more than 26% of older americans have impaired glucose tolerance ( igt ) ( 1 ) , increasing their risk for developing type 2 diabetes .
a sedentary lifestyle may further increase this risk through changes in skeletal muscle morphology and insulin signaling that worsen insulin resistance .
we ( 2,3 ) and others ( 4,5 ) report low skeletal muscle capillarization in sedentary , insulin - resistant subjects that is inversely associated with the degree of glucose tolerance ( 2 ) and directly associated with insulin sensitivity ( 4,5 ) in cross - sectional studies .
as the major interface between the circulation and skeletal muscle , the microvasculature affects a number of physiological processes , including insulin resistance .
transcapillary transport of insulin is an important determinant of glucose uptake in skeletal muscle ( 6 ) and is a rate - limiting step for insulin action ( 7,8 ) .
the reduced capillary surface area in igt and type 2 diabetes is associated with lower glucose uptake during insulin infusion ( 9 ) ; therefore , low skeletal muscle capillarization , along with defects in insulin signaling , can contribute to insulin resistance in igt and type 2 diabetes by decreasing the available surface area for diffusion of insulin and glucose ( 10,11 ) .
lifestyle interventions including aerobic exercise and weight loss ( aex+wl ) are a cornerstone of the treatment of insulin resistance and can prevent progression from igt to type 2 diabetes in older people ( 12,13 ) .
aex training increases skeletal muscle capillarization in healthy adults ( 1418 ) , but less is known about its effects on skeletal muscle capillarization in insulin - resistant subjects .
one study showed that aex training increases skeletal muscle capillarization in subjects with igt ( 19 ) and such an increase in capillarization after aex+wl may represent a mechanism for enhancing insulin and glucose delivery to skeletal muscle to improve insulin sensitivity .
we hypothesized that a 6-month aex+wl program would increase skeletal muscle capillarization in older adults with igt , and that the increase in capillarization would translate to improvements in insulin sensitivity and glucose tolerance . to test this hypothesis , we examined skeletal muscle capillarization , glucose tolerance , and insulin sensitivity during hyperinsulinemic - euglycemic clamps before and after 6 months of aex+wl in overweight - obese , older adults with igt .
men and postmenopausal women 5080 years of age , who were nonsmokers and had no previous diagnosis of diabetes or cardiovascular disease , were recruited from the baltimore , md , regional area to participate in studies examining metabolic responses to aex+wl .
data from eight men and eight women ( mean age 63  2 years ) with igt and skeletal muscle samples for assessment of capillarization are reported herein ; subject characteristics and certain metabolic data from a larger sample of the subjects were previously reported ( 20,21 ) .
subjects were all weight stable ( < 2.0 kg weight change in past year ) , sedentary ( < 20 min of aerobic exercise two times per week ) , and screened by medical history questionnaire , physical examination , and fasting blood profile .
subjects were screened for igt by oral glucose tolerance tests ( ogtts ) according to american diabetes association criteria ( 22 ) .
all subjects were nonsmokers and showed no evidence of cancer ; liver , renal , or hematological disease ; or other medical disorders .
subjects taking medications for hypertension or dyslipidemia were included if medically stable and if medications were not known to affect glucose metabolism .
the research protocols were approved by the institutional review board at the university of maryland school of medicine .
prior to metabolic testing , subjects received 68 weeks of instruction on the therapeutic lifestyle changes diet ( 23 ) in order to minimize potential confounding by changes in dietary composition during the intervention .
subjects followed the dietary guidelines and were weight stable for at least 2 weeks prior to baseline testing .
after baseline testing , subjects were instructed to maintain the diet , and all subjects attended weekly weight loss classes for 6 months led by a registered dietitian .
individuals were counseled to restrict their caloric intake by 300500 kcal / day to achieve > 5% weight loss during the intervention .
compliance was monitored by 7-day food records using the american diabetes association exchange list system .
in addition , all subjects underwent 6 months of supervised aex training on treadmills at the baltimore veterans affairs medical center geriatric research , education , and clinical center exercise facility .
exercise intensity was prescribed by target heart rate range calculated using the karvonen formula ( 24 ) ; heart rates were monitored during exercise with chest - strap heart rate monitors ( polar electro inc . , lake success , ny ) .
