Regulation of exercise carbohydrate metabolism by estrogen
and progesterone in women
Tara M.
D'Eon1,
Carrie
Sharoff1,
Stuart R.
Chipkin2,
Dan
Grow3,
Brent C.
Ruby4, and
Barry
Braun1
1 Department of Exercise Science, University of
Massachusetts, Amherst 01003; 2 Division of
Endocrinology, Diabetes and Metabolism and 3 Department
of Obstetrics and Gynecology, Baystate Medical Center, Springfield,
Massachusetts 01199; and 4 Department of
Health and Human Performance, University of Montana, Missoula,
Montana 59813
 |
ABSTRACT |
To assess the roles of endogenous estrogen
(E2) and progesterone (P4) in regulating
exercise carbohydrate use, we used pharmacological suppression and
replacement to create three distinct hormonal environments: baseline
(B), with E2 and P4 low; estrogen only (E),
with E2 high and P4 low; and
estrogen/progesterone (E + P), with E2 and P4
high. Blood glucose uptake (Rd), total carbohydrate oxidation (CHOox), and estimated muscle glycogen
utilization (EMGU) were assessed during 60 min of submaximal exercise
by use of stable isotope dilution and indirect calorimetry in eight
eumenorrheic women. Compared with B (1.26 ± 0.04 g/min) and E + P
(1.27 ± 0.04 g/min), CHOox was lower with E
(1.05 ± 0.02 g/min). Glucose Rd tended to be lower
with E and E + P relative to B. EMGU was 25% lower with E than with B
or E + P. Plasma free fatty acids (FFA) were inversely related to EMGU
(r2 = 0.49). The data suggest that estrogen
lowers CHOox by reducing EMGU and glucose Rd.
Progesterone increases EMGU but not glucose Rd. The
opposing actions of E2 and P4 on EMGU may be
mediated by their impact on FFA availability or vice versa.
ovarian hormones; menstrual cycle; fat oxidation; stable isotope; glycogen
 |
INTRODUCTION |
THERE IS GROWING
CONSENSUS that the ovarian hormones, estrogen and progesterone,
have important roles in regulating substrate metabolism during exercise
in women (5, 7, 10, 14, 39, 40, 43-46). In animal
models, estrogen promotes lipolysis and increases fatty acid
availability (3, 15, 21, 23, 31) while decreasing the rate
of gluconeogenesis and sparing muscle and liver glycogen use (18,
23, 31, 34, 38). The addition of progesterone has been reported
to antagonize the lipolytic effects of estrogen and reduce fatty acid
availability (23, 29, 33). Conversely, the addition of
progesterone appears to accentuate the carbohydrate-sparing actions of
estrogen by decreasing hepatic glycogenolysis (23, 29,
37). Recently, Campbell and Febbraio (9) showed
that estrogen upregulates mitochondrial enzymes favoring fat oxidation,
whereas progesterone opposed these actions.
Metabolic regulation by endogenous estrogen (estradiol,
E2) and progesterone (P4) has usually been
studied in humans by comparison across different phases of the
menstrual cycle: the menstrual phase (E2 and P4
both low), the midfollicular phase (E2 elevated, P4 low), and the midluteal phase (E2 and
P4 both elevated). High inter- and intrasubject variability
inherent to studying the "natural" hormonal environment complicates
the ability to draw clear conclusions. In some cases (8, 20,
47), researchers observed a shift toward reduced blood glucose
use and increased fat oxidation during submaximal exercise in the
luteal phase of the cycle, but others have reported no significant
differences (2, 4, 6, 25, 30). Even in well designed
studies that minimize confounding variables with dietary control,
careful timing of measurements, and the addition of stable isotope
tracers to indirect calorimetry, the discrepancy remains. Furthermore,
because both E2 and P4 concentrations vary in
different phases of the natural menstrual cycle, it is not possible to
attribute metabolic differences observed to independent effects of
progesterone or estrogen.
Researchers have tried to solve the latter problem by "controlling"
the ovarian hormone environment with estrogen administration. Ruby et
al. (41) found that supplementing amenorrheic women (who
have low E2) with transdermal estrogen reduced blood
glucose flux during submaximal exercise. Carter et al.
(12) supplemented men with estrogen and found comparable
results. These data suggest that estradiol administration reduces the
rate of blood glucose utilization. Neither research group reported a
significant decrease in whole body carbohydrate oxidation, implying
that there was no sparing of muscle glycogen utilization in the
presence of estrogen.
Taken together, results from animal and human studies present a
complicated and contradictory story. Recently, Horton et al. (25), Campbell and Febbraio (10), and D'Eon
and Braun (14) argued that examining the relative ratio of
estrogen to progesterone might clarify some of the discrepancies.
