Divisions of 1 Nutrition/Metabolism, 3 Nuclear Medicine, 4 Diagnostic Radiology, and 2 Unit of Clinical Spectroscopy, Università Vita e Salute San Raffaele, Istituto Scientifico H San Raffaele, 20132 Milan; and 5 International Center for the Assessment of Nutritional Status, Università degli Studi di Milano, 20133 Milan, Italy
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Intramyocellular lipid (IMCL) storage is considered a local marker of whole body insulin resistance; because increments of body weight are supposed to impair insulin sensitivity, this study was designed to assess IMCL content, lipid oxidation, and insulin action in individuals with a moderate increment of body fat mass and no family history of diabetes. We studied 14 young, nonobese women with body fat <30% (n = 7) or >30% (n = 7) and 14 young, nonobese men with body fat <25% (n = 7) or >25% (n = 7) by means of the euglycemic-insulin clamp to assess whole body glucose metabolism, with indirect calorimetry to assess lipid oxidation, by localized 1H NMR spectroscopy of the calf muscles to assess IMCL content, and with dual-energy X-ray absorptiometry to assess body composition. Subjects with higher body fat had normal insulin-stimulated glucose disposal (P = 0.80), IMCL content in both soleus (P = 0.22) and tibialis anterior (P = 0.75) muscles, and plasma free fatty acid levels (P = 0.075) compared with leaner subjects in association with increased lipid oxidation (P < 0.05), resting energy expenditure (P = 0.046), resting oxygen consumption (P = 0.049), and plasma leptin levels (P < 0.01) in the postabsorptive condition. In conclusion, in overweight subjects, preservation of insulin sensitivity was combined with increased lipid oxidation and maintenance of normal IMCL content, suggesting that abnormalities of these factors may mutually determine the development of insulin resistance associated with weight gain.
intramyocellular lipid content; lipid oxidation; insulin
resistance; leptin; tumor necrosis factor-; nuclear magnetic
resonance spectroscopy
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
HIGHER RISK OF TYPE 2 DIABETES is associated with moderate overweight (58), and a detrimental lipid profile is frequently combined with obesity. The abnormalities of lipid metabolism may be primarily involved in the pathogenesis of obesity and type 2 diabetes (32); intramyocellular lipid (triglyceride) (IMCL) storage was proposed to modulate the development of insulin resistance (32). 1H NMR spectroscopy showed the feasibility of identifying and measuring lipids within the fat cells and within the muscle cells separately (8, 45), and this measurement was shown to be comparable to biochemical assay (53). With use of this approach, a relationship between IMCL content and whole body insulin sensitivity was shown in normal humans (28, 50), offspring of type 2 diabetic parents (22, 41), and obese patients with type 2 diabetes mellitus (51). Even if the metabolic effects of IMCL accumulation are intensively investigated, little is known about the rate-limiting factors involved in the regulation of the IMCL content. Increments of IMCL might be due to increased fatty acid flux from the adipose tissue or to a reduced oxidative disposal. Obesity and type 2 diabetes mellitus were proposed to be associated with decreased muscle lipid oxidation (6, 9, 24), but its relationship with insulin sensitivity and the effects on IMCL content are controversial. Increased fat oxidation, because of the increased fat flux, has been considered responsible for insulin resistance (25); recently, insulin resistance was more precisely associated with a metabolic muscle inflexibility characterized by a reduced ability to utilize fat in the postabsorptive state, but at the same time by reduced ability to suppress lipid oxidation in insulin-stimulated states (25). This study aimed to assess the effect of moderate overweight in young healthy subjects on whole body insulin action, IMCL content, lipid oxidation, and plasma free fatty acid (FFA) concentrations in a cross-sectional fashion. To avoid the confounding effect of genetic factors, we carefully selected young, healthy, normal-weight [body mass index (BMI) <27 kg/m2], and nonexercising subjects who had no family history of diabetes or additional metabolic diseases and who were not taking any medications.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Subjects
Twenty-eight healthy men and women were recruited at the Istituto Scientifico H San Raffaele. The criteria for inclusion in the study were 1) no family history of diabetes, obesity, and hypertension traced through their grandparents, 2) age (24-40 yr), 3) white race, 4) BMI <27 kg/m2, 5) sedentary life style, and 6) no history of hypertension, endocrine/metabolic disease, or cigarette smoking. Habitual physical activity was assessed using a questionnaire (1). Body weight was stable for
|
Experimental Protocol
Subjects were asked to consume an isocaloric diet containing 250 g/day of carbohydrates and to abstain from exercise activity for 3 wk before the studies. Women were studied between day 3 and day 8 of the menstrual cycle. Subjects were studied by means of the euglycemic-hyperinsulinemic clamp and indirect calorimetry to assess whole body insulin sensitivity, resting energy expenditure (REE), and glucose and lipid oxidation after a 10-h overnight fast period and during the insulin clamp. Within 2-3 days they were studied by means of 1H NMR spectroscopy to assess IMCL content and by means of dual-energy X-ray absorptiometry (DEXA) to assess body composition. The NMR session was performed in the Division of Diagnostic Radiology of the Istituto Scientifico H San Raffaele.Euglycemic-hyperinsulinemic clamp.