aex training began at a volume of three sessions per week of 20 min at 50% of heart rate reserve , and gradually increased to three sessions per week of 45 min at 85% of heart rate reserve , a level maintained for > 4 months .
fat mass , fat - free mass , and percent body fat were measured with dual - energy x - ray absorptiometry ( prodigy ; lunar radiation corp .
intra - abdominal ( iaf ) and subcutaneous abdominal ( saf ) fat areas were determined by a computed tomography scan at l4-l5 region using a siemens somatom sensation 64 scanner ( fairfield , ct ) and medical image processing , analysis and visualization software ( mipav v.7.0.0 ; nih , bethesda , md ) . maximal oxygen consumption ( vo2max )
was measured by indirect calorimetry during a graded treadmill exercise test on a motorized treadmill .
subjects walked at a constant velocity throughout the protocol ; grade was initially set to 0% and increased every 2 min thereafter to maximal effort .
vo2max was defined as the highest oxygen consumption value obtained for a full 30-s increment .
attainment of vo2max was verified by standard physiological criteria ( respiratory exchange ratio > 1.10 or a plateau in vo2 with an increase in workload ) .
a catheter was placed in an antecubital vein and blood samples were drawn before and 30 , 60 , 90 , and 120 min after the ingestion of a 75-g glucose solution .
plasma glucose levels were analyzed with a glucose analyzer ( 2300 stat plus ; ysi , yellow springs , oh ) .
plasma insulin levels were determined by radioimmunoassay ( millipore , st . charles , mo ) .
glucose ( gauc ) and insulin ( iauc ) areas under the curve during the ogtt were calculated using the trapezoidal method .
the homeostatic model assessment for insulin resistance was calculated as described by matthews et al .
insulin - stimulated glucose uptake ( m ) was measured as an index of insulin sensitivity .
subjects were provided with all meals for the 2 days preceding the clamp to control nutrient intake . after a 12-h overnight fast , subjects underwent the hyperinsulinemic - euglycemic glucose clamp ( 26,27 ) as performed in our laboratory ( 20 ) .
insulin was infused at a rate of 555 pmol / m / min , and m is reported in micromoles of glucose infused per kilogram of fat - free mass per minute ( mol / kg ffm / min ) .
clamp data were not available for one subject after aex+wl due to a technical problem .
plasma glucose levels were analyzed at 5-min intervals using the glucose oxidase method ( beckman instruments , fullerton , ca ) .
plasma insulin levels were determined by radioimmunoassay ( millipore , st . charles , mo ) . mean insulin and glucose levels during the clamp were 1,131  32
mmol / l , respectively , and did not differ before and after aex+wl ( p > 0.4 ) .
percutaneous needle biopsies were obtained from the vastus lateralis , 1213 cm above the patella on the anterolateral aspect of the right thigh using a bergstrom needle ( stille , solna , sweden ) as previously described ( 28 ) .
muscle samples were rapidly embedded in optimal cutting temperature - tragacanth gum mixture , frozen , and stored at 80c for histochemical analyses .
muscle was sectioned to a thickness of 14 m on a cryostat and capillaries were identified using a modified double - stain technique ( 29 ) . in brief ,
sections were then incubated with primary antibodies ( ulex europaeus agglutinin i for endothelial cells and mouse anticollagen iv for muscle fiber perimeters ) and secondary antibodies ( rabbit anti - ulex europaeus , goat anti - rabbit , and goat anti - mouse ) .
sections were then reacted with avidin - biotinylated alkaline phosphatase ( abc / ap ; dako , carpinteria , ca ) and the new fuchsin substrate system ( dako , carpinteria , ca ) .
immunostained muscle sections were viewed under a light microscope and digital images were obtained ( eclipse ti ; nikon instruments inc . ,
quantification of capillarization was performed on at least 50 fibers for each sample ( mean = 69  4 fibers / sample ) ; sampling a larger number of fibers does not improve the estimation of capillarization in human muscle ( 30 ) .
