Mechanisms of glucose intolerance during triglyceride
infusion
V.
Rigalleau1,
M.
Beylot2,
C.
Pachiaudi3,
C.
Guillot4,
G.
Deleris4, and
H.
Gin1
1 Service de Nutrition et
Diabétologie, Hôpital Haut-Lévêque, 33600 Pessac; 2 Institut National de
la Santé et de la Recherche Médicale U-499, Faculté
de Médecine Laennec and
3 Centre de Recherche en Nutrition
Humaine de Lyon, 69372 Lyon; and
4 Laboratoire de Chimie
Bioorganique, Université Victor Segalen, 3300 Bordeaux,
France
 |
ABSTRACT |
Lipid infusions may affect glucose tolerance by
effects on glucose production or utilization. We performed
double-labeled oral glucose tolerance tests with and without a lipid
infusion in eight normal subjects. During the lipid infusion, plasma
glucose and insulin levels were higher, showing some insulin
resistance. The increased glucose level was due to a higher total
glucose appearance rate, partly reproducible by a control infusion
of glycerol [saline 1,181 ± 71 mg · kg
1 · 330 min
1 vs. lipid 1,388 ± 100 (P < 0.05) vs.
glycerol 1,276 ± 126 (NS)]. The tracer-determined appearance
rate of exogenous glucose was higher with lipid infusion but was
probably overestimated because of higher
13C recycling into glucose.
Residual systemic glucose production was increased but was reproducible
by the glycerol infusion. Total glucose disposal was increased. This
was observed despite a lower stimulation of total glucose oxidation as
measured by indirect calorimetry, whereas oxidation of exogenous
glucose was normal after correction for the lipid-induced modification
of excretion rate of
13CO2.
Accordingly, glucose nonoxidative disposal was increased. These
moderate modifications of glucose metabolism (increased appearance,
increased nonoxidative disposal, and lower total oxidation) have been
reported in starvation-induced or spontaneously impaired glucose
tolerance. Further impairment, especially decreased nonoxidative glucose disposal, seems to be required to produce non-insulin-dependent diabetes mellitus.
oral glucose tolerance; Randle cycle; systemic glucose production; glucose disposal; glucose recycling
 |
INTRODUCTION |
MORE THAN THIRTY YEARS AGO, Randle et al. (33) showed
that increased availability of lipid substrates reduced glucose
utilization, which was thought to play a role in insulin-resistant
states, such as obesity, diabetes, or starvation. Numerous studies
examining the influence of lipid infusions on glucose metabolism have
since been devoted to the "glucose-free fatty acid cycle" in
humans.
Lipid infusions have been found to have little effect in the
postabsorptive state (8) unless insulin secretion is disrupted as in
normal subjects by infusion of somatostatin (5) or in patients with
non-insulin-dependent diabetes mellitus (NIDDM) (35). On the other
hand, both insulin-stimulated glucose oxidation and nonoxidative
disposal (6, 7) are impaired by infusion of lipids during euglycemic
hyperinsulinemic clamp. However, these observations of lipid-induced
insulin resistance in situations of euglycemia do not necessarily imply
that lipid-carbohydrate interactions play a significant role in the
alterations of glucose metabolism observed in the postprandial state of
insulin-resistant subjects, since plasma glucose levels are increased
in the latter case. Effects of lipid infusions may be more relevant if
tested under the conditions of the oral glucose tolerance test (OGTT), which is closer to the postprandial state.
The influence of a lipid infusion on glucose tolerance has been studied
by Rousselle et al. (37) and Kruszynska et al. (23) in normal subjects
and Meylan et al. (26) in obese subjects. In these studies, lipids
inhibited glucose oxidation, although glucose nonoxidative disposal was
increased, presumably due to the slight increase in glycemia. However,
indirect calorimetry may lead to erroneous conclusions concerning
glucose oxidation when lipids are infused, since they stimulate
gluconeogenesis (9, 10), which could result in an underestimate of
glucose oxidation if amino acids are the main neoglucogenic precursors used (14). The study from Kruszynska et al. (23) suggested that lipid
infusion also induced a defective suppression of systemic glucose
production (SGP), as observed during clamp studies with lipid infusion
(24), but the potential role of the glycerol infused was not evaluated
in this observation.
