Joanna K Zawadzki

Skeletal muscle capillary density and fiber type are possible determinants of in vivo insulin resistance in man.

We have compared the capillary density and muscle fiber type of musculus vastus lateralis with in vivo insulin action determined by the euglycemic clamp (M value) in 23 Caucasians and 41 Pima Indian nondiabetic men. M value was significantly correlated with capillary density (r = 0.63; P less than or equal to 0.0001), percent type I fibers (r = 0.29; P less than 0.02), and percent type 2B fibers (r = -0.38; P less than 0.003). Fasting plasma glucose and insulin concentrations were significantly negatively correlated with capillary density (r = -0.46, P less than or equal to 0.0001; r = -0.47, P less than or equal to 0.0001, respectively). Waist circumference/thigh circumference ratio was correlated with percent type 1 fibers (r = -0.39; P less than 0.002). These results suggest that diffusion distance from capillary to muscle cells or some associated biochemical change, and fiber type, could play a role in determining in vivo insulin action. The association of muscle fiber type with body fat distribution may indicate that central obesity is only one aspect of a more generalized metabolic syndrome. The data may provide at least a partial explanation for the insulin resistance associated with obesity and for the altered kinetics of insulin action in the obese.

Familial Dependence of the Resting Metabolic Rate

Human obesity is known to be a familial disorder. We studied 130 nondiabetic adult southwestern American Indians (74 men and 56 women) from 54 families to determine whether the resting metabolic rate, as measured by indirect calorimetry, is a familial trait that is independent of individual differences in fat-free mass (estimated mass of metabolically active tissue), age, and sex. We found that most of the variance in the resting metabolic rate (83 percent, P less than 0.0001) was accounted for by three covariates--fat-free mass, age, and sex--and that fat-free mass was the most important determinant. Family membership accounted for an additional 11 percent (P less than 0.0001) of the variance in the resting metabolic rate. Thus, resting metabolic rate is a familial trait in this population, and it is independent of differences in fat-free mass, age, and sex. We also found that persons from families with lower resting metabolic rates were no more obese than persons from families with higher metabolic rates. This finding may be partly explained by the close correlation between fat-free mass and percentage of body fat (r = 0.81, P less than 0.0001), which indicates that the resting metabolic rate, as adjusted for fat-free mass, is already partly adjusted for obesity. Only prospective studies will elucidate whether the familial dependence of the resting metabolic rate is a contributing mechanism to the familial predisposition to obesity.

In Vivo Insulin Action Is Familial Characteristic in Nondiabetic Pima Indians

Non-insulin-dependent diabetes mellitus (NIDDM) is a genetic disorder characterized by two major pathogenic processes: reduced insulin action and a relative or absolute decrease in plasma insulin concentrations. We studied 116 nondiabetic siblings from 45 families to determine if in vivo insulin action showed any aggregation among siblings. Subjects were Pima Indians from the Gila River Indian Community in Arizona who, as a group, have the highest reported incidence and prevalence of NIDDM in the world. In vivo insulin action was determined by the euglycemic-clamp technique at two rates of insulin infusion in each subject with resulting mean plasma insulin concentrations of 119 and 1938 μU/ml. After adjustment for age, sex, and degree of obesity, there was significant aggregation among siblings of in vivo insulin action at the high insulin infusion rate (P < .0001). Family membership independently accounted for ∼34% of the variance in this measure of insulin action. Glucose uptake at the lower insulin infusion rate also showed familial aggregation (P < .01), with family membership independently accounting for ∼15% of the variance of this measurement. We conclude that in vivo insulin action is a familial characteristic. The familial component of insulin action occurs in addition to the effects of obesity, age, and sex on insulin action. Therefore it is not sufficient to simply know that an individual is lean or obese to predict his/her in vivo insulin resistance, because it must also be known whether he/she is from an insulin-resistant or insulin-sensitive family. An alteration of insulin action may be an underlying and potentially genetically determined abnormality in Pima Indians that could help explain the familial aggregation of diabetes in this population. Such a genetic mechanism might also operate in populations with Native American admixture (e.g., Mexicans) as well as other populations.

