A percepção subjetiva de esforço (PSE) foi menor (p < 0,05) para a suplementação com G+F do que com G. Em conclusão, a suplementação com G+F afeta positivamente o metabolismo de lipídios durante o exercício de força e favorece seu metabolismo imediatamente após o esforço, proporcionando condição metabólica que reflete em uma condição que afeta favoravelmente a PSE.

Dysregulation of sterol regulatory element binding protein-1c in livers of morbidly obese women is associated with altered suppressor of cytokine signaling-3 and signal transducer and activator of transcription-1 signaling

We compared hepatic expression of genes that regulate lipid biosynthesis and metabolic signaling in liver biopsy specimens from women who were undergoing gastric bypass surgery (GBP) for morbid obesity with that in women undergoing ventral hernia repair who had experienced massive weight loss (MWL) after prior GBP. Comprehensive metabolic profiles of morbidly obese (MO) (22 subjects) and MWL (9 subjects) were also compared. Analyses of gene expression in liver biopsies from MO and MWL were accomplished by Affymetrix microarray, real-time polymerase chain reaction, and Western blotting techniques. After GBP, MWL subjects had lost on average 102 lb as compared with MO subjects. This was accompanied by effective reversal of the dyslipidemia and insulin resistance that were present in MO. As compared with MWL, livers of MO subjects exhibited increased expression of sterol regulatory element binding protein (SREBP)-1c and its downstream lipogenic targets, fatty acid synthase and acetyl-coenzyme A-carboxylase-1. Livers of MO subjects also exhibited enhanced expression of suppressor of cytokine signaling-3 protein and attenuated Janus kinase signal transducer and activator of transcription (JAK/STAT) signaling. Consistent with these findings, we found that the human SREBP-1c promoter was positively regulated by insulin and negatively regulated by STAT3. These data support the hypothesis that suppressor of cytokine signaling-3-mediated attenuation of the STAT signaling pathway and resulting enhanced expression of SREBP-1c, a key regulator of de novo lipid biosynthesis, are mechanistically related to the development of hepatic insulin resistance and dyslipidemia in MO women.

Fatty liver: role of inflammation and fatty acid nutrition

Nonalcoholic fatty liver disease (NAFLD) refers to a wide spectrum of liver damage, ranging from simple steatosis to nonalcoholic steatohepatitis (NASH), advanced fibrosis, and cirrhosis. NAFLD is strongly associated with insulin resistance and is defined by accumulation of liver fat >5% per liver weight in the presence of <10g of daily alcohol consumption. The exact prevalence of NAFLD is uncertain because of the absence of simple noninvasive diagnostic tests to facilitate an estimate of prevalence but in subgroups of people such as those with type 2 diabetes, the prevalence may be as high as 70%. NASH is an important subgroup within the spectrum of NAFLD that progresses over time with worsening fibrosis and cirrhosis, and NASH is associated with increased risk for cardiovascular disease. It is, therefore, important to understand the pathogenesis of NASH specifically, to develop strategies for interventions to treat this condition. The purpose of this review is to discuss the roles of inflammation, fatty acids and fatty acids in nutrition, in the pathogenesis and potential treatment of NAFLD.

Metabolic effects of fructose and the worldwide increase in obesity

While virtually absent in our diet a few hundred years ago, fructose has now become a major constituent of our modern diet. Our main sources of fructose are sucrose from beet or cane, high fructose corn syrup, fruits, and honey. Fructose has the same chemical formula as glucose (C(6)H(12)O(6)), but its metabolism differs markedly from that of glucose due to its almost complete hepatic extraction and rapid hepatic conversion into glucose, glycogen, lactate, and fat. Fructose was initially thought to be advisable for patients with diabetes due to its low glycemic index. However, chronically high consumption of fructose in rodents leads to hepatic and extrahepatic insulin resistance, obesity, type 2 diabetes mellitus, and high blood pressure. The evidence is less compelling in humans, but high fructose intake has indeed been shown to cause dyslipidemia and to impair hepatic insulin sensitivity. Hepatic de novo lipogenesis and lipotoxicity, oxidative stress, and hyperuricemia have all been proposed as mechanisms responsible for these adverse metabolic effects of fructose. Although there is compelling evidence that very high fructose intake can have deleterious metabolic effects in humans as in rodents, the role of fructose in the development of the current epidemic of metabolic disorders remains controversial. Epidemiological studies show growing evidence that consumption of sweetened beverages (containing either sucrose or a mixture of glucose and fructose) is associated with a high energy intake, increased body weight, and the occurrence of metabolic and cardiovascular disorders. There is, however, no unequivocal evidence that fructose intake at moderate doses is directly related with adverse metabolic effects. There has also been much concern that consumption of free fructose, as provided in high fructose corn syrup, may cause more adverse effects than consumption of fructose consumed with sucrose. There is, however, no direct evidence for more serious metabolic consequences of high fructose corn syrup versus sucrose consumption.

Hepatic lipid composition and stearoyl-coenzyme A desaturase 1 mRNA expression can be estimated from plasma VLDL fatty acid ratios

BACKGROUND

Stearoyl-coenzyme A desaturase 1 (SCD1) catalyzes the limiting step of monounsaturated fatty acid synthesis in humans and is an important player in triglyceride generation. SCD1 has been repeatedly implicated in the pathogenesis of metabolic and inflammatory diseases. Therefore it is of great importance to determine SCD1 activity in human samples. In this study we aimed to evaluate a hepatic SCD1 activity index derived from plasma VLDL triglyceride composition as a tool to estimate hepatic SCD1 expression in humans. Additionally, we further evaluated commonly used fatty acid ratios [elongase, de novo lipogenesis, and Delta5 and Delta6 desaturase] in plasma VLDL and hepatic lipid fractions.

DESIGN AND METHODS

Liver biopsies and plasma samples were simultaneously collected from 15 individuals. Plasma VLDL was obtained by ultracentrifugation. Hepatic and plasma VLDL lipids were fractionated by thin-layer chromatography, and the fatty acid composition of each fraction was analyzed by gas chromatography. Hepatic SCD1 expression was determined by real-time PCR.

RESULTS

Hepatic SCD1 mRNA expression was associated with the product/precursor ratios (16:1/16:0 and 18:1/18:0) of hepatic lipid fractions. The 16:1/16:0 ratio in hepatic and VLDL triglycerides as well as the 18:1/18:0 ratio in plasma VLDL were closely associated with hepatic SCD1 expression. The hepatic de novo lipogenesis index from triglycerides was associated with expression of lipogenic genes [fatty acid synthase (FASN), acetyl-Coenzyme A carboxylase alpha (ACACA), and sterol regulatory element binding transcription factor 1 (SREBP-1)] and is closely reflected by the de novo lipogenesis index in VLDL triglycerides.

CONCLUSION

We demonstrated for the first time that hepatic SCD1 expression can be estimated noninvasively from routine blood samples by measuring the SCD1 activity index in fasting plasma VLDL.

