Dietary fish oil normalize dyslipidemia and glucose intolerance with unchanged insulin levels in rats fed a high sucrose diet

Differential effects of fish-oil and cocoa-butter based high-fat/high-sucrose diets on endocrine pancreas morphology and function in mice

Introduction

A high-fat/high-sucrose diet leads to adverse metabolic changes that affect insulin sensitivity, function, and secretion. The source of fat in the diet might inhibit or increase this adverse effect. Fish oil and cocoa butter are a significant part of our diets. Yet comparisons of these commonly used fat sources with high sucrose on pancreas morphology and function are not made. This study investigated the comparative effects of a fish oil-based high-fat/high-sucrose diet (Fish-HFDS) versus a cocoa butter-based high-fat/high-sucrose diet (Cocoa-HFDS) on endocrine pancreas morphology and function in mice.

Methods

C57BL/6 male mice (n=12) were randomly assigned to dietary intervention either Fish-HFDS (n=6) or Cocoa-HFDS (n=6) for 22 weeks. Intraperitoneal glucose and insulin tolerance tests (IP-GTT and IP-ITT) were performed after 20-21 weeks of dietary intervention. Plasma concentrations of c-peptide, insulin, glucagon, GLP-1, and leptin were measured by Milliplex kit. Pancreatic tissues were collected for immunohistochemistry to measure islet number and composition. Tissues were multi-labelled with antibodies against insulin and glucagon, also including expression on Pdx1-positive cells.

Results and discussion

Fish-HFDS-fed mice showed significantly reduced food intake and body weight gain compared to Cocoa-HFDS-fed mice. Fish-HFDS group had lower fasting blood glucose concentration and area under the curve (AUC) for both GTT and ITT. Plasma c-peptide, insulin, glucagon, and GLP-1 concentrations were increased in the Fish-HFDS group. Interestingly, mice fed the Fish-HFDS diet displayed higher plasma leptin concentration. Histochemical analysis revealed a significant increase in endocrine pancreas β-cells and islet numbers in mice fed Fish-HFDS compared to the Cocoa-HFDS group. Taken together, these findings suggest that in a high-fat/high-sucrose dietary setting, the source of the fat, especially fish oil, can ameliorate the effect of sucrose on glucose homeostasis and endocrine pancreas morphology and function.

Tissue Changes in the Development of Fatty Liver by Chronic Ingestion of Sucrose Associated with Obesity and Dyslipidemia in Rats

A diet high in sucrose, which is a common food constituent, induces obesity and non- alcoholic fatty liver (NFLD) caused by high caloric intake; however, it is important to investigate those sequential changes in the hepatic parenchyma related to sugar consumption which are associated to obesity and dyslipidemia. We analyzed the effects of long-term sucrose intake on fatty liver development, by the administration of 30% sucrose in drinking water in healthy Wistar rats during 30 weeks. Serum variables, body fat index, caloric intake and microscopic examination of liver tissue were monitored. In the first week, grade 1 steatosis was observed with ballooned hepatocytes, with a caloric intake of 125 ± 1.90 kcal / day / 100 g of body weight; together with a gain of 71% in abdominal fat with respect to the control group and dyslipidemia. During the 10 to 20 weeks period, steatosis grade 2 with noticeable inflammation (steatohepatitis), polymorphic cells and ballooned hepatocytes were evident. After 10 weeks, the caloric intake was 72.9 ± 5.99 kcal / day / 100 g of body weight with 199% of gain in abdominal fat in SUC groups with respect control group (p < 0.01) and moderate dyslipidemia; while after 20 weeks, the caloric intake was 61.6 ± 4.65 kcal / day / 100 g of body weight with 208% of gain in abdominal fat and also moderate dyslipidemia. After 30 weeks steatosis grade 3 with marked inflammation (steatohepatitis), periportal fibrosis, globose and fat-filled hepatocytes were observed, with a caloric intake of 52.3 ± 3.05 kcal / day / 100 g of body weight and 232% of gain in abdominal fat that was related to severe dyslipidemia. In conclusion, the sequential changes in the development of NAFLD were associated with the ingestion of sucrose and obesity since the first week of administration.

Influence of feeding a fish oil-containing diet to mature, overweight dogs: Effects on lipid metabolites, postprandial glycaemia and body weight

The objective of this study was to determine the effect of feeding a fish oil (FO)-containing diet on lipid and protein metabolism, postprandial glycaemia and body weight (BW) of mature, overweight dogs. Seven female dogs were randomly assigned to one of two isonitrogenous and isocaloric diets, control (CO) or FO (FO), in a crossover design. Experimental periods were 69 day, separated by a washout period of 30 day. At the beginning of the experiment, and at 30 and 60 day of feeding the experimental diets, the dogs were infused with D-glucose (2 g/kg BW) through an intravenous catheter. Blood samples were collected for 3 hr to perform a glucose tolerance test. Nitrogen balance measurements began at 06:30 on d 63 of each experimental period and ended at 06:30 on d 69. On d 66 of each period, a single dose (7.5 mg/kg) of ¹⁵ N-glycine was administered orally for determination of protein turnover. Incremental area under the curve and glucose concentration at peak did not differ between treatments or among sampling days within treatment. Glucose half-life tended to decrease (p < .10) in the FO treatment on day 30 when compared to baseline (day 0). β-hydroxybutyrate, non-esterified fatty acid (NEFA) and triglycerides did not differ within or between treatments. Cholesterol decreased (p < .05) on the FO treatment on day 30, 60 and 69 when compared to day 0. High-density lipoprotein (HDL) decreased (p < .05) in the FO treatment on day 69 when compared to day 0. Body weight, food intake, faecal excretion, DM and N digestibilities, N balance and protein turnover were not different between diets. Overall, FO-containing diet decreases cholesterol in mature overweight dogs; however, further research is warranted to verify the effects of FO on glucose metabolism.

