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. Author manuscript; available in PMC: 2022 May 1.
Published in final edited form as: Diabetologia. 2021 Jan 29;64(5):1158–1168. doi: 10.1007/s00125-021-05383-w

Heterogeneity in insulin-stimulated glucose uptake among different muscle groups in healthy lean people and people with obesity

Han-Chow E Koh 1, Stephan van Vliet 1, Gretchen A Meyer 2, Richard Laforest 3, Robert J Gropler 3, Samuel Klein 1, Bettina Mittendorfer 1
PMCID: PMC8336476  NIHMSID: NIHMS1723651  PMID: 33511440

Abstract

Aims/hypothesis:

It has been proposed that muscle fibre-type composition and perfusion are key determinants of insulin-stimulated muscle glucose uptake and alterations in muscle fibre-type composition and perfusion contribute to muscle, and consequently whole-body insulin resistance in people with obesity. The goal of the study was to evaluate the relationships among muscle fibre-type composition, perfusion, and insulin-stimulated glucose uptake rates in healthy, lean people and people with obesity.

Methods:

We measured insulin-stimulated whole-body glucose disposal and glucose uptake and perfusion rates in five major muscle groups (erector spinae, obliques, rectus abdominis, hamstrings, quadriceps) in 15 healthy lean people and 37 people with obesity by using the hyperinsulinaemic-euglycaemic clamp procedure in conjunction with [2H]glucose-tracer infusion (to assess whole-body glucose disposal) and positron emission tomography after injections of [15O]H2O (to assess muscle perfusion) and [18F]fluorodeoxyglucose (to assess muscle glucose uptake). A biopsy from the vastus lateralis was obtained to assess fibre-type composition.

Results:

We found: i) a two-fold difference in glucose uptake rates among muscles in both the Lean and Obese groups [rectus abdominis: 67 (51, 78) and 32 (21, 55) μmol/kg/min in the Lean and Obese groups, respectively; erector spinae: 134 (103, 160) and 66 (24, 129) μmol/kg/min, respectively; median(IQR)] that was unrelated to perfusion or fibre-type composition (assessed in the vastus only), ii) the impairment in insulin action in the Obese compared with the Lean group was not different among muscle groups, and iii) insulin-stimulated whole-body glucose disposal expressed per kg fat-free mass was linearly related with muscle glucose uptake rate (R2=0.65, p<0.05).

Conclusion:

Obesity-associated insulin resistance is generalized across all major muscles, and is not caused by alterations in muscle fibre-type composition or perfusion. In addition, insulin-stimulated whole-body glucose disposal relative to fat-free mass provides a reliable index of muscle glucose uptake rate.

Keywords: glucose uptake, glucose disposal, perfusion, insulin resistance

Graphical Abstract

graphic file with name nihms-1723651-f0001.jpg

Introduction

Insulin-stimulated glucose uptake in skeletal muscles is important for maintaining plasma glucose homeostasis. Impaired insulin-stimulated muscle glucose uptake is common in people with obesity and is a major risk factor for type 2 diabetes [1, 2]. Insulin promotes muscle glucose uptake through several mechanisms, including an insulin-mediated increase in muscle perfusion and concomitant increase in both insulin and glucose delivery to muscles, increased GLUT4 translocation to the plasma membrane to facilitate glucose transport into myocytes, and intracellular glucose phosphorylation [3]. In rodent models, insulin-stimulated glucose uptake rate, assessed both in vivo and ex vivo, varies several-fold among different muscles and is generally greater in muscles that contain primarily type I fibres than those that contain primarily fast glycolytic type II fibres, presumably because of greater capillary density (and subsequently perfusion) and GLUT4 content in type I fibre-rich muscles [4–6]. In addition, the susceptibility to lipid and high-fat diet-induced insulin resistance differs among different myofibre types and muscles in rodent models. High-fat feeding impairs insulin-stimulated glucose uptake in isolated type IIx, but not type I and type IIa, fibres [7], and lipid infusion and high-fat feeding impair insulin-stimulated glucose uptake in some but not all muscles [8, 9]. However, it is not known whether insulin-stimulated glucose uptake rates differ among different muscles in people and whether obesity causes insulin resistance in specific muscle groups only, because muscle insulin sensitivity is typically assessed as the glucose infusion rate needed to maintain euglycaemia during a hyperinsulinaemic-euglycaemic clamp procedure or insulin-stimulated whole-body or limb glucose disposal rate [2, 10–13].