The Decrease of n-3 Fatty Acid Energy Percentage in an Equicaloric Diet Fed to B6C3Fe Mice for Three Generations Elicits Obesity

Feeding mice, over 3 generations, an equicaloric diet in which alpha-linolenic acid, the dietary precursor of n-3 polyunsaturated fatty acids, was substituted by linoleic acid, the dietary precursor of n-6 polyunsaturated fatty acids, significantly increased body weight throughout life when compared with standard diet-fed mice. Adipogenesis observed in the low n-3 fatty acid mice was accompanied by a 6-fold upregulation of stearyl-coenzyme A desaturase 1 (Scd1), whose activity is correlated to plasma triglyceride levels. In total liver lipid and phospholipid extracts, the sum of n-3 fatty acids and the individual longer carbon chain acids, eicosapentaenoic acid (20:5n3), docosapentaenoic acid (22:5n3), and docosahexaenoic acid (22:6n3) were significantly decreased whereas arachidonic acid (20:4n6) was significantly increased. In addition, low n-3 fatty acid-fed mice had liver steatosis, heart, and kidney hypertrophy. Hence, reducing dietary alpha-linolenic acid, from 1.02 energy % to 0.16 energy % combined with raising linoleic acid intake resulted in obesity and had detrimental consequences on organ function.

High fat feeding induces hepatic fatty acid elongation in mice

Background

High-fat diets promote hepatic lipid accumulation. Paradoxically, these diets also induce lipogenic gene expression in rodent liver. Whether high expression of these genes actually results in an increased flux through the de novo lipogenic pathway in vivo has not been demonstrated.

Methodology/Principal Findings

To interrogate this apparent paradox, we have quantified de novo lipogenesis in C57Bl/6J mice fed either chow, a high-fat or a n-3 polyunsaturated fatty acid (PUFA)-enriched high-fat diet. A novel approach based on mass isotopomer distribution analysis (MIDA) following 1-¹³C acetate infusion was applied to simultaneously determine de novo lipogenesis, fatty acid elongation as well as cholesterol synthesis. Furthermore, we measured very low density lipoprotein-triglyceride (VLDL-TG) production rates. High-fat feeding promoted hepatic lipid accumulation and induced the expression of lipogenic and cholesterogenic genes compared to chow-fed mice: induction of gene expression was found to translate into increased oleate synthesis. Interestingly, this higher lipogenic flux (+74 µg/g/h for oleic acid) in mice fed the high-fat diet was mainly due to an increased hepatic elongation of unlabeled palmitate (+66 µg/g/h) rather than to elongation of de novo synthesized palmitate. In addition, fractional cholesterol synthesis was increased, i.e. 5.8±0.4% vs. 8.1±0.6% for control and high fat-fed animals, respectively. Hepatic VLDL-TG production was not affected by high-fat feeding. Partial replacement of saturated fat by fish oil completely reversed the lipogenic effects of high-fat feeding: hepatic lipogenic and cholesterogenic gene expression levels as well as fatty acid and cholesterol synthesis rates were normalized.

Conclusions/Significance

High-fat feeding induces hepatic fatty acid synthesis in mice, by chain elongation and subsequent desaturation rather than de novo synthesis, while VLDL-TG output remains unaffected. Suppression of lipogenic fluxes by fish oil prevents from high fat diet-induced hepatic steatosis in mice.

Fructose overconsumption causes dyslipidemia and ectopic lipid deposition in healthy subjects with and without a family history of type 2 diabetes

BACKGROUND

Both nutritional and genetic factors are involved in the pathogenesis of nonalcoholic fatty liver disease and insulin resistance.

OBJECTIVE

The aim was to assess the effects of fructose, a potent stimulator of hepatic de novo lipogenesis, on intrahepatocellular lipids (IHCLs) and insulin sensitivity in healthy offspring of patients with type 2 diabetes (OffT2D)--a subgroup of individuals prone to metabolic disorders.

DESIGN

Sixteen male OffT2D and 8 control subjects were studied in a crossover design after either a 7-d isocaloric diet or a hypercaloric high-fructose diet (3.5 g x kg FFM(-1) x d(-1), +35% energy intake). Hepatic and whole-body insulin sensitivity were assessed with a 2-step hyperinsulinemic euglycemic clamp (0.3 and 1.0 mU x kg(-1) x min(-1)), together with 6,6-[2H2]glucose. IHCLs and intramyocellular lipids (IMCLs) were measured by 1H-magnetic resonance spectroscopy.

RESULTS

The OffT2D group had significantly (P < 0.05) higher IHCLs (+94%), total triacylglycerols (+35%), and lower whole-body insulin sensitivity (-27%) than did the control group. The high-fructose diet significantly increased IHCLs (control: +76%; OffT2D: +79%), IMCLs (control: +47%; OffT2D: +24%), VLDL-triacylglycerols (control: +51%; OffT2D: +110%), and fasting hepatic glucose output (control: +4%; OffT2D: +5%). Furthermore, the effects of fructose on VLDL-triacylglycerols were higher in the OffT2D group (group x diet interaction: P < 0.05).

CONCLUSIONS

A 7-d high-fructose diet increased ectopic lipid deposition in liver and muscle and fasting VLDL-triacylglycerols and decreased hepatic insulin sensitivity. Fructose-induced alterations in VLDL-triacylglycerols appeared to be of greater magnitude in the OffT2D group, which suggests that these individuals may be more prone to developing dyslipidemia when challenged by high fructose intakes. This trial was registered at clinicaltrials.gov as NCT00523562.

Metabolic disturbances in non-alcoholic fatty liver disease

NAFLD (non-alcoholic fatty liver disease) refers to a wide spectrum of liver damage, ranging from simple steatosis to NASH (non-alcoholic steatohepatitis), advanced fibrosis and cirrhosis. NAFLD is strongly associated with insulin resistance and is defined by accumulation of liver fat >5% per liver weight in the presence of <10 g of daily alcohol consumption. The exact prevalence of NAFLD is uncertain because of the absence of simple non-invasive diagnostic tests to facilitate an estimate of prevalence. In certain subgroups of patients, such as those with Type 2 diabetes, the prevalence of NAFLD, defined by ultrasound, may be as high as 70%. NASH is an important subgroup within the spectrum of NAFLD that progresses over time with worsening fibrosis and cirrhosis, and is associated with increased risk for cardiovascular disease. It is, therefore, important to understand the pathogenesis of NASH and, in particular, to develop strategies for interventions to treat this condition. Currently, the 'gold standard' for the diagnosis of NASH is liver biopsy, and the need to undertake a biopsy has impeded research in subjects in this field. Limited results suggest that the prevalence of NASH could be as high as 11% in the general population, suggesting there is a worsening future public health problem in this field of medicine. With a burgeoning epidemic of diabetes in an aging population, it is likely that the prevalence of NASH will continue to increase over time as both factors are important risk factors for liver fibrosis. The purpose of this review is to: (i) briefly discuss the epidemiology of NAFLD to describe the magnitude of the future potential public health problem; and (ii) to discuss extra- and intra-hepatic mechanisms contributing to the pathogenesis of NAFLD, a better understanding of which may help in the development of novel treatments for this condition.

