Opposite Regulation of Insulin Sensitivity by Dietary Lipid Versus Carbohydrate Excess

A 3-Week Ketogenic Diet Increases Skeletal Muscle Insulin Sensitivity in Individuals With Obesity: A Randomized Controlled Crossover Trial

A ketogenic diet (KD) can induce weight loss and improve glycemic regulation, potentially reducing the risk of type 2 diabetes development. To elucidate the underlying mechanisms behind these beneficial effects of a KD, we investigated the impact of a KD on organ-specific insulin sensitivity (IS) in skeletal muscle, liver, and adipose tissue. We hypothesized that a KD would increase IS in skeletal muscle. The study included 11 individuals with obesity who underwent a randomized, crossover trial with two 3-week interventions: 1) a KD and 2) a standard diet. Skeletal muscle IS was quantified as the increase in glucose disposal during a hyperinsulinemic-euglycemic clamp (HEC). Hepatic IS and adipose tissue IS were quantified as the relative suppression of endogenous glucose production (EGP) and the relative suppression of palmitate flux during the HEC. The KD led to a 2.2-kg weight loss and increased insulin-stimulated glucose disposal, whereas the relative suppression of EGP during the HEC was similar. In addition, the KD decreased insulin-mediated suppression of lipolysis. In conclusion, a KD increased skeletal muscle IS in individuals with obesity.

ARTICLE HIGHLIGHTS

Post-translational modifications of pyruvate dehydrogenase complex in cardiovascular disease

Pyruvate dehydrogenase complex (PDC) is a crucial enzyme that connects glycolysis and the tricarboxylic acid (TCA) cycle pathway. It plays an essential role in regulating glucose metabolism for energy production by catalyzing the oxidative decarboxylation of pyruvate to acetyl coenzyme A. Importantly, the activity of PDC is regulated through post-translational modifications (PTMs), phosphorylation, acetylation, and O-GlcNAcylation. These PTMs have significant effects on PDC activity under both physiological and pathophysiological conditions, making them potential targets for metabolism-related diseases. This review specifically focuses on the PTMs of PDC in cardiovascular diseases (CVDs) such as myocardial ischemia/reperfusion injury, diabetic cardiomyopathy, obesity-related cardiomyopathy, heart failure (HF), and vascular diseases. The findings from this review offer theoretical references for the diagnosis, treatment, and prognosis of CVD.

Ketogenic diet but not free-sugar restriction alters glucose tolerance, lipid metabolism, peripheral tissue phenotype, and gut microbiome: RCT

Restricted sugar and ketogenic diets can alter energy balance/metabolism, but decreased energy intake may be compensated by reduced expenditure. In healthy adults, randomization to restricting free sugars or overall carbohydrates (ketogenic diet) for 12 weeks reduces fat mass without changing energy expenditure versus control. Free-sugar restriction minimally affects metabolism or gut microbiome but decreases low-density lipoprotein cholesterol (LDL-C). In contrast, a ketogenic diet decreases glucose tolerance, increases skeletal muscle PDK4, and reduces AMPK and GLUT4 levels. By week 4, the ketogenic diet reduces fasting glucose and increases apolipoprotein B, C-reactive protein, and postprandial glycerol concentrations. However, despite sustained ketosis, these effects are no longer apparent by week 12, when gut microbial beta diversity is altered, possibly reflective of longer-term adjustments to the ketogenic diet and/or energy balance. These data demonstrate that restricting free sugars or overall carbohydrates reduces energy intake without altering physical activity, but with divergent effects on glucose tolerance, lipoprotein profiles, and gut microbiome.

The impact of short-term eucaloric low- and high-carbohydrate diets on liver triacylglycerol content in males with overweight and obesity: a randomized crossover study

BACKGROUND

Intrahepatic triacylglycerol (liver TG) content is associated with hepatic insulin resistance and dyslipidemia. Liver TG content can be modulated within days under hypocaloric conditions.

OBJECTIVES

We hypothesized that 4 d of eucaloric low-carbohydrate/high-fat (LC) intake would decrease liver TG content, whereas a high-carbohydrate/low-fat (HC) intake would increase liver TG content, and further that alterations in liver TG would be linked to dynamic changes in hepatic glucose and lipid metabolism.

METHODS

A randomized crossover trial in males with 4 d + 4 d of LC and HC, respectively, with ≥2 wk of washout. 1H-magnetic resonance spectroscopy (1H-MRS) was used to measure liver TG content, with metabolic testing before and after intake of an LC diet (11E% carbohydrate corresponding to 102 ± 12 {mean ± standard deviation [SD]) g/d, 70E% fat} and an HC diet (65E% carbohydrate corresponding to 537 ± 56 g/d, 16E% fat). Stable [6,6-2H2]-glucose and [1,1,2,3,3-D5]-glycerol tracer infusions combined with hyperinsulinemic-euglycemic clamps and indirect calorimetry were used to measure rates of hepatic glucose production and lipolysis, whole-body insulin sensitivity and substrate oxidation.

RESULTS

Eleven normoglycemic males with overweight or obesity (BMI 31.6 ± 3.7 kg/m2) completed both diets. The LC diet reduced liver TG content by 35.3% (95% confidence interval: -46.6, -24.1) from 4.9% [2.4-11.0] (median interquartile range) to 2.9% [1.4-6.9], whereas there was no change after the HC diet. After the LC diet, fasting whole-body fat oxidation and plasma beta-hydroxybutyrate concentration increased, whereas markers of de novo lipogenesis (DNL) diminished. Fasting plasma TG and insulin concentrations were lowered and the hepatic insulin sensitivity index increased after LC. Peripheral glucose disposal was unchanged.

CONCLUSIONS

Reduced carbohydrate and increased fat intake for 4 d induced a marked reduction in liver TG content and increased hepatic insulin sensitivity. Increased rates of fat oxidation and ketogenesis combined with lower rates of DNL are suggested to be responsible for lowering liver TG. This trial was registered at clinicaltrials.gov as NCT04581421.

The Effect of High-Fat Diet on Intramyocellular Lipid Content in Healthy Adults: A Systematic Review, Meta-Analysis, and Meta-Regression

Fatty acids are stored within the muscle as intramyocellular lipids (IMCL). Some, but not all, studies indicate that following a high-fat diet (HFD), IMCL may accumulate and affect insulin sensitivity. This systematic review and meta-analysis aimed to quantify the effects of an HFD on IMCL. It also explored the potential modifying effects of HFD fat content and duration, IMCL measurement technique, physical activity status, and the associations of IMCL with insulin sensitivity. Five databases were systematically searched for studies that examined the effect of ≥3 d of HFD (>35% daily energy intake from fat) on IMCL content in healthy individuals. Meta-regressions were used to investigate associations of the HFD total fat content, duration, physical activity status, IMCL measurement technique, and insulin sensitivity with IMCL responses. Changes in IMCL content and insulin sensitivity (assessed by hyperinsulinemic-euglycemic clamp) are presented as standardized mean difference (SMD) using a random effects model with 95% confidence intervals (95% CIs). Nineteen studies were included in the systematic review and 16 in the meta-analysis. IMCL content increased following HFD (SMD = 0.63; 95% CI: 0.31, 0.94, P = 0.001). IMCL accumulation was not influenced by total fat content (P = 0.832) or duration (P = 0.844) of HFD, physical activity status (P = 0.192), or by the IMCL measurement technique (P > 0.05). Insulin sensitivity decreased following HFD (SMD = -0.34; 95% CI: -0.52, -0.16; P = 0.003), but this was not related to the increase in IMCL content following HFD (P = 0.233). Consumption of an HFD (>35% daily energy intake from fat) for ≥3 d significantly increases IMCL content in healthy individuals regardless of HFD total fat content and duration of physical activity status. All IMCL measurement techniques detected the increased IMCL content following HFD. The dissociation between changes in IMCL and insulin sensitivity suggests that other factors may drive HFD-induced impairments in insulin sensitivity in healthy individuals. This trial was registered at PROSPERO as CRD42021257984.