In the present study, we performed double-labeled OGTTs with a
naturally 13C-enriched oral load
(cornstarch glucose; Ref. 41) and a dideuterated glucose infusion in
eight normal volunteers. Each subject was studied twice, once during a
saline infusion (controls) and once during a lipid infusion. To
determine the influence of glycerol in the infused lipid emulsion on
glucose appearance, four subjects were studied a third time, during a
glycerol infusion. Four subjects also underwent a lipid infusion
without OGTT to assess the contribution of
13C from infused lipids to expired
13CO2.
Using this methodology, we could determine the respective contribution
of modifications in SGP, glucose oxidation, and nonoxidative disposal
to lipid-induced alteration of glucose tolerance.
 |
METHODS |
Materials.
D-[6,6-2H2]glucose
(99 atom% excess) was obtained from the Commissariat à
l'Energie Atomique (Gif-sur-Yvette, France). It was checked to be
sterile and pyrogen free and was dissolved in sterile normal saline
solution before administration. Pure cornstarch glucose was
obtained from Aguettant (Lyon, France). The triglyceride emulsion
(Ivélip, 20%) was obtained from Cernep Synthélabo
(Montargis, France). It contained 200 g/l long-chain triglycerides
(fatty acids: 11% palmitic acid, 5% stearic acid, 26% oleic acid,
50% linoleic acid, 7% linolenic acid, 0.5% gadoleic acid, and 0.5% erucic acid) and 25 g/l glycerol to stabilize the emulsion. The 100 g/l
glycerol solution was obtained from the Pharmacie Centrale des
Hôpitaux de Bordeaux. Gas exchanges were measured using a Deltatrac metabolic monitor (Datex, France).
Subjects.
Eight subjects were studied twice by doubly labeled OGTTs,
once under infusion of saline (control tests) and once under a lipid
infusion (Ivélip test) 1 mo later. Four of these subjects had two
more tests: one OGTT under a glycerol infusion and one lipid infusion
(Ivélip) without OGTT. They were normal healthy volunteers (5 men
and 3 women, age 23 ± 2 yr, body weight 63.2 ± 2.6 kg, body mass index 21.5 ± 0.4). None of them had a family history
of diabetes or were receiving medication. They were requested to
consume their normal diet but to avoid food containing cane sugar,
cornstarch, or exotic fruits for 1 wk before and between the tests.
They gave their written consent to the study after being informed of
its nature, purpose, and potential risks. The protocol was approved by
the Ethical Committee of Edouard Herriot Hospital (Lyon, France).
Protocol.
Figure 1 shows the study design.
All subjects were studied in the postabsorptive state after a
12-h overnight fast in the metabolic unit of the Service de
Nutrition-Diabétologie (Hôpital Haut-Lévêque, Pessac, France). A retrograde catheter was
inserted in a dorsal hand vein and kept in a hot blanket (55°C)
to collect arterialized venous blood. A forearm vein of the
controlateral arm was catheterized to infuse
D-[6,6-2H2]glucose
and saline, Ivélip, or glycerol.
A priming dose of
D-[6,6-2H2]glucose
(6 mg/kg) was administered at 0800, and
D-[6,6-2H2]glucose
was then infused at a constant rate (0.06 mg · kg
1 · min
1)
using an electric syringe (Harvard, Les Ulis, France) for 450 min. Half
an hour later, a constant-rate infusion (0.015 ml · kg
1 · min
1
as used in previous studies; Refs. 24, 35) of saline (control tests) or
Ivélip (Ivélip tests) began and was continued until the end
of the test by using a volumetric infusion pump (Harvard). Because
heparin has its own effects on insulin binding and biological activity,
we did not use it in combination with the lipid infusion (21). Four
subjects underwent a third test with a constant-rate infusion (0.004 ml · kg
1 · min
1)
of a 100 g/l glycerol solution to test the effect on glucose appearance
of the glycerol used in the triglyceride emulsion. The first 120 min of
the tests (from times
120 to
0) were allowed for isotopic
equilibration and measurement of postabsorptive glucose turnover rate.