the following four indices of capillarization were measured : 1 ) capillary contacts ( the number of capillaries in contact with each muscle fiber ) , 2 ) individual capillary - to - fiber ratio ( the number of whole capillary equivalents in contact with each muscle fiber ) , 3 ) capillary density ( cd ; the number of capillaries per mm of muscle cross - sectional area ) , and 4 ) capillary - to - fiber perimeter exchange index ( the number of capillaries per mm of muscle fiber perimeter ) . for each vastus
lateralis sample , a serial section was obtained and fiber type was determined using a myosin atpase technique . after an initial 5-min incubation in an acid solution ( 0.4% sodium acetate , 0.6% sodium barbital , and 0.04 n hydrochloric acid ) , the samples
were incubated for 45 min in an atp reaction solution ( 0.6% glycine , 0.6% calcium chloride , 0.4% sodium chloride , 0.3% sodium hydroxide , and 0.17% atp ) .
this was followed by a 3-min incubation in 1% calcium chloride , a 3-min incubation in 2% cobalt chloride , and a 1-min incubation in 0.2% ammonium sulfide .
stained sections viewed under a light microscope and digital images were obtained ( eclipse ti ) .
the primary study outcomes were skeletal muscle capillarization , m , fasting plasma glucose , plasma glucose response to an ogtt , and vo2max .
secondary variables included fasting plasma insulin , plasma insulin response to an ogtt , and body composition .
statistical analyses were performed using spss v12.0 ( ibm , armonk , ny ) . repeated - measures anova was used to test for differences in outcome variables after aex+wl , with sex used as a covariate in all analyses .
regression analyses were used to test for associations between capillarization variables and other primary and secondary variables ; multivariable regression accounting for age , sex , and/or baseline levels of primary outcome variables was conducted where indicated . a type i error rate of  = 0.05 was selected , and two - tailed probabilities are reported for all analyses .
men and postmenopausal women 5080 years of age , who were nonsmokers and had no previous diagnosis of diabetes or cardiovascular disease , were recruited from the baltimore , md , regional area to participate in studies examining metabolic responses to aex+wl .
data from eight men and eight women ( mean age 63  2 years ) with igt and skeletal muscle samples for assessment of capillarization are reported herein ; subject characteristics and certain metabolic data from a larger sample of the subjects were previously reported ( 20,21 ) .
subjects were all weight stable ( < 2.0 kg weight change in past year ) , sedentary ( < 20 min of aerobic exercise two times per week ) , and screened by medical history questionnaire , physical examination , and fasting blood profile .
subjects were screened for igt by oral glucose tolerance tests ( ogtts ) according to american diabetes association criteria ( 22 ) .
all subjects were nonsmokers and showed no evidence of cancer ; liver , renal , or hematological disease ; or other medical disorders .
subjects taking medications for hypertension or dyslipidemia were included if medically stable and if medications were not known to affect glucose metabolism .
the research protocols were approved by the institutional review board at the university of maryland school of medicine .
prior to metabolic testing , subjects received 68 weeks of instruction on the therapeutic lifestyle changes diet ( 23 ) in order to minimize potential confounding by changes in dietary composition during the intervention .
subjects followed the dietary guidelines and were weight stable for at least 2 weeks prior to baseline testing .
after baseline testing , subjects were instructed to maintain the diet , and all subjects attended weekly weight loss classes for 6 months led by a registered dietitian .
individuals were counseled to restrict their caloric intake by 300500 kcal / day to achieve > 5% weight loss during the intervention .
compliance was monitored by 7-day food records using the american diabetes association exchange list system .
in addition , all subjects underwent 6 months of supervised aex training on treadmills at the baltimore veterans affairs medical center geriatric research , education , and clinical center exercise facility .
exercise intensity was prescribed by target heart rate range calculated using the karvonen formula ( 24 ) ; heart rates were monitored during exercise with chest - strap heart rate monitors ( polar electro inc . , lake success , ny ) .
aex training began at a volume of three sessions per week of 20 min at 50% of heart rate reserve , and gradually increased to three sessions per week of 45 min at 85% of heart rate reserve , a level maintained for > 4 months .
fat mass , fat - free mass , and percent body fat were measured with dual - energy x - ray absorptiometry ( prodigy ; lunar radiation corp . ,
intra - abdominal ( iaf ) and subcutaneous abdominal ( saf ) fat areas were determined by a computed tomography scan at l4-l5 region using a siemens somatom sensation 64 scanner ( fairfield , ct ) and medical image processing , analysis and visualization software ( mipav v.7.0.0 ; nih , bethesda , md ) .
