The Journal of Clinical Endocrinology & Metabolism Vol. 85, No. 3 1251-1254
Copyright © 2000 by The Endocrine Society
The Relationship between Glucose Disposal in Response to Physiological Hyperinsulinemia and Basal Glucose and Free Fatty Acid Concentrations in Healthy Volunteers1
Fahim Abbasi,
Tracey McLaughlin,
Cindy Lamendola and
Gerald M. Reaven
Department of Medicine, Stanford University School of Medicine,
Stanford, California 94080
Address correspondence and requests for reprints to: Gerald M. Reaven, M.D., Shaman Pharmaceuticals, Inc., 213 East Grand Avenue, South San Francisco, California 94080-4812; E-mail: greaven{at}shaman.com
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Abstract
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This study was initiated to see if defects in the ability of
physiological hyperinsulinemia (
60 µU/mL) to stimulate glucose
uptake in healthy, nondiabetic volunteers are associated with increases
in concentrations of plasma glucose and free fatty acid (FFA) when
measured at basal insulin concentrations (10 µU/mL). We recruited
22 volunteers (12 women and 10 men) for these studies, with a
(mean ± SEM) body mass index of 24.8 ± 0.5
kg/m2. Resistance to insulin-mediated glucose disposal
during physiological hyperinsulinemia was determined by suppressing
endogenous insulin and determining the steady-state plasma glucose
(SSPG) and steady-state plasma insulin (SSPI) concentrations at the end
of a 3-h infusion, period during which glucose (267
mg/m2·min) and insulin (32 mU/m2·min) were
infused at a constant rate. Glucose, insulin and FFA concentrations
were also measured in response to infusion rates of glucose (50
mg/m2·min) and insulin (6 mU/m2·min). The
SSPI concentration (mean ± SEM) during physiological
hyperinsulinemia was 64 ± 3 µU/mL), in contrast to 12 ±
0.4 µU/mL during the basal insulin study. The results demonstrated a
significant relationship between SSPG concentration in response to
physiological hyperinsulinemia (SSPG60) and
SSPGBasal (r = 0.57, P < 0. 01)
and FFABasal (r = 0.73, P <
0.001). Furthermore, FFABasal and SSPGBasal
were significantly correlated (r = 0.47, P <
0.05). Comparison of the seven most insulin-resistant and seven most
insulin sensitive individuals (SSPG60 values of 209 ±
16 vs. 64 ± 8 mg/dL) revealed that the
insulin-resistant group also had significantly higher
SSPGBasal (105 ± 5 vs. 78 ± 7
mg/dL, P < 0.01) and FFABasal
(394 ± 91 vs. 104 ± 41,
P < 0.02) concentrations. However, random
fasting plasma glucose and FFA concentrations of the two groups were
not different. The results presented demonstrate that individual
differences in the ability of elevated insulin concentrations to
stimulate muscle glucose disposal are significantly correlated with
variations in insulin regulation of plasma glucose and FFA
concentrations at basal insulin concentrations.
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Introduction
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PREVIOUS reports from our research
group have emphasized that insulin-mediated glucose disposal in
response to physiological hyperinsulinemia varies widely in
nondiabetic, healthy volunteers (1, 2). Furthermore, differences in the
plasma glucose response to an oral glucose challenge in these normal
glucose tolerant individuals were highly correlated with the variations
in insulin-mediated glucose disposal (2). In contrast, fasting plasma
glucose concentrations in the same individuals were quite similar and
were unrelated to the degree of insulin resistance as estimated during
periods of physiological hyperinsulinemia. There are two obvious
explanations for these findings. On the one hand, dramatic variations
in insulin-mediated glucose disposal in response to physiological
hyperinsulinemia in normal glucose tolerant individuals need not be
paralleled by similar differences at basal insulin concentrations.
