Jeffrey F. Horowitz

Objectives: To quantify the association between skeletal muscle echo intensity (MEI), measured by ultrasound, and clinical markers of insulin resistance and glycemic control, and evaluate MEI's diagnostic accuracy in identifying insulin resistance. Methods: In this cross‐sectional study, 20 adults with obesity (mean body mass index [BMI] 34.4 ± 2.6 kg/m ² , mean age 33 years, 40% female) and 8 healthy, lean adults (mean BMI 22.5 ± 1.4 kg/m ² , mean age 25 years, 75% female), all without diabetes or metabolic disease, underwent laboratory testing (HbA _1c , 2‐hour oral glucose tolerance testing with insulin for Matsuda Index), muscle ultrasound (deltoid, vastus lateralis), and DEXA for sarcopenia indices. Two blinded research assistants independently analyzed 336 ultrasound images to quantify MEI. Results: Increased MEI was significantly associated with greater insulin resistance (lower Matsuda Index; r = −.47, p = .011), particularly in women ( r = −.56, p = .039). MEI z‐scores identified insulin resistance with an AUROC of 0.872 (95% CI 0.742–1.000). At the optimal threshold of z = 1.96, sensitivity was 94.4%, specificity 80%, accuracy 89.3%, and Youden's index 0.744. MEI accurately identified insulin resistance despite normal HbA _1c and fasting glucose. Increased MEI in both muscles suggested global skeletal muscle changes. Among participants with obesity, MEI did not correlate with BMI but was negatively correlated with sarcopenia indices ( r = −.56, p = .0096) and body weight ( r = −.50, p = .0233). Conclusions: MEI is an accurate, noninvasive biomarker for insulin resistance and may detect muscle alterations before conventional markers emerge. Its independence from BMI and conventional markers supports MEI's use in early risk stratification and identification of individuals at risk for metabolic dysfunction who might otherwise go undetected.

The primary aim of this study was to compare the acute effects of three exercise intensities on abdominal subcutaneous adipose tissue (aSAT) transcriptome in regular exercisers. A total of 45 adults who exercise regularly were assigned to perform a single session of either low‐intensity continuous (LOW; 60 min at 30% VO ₂ max; n = 15), moderate‐intensity continuous (MOD; 45 min at 65% VO ₂ max; n = 15) or high‐intensity interval exercise (HIGH; 10 × 1 min at 90% VO ₂ max interspersed with 1 min active recovery; n = 15). aSAT biopsy samples were collected before and 1.5 h after the exercise session for bulk RNA sequencing and targeted protein immunoassays. HIGH upregulated genes were involved in angiogenesis, protein secretion and insulin signalling pathways, whereas MOD and LOW upregulated genes regulated extracellular matrix (ECM) remodelling, ribosome biogenesis and oxidative phosphorylation pathways. Exercise‐induced changes in aSAT clock genes, ERK protein phosphorylation and circulating cytokines were similar after all three exercise treatments. Network analysis identified exercise‐responsive gene clusters linked to cardiometabolic health traits. Cell‐type analysis highlighted a heterogeneous response of aSAT cell types to exercise, with distinct patterns observed across exercise intensities. Collectively, our data characterize early responses in aSAT after a single session of exercise. Because adaptations to exercise training stem from an accrual of responses after each session of exercise, these early responses to exercise are likely important contributors to the long‐term structural and functional changes that occur in adipose tissue in response to exercise training. image Key points: Chronic adaptations in adipose tissue from regular exercise support cardiometabolic health, but the acute molecular triggers of these adaptations remain unclear. We show that acute exercise alters gene expression, along with ERK phosphorylation in adipose tissue of regular exercisers. Exercise intensity shapes the transcriptomic response: high‐intensity exercise induces inflammatory, cytokine and genes, whereas lower intensities upregulate genes involved in protein translation and oxidative phosphorylation. Network and cell‐type analyses highlight intensity‐specific adipose responses, revealing gene modules linked to health traits and differential engagement of adipocyte subpopulations.

