Effects of vanadate on glucose production in cultured hepatocytes isolated from rats on high saturated fat diet

Insulin resistance is a common phenomenon in obesity and Type 2 diabetes. Common factor important for development of diabetes and insulin resistance is intake of saturated fat. Vanadate treatment improves glucose homeostasis in vivo. The aim of this study was to find out changing of hepatic glucose output in dependence of saturated fat diet and possible direct action of vanadate in cultured hepatocytes. Hepatocytes were isolated by a collagenase perfusion technique and cultured for 24 h in M 199 serum-free medium. The glucose production in hepatocytes isolated from rats on high saturated fat diet was significantly 139% higher comparable to standard controls. Glucagon 100% increased glucose production in hepatocytes from rats on standard diet and 200% in hepatocytes on saturated high fat diet. The addition vanadate significantly decreased basic glucose production and did not influence glucagon stimulated glucose production. Presence of insulin did not influence either glucagon or vanadate effect. High saturated fat diet not only increases insulin resistance but also decreases chances of successful therapy of diabetes.     

Low-carbohydrate diets affect energy balance and fuel homeostasis differentially in lean and obese rats

In parallel with increased prevalence of overweight people in affluent societies are individuals trying to lose weight, often using low-carbohydrate diets. Nevertheless, long-term metabolic consequences of those diets, usually high in (saturated) fat, remain unclear. Therefore, we investigated long-term effects of high-fat diets with different carbohydrate/protein ratios on energy balance and fuel homeostasis in obese (fa/fa) Zucker and lean Wistar rats. Animals were fed high-carbohydrate (HC), high-fat (HsF), or low-carbohydrate, high-fat, high-protein (LC-HsF-HP) diets for 60 days. Both lines fed the LC-HsF-HP diet displayed reduced energy intake compared with those fed the HsF diet (Zucker, -3.7%) or the HC diet (Wistar rats, -12.4%). This was not associated with lower weight gain relative to HC fed rats, because of increased food efficiencies in each line fed HsF and particularly LC-HsF-HP food. Zucker rats were less glucose tolerant than Wistar rats. Lowest glucose tolerances were found in HsF and particularly in LC-HsF-HP-fed animals irrespective of line, but this paralleled reduced plasma adiponectin levels, elevated plasma resistin levels, higher retroperitoneal fat masses, and reduced insulin sensitivity (indexed by insulin-induced hypoglycemia) only in Wistar rats. In Zucker rats, however, improved insulin responses during glucose tolerance testing and tendency toward increased insulin sensitivities were observed with HsF or LC-HsF-HP feeding relative to HC feeding. Thus, despite adverse consequences of LC-HsF diets on blood glucose homeostasis, principal differences exist in the underlying hormonal regulatory mechanisms, which could have benefits for B-cell functioning and insulin action in the obese state but not in the lean state.     

Dietary effects on body composition, glucose metabolism, and longevity are modulated by skeletal muscle mitochondrial uncoupling in mice

Little is known about how diet and energy metabolism interact in determination of lifespan under ad libitum feeding. From 12 weeks of age until death, male and female wild-type (WT) and transgenic (TG) mice with increased skeletal muscle mitochondrial uncoupling (HSA-mUCP1 mice) were fed one of three different semisynthetic diets differing in macronutrient ratio: control (high-carbohydrate/low-fat-HCLF) and two high-fat diets: high-carbohydrate/high-fat (HCHF), and low-carbohydrate/high-fat (LCHF). Compared to control and LCHF, HCHF feeding rapidly and significantly increased body fat content in WT. Median lifespan of WT was decreased by 33% (HCHF) and 7% (LCHF) compared to HCLF. HCHF significantly increased insulin resistance (HOMA) of WT from 24 weeks on compared to control. TG mice had lower lean body mass and increased energy expenditure, insulin sensitivity, and maximum lifespan (+10%) compared to WT. They showed a delayed development of obesity on HCHF but reached similar maximum adiposity as WT. TG median lifespan was only slightly reduced by HCHF (-7%) and unaffected by LCHF compared to control. Correlation analyses showed that decreased longevity was more strongly linked to a high rate of fat gain than to adiposity itself. Furthermore, insulin resistance was negatively and weight-specific energy expenditure was positively correlated with longevity. We conclude that (i) dietary macronutrient ratios strongly affected obesity development, glucose homeostasis, and longevity, (ii) that skeletal muscle mitochondrial uncoupling alleviated the detrimental effects of high-fat diets, and (iii) that early imbalances in energy homeostasis leading to increased insulin resistance are predictive for a decreased lifespan.     