The Metabolic Syndrome, a Human Disease

This review focuses on the question of metabolic syndrome (MS) being a complex, but essentially monophyletic, galaxy of associated diseases/disorders, or just a syndrome of related but rather independent pathologies. The human nature of MS (its exceptionality in Nature and its close interdependence with human action and evolution) is presented and discussed. The text also describes the close interdependence of its components, with special emphasis on the description of their interrelations (including their syndromic development and recruitment), as well as their consequences upon energy handling and partition. The main theories on MS's origin and development are presented in relation to hepatic steatosis, type 2 diabetes, and obesity, but encompass most of the MS components described so far. The differential effects of sex and its biological consequences are considered under the light of human social needs and evolution, which are also directly related to MS epidemiology, severity, and relations with senescence. The triggering and maintenance factors of MS are discussed, with especial emphasis on inflammation, a complex process affecting different levels of organization and which is a critical element for MS development. Inflammation is also related to the operation of connective tissue (including the adipose organ) and the widely studied and acknowledged influence of diet. The role of diet composition, including the transcendence of the anaplerotic maintenance of the Krebs cycle from dietary amino acid supply (and its timing), is developed in the context of testosterone and β-estradiol control of the insulin-glycaemia hepatic core system of carbohydrate-triacylglycerol energy handling. The high probability of MS acting as a unique complex biological control system (essentially monophyletic) is presented, together with additional perspectives/considerations on the treatment of this 'very' human disease.

PDHA1 Alleviates Myocardial Ischemia-Reperfusion Injury by Improving Myocardial Insulin Resistance During Cardiopulmonary Bypass Surgery in Rats

OBJECTIVE

Cardiopulmonary bypass (CPB) is a requisite technique for thoracotomy in advanced cardiovascular surgery. However, the consequent myocardial ischemia-reperfusion injury (MIRI) is the primary culprit behind cardiac dysfunction and fatal consequences post-operation. Prior research has posited that myocardial insulin resistance (IR) plays a vital role in exacerbating the progression of MIRI. Nonetheless, the exact mechanisms underlying this phenomenon remain obscure.

METHODS

We constructed pyruvate dehydrogenase E1 α subunit (PDHA1) interference and overexpression rats and used ascending aorta occlusion in an in vivo model of CPB-MIRI. We devised an in vivo model of CPB-MIRI by constructing rat models with both pyruvate dehydrogenase E1α subunit (PDHA1) interference and overexpression through ascending aorta occlusion. We analyzed myocardial glucose metabolism and the degree of myocardial injury using functional monitoring, biochemical assays, and histological analysis.

RESULTS

We discovered a clear downregulation of glucose transporter 4 (GLUT4) protein content expression in the CPB I/R model. In particular, cardiac-specific PDHA1 interference resulted in exacerbated cardiac dysfunction, significantly increased myocardial infarction area, more pronounced myocardial edema, and markedly increased cardiomyocyte apoptosis. Notably, the opposite effect was observed with PDHA1 overexpression, leading to a mitigated cardiac dysfunction and decreased incidence of myocardial infarction post-global ischemia. Mechanistically, PDHA1 plays a crucial role in regulating the protein content expression of GLUT4 on cardiomyocytes, thereby controlling the uptake and utilization of myocardial glucose, influencing the development of myocardial insulin resistance, and ultimately modulating MIRI.

CONCLUSION

Overall, our study sheds new light on the pivotal role of PDHA1 in glucose metabolism and the development of myocardial insulin resistance. Our findings hold promising therapeutic potential for addressing the deleterious effects of MIRI in patients.

Pre-Meal Whey Protein Alters Postprandial Insulinemia by Enhancing β-Cell Function and Reducing Insulin Clearance in T2D

CONTEXT

Treatments that reduce postprandial glycemia (PPG) independent of stimulating insulin secretion are appealing for the management of type 2 diabetes (T2D). Consuming pre-meal whey protein (WP) reduces PPG by delaying gastric emptying and increasing plasma insulin concentrations. However, its effects on β-cell function and insulin kinetics remains unclear.

OBJECTIVE

To examine the PPG-regulatory effects of pre-meal WP by modeling insulin secretion rates (ISR), insulin clearance, and β-cell function.

METHODS

This was a single-blind, randomized, placebo-controlled, crossover design study in 18 adults with T2D (HbA1c, 56.7 ± 8.8 mmol/mol) who underwent 2 240-minute mixed-meal tolerance tests. Participants consumed WP (15 g protein) or placebo (0 g protein) 10 minutes before a mixed-macronutrient breakfast meal. PPG, pancreatic islet, and incretin hormones were measured throughout. ISR was calculated by C-peptide deconvolution. Estimates of insulin clearance and β-cell function were modeled from glucose, insulin, and ISR. Changes in PPG incremental area under the curve (iAUC; prespecified) and insulin clearance (post hoc) were measured.

RESULTS

β-cell function was 40% greater after WP (P = .001) and was accompanied with a -22% reduction in postprandial insulin clearance vs placebo (P < .0001). Both the peak change and PPG iAUC were reduced by WP (-1.5 mmol/L and -16%, respectively; both P < .05). Pre-meal WP augmented a 5.9-fold increase in glucagon and glucagon-like peptide 1 iAUC (both P < .0001), and a 1.5-fold increase in insulin iAUC (P < .001). Although the plasma insulin response was greater following WP, ISR was unaffected (P = .133).

CONCLUSION

In adults with T2D, pre-meal WP reduced PPG by coordinating an enhancement in β-cell function with a reduction in insulin clearance. This enabled an efficient postprandial insulinemic profile to be achieved without requiring further β-cell stimulation.Trial registry ISRCTN ID: ISRCTN17563146 Website link: www.isrctn.com/ISRCTN17563146.

The Impact of Westernization on the Insulin/IGF-I Signaling Pathway and the Metabolic Syndrome: It Is Time for Change

The metabolic syndrome is a cluster of overlapping conditions resulting in an increased incidence of type 2 diabetes, cardiovascular disease, and cancer. In the last few decades, prevalence of the metabolic syndrome in the Western world has reached epidemic proportions and this is likely due to alterations in diet and the environment as well as decreased physical activity. This review discusses how the Western diet and lifestyle (Westernization) has played an important etiological role in the pathogenesis of the metabolic syndrome and its consequences by exerting negative effects on activity of the insulin-insulin-like growth factor-I (insulin-IGF-I) system. It is further proposed that interventions that normalize/reduce activity of the insulin-IGF-I system may play a key role in the prevention and treatment of the metabolic syndrome. For successful prevention, limitation, and treatment of the metabolic syndrome, the focus should be primarily on changing our diets and lifestyle in accordance with our genetic make-up, formed in adaptation to Paleolithic diets and lifestyles during a period of several million years of human evolution. Translating this insight into clinical practice, however, requires not only individual changes in our food and lifestyle, starting in pediatric populations at a very young age, but also requires fundamental changes in our current health systems and food industry. Change is needed: primary prevention of the metabolic syndrome should be made a political priority. New strategies and policies should be developed to stimulate and implement behaviors encouraging the sustainable use of healthy diets and lifestyles to prevent the metabolic syndrome before it develops.

