Plasma free fatty acids do not provide the link between obesity and insulin resistance or β-cell dysfunction: results of the Reading, Imperial, Surrey, Cambridge, Kings (RISCK) study

Adipocyte Insulin Resistance in PCOS: Relationship With GLUT-4 Expression and Whole-Body Glucose Disposal and β-Cell Function

CONTEXT

Impaired sensitivity to the antilipolytic action of insulin in adipose tissue (AT) may play a role in determining metabolic dysfunction in polycystic ovary syndrome (PCOS).

OBJECTIVES

To test the hypothesis that insulin resistance (IR) in AT is associated with whole-body insulin sensitivity and β-cell function in PCOS.

RESEARCH DESIGN AND SETTING

Prospective cross-sectional study.

METHODS

Eighteen participants with PCOS and 18-matched control participants underwent a modified frequently sampled intravenous glucose tolerance test (mFSIVGTT); subgroups underwent single-slice computed tomography scans determining AT distribution and adipocyte glucose transporter type 4 (GLUT-4) expression.

MAIN OUTCOME MEASURES

IR in AT in basal (by the adipose insulin resistance index [Adipo-IR]) and dynamic (mFSIVGTT-derived indices of insulin-mediated nonesterified fatty acids [NEFA] suppression [NEFAnadir, TIMEnadir, and %NEFAsupp]) states; whole-body insulin-mediated glucose uptake and insulin secretion in basal (by homeostatic model assessment [HOMA]-IR and HOMA-β%) and dynamic (mFSIVGTT-derived insulin sensitivity index [Si], acute insulin response to glucose [AIRg], and disposition index [Di]) states.

RESULTS

Participants with PCOS had higher HOMA-IR and HOMA-β%, lower Si and Di, higher longer TIMEnadir, higher Adipo-IR and NEFAnadir, and a trend toward lower GLUT-4, than the control group participants. Adipo-IR was associated with dynamic state IR in AT (NEFAnadir TIMEnadir, and %NEFAsupp), but only in PCOS, and with HOMA-IR and HOMA-β% in both groups. NEFAnadir and TIMEnadir were negatively and %NEFAsupp positively associated with Si only in PCOS, but not with AIRg and Di, or GLUT-4 expression.

CONCLUSION

Women with PCOS demonstrated increased IR in AT, which is closely associated with whole-body IR but not with dynamic state β-cell function or adipocyte GLUT-4 gene expression.

High or low glycemic index (GI) meals at dinner results in greater postprandial glycemia compared with breakfast: a randomized controlled trial

INTRODUCTION

While circadian control of glucose metabolism is well known, how glycemic index (GI) of carbohydrate-rich meals interacts with time of consumption (breakfast or dinner) to influence postprandial (PP) glucose homeostasis is less well established. The objective of the study was to assess markers of PP glucose homeostasis following high or low GI test meals (TM) consumed either at breakfast or at dinner and following consumption of the subsequent standardized meals (SSM).

RESEARCH DESIGN AND METHODS

Randomized crossover trial in 34 healthy, Chinese, elderly volunteers (mean±SEM age, 56.8±0.83 years), who completed 4 separate study sessions per-protocol, consisting of a high-GI breakfast, low-GI breakfast, high-GI dinner and low-GI dinner TM, followed by a SSM at the subsequent eating occasion. Blood samples were taken for 3 hours after each TM and SSM for glucose, insulin, glucagon, free fatty acids (FFA) and triglycerides (TG) measurements.

RESULTS

Consuming TM at dinner produced greater PP glycemia than breakfast both after TM and SSM (both p<0.0001), irrespective of GI. High-GI TM also produced greater PP glycemia than low-GI TM, both after TM and SSM (both p<0.01), irrespective of time of consumption. No interaction between GI and time were found on PP glycemia, indicating parallel, but independent effects. Combined total areas under the curve of TM+SSM for PP glucose (p<0.0001), PP TG (p<0.0001) and PP FFA (p<0.0001) were all greater when TM taken during dinner compared with breakfast.

CONCLUSIONS

Carbohydrate-rich meals consumed at dinner leads to significantly worse PP glucose homeostasis than when consumed at breakfast, on top of the independent GI effect of the meal. This may have implications to future type 2 diabetes risk. Moreover, future studies investigating GI/glycemic load (GL) and disease risk associations should factor in timing of GL consumption as an additional variable.

TRIAL REGISTRATION NUMBER

NCT02927600.

Interactive association of lipopolysaccharide and free fatty acid with the prevalence of type 2 diabetes: A community-based cross-sectional study

AIMS/INTRODUCTION

Increased blood lipopolysaccharide (LPS) or free fatty acid (FFA) levels correlate with an increased risk of type 2 diabetes. The purpose of the present study was to evaluate the interactive effect of serum LPS and FFA levels on the prevalence of type 2 diabetes.