Therefore, the purpose of this study was to assess glucose kinetics and
oxidation during submaximal exercise by use of pharmacological agents
to create three tightly controlled hormonal environments: baseline (low estrogen and progesterone), estrogen only (high estrogen, low progesterone), and estrogen plus progesterone (high estrogen, high
progesterone). Mainly on the basis of menstrual cycle phase studies in
humans, we hypothesized that the addition of progesterone would
potentiate the effects of estrogen alone to lower blood glucose uptake
and total carbohydrate oxidation relative to the baseline condition.
 |
METHODS |
Subjects.
Subjects for this study were healthy, physically active women who
participated in regular aerobic activity
3 times/wk. All of the
subjects were in excellent overall health, had no history of
cardiovascular, metabolic, or hormonal disorders, used no medications other than occasional over-the-counter aspirin or ibuprofen, displayed no evidence of eating disorders and reported normal eating habits, and
had not used oral contraceptives for
6 mo before the study. After
study procedures were explained verbally, subjects signed a written
informed consent document approved by Institutional Review Boards at
both the University of Massachusetts and Baystate Medical Center.
Initially, 12 women enrolled in the study. Two subjects withdrew after
the first test due to discomfort with the blood-drawing procedures
and/or hormonal treatments. In two other subjects who completed the
study, either estrogen or progesterone concentrations were considerably
elevated in a condition when they were expected to be low. Therefore,
data from these two subjects were not included in the analysis, and all
results are reported with n = 8. Characteristics of
these subjects are shown in Table 1.
Pretesting procedures.
Body density was determined by hydrostatic weighing, with residual lung
volume estimated as (0.28 × maximal expired volume). Maximal
oxygen consumption (
O2 max) was
measured on an electronically braked cycle ergometer with an
incremental ramp protocol starting at 60 W and increasing by 30 W every
2 min until subjects could no longer maintain a cadence of 60 rpm.
Oxygen consumption and carbon dioxide production were measured by
indirect calorimetry with a TrueMax 2400 metabolic measurement system
(Parvo Medics, Sandy, UT).
Overall study design.
Subjects were tested in three different conditions: baseline, estrogen
only, and estrogen plus progesterone (see Table
2). The treatments were originally
administered in a double-blind manner, but one author inadvertently
became aware of the code about halfway through the study and was
therefore not blinded for the remainder of the study. The order in
which the E and E+P treatments were administered was balanced across
subjects.
Hormonal control.
Subcutaneous injections of 0.25 mg Ganirelix (Organon, West Orange,
NJ) were begun 2-5 days after the onset of menses to
suppress endogenous production of gonadotropin-releasing hormone, or
GnRH. The injections were given 30-34 and 6-10 h before
baseline testing (B), which occurred on day 3. After B,
subjects were given one of two treatments: estrogen-only (E), in which
subjects wore three transdermal estradiol patches (Vivelle, 0.1 mg
estradiol each) affixed to the skin of the upper pelvis and consumed
an oral placebo for 3 days; and estrogen plus progesterone
(E + P), which was exactly the same treatment but with oral
progesterone (Prometrium, 200 mg/day) replacing the placebo.
Posttreatment testing occurred on day 6, at the same time of
day as baseline testing. The second round of treatment and testing was
begun 2-5 days after the following onset of menses ~1 mo later.
Control of diet and activity.
Subjects were regularly reminded to maintain a similar activity level
and consistent dietary habits throughout the course of the study. They
refrained from exercise for 24 h before testing in all conditions.
Although diet was not rigidly controlled throughout the study, all
subjects consumed the same preexercise meal 3 h before each test.
The meal comprised 35% of estimated daily energy requirements and was
composed of 55% carbohydrate, 15% protein, and 30% fat. The Harris
and Benedict equation specific to women, 655 + 9.5 · (weight) + 1.9 · (height)
4.7 · (age), was used to calculate resting
metabolic rate, and this value was multiplied by an activity factor of
1.7 (assuming women in this study were moderately active) to estimate
daily energy requirements. Subjects were instructed to fast after this
meal until after testing.
Testing procedures.
Subjects reported to the laboratory 3 h after the meal, and a
catheter was inserted into an antecubital vein for infusion of stable
isotope. A second catheter was placed in a forearm or wrist vein of the
contralateral arm for blood sampling. A venous blood sample was
collected before infusion for determination of background isotopic
enrichment, and a priming bolus of 200 mg [6,6-2H]glucose
in 0.9% sterile saline was then rapidly infused into the venous
catheter. To reach and maintain isotopic equilibrium, [6,6-2H]glucose was then continuously infused at 2.5 mg/min with a peristaltic infusion pump (Harvard Apparatus, South
Natick MA). Venous blood samples and 5-min collections of expired
oxygen and carbon dioxide were taken at rest 75 and 90 min after the
start of the infusion.
