Insulin Resistance in Adolescents with Type 2 Diabetes Is Associated with Impaired Exercise Capacity

Abstract

Context: The incidence of pediatric type 2 diabetes (T2D) is rising, with unclear effects on the cardiovascular system. Cardiopulmonary fitness, a marker of morbidity and mortality, is abnormal in adults with T2D, yet the mechanisms are incompletely understood.

Objective: We hypothesized that cardiopulmonary fitness would be reduced in youth with T2D in association with insulin resistance (IR) and cardiovascular dysfunction.

Design, Setting, and Participants: We conducted a cross-sectional study at an academic hospital that included 14 adolescents (age range, 12–19 yr) with T2D, 13 equally obese adolescents and 12 lean adolescents similar in age, pubertal stage, and activity level.

Main Outcome Measures: Cardiopulmonary fitness was measured by peak oxygen consumption (VO₂peak) and oxygen uptake kinetics (VO₂kinetics), IR by hyperinsulinemic clamp, cardiac function by echocardiography, vascular function by venous occlusion plethysmography, body composition by dual-energy x-ray absorptiometry, intramyocellular lipid by magnetic resonance spectroscopy, and inflammation by serum markers.

Results: Adolescents with T2D had significantly decreased VO₂peak and insulin sensitivity, and increased soleus intramyocellular lipid, C-reactive protein, and IL-6 compared to obese or lean adolescents. Adolescents with T2D also had significantly prolonged VO₂kinetics, decreased work rate, vascular reactivity, and adiponectin, and increased left ventricular mass and fatty acids compared to lean adolescents. In multivariate linear regression analysis, IR primarily, and fasting free fatty acids and forearm blood flow secondarily, were significant independent predictors of VO₂peak.

Conclusions: Given the strong relationship between decreased cardiopulmonary fitness and increased mortality, these findings in children are especially concerning and represent early signs of impaired cardiac function.

Adolescents with type 2 diabetes have abnormal exercise function, vascular reactivity, and increased left ventricular mass.

The incidence of pediatric type 2 diabetes (T2D) increased 10-fold from 1982 to 1994 (1), and it continues to rise in parallel with increases in pediatric obesity (2). However, little is known about how T2D affects the cardiovascular system in youth. Because cardiopulmonary fitness is directly linked to mortality, we sought to investigate exercise function in T2D youth. We previously observed that T2D adults have significantly reduced cardiopulmonary fitness (3,4), but younger subjects, with generally lower cardiopulmonary risks, and who are influenced by growth and pubertal hormone changes, may not mirror adults. We hypothesized that cardiopulmonary fitness would be reduced in T2D youth in association with insulin resistance (IR) and markers of cardiovascular function. To test this hypothesis, we measured peak oxygen uptake (VO₂peak), oxygen uptake kinetics (VO₂kinetics) during submaximal constant-load exercise, and IR in obese T2D adolescents and in activity-matched healthy normal-weight and obese adolescents without diabetes. Resting echocardiography and forearm reactive hyperemia were measured to assess possible cardiac and hemodynamic contributors to exercise dysfunction. Additionally, intramyocellular (IMCL) and extramyocellular (EMCL) lipid, serum lipids, and inflammatory markers were measured to assess the potential relations of ectopic fat and inflammation, typically associated with IR, to the observed exercise responses.

Subjects and Methods

Subjects

Adolescents were recruited from the University of Colorado Denver (UCD) pediatric diabetes and obesity clinics and by advertisement. By design, obese and T2D subjects had similar body mass index (BMI). A total of 39 subjects were enrolled: 12 controls, BMI 15–85%ile (six males and six females; one Black, one Hispanic, nine White, one other), 13 obese (six males and seven females; one Black, five Hispanic, one Asian, six White), and 14 T2D (seven males and seven females; one Black, nine Hispanic, three White, one other). Mean age of participants was 15 ± 2 yr (range, 12–19 yr), and mean Tanner stage was 4.7 ± 0.5 for females and 4.1 ± 0.8 for males. Family history of diabetes was similar in obese and T2D subjects, but negative in control subjects. The study was approved by the UCD institutional review board, and appropriate consent was obtained.

