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1600 DIABETES, VOL. 48, AUGUST 1999
Intramyocellular Triglyceride Content Is a
Determinant of in Vivo Insulin Resistance in Humans
A
1
H -
1 3
C Nuclear Magnetic Resonance Spectroscopy
Assessment in Offspring of Type 2 Diabetic Parents
Gianluca Perseghin, Paola Scifo, Francesco De Cobelli, Emanuela Pagliato, Alberto Battezzati,
Cinzia Arcelloni, Angelo Vanzulli, Giulio Testolin, Guido Pozza, Alessandro Del Maschio, and Livio Luzi
Insulin resistance is the best prediction factor for the
clinical onset of type 2 diabetes. It was suggested that
intramuscular triglyceride store may be a primary path-
ogenic factor for its development. To test this hypoth-
esis, 14 young lean offspring of type 2 diabetic parents,
a model of in vivo insulin resistance with increased
risk to develop diabetes, and 14 healthy subjects
matched for anthropomorphic parameters and life
habits were studied with 1) euglycemic-hyperinsuline-
mic clamp to assess whole body insulin sensitivity,
2) localized
1
H nuclear magnetic resonance (NMR)
spectroscopy of the soleus (higher content of fiber
type I, insulin sensitive) and tibialis anterior (higher
content of fiber type IIb, less insulin sensitive) muscles
to assess intramyocellular triglyceride content, 3)
1 3
C
NMR of the calf subcutaneous adipose tissue to assess
composition in saturated/unsaturated carbons of tri-
g l y ceride fatty acid chains, and 4) dual X-ray energy
absorption to assess body composition. Offspring of
diabetic parents, notwithstanding normal fat content
and distribution, were characterized by insulin resis-
tance and increased intramyocellular triglyceride con-
tent in the soleus (P < 0.01) but not in the tibialis ante-
rior (P = 0.19), but showed a normal content of satu-
rated/unsaturated carbons in the fatty acid chain of
subcutaneous adipocytes. Stepwise regression analy-
sis selected intramyocellular triglyceride soleus content
and plasma free fatty acid levels as the main predictors
of whole body insulin sensitivity. In conclusion,
1
H and
1 3
C NMR spectroscopy revealed intramyocellular abnor-
malities of lipid metabolism associated with whole
body insulin resistance in subjects at high risk of devel-
oping diabetes, and might be useful tools for noninva-
sively monitoring these alterations in diabetes and pre-
diabetic states. D i a b e t e s 48:1600–1606, 1999
I
nsulin resistance plays a primary role in the pathogen-
esis of type 2 diabetes (1,2), being the most relevant
abnormality of glucose homeostasis 1–2 decades
before the onset of frank hyperglycemia (3,4). In
type 2 diabetic patients, the major site of impairment of
insulin action resides in the skeletal muscle, where glucose
metabolism is abnormal because of an impairment of insulin-
stimulated glycogen synthesis (5) due to a defect in glucose
transport/phosphorylation (6,7). The same features charac-
terize young nonobese and nondiabetic first-degree relatives
of type 2 diabetic patients (8,9), who have been shown to
have a high risk of developing diabetes, suggesting that these
alterations are fully expressed before the development of dia-
betes and, therefore, would be primary for its pathogenesis.
A competition between plasma glucose and free fatty acids as
fuel for energy production was suggested several years ago
(10); it was also recently demonstrated that in healthy
humans, an increased plasma free fatty acid availability was
acutely able to induce insulin resistance by the same mecha-
nisms responsible for insulin resistance in type 2 diabetic
patients and in their nondiabetic first-degree relatives (11). It
is very well known that type 2 diabetic patients (12) and their
first-degree relatives (13) are characterized by increased
plasma free fatty acid concentrations and plasma levels cor-
related with the severity of insulin resistance (13). We do not
know whether a causal link between free fatty acid–induced
insulin resistance and insulin resistance in type 2 diabetes
exists, nevertheless, the link between increased availability of
fatty acids and muscle insulin resistance has been well estab-
lished. More recently, it was also hypothesized that the incre-
ment of plasma free fatty acids may be paralleled by an
increased storage of intramyocellular triglycerides (14).
Comparison of signals from human skeletal muscle and fat
tissue obtained in vivo by means of
1
H nuclear magnetic res-
onance (NMR) spectroscopy showed that muscle tissue con-
tains two compartments of triglycerides: one of these com-
partments represents the lipids within the fat cells, and the
other compartment, the lipids located as droplets in contact
with mitochondria within the cytoplasm of muscle cells
(15–18). In addition,
1 3
C NMR spectroscopy allows measure-
ments of the major classes of unsaturated and saturated fatty
acids constituting tissue triglycerides (19,20).
From the Divisions of Internal Medicine I (G.Pe., A.B., G.Po., L.L.), Nuclear
Medicine (P.S.), and Diagnostic Radiology (P.S., F.D.C., A.V., A.D.M.) and the
Laboratory of Separative Techniques (C.A.), Istituto Scientifico H San
Raffaele; and the International Center for the Assessment of Nutritional
Status (E.P., G.T.), Università degli Studi di Milano, Milan, Italy.
Address correspondence to Gianluca Perseghin, MD, Division of Internal
Medicine I, Laboratory of Amino Acids and Stable Isotopes/Unit of Clinical
S p e c t r o s c o p y, via Olgettina 60, 20132 Milan, Italy. E-mail: perseghin.
g i a n l u c a @ h s r. i t .
Received for publication 13 January 1999 and accepted in revised form
3 May 1999.
DEXA, dual-energy X-ray absorption; NMR, nuclear magnetic resonance;
TE, echo time; TR, repetition time.
DIABETES, VOL. 48, AUGUST 1999 1601
G. PERSEGHIN AND ASSOCIATES
The aim of this work was to test the hypothesis that intra-
muscular triglyceride content is related to whole body insulin
sensitivity by studying young nonobese and nondiabetic off-
spring of type 2 diabetic parents, who are at high risk of devel-
oping diabetes because of the presence of insulin resistance.
To pursue this goal, we used
1
H NMR spectroscopy of the
skeletal muscle in combination with the euglycemic clamp
technique for the assessment of whole body insulin sensitivity
and with dual-energy X-ray absorption (DEXA) for the assess-
ment of body composition. In addition, we also tested, using
1 3
C NMR spectroscopy, whether this model of insulin resistance
was characterized by an abnormal pattern of saturation/unsat-
uration of the fatty acids constituting tissue triglycerides.