Ghrelin, macronutrient intake and dietary preferences in long-evans rats

Ghrelin is a recently discovered peptide that is primarily produced by the stomach. As a ligand of the growth hormone (GH) secretagogue (GHS) receptor, it stimulates GH secretion but it also stimulates feeding and has adipogenic effects in rodents. Although its circulating levels are modulated by fasting and refeeding, its relationship with diet composition is not known. In the present paper, we measured plasma ghrelin as well as two important hormones (leptin and insulin) in Long-Evans rats placed in two different feeding situations, e.g., either with imposed diets or with food choice. In the first case, the rats were fed unbalanced diets (either high-carbohydrate (HC) or high-fat (HF) diets) for 14 weeks, whereas in the second situation, they had the choice between these 2 diets for 2 weeks and were selected for their fat or carbohydrate preference. The intake of the HF diet for 14 weeks was associated with lower levels of ghrelin (-30% vs control diet; P < 0.01). These levels increased when the percentage of carbohydrate in the diet increased (+26 to +42% vs control diet; P < 0.01 or less). Ghrelin was inversely correlated with plasma leptin (r = -0.55; P < 0.003) and blood glucose (r = -0.58; P < 0.001) as well as with body weight (r = -0.63; P < 0.0001) and body fat content estimated by the sampling of specific fat pads (r = -0.62; P < 0.0001). In the food choice experiment, fat-preferring rats had plasma ghrelin levels lower than the carbohydrate-preferring rats (-33%; P < 0.0002). Ghrelin secretion was therefore very sensitive to the diet composition. Its down-regulation by fat ingestion might serve as a counterregulatory mechanism to limit the development of dietary-induced adiposity. Ghrelin may signal when a high calorie diet is ingested.     

Metabolic Implications of Dietary Trans‐fatty Acids

Dietary trans‐fatty acids are associated with increased risk of cardiovascular disease and have been implicated in the incidence of obesity and type 2 diabetes mellitus (T2DM). It is established that high‐fat saturated diets, relative to low‐fat diets, induce adiposity and whole‐body insulin resistance. Here, we test the hypothesis that markers of an obese, prediabetic state (fatty liver, visceral fat accumulation, insulin resistance) are also worsened with provision of a low‐fat diet containing elaidic acid (18:1t), the predominant trans‐fatty acid isomer found in the human food supply. Male 8‐week‐old Sprague–Dawley rats were fed a 10% trans‐fatty acid enriched (LF‐trans) diet for 8 weeks. At baseline, 3 and 6 weeks, in vivo magnetic resonance spectroscopy (¹H‐MR) assessed intramyocellular lipid (IMCL) and intrahepatic lipid (IHL) content. Euglycemic–hyperinsulinemic clamps (week 8) determined whole‐body and tissue‐specific insulin sensitivity followed by high‐resolution ex vivo ¹H‐NMR to assess tissue biochemistry. Rats fed the LF‐trans diet were in positive energy balance, largely explained by increased energy intake, and showed significantly increased visceral fat and liver lipid accumulation relative to the low‐fat control diet. Net glycogen synthesis was also increased in the LF‐trans group. A reduction in glucose disposal, independent of IMCL accumulation was observed in rats fed the LF‐trans diet, whereas in rats fed a 45% saturated fat (HF‐sat) diet, impaired glucose disposal corresponded to increased IMCL_TA. Neither diet induced an increase in IMCL_soleus. These findings imply that trans‐fatty acids may alter nutrient handling in liver, adipose tissue, and skeletal muscle and that the mechanism by which trans‐fatty acids induce insulin resistance differs from diets enriched with saturated fats.     

Dietary modulation of circulating leptin levels: site-specific changes in fat deposition and ob mRNA expression.