Effects of post-suckling n-3 polyunsaturated fatty acids: prevention of dyslipidemia and liver steatosis induced in rats by a sucrose-rich diet during pre- and post-natal life

The interaction between fetal programming and the post-natal environment suggests that the post-natal diet could amplify or attenuate programmed outcomes. We investigated whether dietary n-3 long-chain polyunsaturated fatty acids (n-3 PUFAs) at weaning resulted in an amelioration of dyslipidemia, adiposity and liver steatosis that was induced by a sucrose-rich diet (SRD; where the fat source is corn oil) from the onset of pregnancy up to adulthood. During pregnancy and lactation, dams were fed an SRD or the standard powdered rodent commercial diet (RD). At weaning and until 150 days of life, male offspring from SRD-dams were divided into two groups and fed an SRD or SRD-with-fish oil [where 6% of the corn oil was partially replaced by fish oil (FO) 5% and corn oil (CO) 1%], forming SRD-SRD or SRD-FO groups. Male offspring from RD-dams continued with RD up to the end of the experimental period, forming an RD-RD group. The presence of FO in the weaning diet showed the following: prevention of hypertriglyceridemia and liver steatosis, together with increased lipogenic enzyme activity caused by a maternal SRD; the complete normalization of CPT I activity and PPARα protein mass levels; a slight but not statistically significant accretion of visceral adiposity; and limited body fat content and reduced plasma free fatty acid levels. All of these results were observed even in the presence of a high-sucrose diet challenge after weaning. SRD-dams' breast milk showed a more saturated fatty acid composition. These results suggest the capacity of n-3 PUFAs to overcome some adverse outcomes induced by a maternal and post-weaning sucrose-rich diet.

The Effect of n-3 Fatty Acids on Glucose Homeostasis and Insulin Sensitivity

Type 2 diabetes (T2D) as well as cardiovascular disease (CVD) represent major complications of obesity and associated metabolic disorders (metabolic syndrome). This review focuses on the effects of long-chain n-3 polyunsaturated fatty acids (omega-3) on insulin sensitivity and glucose homeostasis, which are improved by omega-3 in many animal models of metabolic syndrome, but remain frequently unaffected in humans. Here we focus on: (i) mechanistic aspects of omega-3 action, reflecting also our experiments in dietary obese mice; and (ii) recent studies analysing omega-3’s effects in various categories of human subjects. Most animal experiments document beneficial effects of omega-3 on insulin sensitivity and glucose metabolism even under conditions of established obesity and insulin resistance. Besides positive results obtained in both cross-sectional and prospective cohort studies on healthy human populations, also some intervention studies in prediabetic subjects document amelioration of impaired glucose homeostasis by omega-3. However, the use of omega-3 to reduce a risk of new-onset diabetes in prediabetic subjects still remains to be further characterized. The results of a majority of clinical trials performed in T2D patients suggest that omega-3 have none or marginal effects on metabolic control, while effectively reducing hypertriglyceridemia in these patients. Despite most of the recent randomized clinical trials do not support the role of omega-3 in secondary prevention of CVD, this issue remains still controversial. Combined interventions using omega-3 and antidiabetic or hypolipidemic drugs should be further explored and considered for treatment of patients with T2D and other diseases.

Influence of virgin coconut oil-enriched diet on the transcriptional regulation of fatty acid synthesis and oxidation in rats - a comparative study

The present study was carried out to evaluate the effects of virgin coconut oil (VCO) compared with copra oil, olive oil and sunflower-seed oil on the synthesis and oxidation of fatty acids and the molecular regulation of fatty acid metabolism in normal rats. Male Sprague-Dawley rats were fed the test oils at 8 % for 45 d along with a synthetic diet. Dietary supplementation of VCO decreased tissue lipid levels and reduced the activity of the enzymes involved in lipogenesis, namely acyl CoA carboxylase and fatty acid synthase (FAS) (P< 0·05). Moreover, VCO significantly (P< 0·05) reduced the de novo synthesis of fatty acids by down-regulating the mRNA expression of FAS and its transcription factor, sterol regulatory element-binding protein-1c, compared with the other oils. VCO significantly (P< 0·05) increased the mitochondrial and peroxisomal β-oxidation of fatty acids, which was evident from the increased activities of carnitine palmitoyl transferase I, acyl CoA oxidase and the enzymes involved in mitochondrial β-oxidation; this was accomplished by up-regulating the mRNA expression of PPARα and its target genes involved in fatty acid oxidation. In conclusion, the present results confirmed that supplementation of VCO has beneficial effects on lipid parameters by reducing lipogenesis and enhancing the rate of fatty acid catabolism; this effect was mediated at least in part via PPARα-dependent pathways. Thus, dietary VCO reduces the risk for CHD by beneficially modulating the synthesis and degradation of fatty acids.

Dietary Salba (Salvia hispanica L) seed rich in α-linolenic acid improves adipose tissue dysfunction and the altered skeletal muscle glucose and lipid metabolism in dyslipidemic insulin-resistant rats

This work reports the effect of dietary Salba (chia) seed rich in n-3 α-linolenic acid on the morphological and metabolic aspects involved in adipose tissue dysfunction and the mechanisms underlying the impaired glucose and lipid metabolism in the skeletal muscle of rats fed a sucrose-rich diet (SRD). Rats were fed a SRD for 3 months. Thereafter, half the rats continued with SRD while in the other half, corn oil (CO) was replaced by chia seed for 3 months (SRD+chia). In control group, corn starch replaced sucrose. The replacement of CO by chia seed in the SRD reduced adipocyte hypertrophy, cell volume and size distribution, improved lipogenic enzyme activities, lipolysis and the anti-lipolytic action of insulin. In the skeletal muscle lipid storage, glucose phosphorylation and oxidation were normalized. Chia seed reversed the impaired insulin stimulated glycogen synthase activity, glycogen, glucose-6-phosphate and GLUT-4 protein levels as well as insulin resistance and dyslipidemia.