RESEARCH DESIGN AND METHODS
Subjects. First-degree relatives of type 2 diabetic patients were recruited at Isti-
tuto Scientifico H San Raffaele. The main criteria for their inclusion in the study
were as follows: 1) both parents with type 2 diabetes or one parent and a first- or
second-degree relative with type 2 diabetes (diagnosis of diabetes was based on
fasting glucose concentration >7.8 mmol/l and/or history of oral hypoglycemic
agent assumption); 2) age (24–45 years); 3) white race; 4) body weight within 12%
of their ideal body weight according to the 1983 Metropolitan Life Insurance
tables; 5) sedentary life style; 6) no history of hypertension, endocrine/meta-
bolic disease, or cigarette smoking. Habitual physical activity was assessed using
a questionnaire (21). Fourteen (7 male and 7 female) first-degree relatives of type
2 diabetic parents who were not participating in heavy physical activity were
selected. Each single offspring of type 2 diabetic parents was matched with a sin-
gle control subject for anthropomorphic characteristics, with the difference that the
14 healthy normal subjects had no family history of diabetes, obesity, or hyperten-
sion traced through their grandparents. The clinical and laboratory characteristics
of the two groups of subjects are summarized in Tables 1 and 2. All subjects were
in good health as assessed by medical history, physical examination, hematologi-
cal testing, and urinalysis. Informed consent was obtained from all subjects after
explanation of the purpose, nature, and potential risks of the study. The protocol
was approved by the ethical committee of the Istituto Scientifico H San Raffaele.
Experimental protocol. Subjects were instructed to consume an isocaloric diet
(~250 g carbohydrate/day) for 3 weeks before the studies and to abstain from exer-
cise activity for at least 2 weeks before the studies. Women were studied between
days 3 and 10 of the menstrual cycle. Subjects were studied by means of the eugly-
cemic-hyperinsulinemic clamp to assess whole body insulin sensitivity after a 10-h
overnight fast. Within 2–3 days, patients were studied by means of
1
H and
1 3
C NMR
spectroscopy to assess muscle triglyceride content and degree of saturation/
unsaturation of the fatty acids constituting tissue triglycerides, respectively. NMR
sessions were performed in the Division of Diagnostic Radiology of the Istituto
S c i e n t i fi co H San Raffaele between 7:00 and 9:00 A.M. after a 10-h overnight fast.
For 15 of the 28 patients,
1
H and
1 3
C NMR spectroscopy sessions were performed
on two separate consecutive days because of technical problems. Within 2–3 days,
the patients were also studied by DEXA to assess body composition. DEXA was
performed in the Department of Science, Nutrition and Microbiology, Nutrition
Section, Università degli Studi di Milano.
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 a.m. after a 10-h overnight fast. A Te flon catheter was inserted into an ante-
cubital vein for tracers, glucose, and insulin infusions, and an additional catheter
was inserted retrogradely into a wrist vein for blood sampling. The hand was kept
in a heated box (70°C) throughout the experiment to allow sampling of arterial-
ized venous blood. Blood samples for fasting plasma glucose, HbA
1 c
, total cho-
lesterol, HDL cholesterol, triglycerides, free fatty acids, insulin, proinsulin,
glucagon, cortisol, and leptin were performed to measure them in the fasting con-
dition. Thereafter, a bolus (5 mg/kg body wt) was administered, followed by a con-
tinuous infusion (0.05 mg · kg
– 1
body wt · min
– 1
) of [6,6-
2
H
2
]glucose obtained
from massTrace (Woburn, MA). Blood for determination of plasma glucose and
insulin concentrations and for plasma tracer enrichment was drawn every 15 min
during the last 30 min of the 2.5-h equilibration period. After the basal equilibra-
tion period, a euglycemic-hyperinsulinemic clamp was performed as previously
described (22). Insulin was infused at 1 mU ? k g
– 1
· min
– 1
to reach a plasma insulin
concentration of ~550 pmol/l, and plasma glucose concentration was kept at
~5 mmol/l for an additional 150 min by a variable infusion of 20% dextrose infu-
sion. Blood samples for plasma insulin concentration and plasma [6,6-
2
H
2
] g l u c o s e
enrichment were drawn every 15 min throughout the study.
1
H NMR spectroscopy.
1
H NMR spectroscopy was performed on a Signa 1.5
Tesla scanner (General Electric Medical Systems, Milwaukee, WI) using a con-
ventional linear extremity coil. High resolution T
1
weighted images of the right calf
were obtained before spectroscopic acquisitions to localize the voxel of interest
for the
1
H spectroscopy study. The voxel shimming was executed to optimize the
homogeneity of the magnetic field within the specific volume of interest. Two
1
H spectra were collected from a 15 3 15 3 15 mm
3
volume within the soleus and
tibialis anterior muscles. A PRESS pulse sequence (repetition time [TR] = 2,000 ms
and echo time [TE] = 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 sup-
pressed during the acquisition, since it would dominate the other metabolite’s peak
signals of interest. A third
1
H 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 con-
ditions as the subject’s spectra. Postprocessing of the data, executed with the
TABLE 1
Anthropomorphic characteristics of study groups
Offspring of type 2 diabetic parents Normal subjects
M e n Wo m e n M e n Wo m e n P v a l u e
n 7 7 7 7 —
Age (years) 31 ± 2 29 ± 2 30 ± 2 30 ± 2 0 . 7 8
Body weight (kg) 74 ± 4 58 ± 4 72 ± 3 58 ± 1 0 . 7 6
Height (cm) 177 ± 2 166 ± 2 173 ± 1 165 ± 2 0 . 4 2
BMI (kg/m
2
) 23.4 ± 0.8 21.1 ± 0.7 24.0 ± 0.7 21.2 ± 0.7 0 . 6 9
Waist-to-hip ratio 0.91 ± 0.05 0.80 ± 0.02 0.87 ± 0.03 0.74 ± 0.01 0 . 1 6
Ideal body weight (%) 107 ± 4 102 ± 4 110 ± 3 104 ± 4 0 . 6 6
Total body fat (kg) 16 ± 2 16 ± 2 15 ± 1 16 ± 2 0 . 8 8
Body fat (%) 22.4 ± 2.3 28.1 ± 3.1 21.6 ± 1.4 29.7 ± 3.1 0 . 9 8
Fat content (%)
A r m s 18.5 ± 1.7 23.7 ± 4.0 17.5 ± 1.8 22.5 ± 3.0 0 . 5 6
Tr u n k 24.7 ± 1.7 23.7 ± 3.6 22.9 ± 1.3 26.3 ± 3.3 0 . 9 3
L e g s 22.9 ± 1.0 32.8 ± 3.1 22.0 ± 1.9 34.2 ± 2.2 0 . 8 6
Physical activity index 8.0 ± 0.4 8.5 ± 0.6 7.7 ± 0.5 8.1 ± 0.2 0 . 4 1
Data are means ± SE. The range of possible scores for the physical activity index is 3–15. The lowest value corresponds to the level
of physical activity of a clerical worker who plays a light sport (energy expended is <0.76 mJ/h; e.g., bowling) and who participates
in sedentary activities during leisure time. The highest value corresponds to the level of physical activity of a person who is very
physically active at work (e.g., a construction worker), who plays heavy sports (energy expended is at least 1.76 mJ/h; e.g., boxing,
basketball, football, or rugby), and who is very physically active during leisure time (e.g., walking >1 h/day or biking >45 min/day).