Thereafter, a dose of 1 g/kg of naturally
13C-enriched cornstarch glucose,
diluted in water (30 g/100 ml), was ingested.
In four subjects, the effect of the Ivélip infusion alone (0.015 ml · kg
1 · min
1
during 420 min, without OGTT) on
13C enrichment of expired
CO2 was determined to allow
calculation of a corrected exogenous glucose oxidation rate
(Gexo,corr).
Throughout the test, samples of blood and of expired gases were drawn
for determination of metabolites and hormone levels and for isotopic
enrichments. Except during the glycerol infusion (which was designed to
study glucose appearance), gaseous exchange was measured continuously
using the Deltatrac metabolic monitor, with a 5-min pause every 30 min
to collect expired gases for isotopic enrichment. Urine samples were
collected at times
120,
0, and 330 for urinary nitrogen excretion
determination.
Analytical procedures.
Plasma glucose, free fatty acids (FFA) (25), glycerol, and triglyceride
concentrations were determined enzymatically (2). Urinary nitrogen was
determined by the Kjeldahl method. Insulin and C-peptide levels were
measured by RIA. Plasma
D-[6,6-2H2]glucose
enrichment was measured by conventional selected ion-monitoring gas
chromatography-mass spectrometry (Hewlett-Packard 5971A-MSD, Paris,
France) as described by Bier et al. (3). Plasma and ingested
[13C]glucose isotopic
enrichment was measured by gas chromatography-isotope ratio mass
spectrometry (Sira 12, VG Instrument, Middlewich, UK) as described by
Tissot et al. (41). 13C enrichment
of expired CO2 was determined on
the dual-inlet isotope ratio mass spectrometer described by Guilluy et
al. (19). The 13C-to-12C
ratios of the samples were expressed as differences from the International PDB standard according to the formula,
13Cp × 1,000 = [(13C/12C
sample
13C/12C
standard)/(13C/12C
standard)] × 1,000, and was transformed to atom percent
excess (APE) for ingested and plasma glucose as previously described (41).
To determine the relative 13C
abundance of glucose, glycerol, and lipid emulsions, samples of these
solutions were combusted at 1,020°C. The
CO2 was separated using a liquid
N2 trap and analyzed for
13C abundance. The isotopic
abundance measured after oxidation was
12.6,
30.2,
29.5, and
28.7
13Cp × 1,000 for the glucose, glycerol from the Pharmacie Centrale des
Hôpitaux de Bordeaux, glycerol used for stabilization of Ivélip (furnished by Cernep Synthélabo), and total
Ivélip, respectively.
Calculations.
Rates of total glucose appearance
(RaT) and disappearance
(RdT) were calculated from the
enrichment of blood glucose by
D-[6,6-2H2]glucose,
using the steady-state equation in the postabsorptive state and the
non-steady-state approximation of Steele (39) during the OGTT as
described elsewhere (34).
Rates of exogenous glucose appearance
(RaE) and disappearance
(RdE) were calculated from
isotopic enrichment of blood glucose by
13C glucose, using the
transposition of the Steele equation suggested by Proietto et al.
(32)
where
APE1 and
APE2, correspond to the
13C APE at times
t1 and
t2, respectively,
and G1 and
G2 correspond to the plasma
glucose levels at times
t1 and
t2. The pool
fraction value (p) was taken as 0.75, and the distribution
volume (v) was taken as 0.21/kg.
SGP was calculated as SGP = RaT
RaE.