Screening included a history, physical examination, Tanner staging and fasting laboratory testing to confirm the absence of comorbid conditions. T2D was defined by American Diabetes Association criteria and the absence of glutamic acid decarboxylase, islet cell or insulin autoantibodies, insulin requirement, or secondary causes of diabetes. Absence of diabetes was confirmed in the nondiabetic groups by a 2-h, 75-g oral glucose tolerance test.

Inclusion criteria included pubertal status (Tanner stage >1) to minimize pubertal effects, and sedentary status (<3 h of exercise per week) to minimize training effects. Exclusions included body weight greater than 300 pounds, blood pressure greater than 140/90 mm Hg at rest or greater than 190/100 mm Hg during exercise, hemoglobin less than 9 mg/dl, serum creatinine greater than 1.5 mg/dl, hemoglobin A-1c (HbA-1c) above 12%, smoking, medication-dependent asthma, respiratory or orthopedic conditions precluding exercise testing, medications known to affect IR other than metformin (oral or inhaled steroids, thiazolidinediones, atypical antipsychotics), insulin, other antidiabetic drugs, antihypertensive drugs, pregnancy, breastfeeding, or plans to alter exercise or diet during the study.

All subsequent tests were performed after a 12-h fast, preceded by 3 d of restricted physical activity and a fixed-macronutrient, weight-maintenance diet provided by the Clinical Translational Research Center (CTRC) metabolic kitchen (55% carbohydrates, 30% fat, 15% protein). Metformin was withdrawn during the 3-d study diet. T2D subjects were instructed to monitor blood glucose levels at least four times daily and were excluded for fasting blood glucose greater than 200 mg/dl or random blood glucose greater than 300 mg/dl during the study diet, or if urine ketones were present on admission. There were no significant changes in blood glucose during the study diet.

Activity and diet questionnaires

A 3-d pediatric physical activity recall questionnaire was administered by interview to estimate habitual physical activity (5), reported as a 3-d average of metabolic equivalents (METs) per day, where one MET equals resting VO₂ (3.5 ml · kg⁻¹ · min⁻¹). A food frequency questionnaire designed for lean and obese youth was administered by interview to estimate calorie and macronutrient intake (6).

Insulin sensitivity

Subjects were admitted overnight to the Pediatric CTRC to ensure fasting. Fasting laboratory evaluation included: total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein (HDL) cholesterol, triglycerides, glucose, insulin, HbA-1c, C-peptide, blood urea nitrogen, creatinine, complete blood count, alanine aminotransferase, aspartate aminotransferase, highly sensitive C-reactive protein (hsCRP), IL-6, myeloperoxidase, and adiponectin; assays were performed by standard methods at the UCD CTRC Laboratory. A 3-h hyperinsulinemic euglycemic clamp (80 mU · m⁻² · min⁻¹ insulin) was performed to estimate IR (7). Glycemia was monitored every 5 min using a Yellow Springs Instruments (YSI, Yellow Springs, OH) glucose analyzer, and 20% dextrose infusion was titrated to maintain a blood glucose concentration of 95–105 mg/dl. Blood samples were drawn at baseline, 90, 150, 160, 165, 170, 175, and 180 min to determine glucose, insulin, and free fatty acids (FFA). Glucose disposal rate was determined from the average glucose infusion rate during the last 20 min of the clamp and expressed as milligrams per kilogram body mass and milligrams per kilogram fat-free mass (obtained from dual-energy x-ray absorptiometry scan). There were no significant differences in mean blood glucose or mean insulin levels between the groups at the conclusion of the clamp (Table 1).

Table 1.