P values were obtained from analyses comparing the offspring of type 2 diabetic patients vs. normal subjects.
1602 DIABETES, VOL. 48, AUGUST 1999
INTRAMYOCELLULAR TRIGLYCERIDE CONTENT IN HUMANS
Sage/IDL software, consisted of high pass filtering, spectral apodization, zerofil l-
ing, Fourier transformation, and 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 (–CH
2
) at 1.35 ppm
was used to calculate intramyocellular triglyceride content expressed in arbitrary
units (AU) as ratio to the integral of the peak of the external standard 3 1 , 0 0 0 .
1 3
C NMR spectroscopy.
1 3
C NMR spectroscopy was performed in the same
system after the
1
H spectroscopy session or on the following morning. The dual-
frequency flexible spectroscopy coil (Medical Advances, Milwaukee, WI), con-
sisting of a
1 3
C square surface coil of 11.3 3 11.3 cm and two
1
H 14 3 15.5 cm
Helmholtz-type coils, was positioned around the right calf.
1
H coils were used for
the scout image, localized shimming, and the
1
H decoupling. Carbon spectra
were acquired from a 20-mm slice using a 90° pulse with TR = 3 s.
1
H decoupling
was performed using WA LTZ-4, and an average power of 18 W was applied using
a prototype decoupler (General Electric Medical Systems) during the acquisition.
The spectra were then analyzed using a correction factor to overcome partial sat-
uration effects. This factor was obtained from three volunteers by measuring the
peak areas of spectra acquired at different repetition times (TR = 3 and 10 s) and
calculating the change in the spectra. The calculated factor was then applied for
the analysis of all spectra at TR = 3 s (19,20). A typical spectrum is shown in
Fig. 1. The proportion of saturated or unsaturated fatty acid carbons was
obtained as previously described without making any assumption about the com-
position of tissue under study (23).
Body composition. DEXA was performed with a Lunar-DPX-IQ scanner
( L u n a r, Madison, WI). A different scan mode was chosen with respect to each
s u b j e c t ’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 (24). Fat content is expressed as kilograms of fat mass and as per-
cent of soft tissues.
Analytical procedures. Plasma glucose was measured with a Beckman glucose
analyzer (Fullerton, CA) (25). Plasma free fatty acids were measured by micro-
fluorometric assay (26). Fasting serum triglycerides, total cholesterol, HDL cho-
lesterol, and creatinine were measured as previously described (27). Plasma
insulin, total proinsulin, and intact proinsulin were measured by a highly specific
two-site monoclonal antibody-based immunosorbent assay (ELISA; Dako Diag-
nostics, Cambridgeshire, U.K.) (28). C-peptide was measured by a radioim-
munoassay using a double antibody (25); glucagon and cortisol were measured
as previously described (25). Plasma concentration of leptin was determined by
radioimmunoassay with a human kit (Linco Research, St. Charles, MO) by using
the manufacturer’s assay protocol (29). The [6,6-
2
H
2
]glucose enrichment was
measured by gas chromatography–mass spectrometry as previously described
(30). Glucose kinetics were calculated using Steele’s equations for the non–steady
state (31). M value was calculated by adding the rate of residual endogenous glu-
cose production to the glucose infusion rate.
Statistical analysis. All data are presented as means ± SE. Comparison
between different groups was performed using analysis of variance (ANOVA) with
S c h e f f è ’s post hoc testing when appropriate. Forward and backward stepwise
regression analysis were performed using F ratio-to-remove of 4 and F ratio-to-
enter of 3.996 to assess which variable was most useful in predicting the M value.
R E S U LT S
Clinical and laboratory characteristics. Healthy volun-
teers and offspring of type 2 diabetic parents were selected
to be comparable for age, body weight, height, BMI, waist-to-
hip ratio, and ideal body weight (Table 1). Total body fat,
assessed by DEXA, was similar in the two groups, and the fat
distribution in different body compartments (arms, trunk,
and legs) was also comparable between normal healthy sub-
jects and offspring of type 2 diabetic parents, demonstrating
identical total fat content and a similar pattern of fat distri-
bution. Nonetheless, women were characterized by an
increased fat storage when compared with men (28.9 ± 2.1 vs.
22.0 ± 1.3%, P < 0.01), and this was particularly evident at the
level of the legs (33.5 ± 1.8 vs. 22.5 ± 1.1%, P < 0.01, respec-
t i v e l y, in women and men). Laboratory characteristics are
summarized in Table 2. Fasting plasma glucose concentration,
even if within the normal range, was increased in the offspring
of type 2 diabetic parents (P < 0.01). Glycosylated hemoglo-
bin was, nevertheless, comparable between the two groups.
Fasting plasma free fatty acids were also slightly higher in the
offspring of type 2 diabetic parents compared with healthy
control subjects (P = 0.05). On the contrary, total choles-
terol, HDL cholesterol, and triglycerides were comparable
between the two groups studied. The hormone profile was
also comparable: fasting insulin showed a trend for higher
concentrations in the offspring of type 2 diabetic parents
compared with normal subjects (P = 0.08); total proinsulin
(intact proinsulin and fragments) and intact proinsulin con-
centrations were also comparable. Finally, fasting C-peptide,
leptin, glucagon, and cortisol concentrations were comparable
between the two study groups.