Total lipid (LOx) and glucose
oxidation (GOx) rates were
estimated from gaseous exchanges measurements (14). Suprabasal glucose
oxidation GOxSB was calculated as
GOx during the OGTT
GOx before the OGTT
(times
30 to
0). The exogenous glucose oxidation
rate (Gexo,
mg · kg
1 · min
1)
was also calculated from 13C
enrichment of expired CO2 as
described by Normand et al. (30)
where
p1,000
Gluing is the
value of
ingested glucose measured after complete combustion in the elemental
nitrogen and carbon analyzer on line with the isotope-ratio mass
spectrometer;
p1,000
CO2(t0)
is the
value of the expired
CO2 during times
30 to 0;
p1,000
CO2(t)
is the
value of the sampled
CO2 at time
t (end of each 30-min period);
VCO2(t),
expressed in liters per minute, is the mean volume of exhaled
CO2 during periods of 30 min;
180,000 mg is the molecular weight of glucose; 1/22.4 is for the
conversion into moles of CO2; and
1/6 is for the conversion into moles of glucose.
In four subjects, the effect of Ivélip infusion alone on
13C enrichment of exhaled
CO2 was determined, and
p1,000
CO2(t)
of the OGTT during lipid infusion was corrected before calculation of a
corrected Gexo:
Gexo,corr, as follows
Gexo,corr
was then calculated by the same formula as
Gexo, using values of
p1,000
CO2(t)corr,Ivélip
OGTT instead of values of
p1,000
CO2(t).
This must be considered as a maximal correction for the contribution of
13C from the lipid infusion, since
the infused lipids must be oxidized before their
13C appears in exhaled
CO2, and such oxidation will tend
to be reduced during the OGTT (38).
Glucose nonoxidative disposal was calculated using three different
relationships
which
is a balance between glucose input and glucose disappeared from the
whole organism, as calculated in previous OGTTs under lipid infusions
(26, 37)
as
proposed by Féry et al. (17), which represents
nonoxidized glucose leaving plasma, and takes account of lipid-induced changes in glucose turnover rate
which
represents exogenous glucose which had left plasma but did not
contribute to exhaled CO2 and is
not affected by lipid-induced alterations in gluconeogenesis, since it
is not based on the indirect calorimetric equations.
Gnon-Ox3,corr was also calculated,
Gnon-Ox3,corr = RdE
Gexo,corr.
Turnover and oxidation rates values were calculated for 30-min
intervals; cumulative values for the whole OGTT (330 min) are also
presented.
Statistical analysis.
Results are shown as means ± SE. Comparisons were performed by
one-way ANOVA for repeated measurements followed by a
t-test (within test comparison) and a
t-test for nonpaired data (between test comparison). P < 0.05 was
considered significant.
 |
RESULTS |
Substrates and hormones.
The data for substrates and hormones are presented in Figs.
2 and 3. The
lipid infusion induced a continuous rise in plasma triglycerides and
prevented the fall in FFA during the OGTT. Triglycerides and FFA were
significantly higher from the beginning to the end of the lipid
infusion, except at the last time point (+330 min), when FFA did not
differ between the saline and Ivélip tests. Plasma glucose levels
did not differ before the tests (times
120 and
90) and even before the oral
load, after 90 min of lipid infusion. The lipid infusion induced a
slight impairment in glucose tolerance, as shown by significantly
higher values from times +120 to
+180
(P < 0.05). Insulin and C-peptide
levels were also slightly higher during the lipid infusion, with
significant differences from times
+180 to +240
(P < 0.05). The glycerol infusion
had no significant effect on any substrate or hormone level, except on
plasma glycerol, which was higher than during the saline infusion from
time +120 to the end of the test
(P < 0.05). However, this increase
was less important than that observed during the lipid infusion
(P < 0.05 from
times
30 to
+300).

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Fig. 2.
Time course of plasma glucose (mM), insulin (pM), and C-peptide (nM)
before (times 120 to
0 min) and after (times
0 to 330 min) oral load. ,
Controls; , Ivélip tests; , glycerol tests.
* P < 0.05 between
Ivélip and saline controls.
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Fig. 3.
Time course of plasma glycerol (µM), free fatty acid (FFA, µM), and
triglycerides levels (mM) before (times
120 to 0 min) and after
(times 0 to 330 min) oral load. , Controls; , Ivélip tests; ,
glycerol tests. * P < 0.05 between Ivélip and saline controls.
|
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Isotopic enrichments.