Subjects were admitted to the Metabolic Unit of the Division of
Internal Medicine I of the Istituto Scientifico H San Raffaele at 7:00
AM after a 10-h overnight fast. A Teflon catheter was inserted into an
antecubital vein for infusions, and an additional one was inserted
retrogradely into a wrist vein for blood sampling. The hand was kept in
a heated box (50°C) throughout the experiment to allow sampling of
arterialized venous blood. A bolus (5 mg/kg body wt), followed by a
300-min period of continuous infusion (0.05 mg · kg body
wt1 · min
1) of
[6,6-2H2]glucose obtained from MassTrace
(Woburn, MA), was administered. Basal blood samples for glucose and
tracer enrichment were collected on four occasions before insulin
infusion during the last 45 min of the 150-min tracer equilibration
period; samples for FFA, the lipid profile, insulin, C-peptide, tumor
necrosis factor receptor-2 (TNFR-2), and leptin were drawn twice in the
same 45-min interval (at
30 and time 0). After the 150-min
tracer equilibration period, a euglycemic-hyperinsulinemic clamp was
performed as previously described (10). Insulin was
infused at 40 mU · m
2 · min
1 to reach
a plasma insulin concentration of ~350 pmol/l, and plasma glucose
concentration was kept at ~5 mmol/l for an additional 150 min by
means of a variable infusion of 20% dextrose. Blood samples for plasma
hormones, substrates, and tracer enrichment were drawn every 15 min
throughout the study.
Indirect calorimetry.
While subjects lay quietly for 45 min during the basal equilibration
period and at the end of the euglycemic-hyperinsulinemic clamp,
indirect calorimetry was performed continuously with a ventilated hood
system (Sensor Medics 2900, Metabolic Measurement Cart) to measure
oxygen consumption (O2) and carbon
dioxide production (
CO2) for
calculations of glucose and lipid oxidation. The mean coefficient of
variation within the session for both O2 and
CO2 measurements was below 2%. In our metabolic unit, the daily variability of the REE assessed in 25 subjects in the
1999-2001 period was 3.5 ± 0.3% for
O2 and 4.1 ± 0.5% for
CO2.
1H NMR spectroscopy.
1H NMR spectroscopy was performed on a GE Signa 1.5 Tesla
scanner (General Electric Medical Systems, Milwaukee, WI) with a conventional linear extremity coil, as previously described
(41). High-resolution T1 weighted images of
the right calf were obtained before the spectroscopic acquisitions to
localize the voxel of interest for the 1H spectroscopy
study. The voxel shimming was executed to optimize the homogeneity of
the magnetic field within the specific volume of interest. Two
1H spectra were collected from a 15 × 15 × 15-mm3 volume within the soleus and tibialis anterior
muscles. A PRESS pulse sequence (repetition time = 2,000 ms and
echo time = 60 ms) was used, and 128 averages were accumulated for
each spectrum, with a final acquisition time of 4.5 min. The water
signal was suppressed during the acquisition, because it would dominate
the other metabolites' peak signals of interest. A third
1H spectrum of a triglyceride solution inside a glass
sphere, positioned within the extremity coil next to the calf, was also
obtained during the same session to have an external standard acquired in the same conditions as the subject's spectra. Postprocessing, executed with the Sage/IDL software (GE Medical Systems), consisted of
highpass filtering, spectral apodization, zerofilling, Fourier transformation, and the phasing of the spectra. The integral of the
area under the peak was calculated using a Marquardt fitting with
Lorentzian functions of the peaks of interest. The integral of the
methylene signal (CH2) at 1.35 ppm was used to calculate IMCL content expressed in arbitrary units as a ratio with the integral
of the peak of the external standard × 1,000. Daily variability of the soleus IMCL content assessed in seven subjects in the
1999-2001 period was 11 ± 3%.
Body composition. DEXA was performed with a Lunar-DPX-IQ scanner (Lunar, Madison, WI). A different scan mode was chosen with respect to each subject's body size, as suggested by the manufacturer's operator manual. For regional analysis, three-compartment processing was performed in the arms, trunk, and legs (34). Fat content is expressed as kilograms of fat mass and as a percentage of tissues. Svendsen et al. (52) showed that abdominal fat content assessed by means of DEXA explained ~80% of the variation in the abdominal fat assessed by computed tomography scan in postmenopausal women. We also assessed the abdominal fat content with a procedure available in the software by manually selecting the region of interest (ROI). The ROI was selected by the same operator, as previously described (52), comprising the abdominal tissue between the 1st and 4th lumbar intervertebral disks, and the lines of the rib box were adjusted (standard software option).