Omega-3 fatty acids reduce adipose tissue macrophages in human subjects with insulin resistance

Fish oils (FOs) have anti-inflammatory effects and lower serum triglycerides. This study examined adipose and muscle inflammatory markers after treatment of humans with FOs and measured the effects of ω-3 fatty acids on adipocytes and macrophages in vitro. Insulin-resistant, nondiabetic subjects were treated with Omega-3-Acid Ethyl Esters (4 g/day) or placebo for 12 weeks. Plasma macrophage chemoattractant protein 1 (MCP-1) levels were reduced by FO, but the levels of other cytokines were unchanged. The adipose (but not muscle) of FO-treated subjects demonstrated a decrease in macrophages, a decrease in MCP-1, and an increase in capillaries, and subjects with the most macrophages demonstrated the greatest response to treatment. Adipose and muscle ω-3 fatty acid content increased after treatment; however, there was no change in insulin sensitivity or adiponectin. In vitro, M1-polarized macrophages expressed high levels of MCP-1. The addition of ω-3 fatty acids reduced MCP-1 expression with no effect on TNF-α. In addition, ω-3 fatty acids suppressed the upregulation of adipocyte MCP-1 that occurred when adipocytes were cocultured with macrophages. Thus, FO reduced adipose macrophages, increased capillaries, and reduced MCP-1 expression in insulin-resistant humans and in macrophages and adipocytes in vitro; however, there was no measureable effect on insulin sensitivity.

Fish oil reverses the altered glucose transporter, phosphorylation, insulin receptor substrate-1 protein level and lipid contents in the skeletal muscle of sucrose-rich diet fed rats

The role and underlying mechanisms by which n-3 polyunsaturated fatty acids (PUFA) prevent/reverse SRD-induced insulin resistance (IR) in the muscle are not completely understood. Therefore, we examined: triglyceride, diacylglycerol, PKCθ, Glut-4, enzymatic hexokinase activity, IRS-1 protein mass level, and fatty acid composition of muscle phospholipids. Rats were fed a SRD during 6 months. Thereafter, half the animals continued with SRD up to 8 months; the other half was fed a SRD in which CO (8% wt/wt) was replaced by FO (7%+1% CO) for 2 months. Results were compared with those obtained in rats fed a control diet (CD). In SRD-fed rats, FO oil normalized/improved lipid storage and PKCθ protein mass level. Effects of insulin were comparable with those of CD-fed rats. FO reversed impaired glucose phosphorylation, IRS-1, and, under insulin stimulation, Glut-4 protein mass level. FO normalized insulin resistance and increased n-3 PUFAs in muscle phospholipids.

Fish oil supplementation for two generations increases insulin sensitivity in rats

We investigated the effect of fish oil supplementation for two consecutive generations on insulin sensitivity in rats. After the nursing period (21 days), female rats from the same prole were divided into two groups: (a) control group and (b) fish oil group. Female rats were supplemented with water (control) or fish oil at 1 g/kg body weight as a single bolus for 3 months. After this period, female rats were mated with male Wistar rats fed on a balanced chow diet (not supplemented). Female rats continued to receive supplementation throughout gestation and lactation periods. The same treatment was performed for the next two generations (G1 and G2). At 75 days of age, male offspring from G1 and G2 generations from both groups were used in the experiments. G1 rats did not present any difference with control rats. However, G2 rats presented reduction in glycemia and lipidemia and improvement in in vivo insulin sensitivity (model assessment of insulin resistance, insulin tolerance test) as well as in vitro insulin sensitivity in soleus muscle (glucose uptake and metabolism). This effect was associated with increased insulin-stimulated p38 MAP kinase phosphorylation and lower n-6/n-3 fatty acid ratio, but not with activation of proteins from insulin signaling (IR, IRS-1 and Akt). Global DNA methylation was decreased in liver but not in soleus muscle. These results suggest that long-term fish oil supplementation improves insulin sensitivity in association with increased insulin-stimulated p38 activation and decreased n-6:n-3 ratio in skeletal muscle and decreased global DNA methylation in liver.

Sucrose-rich feeding during rat pregnancy-lactation and/or after weaning alters glucose and lipid metabolism in adult offspring

1. Adverse fetal and early life environments predispose to the development of metabolic disorders in adulthood. The present study examined whether offspring of normal Wistar dams fed a high-sucrose diet (SRD) developed impaired lipid and glucose homeostasis when fed a control diet (CD) after weaning. In addition, we investigated whether there were more pronounced derangements in lipid and glucose homeostasis when offspring of SRD-fed Wistar were fed an SRD after weaning compared with those in offspring of CD-fed dams weaned on an SRD. 2. During pregnancy and lactation, female rats were fed either an SRD or CD. After weaning, half the male offspring from both groups were fed a CD or SRD, up to 100 days of age (CD-CD, CD-SRD, SRD-SRD and SRD-CD groups). 3. Final bodyweight was similar between all groups, although offspring of SRD-fed dams had lighter bodyweight at birth. Plasma lipid and glucose levels were significantly higher (P < 0.05) without changes in insulin levels in the CD-SRD, SRD-SRD and SRD-CD groups compared with the CD-CD group. Dyslipidaemia in the CD-SRD and SRD-SRD groups resulted from increased secretion of very low-density lipoprotein triacylglycerol, as well as decreased triacylglycerol (TAG) clearance that was associated with increased liver TAG content (P < 0.05) compared with the CD-CD group. The hypertriglyceridaemia observed in the SRD-CD group was mostly associated with decreased TAG clearance. Altered glucose and insulin tolerance were observed when the SRD was fed during any period of life. 4. These data support the hypothesis that early life exposure to SRD is associated with changes in lipid and glucose metabolism, leading to an unfavourable profile in adulthood, regardless of whether offspring consumed an SRD after weaning.