In order to study the effects of diet on fat distribution, circulating leptin levels and ob mRNA expression, diets of different macronutrient composition were fed to lean mice and gold thioglucose-obese mice. A high-fat diet and 2 high-carbohydrate diets, one containing mostly high-glycaemic-index starch and the other containing low-glycaemic-index starch were fed ad libitum for 10 weeks and were compared to standard laboratory chow. Weight gain was attenuated by feeding low-glycaemic-index starch in all mice and by feeding a high-fat diet in lean mice. Reduced adiposity was seen in lean mice fed low-glycaemic-index starch, whereas increased adiposity was seen in both lean and obese mice fed on the high-fat diet. Circulating leptin levels, when corrected for adiposity, were decreased in all mice fed either the high-fat diet or the low-GI diet. In epididymal fat pads, decreased ob mRNA expression was seen after both high-fat and high-glycaemic-index starch feeding. These results show that diet macronutrient composition contributes to the variability of circulating leptin levels by the combined effects of diet on fat distribution and on site-specific changes in ob mRNA expression.     

Implications of crosstalk between leptin and insulin signaling during the development of diet-induced obesity

Insulin and leptin play complementary roles in regulating the consumption, uptake, oxidation and storage of nutrients. Chronic consumption of diets that contain a high proportion of calories from saturated fat induces a progressive deterioration in function of both hormones. Certain rat lines and strains of mice are particularly sensitive to the obesogenic and diabetogenic effects of high fat diets, and have been used extensively to study the developmental progression of insulin and leptin resistance in relation to the increasing adiposity that is characteristic of their response to these diets. Some aspects of the diminished efficacy of each hormone are secondary to increased adiposity but a consensus is emerging to support the view that direct effects of dietary components or their metabolites, independent of the resulting obesity, play important roles in development of insulin and leptin resistance. In this minireview, we will examine the implications of crosstalk between leptin and insulin signaling during the development of diet-induced obesity, emphasizing potential interactions between pathways that occur among target sites, and exploring how these interactions may influence the progression of obesity and diabetes.     

Omega 3 rich diet modulates energy metabolism via GPR120-Nrf2 crosstalk in a novel antioxidant mouse model

With obesity rates reaching epidemic proportions, more studies concentrated on reducing the risk and treating this epidemic are vital. Redox stress is an important metabolic regulator involved in the pathophysiology of cardiovascular disease, Type 2 diabetes, and obesity. Oxygen and nitrogen-derived free radicals alter glucose and lipid homeostasis in key metabolic tissues, leading to increases in risk of developing metabolic syndrome. Oxidants derived from dietary fat differ in their metabolic regulation, with numerous studies showing benefits from a high omega 3 rich diet compared to the frequently consumed “western diet” rich in saturated fat. Omega 3 (OM3) fatty acids improve lipid profile, lower inflammation, and ameliorate insulin resistance, possibly through maintaining redox homeostasis. This study is based on the hypothesis that altering endogenous antioxidant production and/or increasing OM3 rich diet consumption will improve energy metabolism and maintain insulin sensitivity. We tested the comparative metabolic effects of a diet rich in saturated fat (HFD) and an omega 3-enriched diet (OM3) in the newly developed ‘stress-less’ mice model that overexpresses the endogenous antioxidant catalase. Eight weeks of dietary intervention showed that mice overexpressing endogenous catalase compared to their wild-type controls when fed an OM3 enriched diet, in contrast to HFD, activated GPR120-Nrf2 cross-talk to maintain balanced energy metabolism, normal circadian rhythm, and insulin sensitivity. These findings suggest that redox regulation of GPR120/FFAR4 might be an important target in reducing risk of metabolic syndrome and associated diseases.     

Pigs fed saturated fat/cholesterol have a blunted hypothalamic-pituitary-adrenal function,are insulin resistant and have decreased expression of IRS-1, PGC1α and PPARα