Analytical Procedures
Plasma glucose was measured with a Beckman glucose analyzer (41). Plasma FFAs and plasma total cholesterol, high-density lipoprotein (HDL)-cholesterol, and triglycerides were measured as previously described (41). Low-density lipoprotein (LDL)-cholesterol was calculated using the Friedewald formula. Serum urea nitrogen was measured in the postabsorptive and hyperinsulinemic conditions by use of an enzymatic method on a Hitachi 747. Plasma insulin was measured with microparticle enzyme immunoassay technology (35) with no cross-reactions with proinsulin, C-peptide, and glucagon (IMx Insulin assay, Abbott Laboratories, Rome, Italy), and C-peptide was measured with a double-antibody RIA (41). Plasma leptin concentrations were determined by RIA (Linco Research, St. Charles, MO) as previously described (41). TNFR-2 was measured with an enzyme immunoassay by following manufacturer (Immunotech Beckman Coulter, Marseille, France) recommendations. The [2H2]glucose enrichment was measured by gas chromatography-mass spectrometry, as previously described (2).Calculations
Glucose turnover was calculated in the basal state by dividing the [6,6-2H2]glucose infusion rate by the steady-state plateau of plasma [6,6-2H2]glucose enrichment achieved during the last 45 min of the basal period. Glucose kinetics during the insulin clamp were calculated by using Steele's equations for the nonsteady state (49). Steady state of plasma enrichments was reached in the study groups during the last 30 min of the insulin clamp. Endogenous glucose production (EGP) was calculated by subtracting the glucose infusion rate (GIR) from the rate of glucose appearance measured with the isotope tracer technique. Total body glucose uptake (Rd) was determined during the clamp by adding the rate of residual EGP to the GIR. Insulin sensitivity, SIP(clamp), was obtained as follows:Statistical Analysis
All data are presented as means ± SE. The steady state for plasma [6,6-2H2]glucose enrichment was defined as a nonsignificant correlation with time (P > 0.05) by use of standard linear regression. Comparisons among groups were performed by use of ANOVA, with Scheffé's post hoc testing when appropriate. Simple regression analysis was performed to assess relationships between variables. ![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Anthropometric Characteristics
Anthropometric parameters of study subjects are summarized in Table 1. Overweight and normal subjects were comparable for age, gender, and physical activity index; in addition, LBM and height were not different. BMI was higher in the overweight than in the normal subjects (23.5 ± 0.8 vs. 21.1 ± 0.6 kg/m2; P = 0.05), even if within the normal range. Total body fat content, regional distribution of fat in the appendicular (arms and legs), trunk, and abdominal areas, and fat content in each area normalized to the total fat content are summarized in Table 1.Postabsorptive Energy Homeostasis
|
Glucose Metabolism in the Postabsorptive State and During the Insulin Clamp
Postabsorptive plasma glucose levels and EGP rates were similar in overweight and normal subjects (Table 3). Insulin-stimulated glucose disposal was comparable in overweight and normal subjects (Fig. 1), even if the overweight subjects showed higher glucose oxidative disposal (Table 3; P < 0.05) and a nonsignificantly lower nonoxidative glucose disposal (Table 3; P = 0.19) compared with normal subjects.
|
|
Plasma Lipid Profile in the Postabsorptive State and During the Insulin Clamp
In the postabsorptive state, the plasma lipid profile was comparable between the groups (Table 3). Plasma FFA concentrations showed a trend to be increased in overweight compared with normal subjects (P = 0.075); on the contrary, circulating glycerol andLipid Oxidation
Postabsorptive lipid oxidation was similar between the overweight and normal subjects when corrected as kilograms of body weight (0.898 ± 0.058 vs. 0.916 ± 0.059 mg · kg
|
IMCL Content
1H NMR spectroscopy of the calf muscle showed that IMCL content in the soleus (Fig. 3A: 62 ± 7 vs. 51 ± 6 AU; P = 0.22) and tibialis anterior (Fig. 3B: 13 ± 3 vs. 12 ± 2 AU; P = 0.67) muscles was similar in the overweight and normal subjects. Normal men showed slightly lower (P = 0.17) IMCL content in the soleus muscle compared with women, and a nonsignificantly lower value compared with overweight men (P = 0.14), in whom the soleus IMCL content showed a threefold larger variability.
|
Plasma Insulin, C-Peptide, and Cortisol Concentrations
Postabsorptive plasma insulin concentrations were comparable in the study groups and similarly increased during the insulin clamp (Table 3), with no significant gender difference. Plasma postabsorptive C-peptide concentrations were also comparable between the groups (0.39 ± 0.03 vs. 0.40 ± 0.05 nmol/l; P = 0.84, respectively, in overweight and normal subjects), and the concentration levels dropped similarly during the clamp (55 ± 5 vs. 58 ± 5%; P = 0.71). Postabsorptive serum cortisol concentration was also similar in the study groups, with no gender-related difference (73 ± 12 vs. 58 ± 9 ng/ml; P = 0.20).Plasma Leptin Concentration
Postabsorptive plasma leptin concentration was reduced in men compared with women in both normal (Fig. 4: 2.43 ± 0.61 vs. 5.18 ± 0.85 ng/ml; P = 0.02) and overweight subjects (Fig. 4: 5.12 ± 0.63 vs. 11.0 ± 2.01 ng/ml, P = 0.05). Overweight subjects had higher circulating plasma leptin than normal subjects (Fig. 4: 8.04 ± 1.30 vs. 3.81 ± 0.63 ng/ml; P < 0.01); nevertheless, when normalized to kilogram of body fat, it was comparable to that of the normal subjects (Fig. 4: 0.37 ± 0.05 vs. 0.34 ± 0.05; P = 0.71).