Mineralocorticoid Receptors Mediate Diet-Induced Lipid Infiltration of Skeletal Muscle and Insulin Resistance

Excess circulating lipids increase total intramyocellular (IMC) lipid content and ectopic fat storage, resulting in lipotoxicity and insulin resistance in skeletal muscle. Consumption of a diet high in fat and refined sugars-a Western diet (WD)-has been shown to activate mineralocorticoid receptors (MRs) and promote insulin resistance. However, our understanding of the precise mechanisms by which enhanced MR activation promotes skeletal muscle insulin resistance remains unclear. In this study, we investigated the mechanisms by which enhanced MR signaling in soleus muscle promotes ectopic skeletal muscle lipid accumulation and related insulin resistance. Six-week-old C57BL/6J mice were fed either a mouse chow diet or a WD with or without spironolactone (1 mg/kg/day) for 16 weeks. Spironolactone attenuated 16 weeks of WD-induced in vivo glucose intolerance and insulin resistance, and improved soleus insulin metabolic signaling. Improved insulin sensitivity was accompanied by increased glucose transporter 4 (Glut4) expression in conjunction with decreased soleus free fatty acid and IMC lipid content, as well as CD36 expression. Additionally, spironolactone prevented WD-induced soleus mitochondria dysfunction. Furthermore, MR signaling also mediated WD/aldosterone-induced reductions in soleus microRNA (miR)-99a, which was identified to negatively target CD36 and prevented palmitic acid-induced increases in CD36 expression, lipid droplet formation, mitochondria dysfunction, and insulin resistance in C2C12 cells. These data indicate that inhibition of MR activation with spironolactone prevented diet-induced abnormal expression of miR-99a, which had the capacity to reduce CD36, leading to reduced IMC lipid content and improved soleus mitochondria function and insulin sensitivity.

Regulation of Insulin Clearance by Non-Esterified Fatty Acids

Insulin stores lipid in adipocytes and prevents lipolysis and the release of non-esterified fatty acids (NEFA). Excessive release of NEFA during sustained energy supply and increase in abdominal adiposity trigger systemic insulin resistance, including in the liver, a major site of insulin clearance. This causes a reduction in insulin clearance as a compensatory mechanism to insulin resistance in obesity. On the other hand, reduced insulin clearance in the liver can cause chronic hyperinsulinemia, followed by downregulation of insulin receptor and insulin resistance. Delineating the cause–effect relationship between reduced insulin clearance and insulin resistance has been complicated by the fact that insulin action and clearance are mechanistically linked to insulin binding to its receptors. This review discusses how NEFA mobilization contributes to the reciprocal relationship between insulin resistance and reduced hepatic insulin clearance, and how this may be implicated in the pathogenesis of non-alcoholic fatty liver disease.

A low n-6 to n-3 polyunsaturated fatty acid ratio diet improves hyperinsulinaemia by restoring insulin clearance in obese youth

AIM

To examine the determinants and metabolic impact of the reduction in fasting and postload insulin levels after a low n-6 to n-3 polyunsaturated fatty acid (PUFA) ratio diet in obese youth.

MATERIALS AND METHODS

Insulin secretion and clearance were assessed by measuring and modelling plasma insulin and C-peptide in 17 obese youth who underwent a nine-point, 180-minute oral glucose tolerance test (OGTT) before and after a 12-week, eucaloric low n-6:n-3 polyunsaturated fatty acid (PUFA) ratio diet. Hepatic fat content was assessed by repeated abdominal magnetic resonance imaging.

RESULTS

Insulin clearance at fasting and during the OGTT was significantly increased after the diet, while body weight, glucose levels, absolute and glucose-dependent insulin secretion, and model-derived variables of β-cell function were not affected. Dietary-induced changes in insulin clearance positively correlated with changes in whole-body insulin sensitivity and β-cell glucose sensitivity, but not with changes in hepatic fat. Subjects with greater increases in insulin clearance showed a worse metabolic profile at enrolment, characterized by impaired insulin clearance, β-cell glucose sensitivity, and glucose tolerance, and benefitted the most from the diet, achieving greater improvements in glucose-stimulated hyperinsulinaemia, insulin resistance, and β-cell function.

CONCLUSIONS

We showed that a 12-week low n-6:n-3 PUFA ratio diet improves hyperinsulinaemia by increasing fasting and postload insulin clearance in obese youth, independently of weight loss, glucose concentrations, and insulin secretion.

Protein-carbohydrate interaction effects on energy balance, FGF21, IGF-1, and hypothalamic gene expression in rats

Amino acids are involved in energy homeostasis, just as are carbohydrates and lipids. Therefore, mechanisms controlling protein intake should operate independently and in combination with systems controlling overall energy intake to coordinate appropriate metabolic and behavioral responses. The objective of this study was to quantify the respective roles of dietary protein and carbohydrate levels on energy balance, plasma fibroblast growth factor 21 (FGF21) and insulin growth factor 1 (IGF-1) concentrations, and hypothalamic neurotransmitters (POMC, NPY, AgRP, and CART). In a simplified geometric framework, 7-wk-old male Wistar rats were fed 12 diets containing 3%-30% protein for 3 wk, in which carbohydrates accounted for 30%-75% of the carbohydrate and fat part of the diet. As a result of this study, most of the studied parameters (body composition, energy expenditure, plasma FGF21 and IGF-1 concentrations, and Pomc/Agrp ratio) responded mainly to the protein content and to a lesser extent to the carbohydrate content in the diet.NEW & NOTEWORTHY As mechanisms controlling protein intake can operate independently and in combination with those controlling energy intakes, we investigated the metabolic and behavioral effects of the protein-carbohydrate interaction. With a simplified geometric framework, we showed that body composition, energy balance, plasma FGF21 and IGF-1 concentrations, and hypothalamic Pomc/Agrp ratio were primarily responsive to protein content and, to a lesser extent, to carbohydrate content of the diet.

Sleeve Gastrectomy Suppresses Hepatic Glucose Production and Increases Hepatic Insulin Clearance Independent of Weight Loss

Bariatric operations induce weight loss, which is associated with an improvement in hepatic steatosis and a reduction in hepatic glucose production. It is not clear whether these outcomes are entirely due to weight loss, or whether the new anatomy imposed by the surgery contributes to the improvement in the metabolic function of the liver. We performed vertical sleeve gastrectomy (VSG) on obese mice provided with a high-fat high-sucrose diet and compared them to diet and weight-matched sham-operated mice (WMS). At 40 days after surgery, VSG-operated mice displayed less hepatic steatosis compared with WMS. By measuring the fasting glucose and insulin levels in the blood vessels feeding and draining the liver, we showed directly that hepatic glucose production was suppressed after VSG. Insulin levels were elevated in the portal vein, and hepatic insulin clearance was elevated in VSG-operated mice. The hepatic expression of genes associated with insulin clearance was upregulated. We repeated the experiment in lean mice and observed that portal insulin and glucagon are elevated, but only insulin clearance is increased in VSG-operated mice. In conclusion, direct measurement of glucose and insulin in the blood entering and leaving the liver shows that VSG affects glucose and insulin metabolism through mechanisms independent of weight loss and diet.

The many actions of insulin in skeletal muscle, the paramount tissue determining glycemia

As the principal tissue for insulin-stimulated glucose disposal, skeletal muscle is a primary driver of whole-body glycemic control. Skeletal muscle also uniquely responds to muscle contraction or exercise with increased sensitivity to subsequent insulin stimulation. Insulin's dominating control of glucose metabolism is orchestrated by complex and highly regulated signaling cascades that elicit diverse and unique effects on skeletal muscle. We discuss the discoveries that have led to our current understanding of how insulin promotes glucose uptake in muscle. We also touch upon insulin access to muscle, and insulin signaling toward glycogen, lipid, and protein metabolism. We draw from human and rodent studies in vivo, isolated muscle preparations, and muscle cell cultures to home in on the molecular, biophysical, and structural elements mediating these responses. Finally, we offer some perspective on molecular defects that potentially underlie the failure of muscle to take up glucose efficiently during obesity and type 2 diabetes.

A novel numerical model of combination levels of C-peptide and insulin in coronary artery disease risk prediction

OBJECTIVE

Insulin resistance is a major risk factor for coronary artery disease (CAD). The C-peptide-to-insulin ratio (C/I) is associated with hepatic insulin clearance and insulin resistance. The current study was designed to establish a novel C/I index (CPIRI) model and provide early risk assessment of CAD.