TABLE 2
Laboratory characteristics of study groups
Offspring of type 2 diabetic parents Normal subjects
M e n Wo m e n M e n Wo m e n P v a l u e
n 7 7 7 7 —
Plasma glucose (mmol/l) 5.1 ± 0.2 5.0 ± 0.2 4.7 ± 0.2 4.6 ± 0.1 0 . 0 1
H b A
1 c
( % ) 5.1 ± 0.1 5.0 ± 0.8 4.9 ± 0.2 4.9 ± 0.2 0 . 3 9
M value (mg/[kg · min]) 4.73 ± 1.39 4.87 ± 0.85 7.01 ± 0.98 5.88 ± 0.69 0 . 0 4
Total cholesterol (mmol/l) 4.81 ± 0.26 4.58 ± 0.31 4.68 ± 0.36 5.25 ± 0.36 0 . 4 5
HDL cholesterol (mmol/l) 1.61 ± 0.23 2.05 ± 0.13 1.33 ± 0.16 2.00 ± 0.21 0 . 4 5
Plasma free fatty acids (mmol/l) 0.636 ± 0.061 0.675 ± 0.049 0.531 ± 0.057 0.563 ± 0.050 0 . 0 5
Plasma triglycerides (mmol/l) 1.90 ± 0.55 0.70 ± 0.08 0.94 ± 0.12 0.87 ± 0.16 0 . 2 0
Insulin (pmol/l) 67 ± 15 49 ± 10 47 ± 8 45 ± 9 0 . 0 8
Total proinsulin (pmol/l) 9.6 ± 2.4 6.0 ± 0.6 6.6 ± 1.5 5.5 ± 1.0 0 . 2 3
Intact proinsulin (pmol/l) 3.6 ± 1.0 2.6 ± 0.3 2.6 ± 0.6 2.5 ± 0.5 0 . 6 1
Total proinsulin/insulin % 14 ± 3 12 ± 3 14 ± 6 12 ± 2 0 . 4 1
Intact proinsulin/insulin % 5 ± 1 5 ± 2 5 ± 1 6 ± 1 0 . 4 9
C-peptide (nmol/l) 0.75 ± 0.19 0.57 ± 0.06 0.67 ± 0.05 0.56 ± 0.06 0 . 1 9
Leptin (ng/ml) 7.2 ± 2.1 7.8 ± 1.5 4.8 ± 0.7 8.6 ± 0.2 0 . 8 1
Glucagon (ng/l) 164 ± 21 67 ± 9 160 ± 15 97 ± 5 0 . 8 9
Cortisol (nmol/l) 3,090 ± 552 2,897 ± 552 4,139 ± 497 2,649 ± 303 0 . 2 2
Creatinine (µmol/l) 84.0 ± 8.0 68.1 ± 4.4 92.8 ± 3.5 74.3 ± 2.7 0 . 3 0
Data are means ± SE. Blood sampling for substrates and hormones was performed after a 10-h overnight fast. P values were
obtained from analyses comparing the offspring of type 2 diabetic patients vs. normal subjects.
DIABETES, VOL. 48, AUGUST 1999 1603
G. PERSEGHIN AND ASSOCIATES
Euglycemic-hyperinsulinemic clamp. During the insulin
clamp, plasma glucose concentration was kept at baseline lev-
els and was comparable between the two groups (4.99 ± 0.75
v s . 4.81 ± 0.74 mmol/l in the offspring of type 2 diabetic par-
ents and normal subjects, respectively). Plasma insulin con-
centration was also comparable (542 ± 22 vs. 527 ± 14 pmol/l
during the last 30 min of the clamp in the offspring of type 2
diabetic parents and normal subjects, respectively). Rate of
glucose appearance (R
a
) was comparable in both postab-
sorptive (2.32 ± 0.21 vs. 2.17 ± 0.15 mg ? k g
– 1
· min
– 1
in the off-
spring of type 2 diabetic parents and normal subjects, respec-
t i v e l y, P = 0.759) and clamp (0.18 ± 0.05 vs. 0.11 ± 0.09 mg ?
k g
– 1
· min
– 1
in the offspring of type 2 diabetic parents and nor-
mal subjects, respectively, P = 0.745) conditions. Insulin sen-
s i t i v i t y, measured as the M value, was impaired in the offspring
of type 2 diabetic parents (4.80 ± 0.91 vs. 6.45 ± 0.72 mg ? k g
– 1
body wt · min
– 1
, P = 0.04) compared with that in the normal
subjects, and the difference was more pronounced in men
(P = 0.01) than in women (P = 0.07).
1
H NMR spectroscopy
Effect of muscle type. Intramyocellular triglyceride content
in the soleus muscle (Fig. 2A, left panel, shaded peak) was
higher than in the tibialis anterior muscle (Fig. 2B, left panel,
shaded peak) in both normal subjects (636 ± 52 vs. 395 ±
51 AU, P < 0.01) and offspring of type 2 diabetic parents
(1,047 ± 40 vs. 493 ± 52 AU, P < 0.01). This is summarized in
the right panels of Fig. 2A (soleus) and B (tibialis anterior),
taking into account that the scale on the y-axes was kept iden-
tical in the two panels. A linear relationship between the
soleus and tibialis anterior muscle lipid content was found in
normal subjects (r = 0.52, P = 0.04), but not in the offspring
of type 2 diabetic parents (r = –0.08, P = 0.79).
Effect of sex. There was a trend for higher intramyocellular
triglyceride content in women than in men in the tibialis
anterior in both normal subjects (489 ± 43 vs. 302 ± 82 AU,
P = 0.07) and offspring of type 2 diabetic parents (614 ± 70 vs.
371 ± 45 AU, P = 0.02). On the contrary, there was no sex effect
with regard to the soleus muscle: the content was similar in
both normal subjects (684 ± 84 vs. 589 ± 64 AU, respectively,
in women and men, P = 0.39) and offspring of type 2 diabetic
parents (999 ± 34 vs. 1094 ± 70 AU, respectively, in women and
men, P = 0.24).
Effect of family history of type 2 diabetes. I n t r a m y-
ocellular triglyceride content in the soleus muscle (Fig. 2A,
right panel) was higher in offspring of type 2 diabetic parents
(1,047 ± 40 AU) than in normal subjects (636 ± 52 AU, P < 0.01).
On the contrary, the tibialis anterior muscle (Fig. 2B, l e f t
p a n e l) showed just a trend for higher content in the offspring
of type 2 diabetic parents (493 ± 52 vs. 395 ± 51 AU, P = 0.19).
1 3
C NMR spectroscopy. Offspring of type 2 diabetic parents
had similar content of saturated (89.74 ± 0.41 vs. 90.19 ±
0.44%, P = 0.44) and unsaturated (10.25 ± 0.41 vs. 9.81 ±
0.38%, P = 0.44) carbons when compared with normal sub-
jects, and the content of polyunsaturated (2.03 ± 0.14 vs.
1.96 ± 0.11%, P = 0.72) and monounsaturated (8.23 ± 0.33 vs.