Demographic characteristics

	Control	Obese	T2D
n	12	13	14
Sex (% female)	50	46	50
Age (yr)	15.6 ± 1.8	14.9 ± 2.1	14.6 ± 2.2
Tanner stage	4.4 ± 0.7	4.6 ± 0.6	4.5 ± 0.8
BMI (kg/m2)	21.0 ± 2.4	32.1 ± 5.4a	33.4 ± 6.2a
BMI percentile	45%	96%	97%
Diabetes duration (yr)			1.6 ± 1.6
HbA-1c (%)	4.9 ± 0.3	5 ± 0.3	8 ± 2.2a,h
Cholesterol (mg/dl)	132 ± 22	178 ± 40b	176 ± 35b
Triglycerides (mg/dl)	82 ± 37	211 ± 168c	200 ± 136d
HDL cholesterol (mg/dl)	45 ± 8	42 ± 14	42 ± 10
Fasting C-peptide (ng/ml)	1.7 ± 0.6	3.8 ± 2.3	7 ± 5.5d
Fasting FFA (μEq/liter)	491 ± 310	583 ± 145	619 ± 87d
FFA during euglycemic clamp (μEq/liter)	48.9 ± 38.9	357 ± 484	241.4 ± 201e
Fasting insulin (μU/ml)	7.7 ± 3.5	27 ± 37	31 ± 20
Insulin during euglycemic clamp (μU/ml)	105 ± 35	134 ± 41	109 ± 44
Fasting glucose (mg/dl)	83 ± 5	86 ± 9	121 ± 26a,g
Glucose during euglycemic clamp (mg/dl)	100 ± 4	100 ± 4	99 ± 3
Glucose disposal rate (mg · kg · min)	15.3 ± 4.7	8.1 ± 2.7a	4.1 ± 2.3a,f
Glucose disposal rate (mg · kg lean · body mass · min)	19.8 ± 4.6	14.6 ± 4.9c	7.8 ± 3.9a,g

^aP < 0.001 vs. controls; 

^bP < 0.01 vs. controls; 

^cP < 0.05 vs. controls; 

^dP < 0.02 vs. controls; 

^eP < 0.04 vs. controls; 

^fP < 0.02 vs. obese; 

^gP < 0.01 vs. obese; 

^hP < 0.001 vs. obese. 

Autonomic insufficiency

Variation in R-R intervals with cycled breathing were obtained in supine subjects breathing five times per minute for 5 min, coordinated with an electronic signal (8). Blood pressure and heart rate responses to standing and R-R variations with Valsalva breathing were measured. Average tachycardic beats per minute/bradycardic beats per minute value of 1.21 or less during three 15-sec Valsalvas or each following 45 sec was considered abnormal. No subjects had autonomic dysfunction by these methods (data not shown).

Echocardiograms

Resting two-dimensional and tissue Doppler echocardiography was performed using Vivid 7 and Echopac (BTO6 6.1.3, General Electric, Milwaukee, WI) to exclude left ventricular systolic dysfunction (ejection fraction <50%), regional wall motion abnormalities, pericardial disease, or significant valvular pathology. Electrocardiograms excluded ischemia. Left ventricular end-systolic and diastolic chamber dimensions and wall thickness, fractional shortening, and biplane Simpson’s method (chamber volumes) were quantitated by standard techniques. Diastolic filling measurements were obtained by velocity of propagation (Vp) and tissue Doppler at the septal and lateral mitral annuli. Mitral valve deceleration time, Vp, peak early mitral inflow velocity:peak late inflow velocity (E/A), and peak early mitral annular velocity (E/E’ septal and E:E’ lateral) were assessed for diastolic dysfunction (9).

Left ventricular posterior wall thickness in diastole (LVPWd), interventricular septal thickness in diastole (IVSd), left ventricular end-diastolic dimension (LVED), and end-systolic dimension were assessed for left ventricular hypertrophy. Left ventricular mass (LVM) was calculated as LVM(g) = (0.8) (1.04) × [(LVED + LVPWd + IVSd)³ − LVED³] + 0.6, and indexed by dividing by height in meters^2.7 (10).

Plethysmography

Forearm reactive hyperemic blood flow was determined in supine participants by venous occlusion strain-gauge plethysmography (D. E. Hokanson, Issaquah, WA) with calibrated mercury SILASTIC (Dow Corning Corp., Midland, MI) strain gauges using standard methods (11). At least five measurements were performed at 10- to 15-sec intervals. The difference between peak (after releasing arterial occluding cuff) and resting forearm blood flow was considered peak hyperemic flow (ml/100ml/min).

Exercise testing

A graded cycle ergometer (Lode, Groningen, The Netherlands) protocol to exhaustion was used to determine VO₂peak using standard methods (12). Oxygen consumption (VO₂), carbon dioxide production, and minute ventilation were measured continuously and recorded breath-by-breath using a metabolic cart (Ultima CPX; Medical Graphics, St. Paul, MN), and perceived exertion was measured every minute using the Borg scale (13). The respiratory exchange ratio (RER) was calculated as carbon dioxide production/VO₂. Work rate was increased in 10, 15, or 20 W/min increments, depending on age and sex, while subjects maintained a speed of 65 rotations per minute. VO₂peak (ml/kg/min) was defined as the highest VO₂ and heart rate averaged over 10 sec at an RER of 1.1 or greater. Subjects were excluded if RER was less than 1.1.