The increasing incidence of insulin resistance has been linked to both increased intake of saturated fatty acids and disruption of the hypothalamic-pituitary-adrenal (HPA) axis. We tested the hypothesis that adding saturated fat/cholesterol to the diet of growing pigs would both disrupt HPA function and cause insulin resistance. Three-month-old pigs were fed either a control (13% energy from fat) or a high saturated fatty acid cholesterol (HSFC) diet (44% energy from fat; 2% cholesterol). After 10 weeks on the diets, intravenous ACTH, insulin and glucose challenges were performed, and after 12 weeks, tissue samples were taken for measurement of mRNA and for lipid-rich aortic lesions. Plasma total, HDL- and LDL-cholesterol were significantly increased in pigs fed the HSFC diet. Cortisol release during the ACTH challenge was suppressed in HSFC-fed pigs which were also more insulin resistant and glucose intolerant than controls. The HSFC diet decreased the expression of insulin receptor (IR) and insulin receptor substrate-1 in muscle and adipose tissue as well as adiponectin and adiponectin receptor 2 expression in fat. The HSFC diet decreased PGC-1α and PPARα expression in muscle but increased PPARα expression in liver. There was a trend for an increase in lipid-stained lesion frequency around the abdominal branches of the aorta in HSFC-fed pigs. We conclude that feeding increased saturated fat to pigs causes disruption in the HPA axis, insulin resistance and decreased muscle and adipose expression of genes controlling insulin signalling and mitochondrial oxidative capacity.     

Improved glucose homeostasis in mice with muscle-specific deletion of protein-tyrosine phosphatase 1B

Obesity and type 2 diabetes are characterized by insulin resistance. Mice lacking the protein-tyrosine phosphatase PTP1B in all tissues are hypersensitive to insulin but also have diminished fat stores. Because adiposity affects insulin sensitivity, the extent to which PTP1B directly regulates glucose homeostasis has been unclear. We report that mice lacking PTP1B only in muscle have body weight and adiposity comparable to those of controls on either chow or a high-fat diet (HFD). Muscle triglycerides and serum adipokines are also affected similarly by HFD in both groups. Nevertheless, muscle-specific PTP1B(-/-) mice exhibit increased muscle glucose uptake, improved systemic insulin sensitivity, and enhanced glucose tolerance. These findings correlate with and are most likely caused by increased phosphorylation of the insulin receptor and its downstream signaling components. Thus, muscle PTP1B plays a major role in regulating insulin action and glucose homeostasis, independent of adiposity. In addition, rosiglitazone treatment of HFD-fed control and muscle-specific PTP1B(-/-) mice revealed that rosiglitazone acts additively with PTP1B deletion. Therefore, combining PTP1B inhibition with thiazolidinediones should be more effective than either alone for treating insulin-resistant states.     

Adropin deficiency is associated with increased adiposity and insulin resistance

Adropin is a secreted peptide that improves hepatic steatosis and glucose homeostasis when administered to diet-induced obese mice. It is not clear if adropin is a peptide hormone regulated by signals of metabolic state. Moreover, the significance of a decline in adropin expression with obesity with respect to metabolic disease is also not clear. We investigated the regulation of serum adropin by metabolic status and diet. Serum adropin levels were high in chow-fed conditions and were suppressed by fasting and diet-induced obesity (DIO). High adropin levels were observed in mice fed a high-fat low carbohydrate diet, whereas lower levels were observed in mice fed a low-fat high carbohydrate diet. To investigate the role of adropin deficiency in metabolic homeostasis, we generated adropin knockout mice (AdrKO) on the C57BL/6J background. AdrKO displayed a 50%-increase in increase in adiposity, although food intake and energy expenditure were normal. AdrKO also exhibited dyslipidemia and impaired suppression of endogenous glucose production (EndoR(a)) in hyperinsulinemic-euglycemic clamp conditions, suggesting insulin resistance. While homo- and heterozygous carriers of the null adropin allele exhibited normal DIO relative to controls, impaired glucose tolerance associated with weight gain was more severe in both groups. In summary, adropin is a peptide hormone regulated by fasting and feeding. In fed conditions, adropin levels are regulated dietary macronutrients, and increase with dietary fat content. Adropin is not required for regulating food intake, however, its functions impact on adiposity and are involved in preventing insulin resistance, dyslipidemia, and impaired glucose tolerance.     