7.85 ± 0.32%, P = 0.43) carbons was also comparable between
the two groups of study.
Multivariate analysis. To establish which variable was
most useful in predicting insulin sensitivity, both forward
and backward stepwise regression analyses were performed
including the following independent variables: age, BMI, total
body fat content, total body percent fat content, percent fat
in the trunk, waist-to-hip ratio, physical activity index, fast-
ing plasma free fatty acids, fasting plasma glucose, serum
triglyceride, plasma leptin, triglyceride content in the soleus
A
FIG. 2.
1
H spectra of the soleus and tibialis anterior muscles. A, left
p a n e l: a spectrum typical of the soleus muscle; B, left panel: a spec-
trum typical of the tibialis anterior muscle. The shaded peaks in the
spectra represent the best fit of the CH
2
– residue peaks of intramy-
ocellular triglycerides (1.35 ppm). At 1.55 ppm, the CH
2
– residue
peak of extramyocellular triglycerides is represented. In the right
panels, bar graphs representing the intramyocellular content in
the soleus (A) and tibialis anterior (B) for each offspring of type 2
diabetic parents (s) and normal subjects (d) and means ± SE for the
series of values are depicted.
FIG. 1. Decoupled
1 3
C spectrum of calf tissues. Signals from saturated
fatty acid carbons are in the region from 10 to 45 ppm; peaks from
unsaturated fatty acid carbons are in the region from 110 to 140 ppm;
and peak from carbonyl is in the region from 165 to 175 ppm. The window
represents an enlargement of the unsaturated region at 110–140 ppm,
which contains one pair of resonances: the smaller peak of carbons on
the inside of consecutive double bonds (–C=C– C–C=C–, “inner”
olefinic carbons) is characteristic of polyunsaturated fatty acids
(poly), and the larger peak of carbons external to the double bonds
( –C= C – C – C =C–) resonates around the same chemical shift as the
olefinic carbons from monounsaturated fatty acids (mono) (–C= C – )
and are called “outer” olefinic carbons.
B
1604 DIABETES, VOL. 48, AUGUST 1999
INTRAMYOCELLULAR TRIGLYCERIDE CONTENT IN HUMANS
and tibialis anterior muscles, percentual composition in
monounsaturated carbons, polyunsaturated carbons, and
saturated carbons in fatty acids constituting the triglyceride
pool of calf tissue. The analysis selected the soleus triglyceride
content as the most useful variable in predicting insulin sen-
sitivity (simple regression analysis: R
2
= 0.29, P < 0.01), and
plasma free fatty acids concentration was also useful (simple
regression analysis: R
2
= 0.21, P = 0.03).
D I S C U S S I O N
In this work,
1
H and
1 3
C NMR spectroscopy methods were
used in offspring of type 2 diabetic parents to look for a rela-
tionship between intramyocellular muscle content and/or
adipose tissue composition in saturated/unsaturated fatty
acids and whole body insulin resistance. This is the fir s t
application of the combination of the two techniques to
s t u d y, noninvasively, intracellular lipid store in physiology and
pathophysiology of diabetes. The major findings of this
research are that healthy young lean offspring of type 2 dia-
betic parents, an in vivo model of increased risk to develop
diabetes in the future, were characterized by 1) increased
muscle triglyceride content that correlated with the severity
of whole body insulin resistance and 2) no change in the
content of unsaturated/saturated carbons of fatty acid chains
in adipose tissue when compared with healthy normal sub-
jects without a family history of diabetes.
The assessment of whole body insulin sensitivity measured
by the clamp technique showed that offspring of type 2 dia-
betic parents were characterized by a reduced glucose dis-
posal rate (Table 2). They had normal serum total cholesterol,
HDL cholesterol, and triglycerides when compared with nor-
mal subjects. On the contrary, plasma free fatty acid levels
were increased notwithstanding a trend for higher fasting
insulin levels (Table 2). It was crucial to determine whether
the storage of this substrate in the skeletal muscle was nor-
mal or somehow defective (14).
1
H NMR spectroscopy allows the distinction of two intra-
muscular triglyceride compartments in human skeletal mus-
cle, and one of these represents the intramyocellular fraction
(15). The study of the soleus muscle resulted in increased
triglyceride content in both the normal subjects and the off-
spring of type 2 diabetic parents compared with the tibialis
a n t e r i o r. All human muscles are of mixed fiber type (32), but
the soleus is prevalently composed of slow-twitch oxidative
fibers (fiber type I), and the tibialis anterior has a higher con-
tent of fast-twitch glycolytic fibers (fiber type IIb) (33). This
is in agreement with previous papers showing that intra-
muscular triglyceride concentrations are graded according to
the following sequence (from highest to lowest concentra-
tion): slow-twitch oxidative, fast-twitch oxidative, and fast-
twitch glycolytic (34,35). We also found a sex effect with
regard to the tibialis anterior, in which the triglyceride con-
tent was higher in women than in men in both offspring of
type 2 diabetic parents and normal subjects.
The most important finding was the higher triglyceride
content in the soleus muscle, but not in the tibialis anterior,
of the offspring of type 2 diabetic parents compared with that
in the normal subjects. We think that this divergent behavior
of the two muscles is due to the fact that different fiber types
have different insulin sensitivity and responsiveness (36).
The observation in animal models that slow-twitch oxidative
(type I) and fast-twitch oxidative (type IIa) fibers have the
greatest insulin sensitivity, whereas the fast-twitch glycolytic
fibers (type IIb) have the least (37–39), supports this theory.
A study in humans (Pima Indians and Caucasians) also
showed a positive correlation between insulin action and
the percentage of fiber type I and a negative correlation with
the percentage of fiber type IIb in biopsies of the vastus lat-
eralis (40). As a consequence, the metabolic abnormality had
to be more evident in the more insulin-sensitive fibers (type I),
and thus in the soleus muscle, which is supposed to contain
more type I fibers than the tibialis anterior muscle. In fact, the
result of the multivariate analysis showed that triglyceride
content in the soleus muscle was the best predictor of whole
body insulin sensitivity; on the contrary, the triglyceride con-
tent of the tibialis anterior was not significantly related. The
relationship between the triglyceride content in the soleus
muscle and whole body insulin sensitivity is in agreement with
a previous study in which the same relationship was found in
male Pima Indians using assessment of triglyceride content
in the vastus lateralis muscle by means of a needle biopsy (41).