The primary goal of the present study was to evaluate: i) glucose uptake rates in different skeletal muscles in healthy lean people (Lean group) and people with obesity (Obese group) and ii) the relationships between muscle perfusion, fibre type composition and glucose uptake. A secondary goal was to evaluate the common assumption that insulin-stimulated whole-body glucose disposal is a reliable surrogate measure of muscle glucose uptake [2, 10, 12, 13]. To this end, we measured insulin-stimulated whole-body glucose disposal and skeletal muscle perfusion and glucose uptake rates by using a hyperinsulinaemic-euglycaemic pancreatic clamp procedure in conjunction with [6,6-2H2]glucose tracer infusion and positron emission tomography (PET) after bolus injections of 15O-water ([15O]H2O) and 18F-fluorodeoxyglucose ([18F]FDG). Specific muscle groups in the torso (erector spinae, obliques, rectus abdominis) and thigh (hamstrings, quadriceps) were studied because they differ in their fibre type composition; muscles along the spine have more type I fibres than abdominal muscles, muscles in the torso have more type I fibres than leg muscles, and the hamstrings have more type I fibres than the quadriceps [14, 15]. We hypothesized that: i) insulin-stimulated muscle glucose uptake rate varies among different muscle groups as a function of muscle perfusion and fibre type composition and is therefore greater in the erector spinae than abdominal muscles, greater in muscles in the torso than leg muscles, and greater in the hamstrings than the quadriceps and ii) obesity-associated muscle insulin resistance is less pronounced in muscles of the torso than the leg. Moreover, we hypothesized that insulin-stimulated glucose uptake rate in the vastus lateralis correlates with vastus lateralis fibre type composition.

Research Design and Methods

Study participants

The data reported here were obtained from fifteen healthy lean people (5 men, 10 women; 39 ± 3 years, mean ± SEM) and thirty-seven people with obesity (9 men, 28 women; 44 ± 1 years) who participated in two different, currently ongoing, larger studies that used the same experimental protocol (NCT02994459 and NCT03408613, ClinicalTrials.gov), which was approved by the Human Research Protection Office at Washington University School of Medicine in St. Louis, MO, USA. All participants completed a dual energy X-ray absorptiometry (DXA, Lunar iDXA, GE Healthcare Lunar, Madison, WI) scan to determine body composition and a comprehensive medical examination, including a history and physical examination, a resting electrocardiogram, standard blood tests, and an oral glucose tolerance test. All participants were sedentary (<1.5 hour of exercise/week), and none had evidence of chronic illness or significant organ dysfunction (including type 2 diabetes), took medications that interfere with insulin action or glucose metabolism, or consumed tobacco products and/or excessive amounts of alcohol. Written informed consent was obtained from all participants before their participation.

Hyperinsulinaemic-euglyacemic pancreatic clamp procedure

Participants were admitted to the Clinical Translational Research Unit the night before the study. At 2000 h, they consumed a standardized meal and then fasted, except for water, until the completion of the study the next day. At ~0600 h, a catheter was inserted into an antecubital vein to infuse the metabolic tracers, hormones, and dextrose; another catheter was inserted into a radial artery for blood sampling. Participants were then transferred to the Center for Clinical Imaging Research, where constant infusions of octreotide (45 ng/kg FFM/min), glucagon (1.5 ng/kg FFM/min), growth hormone (6 ng/kg FFM/min) were started and maintained for 390 min. Insulin was infused at 10 mU/m2 body surface area/min for the first 120 min then at 50 mU/m2 body surface area/min, both initiated with a two-step priming dose; [6,6-2H2]glucose was infused at 0.22 μmol/kg/min for the first 120 min, and then at 0.11 μmol/kg/min [10, 16]. Dextrose, enriched with [6,6-2H2]glucose (2.5%) was infused at a variable rate to maintain plasma glucose concentration (monitored every 10 min) at ~6.1 mmol/l during the hyperinsulinaemic clamp procedure. We chose a pancreatic clamp procedure to minimize differences in plasma insulin concentration during the clamp procedure in healthy lean people and people with obesity [10, 11]. Approximately 270 min after starting the hyperinsulinaemic clamp procedure, participants were transferred to the PET/CT scanner (Siemens Biograph True Point/True View) where ~1.2 GBq [15O]H2O was administered as a bolus and 2 min of dynamic PET scanning of the torso was performed. Subjects were then quickly repositioned and a second dose of [15O]H2O was administered followed by 2 min of PET scanning of the thigh. At ~300 min, ~185 MBq [18F]FDG was administered intravenously and 40 min dynamic PET scanning of the torso, followed by 30 min dynamic PET scanning of the thigh was performed. Low dose CT scans (120 kVp, 50 mAs effective) were performed for attenuation correction and to delineate the muscle regions of interest. Blood samples to determine plasma insulin concentration (Elecsys®, Roche Diagnostics) were collected immediately before the start of the hormone infusions and then every 10 min from 300 – 390 min. A biopsy from the vastus lateralis was obtained during local anaesthesia from 13 of the lean participants and 28 of the participants with obesity for histology.