Effects of fructose and glucose on plasma leptin, insulin, and insulin resistance in lean and VMH-lesioned obese rats

To determine the influence of dietary fructose and glucose on circulating leptin levels in lean and obese rats, plasma leptin concentrations were measured in ventromedial hypothalamic (VMH)-lesioned obese and sham-operated lean rats fed either normal chow or fructose- or glucose-enriched diets (60% by calories) for 2 wk. Insulin resistance was evaluated by the steady-state plasma glucose method and intravenous glucose tolerance test. In lean rats, glucose-enriched diet significantly increased plasma leptin with enlarged parametrial fat pad, whereas neither leptin nor fat-pad weight was altered by fructose. Two weeks after the lesions, the rats fed normal chow had marked greater body weight gain, enlarged fat pads, and higher insulin and leptin compared with sham-operated rats. Despite a marked adiposity and hyperinsulinemia, insulin resistance was not increased in VMH-lesioned rats. Fructose brought about substantial insulin resistance and hyperinsulinemia in both lean and obese rats, whereas glucose led to rather enhanced insulin sensitivity. Leptin, body weight, and fat pad were not significantly altered by either fructose or glucose in the obese rats. These results suggest that dietary glucose stimulates leptin production by increasing adipose tissue or stimulating glucose metabolism in lean rats. Hyperleptinemia in VMH-lesioned rats is associated with both increased adiposity and hyperinsulinemia but not with insulin resistance. Dietary fructose does not alter leptin levels, although this sugar brings about hyperinsulinemia and insulin resistance, suggesting that hyperinsulinemia compensated for insulin resistance does not stimulate leptin production.     

Activation of natural killer T cells promotes M2 Macrophage polarization in adipose tissue and improves systemic glucose tolerance via interleukin-4 (IL-4)/STAT6 protein signaling axis in obesity

Natural killer T (NKT) cells are important therapeutic targets in various disease models and are under clinical trials for cancer patients. However, their function in obesity and type 2 diabetes remains unclear. Our data show that adipose tissues of both mice and humans contain a population of type 1 NKT cells, whose abundance decreases with increased adiposity and insulin resistance. Although loss-of-function of NKT cells had no effect on glucose tolerance in animals with prolonged high fat diet feeding, activation of NKT cells by lipid agonist α-galactosylceramide enhances alternative macrophage polarization in adipose tissue and improves glucose homeostasis in animals at different stages of obesity. Furthermore, the effect of NKT cells is largely mediated by the IL-4/STAT6 signaling axis in obese adipose tissue. Thus, our data identify a novel therapeutic target for the treatment of obesity-associated inflammation and type 2 diabetes.     

Histopathological changes in rat pancreas and skeletal muscle associated with high fat diet induced insulin resistance

The effects of a high fat diet on the development of diabetes mellitus, insulin resistance and secretion have been widely investigated. We investigated the effects of a high fat diet on the pancreas and skeletal muscle of normal rats to explore diet-induced insulin resistance mechanisms. Forty-four male Wistar rats were divided into six groups: a control group fed standard chow, a group fed a 45% fat diet and a group fed a 60% fat diet for 3 weeks to measure acute effects; an additional three groups were fed the same diet regimens for 8 weeks to measure chronic effects. The morphological effects of the two high fat diets were examined by light microscopy. Insulin in pancreatic islets was detected using immunohistochemistry. The homeostasis model assessment of insulin resistance index and insulin staining intensity in islets increased significantly with acute administration of high fat diets, whereas staining intensity decreased with chronic administration of the 45% fat diet. Islet areas increased significantly with chronic administration. High fat diet administration led to islet degeneration, interlobular adipocyte accumulation and vacuolization in the pancreatic tissue, as well as degeneration and lipid droplet accumulation in the skeletal muscle tissue. Vacuolization in the pancreas and lipid droplets in skeletal muscle tissue increased significantly with chronic high fat diet administration. We suggest that the glucolipotoxic effects of high fat diet administration depend on the ratio of saturated to unsaturated fatty acid content in the diet and to the total fat content of the diet.     