Immediately after the last resting measurement, the subject began
submaximal exercise on a bicycle ergometer (LifeFitness888). To
maintain a steady isotopic enrichment of blood glucose, the [6,6-2H]glucose infusion rate was increased to 6.0 mg/min. During the first 15 min of exercise in the B condition, the
intensity was adjusted by manipulating the pedaling resistance until
oxygen consumption reached a steady state at ~60% of the previously
measured
O2 max. Blood and breath
samples (5 min) were collected at 15, 30, 45, and 60 min of exercise
(see Fig. 1).
Biochemical assays.
Samples of venous blood for analysis of glucose, lactate, insulin, and
glucose isotopic enrichment were collected in heparinized syringes and
then transferred to vacutainers containing sodium fluoride (to inhibit
glycolysis). Samples for analysis of free fatty acids (FFA) were
collected in heparinized syringes and then transferred to vacutainers
containing EDTA. Samples for analysis of estrogen, progesterone,
epinephrine, and norepinephrine were collected in nonheparinized
syringes, transferred to vacutainers specific for serum analysis
(glutathione added for epinephrine and norepinephrine), and allowed to
clot. All samples were immediately centrifuged, and the plasma was
transferred to cryogenic vials and frozen at
70°C until analysis.
Glucose and lactate concentrations were determined using a
glucose/lactate analyzer (GL5 Analyzer, Analox Instruments. Lunenberg,
MA). Estradiol and progesterone levels were determined using enzyme
immunoassays (Diagnostic Systems Laboratories, Webster, TX). Insulin
was measured using radioimmunoassay (Linco Research, St. Charles, MO).
FFA concentrations were measured using a standard colorimetric assay
(Wako Chemicals, Richmond, VA). Epinephrine and norepinephrine
concentrations were determined by high performance liquid
chromatography with electrochemical detection.
Glucose isotopic enrichment was measured by GC-MS. Plasma was first
neutralized by back titration with 2 N KOH, passed through anion and
cation exchange resins, lyophilized, reconstituted with acetic
anhydride-pyridine (2:1), dried under a stream of nitrogen, and
reconstructed in 100 µl of ethyl acetate. A 25-µl sample was injected and separated on a gas chromatograph, with spectra recorded on
a mass spectrometer (Hewlett-Packard 6890, Palo Alto, CA). Selected ion
monitoring was used to compare the abundance of the unlabeled fragment
with that of the enriched isotopomer (Chemstation Software). After
correction for background enrichment, the abundance of the dideuterated
isotopomer [mass-to-charge ratio (m/z) = 333] was
expressed as a percentage of total glucose species (m/z = 331 + 332 + 333).
Calculations.
To test the rate at which glucose is taken up from the blood
(rate of disappearance, Rd) and replaced by the liver (rate
of appearance, Ra), equations specifically designed for use
with stable isotopes in biological systems were used
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(1)
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(2)
|
F represents the isotopic infusion rate, IE1 and
IE2 are the enrichments of plasma glucose with dideuterated
glucose at time points t1 and
t2, respectively; C1 and
C2 are the concentrations of plasma glucose at
t1 and t2; and V is the
estimated volume of distribution for glucose (180 ml/kg).
|
(3)
|
where RER is the respiratory exchange ratio.
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(4)
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(5)
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This estimate is based on the assumption that 100% of blood
glucose taken up from the blood is oxidized, which is unlikely to be
true; i.e., the percentage of Rd oxidized is probably
70-90% (16, 28) but may vary across the conditions
used in this study. Thus the calculation underestimates glycogen use
and is best described as minimal muscle glycogen utilization.
Statistical analysis.
All data in Tables 1-4 and in Figs. 2-7 are group means ± SE (SD for demographic information). Summary measures of the
exercise time points for each subject were calculated using the
trapezium rule for area under the curve (AUC). The summary data were
analyzed as raw data by ANOVA with repeated measures by use of the
PROC-MIXED univariate analysis for all variables (SAS Institute, Cary,
NC). Statistical significance was defined as
< 0.05. Post hoc
analysis with planned comparisons was made using Fischer's protected
least significant difference test. Nonlinear regression analysis was performed using SPSS 10.0.7 (SPSS, Chicago, IL).
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Table 4.
Work intensity, oxygen consumption, substrate use, and catecholamine
concentrations during steady-state exercise in the 3 conditions
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Fig. 3.