Muscle histology

Muscle biopsies were embedded in tragacanth gum, flash frozen in liquid nitrogen-cooled isopentane, and then stored at −80 °C. Frozen samples were cut in the axial plane in 10 μm sections, which were immunostained against type I, IIa and IIx fibres (Developmental Studies Hybridoma Bank: BA-F8, SC-71, and 6H1 respectively). Additional sections were stained with fluorescin conjugated lectin (Vector Laboratories FL-1061) to visualize capillaries and counterstained with laminin (Abcam: ab11575) and 4′,6-diamidino-2-phenylindole to visualize fibre boundaries and nuclei, respectively. Image analysis was performed with the public domain software ImageJ [17].

Calculations

Muscle perfusion (i.e., capillary blood flow rate in ml/kg muscle/min) was determined by using a 1-compartment model to fit the arterial blood (corrected for spillover and partial volume) and muscle-specific activity time courses after [15O]H2O injection [18]. Glucose delivery to muscles was calculated as the product of muscle capillary blood flow rate and circulating glucose concentration. The fractional glucose uptake rate for each muscle group was calculated by using Patlak graphical analysis of the muscle and arterial blood specific activity time curves [19, 20]. Absolute rates of glucose uptake (μmol/kg muscle/min) were derived by dividing the product of fractional glucose uptake rate and plasma glucose concentration by 1.2 (lumped constant) [19]. Total whole-body muscle glucose disposal rate was estimated as the product of the average glucose uptake rates in the erector spinae, obliques, rectus abdominis, hamstrings, and quadriceps and muscle mass, which was assumed to be 53% of lean soft tissue mass [21]. We used both the arithmetic average and the weighted average for this estimation and found it made no difference; the arithmetic average and weighted average glucose uptake rates (mean ± SEM) in the five muscles were 100 ± 7 and 103 ± 9 μmol/kg muscle/min in lean people and 63 ± 7 and 67 ± 8 μmol/kg muscle/min in people with obesity, respectively. We therefore only present the arithmetic average values. Unlabelled glucose rate of appearance in plasma (endogenous glucose production and dextrose solution) was calculated by dividing the [2H2]glucose infusion rate (sum of [2H2]glucose infusion and [2H2]glucose in the labelled dextrose solution) by the average plasma glucose tracer-to-tracee ratio [16]. Whole-body glucose disposal rate was calculated as the sum of unlabelled glucose appearance rate and glucose tracer infusion rate.

Statistical analysis

Statistical analyses were performed by using Prism 8 (GraphPad Software, San Diego, CA). All data sets were tested for normality by using the Shapiro-Wilk test. Skewed data sets were log transformed before analysis, and transformation resulted in normally distributed data sets. Accordingly, differences in muscle perfusion and muscle glucose uptake rates between lean and obese participants were evaluated by using analysis of variance (ANOVA) with group and muscle as factors. This analysis identified an effect of obesity, but no obesity × muscle interaction. Tukey’s post-hoc procedure, which adjusts for multiple comparisons, was used to locate differences in both perfusion and glucose uptake rates among muscle groups. The relationships between: i) glucose uptake rates in different muscle groups, ii) muscle perfusion and glucose uptake rate, iii) fibre type composition and glucose uptake rate in the vastus lateralis, and iv) muscle glucose uptake rate and whole-body glucose disposal rate were evaluated by using regression analysis to identify the best fit and the strength of the association. We also explored whether age, sex, and body mass index affect the associations and found they did not improve the model fits. We therefore present the R2-values from simple regression analysis. Bonferroni’s correction was used to account for multiple comparisons. Values are reported as mean ± SEM or median and interquartile range. A p-value <0.05 was considered statistically significant.