METHODS

A total of 865 adults diagnosed with new-onset diabetes mellitus (DM) within one year and 54 healthy controls (HC) were recruited to develop a CPIRI model. The CPIRI model was established with fasting C/I as the independent variable and homeostasis model assessment of insulin resistance (HOMA-IR) as the dependent variable. Associations between the CPIRI model and the severity of CAD events were also assessed in 45 hyperglycemic patients with CAD documented via coronary arteriography (CAG) and whom underwent stress echocardiography (SE) and exercise electrocardiography test (EET).

RESULTS

Fasting C-peptide/insulin and HOMA-IR were hyperbolically correlated in DM patients and HC, and log(C/I) and log(HOMA-IR) were linearly and negatively correlated. The respective correlational coefficients were -0.83 (p < 0.001) and -0.76 (p < 0.001). The equations CPIRI(DM) = 670/(C/I)2.24 + 0.25 and CPIRI(HC) = 670/(C/I)2.24 - 1 (F = 1904.39, p < 0.001) were obtained. Patients with insulin resistance exhibited severe coronary artery impairment and myocardial ischemia. In CAD patients there was no significant correlation between insulin resistance and the number of vessels involved.

CONCLUSIONS

CPIRI can be used to effectively evaluate insulin resistance, and the combination of CPIRI and non-invasive cardiovascular examination is of great clinical value in the assessment of CAD.

The Role of Hepatic Fat Accumulation in Glucose and Insulin Homeostasis-Dysregulation by the Liver

Accumulation of hepatic triacylglycerol (TG) is associated with obesity and metabolic syndrome, which are important pathogenic factors in the development of type 2 diabetes. In this narrative review, we summarize the effects of hepatic TG accumulation on hepatic glucose and insulin metabolism and the underlying molecular regulation in order to highlight the importance of hepatic TG accumulation for whole-body glucose metabolism. We find that liver fat accumulation is closely linked to impaired insulin-mediated suppression of hepatic glucose production and reduced hepatic insulin clearance. The resulting systemic hyperinsulinemia has a major impact on whole-body glucose metabolism and may be an important pathogenic step in the development of type 2 diabetes.

Small Amounts of Dietary Medium-Chain Fatty Acids Protect Against Insulin Resistance During Caloric Excess in Humans

Medium-chain fatty acids (MCFAs) have in rodents been shown to have protective effects on glucose homeostasis during high-fat overfeeding. In this study, we investigated whether dietary MCFAs protect against insulin resistance induced by a hypercaloric high-fat diet in humans. Healthy, lean men ingested a eucaloric control diet and a 3-day hypercaloric high-fat diet (increase of 75% in energy, 81-83% energy [E%] from fat) in randomized order. For one group (n = 8), the high-fat diet was enriched with saturated long-chain FAs (LCSFA-HFD), while the other group (n = 9) ingested a matched diet, but with ∼30 g (5E%) saturated MCFAs (MCSFA-HFD) in substitution for a corresponding fraction of the saturated long-chain fatty acids (LCFAs). A hyperinsulinemic-euglycemic clamp with femoral arteriovenous balance and glucose tracer was applied after the control and hypercaloric diets. In LCSFA-HFD, whole-body insulin sensitivity and peripheral insulin-stimulated glucose disposal were reduced. These impairments were prevented in MCSFA-HFD, accompanied by increased basal fatty acid oxidation, maintained glucose metabolic flexibility, increased nonoxidative glucose disposal related to lower starting glycogen content, and increased glycogen synthase activity, together with increased muscle lactate production. In conclusion, substitution of a small amount of dietary LCFAs with MCFAs rescues insulin action in conditions of lipid-induced energy excess.

The role of the liver in the modulation of glucose and insulin in non alcoholic fatty liver disease and type 2 diabetes

In this review we have discussed how the liver plays a central role in the regulation of glucose metabolism and in insulin clearance. Both non-alcoholic fatty liver disease (NAFLD) and diabetes (T2D) are characterized by high plasma insulin concentrations, hepatic insulin resistance, high hepatic glucose production (HGP), in particular gluconeogenesis (GNG), that are increased proportionally to fasting hyperglycemia, while postprandial hyperglycemia is due to impaired suppression of HGP by insulin, and reduced hepatic glycogen storage. The liver acts also as a modulator of peripheral insulin since most of insulin secreted by the pancreas is cleared by the liver during the first pass. Hepatokines and hepatic lipids can act in either autocrine or paracrine way and can be responsible of the changes in insulin sensitivity and alterations in glucose metabolism.

Tuning fatty acid oxidation in skeletal muscle with dietary fat and exercise

Both the consumption of a diet rich in fatty acids and exercise training result in similar adaptations in several skeletal muscle proteins. These adaptations are involved in fatty acid uptake and activation within the myocyte, the mitochondrial import of fatty acids and further metabolism of fatty acids by β-oxidation. Fatty acid availability is repeatedly increased postprandially during the day, particularly during high dietary fat intake and also increases during, and after, aerobic exercise. As such, fatty acids are possible signalling candidates that regulate transcription of target genes encoding proteins involved in muscle lipid metabolism. The mechanism of signalling might be direct or indirect targeting of peroxisome proliferator-activated receptors by fatty acid ligands, by fatty acid-induced NAD⁺-stimulated activation of sirtuin 1 and/or fatty acid-mediated activation of AMP-activated protein kinase. Lactate might also have a role in lipid metabolic adaptations. Obesity is characterized by impairments in fatty acid oxidation capacity, and individuals with obesity show some rigidity in increasing fatty acid oxidation in response to high fat intake. However, individuals with obesity retain improvements in fatty acid oxidation capacity in response to exercise training, thereby highlighting exercise training as a potential method to improve lipid metabolic flexibility in obesity.

Calorie restriction for enhanced longevity: The role of novel dietary strategies in the present obesogenic environment

Calorie restriction (CR) is a potent modulator of longevity in multiple species. A growing body of evidence shows that sustained periods of CR without malnutrition improves risk factors involved in the pathophysiology of type 2 diabetes, cardiovascular diseases, cancer, and neurological disorders in humans. Innovative dietary strategies such as intermittent fasting and protein restriction have recently emerged as alternative approaches that improve markers of aging. Some of these newer strategies might provide benefits for healthy aging with little to no CR and therefore, compared to traditional CR, may be easier to follow. Further to providing an update of CR studies in humans, the present narrative review appraises the influence of these contemporary dietary strategies on mechanisms posited to drive CR-induced longevity in humans, including those involving energy metabolism, oxidative damage, inflammation, glucose homeostasis, and functional changes in the neuroendocrine systems. The review also discusses the utilization of these diets for populations in the current obesogenic environment, and comments on whether current research can inform an optimal diet that attenuates aging, can be easily followed, and promises to improve longevity in humans.

Bromocriptine mesylate improves glucose tolerance and disposal in a high-fat-fed canine model

Bromocriptine mesylate treatment was examined in dogs fed a high fat diet (HFD) for 8 wk. After 4 wk on HFD, daily bromocriptine (Bromo; n = 6) or vehicle (CTR; n = 5) injections were administered. Oral glucose tolerance tests were performed before beginning HFD (OGTT1), 4 wk after HFD began (Bromo only), and after 7.5 wk on HFD (OGTT3). After 8 wk on HFD, clamp studies were performed, with infusion of somatostatin and intraportal replacement of insulin (4× basal) and glucagon (basal). From 0 to 90 min (P1), glucose was infused via peripheral vein to double the hepatic glucose load; and from 90 to 180 min (P2), glucose was infused via the hepatic portal vein at 4 mg·kg^-1·min^-1, with the HGL maintained at 2× basal. Bromo decreased the OGTT glucose ΔAUC_0-30 and ΔAUC_0-120 by 62 and 27%, respectively, P < 0.05 for both) without significantly altering the insulin response. Bromo dogs exhibited enhanced net hepatic glucose uptake (NHGU) compared with CTR (~33 and 21% greater, P1 and P2, respectively, P < 0.05). Nonhepatic glucose uptake (non-HGU) was increased ~38% in Bromo in P2 (P < 0.05). Bromo vs. CTR had higher (P < 0.05) rates of glucose infusion (36 and 30%) and non-HGU (~40 and 27%) than CTR during P1 and P2, respectively. In Bromo vs. CTR, hepatic 18:0/16:0 and 16:1/16:0 ratios tended to be elevated in triglycerides and were higher (P < 0.05) in phospholipids, consistent with a beneficial effect of bromocriptine on liver fat accumulation. Thus, bromocriptine treatment improved glucose disposal in a glucose-intolerant model, enhancing both NHGU and non-HGU.