Isotopic enrichment data are given in Fig.
4.
D-[6,6-2H2]glucose
enrichments declined comparably during the OGTT under saline, glycerol,
and lipid infusion, reverting to postabsorptive values at the end of
tests. However, as shown in Fig. 4, there was a trend for lower values
under Ivélip. Before the oral load was ingested, no differences
in the 13C level in glucose were
detected among the saline, glycerol, and lipid infusion tests. It rose
during the first 2 h of the OGTT and then declined without returning to
baseline at the end of the tests. This decrease was slower during the
lipid infusion, with significantly higher
13C enrichments of plasma glucose
at times +180,
210,
240,
300 (P < 0.05), and
330 (P < 0.01). During glycerol
infusion, 13C enrichments of
plasma glucose were lower (P < 0.05 at times +60,
90, and
120).
13C enrichment of expired
CO2 was significantly lower during
the last 150 min of the Ivélip tests than during control tests
(P < 0.01). However, the infusion of
Ivélip alone in four subjects produced a progressive decline in
13C enrichment of expired
CO2, with values from
time +90 to the end of test below
those at time
120
(P < 0.05). As shown in Fig. 4, no
difference in 13C enrichment of
expired CO2 was found between the
Ivélip and saline tests after correction for the contribution of
13C from the infused lipids.

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Fig. 4.
Time course of isotopic enrichments of plasma glucose with
D-[6,6-2H2]glucose
(molar ratio × 100), plasma glucose with
[13C] glucose [atom%
excess (APE) × 1,000], and expired
CO2 with
13C
( p × 1,000)
before (times 120 to
0 min) and after (times
0 to 330 min) oral load. ,
Controls; , Ivélip tests; , glycerol tests; ,
Ivélip tests without oral glucose tolerance test; ,
p 1,000 CO2(t)
corrected for contribution of 13C
from Ivélip. * P < 0.05 between Ivélip and saline controls.
|
|
Rates of glucose appearance.
Rates of glucose appearance are given in Table
1. Before the OGTT, postabsorptive SGP
rates were identical during control (2.32 ± 0.11 mg · kg
1 · min
1),
Ivélip (2.32 ± 0.05 mg · kg
1 · min
1), and glycerol (2.46 ± 0.18 mg · kg
1 · min
1)
tests. Cumulative RaT over 330 min
were higher during the lipid infusion and slightly increased during the
glycerol infusion [saline 1,181 ± 71 mg/kg vs. Ivélip
1,388 ± 100 mg/kg (P < 0.05),
glycerol 1,276 ± 126 mg/kg (NS vs. saline)]. Cumulative
RaE over 330 min were higher
during the lipid infusion and lower during the glycerol infusion
[saline 864 ± 38 mg/kg vs. Ivélip 993 ± 67 mg/kg
(P < 0.05), glycerol 698 ± 117 mg/kg (P < 0.05 vs.
saline)]. Cumulative SGP over 330 min were higher during the
lipid and glycerol infusions [saline 317 ± 57 mg/kg vs.
Ivélip 395 ± 58 mg/kg
(P < 0.05), glycerol 570 ± 54 mg/kg (P < 0.05 vs. Ivélip and
P < 0.001 vs. saline)].
Rates of glucose disappearance.
Rates of glucose disappearance are given in Table 1. No glucose was
detected in urine in any of the tests. Cumulative
RdT over 330 min were higher
during the lipid infusion (saline 1,242 ± 67 mg/kg vs. Ivélip
1,401 ± 96 mg/kg; P < 0.05). This was also observed for
RdE but did not reach significance
(saline 810 ± 52 mg/kg vs. Ivélip 885 ± 72 mg/kg; NS). Cumulative GOx over 330 min was lower during lipid infusion (saline 645 ± 19 mg/kg vs.