maximal oxygen consumption ( vo2max ) was measured by indirect calorimetry during a graded treadmill exercise test on a motorized treadmill .
subjects walked at a constant velocity throughout the protocol ; grade was initially set to 0% and increased every 2 min thereafter to maximal effort .
vo2max was defined as the highest oxygen consumption value obtained for a full 30-s increment .
attainment of vo2max was verified by standard physiological criteria ( respiratory exchange ratio > 1.10 or a plateau in vo2 with an increase in workload ) .
a catheter was placed in an antecubital vein and blood samples were drawn before and 30 , 60 , 90 , and 120 min after the ingestion of a 75-g glucose solution .
plasma glucose levels were analyzed with a glucose analyzer ( 2300 stat plus ; ysi , yellow springs , oh ) .
plasma insulin levels were determined by radioimmunoassay ( millipore , st . charles , mo ) .
glucose ( gauc ) and insulin ( iauc ) areas under the curve during the ogtt were calculated using the trapezoidal method .
the homeostatic model assessment for insulin resistance was calculated as described by matthews et al .
insulin - stimulated glucose uptake ( m ) was measured as an index of insulin sensitivity .
subjects were provided with all meals for the 2 days preceding the clamp to control nutrient intake . after a 12-h overnight fast , subjects underwent the hyperinsulinemic - euglycemic glucose clamp ( 26,27 ) as performed in our laboratory ( 20 ) .
insulin was infused at a rate of 555 pmol / m / min , and m is reported in micromoles of glucose infused per kilogram of fat - free mass per minute ( mol / kg ffm / min ) .
clamp data were not available for one subject after aex+wl due to a technical problem .
plasma glucose levels were analyzed at 5-min intervals using the glucose oxidase method ( beckman instruments , fullerton , ca ) .
pmol / l and 5.1  0.1 mmol / l , respectively , and did not differ before and after aex+wl ( p > 0.4 ) .
percutaneous needle biopsies were obtained from the vastus lateralis , 1213 cm above the patella on the anterolateral aspect of the right thigh using a bergstrom needle ( stille , solna , sweden ) as previously described ( 28 ) .
muscle samples were rapidly embedded in optimal cutting temperature - tragacanth gum mixture , frozen , and stored at 80c for histochemical analyses .
muscle was sectioned to a thickness of 14 m on a cryostat and capillaries were identified using a modified double - stain technique ( 29 ) . in brief ,
sections were then incubated with primary antibodies ( ulex europaeus agglutinin i for endothelial cells and mouse anticollagen iv for muscle fiber perimeters ) and secondary antibodies ( rabbit anti - ulex europaeus , goat anti - rabbit , and goat anti - mouse ) .
sections were then reacted with avidin - biotinylated alkaline phosphatase ( abc / ap ; dako , carpinteria , ca ) and the new fuchsin substrate system ( dako , carpinteria , ca ) . immunostained muscle sections were viewed under a light microscope and digital images were obtained ( eclipse ti ; nikon instruments inc . , melville , ny ) .
quantification of capillarization was performed on at least 50 fibers for each sample ( mean = 69  4 fibers / sample ) ; sampling a larger number of fibers does not improve the estimation of capillarization in human muscle ( 30 ) .
the following four indices of capillarization were measured : 1 ) capillary contacts ( the number of capillaries in contact with each muscle fiber ) , 2 ) individual capillary - to - fiber ratio ( the number of whole capillary equivalents in contact with each muscle fiber ) , 3 ) capillary density ( cd ; the number of capillaries per mm of muscle cross - sectional area ) , and 4 ) capillary - to - fiber perimeter exchange index ( the number of capillaries per mm of muscle fiber perimeter ) .
for each vastus lateralis sample , a serial section was obtained and fiber type was determined using a myosin atpase technique . after an initial 5-min incubation in an acid solution ( 0.4% sodium acetate , 0.6% sodium barbital , and 0.04 n hydrochloric acid ) , the samples were incubated for 45 min in an atp reaction solution ( 0.6% glycine , 0.6% calcium chloride , 0.4% sodium chloride , 0.3% sodium hydroxide , and 0.17% atp ) .