Glucose rates of appearance (Ra,
A), disappearance (Rd, B), and
metabolic clearance rate (MCR, C) in the 3 conditions.
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Fig. 4.
Summary of average fuel utilization in the 3 treatment
conditions during steady-state exercise. %Contribution to total energy
expenditure is shown inside each bar. Conditions that do not share a
superscript are significantly different from each other.
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Fig. 5.
Glucose (A), insulin (B), and
lactate (C) concentrations at rest and during exercise.
Insulin concentrations fell during the course of exercise and were
significantly higher in the baseline testing condition at rest and at
15 and 30 min of exercise.
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Fig. 6.
Plasma free fatty acid (FFA) concentrations at rest and
during exercise. FFA were significantly higher (P < 0.05) in E compared with E + P at rest and during exercise.
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Fig. 7.
Relationship between plasma FFA concentrations and the
estimated rate of muscle glycogen use. The power function shown above
the figure describes the relationship between the 2 parameters, and the
correlation coefficient (R2) indicates that
about one-half of the total variance is explained by that
relationship.
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RESULTS |
Hormonal environment.
The treatments resulted in three very distinct hormonal environments
(Table 3). Compared with the baseline
condition (B), the serum concentration of estradiol was considerably
elevated in the estrogen-only condition (E), with no difference in
serum progesterone. With the addition of progesterone (E + P), there was a substantial elevation in serum progesterone, but the serum estrogen concentration was comparable to E. Therefore, E and B differed
only in the levels of serum estradiol, whereas E and E + P differed
only in the levels of progesterone.
Exercise oxygen consumption and heart rate.
Oxygen consumption, percent
O2 max, and
heart rates were almost identical in B, E, and E + P (Table
4), implying that the exercise intensity
did not vary among the three conditions.
Gas exchange.
The RER was significantly lower both at rest and during exercise in E
compared with B (Table 4). Compared with E, the RER in E + P was
elevated and almost indistinguishable from B. As shown in Table 4,
total carbohydrate oxidation was reduced with E relative to B, and the
addition of progesterone did not potentiate the estrogen effects as
expected. Instead, total carbohydrate oxidation was significantly
higher with E + P compared with E, and very similar to B. Total fat
oxidation is the reciprocal of carbohydrate oxidation and so followed
the same pattern, in which fat oxidation was elevated with E relative
to either B or E + P (Table 4).
Blood glucose kinetics and EMGU.
The isotopic enrichment of plasma glucose at rest and during exercise
is shown in Fig. 2. The increase in
isotope infusion rate at the onset of exercise appears to have resulted
in relatively constant isotope enrichment over time. The rate of
glucose appearance into (glucose Ra) and uptake from the
blood (glucose Rd) were not significantly different among
conditions (Fig. 3, A and
B) but tended to be lower with
E and E + P relative to B (0.05 < P < 0.10).
When Rd was scaled to the ambient plasma glucose
concentration to assess the metabolic clearance rate (MCR) of glucose,
there were no differences among conditions (Fig. 3C). Muscle
glycogen use, estimated as the difference between total carbohydrate
oxidation and glucose Rd (EMGU), was significantly lower in
E compared with B (Table 4). Contrary to the stated hypothesis, the
addition of progesterone increased EMGU to values that were very
similar to those observed in B. Therefore, the decrease in total
carbohydrate oxidation with E relative to B resulted from reductions in
both blood glucose and EMGU. Compared with E, total carbohydrate
oxidation was restored by E + P to a level indistinguishable from B and was attributable solely to increased EMGU. A summary of the relative proportions of nonprotein energy expenditure derived from blood glucose, other carbohydrate (i.e., glycogen), and fat oxidation is
shown in Fig. 4.
Plasma glucose and lactate.
Plasma concentrations of glucose did not change from rest to exercise
and were not different among conditions (Fig.
5A). Plasma lactate
concentrations rose steeply in the first 15 min of exercise in all
conditions and then declined toward baseline at each subsequent time
point (Fig. 5B). At 60 min, plasma lactate concentrations were ~2.0-2.5 mM in B and E. Although lactate concentrations
were lowest at every exercise time point with E + P (final
concentration ~1.5 mM), there were no significant differences among conditions.
Glucoregulatory hormones.
Plasma insulin concentrations fell during exercise in all conditions
(Fig. 5C). Insulin levels plateaued at ~25 pM in E and E + P. In B, plasma insulin concentrations were significantly higher at
rest and for the first 30 min of exercise relative to E and E + P, but
there were no differences among conditions at 45 and 60 min. Plasma
concentrations of epinephrine and norepinephrine (Table 4) rose
considerably during exercise compared with rest, and the responses were
very similar in all three conditions.