Effects of dietary coconut oil on the biochemical and anthropometric profiles of women presenting abdominal obesity

The effects of dietary supplementation with coconut oil on the biochemical and anthropometric profiles of women presenting waist circumferences (WC) >88 cm (abdominal obesity) were investigated. The randomised, double-blind, clinical trial involved 40 women aged 20-40 years. Groups received daily dietary supplements comprising 30 mL of either soy bean oil (group S; n = 20) or coconut oil (group C; n = 20) over a 12-week period, during which all subjects were instructed to follow a balanced hypocaloric diet and to walk for 50 min per day. Data were collected 1 week before (T1) and 1 week after (T2) dietary intervention. Energy intake and amount of carbohydrate ingested by both groups diminished over the trial, whereas the consumption of protein and fibre increased and lipid ingestion remained unchanged. At T1 there were no differences in biochemical or anthropometric characteristics between the groups, whereas at T2 group C presented a higher level of HDL (48.7 +/- 2.4 vs. 45.00 +/- 5.6; P = 0.01) and a lower LDL:HDL ratio (2.41 +/- 0.8 vs. 3.1 +/- 0.8; P = 0.04). Reductions in BMI were observed in both groups at T2 (P < 0.05), but only group C exhibited a reduction in WC (P = 0.005). Group S presented an increase (P < 0.05) in total cholesterol, LDL and LDL:HDL ratio, whilst HDL diminished (P = 0.03). Such alterations were not observed in group C. It appears that dietetic supplementation with coconut oil does not cause dyslipidemia and seems to promote a reduction in abdominal obesity.

Sources of hepatic triglyceride accumulation during high-fat feeding in the healthy rat

Hepatic triglyceride (HTG) accumulation from peripheral dietary sources and from endogenous de novo lipogenesis (DNL) was quantified in adult Sprague-Dawley rats by combining in vivo localized (1)H MRS measurement of total hepatic lipid with a novel ex vivo (2)H NMR analysis of HTG (2)H enrichment from (2)H-enriched body water. The methodology for DNL determination needs further validation against standard methodologies. To examine the effect of a high-fat diet on HTG concentrations and sources, animals (n = 5) were given high-fat chow for 35 days. HTG accumulation, measured by in vivo (1)H MRS, increased significantly after 1 week (3.85 +/- 0.60% vs 2.13 +/- 0.34% for animals fed on a standard chow diet, P < 0.05) and was maintained until week 5 (3.30 +/- 0.60% vs 1.12 +/- 0.30%, P < 0.05). Animals fed on a high-fat diet were glucose intolerant (13.3 +/- 1.3 vs 9.4 +/- 0.8 mM in animals fed on a standard chow diet, for 60 min glycemia after glucose challenge, P < 0.05). In control animals, DNL accounted for 10.9 +/- 1.0% of HTG, whereas in animals given the high-fat diet, the DNL contribution was significantly reduced to 1.0 +/- 0.2% (P < 0.01 relative to controls). In a separate study to determine the response of HTG to weaning from a high-fat diet, animals with raised HTG (3.33 +/- 0.51%) after 7 days of a high-fat diet reverted to basal HTG concentrations (0.76 +/- 0.06%) after an additional 7 days of weaning on a standard chow diet. These studies show that, in healthy rats, HTG concentrations are acutely influenced by dietary lipid concentrations. Although the DNL contribution to HTG content is suppressed by a high-fat diet in adult Sprague-Dawley rats, this effect is insufficient to prevent overall increases in HTG concentrations.

Changes of the plasma metabolome during an oral glucose tolerance test: is there more than glucose to look at?

The oral glucose tolerance test (oGTT) is a common tool to provoke a metabolic challenge for scientific purposes, as well as for diagnostic reasons, to monitor the kinetics of glucose and insulin. Here, we aimed to follow the variety of physiological changes of the whole metabolic pattern in plasma during an oGTT in healthy subjects in a nontargeted reversed-phase ultra performance liquid chromatography coupled to electrospray ionization quadrupole time of flight mass spectrometric metabolomics approach. We detected 11,500 metabolite ion masses/individual. Applying multivariate data analysis, four major groups of metabolites have been detected as the most discriminating oGTT biomarkers: free fatty acids (FFA), acylcarnitines, bile acids, and lysophosphatidylcholines. We found in detail 1) a strong decrease of all saturated and monounsaturated FFA studied during the oGTT; 2) a significant faster decline of palmitoleate (C16:1) and oleate (C18:1) FFA levels than their saturated counterparts; 3) a strong relative increase of polyunsaturated fatty acids in the fatty acid pattern at 120 min; and 4) a clear decrease in plasma C10:0, C12:0, and C14:1 acylcarnitine levels. These data reflect the switch from beta-oxidation to glycolysis and fat storage during the oGTT. Moreover, the bile acids glycocholic acid, glycochenodeoxycholic acid, and glycodeoxycholic acid were highly discriminative, showing a biphasic kinetic with a maximum of a 4.5- to 6-fold increase at 30 min after glucose ingestion, a significant decrease over the next 60 min followed by an increase until the end of the oGTT. Lysophosphatidylcholines were also increased significantly. The findings of our metabolomics study reveal detailed insights in the complex physiological regulation of the metabolism during an oGTT offering novel perspectives of this widely used procedure.

Fatty acid synthase: association with insulin resistance, type 2 diabetes, and cancer

BACKGROUND

An emerging paradigm supports the notion that deregulation of fatty acid synthase (FASN)-catalyzed de novo FA biogenesis could play a central role in the pathogenesis of metabolic diseases sharing the hallmark of insulin-resistance.

CONTENT

We reviewed pharmacological and genetic alterations of FASN activity that have been shown to significantly influence energy expenditure rates, fat mass, insulin sensitivity, and cancer risk. This new paradigm proposes that insulin-resistant conditions such as obesity, type 2 diabetes, and cancer arise from a common FASN-driven "lipogenic state". An important question then is whether the development or the progression of insulin-related metabolic disorders can be prevented or reversed by the modulation of FASN status. If we accept the paradigm of FASN dysfunction as a previously unrecognized link between insulin resistance, type 2 diabetes, and cancer, the use of insulin sensitizers in parallel with forthcoming FASN inhibitors should be a valuable therapeutic approach that, in association with lifestyle interventions, would concurrently improve energy-flux status, ameliorate insulin sensitivity, and alleviate the risk of lipogenic carcinomas.

CONCLUSIONS

Although the picture is currently incomplete and researchers in the field have plenty of work ahead, the latest clinical and experimental evidence that we discuss illuminates a functional and drug-modifiable link that connects FASN-driven endogenous FA biosynthesis, insulin action, and glucose homeostasis in the natural history of insulin-resistant pathologies.