|
Plasma Soluble -TNFR-2 Concentration
Regression Analysis
A stepwise regression analysis was performed, with insulin sensitivity selected as the dependent variable, and age, BMI, percent fat content, LBM, IMCL muscle content, lipid oxidation and its insulin-dependent suppression, REE (corrected for kg body wt or lean mass), postabsorptive plasma FFA concentration, plasma total cholesterol (or HDL/LDL fractions) and triglycerides, postabsorptive plasma leptin (or leptin levels corrected to kg of fat mass), and
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The results of this work demonstrate that, in healthy young sedentary subjects with no genetic background of diabetes or other diseases known to impair insulin sensitivity, the lack of deleterious effects of moderately higher body fat content (>25% in men and >30% in women) on insulin sensitivity and fatty acid metabolism was associated with increased lipid oxidation in the postabsorptive condition. To our knowledge, this is the first report in which increased postabsorptive lipid oxidation is associated with a beneficial effect on insulin action in healthy sedentary humans.
Fatty acids may induce insulin resistance by means of inhibition of insulin signaling (11), and their accumulation in the myocytes has been associated with insulin resistance (22, 28, 41, 48, 50, 51). The findings of the present study would support the hypothesis that IMCL accretion in a condition of insulin resistance may derive from reduced lipid oxidation (24, 27). The present study represents the reverse experiment, in which subjects able to trigger the oxidative disposal of fatty acids developed an efficient compensatory mechanism to fight muscle fatty acid accumulation (driven by the higher fat flux) and consequently to avoid insulin resistance. Even in sedentary subjects, the fatty acid oxidation would represent a crucial compensatory mechanism to fight insulin resistance and additional body weight gain in the early stages of the pathogenesis of obesity and type 2 diabetes (5, 7). In agreement with these findings, we recently observed that, in human immunodeficiency virus (HIV)-infected patients with lipodystrophy, insulin resistance and abnormal accumulation of IMCL content were associated with a marked impairment of postabsorptive lipid oxidation (39), probably related in part to the infection per se and in part to the effects of the anti-retroviral drugs.
It was interesting to notice that our overweight subjects were normally sensitive to insulin stimulation, which might be in contrast to previous reports (44). In a recent study (12), metabolically obese but normal-weight women, a subset of the normal-weight general population with increased body fat content similar to the women of our study, showed along with higher body fat content a cluster of phenotypic characteristics that may predispose them to the insulin-resistant syndrome (12). We believe that our subjects showed a different behavior because their increased body fat mass was not located in the abdominal area and, most importantly, they were carefully screened to have no personal or family history of insulin-resistant states, which allowed us the selection of subjects with lower risk to develop the metabolic syndrome and to inherit abnormalities of fatty acid metabolism (fat oxidation).
We (42) and others (15) recently reported that, in nonobese subjects, women showed some degree of protection from fatty acid-induced muscle insulin resistance compared with men; in the present study, we confirm the observation of higher IMCL soleus content in the normal women compared with men, regardless of the fact that insulin sensitivity was similar. Very recently it was shown (4) that fatty acid transport protein-1 mRNA expression in skeletal muscle in lean women was higher than in lean men, and that this might contribute to the higher IMCL content in women. In addition, in the present study we observed that the overweight women compared with men showed a stronger aptitude to increase lipid oxidation (Fig. 3) and therefore to keep IMCL content (Fig. 2) within levels similar to those of normal subjects, suggesting also that, in conditions of moderate increment of body fat mass, women seem better able to handle muscle fatty acid-induced insulin resistance. We think that the protection is due to the effects of estrogens (19, 42) and to a greater ability to store fat in the adipose tissue.
In our study lipid oxidation was assessed using indirect calorimetry, but this method is not a direct measure of leg lipid oxidation (24, 25) or of the metabolic oxidative capacity of the muscle (17, 24, 48); therefore, these data must be taken with caution. Nevertheless, we believe that these limitations have been partially circumvented by correcting the lipid oxidative disposal for LBM. In fact, in the postabsorptive condition, the skeletal muscle and the liver are the tissues contributing most to fat oxidation; meanwhile, the adipose tissue is minimally involved. In addition, female athletes demonstrated higher postabsorptive lipid oxidation than sedentary women, and exercise training is known to be associated with increased insulin sensitivity (40); also in that work (30), differences in lipid oxidation rates were detected by means of indirect calorimetry.