Fish oil normalizes plasma glucose levels and improves liver carbohydrate metabolism in rats fed a sucrose-rich diet

A sucrose-rich diet (SRD) induces insulin resistance and dyslipidemia with impaired hepatic glucose production and gluconeogenesis, accompanied by altered post-receptor insulin signaling steps. The aim of this study was to examine the effectiveness of fish oil (FO) to reverse or improve the impaired hepatic glucose metabolism once installed in rats fed 8 months a SRD. In the liver of rats fed SRD in which FO replaced corn-oil during the last 2 months, as dietary fat, several key enzyme activities and metabolites involved in glucose metabolisms (phosphorylation, glycolysis, gluconeogenesis and oxidative and non oxidative glucose pathway) were measured. The protein mass levels of IRS-1 and αp85 PI-3K at basal conditions were also analyzed. FO improved the altered activities of some enzymes involved in the glycolytic and oxidative pathways observed in the liver of SRD fed rats but was unable to restore the impaired capacity of glucose phosphorylation. Moreover, FO reversed the increase in PEPCK and G-6-Pase and reduced the G-6-Pase/GK ratio. Glycogen concentration and GSa activity returned to levels similar to those observed in the liver of the control-fed rats. Besides, FO did not modify the altered protein mass levels of IRS-1 and αp85 PI-3K. Finally, dietary FO was effective in reversing or improving the impaired activities of several key enzymes of hepatic carbohydrate metabolism contributing, at least in part, to the normalization of plasma glucose levels in the SRD-fed rats. However, these positive effects of FO were not observed under basal conditions in the early steps of insulin signaling transduction.

Lipídeos séricos e morfologia hepática de ratos alimentados com diferentes fontes lipídicas (óleo de soja, gordura de peixe e porco, margarina e manteiga)

OBJETIVO: Analisar as alterações lipídicas séricas e morfológicas hepáticas de ratos alimentados com diferentes fontes lipídicas (óleo de soja, gordura de peixe e porco, margarina e manteiga). MÉTODOS: Os 50 ratos Wistars utilizados no estudo foram divididos em cinco grupos, que durante 28 dias receberam dietas semissintéticas com diferentes fontes lipídicas: óleo de soja, gordura de porco, manteiga, margarina e gordura de peixe. Foram avaliados os pesos corporais, o consumo alimentar e o coeficiente de eficiência alimentar; a atividade da lipase lipoproteica; as concentrações séricas de colesterol total e de lipoproteína de alta densidade - colesterol, triacilgliceróis e albumina, bem como a histologia dos tecidos hepático e cardíaco. RESULTADOS: O tipo de fonte lipídica não influenciou o consumo alimentar, o ganho de peso, o coeficiente de eficiência alimentar dos animais, nem a atividade da lipase lipoproteica, porém promoveu alterações nas concentrações séricas de colesterol total, lipoproteína de alta densidade - colesterol, triacilgliceróis e albumina. Todos os grupos apresentaram gotículas lipídicas nas paredes coronarianas e nos capilares cardíacos, sendo caracterizada como esteatose a deposição de gordura no fígado dos animais que receberam óleo de soja, gordura de peixe, de porco e manteiga. CONCLUSÃO: A fonte lipídica que apresentou os melhores resultados foi o óleo de soja, enquanto o consumo de gordura do peixe causou efeito sérico e tecidual semelhante ao encontrado para as outras fontes lipídicas (gordura de porco, manteiga e margarina), as quais podem contribuir para o surgimento e a progressão das doenças cardiovasculares.

Soya protein reverses dyslipidaemia and the altered capacity of insulin-stimulated glucose utilization in the skeletal muscle of sucrose-rich diet-fed rats

The present study investigates the benefits of dietary intake of soya protein upon dyslipidaemia and insulin resistance in rats chronically (8 months) fed a sucrose-rich (63 %) diet (SRD). For this purpose, we analysed the effectiveness of soya protein isolate in improving or reversing these metabolic abnormalities. Wistar rats were fed a SRD for 4 months. By the end of this period, stable dyslipidaemia and insulin resistance were present in the animals. From months 4 to 8, half the animals continued with the SRD and the other half were fed a SRD in which the source of protein casein was substituted by soya. The control group received a diet in which the source of carbohydrate was maize starch. The results showed that: (1) soya protein normalized plasma TAG, cholesterol and NEFA levels in the SRD-fed rats. Moreover, the addition of soya protein reversed the hepatic steatosis. (2) Glucose homeostasis was normalized without changes in circulating insulin levels. Whole-body peripheral insulin sensitivity substantially improved. Besides, soya protein moderately decreases body weight gain limiting the accretion of visceral fat. (3) By shifting the source of dietary protein from casein to soya during the last 4 months of the feeding period it was possible to reverse both the diminished insulin-stimulated glucose oxidation and disposal in the skeletal muscle of SRD-fed rats. This study provides new data showing the beneficial effect of soya protein upon lipid and glucose homeostasis in the experimental model of dyslipidaemia and insulin resistance.