Stearoyl-CoA desaturase and its relation to high-carbohydrate diets and obesity

Obesity is currently a worldwide epidemic and public health burden that increases the risk for developing insulin resistance and several chronic diseases such as diabetes, cardiovascular diseases and non-alcoholic fatty liver disease. The multifactorial causes of obesity include several genetic, dietary and lifestyle variables that together result in an imbalance between energy intake and energy expenditure. Dietary approaches to limit fat intake are commonly prescribed to achieve the hypocaloric conditions necessary for weight loss. But dietary fat restriction is often accompanied by increased carbohydrate intake, which can dramatically increase endogenous fatty acid synthesis depending upon carbohydrate composition. Since both dietary and endogenously synthesized fatty acids contribute to the whole-body fatty acid pool, obesity can therefore result from excessive fat or carbohydrate consumption. Stearoyl-Coenzyme A desaturase-1 (SCD1) is a delta-9 fatty acid desaturase that converts saturated fatty acids into monounsaturated fatty acids (MUFA) and this activity is elevated by dietary carbohydrate. Mice lacking Scd1 are protected from obesity and insulin resistance and are characterized by decreased fatty acid synthesis and increased fatty acid oxidation. In this review, we address the association of high-carbohydrate diets with increased SCD activity and summarize the current literature on the subject of SCD1 and body weight regulation.

Hepatic stearoyl-CoA desaturase (SCD)-1 activity and diacylglycerol but not ceramide concentrations are increased in the nonalcoholic human fatty liver

OBJECTIVE

To determine whether 1) hepatic ceramide and diacylglycerol concentrations, 2) SCD1 activity, and 3) hepatic lipogenic index are increased in the human nonalcoholic fatty liver.

RESEARCH DESIGN AND METHODS

We studied 16 subjects with (n = 8) and without (n = 8) histologically determined nonalcoholic fatty liver (NAFL(+) and NAFL(-)) matched for age, sex, and BMI. Hepatic concentrations of lipids and fatty acids were quantitated using ultra-performance liquid chromatography coupled to mass spectrometry and gas chromatography.

RESULTS

The absolute (nmol/mg) hepatic concentrations of diacylglycerols but not ceramides were increased in the NAFL(+) group compared with the NAFL(-) group. The livers of the NAFL(+) group contained proportionally less long-chain polyunsaturated fatty acids as compared with the NAFL(-) group. Liver fat percent was positively related to hepatic stearoyl-CoA desaturase 1 (SCD1) activity index (r = 0.70, P = 0.003) and the hepatic lipogenic index (r = 0.54, P = 0.030). Hepatic SCD1 activity index was positively related to the concentrations of diacylglycerols (r = 0.71, P = 0.002) but not ceramides (r = 0.07, NS).

CONCLUSIONS

We conclude that diacylglycerols but not ceramides are increased in NAFL. The human fatty liver is also characterized by depletion of long polyunsaturated fatty acids in the liver and increases in hepatic SCD1 and lipogenic activities.

The evolution of plasma cholesterol: direct utility or a "spandrel" of hepatic lipid metabolism?

Fats provide a concentrated source of energy for multicellular organisms. The efficient transport of fats through aqueous biological environments raises issues concerning effective delivery to target tissues. Furthermore, the utilization of fatty acids presents a high risk of cytotoxicity. Improving the efficiency of fat transport while simultaneously minimizing the cytotoxic risk confers distinct selective advantages. In humans, most of the plasma cholesterol is associated with low-density lipoprotein (LDL), a metabolic by-product of very-low-density lipoprotein (VLDL), which originates in the liver. However, the functions of VLDL are not clear. This paper reviews the evidence that LDL arose as a by-product during the natural selection of VLDL. The latter, in turn, evolved as a means of improving the efficiency of diet-derived fatty acid storage and utilization, as well as neutralizing the potential cytotoxicity of fatty acids while conserving their advantages as a concentrated energy source. The evolutionary biology of lipid transport processes has provided a fascinating insight into how and why these VLDL functions emerged during animal evolution. As causes of historical origin must be separated from current utilities, our spandrel-LDL theory proposes that LDL is a spandrel of VLDL selection, which appeared non-adaptively and may later have become crucial for vertebrate fitness.

Overproduction of very low-density lipoproteins is the hallmark of the dyslipidemia in the metabolic syndrome

Insulin resistance is a key feature of the metabolic syndrome and often progresses to type 2 diabetes. Both insulin resistance and type 2 diabetes are characterized by dyslipidemia, which is an important and common risk factor for cardiovascular disease. Diabetic dyslipidemia is a cluster of potentially atherogenic lipid and lipoprotein abnormalities that are metabolically interrelated. Recent evidence suggests that a fundamental defect is an overproduction of large very low-density lipoprotein (VLDL) particles, which initiates a sequence of lipoprotein changes, resulting in higher levels of remnant particles, smaller LDL, and lower levels of high-density liporotein (HDL) cholesterol. These atherogenic lipid abnormalities precede the diagnosis of type 2 diabetes by several years, and it is thus important to elucidate the mechanisms involved in the overproduction of large VLDL particles. Here, we review the pathophysiology of VLDL biosynthesis and metabolism in the metabolic syndrome. We also review recent research investigating the relation between hepatic accumulation of lipids and insulin resistance, and sources of fatty acids for liver fat and VLDL biosynthesis. Finally, we briefly discuss current treatments for lipid management of dyslipidemia and potential future therapeutic targets.

The secretory function of adipocytes in the physiology of white adipose tissue

White adipose tissue, previously regarded as a passive lipid storage site, is now viewed as a dynamic tissue. It has the capacity to actively communicate by sending and receiving different types of signals. An overview of these signals, the external modulators that affect adipose tissue and the secreted signaling molecules, the adipokines, is presented. The secretory function is highlighted in relation to energy metabolism, inflammation and the extracellular matrix and placed in the context of adipose tissue biology. We observe that the endocrine function of adipocytes receives much attention, while its paracrine and autocrine functions are underestimated. Also, we provide examples that species specificity should not be neglected. We conclude that adipose tissue primarily is an energy storage organ, well supported by its secretory function.

Dietary sugars stimulate fatty acid synthesis in adults

The goal of this study was to determine the magnitude by which acute consumption of fructose in a morning bolus would stimulate lipogenesis (measured by infusion of 13C1-acetate and analysis by GC-MS) immediately and after a subsequent meal. Six healthy subjects [4 men and 2 women; aged (mean +/- SD) 28 +/- 8 y; BMI, 24.3 +/- 2.8 kg/m(2); and serum triacylglycerols (TG), 1.03 +/- 0.32 mmol/L] consumed carbohydrate boluses of sugars (85 g each) in a random and blinded order, followed by a standardized lunch 4 h later. Subjects completed a control test of glucose (100:0) and a mixture of 50:50 glucose:fructose and one of 25:75 (wt:wt). Following the morning boluses, serum glucose and insulin after 100:0 were greater than both other treatments (P < 0.05) and this pattern occurred again after lunch. In the morning, fractional lipogenesis was stimulated when subjects ingested fructose and peaked at 15.9 +/- 5.4% after the 50:50 treatment and at 16.9 +/- 5.2% after the 25:75 treatment, values that were greater than after the 100:0 treatment (7.8 +/- 5.7%; P < 0.02). When fructose was consumed, absolute lipogenesis was 2-fold greater than when it was absent (100:0). Postlunch, serum TG were 11-29% greater than 100:0 and TG-rich lipoprotein-TG concentrations were 76-200% greater after 50:50 and 25:75 were consumed (P < 0.05). The data demonstrate that an early stimulation of lipogenesis after fructose, consumed in a mixture of sugars, augments subsequent postprandial lipemia. The postlunch blood TG elevation was only partially due to carry-over from the morning. Acute intake of fructose stimulates lipogenesis and may create a metabolic milieu that enhances subsequent esterification of fatty acids flowing to the liver to elevate TG synthesis postprandially.