Results

Participant characteristics

Participants’ body composition and basic metabolic characteristics are shown in Table 1. Body fat mass was more than double in the Obese than in the Lean group and fat-free mass was ~25% greater in the Obese than the Lean group. Fasting plasma glucose concentration was not different in the Lean and Obese groups, but glucose tolerance was markedly impaired in the Obese compared with the Lean group. Plasma insulin, triglyceride and LDL-cholesterol concentrations were higher and HDL-cholesterol concentration was lower in the Obese than in the Lean group.

Table 1.

Participants’ body composition, oral glucose tolerance, and plasma metabolic profile

Lean (n=15) Obese (n=37)
Body mass (kg) 64 ± 2 106 ± 3*
Body mass index (kg/m2) 23 ± 1 38 ± 1*
Body fat (%) 31 (29, 37) 48 (42, 51)*
Fat mass (kg) 19 ± 1 48 ± 2*
Fat-free mass (kg) 44 (39, 47) 55 (48, 66)*
Glucose, fasted (mmol/l)a 5.1 (4.7, 5.2) 5.1 (4.8, 5.5)
Glucose, 2-h OGTT (mmol/l) 6.4 (5.1, 7.5) 7.2 (6.3, 8.6)*
Insulin (pmol/l)a 34 ± 3 77 ± 7*
Triglycerides (mmol/l)a 0.74 ± 0.07 1.17 ± 0.07*
LDL-cholesterol (mmol/l)a 2.2 (1.9, 2.9) 3.0 (2.4, 3.4)*
HDL-cholesterol (mmol/l)a 1.69 ± 0.11 1.19 ± 0.04*
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Values are mean ± SEM or median (IQR). OGTT, oral glucose (75 g) tolerance test.

a

Values were obtained after an overnight fast.

*

Value significantly different from value in the Lean group, p<0.05.

Plasma glucose and insulin concentrations during the hyperinsulinaemic-euglycaemic pancreatic clamp procedure

Steady state plasma glucose and insulin concentrations during the clamp procedure were not different in the Lean and Obese groups (glucose: 6.2 ± 0.1 and 6.1 ± 0.1 mmol/l, respectively; insulin: 638 ± 31 and 697 ± 15 pmol/l, respectively).

Muscle perfusion (capillary blood flow rate)

Muscle perfusion rates were up to 5 times different among different muscle groups in both the Lean and Obese groups (Figure 1). Muscle perfusion rates were several-fold higher in the erector spinae, hamstrings and quadriceps than the rectus abdominis and obliques (Figure 1). Perfusion rates were not statistically significantly different in the Lean and Obese groups (Figure 1).

Figure 1.

Figure 1.

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Insulin-stimulated muscle perfusion (a, b) and glucose uptake (c, d) rates in major muscle groups of the torso and thigh in healthy lean people (n=15, left panels, white bars and symbols) and people with obesity (n=37, right panels, orange bars and symbols). Values are median and interquartile range. * significantly different from erector spinae, hamstrings and quadriceps, p<0.05; † significantly different from erector spinae, p<0.05; ‡ significantly different from erector spinae and hamstrings, p<0.05;§ significantly different from corresponding values in lean people, p<0.05.

Muscle glucose uptake

On average, only a small fraction (<20 %; overall median and interquartile range: 6.9 (3.0, 12.2) of glucose delivered to muscles was taken up by muscles in both the Lean and Obese groups. Glucose uptake rate in the erector spinae was about twice as great as in the rectus abdominis and approximately 30% greater than in the obliques and quadriceps (Figure 1). There was considerable variability in glucose uptake rates in the Obese group, but on average, glucose uptake rates were ~50% less in the Obese than in the Lean group (Figure 1), without a difference among muscle groups [erector spinae: 49 (18, 97) %, obliques: 38 (30, 84) %, rectus abdominis: 49 (32, 83) %, hamstrings: 53 (28, 96) %, quadriceps: 49 (27, 73) %; values in obese participants relative to average Lean value]. The results were the same when muscle glucose uptake rates were expressed in relation to plasma insulin concentration during the clamp procedure (data not shown). There were linear relationships among glucose uptake rates in different muscle groups without a difference between the Lean and Obese groups (Figure 2 and ESM Figure 1). There was no association between muscle perfusion and glucose uptake rates in any single muscle group or in all muscles combined (ESM Figure 2).