Lipoprotein metabolism mediates the association of MTP polymorphism with β-cell dysfunction in healthy subjects and in nondiabetic normolipidemic patients with nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) predicts incident diabetes independently of insulin resistance, adiposity and metabolic syndrome through unclear mechanisms. Dietary fat consumption and lipoperoxidative stress predispose to diabetes in the general population and to liver injury in NASH. Microsomal triglyceride transfer protein (MTP) polymorphism modulates lipoprotein metabolism in the general population and liver disease in NASH; a functional MTP polymorphism recently predicted incident diabetes independently of insulin resistance in the general population. We simultaneously assessed the impact of MTP polymorphism, diet, adipokines and lipoprotein metabolism, on glucose homeostasis in NASH.MTP ?493G/T polymorphism, dietary habits, adipokines and postprandial triglyceride-rich lipoproteins, high-density lipoprotein cholesterol (HDL-C) and oxidized low-density lipoprotein (oxLDL) responses to an oral fat load, were cross-sectionally correlated to oral glucose tolerance test- and frequently sampled intravenous glucose tolerance test-derived Minimal Model indexes of glucose homeostasis in 40 nondiabetic normolipidemic patients with NASH and 40 age-,sex- and body mass index-matched healthy controls.Despite comparable insulin resistance, fasting lipids, adipokines and dietary habits, MTP GG genotype had significantly more severe β-cell dysfunction; higher plasma Tg, FFA, intestinal and hepatic very low-density lipoprotein 1 subfractions and oxLDL responses and deeper HDL-C fall than GT/TT carriers in patients and controls.Postprandial HDL-C and oxLDL responses independently predicted β-cell dysfunction and mediated the effect of MTP polymorphism on β-cell function.In nondiabetic normolipidemic NASH, MTP ?493G/T polymorphism modulates β-cell function, an effect mediated by postprandial HDL-C and oxLDL metabolism. The impact of this polymorphism on the risk of diabetes and the efficacy of lipid-lowering therapies in restoring β-cell function in NASH, even with normal fasting lipid values, warrant further investigation.     

Nocturnal free fatty acids are uniquely elevated in the longitudinal development of diet-induced insulin resistance and hyperinsulinemia

Obesity is strongly associated with hyperinsulinemia and insulin resistance, both primary risk factors for type 2 diabetes. It has been thought that increased fasting free fatty acids (FFA) may be responsible for the development of insulin resistance during obesity, causing an increase in plasma glucose levels, which would then signal for compensatory hyperinsulinemia. But when obesity is induced by fat feeding in the dog model, there is development of insulin resistance and a marked increase in fasting insulin despite constant fasting FFA and glucose. We examined the 24-h plasma profiles of FFA, glucose, and other hormones to observe any potential longitudinal postprandial or nocturnal alterations that could lead to both insulin resistance and compensatory hyperinsulinemia induced by a high-fat diet in eight normal dogs. We found that after 6 wk of a high-fat, hypercaloric diet, there was development of significant insulin resistance and hyperinsulinemia as well as accumulation of both subcutaneous and visceral fat without a change in either fasting glucose or postprandial glucose. Moreover, although there was no change in fasting FFA, there was a highly significant increase in the nocturnal levels of FFA that occurred as a result of fat feeding. Thus enhanced nocturnal FFA, but not glucose, may be responsible for development of insulin resistance and fasting hyperinsulinemia in the fat-fed dog model.     

Transgenic MSH overexpression attenuates the metabolic effects of a high-fat diet

To determine whether long-term melanocortinergic activation can attenuate the metabolic effects of a high fat diet, mice overexpressing an NH(2)-terminal POMC transgene that includes alpha- and gamma(3)-MSH were studied on either a 10% low-fat diet (LFD) or 45% high-fat diet (HFD). Weight gain was modestly reduced in transgenic (Tg-MSH) male and female mice vs. wild type (WT) on HFD (P < 0.05) but not LFD. Substantial reductions in body fat percentage were found in both male and female Tg-MSH mice on LFD (P < 0.05) and were more pronounced on HFD (P < 0.001). These changes occurred in the absence of significant feeding differences in most groups, consistent with effects of Tg-MSH on energy expenditure and partitioning. This is supported by indirect calorimetry studies demonstrating higher resting oxygen consumption and lower RQ in Tg-MSH mice on the HFD. Tg-MSH mice had lower fasting insulin levels and improved glucose tolerance on both diets. Histological and biochemical analyses revealed that hepatic fat accumulation was markedly reduced in Tg-MSH mice on the HFD. Tg-MSH also attenuated the increase in corticosterone induced by the HFD. Higher levels of Agrp mRNA, which might counteract effects of the transgene, were measured in Tg-MSH mice on LFD (P = 0.02) but not HFD. These data show that long-term melanocortin activation reduces body weight, adiposity, and hepatic fat accumulation and improves glucose metabolism, particularly in the setting of diet-induced obesity. Our results suggest that long-term melanocortinergic activation could serve as a potential strategy for the treatment of obesity and its deleterious metabolic consequences.     