In animal models, leptin was shown to prevent lipotoxicity
(55) and insulin resistance (36, 46),
repartitioning fatty acids toward oxidation and away from storage
(56) via peroxisome proliferator-activated receptor-
(PPAR
) stimulation (59). Leptin levels, as expected,
were increased in the overweight compared with normal subjects, and
because the ratio of leptin to kilogram of fat mass was unchanged (Fig.
4), the higher levels were proportional to the increased body fat mass.
Because these overweight subjects accumulated the higher fat mass
homogeneously in the body, with no predilection for trunk or abdominal
accretion (Table 1), and because leptin is known to be predominantly
secreted by subcutaneous adipose tissue rather than visceral tissue
(33, 34), it is possible that these subjects were
counteracting body weight gain with appropriate leptin secretion. We
failed to find a relationship between leptin levels and insulin
sensitivity (R2 = 0.21, P = 0.10) or postabsorptive lipid oxidation (R2 = 0.20, P = 0.16). Nevertheless, because the higher
lipid oxidative rates were paralleled by higher postabsorptive
O2 and higher REE (Table 2), we also
tested whether leptin levels were associated with these parameters and
found that the relationships were significant (Fig. 5). Therefore, even
if the fact that leptin was involved in this compensatory mechanism was
not statistically proved, in our opinion this possibility may not be
excluded. Obesity-related insulin resistance may be also mediated by
increased expression of TNF-
(21), and soluble,
circulating
-TNFR-2 has been suggested to be involved in human
obesity in women, modulating the action of TNF-
(20).
The postabsorptive levels of the soluble receptor were similar between
the two groups, suggesting that a major role of the
-TNF system
activity in obesity-induced insulin resistance may become evident in
more severe degrees of obesity.
The higher lipid oxidation rates found in our overweight subjects may
represent a factor that counteracts IMCL accumulation and avoids fatty
acid-induced insulin resistance (38, 43); we must also
emphasize that the overweight subjects had a significant increment of
body fat mass, and this may reflect a successful storing capacity of
the excessive dietary intake in the adipose tissue, sparing its
deposition in the skeletal muscle, the liver, and the -cell. In
lipoatrophic diabetes, a lack of adipose tissue is associated with
severe insulin resistance in humans (23, 31); also, in the
A-ZIP/F-1, a transgenic mouse with no adipose tissue (16),
insulin resistance is mediated by lipotoxicity in the liver and
skeletal muscle (47). The protective role of the adipose
tissue is further reflected by the fact that its surgical implantation
in this animal model reverses diabetes (16), requesting fatty acids from the other peripheral tissues. Moreover, in obese and
diabetic patients, the expression of adipogenic genes was found to be
decreased (37), further supporting the importance of the
normal function of the adipose tissue in controlling energy and glucose
homeostasis (47). It is possible that, in the overweight subjects, a proper adipose tissue storing capacity may have contributed to the counteraction against fatty acid-induced insulin resistance in
the other peripheral tissues.
In conclusion, this work demonstrated that moderately overweight subjects may maintain normal IMCL content and insulin sensitivity in association with increased fasting lipid oxidation; whether this compensatory mechanism is mediated by the increment of the leptin levels is uncertain. We believe that this study suggests that 1) maintenance of normal IMCL content is crucial for preserving insulin sensitivity, and 2) inherited or acquired alterations in the ability of muscle (and possibly liver) to oxidize fat represent predisposing factors for development of abnormal IMCL accretion and, in turn, insulin resistance and obesity.
![]() |
ACKNOWLEDGEMENTS |
---|
We thank Van Chuong Phan, Paola Sandoli, and Sabrina Costa for their skilled work with hormone and substrate assessments, and Antonella Scollo of the Metabolic Unit of the Istituto Scientifico H San Raffaele for nursing assistance.
![]() |
FOOTNOTES |
---|
This work was supported by grants from the Italian Ministry of Health (030.5/RF96.305 and 030.5/RF98.49) and the Italian National Research Council (CNR 97.00485.CT04). The financial support of Telethon, Italy (1032C), is also gratefully acknowledged.
Address for reprint requests and other correspondence: G. Perseghin, Nutrition/Metabolism, Laboratory of Amino Acids and Stable Isotopes/Unit of Clinical Spectroscopy, via Olgettina 60, 20132 Milan, Italy (E-mail: perseghin.gianluca{at}hsr.it).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpendo.00127.2002
Received 22 March 2002; accepted in final form 13 May 2002.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Baecke, JAH,
Burema J,
and
Frijters JER
A short questionnaire for the measurement of habitual physical activity in epidemiological studies.
Am J Clin Nutr
36:
936-942,
1982[Abstract].
2.
Battezzati, A,
Simonson DC,
Luzi L,
and
Matthews DE.
Glucagon increases glutamine uptake without affecting glutamine release in humans.
Metabolism
47:
713-723,
1998[ISI][Medline].
