Koichiro Azuma

Effects of Physical Activity and Weight Loss on Skeletal Muscle Mitochondria and Relationship With Glucose Control in Type 2 Diabetes

OBJECTIVE— Reduced mitochondrial capacity in skeletal muscle occurs in type 2 diabetic patients and in those at increased risk for this disorder, but the extent to which mitochondrial dysfunction in type 2 diabetic patients is remediable by physical activity and weight loss intervention is uncertain. We sought to address whether an intervention of daily moderate-intensity exercise combined with moderate weight loss can increase skeletal muscle mitochondrial content in type 2 diabetic patients and to address the relationship with amelioration of insulin resistance and hyperglycemia. RESEARCH DESIGN AND METHODS— Muscle biopsies were obtained before and after a 4-month intervention to assess mitochondrial morphology, mitochondrial DNA content, and mitochondrial enzyme activities. Glucose control, body composition, aerobic fitness, and insulin sensitivity were measured. RESULTS— In response to a weight loss of 7.1 ± 0.8% and a 12 ± 1.6% improvement in Vo2max (P < 0.05), insulin sensitivity improved by 59 ± 21% (P < 0.05). There were significant increases in skeletal muscle mitochondrial density (by 67 ± 17%, P < 0.01), cardiolipin content (55 ± 17%, P < 0.01), and mitochondrial oxidation enzymes. Energy expenditure during physical activity correlated with the degree of improvement in insulin sensitivity (r = 0.84, P < 0.01), and, in turn, improvement in mitochondrial content was a strong correlate of intervention-induced improvement in A1C and fasting plasma glucose. CONCLUSIONS— Intensive short-term lifestyle modifications can restore mitochondrial content and functional capacity in skeletal muscle in type 2 diabetic patients. The improvement in the oxidative capacity of skeletal muscle may be a key component mediating salutary effects of lifestyle interventions on hyperglycemia and insulin resistance.

Deficiency of electron transport chain in human skeletal muscle mitochondria in type 2 diabetes mellitus and obesity

Insulin resistance in skeletal muscle in obesity and T2DM is associated with reduced muscle oxidative capacity, reduced expression in nuclear genes responsible for oxidative metabolism, and reduced activity of mitochondrial electron transport chain. The presented study was undertaken to analyze mitochondrial content and mitochondrial enzyme profile in skeletal muscle of sedentary lean individuals and to compare that with our previous data on obese or obese T2DM group. Frozen skeletal muscle biopsies obtained from lean volunteers were used to estimate cardiolipin content, mtDNA (markers of mitochondrial mass), NADH oxidase activity of mitochondrial electron transport chain (ETC), and activity of citrate synthase and beta-hydroxyacyl-CoA dehydrogenase (beta-HAD), key enzymes of TCA cycle and beta-oxidation pathway, respectively. Frozen biopsies collected from obese or T2DM individuals in our previous studies were used to estimate activity of beta-HAD. The obtained data were complemented by data from our previous studies and statistically analyzed to compare mitochondrial content and mitochondrial enzyme profile in lean, obese, or T2DM cohort. The total activity of NADH oxidase was reduced significantly in obese or T2DM subjects. The cardiolipin content for lean or obese group was similar, and although for T2DM group cardiolipin showed a tendency to decline, it was statistically insignificant. The total activity of citrate synthase for lean and T2DM group was similar; however, it was increased significantly in the obese group. Activity of beta-HAD and mtDNA content was similar for all three groups. We conclude that the total activity of NADH oxidase in biopsy for lean group is significantly higher than corresponding activity for obese or T2DM cohort. The specific activity of NADH oxidase (per mg cardiolipin) and NADH oxidase/citrate synthase and NADH oxidase/beta-HAD ratios are reduced two- to threefold in both T2DM and obesity.

Mitochondrial capacity in skeletal muscle is not stimulated by weight loss despite increases in insulin action and decreases in intramyocellular lipid content