Short-term High-fat Overfeeding Does Not Induce NF-κB Inflammatory Signaling in Subcutaneous White Adipose Tissue

CONTEXT

It is unclear how white adipose tissue (WAT) inflammatory signaling proteins respond during the early stages of overnutrition.

OBJECTIVE

To investigate the effect of short-term, high-fat overfeeding on fasting abdominal subcutaneous WAT total content and phosphorylation of proteins involved in nuclear factor-κB (NF-κB) inflammatory signaling, systemic metabolic and inflammatory biomarkers.

DESIGN

Individuals consumed a high-fat (65% total energy from total fat), high-energy (50% above estimated energy requirements) diet for 7 days.

RESULTS

Fifteen participants (aged 27 ± 1 years; body mass index 24.4 ± 0.6 kg/m2) completed the study. Body mass increased following high-fat overfeeding (+1.2 ± 0.2 kg; P < 0.0001). However, total content and phosphorylation of proteins involved in NF-κB inflammatory signaling were unchanged following the intervention. Fasting serum glucose (+0.2 ± 0.0 mmol/L), total cholesterol (+0.4 ± 0.1 mmol/L), low-density lipoprotein cholesterol (+0.3 ± 0.1 mmol/L), high-density lipoprotein cholesterol (+0.2 ± 0.0 mmol/L), and lipopolysaccharide-binding protein (LBP; +4.7 ± 2.1 µg/mL) increased, whereas triacylglycerol concentrations (-0.2 ± 0.1 mmol/L) decreased following overfeeding (all P < 0.05). Systemic biomarkers (insulin, soluble cluster of differentiation 14 [CD14], C-reactive protein, interleukin-6, tumor necrosis factor-α and monocyte chemoattractant protein-1) and the proportion and concentration of circulating CD14+ monocytes were unaffected by overfeeding.

CONCLUSION

Acute lipid oversupply did not impact on total content or phosphorylation of proteins involved in WAT NF-κB inflammatory signaling, despite modest weight gain and metabolic alterations. Systemic LBP, which is implicated in the progression of low-grade inflammation during the development of obesity, increased in response to a 7-day high-fat overfeeding period.

Association of increased hepatic insulin clearance and change in serum triglycerides or β-hydroxybutyrate concentration via the sodium/glucose-cotransporter 2 inhibitor tofogliflozin

AIMS

Obesity and hepatic fat accumulation diminish hepatic insulin clearance, which can cause hyperinsulinaemia. Sodium/glucose-cotransporter 2 inhibitors (SGLT2-is) improve insulin resistance and hyperinsulinaemia by weight loss via increased urinary glucose excretion in type 2 diabetes. However, there are few reports of the influence of SGLT2-is on hepatic insulin clearance. We examined the impact of an SGLT2-i on hepatic insulin clearance and explored the clinical influence associated with changes in hepatic insulin clearance via an SGLT2-i and the mechanism of the effects of SGLT2-i.

MATERIALS AND METHODS

Data were analysed from 419 patients with type 2 diabetes controlled by diet and exercise. Patients received a placebo or the SGLT2-i tofogliflozin (TOFO) (placebo: n = 56; TOFO: n = 363) orally once daily for ≥24 weeks. Hepatic insulin clearance was calculated from the ratio of areas under the curve (AUC) of C-peptide and insulin levels derived from oral meal tolerance test data (C-peptide AUC_{0-120 min} /insulin AUC_{0-120 min} : HIC_CIR ). The correlation of HIC_CIR via the SGLT2-i with other clinical variables was analysed using multivariate analysis.

RESULTS

HIC_CIR was significantly increased via TOFO at week 24. Furthermore, with TOFO insulin and triglyceride (TG) levels were significantly reduced (P < 0.001) and β-hydroxybutyrate (BHB) was significantly elevated (P < 0.001). Changes in HIC_CIR were significantly correlated with changes in TG and BHB via TOFO.

CONCLUSIONS

Increased HIC_CIR was significantly associated with reduced TG via TOFO and contributed to the greater increase in BHB compared with placebo in addition to the correction of hyperinsulinaemia.

The (pro)renin receptor in health and disease

The (pro)renin receptor ((P)RR) was first identified as a single-transmembrane receptor in human kidneys and initially attracted attention owing to its potential role as a regulator of the tissue renin-angiotensin system (RAS). Subsequent studies found that the (P)RR is widely distributed in organs throughout the body, including the kidneys, heart, brain, eyes, placenta and the immune system, and has multifaceted functions in vivo. The (P)RR has roles in various physiological processes, such as the cell cycle, autophagy, acid-base balance, energy metabolism, embryonic development, T cell homeostasis, water balance, blood pressure regulation, cardiac remodelling and maintenance of podocyte structure. These roles of the (P)RR are mediated by its effects on important biological systems and pathways including the tissue RAS, vacuolar H⁺-ATPase, Wnt, partitioning defective homologue (Par) and tyrosine phosphorylation. In addition, the (P)RR has been reported to contribute to the pathogenesis of diseases such as fibrosis, hypertension, pre-eclampsia, diabetic microangiopathy, acute kidney injury, cardiovascular disease, cancer and obesity. Current evidence suggests that the (P)RR has key roles in the normal development and maintenance of vital organs and that dysfunction of the (P)RR is associated with diseases that are characterized by a disruption of the homeostasis of physiological functions.

Low-attenuation muscle is a predictor of diabetes mellitus: A population-based cohort study

OBJECTIVES

Diabetes mellitus is a major global public health issue. Cross-sectional studies have demonstrated that skeletal muscle quality and quantity by computed tomography (CT) is related to glucose metabolism. However, to our knowledge, no longitudinal study has yet to elucidate the association between muscle quality determined by CT and glucose metabolism. Thus, the aim of this study was to evaluate the association between muscle quality and glucose metabolism.

METHODS

In this retrospective study, we evaluated data from 621 middle-aged Japanese individuals without diabetes mellitus from a cohort of employees undergoing annual physical examinations. The cross-sectional skeletal muscle area was determined based on CT data at the level of the third lumbar vertebrae, and the skeletal muscle index (SMI) and density (SMD) were calculated. Low-attenuation muscle (LAM) and normal-attenuation muscle (NAM) were identified and quantified using thresholds of -29 to +29 HU and +30 to +150 HU, respectively.

RESULTS

We followed the individuals for a mean period of 3 y, and 27 of them developed diabetes mellitus during this period. Multiple Cox regression analyses revealed that, even after adjustment for visceral fat area index, the LAM index (hazard ratio [HR], 3.07; 95% confidence interval [CI], 1.00-7.52) showed an increased adjusted HR for incident diabetes mellitus. When total SMI and SMD were used in the same models, only total SMD (HR, 0.90; 95% CI, 0.81-0.99) showed a decreased adjusted HR for incident diabetes mellitus.

CONCLUSIONS

Both LAM index and total SMD were associated with a higher risk for incident diabetes mellitus, whereas NAM index and total SMI were not.