With respect to that article, our study was performed in both
male and female Caucasians, which group is not as diabetes-
prone as Pima Indians; in addition, we specifically selected
lean subjects (BMI 22.2 ± 0.6 in the offspring of type 2 diabetic
parents and 22.6 ± 0.6 in normal subjects vs. 32.7 ± 1.1 kg/m
2
in subjects studied by Pan et al. [41]) to avoid any con-
founding effect of adiposity, which is also well known to
affect insulin action. All study subjects performed a DEXA
exam to properly assess whether total and regional body fat
(in particular the trunk) were comparable between offspring
of type 2 diabetic parents and normal subjects. The two
groups of study were tightly controlled for these variables,
allowing us to demonstrate the strong relationship between
intramyocellular triglyceride levels and insulin sensitivity
independent of total body fat content.
The mechanisms responsible for abnormal storage of
triglycerides in the cytoplasm of skeletal muscle cell are
u n c l e a r. This abnormality may simply reflect a different fib e r
type composition in the muscle under study: it was recently
found in a Danish population of insulin-resistant fir s t - d e g r e e
relatives of patients with type 2 diabetes that the vastus lat-
eralis muscle had an increased fraction of fiber type IIb com-
pared with healthy subjects without a family history of dia-
betes (42), but the study did not determine whether this was
due to reduced physical fitness. Physical exercise, which
may also have a profound effect on intramyocellular lipid stor-
age (33), was comparable, as assessed by a questionnaire
(21), in offspring of type 2 diabetic parents and normal sub-
jects because we specifically selected subjects with a seden-
tary lifestyle. Therefore, we do not think that a genetic or fit-
ness-related change in muscle fiber type composition may
explain our findings without a metabolic alteration standing
as a background. The main reason for this being that we
found that a muscle rich in fiber type IIb (tibialis anterior) had
a lower triglyceride content than a muscle rich in fiber
type I (soleus), and, as a consequence, if offspring of type 2
diabetic parents had higher type IIb fibers, they should have
been characterized by a lower rather than a higher content of
intramyocellular lipids, for the reasons previously discussed.
Because intramuscular fat content was always assessed on
bioptic specimens and different fiber types, and therefore
different muscles, have different behavior, much of the data
regarding the effects of the hormone milieu is controversial.
DIABETES, VOL. 48, AUGUST 1999 1605
G. PERSEGHIN AND ASSOCIATES
In our study, the hormone profile was similar between the two
study groups (Table 2), fasting plasma insulin concentration
being the only exception because of a trend for higher con-
centration (P = 0.08) in the offspring of type 2 diabetic par-
ents. Proinsulin fractions, C-peptide, glucagon, and cortisol
were comparable. Because leptin directly regulates both adi-
posity and energy homeostasis (43,44) and its gene expression
takes place not only in the adipose tissue, but also in the mus-
cle (45), we also measured plasma leptin concentrations in
our study groups to test whether the increased muscle
triglyceride store in offspring of type 2 diabetic parents could
be associated with abnormal leptin expression, but we did not
find any significant difference (Table 2). If fat deposition
within muscle is increased, fatty acids uptake has to be
increased or fatty acid oxidation has to be reduced. A
reduced Hounsfield attenuation on computed tomography
scans (which would stand for increased fat depositions) was
found in the muscle of nondiabetic obese women and was
strongly related to reduced muscle oxidative capacity, sug-
gesting that an impaired capacity of skeletal muscle for fat oxi-
dation would drive fat to intramuscular accumulation (46). On
the other hand, an increased muscle lipoprotein lipase activ-
ity might lead to increased lipid uptake in muscle; in this
regard, insulin-resistant subjects (Pima Indians) showed
increased muscle lipoprotein lipase activity during insulin
administration with respect to the postabsorptive condition
rather than a decrement, as observed in normal subjects (47).
This point of discussion on the basis of our data is just a mat-
ter of speculation, since this work was not specific a l l y
designed to address these issues.
In muscle biopsies of Pima Indians (48) and Caucasians
(49), an inverse relationship between insulin sensitivity and
saturated fats of muscle membrane phospholipids was
found.
1 3
C NMR spectroscopy gives the unique possibility of
noninvasively assessing the fatty acid status (19,20,23,50,51)
because the main
1 3
C NMR signals visible in in vivo spectra
are from fatty acids in subcutaneous fat. Therefore, we also
performed
1 3
C NMR spectroscopy of the calf in the study
groups but did not find any abnormality of the percentage of
saturated/unsaturated (mono- and poly-) carbons of fatty
acid chain in the offspring of type 2 diabetic patients. In the
multivariate analysis, they did not reveal any significant role
in explaining whole body insulin sensitivity. The dichotomy
with the results of the plasma membrane of insulin-resistant
subjects (48,49) may be explained by two factors. First, we
assessed the degree of saturation/unsaturation of the fatty
acids of triglycerides mainly stored in adipocytes; therefore,
the sampled pools (adipose vs. muscle tissue) and the com-
partment (cytoplasm vs. plasma membrane) were different.
Second, polyunsaturated fatty acids are derived exclusively
from the diet because essential fatty acids have a concen-
tration that is proportional to the dietary intake (52). There-
fore, the similar fatty acid composition in the study groups
was probably due to the similar diet habits. The normal pat-
tern of fatty acid composition found in the adipose tissue of
the offspring of type 2 diabetic parents taken together with
the alterations found in the plasma membrane phospholipids
suggests that the metabolic alterations are differently
expressed in body tissues and compartments (membranes
and cytoplasm).
In conclusion, the results of this work demonstrate that sub-
jects at high risk of developing type 2 diabetes have abnormal
intramuscular triglyceride storage that seems to be selective
for muscle (or fiber) type. The strong relationship between
this abnormal muscle triglyceride storage and whole body
insulin sensitivity suggests that it may play a major role in the
pathogenesis of type 2 diabetes. Whether these differences
might be fully due to a genetic background or to an environ-
mental factor is not known, but
1
H and
1 3
C NMR spec-
t r o s c o p y, noninvasive successful techniques for the study of
muscle and liver glucose metabolism, have been shown to be
useful tools for the study of intracellular lipids and might
eventually be useful for monitoring the effects of preventive
interventions in diabetes and prediabetic states.
A C K N O W L E D G M E N T S
This work was supported by the Istituto Scientifico H San
Raffaele (PZ709 and PZ806) and by grants from the Italian
Minister of Health (030.5/RF96.305) and the Italian National
Research Council (CNR 97.00485.CT04). The financial support
of Telethon–Italy (1032C) is also gratefully acknowledged. L.L.
is a recipient of a grant from the Associazione Italiana
Ricerca Cancro.