Kinetic measurements during constant-load exercise

Subjects performed three identical exercise bouts on a cycle ergometer (Lode) at a constant work rate equal to 85% of their lactate threshold (14), as estimated by the V-slope method during maximal testing (15). Each bout consisted of 2 min of resting gas exchange, 6 min of exercise at 65 rotations per minute, and 10 min of rest. Oxygen consumption data were processed and analyzed using nonlinear regression techniques for kinetic parameters as previously described (14). The phase 2 time constant of oxygen uptake (τ2), a coordinated marker of the ability to deliver and use oxygen at the level of the exercising skeletal muscle, is reported in seconds.

Fat deposition

Body composition by dual-energy x-ray absorptiometry was performed according to standard methods (16). IMCL and EMCL were measured via ¹H magnetic resonance spectroscopy using 3.0 T whole-body magnetic resonance imaging (GE Healthcare, Waukesha, WI) (17,18). The soleus muscle was centered in a standard quadrature lower extremity coil. Scout images obtained in all three orthogonal planes determined the approximately 5-cm³ spectroscopy voxel in soleus or tibialis anterior muscle, avoiding visible adipose, vascular structures, and fasciae lines. Spectroscopic acquisitions with PROBE-P (PRESS) (GE Healthcare) used pulse sequence parameters (repetition time/echo time = 2000/100 msec; 128 averages; total acquisition time, 7 min) optimized to avoid contaminating bulk fat signals and a clean spectral baseline. The PROBE-P sequence automatically collects non-water suppressed frames for internal water referencing (see below). Spectra degraded by subject motion were repeated.

Spectra were analyzed with LCModel software (version 6.1-4A) (Provencher, SW, Gottingen, Germany), widely used for analysis of brain ¹H magnetic resonance spectroscopy, using solution metabolite spectra as basis sets for Bayesian analysis to provide automatic quantification of in vivo proton magnetic resonance spectra without operator bias or intervention using time domain input (19,20,21). More recent versions of LCModel accommodate muscle spectra analysis, using a simulated muscle basis set analyzing for EMCL, IMCL, creatine, choline, and taurine. IMCL and EMCL concentrations were obtained by referencing spectra peak areas at 1.3 ppm and 1.5 ppm to the unsuppressed water peak area at 4.7 ppm (internal water referencing) (22,23) using the unsuppressed water reference frames from the PROBE-P acquisition (18,22,24,25,26). Magnetic resonance relaxation times of water and lipid signals are not corrected for because repetition time/echo time remains constant (18,22,23,24,25,27). Results are reported as concentrations of methylene (-CH₂) groups in arbitrary concentration units, not directly comparable to traditional water/fat % absolute concentrations (mm). Arbitrary units are useful for accessing differences between individuals and groups (24,25,28) but did not allow reporting of IMCL or EMCL fat measurements as a percentage of water signal.

Statistical analysis

Data are reported as mean ± sd. An α level of less than 0.05 was considered statistically significant. Using SPSS 13.0 (SPSS, Inc., Chicago, IL), variances between groups were compared using the Lavine statistic and normality assessed by the Shapiro-Wilks test. A priori comparisons were made using ANOVA and post hoc analysis to correct for multiple comparisons (Bonferroni when variances were equal, Dunnett T3 when unequal). Planned univariate correlations between VO₂peak and variables hypothesized to be causally associated with VO₂peak were analyzed (Pearson correlation coefficients when data were normally distributed, Spearman’s rho when lacking normality). Step-down multivariate linear regression analysis was performed with SAS 9.1 (SAS Institute, Inc., Cary, NC), including biologically plausible variables univariately correlated with VO₂peak, with an α of less than 0.01 or smaller, to determine independent predictors of VO₂peak.

Results

Metabolic parameters

Groups were similar in age, Tanner stage, gender (Table 1), blood urea nitrogen, creatinine, aspartate aminotransferase, alanine aminotransferase, hemoglobin, hematocrit, platelets, urine microalbumin:creatinine, HDL, calorie and macronutrient intake, and no subjects had anemia (data not shown) (Table 1). T2D subjects had significantly higher fasting glucose and HbA-1c than control or obese subjects (Table 1).

