Immunometabolic Signatures of Circulating Monocytes in Humans with Obesity and Insulin Resistance

Obesity is associated with chronic inflammation and metabolic complications, including insulin resistance (IR). Immune cells drive inflammation through the rewiring of intracellular metabolism. However, the impact of obesity-related IR on the metabolism and functionality of circulating immune cells, like monocytes, remains poorly understood. To increase insight into the inter-individual variation of immunometabolic signatures among individuals and their role in the development of IR, we assessed systemic and tissue-specific IR and circulating immune markers, and we characterized metabolic signatures and cytokine secretion of circulating monocytes from 194 individuals with a BMI≥25kg/m2. Monocyte metabolic signatures were defined using extracellular acidification rates (ECAR) to estimate glycolysis and oxygen consumption rates (OCR) for oxidative metabolism. Although monocyte metabolic signatures and function based on cytokine secretion varied greatly among subjects, they were strongly associated with each other. The ECAR/OCR ratio, representing the balance between glycolysis and oxidative metabolism, was negatively associated with fasting insulin, systemic IR, and liver-specific IR. These results indicate that monocytes from individuals with IR were relatively more dependent on oxidative metabolism, while monocytes from more insulinsensitive individuals were more dependent on glycolysis. Additionally, circulating CXCL11 was negatively associated with the degree of systemic IR and positively with the ECAR/OCR ratio in monocytes, suggesting that individuals with high IR and a monocyte metabolic dependence on oxidative metabolism also have lower levels of circulating CXCL11. Our findings suggest that monocyte metabolism is related to obesity-associated IR progression and deepen insights into the interplay between innate immune cell metabolism and IR development in humans.

Physical activity and sedentary behavior show distinct associations with tissue-specific insulin sensitivity in adults with overweight

AIM

The aim of this study is to investigate associations between the physical activity (PA) spectrum (sedentary behavior to exercise) and tissue-specific insulin resistance (IR).

METHODS

We included 219 participants for analysis (median [IQR]: 61 [55; 67] years, BMI 29.6 [26.9; 32.0] kg/m² ; 60% female) with predominant muscle or liver IR, as determined using a 7-point oral glucose tolerance test (OGTT). PA and sedentary behavior were measured objectively (ActivPAL) across 7 days. Context-specific PA was assessed with the Baecke questionnaire. Multiple linear regression models (adjustments include age, sex, BMI, site, season, retirement, and dietary intake) were used to determine associations between the PA spectrum and hepatic insulin resistance index (HIRI), muscle insulin sensitivity index (MISI) and whole-body IR (HOMA-IR, Matsuda index).

RESULTS

In fully adjusted models, objectively measured total PA (standardized regression coefficient β = 0.17, p = 0.020), light-intensity PA (β = 0.15, p = 0.045) and moderate-to-vigorous intensity PA (β = 0.13, p = 0.048) were independently associated with Matsuda index, but not HOMA-IR (p > 0.05). A higher questionnaire-derived sport index and leisure index were associated with significantly lower whole-body IR (Matsuda, HOMA-IR) in men but not in women. Results varied across tissues: more time spent sedentary (β = -0.24, p = 0.045) and a higher leisure index (β = 0.14, p = 0.034) were respectively negatively and positively associated with MISI, but not HIRI. A higher sport index was associated with lower HIRI (β = -0.30, p = 0.007, in men only).

CONCLUSION

While we confirm a beneficial association between PA and whole-body IR, our findings indicate that associations between the PA spectrum and IR seem distinct depending on the primary site of insulin resistance (muscle or liver).

Cardiometabolic health improvements upon dietary intervention are driven by tissue-specific insulin resistance phenotype: A precision nutrition trial