Dietary fish oil reverses lipotoxicity, altered glucose metabolism, and nPKCepsilon translocation in the heart of dyslipemic insulin-resistant rats

The present study analyzes several markers of energy metabolism in the heart muscle of dyslipemic insulin-resistant rats fed a sucrose-rich diet (SRD, 62.5% wt/wt) for 8 months. It also explores the possible beneficial effects of dietary fish oil supplementation on cardiac lipids and glucose metabolism. With this purpose, male Wistar rats were fed an SRD for 6 months. Whereas half of the animals continued with the same diet for up to 8 months, the other half was fed an SRD in which fish oil (7% + 1% corn oil wt/wt) replaced corn oil (8% wt/wt) from months 6 to 8. The results were compared with rats fed a control diet (starch 62.5% wt/wt). The cardiac muscle of SRD-fed rats showed (1) a significant reduction (P < .05) in key enzymes activities and metabolites involved in glucose metabolism, accompanied by a significant (P < .05) increase of lipid storage (triglyceride, long-chain acyl coenzyme A, and diacylglycerol), and (2) a significant increase (P < .05) of nPKCepsilon protein mass expression in the membrane fraction without changes in the cPKCbetaII. Dietary fish oil, which reduces the availability of plasma lipid flux and normalizes glucose homeostasis, was able to reverse heart muscle lipotoxicity. Fish oil benefits key enzymes activities in glucose metabolism and normalizes glycogen and glucose-6-phosphate concentration, and the altered nPKCepsilon protein mass expression translocation in the heart of SRD-fed rats. Our findings suggest that manipulation of dietary fats may play a key role in the management of lipid disorders, offering a protection against the development of cardiovascular diseases.

Dietary chia seed (Salvia hispanica L.) rich in alpha-linolenic acid improves adiposity and normalises hypertriacylglycerolaemia and insulin resistance in dyslipaemic rats

The present study investigates the benefits of the dietary intake of chia seed (Salvia hispanica L.) rich in alpha-linolenic acid and fibre upon dyslipidaemia and insulin resistance (IR), induced by intake of a sucrose-rich (62.5 %) diet (SRD). To achieve these goals two sets of experiments were designed: (i) to study the prevention of onset of dyslipidaemia and IR in Wistar rats fed during 3 weeks with a SRD in which chia seed was the dietary source of fat; (ii) to analyse the effectiveness of chia seed in improving or reversing the metabolic abnormalities described above. Rats were fed a SRD during 3 months; by the end of this period, stable dyslipidaemia and IR were present in the animals. From months 3-5, half the animals continued with the SRD and the other half were fed a SRD in which the source of fat was substituted by chia seed (SRD+chia). The control group received a diet in which sucrose was replaced by maize starch. The results showed that: (i) dietary chia seed prevented the onset of dyslipidaemia and IR in the rats fed the SRD for 3 weeks--glycaemia did not change; (ii) dyslipidaemia and IR in the long-term SRD-fed rats were normalised without changes in insulinaemia when chia seed provided the dietary fat during the last 2 months of the feeding period. Dietary chia seed reduced the visceral adiposity present in the SRD rats. The present study provides new data regarding the beneficial effect of chia seed upon lipid and glucose homeostasis in an experimental model of dislipidaemia and IR.

The reduction of dietary sucrose improves dyslipidemia, adiposity, and insulin secretion in an insulin-resistant rat model

OBJECTIVE

The purpose of the present work was to investigate whether changes in the type of carbohydrate in the diet are able to improve and/or reverse hyperlipemia, impaired glucose homeostasis, and insulin secretion from beta-cells induced in rats by chronically feeding a high sucrose intake.

METHODS

For 30 wk male Wistar rats received a sucrose-rich diet (63% w/w) or a control diet in which sucrose was replaced by starch. After this period, the sucrose-fed animals were randomly divided into two groups: the first group continued with this diet up to 42 wk and the other received the same diet but with a 20% reduction in the amount of sucrose and the rest of the carbohydrate being replaced by starch. Rats were fed with this diet for the next 12 wk.

RESULTS

The reduction of the amount of sucrose in the diet showed a substantial improvement (P < 0.05) of dyslipidemia associated with an amelioration of "in vivo" very low-density lipoprotein-triacylglycerol secretion and triacylglycerol removal rate from the circulation. Glucose homeostasis and glucose-induced insulin release from beta-cells were improved (P < 0.05), although these values did not reach those observed in rats fed a control diet. Visceral adiposity was also significantly reduced (P < 0.05).

CONCLUSION

These data are consistent with the suggestion that the composition of the diet could contribute to improvements in dyslipidemia, insulin resistance, and adiposity by direct effects on the lipid metabolism and insulin action and indirectly through the reduction of visceral fat mass and distribution.

Metabolic Syndrome: Effects of n-3 PUFAs on a Model of Dyslipidemia, Insulin Resistance and Adiposity

Both genetic and environmental factors (e.g. nutrition, life style) contribute to the development of the plurimetabolic syndrome, which has a high prevalence in the world population. Dietary n-3 PUFAs specially those from marine oil (EPA and DHA) appear to play an important role against the adverse effects of this syndrome. The present work examined the effectiveness of fish oil (FO) in reversing or improving the dyslipidemia, insulin resistance and adiposity induced in rats by long-term feeding a sucrose-rich diet (SRD). We studied several metabolic and molecular mechanisms involved in both lipid and glucose metabolisms in different tissues (liver, skeletal muscle, fat pad) as well as insulin secretion patterns from perifused islets under the stimulation of different secretagogues. Dietary FO reverses dyslipidemia and improves insulin action and adiposity in the SRD fed rats. FO reduces adipocytes cell size and thus, the smaller adipocytes are more insulin sensitive and the release of fatty acids decreases. In muscle, FO normalizes both the oxidative and non-oxidative glucose pathways. Moreover, FO modifies the fatty acid composition of membrane phospholipids. In isolated beta cells, lipid contents and glucose oxidation return to normal. All these effects could contribute to the normalization of glucose-stimulated insulin secretion and muscle insulin insensitivity.