BMI, total cholesterol, and triglyceride concentrations were significantly lower in control subjects than T2D or obese subjects. T2D subjects had higher fasting FFA, C-peptide, and FFA at 180 min of the insulin clamp vs. lean controls (Table 1). BMI, total cholesterol, triglycerides, fasting FFA, and FFA at 180 min of the hyperinsulinemic clamp were similar in T2D and obese subjects (Table 1). Baseline biochemical parameters did not differ in T2D subjects on metformin vs. those not on metformin.

Exercise testing

Peak RER and heart rate were similar between groups, indicating similar maximal effort at VO₂peak (Table 2). There were no significant differences between groups in baseline habitual physical activity (Table 2).

Table 2.

Exercise and cardiovascular results

	Control	Obese	T2D
Mean habitual activity (METs)	64 ± 11.2	59 ± 14.6	64 ± 18.8
RER	1.18 ± 0.07	1.13 ± 0.07	1.13 ± 0.05
Peak HR (beats/min)	184 ± 11	188 ± 13	183 ± 8
VO2 (ml/kg body mass/min)	40.4 ± 9.9	27.2 ± 5.3a	21.8 ± 4.2a,b
VO2 (ml/kg lean body mass/min)	53.1 ± 8.5	49.5 ± 5.6	42.8 ± 7.6c
Peak work rate (Watts)	220 ± 48.7	167 ± 44.5	149 ± 33.6d
τ2 (sec)	28.6±10	34.9±10.5	40.2±9.7e
Resting systolic blood pressure (mm Hg)	109 ± 6	119 ± 7e	124 ± 10e
Resting diastolic blood pressure (mm Hg)	63 ± 6	74 ± 7e	72 ± 11b
Limb blood flow (ml/100 ml/min)	17.6 ± 6.9	10 ± 5e	10 ± 5.6e
IVSd (cm)	0.71 ± 0.12	0.69 ± 0.13	0.78 ± 0.15
LVPWd (cm)	0.76 ± 0.10	0.8 ± 0.17	0.9 ± 0.18b
LVM (g)	100 ± 20.7	107 ± 25.6	123 ± 35.8
Indexed LVM (g/m2.7)	24 ± 4.3	27 ± 7.6	32 ± 8e

HR, Heart rate; VO₂, oxygen uptake. 

^aP < 0.0001 vs. controls; 

^bP < 0.04 vs. obese; 

^cP < 0.05 vs. obese; 

^dP < 0.02 vs. controls; 

^eP < 0.01 vs. controls. 

VO₂peak was significantly lower in T2D subjects vs. control or obese subjects (Fig. 1). VO₂peak per kilogram of fat-free mass was also significantly lower in T2D subjects vs. control or obese subjects. Peak work rate was significantly less in T2D subjects than controls (Table 2). No significant differences were noted in resting VO₂, heart rate, recovery heart rate at 2 min, or perceived exertion between groups (data not shown).

Figure 1.

Maximal exercise capacity (VO2peak) in control (white bar), obese (hatched bar), and T2D (black bar) adolescents, expressed in milliliters per kilogram per minute. *, P values compared to control adolescents; †, P values compared to obese adolescents.

T2D subjects had significantly prolonged VO₂ kinetics, (e.g. τ2) vs. controls (Table 2).

Insulin sensitivity

Glucose disposal rates during the hyperinsulinemic euglycemic clamp expressed as milligrams per kilogram per minute or as milligrams per kilogram of fat-free mass per minute were significantly lower in obese and T2D subjects than controls. Glucose disposal rates in milligrams per kilogram per minute and milligrams per kilogram of fat-free mass per minute were also significantly lower in T2D than obese subjects (Table 1).

Fat deposition

There were no significant differences between groups in fat-free mass (Table 3). Percentage body fat and fat-free mass were similar in obese and T2D groups and significantly higher in obese and T2D groups compared with controls (Table 3). Adiponectin was significantly lower in T2D subjects than controls (Table 3). EMCL content of the soleus muscle was similar in T2D and obese subjects, with both groups being significantly higher than controls (Table 3). IMCL content of the soleus and tibialis anterior muscles (Table 3) was significantly higher only in the T2D subjects. T2D subjects also had higher waist:hip ratio (Table 3) when compared with obese or control subjects.