Figure 2.

Figure 2.

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Relationship between insulin-stimulated glucose uptake rates in the erector spinae and obliques in healthy lean people (open circles; n=15) and people with obesity (orange filled circles; n=37).

Vastus lateralis morphology and relationship between fibre type composition and glucose uptake rate in the vastus lateralis

There was considerable interindividual variation in vastus lateralis fibre type composition in both the Lean and Obese groups. On average, type I fibres accounted for ~40% and type II fibres accounted for ~60% of all fibres in both the Lean and Obese groups. The contribution of type IIx fibres to total fibre number was double in the Obese compared with the Lean group, and type IIx fibres replaced both type I and type IIa fibres proportionally (Table 2). There was also considerable interindividual variation in the number of capillaries in the vastus lateralis in both the Lean and Obese groups, and, on average, capillary density (both per cross-sectional area and per fibre) was not different in the Lean than in the Obese group (Table 2). There was no association between the proportion of type I, IIa, or IIx) fibres and glucose uptake rate in the vastus lateralis in either the Lean or the Obese groups or the entire study cohort (ESM Figure 3).

Table 2.

Vastus lateralis morphology

Lean (n=13) Obese (n=28)
Myofibre composition
 Type I fibre (% total fibre area) 41 ± 5 35 ± 3
 Type IIa fibre (% total fibre area) 44 ± 4 37 ± 2
 Type IIx fibre (% total fibre area) 16 ± 4 28 ± 3*
Myofibre size
 Type I fibre (µm2) 4,873 (3,890, 6,160) 4,315 (3,638, 5,185)
 Type IIa fibre (µm2) 4,098 (3,051, 5,348) 4,066 (3,232, 5,866)
 Type IIx fibre (µm2) 2,787 (2,273, 4,286) 3,219 (2,708, 4,225)
Capillary density
 Capillaries per cross-sectional area (n per mm2) 265 (201, 281) 218 (193, 253)
 Capillaries per fibre (n per fibre) 1.5 ± 0.1 1.5 ± 0.1
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Values are mean ± SEM or median (IQR).

*

Value significantly different from value in the Lean group, p<0.05.

Whole-body glucose kinetics and relationship between whole-body glucose kinetics and muscle glucose uptake rates

The glucose infusion rate needed to maintain euglycaemia and insulin-stimulated whole-body glucose disposal rate were significantly lower in the Obese than the Lean group, and the relative obesity-associated reduction in insulin sensitivity varied from ~30% to ~70% according to how the data were expressed (i.e., as total flux, in μmol/min, or adjusted for differences in body size or composition) (Table 3). There was a linear relationship (R2=0.72, p<0.05) between total muscle glucose uptake (expressed in μmol per min) and total whole-body glucose disposal rate (expressed in μmol per min), without a difference between the Lean and Obese groups. However, total muscle glucose uptake was lower than whole-body glucose disposal, and accounted for only ~60% of whole-body glucose disposal in both the Lean (57 ± 5%) and Obese (56 ± 4%) groups (Figure 3). There was also a linear relationship (R2=0.83, p<0.05) between muscle glucose uptake rate (expressed in μmol/kg muscle/min) and whole-body glucose disposal rate in relation to total body weight (expressed in μmol/kg body weight/min) in the Obese but not the Lean group. However, muscle glucose uptake rate was greater than whole-body glucose disposal rate per kg body weight (Figure 3). There was a linear relationship (R2=0.65, p<0.05) between muscle glucose uptake rate (expressed in μmol/kg muscle/min) and whole-body glucose disposal rate in relation to fat-free mass (expressed in μmol/kg fat-free mass/min). Moreover, the values for whole-body glucose disposal in relation to fat-free mass nearly matched the values for muscle glucose uptake rate in both the Lean and Obese groups, without a difference between the groups (Figure 3). The relationships between the glucose infusion rate needed to maintain euglycaemia, expressed as total flux (in μmol/min), or adjusted for differences in body size or composition (expressed in μmol/kg boy weight/min or μmol/kg fat-free mass/min), and the average muscle glucose uptake rate (expressed in μmol/kg muscle/min) mirrored the relationships between whole-body glucose disposal rate and muscle glucose uptake rate (ESM Figure 4).