3.
Bergman, RN,
Finegood DT,
and
Ader M.
Assessment of insulin sensitivity in vivo.
Endocr Rev
6:
45-86,
1985[ISI][Medline].
4.
Binnert, C,
Koistinen HA,
Martin G,
Andreelli F,
Ebeling P,
Koivisto VA,
Laville M,
Auwerx J,
and
Vidal H.
Fatty acid transport protein-1 mRNA expression in skeletal muscle and in adipose tissue in humans.
Am J Physiol Endocrinol Metab
279:
E1072-E1079,
2000
5.
Blaak, EE,
and
Wagenmakers AJ.
The fate of [U-13C]palmitate extracted by skeletal muscle in subjects with type 2 diabetes and control subjects.
Diabetes
51:
784-789,
2002
6.
Blaak, EE,
Wagenmakers AJM,
Glatz JFC,
Wolffenbuttel BHR,
Kemerink GJ,
Langerberg CJM,
Heidendal GAK,
and
Saris WHM
Plasma FFA utilization and fatty acid-binding protein content are diminished in type 2 diabetic muscle.
Am J Physiol Endocrinol Metab
279:
E146-E154,
2000
7.
Blaak, EE,
Wolffenbuttel BH,
Saris WH,
Pelsers MM,
and
Wagenmakers AJ.
Weight reduction and the impaired plasma-derived free fatty acid oxidation in type 2 diabetic subjects.
J Clin Endocrinol Metab
86:
1638-1644,
2001
8.
Boesch, C,
Slotboom J,
Hoppeler H,
and
Kreis R.
In vivo determination of intramyocellular lipids in human muscle by means of localized 1H-MR-spectroscopy.
Magn Reson Med
37:
484-493,
1997[ISI][Medline].
9.
Coldberg, SR,
Simoneau JA,
Thaete FL,
and
Kelley DE.
Skeletal muscle utilization of free fatty acids in women with visceral obesity.
J Clin Invest
95:
1846-1853,
1995[ISI][Medline].
10.
DeFronzo, RA,
Tobin JD,
and
Andres R.
Glucose clamp technique: a method for quantifying insulin secretion and resistance.
Am J Physiol
6:
214-223,
1979.
11.
Dresner, A,
Laurent D,
Marcucci M,
Griffin ME,
Dufour S,
Cline GW,
Slezak LA,
Andersen DK,
Hundal RS,
Rothman DL,
Petersen KF,
and
Shulman GI.
Effects of free fatty acids on glucose transport and IRS-1-associated phosphatidylinositol 3-kinase activity.
J Clin Invest
103:
253-259,
1999
12.
Dvorak, RV,
DeNino WF,
Ades PA,
and
Poehlam ET.
Phenotypic characteristics associated with insulin resistance in metabolically obese but normal-weight young women.
Diabetes
48:
2210-2214,
1999[Abstract].
13.
Elia, M.
Organ and tissue contribution to metabolic rate.
In: Energy Metabolism: Tissue Determinants and Cellular Corollaries, , edited by Kinney JM,
and Tucker HN.. New York: Raven, 1992, p. 61-79.
14.
Frayn, KN.
Calculation of substrate oxidation rates in vivo from gaseous exchange.
J Appl Physiol
55:
628-634,
1983
15.
Frias, JP,
Macaraeg GB,
Ofrecio J,
Yu JG,
Olefsky JM,
and
Kruszynska YT.
Decreased susceptibility to fatty acid-induced peripheral tissue insulin resistance in women.
Diabetes
50:
1344-1350,
2001
16.
Gavrilova, O,
Marcus-Samuels B,
Graham D,
Kim JK,
Shulman GI,
Castle AL,
Vinson C,
Eckhaus M,
and
Reitman ML.
Surgical implantation of adipose tissue reverses diabetes in lipoatrophic mice.
J Clin Invest
105:
271-278,
2000
17.
Goodpaster, BH,
He J,
Watkins S,
and
Kelley DE.
Skeletal muscle lipid content and insulin resistance: evidence for a paradox in endurance-trained athletes.
J Clin Endocrinol Metab
86:
5755-5761,
2001
18.
Hawk, PD.
Kjeldhal method.
In: Practical Physiological Chemistry (12th ed.). Toronto: Blakiston, 1947, p. 814-822.
19.
Hevener, AL,
Wong JC,
Janez A,
and
Olefsky J.
Estrogen protects against free fatty acid-induced insulin resistance (Abstract A307).
Diabetes
50, Suppl2:
1227,
2001
20.
Hotamisligil, GS,
Arner P,
Atkinson RL,
and
Spiegelman BM.
Differential regulation of the p80 tumor necrosis factor receptor in human obesity and insulin resistance.
Diabetes
46:
451-455,
1997[Abstract].
21.
Hotamisligil, GS,
Arner P,
Caro JF,
Atkinson RL,
and
Spiegelman BM.
Increased adipose tissue expression of tumor necrosis factor- in human obesity and insulin resistance.