The primary aim of this study was to compare the acute effects of three exercise intensities on abdominal subcutaneous adipose tissue (aSAT) transcriptome in regular exercisers. A total of 45 adults who exercise regularly were assigned to perform a single session of either low-intensity continuous (LOW; 60min at 30% VO ₂ max; n=15), moderate-intensity continuous (MOD; 45min at 65% VO ₂ max; n=15), or high-intensity interval exercise (HIGH; 10×1min at 90% VO ₂ max interspersed with 1min active recovery; n=15). aSAT biopsy samples were collected before and 1.5hours after the exercise session for bulk RNA sequencing and targeted protein immunoassays. HIGH upregulated genes involved in cytokine secretion, insulin signaling, and proteolysis while MOD and LOW upregulated genes regulating ECM remodeling, ribosome biogenesis, and oxidative phosphorylation pathways. Exercise-induced changes in aSAT angiogenic, MAPK cascade, and clock genes, ERK protein phosphorylation, and circulating cytokines were similar after all three exercise treatments. Network analysis identified exercise-responsive gene clusters linked to cardiometabolic health traits. Cell-type analysis highlighted a heterogeneous response of aSAT cell types to exercise, with distinct patterns observed across exercise intensities. Collectively, our data characterizes early responses in aSAT after a single session of exercise. Because adaptations to exercise training stem from an accrual of responses after each session of exercise, these early responses to exercise are likely important contributors to the long-term structural and functional changes that occur in adipose tissue in response to exercise training.

Endurance exercise is widely recognized for its role in mitigating insulin resistance, yet the precise mechanisms remain unclear. In this Classics in Diabetes article, we revisit the article by Amati et al., “Skeletal Muscle Triglycerides, Diacylglycerols, and Ceramides in Insulin Resistance: Another Paradox in Endurance-Trained Athletes?” Published in the October 2011 issue of Diabetes, this article was among the first to highlight the nuanced roles of exercise-induced changes in bioactive lipids such as ceramide and diacylglycerol (DAG) in insulin signaling. The authors’ groundbreaking work challenged some existing paradigms, revealing a more complex relationship between DAGs and insulin resistance than previously thought. Their findings helped lay the foundation for further exploration to unravel the intricate biochemical pathways through which exercise influences insulin sensitivity and metabolic health.

Aim: To examine factors underlying why most, but not all, adults with obesity exhibit impaired insulin‐mediated glucose uptake, we compared: (1) adipose tissue fatty acid (FA) release, (2) skeletal muscle lipid droplet (LD) characteristics, and (3) insulin signalling events, in skeletal muscle of adults with obesity with relatively high versus low insulin‐mediated glucose uptake.Methods: Seventeen adults with obesity (BMI: 36 ± 3 kg/m²) completed a 2 h hyperinsulinemic–euglycemic clamp with stable isotope tracer infusions to measure glucose rate of disappearance (glucose Rd) and FA rate of appearance (FA Ra). Skeletal muscle biopsies were collected at baseline and 30 min into the insulin infusion. Participants were stratified into HIGH (n = 7) and LOW (n = 10) insulin sensitivity cohorts by their glucose Rd during the hyperinsulinemic clamp (LOW< 400; HIGH >550 nmol/kgFFM/min/[μU/mL]).Results: Insulin‐mediated suppression of FA Ra was lower in LOW compared with HIGH (p < 0.01). In skeletal muscle, total intramyocellular lipid content did not differ between cohorts. However, the size of LDs in the subsarcolemmal region (SS) of type II muscle fibres was larger in LOW compared with HIGH (p = 0.01). Additionally, insulin receptor‐β (IRβ) interactions with regulatory proteins CD36 and Fyn were lower in LOW versus HIGH (p < 0.01), which aligned with attenuated insulin‐mediated Tyr phosphorylation of IRβ and downstream insulin‐signalling proteins in LOW.Conclusion: Collectively, reduced ability for insulin to suppress FA mobilization, with accompanying modifications in intramyocellular LD size and distribution, and diminished IRβ interaction with key regulatory proteins may be key contributors to impaired insulin‐mediated glucose uptake commonly found in adults with obesity.