Effects of dietary polyunsaturated n-3 fatty acids on dyslipidemia and insulin resistance in rodents and humans. A review

For many years, clinical and animal studies on polyunsaturated n-3 fatty acids (PUFAs), especially those from marine oil, eicosapentaenoic acid (20:5,n-3) and docosahexaenoic acid (22:6,n-3), have reported the impact of their beneficial effects on both health and diseases. Among other things, they regulate lipid levels, cardiovascular and immune functions as well as insulin action. Polyunsaturated fatty acids are vital components of the phospholipids of membrane cells and serve as important mediators of the nuclear events governing the specific gene expression involved in lipid and glucose metabolism and adipogenesis. Besides, dietary n-3 PUFAs seem to play an important protecting role against the adverse symptoms of the Plurimetabolic syndrome. This review highlights some recent advances in the understanding of metabolic and molecular mechanisms concerning the effect of dietary PUFAs (fish oil) and focuses on the prevention and/or improvement of dyslipidemia, insulin resistance, impaired glucose homeostasis, diabetes and obesity in experimental animal models, with some extension to humans.

Fish Oil Affects Pancreatic Fat Storage, Pyruvate Dehydrogenase Complex Activity and Insulin Secretion in Rats Fed a Sucrose-Rich Diet

Rats fed a sucrose-rich diet (SRD) develop hypertriglyceridemia and a marked decline in beta cell function. The purpose of this study was to determine whether changes in triglyceride concentration and/or altered pyruvate dehydrogenase complex (PDHc) activity contribute to the beta cell dysfunction, and to analyze the effect of dietary fish oil on the altered patterns of insulin secretion and peripheral insulin resistance. Rats were fed an SRD for 210 d. One-half of the rats continued consuming the SRD until d 270. The other half received an SRD in which fish oil (FO) was partially substituted for corn oil until d 270. A group of rats was fed a control diet (CD) throughout the experiment. The islets of rats fed the SRD had a greater triglyceride concentration and lower PDHc activity than those fed the CD. Insulin secretion patterns under the stimulus of glucose, palmitate or L-arginine were impaired in SRD-fed compared with CD-fed rats. This was accompanied by peripheral insulin resistance, mild hyperglycemia, a sharp increase of plasma triglyceride and free fatty acid levels and greater epididymal and retroperitoneal fat weights. FO normalized and/or improved these variables. Our results indicate that the increased fat storage and decreased PDHc activity in the beta cells play a key role in the abnormal insulin secretion of rats chronically fed an SRD. This is consistent with the reversion of these alterations by dietary FO.

Muscle lipid metabolism and insulin secretion are altered in insulin-resistant rats fed a high sucrose diet

Feeding rats a sucrose rich diet (SRD) induces hypertriglyceridemia and insulin resistance. The purposes of this study were to determine the time course of changes in lipid and glucose metabolism in the gastrocnemius muscle, both in the basal state and after the euglycemic hyperinsulinemic clamp, in rats fed a SRD for 3, 15 or 30 wk, and to analyze the changes in glucose-stimulated insulin secretion from perifused isolated islets from SRD-fed rats and their relationships to peripheral insulin insensitivity. A control group of rats was fed a control diet (CD) for the same period of time. After 3 wk of consuming the SRD, long-chain acyl CoA (LCACoA) levels in muscle were greater than in rats fed the CD, an early indication of the disturbance of lipid metabolism. Neither glycogen storage nor glucose oxidation were impaired at this time. Moreover, the biphasic patterns of glucose-stimulated insulin secretion showed a marked increase in the first peak, which helped maintain normoglycemia in SRD-fed rats. After 15 or 30 wk of consuming the SRD, triglyceride and LCACoA levels in muscles were greater than in rats fed the CD. Glucose oxidation as well as insulin-stimulated glycogen synthase activity and glycogen storage were lower than in rats fed the CD. Moreover, the altered pattern of insulin secretion further deteriorated. This was accompanied by peripheral insulin resistance and moderate hyperglycemia. Our results indicate that the dyslipemia present in rats chronically fed a SRD may play an important role in the progressive deterioration of insulin secretion and sensitivity in this animal model.

Comparison of the effects of sucrose and fructose on insulin action and glucose tolerance

The purpose of the present study was to determine whether fructose is the nutrient mediator of sucrose-induced insulin resistance and glucose intolerance. Toward this end, male rats were fed a purified starch diet (68% of total calories) for a 2-wk baseline period. After this, rats either remained on the starch (ST) diet or were switched to a sucrose (SU, 68% of total calories), fructose/glucose (F/G, 34/34% of total calories), or fructose/starch (F/ST, 34/34% of total calories) diet for 5 wk. Rats then underwent either an intravenous glucose tolerance test (n = 10/diet) or a euglycemic, hyperinsulinemic clamp (n = 8 or 9/diet). Incremental glucose and insulin areas under the curve in SU, F/G, and F/ST were on average 61 and 29% greater than ST, respectively, but not significantly different from one another. During clamps, glucose infusion rates (mg. kg(-1). min(-1)) required to maintain euglycemia were significantly lower (P < 0.05) in SU, F/G, and F/ST (13.4 +/- 0.9, 9. 5 +/- 1.7, 11.3 +/- 1.3, respectively) compared with ST (22.8 +/- 1. 1). Insulin suppression of glucose appearance (mg. kg(-1). min(-1)) was significantly lower (P < 0.05) in SU, F/G, and F/ST (5.6 +/- 0.5, 2.2 +/- 1.2, and 6.6 +/- 0.7, respectively) compared with ST (9.6 +/- 0.4). Insulin-stimulated glucose disappearance (mg. kg(-1). min(-1)) was significantly lower (P < 0.05) in SU, F/G, and F/ST (17. 9 +/- 0.6, 16.2 +/- 1.3, 15.3 +/- 1.8, respectively) compared with ST (24.7 +/- 1.2). These data suggest that fructose is the primary nutrient mediator of sucrose-induced insulin resistance and glucose intolerance.