Alternatively, differences in plasma insulin concentrations after an
overnight fast could explain why fasting glucose concentrations can be
so similar in nondiabetic subjects. We believed this latter possibility
more likely based on the following evidence. We have previously defined
in normal volunteers the presence of a statistically significant
relationship between plasma glucose concentrations during steady states
of physiological hyperinsulinemia and plasma free fatty acid (FFA)
concentrations when measured at the same basal insulin concentration in
all volunteers (3). Demonstration of a defect in the ability of insulin
to maintain plasma FFA concentrations when basal plasma insulin
concentrations were normalized suggested that this might also be true
of the ability of basal plasma insulin concentrations to regulate
plasma glucose concentrations. In further support of this possibility
is the fact that fasting plasma insulin concentrations are
significantly correlated with measurements of insulin-mediated
glucose disposal in nondiabetic volunteers (4, 5). Thus, basal
hyperinsulinemia is capable of maintaining glucose concentrations in
the normal range in insulin-resistant individuals. The current studies
were initiated to extend our previous observations (3) and consisted of
experiments aimed at testing the hypothesis that the greater the defect
in insulin-mediated glucose disposal in response to physiological
hyperinsulinemia, the higher will be plasma glucose and FFA
concentrations when measured at the same basal insulin level.
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Materials and Methods
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The study participants consisted of 22 volunteers (10 men and 12
women) recruited from the San Francisco Bay Area who responded to
newspaper advertisements indicating our interest in studying insulin
resistance in healthy, nondiabetic individuals. Their mean ±
SEM age was 54 ± 2 yr (range 28–70), and their body
mass index (BMI) was 24.8 ± 0.5 kg/m2
(range 20.2–30.0 kg/m2). All participants were
in good general health, with normal medical histories and physical
examinations, and normal values on a routine hematological survey and
chemical screening battery. They were determined to be nondiabetic on
the basis of at least 2 fasting blood glucose values of less than 126
mg/dL (1). Informed consent was obtained before the initiation of any
study.
Participants were admitted to the General Clinical Research Center on
two occasions after an overnight fast for two experimental infusion
procedures, performed in random order, 5–7 days apart. On one occasion
insulin-mediated glucose disposal was evaluated by modification (6) of
an insulin sensitivity test, as previously described, and was validated
by our laboratory (2). Briefly, an iv catheter was placed in each of
the patients’ arms. Blood was sampled from one arm for measurement of
plasma glucose (7) and insulin (8) concentrations, and the
contralateral arm was used for administration of test substances.
Somatostatin (octreotide acetate) was administered at the
rate of 0.27 µg/m2·min to suppress endogenous
insulin secretion. Simultaneously, insulin and glucose were infused at
rates of 32 mU/m2·min and 267
mg/m2·min, respectively. Blood was sampled
every 30 min until 150 min into the study, then every 10 min until 180
min had elapsed. The four values obtained from 150–180 min were
averaged and considered to represent the steady-state plasma glucose
(SSPG) and steady-state plasma insulin (SSPI) concentrations achieved
during the infusion. SSPI concentrations approximated 60 µU/mL under
these conditions, and were comparable in all individuals. The SSPG
concentrations provided a direct estimate of insulin-mediated glucose
disposal in each individual at these levels of insulin concentrations
(SSPG60); the higher the
SSPG60, the more insulin-resistant the
individual.
On the other occasion, plasma glucose and FFA concentrations at a basal
insulin level were measured during a 120-min infusion study. Endogenous
insulin secretion was suppressed with a continuous infusion of
octreotide acetate (0.27
µg/m2·min). To achieve basal insulin
concentrations, insulin and glucose were infused at rates of 6
mU/m2·min and 50
mg/m2·min, respectively. After obtaining blood
for baseline measurements, samples for plasma glucose, FFA (9), and
insulin were drawn every 30 min until 100 min into the study, then
every 10 min until 120 min had elapsed. The 3 values obtained from
100–120 min were averaged to represent the basal
SSPGBasal, FFABasal, and
insulin concentrations.