J Clin Invest
95:
2409-2415,
1995[ISI][Medline].
22.
Jacob, S,
Machann J,
Rett K,
Brechtel K,
Volk A,
Renn W,
Maerker E,
Matthaei S,
Schick F,
Claussen CD,
and
Häring HU.
Association of increased intramyocellular lipid content with insulin resistance in lean nondiabetic offspring of type 2 diabetic subjects.
Diabetes
48:
1113-1119,
1999[Abstract].
23.
Kahn, BB,
and
Flier JS.
Obesity and insulin resistance.
J Clin Invest
106:
473-478,
2000
24.
Kelley, DE,
Goodpaster B,
Wing RR,
and
Simoneau JA.
Skeletal muscle fatty acid metabolism in association with insulin resistance, obesity and weight loss.
Am J Physiol Endocrinol Metab
277:
E1130-E1141,
1999
25.
Kelley, DE,
and
Mandarino LJ.
Fuel selection in human skeletal muscle in insulin resistance. A reexamination.
Diabetes
49:
677-683,
2000[Abstract].
26.
Kim, JK,
Gavrilova O,
Chen Y,
Reitman ML,
and
Shulman GI.
Mechanism of insulin resistance in A-ZIP/F-1 fatless mice.
J Biol Chem
275:
8456-8460,
2000
27.
Kim, JY,
Hickner RC,
Cortright RL,
Dohm GL,
and
Houmard JA.
Lipid oxidation is reduced in obese human skeletal muscle.
Am J Physiol Endocrinol Metab
279:
E1039-E1044,
2000
28.
Krssak, M,
Petersen KF,
Dresner A,
DiPietro L,
Vogel SM,
Rothman DL,
Shulman GI,
and
Roden M.
Intramyocellular lipid concentrations are correlated with insulin sensitivity in humans: a 1H NMR spectroscopy study.
Diabetologia
42:
113-116,
1999[ISI][Medline].
29.
Ley, CJ,
Lees B,
and
Stevenson JC.
Sex- and menopause-associated changes in body-fat distribution.
Am J Clin Nutr
55:
950-954,
1992[Abstract].
30.
Lopez, P,
Ledoux M,
and
Garrel DR.
Increased thermogenic response to food and fat oxidation in female athletes: relationship with O2 max.
Am J Physiol Endocrinol Metab
279:
E601-E607,
2000
31.
Luzi, L,
Dozio N,
Battezzati A,
Perseghin G,
Sarugeri E,
Terruzzi I,
and
Spotti D.
Anomalous leucine metabolism in total lipoatrophic diabetes: a possible mechanism of muscle mass hypertrophy.
Acta Diabetologica
29:
86-93,
1992[ISI].
32.
McGarry, JD.
What if Minkowski had been ageusic? An alternative angle on diabetes.
Science
258:
766-770,
1992[ISI][Medline].
33.
Montague, CT,
and
O'Rahilly S.
The perils of portliness. Causes and consequences of visceral adiposity.
Diabetes
49:
883-888,
2000[Abstract].
34.
Montague, CT,
Prins JB,
Sanders L,
Digby JE,
and
O'Rahilly S.
Depot- and sex-specific differences in human leptin mRNA expression. Implications for the control of regional fat distribution.
Diabetes
46:
342-347,
1997[Abstract].
35.
Monti, LD,
Sandoli EP,
Phan VC,
Piatti PM,
Costa S,
Secchi A,
and
Pozza G.
A sensitive and reliable method for assaying true human insulin without interaction with human proinsulin-like molecules.
Acta Diabetol
32:
57-63,
1995[ISI][Medline].
36.
Muoio, DM,
Dohm GL,
Fiedorek FT, Jr,
Tapscott EB,
and
Coleman RA.
Leptin directly alters lipid partitioning in skeletal muscle.
Diabetes
46:
1360-1363,
1997[Abstract].
37.
Nadler, ST,
Stoehr JP,
Schueler KL,
Tanimoto G,
Yandell BS,
and
Attie AD.
The expression of adipogenic genes is decreased in obesity and diabetes mellitus.
Proc Natl Acad Sci USA
97:
11371-11376,
2000
38.
Perseghin, G,
Ghosh S,
Gerow K,
and
Shulman GI.
Metabolic defects in lean nondiabetic offspring of NIDDM parents. A cross-sectional study.
Diabetes
46:
1001-1009,
1997[Abstract].
39.
Perseghin, G,
Meneghini E,
Scifo P,
Pagliato E,
Mignogna G,
Tambussi G,
Del Maschio A,
Lazzarin A,
and
Luzi L.
Intramyocellular triglyceride content is increased in HIV-1 patients with peripheral lipodystrophy (Abstract A71).
Diabetes
49, Suppl 1:
289,
2000.
40.
Perseghin, G,
Price TB,
Petersen KF,
Roden M,
Cline GW,
Gerow K,
Rothman DL,
and
Shulman GI.