Precision nutrition based on metabolic phenotype may increase the effectiveness of interventions. In this proof-of-concept study, we investigated the effect of modulating dietary macronutrient composition according to muscle insulin-resistant (MIR) or liver insulin-resistant (LIR) phenotypes on cardiometabolic health. Women and men with MIR or LIR (n = 242, body mass index [BMI] 25-40 kg/m², 40-75 years) were randomized to phenotype diet (PhenoDiet) group A or B and followed a 12-week high-monounsaturated fatty acid (HMUFA) diet or low-fat, high-protein, and high-fiber diet (LFHP) (PhenoDiet group A, MIR/HMUFA and LIR/LFHP; PhenoDiet group B, MIR/LFHP and LIR/HMUFA). PhenoDiet group B showed no significant improvements in the primary outcome disposition index, but greater improvements in insulin sensitivity, glucose homeostasis, serum triacylglycerol, and C-reactive protein compared with PhenoDiet group A were observed. We demonstrate that modulating macronutrient composition within the dietary guidelines based on tissue-specific insulin resistance (IR) phenotype enhances cardiometabolic health improvements. Clinicaltrials.gov registration: NCT03708419, CCMO registration NL63768.068.17.

The PERSonalized Glucose Optimization Through Nutritional Intervention (PERSON) Study: Rationale, Design and Preliminary Screening Results

Background: It is well-established that the etiology of type 2 diabetes differs between individuals. Insulin resistance (IR) may develop in different tissues, but the severity of IR may differ in key metabolic organs such as the liver and skeletal muscle. Recent evidence suggests that these distinct tissue-specific IR phenotypes may also respond differentially to dietary macronutrient composition with respect to improvements in glucose metabolism.

Objective: The main objective of the PERSON study is to investigate the effects of an optimal vs. suboptimal dietary macronutrient intervention according to tissue-specific IR phenotype on glucose metabolism and other health outcomes.

Methods: In total, 240 overweight/obese (BMI 25 – 40 kg/m²) men and women (age 40 – 75 years) with either skeletal muscle insulin resistance (MIR) or liver insulin resistance (LIR) will participate in a two-center, randomized, double-blind, parallel, 12-week dietary intervention study. At screening, participants undergo a 7-point oral glucose tolerance test (OGTT) to determine the hepatic insulin resistance index (HIRI) and muscle insulin sensitivity index (MISI), classifying each participant as either “No MIR/LIR,” “MIR,” “LIR,” or “combined MIR/LIR.” Individuals with MIR or LIR are randomized to follow one of two isocaloric diets varying in macronutrient content and quality, that is hypothesized to be either an optimal or suboptimal diet, depending on their tissue-specific IR phenotype (MIR/LIR). Extensive measurements in a controlled laboratory setting as well as phenotyping in daily life are performed before and after the intervention. The primary study outcome is the difference in change in disposition index, which is the product of insulin sensitivity and first-phase insulin secretion, between participants who received their hypothesized optimal or suboptimal diet.

Discussion: The PERSON study is one of the first randomized clinical trials in the field of precision nutrition to test effects of a more personalized dietary intervention based on IR phenotype. The results of the PERSON study will contribute knowledge on the effectiveness of targeted nutritional strategies to the emerging field of precision nutrition, and improve our understanding of the complex pathophysiology of whole body and tissue-specific IR.

Clinical Trial Registration:https://clinicaltrials.gov/ct2/show/NCT03708419, clinicaltrials.gov as NCT03708419.

Skeletal muscle fatty acid handling in insulin resistant men

Disturbances in skeletal muscle lipid metabolism may precede or contribute to the development of whole body insulin resistance. In this study, we examined fasting and postprandial skeletal muscle fatty acid (FA) handling in insulin resistant (IR) men. Thirty men with the metabolic syndrome (MetS) (National Cholesterol Education Program-Adult Treatment Panel III) were included in this sub-study to the LIPGENE study, and divided in two groups (IR and control) based on the median of insulin sensitivity (S(I) = 2.06 (mU/l(-1))·min(-1)·10(-4)). Fasting and postprandial skeletal muscle FA handling were examined by combining the forearm balance technique with stable isotopes of palmitate. [(2)H(2)]-palmitate was infused intravenously to label endogenous triacylglycerol (TAG) and free FAs (FFAs) in the circulation and [U-(13)C]-palmitate was incorporated in a high-fat mixed meal (2.6 MJ, 61 E% fat) to label chylomicron-TAG. Muscle biopsies were taken to determine muscle TAG, diacylglycerol (DAG), FFA, and phospholipid (PL) content, their fractional synthetic rates (FSRs) and degree of saturation, as well as messenger RNA (mRNA) expression of genes involved in lipid metabolism. In the first 2 h after meal consumption, forearm muscle [(2)H(2)]-labeled TAG extraction was higher in IR vs. control (P = 0.05). Fasting percentage saturation of muscle DAG was higher in IR vs. control (P = 0.016). No differences were observed for intramuscular TAG, DAG, FFA, and PL content, FSR, and muscle mRNA expression. In conclusion, increased muscle (hepatically derived) TAG extraction during postprandial conditions and increased saturation of intramuscular DAG are associated with insulin resistance, suggesting that disturbances in skeletal muscle FA handling could play a role in the development of whole body insulin resistance and type 2 diabetes.