Data are expressed as mean ± SE and were analyzed
with Systat 7.0 package for Windows. Pearson’s Correlation
coefficients were calculated between the variables of interest, and a
multiple regression analysis was performed with
SSPGBasal and FFABasal as
the dependent variables. P less than 0.05 was considered
statistically significant.
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Results
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The relationships between the SSPG60
concentration during the period of physiological hyperinsulinemia
(64 ± 3 µU/mL) and the plasma glucose and FFA concentrations at
basal insulin concentration (12 ± 0.4 µU/mL) are shown in
Figure 1
. It can be seen that both
SSPGBasal and FFABasal were
correlated with SSPG60. When adjusted for
differences in age, gender, and BMI, the correlation coefficients
between SSPG60, SSPGBasal,
and FFABasal remained highly related with
correlation coefficients of r = 0.60, (P = 0.007),
and r = 0.72 (P = 0.001), respectively. Given
these results, it was not surprising that
SSPGBasal and FFABasal were
also highly significantly correlated (r = 0.47, P
= 0.03).
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Figure 1. Relationship between glucose disposal in
response to physiological hyperinsulinemia (SSPG60) and
plasma glucose (SSPGBasal) and FFA (FFABasal)
at a fixed, basal insulin concentration.
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In light of the close association between the three experimental
variables—SSPG60,
SSPGBasal, and FFABasal—several multiple
regression models were created in an effort to gain additional insight
into the nature of their relationship. The first model tested had
SSPGBasal as the dependent variable, and the
results are shown in Table
1. The
results in Table 1A document an independent relationship between
SSPGBasal and FFABasal.
Replacing FFABasal with
SSPG60 (1B) demonstrates that
SSPGBasal and SSPG60 are
also significantly related. When both FFABasal
and SSPG60 are in the model, none of the
variables are significantly related to
SSPGBasal.
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Table 1B. TABLE 1B. Multiple regression analysis of the
relationship between SSPGBasal and age, gender, BMI, and
SSPG60
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Results of a similar analytical approach, with
FFABasal as the dependent variable, are shown in
Table 2.
Table 2A indicates that there is a statistically significant
relationship between FFABasal and
SSPGBasal. Table 2B displays the results when
SSPG60 replaces SSPGBasal
in the model and demonstrates a highly significant relationship between
SSPG60 and FFABasal.
Furthermore, when both SSPG60 and
SSPGBasal are in the model, the results in Table 3 indicate that
SSPG60 remains significantly related to
FFABasal, whereas SSPGBasal
does not.
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Table 3. Multiple regression analysis of the relationship
between FFABasal and age, gender, BMI,
SSPGBasal and SSPG60
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To further explore the relationship between the ability of
physiological hyperinsulinemia to stimulate glucose disposal and
regulate plasma glucose and FFA concentrations at a basal insulin
level, we created two groups of seven persons each. For this purpose we
selected the three men and four women with either the highest or the
lowest SSPG concentrations. The two groups thus formed are compared in
Table 4. As can be seen, they were
identical in terms of age, gender distribution, and BMI. By selection,
SSPG60 was approximately 3-fold greater in the
insulin-resistant individuals. Despite the similarity in age, gender
and BMI, concentrations of both SSPGBasal and
FFABasal were significantly higher
(P = 0.02) in the insulin-resistant group at basal
insulin concentrations. It should be emphasized that fasting plasma
glucose and FFA concentrations, under conditions in which the insulin
level was not fixed, were similar, presumably due to the ability of the
higher fasting plasma insulin concentrations to overcome the defect in
insulin action in the insulin-resistant individuals.