Increased glucose transport-phosphorylation and muscle glycogen synthesis after exercise training in insulin-resistant subjects.
N Engl J Med
335:
1357-1362,
1996
41.
Perseghin, G,
Scifo P,
De Cobelli F,
Pagliato E,
Battezzati A,
Arcelloni C,
Vanzulli A,
Testolin G,
Pozza G,
Del Maschio A,
and
Luzi L.
Intramyocellular triglyceride content is a determinant of in vivo insulin resistance in humans: a 1H-13C NMR spectroscopy assessment in offspring of type 2 diabetic parents.
Diabetes
48:
1600-1606,
1999[Abstract].
42.
Perseghin, G,
Scifo P,
Pagliato E,
Battezzati A,
Soldini L,
Benedini S,
Testolin G,
Del Maschio A,
and
Luzi L.
Gender factors affect fatty acids-induced insulin resistance in nonobese humans: effects of oral steroidal contraception.
J Clin Endocrinol Metab
86:
3188-3196,
2001
43.
Roden, M,
Price TB,
Perseghin G,
Petersen KF,
Rothman DL,
Cline GW,
and
Shulman GI.
Mechanism of free fatty acid induced insulin resistance in humans.
J Clin Invest
97:
2859-2866??,
1996
44.
Ruderman, NB,
Chisholm D,
Pi-Sunyer X,
and
Schneider SH.
The metabolically obese, normal-weight individual revisited.
Diabetes
47:
699-713,
1998[Abstract].
45.
Schick, F,
Eismann B,
Jung WI,
Bongers H,
Bunse M,
and
Lutz O.
Comparison of localized proton NMR signals of skeletal muscle and fat tissue in vivo: two lipid compartments in muscle tissue.
Magn Reson Med
29:
158-167,
1993[ISI][Medline].
46.
Shimabukuro, M,
Koyama K,
Chen G,
Wang MY,
Trieu F,
Lee Y,
Newgard CB,
and
Unger RH.
Direct antidiabetic effect of leptin through triglyceride depletion of tissues.
Proc Natl Acad Sci USA
94:
4637-4641,
1997
47.
Shulman, GI.
Cellular mechanisms of insulin resistance.
J Clin Invest
106:
171-176,
2000
48.
Simoneau, JA,
Colberg SR,
Thaete FL,
and
Kelley DE.
Skeletal muscle glycolytic and oxidative enzyme capacities are determinants of insulin sensitivity and muscle composition in obese women.
FASEB J
9:
273-278,
1995
49.
Steele, R.
Influence of glucose loading and of injected insulin on hepatic glucose output.
Ann NY Acad Sci
82:
420-431,
1959[ISI].
50.
Stein, DT,
Dobbins R,
Szczepaniak L,
Malloy C,
and
McGarry JD.
Skeletal muscle triglyceride stores are increased in insulin resistance (Abstract).
Diabetes
46, Suppl 1:
23A,
1997.
51.
Stein, DT,
Szczepaniak L,
Dobbins R,
and
McGarry JD.
Increasing intramyocellular triglyceride stores are associated with impaired glucose tolerance and NIDDM (Abstract).
Diabetes
48, Suppl 1:
287A,
1999[ISI].
52.
Svendsen, OL,
Hassager C,
Bergmann I,
and
Christiansen C.
Measurement of abdominal and intra-abdominal fat in postmenopausal women by DEXA and anthropometry: comparison with computed tomography.
Int J Obes
17:
45-51,
1993[ISI].
53.
Szczepaniak, LS,
Babcock EE,
Schick F,
Dobbins RL,
Garg A,
Burns DK,
McGarry JD,
and
Stein DT.
Measurement of intracellular triglyceride stores by 1H spectroscopy: validation in vivo.
Am J Physiol Endocrinol Metab
276:
E977-E989,
1999
54.
Tappy, L,
Owen OE,
and
Boden G.
Effect of hyperinsulinemia on urea pool size and substrate oxidation rates.
Diabetes
37:
1212-1216,
1988[Abstract].
55.
Unger, RH.
Lipotoxicity in the pathogenesis of obesity-dependent NIDDM. Genetic and clinical implications.
Diabetes
44:
863-870,
1995[Abstract].
56.
Unger, RH,
Zhou YT,
and
Orci L.
Regulation of fatty acid homeostasis in cells: novel role of leptin.
Proc Natl Acad Sci USA
96:
2327-2332,
1999
57.
Weir, JB.
New methods for calculating metabolic rate with special reference to protein metabolism.
J Physiol (Lond)
109:
1-9,
1949[ISI].
58.
Willett, WC,
Dietz WH,
and
Colditz GA.
Guidelines for healthy weight.
N Engl J Med
341:
427-434,
1999
59.
Zhou, YT,
Wang ZW,
Higa M,
Newgard CB,
and
Unger RH.
Reversing adipocyte differentiation: implications for treatment of obesity.
Proc Natl Acad Sci USA
96:
2391-2395,
1999