The Importance of Fatty Acids as Nutrients during Post-Exercise Recovery

It is well recognized that whole-body fatty acid (FA) oxidation remains increased for several hours following aerobic endurance exercise, even despite carbohydrate intake. However, the mechanisms involved herein have hitherto not been subject to a thorough evaluation. In immediate and early recovery (0–4 h), plasma FA availability is high, which seems mainly to be a result of hormonal factors and increased adipose tissue blood flow. The increased circulating availability of adipose-derived FA, coupled with FA from lipoprotein lipase (LPL)-derived very-low density lipoprotein (VLDL)-triacylglycerol (TG) hydrolysis in skeletal muscle capillaries and hydrolysis of TG within the muscle together act as substrates for the increased mitochondrial FA oxidation post-exercise. Within the skeletal muscle cells, increased reliance on FA oxidation likely results from enhanced FA uptake into the mitochondria through the carnitine palmitoyltransferase (CPT) 1 reaction, and concomitant AMP-activated protein kinase (AMPK)-mediated pyruvate dehydrogenase (PDH) inhibition of glucose oxidation. Together this allows glucose taken up by the skeletal muscles to be directed towards the resynthesis of glycogen. Besides being oxidized, FAs also seem to be crucial signaling molecules for peroxisome proliferator-activated receptor (PPAR) signaling post-exercise, and thus for induction of the exercise-induced FA oxidative gene adaptation program in skeletal muscle following exercise. Collectively, a high FA turnover in recovery seems essential to regain whole-body substrate homeostasis.

High-fat Overfeeding Does Not Exacerbate Rapid Changes in Forearm Glucose and Fatty Acid Balance During Immobilization

CONTEXT

Physical inactivity and high-fat overfeeding have been shown to independently induce insulin resistance.

OBJECTIVE

Establish the contribution of muscle disuse and lipid availability to the development of inactivity-induced insulin resistance.

DESIGN, SETTING, PARTICIPANTS, AND INTERVENTIONS

20 healthy males underwent 7 days of forearm cast immobilization combined with a fully controlled eucaloric diet (n = 10, age 23 ± 2 yr, body mass index [BMI] 23.8 ± 1.0 kg·m-2) or a high-fat diet (HFD) providing 50% excess energy from fat (high-fat diet, n = 10, age 23 ± 2 yr, BMI 22.4 ± 0.8 kg·m-2).

MAIN OUTCOME MEASURES

Prior to casting and following 2 and 7 days of immobilization, forearm glucose uptake (FGU) and nonesterified fatty acid (NEFA) balance were assessed using the arterialized venous-deep venous (AV-V) forearm balance method following ingestion of a mixed macronutrient drink.

RESULTS

7 days of HFD increased body weight by 0.9 ± 0.2 kg (P = 0.002), but did not alter fasting, arterialized whole-blood glucose and serum insulin concentrations or the associated homeostatic model assessment of insulin resistance or Matsuda indices. Two and 7 days of forearm immobilization led to a 40 ± 7% and 52 ± 7% decrease in FGU, respectively (P < 0.001), with no difference between day 2 and 7 and no effect of HFD. Forearm NEFA balance tended to increase following 2 and 7 days of immobilization (P = 0.095).

CONCLUSIONS

Forearm immobilization leads to a rapid and substantial decrease in FGU, which is accompanied by an increase in forearm NEFA balance but is not exacerbated by excess dietary fat intake. Altogether, our data suggest that disuse-induced insulin resistance of glucose metabolism occurs as a physiological adaptation in response to the removal of muscle contraction.

High-Fat Overfeeding Impairs Peripheral Glucose Metabolism and Muscle Microvascular eNOS Ser1177 Phosphorylation

CONTEXT

The mechanisms responsible for dietary fat-induced insulin resistance of skeletal muscle and its microvasculature are only partially understood.

OBJECTIVE

To determine the impact of high-fat overfeeding on postprandial glucose fluxes, muscle insulin signaling, and muscle microvascular endothelial nitric oxide synthase (eNOS) content and activation.

DESIGN

Fifteen non-obese volunteers consumed a high-fat (64%) high-energy (+47%) diet for 7 days. Experiments were performed before and after the diet. Stable isotope tracers were used to determine glucose fluxes in response to carbohydrate plus protein ingestion. Muscle insulin signaling was determined as well as the content and activation state of muscle microvascular eNOS.

RESULTS

High-fat overfeeding impaired postprandial glycemic control as demonstrated by higher concentrations of glucose (+11%; P = 0.004) and insulin (+19%; P = 0.035). Carbohydrate plus protein ingestion suppressed endogenous glucose production to a similar extent before and after the diet. Conversely, high-fat overfeeding reduced whole-body glucose clearance (-16%; P = 0.021) and peripheral insulin sensitivity (-26%; P = 0.006). This occurred despite only minor alterations in skeletal muscle insulin signaling. High-fat overfeeding reduced eNOS content in terminal arterioles (P = 0.017) and abolished the increase in eNOS Ser1177 phosphorylation that was seen after carbohydrate plus protein ingestion.

CONCLUSION

High-fat overfeeding impaired whole-body glycemic control due to reduced glucose clearance, not elevated endogenous glucose production. The finding that high-fat overfeeding abolished insulin-mediated eNOS Ser1177 phosphorylation in the terminal arterioles suggests that impairments in the vasodilatory capacity of the skeletal muscle microvasculature may contribute to early dietary fat-induced impairments in glycemic control.

Reassessing the Role of Diacylglycerols in Insulin Resistance

Skeletal muscle (SM) insulin resistance (IR) plays an important role in the burden of obesity, particularly because it leads to glucose intolerance and type 2 diabetes. Among the mechanisms thought to link IR to obesity is the accumulation, in muscle cells, of different lipid metabolites. Diacylglycerols (DAGs) are subject of particular attention due to reported interactions with the insulin signaling cascade. Given that SM accounts for the majority of insulin-stimulated glucose uptake, this review integrates recent observational and mechanistic works with the sole focus on questioning the role of DAGs in SM IR. Particular attention is given to the subcellular distributions and specific structures of DAGs, highlighting future research directions towards reaching a consensus on the mechanistic role played by DAGs.

Relationship between metabolic syndrome and trunk muscle quality as well as quantity evaluated by computed tomography

BACKGROUND & AIMS

Metabolic syndrome is a cluster of metabolic abnormalities. Skeletal muscle attenuation and area evaluated by computer tomography (CT) may provide valuable information about patients with metabolic disorder. Therefore, we examined the association between skeletal muscle characteristics and metabolic syndrome, and investigated the combined effect of quantitative and qualitative muscle parameters.

METHODS

In this cross-sectional study, we analyzed 808 middle-aged Japanese subjects. The cross-sectional area of skeletal muscle was evaluated based on CT at the third lumbar vertebrae. The subjects were divided into four groups according to the median levels of skeletal muscle index (SMI) and density (SMD).

RESULTS

Eighty-five men and twenty-two women had metabolic syndrome. In the adjusted model, SMI and SMD had an interaction effect on the number of metabolic syndrome components (p = 0.0398 in men and p = 0.0306 in women). When SMI and SMD were considered as independent variables for multiple regression analysis, SMI (β = -0.155, p = 0.0019 in men and β = -0.295, p < 0.0001 in women) and SMD (β = -0.355, p < 0.0001 in men and β = -0.324, p < 0.0001 in women) were both independently associated with the number of metabolic syndrome components. The subgroup with presence of low SMI and low SMD levels had a significantly higher prevalence of metabolic syndrome than that observed in other groups.

CONCLUSIONS

Therefore, we suggest that not only muscle quantity but also quality is independently associated with metabolic syndrome.

Dietary Fuels in Athletic Performance

Focusing on daily nutrition is important for athletes to perform and adapt optimally to exercise training. The major roles of an athlete's daily diet are to supply the substrates needed to cover the energy demands for exercise, to ensure quick recovery between exercise bouts, to optimize adaptations to exercise training, and to stay healthy. The major energy substrates for exercising skeletal muscles are carbohydrate and fat stores. Optimizing the timing and type of energy intake and the amount of dietary macronutrients is essential to ensure peak training and competition performance, and these strategies play important roles in modulating skeletal muscle adaptations to endurance and resistance training. In this review, recent advances in nutritional strategies designed to optimize exercise-induced adaptations in skeletal muscle are discussed, with an emphasis on mechanistic approaches, by describing the physiological mechanisms that provide the basis for different nutrition regimens.