Table 3.

Whole-body glucose kinetics

Lean (n=15) Obese (n=37) Obese relative to Lean (%)a
Glucose infusion rate
 µmol/min 3,820 ± 264 2,591 ± 203* 68 ± 5
 (µmol/min)/(pmol insulin/l) 7.1 (4.0, 7.8) 3.4 (2.4, 5.4)* 54 ± 4
 µmol/kg body weight/min 60 ± 4 25 ± 2* 41 ± 3
 (µmol/kg body weight/min)/(pmol insulin/l) × 102 10.3 ± 0.8 3.8 ± 0.3* 36 ± 3
 µmol/kg fat-free mass/min 86 ± 6 48 ± 4* 55 ± 4
 (µmol/kg fat-free mass/min)/(pmol insulin/l) × 102 15.0 ± 1.4 7.1 ± 0.6* 48 ± 4
 µmol/m2 body surface area/min 2,205 ± 143 1,219 ± 93* 55 ± 4
 (µmol/m2 body surface area)/(pmol insulin/l) 3.8 ± 3.2 1.8 ± 1.4* 47 ± 4
Glucose disposal rate
 µmol/kg body weight/min 62 ± 4 26 ± 2* 42 ± 3
 (µmol/kg body weight/min)/(pmol insulin/l) × 102 10.5 ± 1.0 3.8 ± 0.3* 36 ± 3
 µmol/kg fat-free mass/min 89 ± 7 50 ± 4* 55 ± 4
 (µmol/kg fat-free mass/min)/(pmol insulin/l) × 102 15.4 ± 1.5 7.4 ± 0.6* 47 ± 4
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Values are mean ± SEM or median (IQR).

a

Values in participants with obesity expressed relative to the average Lean value.

*

Value significantly different from value in the Lean group, p<0.05.

Figure 3.

Figure 3.

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Relationships between insulin-stimulated total muscle glucose uptake (product of muscle mass and average glucose uptake rate in the erector spinae, obliques, rectus abdominis, hamstrings, and quadriceps) and whole-body glucose disposal rate (a), and between the composite average insulin-stimulated glucose uptake rate in the five muscle groups and whole-body glucose disposal rate in relation to body weight (b), and fat-free mass (c) in healthy lean people (open circles; n=15) and people with obesity (orange filled circles; n=37). Dashed lines represent the lines of identity.

Discussion

Insulin-stimulated whole-body glucose disposal is commonly used as an assessment of insulin-stimulated muscle glucose uptake [2, 10, 12, 13]. However, it does not provide a direct measure of insulin sensitivity in skeletal muscles and prevents an assessment of whether some muscles are more important than others in causing whole-body insulin resistance in people with obesity; it also prevents an assessment of how muscle perfusion and fibre type composition affect muscle glucose uptake. We used a hyperinsulinaemic-euglycaemic clamp procedure in conjunction with stable isotope- and radio-labelled tracer infusions and PET technology to evaluate insulin-stimulated whole-body glucose disposal and muscle perfusion, glucose delivery, and glucose uptake rates in five major muscle groups in the torso and thigh. We found considerable heterogeneity in insulin-stimulated glucose uptake rates among individual muscle groups that was not due to differences in muscle perfusion (insulin and glucose delivery) or fibre type composition (assessed in the vastus only). We also found that the impairment in skeletal muscle insulin-stimulated glucose uptake in people with obesity is generalized across all major muscles, without specific defects in any particular muscle group, and that insulin-stimulated whole-body glucose disposal rate expressed in relation to fat-free mass correlated well with insulin-stimulated glucose uptake rates in the five muscle groups.