Relationships between plasma lipoprotein concentrations and insulin action in an obese hyperinsulinemic population

Relationships have been observed between lipoprotein concentrations and insulin action. These relationships may be important in explaining the association of insulin resistance and abnormalities of lipoprotein metabolism found in obesity, diabetes, and hypertriglyceridemia. We have measured plasma lipoprotein concentrations and indices of insulin action in 85 men and 56 women, all of whom were normolipidemic and had normal glucose tolerance. The subjects were obese Southwestern American Indians (body mass index 34 ± 1). Insulin action was measured via the hyperinsulinemic clamp with simultaneous indirect calorimetry. Triglyceride concentrations were inversely related to rates of total insulin-mediated glucose disposal (in men and women, respectively, r = –.37, P < .01; r = –.24, P < .10), glucose storage (r = –.31, P < .01; r = –.25, P < .10), increase in glucose oxidation (r = –.29, P < .01; r = –.24, P < .10), and, in men only, suppression of endogenous glucose production (r = –.32, P < .01). High-density lipoprotein (HDL) cholesterol concentration was positively related to rates of total insulin-mediated glucose disposal (r –.35, P < .01; r = .33, P < .05), increase in carbohydrate oxidation (r = .40, P < .001; r = .39, P < .001), suppression of endogenous glucose production (r = .24, P < .05; r = .29, P < .05), and, in men only, glucose storage (r = .35, P < .001). The relationships between lipoprotein concentrations and insulin action were independent of adiposity and insulin concentrations (with the exception of HDL cholesterol in women), and the relationships of Triglyceride and HDL cholesterol with insulin action were independent of each other. These relationships indicate that there may be a link between the development of insulin resistance and the abnormalities in lipoprotein metabolism associated with obesity, non-insulin-dependent diabetes mellitus, and hypertriglyceridemia.

Increased Resting Metabolic Rates in Obese Subjects with Non-insulin-dependent Diabetes Mellitus and the Effect of Sulfonylurea Therapy

Obese subjects with non-insulin-dependent diabetes mellitus (NIDDM) lose weight soon after diagnosis and tend to gain weight during hypoglycemic therapy. One explanation for these weight shifts is the change in caloric loss from glycosuria. We compared 24 obese Pima Indians with NIDDM to 24 Pima Indians with normal glucose tolerance to determine whether resting metabolic rate changes may be an additional factor influencing the weight shifts. The diabetic and nondiabetic subjects were equally obese, body fat 38 ± 1% versus 37 ± 1% (mean ± SEM), respectively, as determined by densitometry. In the morning after an overnight fast, resting metabolic rate (RMR) was measured by indirect calorimetry. The mean RMR of the diabetic subjects, 32.9 ± 0.5 kcal/day · kg fat-free mass (FFM), was 5% higher than that of the nondiabetic subjects, 31.4 ± 0.5 kcal/day kg FFM (P < 0.05). In nine of the diabetic subjects, 6 wk of tolazamide therapy was associated with reductions in mean FPG, 253 ± 16 to 144 ± 14 mg/dl (P < 0.01), mean daily urine glucose loss, 128 ± 26 to 11 ± 4 g (P < 0.01), and mean RMR, 31.9 ± 0.8 to 30.2 ± 0.6 kcal/day kg FFM (P < 0.04). Weight of the subjects was maintained constant from beginning to end of therapy (106.5 ± 9.6 versus 108.1 ± 9.9 kg) by decreasing daily calorie intake from 3070 ± 103 to 2784 ± 163 kcal (P < 0.01). We conclude that RMRs of obese, NIDDM subjects are increased compared with the RMRs of equally obese, nondiabetic subjects and that tolazamide therapy that decreases FPG reduces RMR in obese subjects with NIDDM.