Role of skeletal muscle on impaired insulin sensitivity in rats fed a sucrose-rich diet: effect of moderate levels of dietary fish oil

In the present study we investigated: (1) the contribution of the skeletal muscle to the mechanisms underlying the impaired glucose homeostasis and insulin sensitivity present in dyslipemic rats fed a sucrose-rich diet (SRD) over a long period of time and (2) the effect of fish oil on these parameters when there was a stable hypertriglyceridemia before the source of fat (corn oil) in the diet was replaced by isocaloric amounts of cod liver oil. Our results show an increased triglyceride content in the gastrocnemius muscle with an impaired capacity for glucose oxidation in the basal state and during euglycemic clamp. This was mainly due to a decrease of the active form of pyruvate dehydrogenase complex (PDHa) and an increase of PDH kinase activities. Hyperglycemia, normoinsulinemia, and diminished peripheral insulin sensitivity also were found. Even though there were no changes in the insulin levels, the former metabolic abnormalities were completely reversed when the source of fat was changed from corn oil to cod liver oil. The data also suggest that in the gastrocnemius muscle of rats fed a SRD over an extended period, an increased availability and oxidation of the lipid fuel, which in turn impairs the glucose oxidation, contributes to the abnormal glucose homeostasis and to the peripheral insulin insensitivity. Moreover, the parallel effect on insulin sensitivity, glucose, and lipid homeostasis attained through the manipulation of dietary fat (n-3) in the SRD suggests a role of n-3 fatty acid in the management of dyslipidemia and insulin resistance.

Diet composition and insulin action in animal models

Critical insights into the etiology of insulin resistance have been gained by the use of animal models where insulin action has been modulated by strictly controlled dietary interventions not possible in human studies. Overall, the literature has moved from a focus on macronutrient proportions to understanding the unique effects of individual subtypes of fats, carbohydrates and proteins. Substantial evidence has now accumulated for a major role of dietary fat subtypes in insulin action. Intake of saturated fats is strongly linked to development of obesity and insulin resistance, while that of polyunsaturated fats (PUFAs) is not. This is consistent with observations that saturated fats are poorly oxidized for energy and thus readily stored, are poorly mobilized by lipolytic stimuli, impair membrane function, and increase the expression of genes associated with adipocyte profileration (making their own home). PUFAs have contrasting effects in each instance. It is therefore not surprising that increased PUFA intake in animal models is associated with improved insulin action and reduced adiposity. Less information is available for carbohydrate subtypes. Early work clearly demonstrated that diets high in simple sugars (in particular fructose) led to insulin resistance. However, again attention has rightly shifted to the very interesting issue of subtypes of complex carbohydrates. While no differences in insulin action have yet been shown, differences in substrate flux suggest there could be long-term beneficial effects on the fat balance of diets enhanced in slowly digested/resistant starches. A new area of major interest is in protein subtypes. Recent results have shown that rats fed high-fat diets where the protein component was from casein or soy were insulin-resistant, but when the protein source was from cod they were not. These are exciting times in our growing understanding of dietary factors and insulin action. While it has been clear for some time that 'oils ain't oils', the same is now proving true for carbohydrates and proteins.

An overview of Argentine contributions to diabetes research in the decade of the 1990s

Argentina has a longstanding tradition of diabetes research, beginning with the seminal work of Prof. Bernardo A. Houssay, who was awarded the first Nobel Prize in Medical Sciences for his studies on the relationship between diabetes and pituitary function. Prof. Luis F. Leloir, who was also awarded the Nobel Prize for his work in carbohydrate metabolism, also inspired younger generations of biologists to work in the field of diabetes research. The aim of this paper is to provide a review of the contributions of Argentine researchers during the 1990s. This manuscript includes only reports of Argentine researchers working on diabetes in local laboratories and quoted in Medline. Thus, important contributions not reported in journals included in Medline or produced by Argentine researchers working abroad may have been omitted. The material consists of a brief description of clinical research (epidemiology and costs, metabolic control, associated risk factors, immunological aspects, and other clinical studies) and basic research (animal model with spontaneous diabetes, islet morphology and function in normal and pathological conditions, insulin action, metabolic disorders related to diabetes, and some miscellaneous effects related to drug-induced diabetes). Altogether, a broad idea of the continuous contribution of our national research to the international field of diabetes is provided, as well as a list of Argentine researchers and research centers devoted to the study of diabetes.