Diacylglycerol acyltransferases: Potential roles as pharmacological targets

Triglyceride (TG) synthesis occurs in many cell-types, but only the adipocyte is specialised for TG storage. The increased incidence of obesity and its attendant pathologies have increased interest in pharmacological strategies aimed at inhibition of triglyceride synthesis. In the liver this would also appear to offer the advantages of the prevention of steatosis and/or dyslipidaemia. The two major enzymes that have DGAT activity appear to have specialised functions, that are most evident in triglyceride-secreting tissues. The presence of triglyceride in non-adipose cells can lead to (through lipolysis), or be a marker for, undesirable complications such as insulin resistance, or can be indicative of simultaneously high capacities for triglyceride synthesis, lipolysis and oxidation of fatty acids as in highly aerobic, trained muscle. Consequently, inhibition of triglyceride synthesis may not be a straightforward strategy, either in terms of its achievement pharmacologically or in its anticipated outcomes. The metabolic complexities of triglyceride synthesis, with particular reference to the diacylglycerol acyltransferases (DGATs) are considered in this short review.

Fatty acid composition of adipose tissue and blood in humans and its use as a biomarker of dietary intake

Accurate assessment of fat intake is essential to examine the relationships between diet and disease risk but the process of estimating individual intakes of fat quality by dietary assessment is difficult. Tissue and blood fatty acids, because they are mainly derived from the diet, have been used as biomarkers of dietary intake for a number of years. We review evidence from a wide variety of cross-sectional and intervention studies and summarise typical values for fatty acid composition in adipose tissue and blood lipids and changes that can be expected in response to varying dietary intake. Studies in which dietary intake was strictly controlled confirm that fatty acid biomarkers can complement dietary assessment methodologies and have the potential to be used more quantitatively. Factors affecting adipose tissue and blood lipid composition are discussed, such as the physical properties of triacylglycerol, total dietary fat intake and endogenous fatty acid synthesis. The relationship between plasma lipoprotein concentrations and total plasma fatty acid composition, and the use of fatty acid ratios as indices of enzyme activity are also addressed.

Reduced oxidation of dietary fat after a short term high-carbohydrate diet

BACKGROUND

Short-term high-carbohydrate (HC) diets induce metabolic alterations, including hypertriacylglycerolemia, in both the fasting and postprandial states. The underlying tissue-specific alterations in fatty acid metabolism are not well understood.

OBJECTIVE

We investigated alterations in exogenous and endogenous fatty acid metabolism by using stable isotope tracers to label meal triacylglycerol and plasma fatty acids.

DESIGN

Eight healthy subjects consumed isocaloric diets containing a high percentage of energy from carbohydrates or a higher percentage of energy from fat for 3 d in a randomized crossover dietary intervention study. A test meal containing [U-13C] palmitate was combined with intravenous infusion of [2H2] palmitate to label plasma fatty acids and VLDL triacylglycerol. Blood and breath samples were taken before the meal and for 6 h postprandially. Blood samples were drawn from the femoral artery and from veins draining subcutaneous adipose tissue and forearm muscle for monitoring of tissue-specific metabolic substrate partitioning.

RESULTS

Systemic triacylglycerol concentrations were increased in both fasting (P = 0.02) and postprandial (P = 0.02) periods, and a greater amount of infused labeled fatty acid appeared in VLDL triacylglycerol after the HC diet than after the higher-fat diet (P = 0.05). Significantly less 13CO2 was exhaled after the HC diet (P = 0.04) and significantly less production of 13CO2 was seen across forearm muscle (P = 0.04). Systemic 3-hydroxybutyrate was significantly lower, postprandially, after the HC diet (P = 0.02).

CONCLUSION

Metabolic alterations suggestive of repartitioning of fatty acids away from oxidation toward esterification in both liver and muscle occur in response to short-term adaptation to a HC diet.

Parallel activation of de novo lipogenesis and stearoyl-CoA desaturase activity after 3 d of high-carbohydrate feeding

BACKGROUND

High-carbohydrate (HC) diets increase de novo lipogenesis (DNL), but effects on stearoyl-CoA desaturase (SCD) are not so well studied.

OBJECTIVE

The objective was to investigate DNL and SCD in liver and adipose tissue by using fatty acid ratios after short-term dietary intervention.

DESIGN

Eight subjects consumed isoenergetic 3-d HC (10% fat; 75% carbohydrates) or higher fat (HF; 40% fat; 45% carbohydrates) diets (sugar to starch ratio: 60:40 for both) in a crossover study. Blood was taken from an artery and a vein draining subcutaneous adipose tissue. DNL and SCD activity were investigated by using the ratios of 16:0 to 18:2n-6 and of 16:1n-7 to 16:0, respectively. A test meal, including [U-(13)C]palmitate was given to trace dietary fatty acid incorporation into VLDL-triacylglycerol (TG). The conversion of intravenously infused [(2)H(2)]palmitic acid to [(2)H(2)]palmitoleic acid in VLDL-TG was quantified as a specific marker of hepatic SCD activity.

RESULTS

The VLDL-TG 16:0/18:2n-6 ratio, which reflects hepatic DNL, was greater after the HC diet than after the HF diet (P = 0.02). With the HC diet, increased plasma TG concentrations correlated with 16:0/18:2n-6 ratios (r = 0.76, P = 0.028). Plasma VLDL-TG and adipose venous nonesterified fatty acid (NEFA) 16:1n-7/16:0 ratios were higher after the HC diet (fasting: P = 0.01 and P = 0.05, respectively; postprandial: P = 0.03 and P = 0.05, respectively). Changes in fasting VLDL-TG 16:0/18:2n-6 and 16:1n-7/16:0 ratios were associated (P = 0.06). The contribution of total fatty acids from splanchnic sources (including DNL) was higher after the HC diet (P = 0.02). Expression of lipogenic genes in subcutaneous adipose tissue was not significantly affected by diet.

CONCLUSION

Parallel activation of DNL and SCD was found after a short period of HC feeding.

Low hepatic stearoyl-CoA desaturase 1 activity is associated with fatty liver and insulin resistance in obese humans

AIMS/HYPOTHESIS

Stearoyl-CoA desaturase 1 (SCD1) is the rate-limiting enzyme in monounsaturated fatty acid synthesis. It is imperative for the assembly of VLDL particles, which transport triacylglycerol (TG) from liver to adipose tissue and other sites. We aimed to determine the role of hepatic SCD1 activity in human glucose and lipid metabolism.