Ivélip 405 ± 33 mg/kg; P < 0.001). This was also observed for
Gexo (saline 423 ± 39 mg/kg
vs. Ivélip 266 ± 60 mg/kg;
P < 0.05), but the difference was
smaller and was abolished by correction for the contribution of
13C from the lipids
(Gexo,corr: 360 ± 92 mg/kg; NS vs. saline). Although different results were obtained from
the three different relationships used to calculate nonoxidative
glucose disposal, in every case the values were significantly higher
with the lipid infusion (Gnon-Ox1:
saline 666 ± 51 mg/kg vs. Ivélip 806 ± 28 mg/kg,
P < 0.05;
Gnon-Ox2: saline 597 ± 78 mg/kg vs. Ivélip 996 ± 0.95 mg/kg,
P < 0.001;
Gnon-Ox3: saline 392 ± 100 mg/kg vs. Ivélip 619 ± 95 mg/kg,
P < 0.001). This was also true when
Gexo,corr was used in place of
Gexo
(Gnon-Ox3,corr: 545 ± 83 mg/kg; P < 0.05 vs. saline).
 |
DISCUSSION |
The present study was designed to shed more light on the mechanism of
the moderate glucose intolerance induced by a lipid infusion during an
OGTT. The results of our control OGTTs are in agreement with those of
other studies using the same duration (5-6 h) and 1 g/kg oral
glucose load (16, 27, 41). In line with Rousselle et al. (37) and
Kruszynska et al. (23), we found that lipid infusion produced a
moderate glucose intolerance. This was due to a higher rate of total
glucose appearance, an impairment in the suppression of EGP, and an
apparent higher appearance of exogenous glucose. The higher rate of
total glucose appearance was partially reproducible by a glycerol
infusion. The total glucose disappearance rate was also higher during
lipid infusion despite a decrease in glucose oxidation, due to an
increase in nonoxidative disposal. Total glucose oxidation, evaluated
by indirect calorimetry, was more inhibited than exogenous glucose
oxidation evaluated by the recovery of
13C in exhaled
CO2. Although nonoxidative glucose
disposal was only evaluated indirectly, the three different methods of
calculation led to the same conclusion.
The slight defect in the suppression of SGP induced by the lipid
infusion was observed despite slightly higher plasma levels of insulin
and glucose, which are known to suppress SGP (11). This demonstrates
that lipids can produce a state of hepatic resistance to insulin (24)
but also resistance to the suppressive effect of glucose. This effect
(395
317
78 mg · kg
1 · 330 min
1) seems moderate, but
it was probably underestimated. SGP was calculated by substracting
RaE from
RaT. We found an increased RaE under lipid infusion. This
unexpected finding might reflect a higher absorption of glucose by the
gut as proposed by Meylan et al. (26) but has yet to be demonstrated.
Lipid infusion could also have, contrary to the total body glucose
uptake, lowered the splanchnic uptake of the oral load. In fact,
peripheral and splanchnic glucose uptake have been shown to be
regulated differently (11). However, this would not explain why the
lipid-induced increase in RaE only
appeared at the end of the tests (times
+150 to +240). In
agreement with the results of Tissot et al. (41), calculated
RaE remains positive 330 min after
the oral load had been ingested in our control tests (0.65 ± 0.22 mg · kg
1 · min
1),
when all metabolic parameters had returned to initial values. At this
time, glucose is no longer absorbed (1), and the persistence of a
significant enrichment in 13C of
plasma glucose is an indication of recycling of the label through the
gluconeogenic pathway. This rate of recycling has been estimated to be
~10% of the metabolism of the glucose load (15). It could have been
increased by the lipid infusion, since FFA can stimulate of
gluconeogenesis. Indeed Clore et al. (10) have demonstrated that the
activity of the Cori cycle can be doubled by a lipid-heparin infusion
in normal subjects. It is thus quite possible that an enhanced
gluconeogenesis during lipid infusion led to an increased recycling of
the 13C label, giving an
erroneously high RaE and an
underestimate of the calculated SGP (SGP = RaT
RaE).