OBJECTIVE— In obesity and type 2 diabetes, exercise combined with weight loss increases skeletal muscle mitochondrial capacity. It remains unclear whether mitochondrial capacity increases because of weight loss, improvements in insulin resistance, or physical training. In this study, we examined the effects of an intervention of weight loss induced by diet and compared these with those of a similar intervention of weight loss by diet with exercise. Both are known to improve insulin resistance, and we tested the hypothesis that physical activity, rather than improved insulin resistance, is required to increase mitochondrial capacity of muscle. RESEARCH DESIGN AND METHODS— Sixteen sedentary overweight/obese volunteers were randomized to a 16-week intervention of diet (n = 7) or diet plus exercise (n = 9). Insulin sensitivity was measured using euglycemic clamps. Mitochondria were examined in muscle biopsies before and after intervention. We measured mitochondrial content and size by electron microscopy, electron transport chain (ETC) activity, cardiolipin content, and mitochondrial DNA content. Intramyocellular content of lipid (IMCL) and fiber-type distribution were determined by histology. RESULTS— The diet-only and diet plus exercise groups achieved similar weight loss (10.8 and 9.2%, respectively); only the diet plus exercise group improved aerobic capacity. Insulin sensitivity improved similarly in both groups. Mitochondrial content and ETC activity increased following the diet plus exercise intervention but remained unchanged following the diet-only intervention, and mitochondrial size decreased with weight loss despite improvement in insulin resistance. IMCL decreased in the diet-only but not in the diet plus exercise intervention. CONCLUSIONS— Despite similar effects to improve insulin resistance, these interventions had differential effects on mitochondria. Clinically significant weight loss in the absence of increased physical activity ameliorates insulin resistance and IMCL but does not increase muscle mitochondrial capacity in obesity.

Metabolic Flexibility in Response to Glucose Is Not Impaired in People With Type 2 Diabetes After Controlling for Glucose Disposal Rate

OBJECTIVE—Compared with nondiabetic subjects, type 2 diabetic subjects are metabolically inflexible with impaired fasting fat oxidation and impaired carbohydrate oxidation during a hyperinsulinemic clamp. We hypothesized that impaired insulin-stimulated glucose oxidation is a consequence of the lower cellular glucose uptake rate in type 2 diabetes. Therefore, we compared metabolic flexibility to glucose adjusted for glucose disposal rate in nondiabetic versus type 2 diabetic subjects and in the latter group after 1 year of lifestyle intervention (the Look AHEAD [Action For Health in Diabetes] trial). RESEARCH DESIGN AND METHODS—Macronutrient oxidation rates under fasting and hyperinsulinemic conditions (clamp at 80 mU/m2 per min), body composition (dual-energy X-ray absorptiometry), and relevant hormonal/metabolic blood variables were assessed in 59 type 2 diabetic and 42 nondiabetic individuals matched for obesity, sex, and race. Measures were repeated in diabetic participants after weight loss. RESULTS—Metabolic flexibility to glucose (change in respiratory quotient [RQ]) was mainly related to insulin-stimulated glucose disposal rate (R2 = 0.46, P < 0.0001) with an additional 3% of variance accounted for by plasma free fatty acid concentration at the end of the clamp (P = 0.03). The impaired metabolic flexibility to glucose observed in type 2 diabetic versus nondiabetic subjects (ΔRQ 0.06 ± 0.01 vs. 0.10 ± 0.01, respectively, P < 0.0001) was no longer observed after adjusting for glucose disposal rate (P = 0.19). Additionally, the increase in metabolic flexibility to glucose after weight loss was accounted for by the concomitant increase in insulin-stimulated glucose disposal rate. CONCLUSIONS—This study suggests that metabolic inflexibility to glucose in type 2 diabetic subjects is mostly related to defective glucose transport.

Metabolic Changes Following a 1-Year Diet and Exercise Intervention in Patients With Type 2 Diabetes

OBJECTIVE To characterize the relationships among long-term improvements in peripheral insulin sensitivity (glucose disposal rate [GDR]), fasting glucose, and free fatty acids (FFAs) and concomitant changes in weight and adipose tissue mass and distribution induced by lifestyle intervention in obese individuals with type 2 diabetes. RESEARCH DESIGN AND METHODS We measured GDR, fasting glucose, and FFAs during a euglycemic clamp and adipose tissue mass and distribution, organ fat, and adipocyte size by dual-energy X-ray absorptiometry, CT scan, and adipose tissue biopsy in 26 men and 32 women in the Look-AHEAD trial before and after 1 year of diet and exercise aimed at weight loss. RESULTS Weight and fasting glucose decreased significantly (P < 0.0001) and significantly more in men than in women (−12 vs. −8% and −16 vs. −7%, respectively; P < 0.05), while FFAs during hyperinsulinemia decreased and GDR increased significantly (P < 0.00001) and similarly in both sexes (−53 vs. −41% and 63 vs. 43%; P = NS). Men achieved a more favorable fat distribution by losing more from upper compared with lower and from deeper compared with superficial adipose tissue depots (P < 0.01). Decreases in weight and adipose tissue mass predicted improvements in GDR but not in fasting glucose or fasting FFAs; however, decreases in FFAs during hyperinsulinemia significantly determined GDR improvements. Hepatic fat was the only regional fat measure whose change contributed independently to changes in metabolic variables. CONCLUSIONS Patients with type 2 diabetes undergoing a 1-year lifestyle intervention had significant improvements in GDR, fasting glucose, FFAs and adipose tissue distribution. However, changes in overall weight (adipose tissue mass) and hepatic fat were the most important determinants of metabolic improvements.