Effects of dietary fat modification on skeletal muscle fatty acid handling in the metabolic syndrome

OBJECTIVE

In the metabolic syndrome (MetS), increased fat storage in 'nonadipose' tissues such as skeletal muscle may be related to insulin resistance ('lipid overflow' hypothesis). The objective of this study was to examine the effects of dietary fat modification on the capacity of skeletal muscle to handle dietary and endogenous fatty acids (FAs).

SUBJECTS AND METHODS

In total, 29 men with the MetS were randomly assigned to one of four diets for 12 weeks: a high-fat saturated fat diet (HSFA, n=6), a high-fat monounsaturated fat diet (HMUFA, n=7) and two low-fat high-complex carbohydrate diets supplemented with (LFHCCn-3, n=8) or without (LFHCC, n=8) 1.24 g per day docosahexaenoic and eicosapentaenoic acid. Fasting and postprandial skeletal muscle FA handling was examined by measuring arteriovenous concentration differences across the forearm muscle. [(2)H(2)]-palmitate was infused intravenously to label endogenous triacylglycerol (TAG) and free fatty acids in the circulation and subjects received a high-fat mixed meal (2.6 MJ, 61 energy% fat) containing [U-(13)C]-palmitate to label chylomicron-TAG.

RESULTS

Postprandial circulating TAG concentrations were significantly lower after dietary intervention in the LFHCCn-3 group compared to the HSFA group (DeltaiAUC -139+/-67 vs 167+/-70 micromol l(-1) min(-1), P=0.009), together with decreased concentrations of [U-(13)C]-labeled TAG, representing dietary FA. Fasting TAG clearance across forearm muscle was decreased on the HSFA diet, whereas no differences were observed in postprandial forearm muscle FA handling between diets.

CONCLUSION

Chronic manipulation of dietary fat quantity and quality did not affect forearm muscle FA handling in men with the MetS. Postprandial TAG concentrations decreased on the LFHCCn-3 diet, which could be (partly) explained by lower concentration of dietary FA in the circulation.

Fasting and postprandial remnant-like particle cholesterol concentrations in obese participants are associated with plasma triglycerides, insulin resistance, and body fat distribution

Elevated plasma concentrations of remnant-like particle cholesterol (RLP-C) are atherogenic. However, factors that determine RLP-C are not fully understood. This study evaluates which factors affect RLP-C in the fasting and postprandial state, using multiple regression analyses in a large cohort of lean and obese participants. All participants (n = 740) underwent a test meal challenge containing 95 energy % (en%) fat (energy content 50% of predicted daily resting metabolic rate). Fasting and postprandial concentrations of circulating metabolites were measured over a 3-h period. Obese participants (n = 613) also participated in a 10-wk weight loss program (-2510 kJ/d), being randomized to either a low-fat or a high-fat diet (20-25 vs. 40-45en% fat). Postprandial RLP-C was associated with fasting RLP-C, waist:hip ratio (WHR), HOMA(IR) (homeostasis model assessment index for insulin resistance) (P < 0.001), and age, independently of BMI and gender [adjusted R(2) (adj. R(2)) = 0.70). These factors were also related to fasting RLP-C (P < 0.010), along with gender and physical activity (adj. R(2) = 0.23). The dietary intervention resulted in significantly lower fasting RLP-C concentrations, independently mediated by weight loss, improvements in HOMA(IR), and the fat content of the prescribed diet. However, after inclusion of plasma triglyceride (TG), HDL-cholesterol, and FFA concentrations in the models, HOMA(IR) and WHR no longer significantly predicted fasting RLP-C, although WHR remained a predictor of postprandial RLP-C (P = 0.002). Plasma TG was strongly associated with both fasting and postprandial RLP-C (P < 0.001). In conclusion, plasma RLP-C concentrations are mainly associated with plasma TG concentrations. Interestingly, the high-fat diet was more effective at decreasing fasting RLP-C concentrations in obese participants than the low-fat diet.