Increased Rate of Cori Cycle in Obese Subjects With NIDDM and Effect of Weight Reduction

To determine the contribution of the rate of glucose recycling via the Cori cycle (glucose→3-carbon compounds→glucose) to the higher rate of endogenous glucose production (EGPR) in subjects with non-insulin-dependent diabetes mellitus (NIDDM), we studied eight obese, weight-stabilized diabetic Pima Indians before [93.1 ± 5.4 kg, 38 ± 2% body fat, fasting plasma glucose (FPG) 254 ± 11 mg/dl] and after (87.7 ± 4.7 kg, 36 ± 2% body fat, FPG 153 ± 17 mg/dl) a 5-wk weight-loss diet and eight obese Indians (95.0 ± 4.2 kg, 36 ± 2% body fat, FPG 97 ± 1 mg/dl) with normal glucose tolerance. EGPR and glucose recycling rate were measured during a 4-h primed continuous tracer infusion of [1-13C]glucose, and the rate of reincorporation of 1-13C of glucose into C2–6 positions in glucose was quantified by gas chromatography mass spectrometry. Substrate utilization rates were measured by simultaneous indirect calorimetry. EGPR (corrected for measured rate of recycling) decreased in the diabetic subjects from 3.80 to 2.74 mg·min−1·kg−1 fat-free mass (FFM) (P < .01) after weight loss, approaching the rate observed in nondiabetic subjects (2.09 mg·min−1·kg−1 FFM). The decrease in EGPR was associated with a decrease in nonoxidative carbohydrate disposal. Before weight loss, the measured rate of [1-13C]glucose recycling was higher in diabetic (0.23 ± 0.02 mg · min−1 · kg−1 FFM) than in nondiabetic (0.14 ± 0.01 mg min−1 · kg−1 FFM, P < .005) subjects. The glucose recycling rate did not change in diabetic subjects after weight loss (0.20 mg · min−1 · kg−1 FFM). We conclude that the rates of endogenous glucose production and the Cori cycle are increased in subjects with NIDDM compared with the rates observed in obese subjects with normal glucose tolerance. After a modest weight loss, the endogenous glucose production rate, but not the glucose recycling rate, in diabetic subjects approaches the rate observed in obese nondiabetic subjects.

Insulin Action in Obese Non-Insulin-Dependent Diabetics and in Their Isolated Adipocytes Before and After Weight Loss

To determine the effects of weight loss on insulin action in patients with non-insulin-dependent diabetes mellitus (NIDDM) and in their isolated adipocytes, we studied nine weight-stabilized Pima Indians [7 females and 2 males; age 39 ± 3 yr; wt 99.9 ± 8.2 kg; body fat 39 ± 2% (means ± SE)] before and after a 6.7 ± 1.3-kg weightloss and decrease in fasting plasma glucose from 250 ± 11 to 148 ± 15 mg/dl. Invivo insulin action was measured during a 3-insulin-step, hyperglycemic (∼310 mg/dl) clamp with somatostatin (250 μ/h). At a clamp plasma insulin concentration of ∼10 μU/ml, glucose disposal rates did not change after weight loss; at ∼100 μU/ml, glucose disposal rates increased by 21% [from 4.3 ± 0.2 to 5.3 ± 0.4 mg · min−1 · kg−1 of fat-free mass (FFM), P < .01] mostly due to increased carbohydrate oxidation rates (2.0 ± 0.3 to 2.8 ± 0.3 mg min−1 kg−1 FFM, P < .02); at ∼2400 μU/ml, glucose disposal rates increased by 37% (11.4 ± 0.6 to 15.6 ± 1.4 mg min−1 kg−1 FFM, P < .02) mostly due to increased nonoxidative carbohydrate disposal rates or storage (7.5 ± 0.6 to 10.9 ± 1.3 mg min−1 kg−1 FFM, P < .04). Sensitivity of glucose disposal to insulin in the physiologic range (measured as change in glucose disposal rate per unit change in insulin concentration between clamps at ∼10 and ∼100μU/ml) was very low in these diabetic subjects and did not change after weight loss. Adipocyte cell size, basal and maximal insulin-stimulated glucose transport, and half-miximal rate for transport did not change after weight loss. The data suggest that insulin in the physiologic range has no apparent effect on glucose disposal in patients with NIDDM before or after weight loss. However, a moderate weight loss isassociated with enhanced capacity to transport and metabolizeglucose in vivo. The discrepancy between in vivo and in vitro results suggests that the adipocytemay not always reflect in vivo insulin action.