Lower thigh subcutaneous and higher visceral abdominal adipose tissue content both contribute to insulin resistance

It is well known that visceral adipose tissue (VAT) is associated with insulin resistance (IR). Considerable debate remains concerning the potential positive effect of thigh subcutaneous adipose tissue (TSAT). Our objective was to observe whether VAT and TSAT are opposite, synergistic or additive for both peripheral and hepatic IR. Fifty‐two volunteers (21 male/31 female) between 30 and 75 years old were recruited from the general population. All subjects were sedentary overweight or obese (mean BMI 33.0 ± 3.4 kg/m2). Insulin sensitivity was determined by a 4‐h hyperinsulinemic–euglycemic clamp with stable isotope tracer dilution. Total body fat and lean body mass were determined by dual X‐ray absorptiometry. Abdominal and mid‐thigh adiposity was determined by computed tomography. VAT was negatively associated with peripheral insulin sensitivity, while TSAT, in contrast, was positively associated with peripheral insulin sensitivity. Subjects with a combination of low VAT and high TSAT had the highest insulin sensitivity, subjects with a combination of high VAT and low TSAT were the most insulin resistant. These associations remained significant after adjusting for age and gender. These data confirm that visceral excess abdominal adiposity is associated with IR across a range of middle‐age to older men and women, and further suggest that higher thigh subcutaneous fat is favorably associated with better insulin sensitivity. This strongly suggests that these two distinct fat distribution phenotypes should both be considered in IR as important determinants of cardiometabolic risk.

Interactions Between Delivery, Transport, and Phosphorylation of Glucose in Governing Uptake Into Human Skeletal Muscle

Skeletal muscle accounts for a large proportion of insulin-stimulated glucose utilization. It is generally regarded that much of the control over rates of uptake is posited within the proximal steps of delivery, transport, and phosphorylation of glucose, with glucose transport as the main locus of control. Whether insulin modulates the distribution of control across these steps and in what manner remains uncertain. The current study addressed this in vivo using dynamic positron emission tomography (PET) imaging of human muscle with sequential injections of three tracers ([15O]H2O, [11C]3-O-methyl glucose [3-OMG], and [18F]fluoro-deoxy glucose [FDG]) that enabled quantitative determinations of glucose delivery, transport, and its phosphorylation, respectively. Lean, healthy, research volunteers were studied during fasting conditions (n = 8) or during a euglycemic insulin infusion at 30 mU/min per m2 (n = 8). PET images were coregistered with magnetic resonance imaging to contrast glucose kinetics in soleus, a highly oxidative muscle, with tibialis anterior, a less oxidative muscle. During fasting conditions, uptake of [11C]3-OMG was similar in soleus and tibialis anterior muscles, despite higher delivery to soleus (by 35%; P < 0.01). Uptake of [18F]FDG was also similar between muscle during fasting, and glucose transport was found to be the dominant locus of control (90%) for glucose uptake under this condition. Insulin increased uptake of [11C]3-OMG substantially and strongly stimulated the kinetics of bidirectional glucose transport. Uptake of [11C]3-OMG was higher in soleus than tibialis anterior muscle (by 22%; P < 0.01), a difference partially due to higher delivery, which was again found to be 35% higher to soleus (P < 0.01). The uptake of [18F]FDG was 65% greater in soleus compared with tibialis anterior muscle, a larger difference than for [11C]3-OMG (P < 0.01), indicating an added importance of glucose phosphorylation in defining insulin sensitivity. Analysis of the distribution of control during insulin-stimulated conditions revealed that most of the control was posited at delivery and transport and was equally divided between these steps. Thus, insulin evokes a broader distribution of control than during fasting conditions in governing glucose uptake into skeletal muscle. This redistribution of control is triggered by the robust stimulation of glucose transport, which in turn unmasks a greater dependence upon delivery and glucose phosphorylation.