Lipid emulsions contain free glycerol, which could contribute to the
enhanced SGP. Had all this glycerol been used for gluconeogenesis, it
would represent a production of 62 mg · kg
1 · 330 min
1 glucose, thus
explaining the main part of the increased SGP (78 mg · kg
1 · 330 min
1). However, only
45-50% of infused glycerol is converted to glucose, even at high
rates of glycerol infusion (31). It is unlikely that this proportion
was increased by the presence of high FFA levels, since glycerol
metabolism already generates NADH. To solve this issue, we performed
glucose tolerance tests with glycerol infusion. As shown in Fig. 3,
plasma glycerol rose only slowly compared with an almost square-wave
rise in concentration during Ivélip infusion. Presumably, during
Ivélip infusion, glycerol is released from the infused
triglycerides by the action of lipoprotein lipase in peripheral
tissues, which also explains the rise in FFA. A primed infusion of
glycerol, with a higher infusion rate, would therefore be necessary to
match the systemic glycerol concentrations seen during Ivélip
infusion. However, important effects on glucose metabolism were
observed during our glycerol infusion. We observed a moderate increase
in glucose RaT;
RaE was surprisingly low and therefore calculated SGP high. In fact, the increase in SGP compared with saline glucose tolerance tests cannot be accounted for by the
infused glycerol. The 13C
abundance in this glycerol was only slightly less than in the glycerol
present in the lipid emulsion infused (
30.2 vs.
29.5
13Cp × 1,000). Thus this cannot explain why
13C abundance in plasma glucose
during the OGTT with glycerol infusion was so low. We think that, in
the absence of high FFA levels, the recycling of
[13C]glucose through
gluconeogenesis from
[13C]lactate was
decreased by the glycerol infusion. This possibility is supported by
previous demonstrations that glycerol infusion inhibits gluconeogenesis
from other precursors (20, 40). Whatever the exact mechanism, this
makes difficult to quantify accurately the contribution of glycerol to
SGP. It is probable that the higher glucose
RaT which we found during glycerol
infusion reflects an enhanced SGP. We therefore think, in accordance
with results from Boden and Chen (4), that the glycerol content of the
lipid emulsion probably played a role in the modification of SGP.
RdT was not evaluated in previous
studies on lipid-induced glucose intolerance (26, 37), except for the
study of Kruszynska et al. (23). In agreement with this last study, we
found that the lipid infusion increased glucose
RdT during OGTT. This contrasts with results obtained during hyperinsulinemic euglycemic clamps (24).
Using the glucose-clamp technique, Boden et al. (6) and Bonadonna et
al. (7) have shown that a lipid infusion takes 3-4 h to inhibit
glucose uptake. This may account for the failure to observe an
inhibitory effect of lipids on glucose uptake in some studies but not
in the present one, since we infused lipids for 7 h (beginning 90 min
before glucose ingestion) and RdT
was higher at the end of the Ivélip tests. Increased plasma
insulin and glucose levels are recognized stimulators of peripheral
glucose uptake (11). This may well account for the higher
RdT we found: plasma insulin and
glucose levels were significantly higher under lipid infusion in our
study (which was not the case in clamp experiments). This higher
RdT was observed despite a
decrease in glucose oxidation as determined by indirect calorimetry,
which is an early and consistent consequence of a lipid infusion (4, 6,
23, 24, 26, 37). Measurement of expired
13CO2
can also be used to measure glucose oxidation after an oral glucose
load enriched with
[13C]glucose. At first
glance, both techniques gave the same result: the lipid infusion led to
a ~40% inhibition in GOx and
Gexo. However, the lipid infusion
had a low abundance in 13C, which
had its own effect on
13CO2
excretion rate independent of any lipid-glucose interaction, as we
noted in the four subjects studied without the oral glucose load.
