Effects Of Dietary Fat Acids On Insulin Sensitivity In Healthy Adults

Type 2 diabetes is rapidly emerging as one of the greatest global health challenges of the 21st century. The World Health Organization estimates that by the year 2030, 366 million people will be afflicted with diabetes. This looming epidemic is also expected to trigger a steep rise in the complications associated with diabetes. Developing better treatments and novel prevention strategies for type 2 diabetes is therefore a matter of great urgency. It is well known that insulin resistance plays a critical role in the development of type 2 diabetes.High-fat diets have been shown to produce insulin resistance,animal experiments also prove that high-fat exposure on impaires insulin sensitivity in several insulin target organs, including skeletal muscles, liver, adipocytes etc. Certain fatty acids may have a more deleterious effect on insulin action than others. In animal models, high intake of saturated and polyunsaturated fats induce severe insulin resistance, whereas monounsaturated fats and n-3 fatty acids are less detrimental. However, the relationship between dietary fat acids and insulin sensitivity is not clear, and there are few studies in this field.With the development of molecular biological technology, many specialists discovered the defects in the insulin signaling pathway, abnormal secretion of adipokines and oxidative stress all contributed to the development of insulin resistance. More and more people realized that adipose tissue was an endocrine organ. Many cytokines secreted by adipocyte such as leptin, adiponectin, tumor necrosis factor alpha (TNF-α), interleukin 6(IL-6), plasminogen activator Inhibitor(PAI-1), resistin, visfatin, and retinol binding protein 4(RBP-4) are all related with the insulin resistant state. The present studies demonstrated that these adipokines performed different actions in IR; adiponectin can improve the insulin sensitivity, while the increase of other cytokines can impair the insulin sensitivity. Oxidative stress is caused by an imbalance between the production of reactive oxygen and a biological system's ability to readily detoxify the reactive intermediates or easily repair the resulting damage. It is uncertain that ROS indirectly induce damage to tissues by activating a number of cellular stress-sensitive pathways. These pathways include nuclear factor-kappaB(NF-κB), p38 mitogen-activated protein kinase(MAPK), NH2-terminal Jun kinases/stress-activated protein kinases(JNK), hexosamines, and others.Endoplasmic reticulum(ER) is a membranous network that functions in the synthesis and processing of secretory and membrane proteins. Certain pathological stress conditions disrupt ER homeostasis and lead to accumulation of unfolded or misfolded proteins in the ER lumen. A recent study has uncovered a link between insulin resistance and endoplasmic reticulum (ER) stress in liver and adipose cells. Therefore, some learners postulated that ER stress is probably a core mechanism involved in triggering insulin resistance and type 2 diabetes. Induction of ER stress leads to hyperactivation of JNK, reduced insulin receptor signaling, systemic insulin resistance and type 2 diabetes. In vivo, palmitate but not oleate induces significant ER stress, but the precise mechanism needs to be further studied.In the present study, we explore influence of dietary fat acids on insulin sensitivity in healthy adults. Given the link between hepatic cell ER stress and insulin resistance, we examine the effects of chronic FFA treatment on ER stress to provide a new study direction to improve insulin resistance and type 2 diabetes. The paper contains four parts below:Part one: Effects of dietary fat acids on insulin sensitivity in healthy adultsObjective: To investigate the effects of saturated fats diets, polyunsaturated fats diets and monounsaturated fats diets on insulin sensitivity in healthy adults.Methods: 20 subjects were recruited from Hebei General Hospital, all subjects were healthy. Each of the three experimental diets was fed for 3 days with a minimum 1-week washout between diets. All subjects completed each of the diets in order. At the end of each 3-day diet period, subjects returned to the clinic for measurement of serum glucose, lipids and insulin sensitivity. The target nutrient composition for all three diets was 30% carbohydrate, 50% fat, 20% protein. Monounsaturated fat acid diet (M) was provided by tea oil, polyunsaturated fat acid diet (P) was provided by soybean oil, Saturated fat acid diet (S) was provided by lard oil.Results:1. Fasting blood glucose(FBG) in S group adults were higher than that in isocaloric M and P group. Compared with S group, serum total cholesterol(TC), Low-density lipoprotein(LDL) and apolipoprotein B(apoB) were significantly lower in M and P group,but triglyceride(TG),high-density lipoprotein(HDL), very low-density lipoprotein(VLDL) and apolipoprotein A(apoA) in both groups didn't increase significantly (p>0.05). 2.Compared with M group, Fasting insulin(FINS), Homeostasis Model Assessment insulin resistance index (HOMA-IR), (AUCins) and whole body insulin sensitivity index(QUCKI) were higher in P and S group(p<0.01). There was no detectable difference in AUCglu,HOMA-B,△Ins/△FPG among three groups (p>0.05).Conclusion: 1.Short-term MUFA-rich and PUFA-rich diets decrease fasting blood glucose and TC, LDL and apoB, which demonstrated that UFA diets may have anti-atherosclerosis effects. 2. MUFA-rich diet improves insulin sensitivity, which probably fits for patients at high risk of diabetes or diabetics.Part two: Effects of dietary fat acids on oxidative stress in healthy adultsObjective: To observe the effects of saturated fats diets, polyunsaturated fats diets and monounsaturated fats diets on oxidative stress in healthy adults.Methods: Subjects grouping and the samples acquirement were the same as those in part one. Fasting serum malondialdehyde (MDA),glutathione peroxidase (GSH-PX), and superoxide dismutase (SOD) were measured by Colorimetric method. 