Molecular Mechanisms in Skeletal Muscle Underlying Insulin Resistance in Women Who Are Lean With Polycystic Ovary Syndrome

CONTEXT

Skeletal muscle molecular mechanisms underlying insulin resistance in women with polycystic ovary syndrome (PCOS) are poorly understood.

OBJECTIVE

To provide insight into mechanisms regulating skeletal muscle insulin resistance in women who are lean with PCOS.

PARTICIPANTS AND METHODS

A hyperinsulinemic-euglycemic clamp with skeletal muscle biopsies was performed. Thirteen women who are lean who have hyperandrogenism and PCOS and seven age- and body mass index-matched healthy control subjects were enrolled. Skeletal muscle protein expression and phosphorylation were analyzed by Western blotting and intramuscular lipid content was measured by thin-layer chromatography.

RESULTS

Women with PCOS had 25% lower whole-body insulin sensitivity and 40% lower plasma adiponectin concentration than in control subjects. Intramuscular triacylglycerol, sn-1.3 diacylglycerol, and ceramide contents in skeletal muscle were higher (40%, 50%, and 300%, respectively) in women with PCOS than in control subjects. Activation of insulin signaling did not differ between groups. In women with PCOS, the insulin-stimulated glucose oxidation was reduced and insulin-stimulated dephosphorylation of pyruvate dehydrogenase (PDH) Ser293 was absent. AMP-activated protein kinase (AMPK) α2 protein expression and basal Thr172 phosphorylation were 45% and 50% lower in women with PCOS than in control subjects, respectively.

CONCLUSIONS

Whole-body insulin resistance in women who are lean who have hyperandrogenism and PCOS was not related to changes in the proximal part of the insulin signaling cascade in skeletal muscle despite lipid accumulation. Rather, reduced insulin sensitivity was potentially related to plasma adiponectin levels playing a modulating role in human skeletal muscle via AMPK. Furthermore, abnormal PDH regulation may contribute to reduced whole-body metabolic flexibility and thereby insulin resistance.

Reduced insulin action in muscle of high fat diet rats over the diurnal cycle is not associated with defective insulin signaling

Objective

Energy metabolism and insulin action follow a diurnal rhythm. It is therefore important that investigations into dysregulation of these pathways are relevant to the physiology of this diurnal rhythm.

Methods

We examined glucose uptake, markers of insulin action, and the phosphorylation of insulin signaling intermediates in muscle of chow and high fat, high sucrose (HFHS) diet-fed rats over the normal diurnal cycle.

Results

HFHS animals displayed hyperinsulinemia but had reduced systemic glucose disposal and lower muscle glucose uptake during the feeding period. Analysis of gene expression, enzyme activity, protein abundance and phosphorylation revealed a clear diurnal regulation of substrate oxidation pathways with no difference in Akt signaling in muscle. Transfection of a constitutively active Akt2 into the muscle of HFHS rats did not rescue diet-induced reductions in insulin-stimulated glucose uptake.

Conclusions

These studies suggest that reduced glucose uptake in muscle during the diurnal cycle induced by short-term HFHS-feeding is not the result of reduced insulin signaling.

•

Investigating metabolism in rodents over the diurnal cycle more accurately models normal animal physiology.

•

Diurnal regulation of substrate oxidation is altered in muscle of HFHS-fed rats.

•

There is a disconnect between glucose uptake and canonical insulin signaling in muscle.

•

Activation of Akt2 does not rescue diet-induced reductions in insulin-stimulated glucose uptake in muscle.

Amino acid transporters in the regulation of insulin secretion and signalling

Amino acids are increasingly recognised as modulators of nutrient disposal, including their role in regulating blood glucose through interactions with insulin signalling. More recently, cellular membrane transporters of amino acids have been shown to form a pivotal part of this regulation as they are primarily responsible for controlling cellular and circulating amino acid concentrations. The availability of amino acids regulated by transporters can amplify insulin secretion and modulate insulin signalling in various tissues. In addition, insulin itself can regulate the expression of numerous amino acid transporters. This review focuses on amino acid transporters linked to the regulation of insulin secretion and signalling with a focus on those of the small intestine, pancreatic β-islet cells and insulin-responsive tissues, liver and skeletal muscle. We summarise the role of the amino acid transporter B⁰AT1 (SLC6A19) and peptide transporter PEPT1 (SLC15A1) in the modulation of global insulin signalling via the liver-secreted hormone fibroblast growth factor 21 (FGF21). The role of vesicular vGLUT (SLC17) and mitochondrial SLC25 transporters in providing glutamate for the potentiation of insulin secretion is covered. We also survey the roles SNAT (SLC38) family and LAT1 (SLC7A5) amino acid transporters play in the regulation of and by insulin in numerous affective tissues. We hypothesise the small intestine amino acid transporter B⁰AT1 represents a crucial nexus between insulin, FGF21 and incretin hormone signalling pathways. The aim is to give an integrated overview of the important role amino acid transporters have been found to play in insulin-regulated nutrient signalling.

ADAMTS9 Regulates Skeletal Muscle Insulin Sensitivity Through Extracellular Matrix Alterations

The ADAMTS9 rs4607103 C allele is one of the few gene variants proposed to increase the risk of type 2 diabetes through an impairment of insulin sensitivity. We show that the variant is associated with increased expression of the secreted ADAMTS9 and decreased insulin sensitivity and signaling in human skeletal muscle. In line with this, mice lacking Adamts9 selectively in skeletal muscle have improved insulin sensitivity. The molecular link between ADAMTS9 and insulin signaling was characterized further in a model where ADAMTS9 was overexpressed in skeletal muscle. This selective overexpression resulted in decreased insulin signaling presumably mediated through alterations of the integrin β1 signaling pathway and disruption of the intracellular cytoskeletal organization. Furthermore, this led to impaired mitochondrial function in mouse muscle-an observation found to be of translational character because humans carrying the ADAMTS9 risk allele have decreased expression of mitochondrial markers. Finally, we found that the link between ADAMTS9 overexpression and impaired insulin signaling could be due to accumulation of harmful lipid intermediates. Our findings contribute to the understanding of the molecular mechanisms underlying insulin resistance and type 2 diabetes and point to inhibition of ADAMTS9 as a potential novel mode of treating insulin resistance.

A single day of mixed-macronutrient overfeeding does not elicit compensatory appetite or energy intake responses but exaggerates postprandial lipaemia during the next day in healthy young men

Discrete episodes of overconsumption may induce a positive energy balance and impair metabolic control. However, the effects of an ecologically relevant, single day of balanced macronutrient overfeeding are unknown. Twelve healthy men (of age 22 (sd 2) years, BMI 26·1 (sd 4·2) kg/m2) completed two 28 h, single-blind experimental trials. In a counterbalanced repeated measures design, participants either consumed their calculated daily energy requirements (energy balance trial (EB): 10 755 (sd 593) kJ) or were overfed by 50 % (overfeed trial (OF): 16 132 (sd 889) kJ) under laboratory supervision. Participants returned to the laboratory the next day, after an overnight fast, to complete a mixed-meal tolerance test (MTT). Appetite was not different between trials during day 1 (P&gt;0·211) or during the MTT in the fasted or postprandial state (P&gt;0·507). Accordingly, plasma acylated ghrelin, total glucagon-like peptide-1 and total peptide YY concentrations did not differ between trials during the MTT (all P&gt;0·335). Ad libitum energy intake, assessed upon completion of the MTT, did not differ between trials (EB 6081 (sd 2260) kJ; OF 6182 (sd 1960) kJ; P=0·781). Plasma glucose and insulin concentrations were not different between trials (P&gt;0·715). Fasted NEFA concentrations were lower in OF compared with EB (P=0·005), and TAG concentrations increased to a greater extent on OF than on EB during the MTT (P=0·009). The absence of compensatory changes in appetite-related variables after 1 d of mixed macronutrient overfeeding highlights the limited physiological response to defend against excess energy intake. This supports the concept that repeated discrete episodes of overconsumption may promote weight gain, while elevations in postprandial lipaemia may increase CVD risk.