It has been proposed that whole-body insulin resistance in people with obesity is due to alterations in muscle fibre type based on the following observations: i) fibre type composition is an important determinant of insulin-stimulated muscle glucose uptake in rodent muscles, and type IIx fibres isolated from rodent muscles are more insulin resistant than isolated type I fibres [4–7]; ii) people with obesity have more type IIx fibres than lean people [22]; and iii) studies conducted in people with a wide range of adiposity have shown whole body insulin-stimulated glucose disposal is directly correlated with the proportion of type I fibres and inversely correlated with the proportion of type IIx fibres in the vastus lateralis [23, 24]. However, the results from these studies in people do not prove that the effect of fibre type on muscle insulin sensitivity observed in rodents translates to physiologically meaningful effects of fibre type composition on muscle insulin sensitivity in people. It has even been proposed that insulin resistance and hyperinsulinemia cause the increase in type IIx fibres in people with insulin resistance [25, 26]. We are not aware of any studies that directly evaluated the effect of fibre type on insulin-stimulated muscle glucose uptake in specific muscles in people. In the present study, we measured glucose uptake in muscle groups that are known to differ in fibre type composition; muscles along the spine (erector spinae) have more type I fibres than abdominal muscles (rectus abdominis, obliques), muscles in the torso have more type I fibres than thigh muscles (hamstrings, quadriceps), and the hamstrings have more type I fibres than the quadriceps [14, 15]. We also obtained muscle biopsies from the vastus lateralis so that we could directly evaluate the relationship between measured fibre type composition and insulin-stimulated muscle glucose uptake rate in that muscle. Our data do not support an influence of muscle fibre type composition on muscle insulin sensitivity. Although glucose uptake in the erector spinae was greater than in abdominal muscles, glucose uptake in abdominal muscles was not different from or even lower than glucose uptake in thigh muscles, and glucose uptake in the hamstrings was not different from glucose uptake in the quadriceps. Moreover, we did not find an association between measured fibre type composition and insulin stimulated glucose uptake in the vastus lateralis. These data demonstrate muscle fibre type composition is not an important determinant of insulin-stimulated muscle glucose disposal, and glucose uptake is increased in postural back muscles and weight bearing leg muscles, possibly because insulin-stimulated muscle glucose uptake is more closely related to habitual muscle use [3, 27, 28] than fibre type. These findings are supported by the results from a study that evaluated insulin signalling in type I and type II fibres isolated from the vastus lateralis in healthy lean people and people with obesity and insulin resistance [29]. It was found that compared with type II fibres, type I fibres have higher insulin receptor, GLUT4, and hexokinase protein contents, but both Akt2 protein content and insulin-stimulated AktThr308 phosphorylation, a measure of insulin action, were lower in type I than type II fibres and the protein content and insulin-stimulated phosphorylation of other insulin-responsive signalling proteins were not different between type I and type II fibres [29].

Muscle perfusion is considered a major determinant of insulin-stimulated muscle glucose uptake because it regulates both insulin and glucose delivery to muscles [30, 31]. However, we did not detect an association between muscle perfusion and glucose uptake rates in individual muscle groups or all muscles combined in either the Lean or Obese groups or all subjects combined. These findings confirm and extend the results from previous studies that found there was no association between insulin-stimulated thigh muscle perfusion and thigh muscle glucose uptake rates [32–34]. These data suggest insulin and glucose delivery to muscles are not rate limiting for insulin-stimulated glucose uptake and local factors likely determine insulin action and glucose transport into myocytes. In fact, only a small fraction of both insulin and glucose delivered to muscles are extracted by muscle ([32, 35–37] and present study), and defects in endothelial insulin transport, intracellular insulin signal transduction, and intracellular glucose metabolism have been identified as key regulatory steps in determining muscle insulin availability and insulin-stimulated muscle glucose uptake [1, 3, 38–44].

We found insulin-stimulated muscle glucose uptake accounted for ~60% of whole-body insulin-stimulated glucose disposal in both the Lean and the Obese groups, which is consistent with the results from other studies [32, 45, 46]. The difference between whole-body glucose disposal and muscle glucose uptake is presumably a function of glucose uptake in other organs, including intestine, heart, liver, kidneys, brain, and adipose tissue [32, 47–49]. Nonetheless, we found that the whole-body glucose disposal rate, expressed in relation to fat-free mass, correlated closely and almost along the line of identity with the average muscle glucose uptake rate in both the Lean and the Obese groups, without a difference between groups. This close association between the two measurements is likely due to a similarity in the rate of insulin-stimulated glucose uptake in skeletal muscles and the rate of insulin-stimulated glucose uptake in other organs that constitute fat-free mass, which are also affected by obesity-associated insulin resistance [32, 47–49]. In contrast, insulin-stimulated whole-body glucose disposal rate, expressed per kg body weight, markedly underestimated the average muscle glucose uptake rate, particularly in people with obesity. This difference is likely related to the low rate of insulin-stimulated glucose disposal in adipose tissue and the small contribution of adipose tissue to total whole-body insulin-stimulated glucose disposal, even when adipose tissue mass is large [32, 45, 50]. Therefore, our data support the use of insulin-stimulated whole-body glucose disposal expressed in relation to fat-free mass, but not in relation to total body weight, as a robust surrogate measure of muscle glucose uptake rate.