Correction for this effect abolished the apparent inhibitory effect of
the lipid infusion on Gexo. This
subtraction of the effect of the lipid infusion alone on
13CO2
excretion is maximal estimate of the correction, since it does not take
account the decrease of lipid oxidation due to enhanced glucose
metabolism during the OGTT (38). However, our result suggests that the
inhibitory effect of lipid infusion is more pronounced on total than on
exogenous glucose oxidation. It may therefore specially affect the rate
of glycogen oxidation. Because glucose oxidation is decreased or
unchanged depending on the method used, the higher
RdT during lipid infusion appears
to derive from an increase in nonoxidative disposal of glucose. This
increase was significant regardless of the method of calculation
(Gnon-Ox1
+20%,
Gnon-Ox2 and
Gnon-Ox3
+40%). Because it
does not take account of the higher
RdT, the first method of
calculation underestimates the effect of the lipid infusion; this
explains why Gnon-Ox was not
always significantly higher in other lipid-modified OGTTs (26, 37). Our
result contrasts with reports that lipids inhibit glucose storage
either evaluated indirectly, as we did, or more directly by measuring
glycogen synthase activity on muscle biopsies (6) or by NMR (36).
However, all these results were obtained during euglycemic clamp and
not with a simultaneous increase in plasma glucose and insulin levels
as in the present study. As discussed previously, the lack of
inhibition of glucose nonoxidative disposal in our study cannot,
contrary to other studies (23), be explained by an insufficient period
of lipid infusion, since we infused lipids for 7 h. A glycogen-sparing
effect of a lipid infusion has been described in animals (22) and
suggested in humans (29). Our findings suggest that the slight
lipid-induced increases in glucose and insulin levels divert glucose
metabolism toward storage. This is consistent with the metabolic origin
of insulin resistance proposed by Felber et al. (13).
It is of interest to compare the abnormalities in glucose tolerance
induced by lipid infusion with those observed during starvation and in
subjects with impaired glucose tolerance or NIDDM. Féry et al.
(16) studied the mechanism of starvation diabetes by performing
double-labeled OGTTs in normal subjects before and after 4 days of
fast. The effect of starvation was similar to our observations with the
lipid infusion, namely, higher residual SGP, lower glucose oxidation,
and higher nonoxidative disposal. RaE was not affected by fasting,
but the use of
[1-14C]glucose enabled
correction for recycling, which was indeed slightly higher during
fasting. Similar results have been reported by Mitrakou et al. (27) in
"spontaneous" impaired glucose tolerance. The fate (oxidative or
not) of utilized glucose was not reported in that study, but Felber et
al. (12), using indirect calorimetry, found lowered values for
oxidation but not for glucose nonoxidative disposal in obese subjects
with impaired glucose tolerance. The results obtained in patients with
NIDDM were different. In all cases (17, 18, 28),
RaT and SGP were found elevated
(although Féry et al. observed this only in severely
hyperglycemic patients; Ref. 17), but
RdT was not higher (at least after
correction for glycosuria; Ref. 18). Glucose oxidation was lower (17), but there was also an impaired glucose nonoxidative disposal, as shown
by similar (18) or even lower (28) values despite the hyperglycemia.
This last abnormality cannot be explained by the effect of lipids, at
least during acute administration.
In summary, a lipid infusion led to a moderate impairment in glucose
tolerance in normal subjects. This was due to a defect in both the
suppression of systemic glucose production and the stimulation of
glucose oxidation. Infused glycerol contributed to the effect on
systemic glucose production. The impairment in glucose oxidation is in
accordance with Randle hypothesis. But this defect in glucose oxidation
did not decrease insulin stimulated glucose disposal as initially
proposed by Randle et al. (33) In fact, glucose intolerance was
moderate because of a concomitant increase in glucose nonoxidative
disposal. A further defect, concerning glucose nonoxidative disposal,
therefore appears to be required for the development of NIDDM.
 |
ACKNOWLEDGEMENTS |
The authors acknowledge S. P. Jarman for translation.
 |
FOOTNOTES |
This study was promoted by Institut National de la Santé et de la
Recherche Médicale and supported by a grant from Association de
Langue Française pour l'Etude du Diabete et des Maladies
Métaboliques Institut Servier du Diabète and Comité
Interprofessionnel de la Dinde Française.
Address for reprint requests: V. Rigalleau, Service de Nutrition et
Diabétologie, USN, Hôpital Haut-Lévêque, Ave.
de Magellan, 33600 Pessac, France.
Received 7 October 1997; accepted in final form 18 June 1998.
 |
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