this was followed by a 3-min incubation in 1% calcium chloride , a 3-min incubation in 2% cobalt chloride , and a 1-min incubation in 0.2% ammonium sulfide .
stained sections viewed under a light microscope and digital images were obtained ( eclipse ti ) .
the primary study outcomes were skeletal muscle capillarization , m , fasting plasma glucose , plasma glucose response to an ogtt , and vo2max .
secondary variables included fasting plasma insulin , plasma insulin response to an ogtt , and body composition .
repeated - measures anova was used to test for differences in outcome variables after aex+wl , with sex used as a covariate in all analyses .
regression analyses were used to test for associations between capillarization variables and other primary and secondary variables ; multivariable regression accounting for age , sex , and/or baseline levels of primary outcome variables was conducted where indicated .
a type i error rate of  = 0.05 was selected , and two - tailed probabilities are reported for all analyses .
subject characteristics , body composition , and cardiorespiratory fitness levels before and after 6-month aex+wl are presented in table 1 . in response to the aex+wl intervention
, subjects reduced their body weight and fat mass by 8% and their bmi by 7% ( p
the subjects did not have a statistically significant reduction in iaf , but did reduce saf by 9% ( p = 0.007 ) .
the subjects significantly increased vo2max ( l / min ) by 18% ( p = 0.02 ) .
subject characteristics and responses to 6-month aex+wl aex+wl significantly increased skeletal muscle capillarization ( table 2 ) , with a 15% increase in cd ( p = 0.01 ) , a 14% increase in capillary - to - fiber perimeter exchange index ( p = 0.004 ) , and a greater number of capillaries in contact with each skeletal muscle fiber ( capillary contacts and capillary - to - fiber ratio , p < 0.05 ) . the proportion of type i or ii muscle fibers did not change after aex+wl ( 65  4 vs. 67  5% type i fibers , p = 0.75 ) nor did skeletal muscle fiber area and perimeter ( 2% difference , p > 0.68 ) .
these results indicate that the increase in cd was due to an increase in the number of capillaries , not a reduction in muscle fiber size .
skeletal muscle capillarization , glucose tolerance , and insulin sensitivity before and after 6-month aex+wl data from glucose tolerance tests and hyperinsulinemic - euglycemic clamps are presented in table 2 .
fasting plasma glucose and insulin concentrations were numerically lower after aex+wl , but the differences were not statistically significant .
aex+wl reduced 120-min postprandial glucose concentrations ( g120 ) by 16% ( p = 0.008 ) , reduced 120-min postprandial insulin concentrations by 31% ( p = 0.04 ) , and reduced gauc by 10% ( p = 0.03 ) .
likewise , aex+wl increased m ( mol / kg ffm / min ) by 21% ( p = 0.04 ) . at baseline , m ( mol / kg ffm / min ) directly correlated with cd ( r = 0.62 , p = 0.01 ) and tended to inversely correlate with iaf ( r = 0.49 , p = 0.07 ) in bivariate analyses .
m was not associated with the proportion of type i or ii muscle fibers ( r = 0.00.27 , p > 0.33 for all ) .
baseline g120 correlated only with cd ( r = 0.68 , p = 0.004 ) .
in multivariable regression analyses accounting for age , sex , and body weight in the model , baseline cd was the only variable independently associated with m ( partial r = 0.58 , p = 0.04 ) ( fig .
1a ) and g120 ( partial r = 0.60 , p = 0.03 ) ( fig .
inclusion of iaf as an independent variable slightly reduced the overall r values in the regression models but did not affect the independent relationships ( partial r values ) between cd and m or g120 in the models .
scatterplots depicting the relationships between skeletal muscle cd and insulin sensitivity ( m ) ( a ) and 120-min postprandial glucose ( g120 ) ( b ) at baseline in overweight - obese , older adults with igt .
bivariate correlation coefficients are shown in the figures ; partial correlation coefficients from regression models are r = 0.58 and r = 0.60 , respectively .
after aex+wl , the change in m ( mol / kg ffm / min ) directly correlated with the change in cd ( r = 0.53 , p = 0.04 ) but not with the changes in vo2max , body weight , body fat , iaf , or saf ( r = 0.19 to 0.29 , p > 0.33 ) in bivariate analyses .
likewise , the change in g120 inversely correlated with the change in cd ( r = 0.51 , p = 0.04 ) . in multivariable regression analyses accounting for age , sex , and the change in body weight , the aex+wl - induced increase in cd independently correlated with the increase in m ( partial r = 0.74 , p = 0.006 ) ( fig
. 2a ) and the decrease in g120 ( partial r = 0.55 , p = 0.04 ) ( fig .