Mechanisms Preserving Insulin Action during High Dietary Fat Intake

Prolonged intervention studies investigating molecular metabolism are necessary for a deeper understanding of dietary effects on health. Here we provide mechanistic information about metabolic adaptation to fat-rich diets. Healthy, slightly overweight men ingested saturated or polyunsaturated fat-rich diets for 6 weeks during weight maintenance. Hyperinsulinemic clamps combined with leg balance technique revealed unchanged peripheral insulin sensitivity, independent of fatty acid type. Both diets increased fat oxidation potential in muscle. Hepatic insulin clearance increased, while glucose production, de novo lipogenesis, and plasma triacylglycerol decreased. High fat intake changed the plasma proteome in the immune-supporting direction and the gut microbiome displayed changes at taxonomical and functional level with polyunsaturated fatty acid (PUFA). In mice, eucaloric feeding of human PUFA and saturated fatty acid diets lowered hepatic triacylglycerol content compared with low-fat-fed control mice, and induced adaptations in the liver supportive of decreased gluconeogenesis and lipogenesis. Intake of fat-rich diets thus induces extensive metabolic adaptations enabling disposition of dietary fat without metabolic complications.

Interorgan Metabolic Crosstalk in Human Insulin Resistance

Excessive energy intake and reduced energy expenditure drive the development of insulin resistance and metabolic diseases such as obesity and type 2 diabetes mellitus. Metabolic signals derived from dietary intake or secreted from adipose tissue, gut, and liver contribute to energy homeostasis. Recent metabolomic studies identified novel metabolites and enlarged our knowledge on classic metabolites. This review summarizes the evidence of their roles as mediators of interorgan crosstalk and regulators of insulin sensitivity and energy metabolism. Circulating lipids such as free fatty acids, acetate, and palmitoleate from adipose tissue and short-chain fatty acids from the gut effectively act on liver and skeletal muscle. Intracellular lipids such as diacylglycerols and sphingolipids can serve as lipotoxins by directly inhibiting insulin action in muscle and liver. In contrast, fatty acid esters of hydroxy fatty acids have been recently shown to exert a series of beneficial effects. Also, ketoacids are gaining interest as potent modulators of insulin action and mitochondrial function. Finally, branched-chain amino acids not only predict metabolic diseases, but also inhibit insulin signaling. Here, we focus on the metabolic crosstalk in humans, which regulates insulin sensitivity and energy homeostasis in the main insulin-sensitive tissues, skeletal muscle, liver, and adipose tissue.

24-h severe energy restriction impairs postprandial glycaemic control in young, lean males

Intermittent energy restriction (IER) involves short periods of severe energy restriction interspersed with periods of adequate energy intake, and can induce weight loss. Insulin sensitivity is impaired by short-term, complete energy restriction, but the effects of IER are not well known. In randomised order, fourteen lean men (age: 25 (sd 4) years; BMI: 24 (sd 2) kg/m2; body fat: 17 (4) %) consumed 24-h diets providing 100 % (10 441 (sd 812) kJ; energy balance (EB)) or 25 % (2622 (sd 204) kJ; energy restriction (ER)) of estimated energy requirements, followed by an oral glucose tolerance test (OGTT; 75 g of glucose drink) after fasting overnight. Plasma/serum glucose, insulin, NEFA, glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP) and fibroblast growth factor 21 (FGF21) were assessed before and after (0 h) each 24-h dietary intervention, and throughout the 2-h OGTT. Homoeostatic model assessment of insulin resistance (HOMA2-IR) assessed the fasted response and incremental AUC (iAUC) or total AUC (tAUC) were calculated during the OGTT. At 0 h, HOMA2-IR was 23 % lower after ER compared with EB (P<0·05). During the OGTT, serum glucose iAUC (P<0·001), serum insulin iAUC (P<0·05) and plasma NEFA tAUC (P<0·01) were greater during ER, but GLP-1 (P=0·161), GIP (P=0·473) and FGF21 (P=0·497) tAUC were similar between trials. These results demonstrate that severe energy restriction acutely impairs postprandial glycaemic control in lean men, despite reducing HOMA2-IR. Chronic intervention studies are required to elucidate the long-term effects of IER on indices of insulin sensitivity, particularly in the absence of weight loss.

Hepatic Insulin Clearance in Regulation of Systemic Insulin Concentrations-Role of Carbohydrate and Energy Availability

Hyperinsulinemia is the hallmark of insulin resistance in obesity, and the relative importance of insulin clearance, insulin resistance, and insulin hypersecretion has been widely debated. On the basis of recent experimental evidence, we summarize existing evidence to suggest hepatic insulin clearance as a major and immediate regulator of systemic insulin concentrations responding within days to altered dietary energy and, in particular, carbohydrate intake. Hepatic insulin clearance seems to be closely associated with opposite alterations in hepatic lipid content and glucose production, providing a potential mechanistic link to hepatic insulin sensitivity. The molecular regulation of insulin clearance in the liver is likely to involve changes in insulin binding and receptor internalization in response to the dietary alterations, the molecular mechanisms of which await further research.

Defining lipid mediators of insulin resistance - controversies and challenges

Essential elements of all cells, lipids play important roles in energy production, signalling and as structural components. Despite these critical functions, excessive availability and intracellular accumulation of lipid is now recognised as a major factor contributing to many human diseases, including obesity and diabetes. In the context of these metabolic disorders, ectopic deposition of lipid has been proposed to have deleterious effects of insulin action. While this relationship has been recognised for some time now, there is currently no unifying mechanism to explain how lipids precipitate the development of insulin resistance. This review summarises the evidence linking specific lipid molecules to the induction of insulin resistance, describing some of the current controversies and challenges for future studies in this field.

Post-Exercise Carbohydrate-Energy Replacement Attenuates Insulin Sensitivity and Glucose Tolerance the Following Morning in Healthy Adults

The carbohydrate deficit induced by exercise is thought to play a key role in increased post-exercise insulin action. However, the effects of replacing carbohydrate utilized during exercise on postprandial glycaemia and insulin sensitivity are yet to be determined. This study therefore isolated the extent to which the insulin-sensitizing effects of exercise are dependent on the carbohydrate deficit induced by exercise, relative to other exercise-mediated mechanisms. Fourteen healthy adults performed a 90-min run at 70% V˙O2max starting at 1600–1700 h before ingesting either a non-caloric artificially-sweetened placebo solution (CHO-DEFICIT) or a 15% carbohydrate solution (CHO-REPLACE; 221.4 ± 59.3 g maltodextrin) to precisely replace the measured quantity of carbohydrate oxidized during exercise. The alternate treatment was then applied one week later in a randomized, placebo-controlled, and double-blinded crossover design. A standardized low-carbohydrate evening meal was consumed in both trials before overnight recovery ahead of a two-hour oral glucose tolerance test (OGTT) the following morning to assess glycemic and insulinemic responses to feeding. Compared to the CHO-DEFICIT condition, CHO-REPLACE increased the incremental area under the plasma glucose curve by a mean difference of 68 mmol·L⁻¹ (95% CI: 4 to 132 mmol·L⁻¹; p = 0.040) and decreased the Matsuda insulin sensitivity index by a mean difference of −2 au (95% CI: −1 to −3 au; p = 0.001). This is the first study to demonstrate that post-exercise feeding to replaceme the carbohydrate expended during exercise can attenuate glucose tolerance and insulin sensitivity the following morning. The mechanism through which exercise improves insulin sensitivity is therefore (at least in part) dependent on carbohydrate availability and so the day-to-day metabolic health benefits of exercise might be best attained by maintaining a carbohydrate deficit overnight.