Higher liver fat content among Japanese in Japan compared with non-Hispanic whites in the United States

Among Asians, including Japanese, obesity is related to dyslipidemia and insulin resistance at a lower level of body mass index (BMI) compared with non-Hispanic whites (NHW). We hypothesize that this ethnic difference in the relationship between BMI and metabolic risks is partly associated with the ethnic difference in fat distribution, namely, liver fat as well as visceral adipose tissue. To compare liver fat content among Japanese vs NHW men, regional computed tomographic images were taken to measure liver computed tomographic density in population-based samples of 313 Japanese and 288 NHW men aged 40 to 49 years, along with the assessment of metabolic parameters. Liver fat content was higher in Japanese than NHW men (liver to spleen attenuation ratio [lower value means higher liver fat content]: 1.01 +/- 0.16 vs 1.07 +/- 0.15, respectively; P < .01), despite a lower mean BMI in Japanese men (BMI: 23.6 +/- 2.9 vs 27.8 +/- 4.2 kg/m(2), P < .01). Moreover, Japanese men had a greater disposition for fatty liver with a small increase in BMI than NHW (P < .01), whereas both groups had a similar relationship between visceral adipose tissue and BMI. In both groups, liver fat content correlated with triglycerides, homeostasis model assessment of insulin resistance, and C-reactive protein. Liver fat content is higher among Japanese than NHW; and this ethnic difference becomes more robust with a small increase in BMI, suggesting that fatty liver is a sensitive marker for the failure of the adipose tissue to expand to accommodate an increased energy influx, and is associated with similar metabolic risk in Japanese despite lower BMI compared with NHW men.

PET imaging reveals distinctive roles for different regional adipose tissue depots in systemic glucose metabolism in nonobese humans

Excess amounts of abdominal subcutaneous (SAT) and visceral (VAT) adipose tissue (AT) are associated with insulin resistance, even in normal-weight subjects. In contrast, gluteal-femoral AT (GFAT) is hypothesized to offer protection against insulin resistance. Dynamic PET imaging studies were undertaken to examine the contributions of both metabolic activity and size (volume) of these depots in systemic glucose metabolism. Nonobese, healthy volunteers (n = 15) underwent dynamic PET imaging uptake of [¹⁸F]FDG at a steady-state (20 mU·m⁻²·min⁻¹) insulin infusion. PET images of tissue [¹⁸F]FDG activity were coregistered with MRI to derive K values for insulin-stimulated rates of fractional glucose uptake within tissue. Adipose tissue volume was calculated from DEXA and MRI. VAT had significantly higher rates of fractional glucose uptake per volume than SAT (P < 0.05) or GFAT (P < 0.01). K(GFAT) correlated positively (r = 0.67, P < 0.01) with systemic insulin sensitivity [glucose disappearance rate (R(d))] and negatively with insulin-suppressed FFA (r = -0.71, P < 0.01). SAT (r = -0.70, P < 0.01) and VAT mass (r = -0.55, P < 0.05) correlated negatively with R(d), but GFAT mass did not. We conclude that rates of fractional glucose uptake within GFAT and VAT are significantly and positively associated with systemic insulin sensitivity in nonobese subjects. Furthermore, whereas SAT and VAT amounts are confirmed to relate to systemic insulin resistance, GFAT amount is not associated with insulin resistance. These dynamic PET imaging studies indicate that both quantity and quality of specific AT depots have distinct roles in systemic insulin resistance and may help explain the metabolically obese but normal-weight phenotype.

Interactions among Glucose Delivery, Transport and Phosphorylation that Underlie Skeletal Muscle Insulin Resistance in Obesity and Type 2 Diabetes: Studies with Dynamic PET Imaging

Dynamic positron emission tomography (PET) imaging was performed using sequential tracer injections ([15O]H2O, [11C]3-O-methylglucose [3-OMG], and [18F]fluorodeoxyglucose [FDG]) to quantify, respectively, skeletal muscle tissue perfusion (glucose delivery), kinetics of bidirectional glucose transport, and glucose phosphorylation to interrogate the individual contribution and interaction among these steps in muscle insulin resistance (IR) in type 2 diabetes (T2D). PET imaging was performed in normal weight nondiabetic subjects (NW) (n = 5), obese nondiabetic subjects (OB) (n = 6), and obese subjects with T2D (n = 7) during fasting conditions and separately during a 6-h euglycemic insulin infusion at 40 mU·m−2·min−1. Tissue tracer activities were derived specifically within the soleus muscle with PET images and magnetic resonance imaging. During fasting, NW, OB, and T2D subjects had similar [11C]3-OMG and [18F]FDG uptake despite group differences for tissue perfusion. During insulin-stimulated conditions, IR was clearly evident in T2D (P < 0.01), and [18F]FDG uptake by muscle was inversely correlated with systemic IR (P < 0.001). The increase in insulin-stimulated glucose transport was less (P < 0.01) in T2D (twofold) than in NW (sevenfold) or OB (sixfold) subjects. The fractional phosphorylation of [18F]FDG during insulin infusion was also significantly lower in T2D (P < 0.01). Dynamic triple-tracer PET imaging indicates that skeletal muscle IR in T2D involves a severe impairment of glucose transport and additional impairment in the efficiency of glucose phosphorylation.