Insulin-induced endothelin release and vasoreactivity in hypertriglyceridemic and hypertensive rats

Insulin-elicited endothelin release in hypertriglyceridemic, hypertensive, hyperinsulinemic (HTG) rats was shown. Weanling male Wistar rats were given 30% sucrose in their drinking water for 20-24 wk. In vitro contractions of aorta and femoral arteries were elicited with 40 mM KCl. Endothelin release induced with KCl plus 50 microU/ml insulin resulted in increases in contractile responses: 41 +/- 5.9 and 57 +/- 6% for control and 65.5 +/- 6 and 95 +/- 9% for HTG aortas and femoral arteries, respectively. The endothelin ET(B)-receptor blocker BQ-788 decreased responses to KCl + insulin by 39 +/- 8 and 53 +/- 5% in control and 48 +/- 13 and 79 +/- 3.5% in HTG aortas and femoral arteries, respectively. The ET(A)-receptor antagonist PD-151242 inhibited these responses by 12 +/- 10 and 1 +/- 9% in control and by 51.5 +/- 9 and 58.5 +/- 1% in HTG aortas and femoral arteries, respectively. These results suggest that endothelin may contribute to the hypertension in this model.

Chronic physiologic hyperinsulinemia impairs suppression of plasma free fatty acids and increases de novo lipogenesis but does not cause dyslipidemia in conscious normal rats

Type 2 diabetes mellitus and obesity are characterized by fasting hyperinsulinemia, insulin resistance with respect to glucose metabolism, elevated plasma free fatty acid (FFA) levels, hypertriglyceridemia, and decreased high-density lipoprotein (HDL) cholesterol. An association between hyperinsulinemia and dyslipidemia has been suggested, but the causality of the relationship remains uncertain. Therefore, we infused eight 12-week-old male catheterized conscious normal rats with insulin (1 mU/min) for 7 days while maintaining euglycemia using a modification of the glucose clamp technique. Control rats (n = 8) received vehicle infusion. Baseline FFAs were 1.07+/-0.13 mmol/L, decreased to 0.57+/-0.10 (P < .05) upon initiation of the insulin infusion, and gradually increased to 0.95+/-0.12 by day 7 (P = NS vbaseline). On day 7 after a 6-hour fast, plasma insulin, glucose, and FFA levels in control and chronically hyperinsulinemic rats were 32+/-5 versus 116+/-21 mU/L (P < .005), 122+/-4 versus 129+/-8 mg/dL (P = NS), and 1.13+/-0.18 versus 0.95+/-0.12 mmol/L (P = NS); total plasma triglyceride and cholesterol levels were 78+/-7 versus 66+/-9 mg/dL (P = NS) and 50+/-3 versus 47+/-2 mg/dL (P = NS), respectively. Very-low-density lipoprotein (VLDL) + intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and HDL2 and HDL3 subfractions of plasma triglyceride and cholesterol were similar in control and hyperinsulinemic rats. Plasma FFA correlated positively with total (r = .61, P < .005) triglycerides. On day 7 after an 8-hour fast, hyperinsulinemic-euglycemic clamps with 3-3H-glucose infusion were performed in all rats. Chronically hyperinsulinemic rats showed peripheral insulin resistance (glucose uptake, 15.8+/-0.8 v 19.3+/-1.4 mg/kg x min, P < .02) but normal suppression of hepatic glucose production (HGP) compared with control rats (4.3+/-1.0 v 5.6+/-1.4 mg/kg x min, P = NS). De novo tissue lipogenesis (3-3H-glucose incorporation into lipids) was increased in chronically hyperinsulinemic versus control rats (0.90+/-0.10 v 0.44+/-0.08 mg/kg x min, P < .005). In conclusion, chronic physiologic hyperinsulinemia (1) causes insulin resistance with regard to the suppression of plasma FFA levels and increases lipogenesis; (2) induces peripheral but not hepatic insulin resistance with respect to glucose metabolism; and (3) does not cause an elevation in VLDL-triglyceride or a reduction in HDL-cholesterol.

Menhaden oil prevents but does not reverse sucrose-induced insulin resistance in rats

Although fish oil supplementation may prevent the onset of diet-induced insulin resistance in rats, it appears to worsen glycemic control in humans with existing insulin resistance. In the present study, the euglycemic, hyperinsulinemic (4x basal) clamp technique with [3-3H]glucose and 2-deoxy-[1-14C]glucose was used to directly compare the ability of fish oil to prevent and reverse sucrose-induced insulin resistance. In study 1 (prevention study), male Wistar rats were fed a purified high-starch diet (68% of total energy), high-sucrose diet (68% of total energy), or high-sucrose diet in which 6% of the fat content was replaced by menhaden oil for 5 wk. In study 2 (reversal study), animals were fed the high-starch or high-sucrose diets for 5 wk and then the sucrose animals were assigned to one of the following groups for an additional 5 wk: high starch, high sucrose, or high sucrose with 6% menhaden oil. Rats fed the high-starch diet for 10 wk served as controls. In study 3 (2nd reversal study), animals followed a similar diet protocol as in study 2; however, the reversal period was extended to 15 wk. In study 1, the presence of the fish oil in the high-sucrose diet prevented the development of insulin resistance. Glucose infusion rates (GIR, mg.kg-1.min-1) were 17.0 +/- 0.9 in starch, 10.6 +/- 1.7 in sucrose, and 15.1 +/- 1.5 in sucrose with fish oil animals. However, in study 2, this same diet was unable to reverse sucrose-induced insulin resistance (GIR, 16.7 +/- 1.4 in starch, 7.1 +/- 1.5 in sucrose, and 4.8 +/- 0.9 in sucrose with fish oil animals). Sucrose-induced insulin resistance was reversed in rats that were switched back to the starch diet (GIR, 18.6 +/- 3.0). Results from study 3 were similar to those observed in study 2. In summary, fish oil was effective in preventing diet-induced insulin resistance but not able to reverse it. A preexisting insulin-resistant environment interferes with the positive effects of menhaden oil on insulin action.