Table 3.

Body composition, muscle fat distribution, and inflammation

	Control	Obese	T2D
Fat-free mass (kg)	44.8 ± 8.4	47.3 ± 10.1	47.0 ± 10.8
Fat mass (kg)	11.01 ± 6.4	33.3 ± 11.3a	37.6 ± 13.1a
% Fat-free	79.4 ± 10.6	59.2 ± 5.8a	56.8 ± 7.3a
% Fat	20.6 ± 10.6	40.8 ± 5.8a	43.2 ± 7.3a
Waist circumference (cm)	72.9 ± 7.5	97.0 ± 12.5a	105.3 ± 15.4a
Hip circumference (cm)	83.2 ± 6.1	106.5 ± 11.3a	107.3 ± 12.7a
Waist:hip	0.88 ± 0.06	0.91 ± 0.06	0.98 ± 0.05a,b
Soleus EMCL (ACU)	1450 ± 863	2863 ± 1312c	3169 ± 1014d
Tibialis EMCL (ACU)	926 ± 723	1763 ± 1664e	2220 ± 1027f
Soleus IMCL (ACU)	1225 ± 476	1788 ± 792	2539 ± 910a,h
Tibialis IMCL (ACU)	373 ± 116	434 ± 268	657 ± 256f
Adiponectin (μg/ml)	8.4 ± 3.1	9.9 ± 3.3	6.4 ± 1.4e
WBC (103/μl)	6.0 ± 1.0	6.9 ± 1.9	8.8 ± 2.9f
hsCRP (mg/liter)	0.57 ± 0.3	1.1 ± 1.5	5.2 ± 4.4b,f
Myeloperoxidase (pmol/liter)	694 ± 396	651 ± 270	740 ± 216
IL-6 (pg/ml)	1.2 ± 1.8	0.8 ± 1.6	3.4 ± 1.8g

ACU, Arbitrary concentration units, WBC, white blood cells. 

^aP < 0.0001 vs. controls; 

^bP < 0.01 vs. obese; 

^cP < 0.02 vs. controls; 

^dP < 0.002 vs. controls; 

^eP < 0.05 vs. controls; 

^fP < 0.01 vs. controls; 

^gP < 0.03 vs. obese; 

^hP < 0.05 vs. obese. 

Cardiovascular function

Systolic blood pressure and diastolic blood pressure were similar in the T2D and obese groups (Table 2), and significantly higher than controls. Peak forearm vascular reactivity was significantly reduced in the T2D and obese groups compared with controls (Table 2).

LVPWd and indexed LVM were significantly greater in T2D subjects vs. controls (Table 2). There was evidence of left ventricular hypertrophy (IVSd or LVPWd >1 cm) in 29% of T2D subjects. Using tissue Doppler, measurement of systolic and diastolic parameters (fractional shortening, E/A, Vp, and E/E’) was not significantly different between the three groups (data not shown).

Inflammation

hsCRP and IL-6 levels were significantly higher in T2D subjects compared with obese and control subjects (Table 3). White blood cell count was also significantly higher in T2D subjects than controls (Table 3).

Correlates of VO₂peak

Insulin sensitivity (mg/kg/min) was the strongest predictor of VO₂peak (r = 0.84; P < 0.0001) (Fig. 2) in a multivariate regression analysis controlled for age, Tanner stage, BMI, and baseline habitual physical activity. Other a priori univariate correlates of VO₂peak included fasting insulin (r = −0.75; P < 0.0001), FFA (r = −0.527; P = 0.001), HbA-1c (r = −0.654; P < 0.0001), IMCL (soleus, r = −0.63, P < 0.0001; tibialis anterior, r = −0.46, P < 0.01), forearm blood flow (r = 0.610; P < 0.0001), and hsCRP (r = −0.578; P < 0.0001). Waist:hip ratio did not correlate with VO₂peak. Among T2D subjects, HbA-1c and diabetes duration were not correlated with VO₂peak. In a step-down multivariate linear regression analysis, IR (Fig. 2), forearm blood flow (Fig. 3A), and fasting FFA (Fig. 3B) remained as independent predictors of VO₂peak.