Adipose triglyceride lipase and hormone-sensitive lipase protein expression is decreased in the obese insulin-resistant state

AIM/HYPOTHESIS

Obesity is associated with increased triacylglycerol (TAG) storage in adipose tissue and insulin resistance. The mobilization of stored TAG is mediated by hormone-sensitive lipase (HSL) and the recently discovered adipose triglyceride lipase (ATGL). The aim of the present study was to examine whether ATGL and HSL mRNA and protein expression are altered in insulin-resistant conditions. In addition, we investigated whether a possible impaired expression could be reversed by a period of weight reduction.

METHODS

Adipose tissue biopsies were taken from obese subjects (n = 44) with a wide range of insulin resistance, before and just after a 10-wk hypocaloric diet. ATGL and HSL protein and mRNA expression was determined by Western blot and quantitative RT-PCR, respectively.

RESULTS

Fasting insulin levels and the degree of insulin resistance (using the homeostasis model assessment index for insulin resistance) were negatively correlated with ATGL and HSL protein expression, independent of age, gender, fat cell size, and body composition. Both mRNA and protein levels of ATGL and HSL were reduced in insulin-resistant compared with insulin-sensitive subjects (P < 0.05). Weight reduction significantly decreased ATGL and HSL mRNA and protein expression. A positive correlation between the decrease in leptin and the decrease in ATGL protein level after weight reduction was observed. Finally, ATGL and HSL mRNA and protein levels seem to be highly correlated, indicating a tight coregulation and transcriptional control.

CONCLUSIONS

In obese subjects, insulin resistance and hyperinsulinemia are strongly associated with ATGL and HSL mRNA and protein expression, independent of fat mass. Data on weight reduction indicated that also other factors (e.g. leptin) relate to ATGL and HSL protein expression.

Effect of beta1- and beta2-adrenergic stimulation on energy expenditure, substrate oxidation, and UCP3 expression in humans

In humans, beta-adrenergic stimulation increases energy and fat metabolism. In the case of beta1-adrenergic stimulation, it is fueled by an increased lipolysis. We examined the effect of beta2-adrenergic stimulation, with and without a blocker of lipolysis, on thermogenesis and substrate oxidation. Furthermore, the effect of beta1-and beta2-adrenergic stimulation on uncoupling protein 3 (UCP3) mRNA expression was studied. Nine lean males received a 3-h infusion of dobutamine (DOB, beta1) or salbutamol (SAL, beta2). Also, we combined SAL with acipimox to block lipolysis (SAL+ACI). Energy and substrate metabolism were measured continuously, blood was sampled every 30 min, and muscle biopsies were taken before and after infusion. Energy expenditure significantly increased approximately 13% in all conditions. Fat oxidation increased 47 +/- 7% in the DOB group and 19 +/- 7% in the SAL group but remained unchanged in the SAL+ACI condition. Glucose oxidation decreased 40 +/- 9% upon DOB, remained unchanged during SAL, and increased 27 +/- 11% upon SAL+ACI. Plasma free fatty acid (FFA) levels were increased by SAL (57 +/- 11%) and DOB (47 +/- 16%), whereas SAL+ACI caused about fourfold lower FFA levels compared with basal levels. No change in UCP3 was found after DOB or SAL, whereas SAL+ACI downregulated skeletal muscle UCP3 mRNA levels 38 +/- 13%. In conclusion, beta2-adrenergic stimulation directly increased energy expenditure independently of plasma FFA levels. Furthermore, this is the first study to demonstrate a downregulation of skeletal muscle UCP3 mRNA expression after the lowering of plasma FFA concentrations in humans, despite an increase in energy expenditure upon beta2-adrenergic stimulation.