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Discussion
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These studies were performed in order to quantify the ability of
basal insulin concentrations to regulate SSPG and FFA concentrations in
nondiabetic volunteers, under conditions in which plasma insulin
concentrations were normalized. By so doing, we were able to avoid the
confounding variable of differences in insulin concentration from
person to person. It should also be realized that our use of octreotide
to normalize basal insulin concentration meant that we had also
normalized the potential impact of other hormones that would have
affected glucose and lipid metabolism. Thus, our results are based upon
the attainment of similar basal insulin concentrations in the study
population, not upon achieving basal conditions. Given this explicit
definition, it is clear that our results were quite straight-forward at
the descriptive level. The greater the defect in glucose disposal in
response to physiological hyperinsulinemia
(SSPG60) in a given individual, the higher their
glucose and FFA concentrations were at a basal insulin level (10
µU/mL). The fact that fasting plasma glucose and FFA concentrations
are not usually discerned to be higher in insulin-resistant individuals
is almost certainly related to the higher fasting insulin
concentrations in these individuals. This point is made explicit by the
results in Table 4
, showing that fasting plasma insulin concentrations
are significantly greater in the insulin-resistant subgroup, whereas
the fasting plasma glucose and FFA concentrations are similar in the
insulin-resistant and insulin-sensitive subgroups. Conducting these
studies at a fixed basal insulin concentration permitted us to observe
the defect in the ability of basal insulin concentrations to maintain
basal glucose and FFA concentrations in individuals resistant to
glucose disposal in response to physiological hyperinsulinemia.
Furthermore, at that fixed insulin level, the higher the
SSPGBasal, the higher the
FFABasal.
The description "insulin-resistant" is most often used to designate
an individual or group that disposes of an infused glucose load less
efficiently in response to a physiological increase in plasma insulin
concentration, compared with another individual or group. The enormous
variability in this facet of in vivo insulin action has been
emphasized in previous reports from our research group (1, 2) and is
certainly apparent from inspection of the range of
SSPG60 values illustrated in Fig. 1. However, the
limitation of this definition is apparent from the results of our
study. Specifically, plasma glucose and FFA concentrations of healthy,
nondiabetic volunteers are also significantly different when determined
at the same basal insulin concentration. It is certainly possible that
the description "insulin-resistant" should not be used to describe
an individual whose plasma FFA concentration is twice as high when
compared to another individual at a basal insulin concentration. On the
other hand, the results presented demonstrate that this certainly can
occur and that there is a significant correlation between variations in
insulin-stimulated glucose disposal at elevated insulin concentrations
and plasma glucose and FFA concentrations at basal insulin levels.
Correlation coefficients do not define causal relationships between
associated variables, and we can only speculate as to how the three
outcome variables we measured are related. However, based upon the
results presented, it appears that the closest relationship is between
glucose disposal in response to physiological hyperinsulinemia
(SSPG60) and FFABasal. This
relationship was also seen in a previous study from our research group
(3) and is consistent with the view that defects in the ability of
insulin to stimulate glucose disposal by muscle in an individual will
be associated with similar abnormalities in insulin regulation of
adipose tissue lipolysis at a basal insulin concentration. More simply
put, differences in insulin action on muscle and adipose tissue are
highly correlated in nondiabetic individuals. There is also a
relationship between plasma glucose concentrations at elevated and
basal insulin concentrations, but the degree of the association is of
lesser magnitude. Based upon these findings, we think it possible that
the relationship between SSPG60 and
SSPGBasal is an indirect one. Namely, we
speculate that elevated FFABasal is a fundamental
abnormality, in individuals with high SSPG60
concentrations, that, in turn, increases
SSPGBasal concentrations because of the
inhibitory effect of higher FFA concentrations on glucose disposal
rates at basal insulin concentrations. The proposed sequence of events
outlined above may eventually prove not to be the best explanation to
account for our results. However, the speculative nature of our effort
to formulate a hypothesis to provide a conceptual framework for our
data should not obscure the new insights as to the regulatory role of
insulin. It is now quite clear from the results presented that
individual differences in the ability of elevated insulin
concentrations to stimulate muscle glucose disposal are significantly
correlated with variations in insulin regulation of plasma glucose and
FFA concentrations at basal insulin concentrations.
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Footnotes
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1 This work was supported by research grants (DK-30732 and RR-00070)
from the National Institutes of Health. 
Received June 17, 1999.
Revised September 15, 1999.
Accepted June 17, 1999.
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Abstract
Introduction