Figure 2.

Correlation between maximal exercise capacity (VO2peak) expressed in milliliters per kilogram per minute and glucose disposal rate from the hyperinsulinemic euglycemic clamp, expressed in milligrams per kilogram per minute. •, Control males; ○, control females; ▪, obese males; □, obese females; ▴, T2D males; ▵, T2D females.

Figure 3.

A, Correlation between maximal exercise capacity (VO2peak) expressed in milliliters per kilogram per minute and peak hyperemic forearm blood flow from venous plethysmography, expressed in milliliters per 100 ml per minute. B, Correlation between maximal exercise capacity (VO2peak) expressed in milliliters per kilogram per minute and fasting FFA, expressed in microequivalents per liter. •, Control males; ○, control females; ▪, obese males; □, obese females; ▴, T2D males; ▵, T2D females.

Discussion

We report significant impairment in functional exercise capacity in well-characterized T2D youth, compared with similarly obese or lean nondiabetic controls of similar pubertal stage and baseline activity level. Our results indicate that the presence of T2D in youth confers an additional reduction in exercise capacity, beyond that seen with obesity alone, despite a relatively short duration of disease. Supporting this observation is the key finding that VO₂peak correlated strongly with IR and its manifestations. Additionally, the exclusion of other univariate correlates from multivariate analysis can potentially be explained by their correlations with IR. Early cardiovascular abnormalities, demonstrated by impaired vascular reactivity and cardiac hypertrophy, are consistent with observations in adults with more advanced T2D and may negatively impact exercise function of T2D youth. Together, these findings in children represent early signs of impaired cardiovascular function and a poor prognosis that accompanies decreased cardiovascular fitness (29,30).

Some abnormalities noted may be more related to obesity, and others specific to T2D. Obese and T2D subjects had similarly abnormal cholesterol, triglycerides, HDL, fat mass, EMCL, blood pressure, and vascular reactivity. Exercise capacity and IR were more abnormal in T2D than obese youth. Only T2D subjects had significantly abnormal FFA, HbA-1c, C-peptide, IMCL, inflammation, waist:hip circumference, adiponectin, VO₂kinetics, and left ventricular hypertrophy.

Limited data are available regarding exercise function in T2D youth. Gusso et al. (31) reported no differences between VO₂peak or peak watts in eight T2D adolescents relative to 10 obese or 10 control adolescents, yet lower VO₂peak in T2D vs. 12 adolescents with type 1 diabetes. Combining all diabetic subjects, VO₂peak (ml/fat-free kg/min but not ml/kg/min or peak watts) was lower than all nondiabetic subjects, suggesting an effect of diabetes independent of body weight. Similarly, Faulkner et al. (32) found that fitness and habitual physical activity were lower in T2D adolescents compared with adolescents with type 1 diabetes, but baseline physical activity and BMI were not controlled. In contrast, controlling for these variables as well as pubertal stage and dietary intake, we found reduced VO₂peak in T2D youth vs. obese or lean controls, despite below average VO₂peak in controls due to their sedentary status (12). These results are consistent with the findings of Regensteiner et al. (3,4) and others, who demonstrated that adults with uncomplicated T2D have reduced exercise function. Moreover, even when corrected for fat-free mass, T2D youth display significant performance deficits that cannot be explained by sedentary or obese status.

Our pulmonary VO₂kinetic data are the first to extend the findings of abnormally slowed oxygen uptake kinetics during submaximal exercise in T2D adults (4) to youth and support the observed peak exercise defect. The practical implication of low-level exercise impairment is a decreased ability to accommodate and perform simple physical tasks, which may decrease the likelihood of children engaging in exercise. The same independent predictors reported for VO₂peak were also predictors of VO₂kinetics (τ2): IR (r = −0.36; P = 0.04), reactive hyperemia (r = −0.412; P = 0.01), and FFA after the hyperinsulinemic clamp (r = 0.582; P = 0.001), indicating that similar components may affect VO₂kinetics and VO₂peak.