2b ) . scatterplots depicting the relationships between the aex+wl - induced change in skeletal muscle cd and insulin sensitivity ( m ) ( a ) and 120-min postprandial glucose ( g120 ) ( b ) in overweight - obese older adults with igt .
bivariate correlation coefficients are shown in the figures ; partial correlation coefficients from regression models are r = 0.74 , p = 0.006 and r = 0.55 , p = 0.04 , respectively .
subject characteristics , body composition , and cardiorespiratory fitness levels before and after 6-month aex+wl are presented in table 1 . in response to the aex+wl intervention
, subjects reduced their body weight and fat mass by 8% and their bmi by 7% ( p
the subjects did not have a statistically significant reduction in iaf , but did reduce saf by 9% ( p = 0.007 ) .
the subjects significantly increased vo2max ( l / min ) by 18% ( p = 0.02 ) .
subject characteristics and responses to 6-month aex+wl aex+wl significantly increased skeletal muscle capillarization ( table 2 ) , with a 15% increase in cd ( p = 0.01 ) , a 14% increase in capillary - to - fiber perimeter exchange index ( p = 0.004 ) , and a greater number of capillaries in contact with each skeletal muscle fiber ( capillary contacts and capillary - to - fiber ratio , p < 0.05 ) . the proportion of type i or ii muscle fibers did not change after aex+wl ( 65  4 vs. 67  5% type i fibers , p = 0.75 ) nor did skeletal muscle fiber area and perimeter ( 2% difference , p > 0.68 ) .
these results indicate that the increase in cd was due to an increase in the number of capillaries , not a reduction in muscle fiber size .
skeletal muscle capillarization , glucose tolerance , and insulin sensitivity before and after 6-month aex+wl data from glucose tolerance tests and hyperinsulinemic - euglycemic clamps are presented in table 2 .
fasting plasma glucose and insulin concentrations were numerically lower after aex+wl , but the differences were not statistically significant .
aex+wl reduced 120-min postprandial glucose concentrations ( g120 ) by 16% ( p = 0.008 ) , reduced 120-min postprandial insulin concentrations by 31% ( p = 0.04 ) , and reduced gauc by 10% ( p = 0.03 ) .
likewise , aex+wl increased m ( mol / kg ffm / min ) by 21% ( p = 0.04 ) .
at baseline , m ( mol / kg ffm / min ) directly correlated with cd ( r = 0.62 , p = 0.01 ) and tended to inversely correlate with iaf ( r = 0.49 , p = 0.07 ) in bivariate analyses .
m was not associated with the proportion of type i or ii muscle fibers ( r = 0.00.27 , p > 0.33 for all ) .
baseline g120 correlated only with cd ( r = 0.68 , p = 0.004 ) .
in multivariable regression analyses accounting for age , sex , and body weight in the model , baseline cd was the only variable independently associated with m ( partial r = 0.58 , p = 0.04 ) ( fig .
1a ) and g120 ( partial r = 0.60 , p = 0.03 ) ( fig .
inclusion of iaf as an independent variable slightly reduced the overall r values in the regression models but did not affect the independent relationships ( partial r values ) between cd and m or g120 in the models .
scatterplots depicting the relationships between skeletal muscle cd and insulin sensitivity ( m ) ( a ) and 120-min postprandial glucose ( g120 ) ( b ) at baseline in overweight - obese , older adults with igt .
bivariate correlation coefficients are shown in the figures ; partial correlation coefficients from regression models are r = 0.58 and r = 0.60 , respectively .
after aex+wl , the change in m ( mol / kg ffm / min ) directly correlated with the change in cd ( r = 0.53 , p = 0.04 ) but not with the changes in vo2max , body weight , body fat , iaf , or saf ( r = 0.19 to 0.29 , p > 0.33 ) in bivariate analyses .
likewise , the change in g120 inversely correlated with the change in cd ( r = 0.51 , p = 0.04 ) . in multivariable regression analyses accounting for age , sex , and the change in body weight , the aex+wl - induced increase in cd independently correlated with the increase in m ( partial r = 0.74 , p = 0.006 ) ( fig .