MATERIALS AND METHODS

This cross-sectional study included 2,553 community-dwelling Chinese adults. Fasting serum LPS levels were determined using the Limulus Amebocyte Lysate Chromogenic Endpoint assay, and FFA levels were determined using an enzymatic method. The participants were divided into three groups according to the tertiles of LPS or FFA levels or nine groups according to the tertiles of LPS and FFA levels. The odd ratios (ORs) for type 2 diabetes were estimated using logistic regression analysis.

RESULTS

We found that higher serum LPS or FFA levels were associated with higher high-sensitivity C-reactive protein levels (P < 0.001), homeostatic model assessment of insulin resistance levels (P < 0.001) and ORs for type 2 diabetes (P < 0.01). Meanwhile, there were significant interactions between LPS and FFA in terms of the high-sensitivity C-reactive protein level (P < 0.001), homeostatic model assessment of insulin resistance level (P < 0.001) and ORs for type 2 diabetes (P < 0.001). In the fully adjusted logistic regression model, the OR for participants with type 2 diabetes in the higher LPS and FFA level group were 6.58 (95% confidence interval 3.05-14.18, P < 0.001) compared with that in participants in the lower LPS and FFA level group.

CONCLUSIONS

The interaction between LPS and FFA was associated with an increased risk of type 2 diabetes in community-dwelling Chinese adults.

Elevated free fatty acid level is associated with insulin-resistant state in nondiabetic Chinese people

BACKGROUND

Free fatty acids (FFAs) are associated with insulin secretion and insulin resistance. However, the associations among FFAs, obesity, and progression from a normal to a prediabetic state are unclear.

METHODS

Nondiabetic subjects (5,952) were divided in two groups according to their body mass index (BMI): obese subjects (BMI ≥24 kg/m²) and nonobese subjects (BMI <24 kg/m²). Clinical and multiple glucolipid metabolism data were collected. The homeostasis model assessment for insulin resistance (HOMA-IR) and β-cell function (HOMA-β) was used. HbA1c level between 5.7% and 6.4% was considered prediabetic. Nonparametric tests, one-way ANOVA, and linear correlation analysis were performed. R and SPSS 23.0 software programs were used to analyze the results.

RESULTS

A U-shaped relationship between FFAs and HOMA-IR was observed. After adjusting for potential confounders, the turning points of FFA levels in the curves were 0.54 mmol/L in the nonobese group and 0.61 mmol/L in the obese group. HOMA-IR levels decreased with increasing FFA concentrations before the turning points (regression coefficient [β]= - 0.9, P=0.0111, for the nonobese group; β=0.2, P=0.5094, for the obese group) and then increased (β=0.9, P=0.0069, for the nonobese group; β=1.5, P=0.0263 for the obese group) after the points. Additionally, our study also identified that FFAs were associated with the prediabetes status in obese individuals.

CONCLUSION

FFA levels were associated with insulin resistance in nondiabetic subjects, and HOMA-IR in nonobese individuals was more sensitive to FFA changes. Monitoring and controlling plasma FFA levels in obese subjects is significant in decreasing insulin resistance and preventing diabetes.

Fatty Acid-Stimulated Insulin Secretion vs. Lipotoxicity

Fatty acid (FA)-stimulated insulin secretion (FASIS) is reviewed here in contrast to type 2 diabetes etiology, resulting from FA overload, oxidative stress, intermediate hyperinsulinemia, and inflammation, all converging into insulin resistance. Focusing on pancreatic islet β-cells, we compare the physiological FA roles with the pathological ones. Considering FAs not as mere amplifiers of glucose-stimulated insulin secretion (GSIS), but as parallel insulin granule exocytosis inductors, partly independent of the K_ATP channel closure, we describe the FA initiating roles in the prediabetic state that is induced by retardations in the glycerol-3-phosphate (glucose)-promoted glycerol/FA cycle and by the impaired GPR40/FFA1 (free FA1) receptor pathway, specifically in its amplification by the redox-activated mitochondrial phospholipase, iPLA2γ. Also, excessive dietary FAs stimulate intestine enterocyte incretin secretion, further elevating GSIS, even at low glucose levels, thus contributing to diabetic hyperinsulinemia. With overnutrition and obesity, the FA overload causes impaired GSIS by metabolic dysbalance, paralleled by oxidative and metabolic stress, endoplasmic reticulum stress and numerous pro-apoptotic signaling, all leading to decreased β-cell survival. Lipotoxicity is exerted by saturated FAs, whereas ω-3 polyunsaturated FAs frequently exert antilipotoxic effects. FA-facilitated inflammation upon the recruitment of excess M1 macrophages into islets (over resolving M2 type), amplified by cytokine and chemokine secretion by β-cells, leads to an inevitable failure of pancreatic β-cells.

Association of NEFA composition with insulin sensitivity and beta cell function in the Prospective Metabolism and Islet Cell Evaluation (PROMISE) cohort

AIMS/HYPOTHESIS

Our aim was to determine the longitudinal associations of individual NEFA with the pathogenesis of diabetes, specifically with differences in insulin sensitivity and beta cell function over 6 years in a cohort of individuals who are at risk for diabetes.