8-iso-PG-F2αwas measured by enzyme-linked immunosorbent assay( ELISA). Results : 1.Compared with M group, MDA and 8-iso-PG-F2αwere significantly higher in S and P group(p<0.01). 2.Compared with M group, SOD and GSH-PX were lower in P and S group(p<0.01), but there was no detectable difference in SOD and GSH-PX between P and S groups (p>0.05). 3. All four markers were significantly associated with HOMA-IR, and 8-iso-PG-F2αwas correlated to HOMA-IR independently.Conclusion: The present study indicates that ingestion of PUFA and SFA diets could result in an imbalance between oxidant and antioxidant, and thus induce oxidative stress in healthy adults, but MUFA diets can reduce oxidative stress.Part three: Effects of dietary fat acids on adipocytokines in healthy adultsObjective: To explore the effects of saturated fats diets, polyunsaturated fats diets and monounsaturated fats diets on adipocytokines in healthy adults.Methods: Subjects grouping and the samples acquirement were the same as those in part one. Samples were analyzed for adiponectin (APN), TNF-α, visfatin (VIS), high-sensitivity C-RP (C-RP), FFA and retinol binding protein 4 (RBP-4). Adiponectin, TNF-α, visfatin, hsC-RP and RBP-4 were analyzed by enzyme-linked immunosorbent assay(ELISA). Free fatty acids (FFA) was measured by Colorimetric method .Results:1. Compared with M group, the levels of RBP4 were significantly higher in S and P groups, and VIS and APN were significantly lower in S and P groups(p<0.05). 2. Compared with S group, TNF-αand FFA were lower in P and M groups(p<0.01), but there was no detectable difference in TNF-αand FFA between P and M groups (p>0.05). No significantly difference in CRP was observed among three groups (p>0.05). 3. Serum TNF-αand FFA were positvely correlated with HOMA-IR. Serum VIS and APN was negatively correlated with HOMA-IR. Multiple regression analysis showed that visfatin was independent related factors influencing HOMA-IR.Conclusion: Fasting serum CRP was not detectably affected by alterations in dietary fatty acid profile in healthy adults. MUFA diets improves insulin sensitivity by decreasing RBP4,TNF-αand increasing VIS,APN. Part four: Effects of dietary fat acids on eIF2αand XBP-1 in liver tissues in ratsObjective: To explore the effects of saturated fats diets, polyunsaturated fats diets and monounsaturated fats diets on eukaryotic initiation factor 2alpha (eIF2α) and X-box binding protein 1(XBP-1) in liver tissues in rats.Methods: Forty-eight male Wistar rats were randomly divided into normal control (N) group (n=12), saturated fatty acid diets group(S)(n=12), monounsaturated fatty acid diets group(M) (n=12),and polyunsaturated fatty acid diets group(P) (n=12). The rats in control group were fed with a regular low fatty acids diet containing 10.3% fat, 24.2% protein, and 65.5% carbohydrate as percentage of total calories. The rats in M, P and S group were fed regular diets mixed with 15% tea oil, soybean oil, lard oil respectively, containing 35.4% fat, 17.4% protein and 47.2% carbohydrate as percentage of total calories. The rats in every group were fed equal calories every day. The body weights were determined weekly for 8 weeks. The blood sample was collected by cardiac puncture after rats were anesthetized with diethyl ether for the biochemical analysis, insulin resistance was evaluated by glucose infusion rate (GIR) of hyperinsulinemic euglycemic clamp technique. At the end of 8 week, the rats were killed after anesthetized with phenobarbital sodium after hyperinsulinemic euglycemic clamp test, and liver tissues were taken out freeze-clamped with copper clamps precooled in liquid N2 and were stored in -70℃refrigerator. Liver XBP-1 was measured by Western-blot method. The SP immunohischemistry methods were used to detect the distribution changes of p-eIF2αand XBP-1 in the rats liver.Results:1. Compared with N group, the amount of food intake were significantly lower in S, M and P groups(p<0.05). Adjusted for the amount of food intake, Compared with S group, serum TC, TG were significantly lower in N, M and P groups(p<0.05), but there was no detectable difference in those among N, P and M groups (p>0.05). Compared with N and M groups, FINS and FBG were significantly higher in S and P groups (p<0.05) , GIR was significantly lower in S and P groups (p<0.05). Compared with N group, GIR were significantly lower in M group (p<0.05). 2.The change of liver ultrastructure in P and S groups haved rough endoplasmic reticulum enlargement, taking off grain. The change in M group were nearly the same as those in N group. 3.Immunohistochemistry analysis results showed that cells positive of p-eIF2αwere intense brown staining and localized mainly in cytoblast,cells positive of XBP-1 were intense brown staining and localized both in cytoblast and cytoplasm. Further, western blot for XBP-1 in liver tissues showed that XBP-1 protein levels increased in P and S groups compared with N and M groups(P<0.05).Conclusions: 1.Long-term high fat diet can lead to insulin resistance,and different kinds of acids may have different impact on insulin sensitivity; diets rich in saturated and polyunsaturated,induce more insulin resistance than other kinds of diets. 2.It may be one of the mechanisms of insulin resistance that PUFA and SFA diets can induce higher expression of XBP-1 and p-eIF2αin liver tissues, simultaneously, which suggested that endoplasmic reticulum stress(ERS) maybe a core mechanism involved in triggering insulin resistance.