2a ) and the decrease in g120 ( partial r = 0.55 , p = 0.04 ) ( fig .
scatterplots depicting the relationships between the aex+wl - induced change in skeletal muscle cd and insulin sensitivity ( m ) ( a ) and 120-min postprandial glucose ( g120 ) ( b ) in overweight - obese older adults with igt .
bivariate correlation coefficients are shown in the figures ; partial correlation coefficients from regression models are r = 0.74 , p = 0.006 and r = 0.55 , p = 0.04 , respectively .
insulin resistance and diabetes are health problems that affect more than one - third of older adults .
although lifestyle interventions including exercise and weight loss can improve glucose tolerance and reduce risk for type 2 diabetes ( 12,13 ) , many of the underlying mechanisms remain elusive .
the current study shows that despite the capillary rarefaction found in the skeletal muscle of sedentary , older adults with igt , aex+wl significantly increases skeletal muscle capillarization .
furthermore , this is the first study to show that increases in capillarization are directly associated with improvements in glucose tolerance and insulin sensitivity measured during a hyperinsulinemic - euglycemic clamp to our knowledge .
therefore , despite impaired angiogenesis and capillary rarefaction in sedentary people with insulin resistance and igt , the ability to increase capillarization in skeletal muscle is maintained and likely contributes to aex+wl - induced improvements in insulin sensitivity in older adults at risk for type 2 diabetes .
skeletal muscle capillarization increases after aex training in healthy young ( 14,18 ) and older ( 15,17 ) subjects ; however , few studies have examined the effects of aex training with or without wl on capillarization in subjects with igt or type 2 diabetes to date . in one study ,
aex training increased capillarization by 10% in a small group of men with igt ( 19 ) .
our results are concordant with these findings , indicating that skeletal muscle capillarization does increase with aex+wl in both men and women with igt .
another study using a combined aex and strength training intervention showed an increase in the number of capillaries per fiber and in muscle fiber size , but no change in cd in subjects with type 2 diabetes ( 31 ) .
the maintenance of cd with increases in fiber size in that study suggests that angiogenesis occurred in skeletal muscle and that diabetes - associated capillary rarefaction may be reversible . as there were no changes in fiber size after aex+wl in our study , increases in capillarization were consistent across the measurements we report .
weight loss independent of aex may not be associated with an increase in skeletal muscle capillarization ( 32 ) ; however , one study showed increases in cd with large amounts ( > 20% ) of weight loss ( 33 ) . additional study is required to distinguish the independent effects of aex and wl on skeletal muscle capillarization in older subjects with igt .
previous cross - sectional studies from our group and others demonstrate relationships between skeletal muscle capillarization and insulin sensitivity or glucose tolerance ( 25 ) .
the longitudinal study design in the present report allowed us to test the effects of an aex+wl intervention that targets skeletal muscle , and to examine its effects on skeletal muscle capillarization and changes in glucose metabolism .
concordant with previous studies ( 25 ) , we find a direct relationship between capillarization and insulin sensitivity at baseline .
furthermore , our results show a strong relationship between aex+wl - induced changes in capillarization and insulin sensitivity in people at high risk for type 2 diabetes .
although these data do not demonstrate true causality , data from animal models do demonstrate a direct role of capillarization in determining insulin sensitivity .
for example , vollus et al . ( 34 ) show that a graded occlusion of capillaries in a rodent model reduces insulin action and causes a stepwise reduction in insulin - stimulated glucose uptake in skeletal muscle ranging from 9 to 60% .
impairments in the recruitment and perfusion of capillaries by insulin also contribute to insulin resistance in humans and animals ( see reference 35 for review ) ; thus , it stands to reason that by having more capillaries in skeletal muscle , there is a larger capillary surface area that can be recruited and perfused in response to insulin .