METHODS

We studied 54 people participating in a lifestyle intervention programme with diet modification and increased physical activity. Insulin sensitivity was determined during a euglycaemic-hyperinsulinaemic clamp and estimated from an OGTT. Liver fat was quantified by (1)H-magnetic resonance spectroscopy at baseline and after 9 months of intervention. The pattern of fatty acids in serum VLDL-TGs was determined by ultracentrifugation followed by thin layer and gas chromatography, with the 18:1 n-9: 18:0 ratio providing an index of hepatic SCD1 activity.

RESULTS

The hepatic SCD1 activity index correlated negatively with liver fat (r= -0.29, p=0.04) and positively with insulin sensitivity, both OGTT-derived (r=0.42, p=0.003) and clamp-derived (r=0.27, p=0.07). These correlations depended on overall adiposity. They were absent in leaner participants (n=27, liver fat: p=0.34, insulin sensitivity [OGTT]: p=0.75, insulin sensitivity [clamp]: p=0.24), but were strong in obese individuals (n=27, p=0.004, p=0.0002 and p=0.006, respectively). Furthermore, during intervention a high SCD1 activity index at baseline predicted a decrease in liver fat only in obese participants (r= -0.46, p=0.02).

CONCLUSIONS/INTERPRETATION

Our data suggest that high hepatic SCD1 activity may regulate fat accumulation in the liver and possibly protects from insulin resistance in obesity.

Lipid accumulation in non-adipose tissue and lipotoxicity

Obesity is a well-known risk factor for the development of type 2 diabetes mellitus and cardiovascular disease. Importantly, obesity is not only associated with lipid accumulation in adipose tissue, but also in non-adipose tissues. The latter is also known as ectopic lipid accumulation and may be a possible link between obesity and its comorbidities such as insulin resistance, type 2 diabetes mellitus and cardiovascular disease. In skeletal muscle and liver, lipid accumulation has been associated with the development of insulin resistance, an early hallmark of developing type 2 diabetes mellitus. More specifically, accumulation of intermediates of lipid metabolism, such as diacylglycerol (DAG) and Acyl-CoA have been shown to interfere with insulin signaling in these tissues. Initially, muscular and hepatic insulin resistance can be overcome by an increased insulin production by the pancreas, resulting in hyperinsulinemia. However, during the progression towards overt type 2 diabetes, pancreatic failure occurs resulting in reduced insulin production. Interestingly, also in the pancreas lipid accumulation has been shown to precede dysfunction. Finally, accumulation of fat in the heart has been associated with cardiac dysfunction and heart failure, which may be an explanation for diabetic cardiomyopathy. Taken together, we conclude that evidence for deleterious effects of lipid accumulation in non-adipose tissue (lipotoxicity) is strong. However, while ample human data is available for skeletal muscle and the liver, future research should focus on lipid accumulation in the pancreas and the heart.

Genome-wide transcriptome expression in the liver of a mouse model of high carbohydrate diet-induced liver steatosis and its significance for the disease

PURPOSE

To perform a large-scale gene profiling of the liver in a mouse model of fatty liver induced by high carbohydrate (sucrose) diet (HCD) to gain a deeper insight into potential mechanisms of diet-induced hepatic steatosis.

METHODS

C57BL/6 male mice were fed either a purified, control diet or a HCD for 16 weeks. HCD feeding led to marked liver steatosis without inflammation or necrosis. The expression of 42,500 genes/sequences was assessed.

RESULTS

A number of genes (471) underwent significant expression changes in HCD- as compared to standard diet-fed mice (n = 5/group; P < 0.01). Of these genes, 211 were down- and 260 up-regulated. The latter group includes 20 genes encoding enzymes involved in carbohydrate conversion to fat. The genes that underwent expression changes perform a large variety of molecular functions, and the vast majority of these have never been tested before in non-alcoholic fatty liver of nutritional origin. They reveal novel aspects of the disease and allow identification of candidate genes that may underlie the initiation of hepatic steatosis and progression to non-alcoholic steatohepatitis.

CONCLUSIONS

HCD-fed laboratory animals provide a model of early non-alcoholic fatty liver disease resembling the disease in humans. The genome wide gene profiling of the liver reveals the complexity of the disease, unravels novel aspects of HCD-induced hepatic steatosis, and helps elucidate its nature and mechanisms.

Effect of short-term Pritikin diet therapy on the metabolic syndrome

The Pritikin Program (Aventura, FL) involves the use of a very-low-fat, low-sodium, high-fiber diet and exercise to decrease the risk of coronary heart disease (CHD). This study evaluated the effect of short-term Pritikin therapy on the metabolic risk factors for CHD in patients with the metabolic syndrome. Sixty-seven subjects who had the metabolic syndrome and attended the Pritikin Longevity Center & Spa for 12-15 days were studied. Short-term Pritikin therapy improved most CHD risk factors: body mass index decreased by 3% (P<.001); systolic and diastolic blood pressure, and serum glucose and low-density lipoprotein cholesterol concentrations decreased by 10%-15% (P<.001); serum triglyceride concentration decreased by 36% (P<.001); and 37% of subjects no longer met National Cholesterol Education Program criteria for the metabolic syndrome. Serum high-density lipoprotein cholesterol, however, decreased by 3% (P<.05). These data demonstrate that brief treatment with a very-low-fat, low-sodium, high-fiber diet and regular exercise simultaneously improves multiple CHD risk factors in patients with the metabolic syndrome.

Long-term administration of inulin-type fructans has no significant lipid-lowering effect in normolipidemic humans

Short-term studies have shown that the addition to diet of inulin-type fructans, a nondigestible carbohydrate, may have a plasma lipid-lowering effect in humans. Whether this beneficial effect persists during long-term administration has not been determined. The study was aimed at determining whether a prolonged (6 months) administration of inulin-type fructans to healthy subjects has a lipid-lowering action. In a double-blind, randomized, placebo-controlled study, 17 healthy subjects were studied before and after 6 months of daily administration of placebo (8 subjects) or 10 g of a mix of inulin and oligofructose (9 subjects). During this 6-month period, they consumed their usual diet and did not modify their everyday way of life. We measured plasma lipid concentrations; cholesterol synthesis and hepatic lipogenesis; and adipose tissue and circulating mononuclear cell messenger RNA concentrations of key regulatory genes of cholesterol metabolism. Compared with the administration of placebo, the administration of inulin-type fructans had no effect on plasma triacylglycerol concentrations and hepatic lipogenesis and induced only a nonsignificant trend for decreased plasma total and low-density lipoprotein cholesterol levels and increased high-density lipoprotein cholesterol concentration. Cholesterol synthesis was not significantly modified. Of all the messenger RNA concentrations measured, none was significantly modified by the administration of inulin-type fructans. In conclusion, contrary to what was observed in short-term studies, we observed no significant beneficial effect of a long-term (6-month) administration of inulin-type fructans on plasma lipids in healthy human subjects.