Abnormalities in the structure and metabolic function of abdominal subcutaneous adipose tissue (aSAT) underlie many obesity-related health complications. Endurance exercise improves cardiometabolic health in adults with overweight or obesity, but the effects of endurance training on aSAT are unclear. We included male and female participants who were regular exercisers with overweight or obesity who exercised for >2 years, and cross-sectionally compared them with well-matched non-exercisers with overweight or obesity. Here we show aSAT from exercisers has a higher capillary density, lower Col6a abundance and fewer macrophages compared with non-exercisers. This is accompanied by a greater abundance of angiogenic, ribosomal, mitochondrial and lipogenic proteins. The abundance of phosphoproteins involved in protein translation, lipogenesis and direct regulation of transcripts is also greater in aSAT collected from exercisers. Exploratory ex vivo experiments demonstrate greater angiogenic capacity and higher lipid-storage capacity in samples cultured from aSAT collected from exercisers versus non-exercisers. Regular exercise may play a role in remodelling aSAT structure and proteomic profile in ways that may contribute to preserved cardiometabolic health.

To examine factors underlying why most, but not all adults with obesity exhibit impaired insulin-mediated glucose uptake, we compared: 1) rates of fatty acid (FA) release from adipose tissue, 2) skeletal muscle lipid droplet (LD) characteristics, and 3) insulin signaling events in skeletal muscle collected from cohorts of adults with obesity with “HIGH” versus “LOW” insulin sensitivity for glucose uptake. Seventeen adults with obesity (BMI: 36±3kg/m ² ) completed a 2h hyperinsulinemic-euglycemic clamp with stable isotope tracer infusions to measure glucose rate of disappearance (glucose Rd) and FA rate of appearance (FA Ra). Skeletal muscle biopsies were collected at baseline and 30min into the insulin infusion. Participants were stratified into HIGH (n=7) and LOW (n=10) insulin sensitivity cohorts by their glucose Rd during the hyperinsulinemic clamp (LOW550 nmol/kgFFM/min/[µU/mL]). Insulin-mediated suppression of FA Ra was lower in LOW compared with HIGH (p<0.01). In skeletal muscle, total intramyocellular lipid content did not differ between cohorts. However, the size of LDs in the subsarcolemmal region (SS) of type II muscle fibers was larger in LOW compared with HIGH (p=0.01). Additionally, insulin receptor (IR) interactions with regulatory proteins CD36 and Fyn were lower in LOW versus HIGH (p<0.01), which aligned with attenuated insulin-mediated Tyr phosphorylation of IRβ and downstream insulin-signaling proteins in LOW. Collectively, reduced ability for insulin to suppress FA mobilization, with accompanying modifications in intramyocellular LD size and distribution, and diminished IR interaction with key regulatory proteins may be key contributors to impaired insulin-mediated glucose uptake commonly found in adults with obesity. KEY POINTS: Although most adults with obesity exhibit impaired insulin-mediated glucose uptake (insulin resistance), some remain sensitive to insulin. Factors that “protect” adults with obesity from developing resistance to insulin-mediated glucose uptake are poorly understood. Potent suppression of fatty acid (FA) mobilization from adipose tissue by insulin is a strong predictor of whole-body insulin-mediated glucose uptake. Participants with higher sensitivity for insulin-mediated glucose uptake had smaller intramyocellular lipid droplets (LDs) within the subsarcolemmal region of type II skeletal muscle fibers. Novel findings revealed that insulin receptor (IR) interaction with the long-chain fatty acid transport protein, CD36, and the Src-family kinase, Fyn, directly associated with higher rates of glucose uptake under basal and hyperinsulinemic conditions. Together, the findings suggest impaired suppression of FA release from adipose tissue associates with reduced glucose uptake in skeletal muscle due in part to a defect in IR activation by CD36/Fyn and altered subcellular LD characteristics.