METHODS

In the Prospective Metabolism and Islet Cell Evaluation (PROMISE) longitudinal cohort, 477 participants had serum NEFA measured at the baseline visit and completed an OGTT at three time points over 6 years. Outcome variables were calculated using the OGTT values. At each visit, insulin sensitivity was assessed using the HOMA2 of insulin sensitivity (HOMA2-%S) and the Matsuda index, while beta cell function was assessed using the insulinogenic index over HOMA-IR (IGI/IR) and the insulin secretion-sensitivity index-2 (ISSI-2). Generalised estimating equations were used, adjusting for time, waist, sex, ethnicity, baseline age, alanine aminotransferase (ALT) and physical activity. NEFA were analysed as both concentrations (nmol/ml) and proportions (mol%) of the total fraction.

RESULTS

Participants' (73% female, 70% with European ancestry) insulin sensitivity and beta cell function declined by 14-21% over 6 years of follow-up. In unadjusted models, several NEFA (e.g. 18:1 n-7, 22:4 n-6) were associated with lower insulin sensitivity, however, nearly all of these associations were attenuated in fully adjusted models. In adjusted models, total NEFA, 16:0, 18:1 n-9 and 18:2 n-6 (as concentrations) were associated with 3.7-8.0% lower IGI/IR and ISSI-2, while only 20:5 n-3 (as mol%) was associated with 7.7% higher HOMA2-%S.

CONCLUSIONS/INTERPRETATION

Total NEFA concentration was a strong predictor of lower beta cell function over 6 years. Our results suggest that the association with beta cell function is due to the absolute size of the serum NEFA fraction, rather than the specific fatty acid composition.

CD40 signaling and hepatic steatosis: Unanticipated links

Obesity predisposes to an increased risk of nonalcoholic fatty liver disease (NAFLD). Hepatic steatosis is the key pathological feature of NAFLD and has emerged as a metabolic disorder in which innate and adaptive arms of the immune response play a central role in disease pathogenesis. Recent studies have revealed unexpected relationships between CD40 signaling and hepatic steatosis in high fat diet rodent models. CD154, the ligand of CD40, is a mediator of inflammation and controls several critical events of innate and adaptive immune responses. In the light of these reports, we discuss potential links between CD40 signaling and hepatic steatosis in NAFLD.

How much variance in insulin resistance is explained by obesity?

Background: Obesity is believed to be the major cause of insulin resistance, although many other obesity-independent signals are shown to affect insulin sensitivity.Aim: We address the degree to which variation in insulin resistance is explained by morphometric and biochemical measures of obesity.Methods: PubMed and Google Scholar were searched for epidemiological studies published between 1994 and 2015 that report correlations between at least one measure of obesity and that of insulin resistance.Results: A total of 63 studies satisfied inclusion criteria. Frequency distribution of coefficients of determination between morphometric measures of obesity and insulin resistance was skewed with the mode being less than 10%, class and median being 17.3%. Plasma leptin concentration, but not plasma non-esterified fatty acid level, was better correlated with insulin resistance, the median variance explained being 33.29%. Morphometric measures alone had a median variance explained of 16%. Ethnicity explained part of the variance across studies with the correlation being significantly poorer in Asians.Conclusion: The extremely limited predictive power of morphometric and biochemical measures of obesity suggests that more research needs to focus on the obesity-independent signals that affect insulin sensitivity as well as leptin expression.

Blood free fatty acids were not increased in high-fat diet induced obese insulin-resistant animals

OBJECTIVE

The close connection between high blood FFA and insulin resistance (IR) in obese individuals is well-known. The purpose of this study was to identify whether the blood FFA increased in obese-IR animals.

METHODS

Obese-IR animal models were established using high-fat diet (HFD) or HFD and streptozocin, and treated with drugs.

RESULTS

The serum FFA of obese-IR animals was not increased, even significantly lower than that of normal animals, and were not significantly decreased when insulin sensitivity and obesity-related indices were ameliorated after treatment.

CONCLUSION

The results suggest that blood FFA are unlikely the link between obesity and insulin resistance.

Are we waking up to the effects of NEFA?

NEFA are mobilised from adipose tissues during fasting or stress. Under conditions of acute or chronic NEFA excess, skeletal muscle and hepatic insulin resistance may ensue. Hence, a wealth of literature has focused on the crosstalk between NEFA and glucose in the pathogenesis of insulin resistance. Sleep restriction has also been shown to acutely induce insulin resistance, and self-reported short sleep duration is associated with diabetes. In this issue of Diabetologia (DOI: 10.1007/s00125-015-3500-4), Broussard and colleagues examine the impact of acute sleep restriction on detailed 24 h metabolic profiles, including plasma NEFA. Here, we address the potential clinical relevance of these findings and pose questions for further research.