In summary, we found considerable heterogeneity in insulin-stimulated glucose uptake rates among different muscle groups in people that was not related to muscle perfusion or fibre-type composition, presumably because local tissue and/or myocyte-specific factors, such as endothelial insulin transport, intracellular insulin signal transduction and glucose metabolism regulate both insulin availability and action and glucose uptake. Furthermore, we found the impairment in skeletal muscle insulin-stimulated glucose uptake in people with obesity is generalized across all major muscles, without specific defects in any one muscle group. Our data also demonstrate that although muscle accounts for only about two-thirds of whole-body glucose disposal rate, insulin-stimulated whole-body glucose uptake expressed per kg of fat-free mass provides a reliable index of insulin-stimulated muscle glucose uptake.

Supplementary Material

1

Research in context.

What is already known about this subject?

  • In rodent models, insulin-stimulated glucose uptake rate varies several-fold among different muscles, presumably because of differences in fibre type composition and perfusion.

  • In addition, the susceptibility to obesity-associated insulin resistance differs among different myofibre types and muscles in rodent models.

What is the key question?

  • Are the observations made in animal models true in people and, if so, how might it affect the interpretation of insulin-stimulated whole-body glucose disposal rate as an index of insulin-stimulated muscle glucose uptake rate?

What are the new findings?

  • A two-fold difference in insulin-stimulated glucose uptake rates among different muscles in people that is not related to perfusion or fibre-type composition (assessed in the vastus only).

  • The obesity-associated impairment in insulin action is consistent among different muscle groups.

  • Insulin-stimulated whole-body glucose disposal relative to fat-free mass provides a reliable index of muscle glucose uptake rate.

How might this impact on clinical practice in the foreseeable future?

Muscle specific insulin-resistant glucose metabolism occurs across all major muscle groups in people with obesity and is an important target for treating insulin resistance, because it accounts for two-thirds of whole-body insulin-mediated glucose disposal.

Acknowledgements

The authors thank the staff of the Center for Human Nutrition, the Clinical Translational Research Unit, the Clinical Translational Imaging Unit, and the Division of Radiological Sciences for help with participant recruitment, scheduling, and testing and technical assistance with sample processing and data analysis, and the study subjects for their time and effort.

The work presented in this manuscript was supported by National Institutes of Health grants R01 DK115400, P30 DK56341 (Nutrition Obesity Research Center), P30 DK020579 (Diabetes Research Center), P30 AR074992 (Musculoskeletal Research Center), and UL1TR000448 (Clinical Translational Science Award), and a grant from the American Diabetes Association (1-18-ICTS-119).

Funding: The work presented in this manuscript was supported by National Institutes of Health grants R01 DK115400, P30 DK56341 (Nutrition Obesity Research Center), P30 DK020579 (Diabetes Research Center), P30 AR074992 (Musculoskeletal Research Center), and UL1TR000448 (Clinical Translational Science Award), and a grant from the American Diabetes Association (1-18-ICTS-119).

Footnotes

Disclosure: The authors have nothing to disclose

Tweet: Researchers @WUSTLmed found insulin resistance in people with obesity is generalized across muscles and not related to fibre-type composition or perfusion. Insulin-stimulated glucose Rd/kg FFM is a reliable index of muscle insulin sensitivity. #InsulinResistance; #Type2Diabetes; #Obesity

Data and Resource Availability

The data generated during the current study are available from the corresponding author upon reasonable request. No resources were generated.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

1
NIHMS1723651-supplement-1.pdf (925.9KB, pdf)

Data Availability Statement

The data generated during the current study are available from the corresponding author upon reasonable request. No resources were generated.

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