aex training also improves endothelial vasoreactivity in subjects with igt , which may play a role in improving insulin sensitivity through increased blood flow ( 36 ) ; however , the effects of capillarization and capillary recruitment appear to be independent of blood flow ( 11 ) .
collectively , these findings support the conclusion that aex+wl - induced increases in capillarization contribute to improvements in insulin sensitivity in older subjects with igt . in this study , subjects with igt improved insulin sensitivity by > 20% and improved glucose tolerance to near - normal levels ( g120 = 7.85 mmol / l ) , with 9 of the 16 subjects reverting to normal glucose tolerance after 6 months of aex+wl . apart from skeletal muscle capillarization , other mechanisms including improvements in insulin signaling , gene and protein expression , glut4 content and translocation ( see reference 37 for review ) , and body composition ( 3840 )
we did not find a significant relationship between changes in insulin sensitivity and body weight or body composition in the current study , but our group previously showed that aex+wl - induced increases in glycogen synthase activity are directly associated with improvements in insulin sensitivity ( 20 ) .
we did not assess these intramuscular mechanisms in the current study ; thus , it is likely that increases in skeletal muscle capillarization work in tandem with these mechanisms to increase insulin action and improve insulin sensitivity in people with igt .
studies are in progress to determine the relative contribution of capillarization and intramuscular mechanisms to aex- and wl - induced improvements in insulin sensitivity . in summary , these results show that skeletal muscle capillarization significantly increases after a 6-month aex+wl intervention in previously sedentary older adults with igt , and that there is a direct relationship between the increase in capillarization and the improvements in glucose tolerance and insulin sensitivity .
these findings add further support to the concept that lifestyle interventions including aex+wl will improve insulin sensitivity and reduce risk for type 2 diabetes , and indicate that increased skeletal muscle capillarization is one mechanism by which these metabolic improvements occur in overweight - obese , older adults with igt .
 objectivetranscapillary transport of insulin is one determinant of glucose uptake by skeletal muscle ; thus , a reduction in capillary density ( cd ) may worsen insulin sensitivity . 
 skeletal muscle cd is lower in older adults with impaired glucose tolerance ( igt ) compared with those with normal glucose tolerance and may be modifiable through aerobic exercise training and weight loss ( aex+wl ) . 
 we tested the hypothesis that 6-month aex+wl would increase cd to improve insulin sensitivity and glucose tolerance in older adults with igt.research design and methodssixteen sedentary , overweight - obese ( bmi 2735 kg / m2 ) , older ( 63  2 years ) men and women with igt underwent hyperinsulinemic - euglycemic clamps to measure insulin sensitivity , oral glucose tolerance tests , exercise and body composition testing , and vastus lateralis muscle biopsies to determine cd before and after 6-month aex+wl.resultsinsulin sensitivity ( m ) and 120-min postprandial glucose ( g120 ) correlated with cd at baseline ( r = 0.58 and r = 0.60 , respectively , p < 0.05 ) . 
 aex+wl increased maximal oxygen consumption ( vo2max ) 18% ( p = 0.02 ) and reduced weight and fat mass 8% ( p < 0.02 ) . 
 cd increased 15% ( 264  11 vs. 304  14 capillaries / mm2 , p = 0.01 ) , m increased 21% ( 42.4  4.0 vs. 51.4  4.3 mol / kg ffm / min , p < 0.05 ) , and g120 decreased 16% ( 9.35 
  0.5 vs. 7.85  0.5 mmol / l , p = 0.008 ) after aex+wl . 
 regression analyses showed that the aex+wl - induced increase in cd independently predicted the increase in m ( r = 0.74 , p < 0.01 ) as well as the decrease in g120 ( r = 0.55 , p < 0.05).conclusionssix - month aex+wl increases skeletal muscle cd in older adults with igt . 
 this represents one mechanism by which aex+wl improves insulin sensitivity in older adults with igt . 
Introduction
Research Design and Methods
Subjects
AEX+WL Intervention
Body Composition
Maximal Oxygen Consumption
OGTT
Hyperinsulinemic-Euglycemic Clamp
Muscle Biopsies
Capillary Density
Skeletal Muscle Fiber Type
Statistical Analyses
Results
Subject Characteristics and Responses to AEX+WL
Relationships Between Metabolic Variables and Skeletal Muscle Capillarization
Conclusions