In this study, 12 wk of either moderate-intensity continuous training (MICT) or high-intensity interval training (HIIT) enhanced skeletal muscle lipid abundance by increasing lipid droplet number within the intramyofibrillar (IMF) region of muscle. Because the IMF associates with high energy production during muscle contraction, this adaptation may enhance lipid oxidation during exercise. Despite differences in training intensity and energy expenditure between MICT and HIIT, their effects on muscle lipid abundance and metabolism were remarkably similar.

Objective: The aims of this study were: 1) to assess relationships among insulin‐mediated glucose uptake with standard clinical outcomes and deep‐phenotyping measures (including fatty acid [FA] rate of appearance [FA Ra] into the systemic circulation); and 2) to examine the contribution of adipocyte size, fibrosis, and proteomic profile to FA Ra regulation.Methods: A total of 66 adults with obesity (BMI = 34 [SD 3] kg/m²) were assessed for insulin sensitivity (hyperinsulinemic‐euglycemic clamp), and stable isotope dilution methods quantified glucose, FA, and glycerol kinetics in vivo. Abdominal subcutaneous adipose tissue (aSAT) and skeletal muscle biopsies were collected, and magnetic resonance imaging quantified liver and visceral fat content.Results: Insulin‐mediated FA Ra suppression associated with insulin‐mediated glucose uptake (r = 0.51; p < 0.01) and negatively correlated with liver (r = −0.36; p < 0.01) and visceral fat (r = −0.42; p < 0.01). aSAT proteomics from subcohorts of participants with low FA Ra suppression (n = 8) versus high FA Ra suppression (n = 8) demonstrated greater extracellular matrix collagen protein in low versus high FA Ra suppression. Skeletal muscle lipidomics (n = 18) revealed inverse correlations of FA Ra suppression with acyl‐chain length of acylcarnitine (r = −0.42; p = 0.02) and triacylglycerol (r = −0.51; p < 0.01), in addition to insulin‐mediated glucose uptake (acylcarnitine: r = −0.49; p < 0.01, triacylglycerol: r = −0.40; p < 0.01).Conclusions: Insulin's ability to suppress FA release from aSAT in obesity is related to enhanced insulin‐mediated glucose uptake and metabolic health in peripheral tissues.

Effective storage of excess energy in abdominal subcutaneous adipose tissue during periods of overeating may help attenuate weight-gain-related insulin resistance. The objective of this study was to assess changes in the expression of factors regulating abdominal subcutaneous adipose tissue storage capacity in response to a brief exposure to overeating in nonobese adults. Because exercise can alter the expression of genes involved in regulating adipose tissue storage capacity, we compared the responses to overeating in regular exercisers (EX, n = 11) and nonexercisers (nonEX, n = 11). Abdominal subcutaneous adipose tissue samples and oral glucose tolerance tests were performed before and after participants ate 30% above their estimated daily energy requirements for 1 week. Both EX and nonEX gained ∼1 kg (P < 0.01), and Matsuda insulin sensitivity index was reduced ∼15% (P = 0.04) in both groups. Gene expression of factors involved in lipid metabolism (HSL, ATGL, DGAT, and PPARγ) and angiogenesis (HIF1α and KDR) were increased (P < 0.05), with no differences observed between EX and nonEX. In contrast, protein abundance of these factors did not change. The modest overeating stimulus did not increase markers of inflammation in the systemic circulation or adipose tissue. Overall, our findings indicate that a brief and modest overeating stimulus can impair insulin sensitivity and upregulate genes involved in abdominal adipose tissue storage capacity similarly in exercisers and nonexercisers. ClinicalTrials.gov ID#: NCT02701738.