Long-term effects of low-intensity exercise training on fat metabolism in weight-reduced obese men

The aim of the present study was to investigate the effect of long-term continuation of low-intensity exercise training on weight maintenance, substrate metabolism, and beta-adrenergic-mediated fat oxidation in weight-reduced obese men. Preceding this part of the study, subjects lost 15 +/- 6 kg of body weight by energy restriction with or without low-intensity exercise training. Twenty-nine subjects (diet group, n = 15; diet + exercise group, n = 14) participated in the follow-up study of 40 weeks in which the former diet + exercise group continued their exercise training program. Pre- and postfollow-up, measurements of body weight, body composition, maximal aerobic capacity and substrate oxidation during rest, exercise, and recovery with or without infusion of the beta-adrenergic antagonist, propranolol (PRP), were performed. Over the follow-up period, body weight, fat mass, and fat free mass increased in both groups (P <.0001) without differences between groups. Attendance at exercise training sessions was negatively correlated with regain of body weight (r = -.6, P <.05). Relative fat oxidation, energy expenditure, and beta-adrenergic-mediated fat oxidation during rest, exercise, and recovery were maintained over the follow-up period in both groups. Continuation of low-intensity exercise training after weight reduction did not limit regain of body weight, unless exercise training was frequently performed. Relative (beta-adrenergic-mediated) fat oxidation and energy expenditure were maintained at postdiet level whether or not low-intensity exercise training was performed during follow-up.

Short-term effects of weight loss with or without low-intensity exercise training on fat metabolism in obese men

BACKGROUND

Energy restriction is known to induce a decline in fat oxidation during the postdiet period. Reduced fat oxidation may contribute to weight regain.

OBJECTIVE

The present study investigated the effect of the addition of low-intensity exercise training to energy restriction on postdiet fat oxidation and on the contribution of the sympathetic nervous system to fat oxidation.

DESIGN

Forty obese men were divided randomly into 2 groups: diet (D) and diet plus exercise (DE). Both groups followed an energy restriction program for 10 wk. Subjects in the DE group also participated in a low-intensity exercise training program [40% maximal oxygen uptake (VO2max)] for 12 wk. Before the intervention and after 12 wk, with subjects at stable body weights, we measured body composition, VO2max, and substrate oxidation at rest, during exercise at 50% VO2max, and during recovery. Measurements were made with and without administration of the beta-adrenergic antagonist propranolol.

RESULTS

Both interventions led to significant decreases in body weight, fat mass, and fat-free mass (P < 0.001); these decreases did not differ significantly between the D and DE groups. Neither intervention significantly affected VO2max. The effect of the intervention on the respiratory exchange ratio differed significantly between the D and DE groups [two-way analysis of variance (ANOVA), P < 0.05]. The effect on the beta-adrenergic-mediated respiratory exchange ratio tended to be different between the 2 groups (two-way ANOVA, P = 0.09).

CONCLUSION

Addition of low-intensity exercise training to energy restriction counteracts the decline in fat oxidation during the postdiet period.

The effect of low-intensity exercise training on fat metabolism of obese women

OBJECTIVE

Previous studies have shown that fat metabolism is different in upper body (UB) and lower body (LB) obese women. The present study investigated whether the effect of low-intensity exercise training on fat metabolism is different in UB and LB obese premenopausal women.

RESEARCH METHODS AND PROCEDURES

Twenty-one healthy, premenopausal women with either LB obesity (waist-to-hip ratio of < or =0.79; n = 8) or UB obesity (waist-to-hip ratio of > or =0.85; n = 13) participated in the present study. The UB obese women were matched and randomly divided in an exercise training group (UB) and a nonexercising control group (UB-C). Subjects in the UB and LB groups participated in a low-intensity exercise training program (40% VO2max) three times per week for 12 weeks. Before and after the intervention, measurements of fat metabolism at rest and during exercise, body composition, and maximal aerobic capacity were performed.

RESULTS

Exercise training did not change the respiratory exchange ratio at rest in the UB and LB groups. During exercise, relative fat oxidation increased in the UB group by 19% (p < 0.05), whereas no change in the LB and UB-C groups was found. Plasma free fatty acid oxidation did not change by exercise training, and nonplasma fatty acid oxidation tended to increase in the UB group compared with the UB-C group (p = 0.08).

DISCUSSION

Low-intensity exercise training increased the contribution of fat oxidation to total energy expenditure during exercise but not at rest in UB obese women. Exercise training had no significant effect on fat metabolism in the LB obese women.