We wish to thank Van Chuong Phan, Paola Sandoli, Sabrina
Costa, and the nursing staff of the Metabolic Unit of the
Istituto Scientifico H San Raffaele for excellent assistance.
R E F E R E N C E S
1 . De Fronzo RA: Banting Lecture: the triumvirate: b-cell, muscle, liver: a
collusion responsible for NIDDM. D i a b e t e s 37:667–687, 1988
2 . Kahn CR: Banting Lecture: insulin action, diabetogenes, and the cause of
type II diabetes. D i a b e t e s 43:1066–1084, 1994
3 . Warram JH, Martin BC, Krolewski AS, Soeldner JS, Kahn CR: Slow glucose
removal rate and hyperinsulinemia precede the development of type II diabetes
in the offspring of diabetic parents. Ann Int Med 113:909–915, 1990
4 . Martin BC, Warram JH, Krolewski AS, Bergman RN, Soeldener JS, Kahn CR:
Role of glucose and insulin resistance in development of type II diabetes
mellitus: results of a 25-year follow-up study. L a n c e t 340:925–929, 1992
5 . Shulman GI, Rothman DL, Jue T, Stein P, De Fronzo RA, Shulman RG: Quan-
titation of muscle glycogen synthesis in normal subjects and subjects with
NIDDM by
1 3
C NMR spectroscopy. N Engl J Med 322:223–228, 1990
6 . Rothman DL, Shulman RG, Shulman GI:
3 1
P nuclear magnetic resonance
measurements of muscle glucose-6-phosphate: evidence for reduced insulin
dependent muscle glucose transport or phosphorylation activity in NIDDM.
J Clin Invest 89:1069–1075, 1992
7 . Bonadonna RC, Del Prato S, Saccomani MP, Bonora E, Gulli G, Ferrannini E,
Bier D, Cobelli C, De Fronzo RA: Transmembrane glucose transport in skele-
tal muscle of patients with NIDDM. J Clin Invest 92:486–494, 1993
8 . Rothman DL, Magnusson I, Cline GW, Gerard R, Kahn CR, Shulman RG,
Shulman GI: Decreased muscle glucose transport/phosphorylation is an early
defect in the pathogenesis of NIDDM. Proc Natl Acad Sci U S A 9 2 : 9 8 3 – 9 8 7 ,
1 9 9 5
9 . Perseghin G, Price T, Petersen KF, Roden M, Cline GW, Gerow K, Rothman DL,
Shulman GI: Increased glucose transport/phosphorylation and muscle glyco-
gen synthesis after exercise training in insulin resistant subjects. N Engl J Med
335:1357–1362, 1996
1 0 . Randle PJ, Garland PB, Hales CN, Newsholme EA: The glucose-fatty acid cycle:
its role in insulin sensitivity and the metabolic disturbances of diabetes
mellitus. L a n c e t i:785–789, 1963
1 1 . Roden M, Price TB, Perseghin G, Petersen KF, Rothman DL, Cline GW,
Shulman GI: Mechanism of free fatty acid induced insulin resistance in
humans. J Clin Invest 97:2859–2865, 1996
1 2 . Reaven GM: Role of insulin resistance in human disease. D i a b e t e s 3 7 :
1595–1607, 1988
1 3 . Perseghin G, Ghosh S, Gerow K, Shulman GI: Metabolic defects in lean
nondiabetic offspring of NIDDM parents: a cross-sectional study. D i a b e t e s
46:1001–1009, 1997
1 4 . Mc Garry JD: What if Minkowski had been ageusic? An alternative angle on
diabetes. S c i e n c e 258:766–770, 1992
1 5 . Schick F, Eismann B, Jung W-I, Bongers H, Bunse M, 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
1606 DIABETES, VOL. 48, AUGUST 1999
INTRAMYOCELLULAR TRIGLYCERIDE CONTENT IN HUMANS
1 6 . Boesch C, Slotboom J, Hoppeler H, Kreis R: In vivo determination of
intra-myocellular lipids in human muscle by means of localized
1
H - M R -
s p e c t r o s c o p y. Magn Reson Med 37:484–493, 1997
1 7 . Stein DT, Szczepaniak LS, Schick F, Dobbins R, Babcock EE, Malloy CR,
McGarry JD: Validation of
1
H MRS measurement of intracellular lipids in vivo
( A b s t r a c t ) . Proc Soc Magn Reson Med 1333, 1997
1 8 . Vock R, Hoppeler H, Claassen H, Wu DXY, Weber JM, Taylor CR, Weibel ER:
Design of the oxygen and substrate pathways. VI. Structural basis of intra-
cellular substrate supply to mitochondria in muscle cell. J Exp Biol 1 9 9 :
1689–1697, 1996
1 9 . Beckmann N, Brocard JJ, Keller U, Seelig J: Relationship between the degree
of unsaturation of dietary fatty acids and adipose tissue fatty acids assessed
by natural abundance
1 3
C MRS in man. Magn Reson Med 27:97–106, 1992
2 0 . Moonen CTW, Dimand RJ, Cox KL: The non-invasive determination of linoleic
acid content of human adipose tissue by natural abundance
1 3
C nuclear
magnetic resonance. Magn Reson Med 6:140–157, 1988
2 1 . Baecke JAH, Burema J, Frijters JER: A short questionnaire for the measure-
ment of habitual physical activity in epidemiological studies. Am J Clin Nutr
36:936–942, 1982
2 2 . De Fronzo RA, Tobin JD, Andres R: Glucose clamp technique: a method for
quantifying insulin secretion and resistance. Am J Physiol 6:E214–E223, 1979
2 3 . Thomas EL, Frost G, Barnard ML, Bryant DJ, Ta y l o r -Robinson SD, Simbrun-
ner J, Coutts GA, Burl M, Bloom SR, Sales KD, Bell JD: An in vivo
1 3
C magnetic
resonance spectroscopic study of the relationship between diet and adipose
tissue composition. L i p i d s 31:145–151, 1996
2 4 . Ley CJ, Lees B, Stevenson JC: Sex- and menopause-associated changes in
body-fat distribution. Am J Clin Nutr 55:950–954, 1992
2 5 . Luzi L, Secchi A, Facchini F, Battezzati A, Staudacher C, Spotti D, Castoldi R,
Ferrari G, Di Carlo V, Pozza G: Reduction of insulin resistance by combined
kidney-pancreas transplantation in type 1 (insulin-dependent) diabetic
patients. D i a b e t o l o g i a 33:549–556, 1990
2 6 . Miles J, Glasscok R, Aikens J, Gerich J, Haymond M: A microflu o r o m e t r i c
method for the determination of FFA in plasma. J Lipid Res 24:96–99, 1983
2 7 . Luzi L, Perseghin G, Regalia E, Sereni LP, Battezzati A, Baratti D, Bianchi E,
Terruzzi I, Hilden H, Groop LC, Pulvirenti A, Taskinen M-R, Gennari L,
Mazzaferro V: Metabolic effects of liver transplantation in cirrhotic patients.