Impairment of cardiac insulin signaling and myocardial contractile performance in high-cholesterol/fructose-fed rats

Although insulin resistance is recognized as a potent and prevalent risk factor for coronary heart disease, less is known as to whether insulin resistance causes an altered cardiac phenotype independent of coronary atherosclerosis. In this study, we investigated the relationship between insulin resistance and cardiac contractile dysfunctions by generating a new insulin resistance animal model with rats on high cholesterol-fructose diet. Male Sprague-Dawley rats were given high cholesterol-fructose (HCF) diet for 15 wk; the rats developed insulin resistance syndrome characterized by elevated blood pressure, hyperlipidemia, hyperinsulinemia, impaired glucose tolerance, and insulin resistance. The results show that HCF induced insulin resistance not only in metabolic-response tissues (i.e., liver and muscle) but also in the heart as well. Insulin-stimulated cardiac glucose uptake was significantly reduced after 15 wk of HCF feeding, and cardiac insulin resistance was associated with blunted Akt-mediated insulin signaling along with glucose transporter GLUT4 translocation. Basal fatty acid transporter FATP1 levels were increased in HCF rat hearts. The cardiac performance of the HCF rats exhibited a marked reduction in cardiac output, ejection fraction, stroke volume, and end-diastolic volume. It also showed decreases in left ventricular end-systolic elasticity, whereas the effective arterial elasticity was increased. In addition, the relaxation time constant of left ventricular pressure was prolonged in the HCF group. Overall, these results indicate that insulin resistance reduction of cardiac glucose uptake is associated with defects in insulin signaling. The cardiac metabolic alterations that impair contractile functions may lead to the development of cardiomyopathy.

The effect of dietary carbohydrate on genes for fatty acid synthase and inflammatory cytokines in adipose tissues from lean and obese subjects

BACKGROUND

Hepatic de novo lipogenesis (DNL) is markedly stimulated in humans by low-fat diets enriched in simple sugars. However, the dietary responsiveness of the key enzyme controlling DNL in human adipose tissue, fatty acid synthase (FAS), is uncertain.

HYPOTHESIS

Adipose tissue mRNA for FAS is increased in lean and obese subjects when hepatic DNL is elevated by a eucaloric, low-fat, high-sugar diet.

DESIGN

Twelve lean and seven obese volunteers were given two eucaloric diets (10% vs. 30% fat; 75% vs. 55% carbohydrate; sugar/starch 60/40) each for 2 weeks by a random-order cross-over design. FAS mRNA in abdominal and gluteal adipose tissues was compared to hepatic DNL measured in serum by isotopic and nonisotopic methods. Adipose tissue mRNA for tumor necrosis factor-alpha and IL-6, which are inflammatory cytokines that modulate DNL, was also assayed.

RESULTS

The low-fat high-sugar diet induced a 4-fold increase in maximum hepatic DNL (P<.001) but only a 1.3-fold increase in adipose tissue FAS mRNA (P=.029) and no change in cytokine mRNA. There was a borderline significant positive correlation between changes in FAS mRNA and hepatic DNL (P=.039). Compared to lean subjects, obese subjects had lower levels of FAS mRNA and higher levels of cytokine mRNA (P<.001).

CONCLUSIONS

The results suggest that key elements of human adipose tissue DNL are less responsive to dietary carbohydrate than is hepatic DNL and may be regulated by diet-independent factors. Irrespective of diet, there is reduced expression of the FAS gene and increased expression of cytokine genes in adipose tissues of obese subjects.

Effects of glucose metabolism on the regulation of genes of fatty acid synthesis and triglyceride secretion in the liver

Glucose disposal induces a signal that modulates the transcriptional regulation of genes involved in the glycolysis and lipogenesis pathways. To investigate the role of glucose metabolism on hepatic gene expression independently from insulin action, we overexpressed glucokinase, the limiting enzyme in the glycolysis pathway, in the liver of streptozotocin-induced type 1 diabetic rats. By microarray analysis, we observed that critical genes such as liver-type pyruvate kinase, malic enzyme, fatty acid synthase, and stearoyl-CoA desaturase 1 were enhanced multiple-fold, whereas genes involved in mitochondrial fatty acid oxidation and the Krebs cycle were downregulated. Despite the increase in expression of fatty acid synthesis genes and the presence of steatosis, no major alterations to the levels of genes involved in VLDL assembly and secretion, such as diacylglycerol acyltransferases 1 and 2 and microsomal triglyceride transfer protein, were observed. Overall, our data suggest that the gene expression pattern induced by glucose metabolism favors fatty acid storage in the liver rather than secretion into the circulation.

Fish oil supplementation does not alter energy efficiency in healthy males

BACKGROUND AND AIMS

Fish oil (FO) supplementation prevents the development of obesity and insulin resistance, and upregulate the expression of UCP3 in skeletal muscle in rodents. This may represent indirect evidence that FO promotes fat oxidation and/or alter energy efficiency. The aim of this study was to evaluate whether such effects can be observed in humans. The metabolic effects of FO were assessed during exercise in order to obtain a direct measurement of energy efficiency.

METHODS

Eight healthy male volunteers were studied with and without supplementation with 7.2 g/day FO (including 1.1 g/day eicosopentaenoic acid and 0.7 g/day decosahexaenoic acid) during 14 days. Their VO(2 max) was measured on cycle ergometer. Thereafter, energy metabolism (substrate oxidation, energy expenditure and energy efficiency) was assessed during a 30 min cycling exercise at 50% VO(2 max) performed 2 h 30 after a standardized, high carbohydrate breakfast.

RESULTS

VO(2 max) was 38.6+/-2.2 after FO and 38.4+/-2.0 (mL x kg(-1) x min(-1)) in control conditions (NS). Basal plasma glucose, insulin and NEFA concentrations, and energy metabolism were similar with FO and in controls. During exercise, the increases in plasma NEFA concentrations, energy expenditure, glucose and lipid oxidation, and the decreases in glycaemia and insulinemia were not altered by FO intake. Energy efficiency was 22.4+/-0.6% after FO vs 21.8+/-0.7% in controls. In order to ascertain that the absence of effects of FO was not due to consumption of a carbohydrate meal immediately before exercise, 4 of the 8 subjects were re-studied in fasting conditions, FO also failed to alter energy efficiency in this subset of studies.

CONCLUSION

FO supplementation did not significantly alter energy metabolism and energy efficiency during exercise in healthy humans.