Evidence for reduced thermic effect of insulin and glucose infusions in Pima Indians.

Several authors have reported a reduced thermic effect of food in obese subjects. The hyperinsulinemic-euglycemic clamp technique has been used to measure one component of the thermic effect of food, insulin and insulin-mediated glucose disposal. We used this technique to measure the thermic responses to insulin and glucose infusions in 120 glucose-tolerant Pima Indians, a population with a high prevalence of obesity. During high-dose insulin infusions (400 mU/m2 per min) the measured increase in energy expenditure (MEE), 150 +/- 6 cal/min, was greater than the predicted increase in energy expenditure (PEE), 72 +/- 2 cal/min, for glucose storage as glycogen. During low-dose insulin infusions (40 mU/m2 per min) the mean MEE, 6 +/- 5 cal/min, was not significantly different from zero and was not greater than the mean PEE, 9 +/- 1 cal/min. These data were in contrast to results obtained from Caucasians by others and suggested a markedly reduced thermic effect of low-dose insulin and glucose infusions in Pima Indians. We also studied 23 glucose-tolerant male Caucasians and compared their results with the results from male Indians matched for glucose storage rates and obesity. The results showed that the thermic response to insulin and glucose infusions was similar in the two racial groups during high-dose insulin infusions but was markedly reduced in the Indians compared with the Caucasians during low-dose insulin infusions.

Thermic Effect of Glucose in Obese Subjects With Non-Insulin-Dependent Diabetes Mellitus

A reduced thermic effect of glucose in non-insulindependent diabetes mellitus (NIDDM) has been previously reported. To investigate whether this defect is related to 1) a decreased rate of glucose storage, 2) a reduced energy cost of glucose storage, or 3) a defect in the sympathetically mediated component of thermogenesis, we studied the thermic effect of ingested and infused glucose in nine NIDDM obese Pima Indians [90.5 ± 3.9 kg, 39 ± 2% fat, fasting plasma glucose (FPG) 130 ± 10 mg/dl] and in nine nondiabetic obese Pima Indians (99.3 ± 7.2 kg, 38 ± 2% fat, FPG 103 ± 2 mg/dl). Energy expenditure and glucose storage were derived from indirect calorimetry measurements. The thermic effect of 100 g of glucose was found to be similar in both groups (4.0 ± 0.9 vs. 5.0 ± 1.3% of energy ingested in diabetic and nondiabetic subjects, respectively) but lower than that previously reported in nonobese subjects. The cost of glucose storage calculated after stimulating storage by constant glucose infusion (0.46 g/min) and variable insulin infusion (19.5 ± 3.8 vs. 2.9 ± 0.6 mU · kg−1 · min−1 in diabetic and nondiabetic subjects, respectively; P < .01) was similar in both groups (∼0.35 kcal/g glucose stored) and not different from that reported in lean subjects. As opposed to lean and obese Caucasian subjects, energy expenditure failed to markedly decrease during propranolol infusion in both nondiabetic and diabetic Pima Indians. We postulate that the decreased rate of tissue glucose uptake and storage associated with insulin resistance is the major cause of the lower thermic effect of ingested glucose in NIDDM. It is unlikely that a reduced thermic effect of glucose causes or perpetuates obesity in NIDDM, because a low thermic response is the consequence of the increased insulin resistance and is opposed by greater increases in resting metabolic rate.

Comparison of glucose metabolism in adipocytes from Pima Indians and Caucasians

The present study was designed to compare various aspects of glucose metabolism of adipocytes isolated from 10 obese Pima Indians and 10 Caucasians. All 20 subjects had normal glucose tolerance, and the two groups were matched for sex, age, degree of obesity, and fasting plasma glucose and insulin concentrations. The results indicated that adipocytes from the two groups were comparable in average cell size, basal and maximum insulin-stimulated glucose transport, glucose metabolism at 5.5 mmol/L glucose, ED50 of insulin for glucose transport and the inhibition of lipolysis, basal lipolysis rates, and insulin binding. The similarity between the metabolic behavior of adipocytes isolated from Pimas and Caucasians suggests that the study of adipocytes from Pima Indians yields information that is relevant to the understanding of obesity in other populations.