J Clin Invest 99:692–700, 1997
2 8 . Sobey WJ, Beer SF, Carrington CA, Clark PMS, Frank BH, Gray IP, Luzio SD,
Owens DR, Schneider AE, Siddle K, Temple RC, Hales CN: Sensitive and
s p e c i fi c two site immunoradiometric assays for human insulin, proinsulin,
65–66 split and 32–33 split proinsulins. Biochem J 260:535–541, 1991
2 9 . Ma Z, Gingerich RL, Santiago JV, Klein S, Smith CH, Landt M: Radioim-
munoassay of leptin in human plasma. Clin Chem 42:942–946, 1996
3 0 . Battezzati A, Simonson DC, Luzi L, Matthews DE: Glucagon increases gluta-
mine uptake without affecting glutamine release in humans. M e t a b o l i s m
47:713–723, 1998
3 1 . Steele R: Influence of glucose loading and of injected insulin on hepatic
glucose output. Ann N Y Acad Sci 82:420–431, 1959
3 2 . Johnson MA, Polgar J, Weightman D, Appleton D: Data on the distribution of
fiber types in thirty-six human muscles: an autopsy study. J Neurol Sci 1 8 :
111–129, 1973
3 3 . Gorski J: Muscle triglyceride metabolism during exercise. Can J Physiol
P h a r m a c o l 70:123–131, 1992
3 4 . Essen B, Jansson E, Henriksson J, Taylor AW, Saltin B: Metabolic character-
istics of fibre types in human skeletal muscle. Acta Physiol Scand 9 5 : 1 5 3 – 1 6 5 ,
1 9 7 5
3 5 . Essen B: Intramuscular substrate utilization during prolonged exercise: a
biochemical and morphological study. Ann N Y Acad Sci 301:30–44, 1977
3 6 . Hickey MS, Weidner MD, Gavigan KE, Zheng D, Tyndall GL, Houmard JA: The
insulin action-fiber type relationship in humans is muscle group specific.
Am J Physiol 269:E150–E154, 1995
3 7 . Saltin B, Gollnick PD: Skeletal muscle adaptability: significance for metabo-
lism and performance. In Handbook of Physiology: Skeletal Muscle. Peachy
LDE, Adrian RH, Geiger SR, Eds. Baltimore, MD, Williams and Wilkins, 1983,
p. 555–631
3 8 . James DE, Jenkins AB, Kraegen EW: Heterogeneity of insulin action in
individual muscles in vivo: euglycemic clamp studies in rats. Am J Physiol
248:E567–E574, 1985
3 9 . Hom FG, Goodner CJ: Insulin dose-response characteristics among individ-
ual muscle and adipose tissues measured in the rat in vivo with
3
( H ) 2 -
deoxyglucose. D i a b e t e s 33:153–159, 1984
4 0 . Lillioja S, Young AA, Culter CL, Ivy JL, Abbott WGH, Zawadzki JK, Yki-Jarvi-
nen H, Christin L, Secomb TW, Bogardus C: Skeletal muscle capillary density
and fiber type are possible determinants of in vivo insulin resistance in man.
J Clin Invest 80:415–424, 1987
4 1 . Pan DA, Lillioja S, Kriketos AD, Milner MR, Baur LA, Bogardus C, Jenkins AB,
Storlien LH: Skeletal muscle triglyceride levels are inversely related to insulin
action. D i a b e t e s 46:983–988, 1997
4 2 . Nyholm B, Qu Z, Kaal A, Pedersen SB, Gravholt CH, Andersen JL, Saltin B,
Schmitz O: Evidence for an increased number of type IIb muscle fibers in
insulin-resistant first-degree relatives of patients with NIDDM. D i a b e t e s
46:1822–1828, 1997
4 3 . Pelleymounter MA, Cullen MJ, Baker MB, Hecht R, Winters D, Boone T,
Collins F: Effects of the obese gene product on body weight regulation in ob/ob
mice. S c i e n c e 269:540–543, 1995
4 4 . Halaas JL, Gajiwala KS, Maffei M, Cohen SL, Chait BT, Rabinowitz D, Lallone
RL, Burley SK, Friedman JM: Weight reducing effects of the plasma protein
encoded by the obese gene. S c i e n c e 269:543–546, 1995
4 5 . Wang J, Liu R, Hawkins M, Barzilai N, Rossetti L: A nutrient-sensing pathway
regulates leptin gene expression in muscle and fat. N a t u r e 393:684–688, 1998
4 6 . Simoneau JA, Colberg SR, Thaete FL, 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
4 7 . Ferraro RT, Eckel RH, Larson DE, Fontvieille A-M, Rising R, Jensen DR,
Ravussin E: Relationship between skeletal muscle lipoprotein lipase activity
and 24-hour macronutrient oxidation. J Clin Invest 92:441–445, 1993
4 8 . Pan DA, Lillioja S, Milner MR, Kriketos AD, Baur LA, Bogardus C, Storlien LH:
Skeletal muscle membrane lipid composition is related to adiposity and
insulin action. J Clin Invest 96:2802–2808, 1995
4 9 . Borkman M, Storlien LH, Pan DA, Jenkins AB, Chisholm DJ, Campbell LV: The
relation between insulin sensitivity and the fatty acid composition of skele-
tal muscle phospholipids. N Engl J Med 328:238–244, 1993
5 0 . Cunnane SC, Likhodii SS, Moine G: In vivo
1 3
C nuclear magnetic resonance:
applications and current limitations for noninvasive assessment of fatty acid
status. L i p i d s 31 (Suppl.):127–130, 1996
5 1 . Thomas EL, Hanrahan JD, Ala-Korpela M, Jenkinson G, Azzopardi D, Iles RA,
Bell JD: Noninvasive characterization of neonatal adipose tissue by
1 3
C mag-
netic resonance spectroscopy. L i p i d s 32:645–651, 1997
5 2 . van Staveren WA, Deurenberg P, Katan MB, Burema J, de Groot LC, Hoffmans
MD: Validity of the fatty acid composition of subcutaneous fat tissue micro-
biopsies as an estimate of the long-term average fatty acid composition of the
diet of separate individuals. Am J Epidemiol 123:455–463, 1986