Saturated, but not n-6 polyunsaturated, fatty acids induce insulin resistance: role of intramuscular accumulation of lipid metabolites.

Consumption of a Western diet rich in saturated fats is associated with obesity and insulin resistance. In some insulin-resistant phenotypes this is associated with accumulation of skeletal muscle fatty acids. We examined the effects of diets high in saturated fatty acids (Sat) or n-6 polyunsaturated fatty acids (PUFA) on skeletal muscle fatty acid metabolite accumulation and whole-body insulin sensitivity. Male Sprague-Dawley rats were fed a chow diet (16% calories from fat, Con) or a diet high (53%) in Sat or PUFA for 8 wk. Insulin sensitivity was assessed by fasting plasma glucose and insulin and glucose tolerance via an oral glucose tolerance test. Muscle ceramide and diacylglycerol (DAG) levels and triacylglycerol (TAG) fatty acids were also measured. Both high-fat diets increased plasma free fatty acid levels by 30%. Compared with Con, Sat-fed rats were insulin resistant, whereas PUFA-treated rats showed improved insulin sensitivity. Sat caused a 125% increase in muscle DAG and a small increase in TAG. Although PUFA also resulted in a small increase in DAG, the excess fatty acids were primarily directed toward TAG storage (105% above Con). Ceramide content was unaffected by either high-fat diet. To examine the effects of fatty acids on cellular lipid storage and glucose uptake in vitro, rat L6 myotubes were incubated for 5 h with saturated and polyunsaturated fatty acids. After treatment of L6 myotubes with palmitate (C16:0), the ceramide and DAG content were increased by two- and fivefold, respectively, concomitant with reduced insulin-stimulated glucose uptake. In contrast, treatment of these cells with linoleate (C18:2) did not alter DAG, ceramide levels, and glucose uptake compared with controls (no added fatty acids). Both 16:0 and 18:2 treatments increased myotube TAG levels (C18:2 vs. C16:0, P < 0.05). These results indicate that increasing dietary Sat induces insulin resistance with concomitant increases in muscle DAG. Diets rich in n-6 PUFA appear to prevent insulin resistance by directing fat into TAG, rather than other lipid metabolites.     

Dietary monounsaturated fat in early life regulates IGFBP2: implications for fat mass accretion and insulin sensitivity

The aim of this study was to investigate effects of dietary supplementation with fat or sugar on body composition (BC) and insulin sensitivity (IS) in maturing pigs. Fifty newborn pigs randomized to a control diet or 18% saturated fat (SF), 18% monounsaturated fat (MUF), 18% mixed fat (MF), or 50% sucrose (SUC), from 1 to 16 weeks of age. Outcomes included weight gain, BC (dual energy X-ray absorptiometry, DXA), IS (fasting insulin and hyperinsulinaemic-euglycaemic clamps), fasting Non-Esterified Fatty Acid (NEFA) concentrations, and mRNA expression of genes involved in lipogenesis and IS in skeletal muscle (SM), subcutaneous (SAT), and visceral adipose tissue (VAT). In vitro studies examined direct effects of fatty acids on insulin-like growth factor-binding protein 2 (IGFBP2) mRNA in C2C12 myotubes. While SUC-fed pigs gained most weight (due to larger quantities consumed; P < 0.01), those fed fat-enriched diets exhibited more weight gain per unit energy intake (P < 0.001). Total (P = 0.03) and visceral (P = 0.04) adiposity were greatest in MUF-fed pigs. Whole-body IS was decreased in those fed fat (P = 0.04), with fasting insulin increased in MUF-fed pigs (P = 0.03). SM IGFBP2 mRNA was increased in MUF-fed pigs (P = 0.009) and, in all animals, SM IGFBP2 mRNA correlated with total (P = 0.007) and visceral (P = 0.001) fat, fasting insulin (r = 0.321; P = 0.03) and change in NEFA concentrations (r = 0.285; P = 0.047). Furthermore, exposure of in vitro cultured myotubes to MUF, but not SF, reduced IGFBP2 mRNA suggesting a converse direct effect. In conclusion, diets high in fat, but not sugar, promote visceral adiposity and insulin resistance in maturing pigs, with evidence that fatty acids have direct and indirect effects on IGFBP2 mRNA expression in muscle.