The mechanisms of exercise dysfunction in T2D are not well understood. Our previous findings in adults treated with rosiglitazone showed that change in fasting insulin and homeostasis model of assessment for insulin resistance, but not glucose, correlated with improvements in exercise capacity, implying that IR may predominate over glycemia (33). Several studies examined the relationship between surrogate IR markers and exercise in obese nondiabetic youth. Physical work capacity was decreased in children with hyperinsulinemia (34), and exercise duration and VO₂peak were reduced in adolescent boys with metabolic syndrome, vs. obese controls (35). Furthermore, baseline and change in predicted VO₂peak correlated with fasting insulin levels in 11 to 14 yr olds (36). These BMI-controlled studies support our finding of IR rather than obesity, per se, as a potential contributor to exercise defects. However, Shaibi et al. (37) found no differences in treadmill VO₂peak between obese Hispanic 8 to 13 yr olds with a family history of T2D, with or without the metabolic syndrome, and no correlation between VO₂peak and IR (measured by iv glucose tolerance test) (38). Possible explanations for this discrepancy include much younger subjects, who therefore had not yet developed IR comorbidities, and no control for pubertal stage or dietary intake. Through intensive control of diet and pubertal stage and direct measurement of IR by hyperinsulinemic euglycemic clamp, our present data provide compelling support for the role of IR as an independent contributor to exercise dysfunction in youth, and further research will help clarify this question.

IMCL is typically associated with obesity and IR (39,40) but has not been studied in T2D youth. We now report the novel finding of increased IMCL in T2D youth relative to controls and even similarly obese adolescents. These results cannot be explained by training differences because all subjects were similarly inactive. This finding is important because decreased turnover and accumulation of lipid byproducts reportedly impairs muscle insulin signaling (41) and mitochondrial function (42), which could affect exercise performance in these youth.

Cardiovascular abnormalities are notable as known contributors to exercise dysfunction. In this study, we report concerning evidence of left ventricular hypertrophy in T2D youth. In support of these findings, left ventricular hypertrophy was reported in 28% of T2D adults (43) and in extremely morbidly obese adolescents (BMI = 60 ± 9 kg/m²) (44). However, unlike reports in T2D adults (45), we found no resting diastolic abnormalities, potentially because of shorter T2D duration and less obesity than was observed in these previous studies. Alternatively, resting echocardiography may be inadequate to exclude diastolic dysfunction in T2D youth. For example, we previously reported that adults with T2D of short duration had normal resting echocardiograms, yet increased pulmonary capillary wedge pressure during exercise, possible early evidence of diastolic dysfunction (46). Indeed, reduced central hemodynamic responses during submaximal exercise have been reported in T2D adolescents. These data argue that cardiac abnormalities secondary to T2D may significantly contribute to the observed exercise defects in youth.

Vascular dysfunction is associated with obesity and T2D (47,48). In the present study, reactive hyperemia was reduced in obese and T2D adolescents, correlating independently with exercise dysfunction. Our results also show a relationship between IR and inflammation in T2D youth, independent of obesity. Increased visceral fat in T2D youth, implied by increased waist:hip circumference, is a possible explanation for inflammation. A provocative question is whether inflammation is a central mechanism uniting vascular and cardiac dysfunction in T2D, making systemic inflammation a potential therapeutic target for improving exercise capacity.

The lack of differences in resting heart rate, peak heart rate, or recovery heart rate after exercise, or abnormalities in autonomic screening make autonomic dysfunction an unlikely explanation for the reduced exercise capacity we observed, but it cannot be entirely excluded. Inclusion criteria and 3-d pediatric physical activity recalls revealed no differences in baseline physical activity, but undetected differences could still impact exercise function. Future studies will employ accelerometers to monitor physical activity.

Taken together, our data demonstrate that T2D youth have decreased exercise performance, potentially explained by IR and its manifestations, as well as vascular dysfunction. T2D adolescents also have left ventricular hypertrophy, increased IMCL, inflammation, and hypoadiponectinemia. Importantly, these abnormalities extend beyond what might be contributed by inactivity or obesity alone. These findings are especially concerning in very young patients, with no other comorbidities to explain exercise dysfunction, because they forecast early morbidity and mortality. Studies in T2D adults suggest the reversibility of exercise defects (33), although acute and chronic metformin treatment failed to improve peak exercise capacity in normal adults (49). Future studies are needed to further evaluate the cardiac abnormalities in T2D adolescents and the effects of interventions aimed at improving exercise capacity to determine the reversibility of exercise defects in youth.