Effects of isoenergetic overfeeding of either carbohydrate or fat in young men

Ten pairs of normal men were overfed by 5 MJ/d for 21 d with either a carbohydrate-rich or a fat-rich diet (C- and F-group). The two subjects in each pair were requested to follow each other throughout the day to ensure similar physical activity and were otherwise allowed to maintain normal daily life. The increase in body weight, fat free mass and fat mass showed great variation, the mean increases being 1·5 kg, 0·6 kg and 0·9 kg respectively. No significant differences between the C- and F-group were observed. Heat production during sleep did not change during overfeeding. The RQ during sleep was 0·86 and 0·78 in the C- and F-group respectively. The accumulated faecal loss of energy, DM, carbohydrate and protein was significantly higher in the C- compared with the F-group (30, 44, 69 and 51 % higher respectively), whereas the fat loss was the same in the two groups. N balance was not different between the C- and F-group and was positive. Fractional contribution from hepatic de novo lipogenesis, as measured by mass isotopomer distribution analysis after administration of [1-13C]acetate, was 0·20 and 0·03 in the C-group and the F-group respectively. Absolute hepatic de novo lipogenesis in the C-group was on average 211 g per 21 d. Whole-body de novo lipogenesis, as obtained by the difference between fat mass increase and dietary fat available for storage, was positive in six of the ten subjects in the C-group (mean 332 (SEM 191) g per 21 d). The change in plasma leptin concentration was positively correlated with the change in fat mass. Thus, fat storage during overfeeding of isoenergetic amounts of diets rich in carbohydrate or in fat was not significantly different, and carbohydrates seemed to be converted to fat by both hepatic and extrahepatic lipogenesis.

Metabolic interaction of dietary sugars and plasma lipids with a focus on mechanisms andde novolipogenesis

The elevation of blood lipid concentrations in response to the consumption of low-fat high-carbohydrate diets is known as carbohydrate-induced hypertriacylglycerolaemia (HPTG). An understanding of the mechanisms involved in the interaction between carbohydrates and plasma lipids may help determine whether carbohydrate-induced HPTG would increase cardiovascular risk. There is growing evidence to suggest that the sugar component of the diet may be largely responsible, rather than the total carbohydrate. In most studies designed to investigate the mechanisms of carbohydrate-induced HPTG, the amounts and types of sugars and starches used in the diets are not specified. Findings have been mixed and inconsistent. It is proposed that the elucidation of mechanisms from current studies could have been confounded by the different ways in which sugars are metabolized in the body. At present, there are few studies that have evaluated the independent effects of dietary sugars. Interest has been focused onde novolipogenesis (DNL), as it has recently been found to be positively correlated with increases in fasting TAG levels produced on high-carbohydrate diets, indicating that DNL may contribute to carbohydrate-induced HPTG. DNL has been found to be determined by starch:sugar in a high-carbohydrate diet and affected by different types of sugars. The presence of DNL in adipose tissue is supported by emerging gene-expression studies in human subjects. In the wake of rising intakes of sugars, further research is needed to investigate the mechanisms associated with different sugars, so that appropriate therapeutic strategies can be adopted.

A moderate-fat diet for combined hyperlipidemia and metabolic syndrome

A low-fat diet is recommended for hyperlipidemia. However, low-density lipoprotein (LDL) responses depend on the type of hyperlipidemia (ie, simple hypercholesterolemia or combined hyperlipidemia). In combined hyperlipidemia, which is typical of patients with metabolic syndrome, LDL levels are only one third as responsive to fat and cholesterol as simple hypercholesterolemia. The diminished dietary sensitivity of combined hyperlipidemia is explained by diminished intestinal absorption of cholesterol, a feature of metabolic syndrome. In turn, combined hyperlipidemia is caused by heightened lipid secretion by the liver. A moderate-fat, moderate-carbohydrate diet employing allowable fats has the promise of reducing endogenous lipoprotein production in combined hyperlipidemia. Triglyceride, LDL, and small-dense LDL should be lower, and high-density lipoprotein, apoprotein A-I, and buoyant LDL should be higher. A test of this dietary strategy on lipoproteins and downstream benefits on inflammatory mediators, oxidative stress, and vascular reactivity is now underway.

Review: The role of insulin resistance in nonalcoholic fatty liver disease

CONTEXT

Insulin resistance is an almost universal finding in nonalcoholic fatty liver disease (NAFLD). This review outlines the evidence linking insulin resistance and NAFLD, explores whether liver fat is a cause or consequence of insulin resistance, and reviews the current evidence for treatment of NAFLD.

EVIDENCE ACQUISITION

Evidence from epidemiological, experimental, and clinical research studies investigating NAFLD and insulin resistance was reviewed.

EVIDENCE SYNTHESIS

Insulin resistance in NAFLD is characterized by reductions in whole-body, hepatic, and adipose tissue insulin sensitivity. The mechanisms underlying the accumulation of fat in the liver may include excess dietary fat, increased delivery of free fatty acids to the liver, inadequate fatty acid oxidation, and increased de novo lipogenesis. Insulin resistance may enhance hepatic fat accumulation by increasing free fatty acid delivery and by the effect of hyperinsulinemia to stimulate anabolic processes. The impact of weight loss, metformin, and thiazolidinediones, all treatments aimed at improving insulin sensitivity, as well as other agents such as vitamin E, have been evaluated in patients with NAFLD and have shown some benefit. However, most intervention studies have been small and uncontrolled.

CONCLUSION

Insulin resistance is a major feature of NAFLD that, in some patients, can progress to steatohepatitis. Treatments aimed at reducing insulin resistance have had some success, but larger placebo-controlled studies are needed to fully establish the efficacy of these interventions and possibly others in reducing the deleterious effects of fat accumulation in the liver.

Adipose fatty acid composition and rate of incorporation of alpha-linolenic acid differ between normal and lipoprotein lipase-deficient cats

Normal adiposity occurs in humans and mice deficient of adipose lipoprotein lipase (LPL) activity. Subnormal adiposity found in LPL-deficient cats is indicative of limited de novo synthesis of fatty acids (FAs). In 14 LPL-deficient (3.0 +/- 0.1 kg) and 8 normal (3.7 +/- 0.1 kg) queens, FAs in triacylglycerol (TAG), phospholipid (PL), and nonesterified FAs (NEFAs) of plasma and inguinal subcutaneous adipose were determined before and after (d 38, 61, 110, 117, and 251) dietary linseed oil supplementation (30 g/kg). By d 60, LPL-deficient queens gained body weight (+0.4 +/- 0.1 kg), developed normal body fat mass (25 +/- 2%), and were enriched in 18:3(n-3) in their plasma and adipose lipids. Adipose TAG 18:3(n-3) enrichment in LPL-deficient queens was subnormal at all sampling times and, as observed in normal queens, apparently not equilibrated by d 251. Adipose FA profiles in TAG but not PL were substantially different (P < 0.05) between LPL-deficient and normal queens; the 16:0 to 18:2(n-6) ratio was high in LPL-deficient (2.4-4.4) relative to normal queens (1.0-1.4). In LPL-deficient queens, fed-state plasma NEFA (n-6) and (n-3) enrichments were similar to those in adipose TAG, and plasma NEFA concentration was high (0.62 +/- 0.05 mmol/L) and similar to that in normal queens after withholding diet for 16 h. These data indicate that LPL deficiency in cats reduces dietary FA storage efficiency, favors storage of saturated over unsaturated FAs, and stimulates de novo FA synthesis substantive enough to support normal adiposity.