Plasma FFA.
Relative to E + P, the FFA concentration was significantly elevated
with E (Fig. 6). As shown in
Fig. 7, there was an inverse nonlinear
(power function) relationship between the concentration of fatty acids
in plasma and the EMGU rate.
 |
DISCUSSION |
The main findings in this study were that 1) high
circulating levels of estrogen reduced total carbohydrate oxidation
during exercise compared with a low-estrogen condition, with decreases in both estimated muscle glycogen use and (a tendency toward) blood
glucose uptake, and 2) when high levels of progesterone were
added to the high-estrogen environment, there was no measurable impact
on blood glucose uptake but a complete reversal of the muscle glycogen
sparing induced by estrogen alone, which restored total carbohydrate
oxidation to baseline values. The first finding supports the stated
hypothesis and is generally consistent with the majority of the animal
and human literature. The second finding is contrary to the stated
hypothesis and is inconsistent with human studies across phases of the
menstrual cycle, but it is concordant with the bulk of the animal literature.
Control of confounding variables.
There are several limitations to this study that could potentially
confound the interpretation of the data. Systematic changes in energy
balance or carbohydrate intake could influence the results obtained.
Although it would have been desirable, dietary energy intake and
composition were not controlled throughout the study. However, to
minimize the contribution of dietary variation to the observed results,
the final meal before each test was of a standardized energy content
and nutrient composition, and it was consumed by the subjects at the
same time before the infusion began. Similarly, changes in physical
activity patterns over the course of the study could alter the relative
intensity of the submaximal exercise and affect substrate utilization.
The observations that submaximal heart rate and oxygen consumption were
almost identical in all three conditions imply that the exercise
intensity did not vary and that the fitness level of the subjects did
not change between tests. In addition, the use of a crossover design, with the order of E and E + P balanced across subjects, minimizes the
possibility that variations in diet or training across conditions could
explain the results.
In eumenorrheic women, both estrogen and progesterone levels vary
across phases of the menstrual cycle, making it difficult to tease
apart their independent and combined actions. Using a suppression/replacement model in the present study, we were able to
create hormonal environments in which the only differences between
conditions were the estrogen concentration (E relative to B) or the
progesterone concentration (E + P relative to E).
Other hormone supplementation studies in humans.
The tendency toward a lower blood glucose Rd with estrogen
observed in the present study is concordant with results reported in
previous estrogen supplementation studies (12, 41). Ruby et al. (41) and Carter et al. (12) both found
that glucose Rd was lower after several days of estrogen
supplementation in amenorrheic women (41) or men
(12) compared with the baseline condition. The sample size
in the present study was equal to or greater than those prior studies,
but we may still have lacked sufficient statistical power to detect a
significant difference in glucose Rd. In addition, the
reduction in blood glucose might have been accentuated if the exercise
protocol had extended beyond 60 min.
Total carbohydrate oxidation was not different from baseline in those
studies, however. In the present study, we report that carbohydrate
oxidation was reduced by estrogen. In addition, we observed a reduction
in estimated muscle glycogen use with estrogen that was not seen by
Ruby et al. (41) or Carter et al. (12). The
differences between the studies are unlikely to be explained by simple
differences in circulating estrogen concentrations. Blood estrogen
concentrations were raised 2- to 3-fold after supplementation in our
subjects, which is within the range of >2-fold (41) and >10-fold (12) spanned by the two prior studies. Subjects
in the other studies were fasted overnight before exercise testing, whereas in the current study, subjects were given a preevent meal 4 h before exercise, which may have influenced reliance on blood glucose vs. muscle glycogen. The differences between the studies are
likely more quantitative, in terms of the magnitude of the differences
observed, than qualitative discrepancies. There are consistent data
from animal studies that estrogen spares endogenous carbohydrate use in
both liver and muscle (23, 31, 32, 37, 38). Because Ruby
et al. and Carter et al. supplemented subjects with estrogen only,
there are no human data to which our estrogen + progesterone
condition can be compared.
Studies in animal models.
Results from the present study generally parallel results from
experiments in which circulating estradiol concentrations were suppressed and reintroduced in animals. In two separate studies, Kendrick and colleagues (31, 32) showed that muscle
glycogen utilization during exercise was lower after estradiol
administration in male rats. These results were confirmed by Rooney et
al. (38), who also found that resting intramuscular lipid
levels were higher in estradiol-treated rats, suggesting that
carbohydrate conservation was secondary to increased fat availability.
Hatta et al. (23) found that estradiol administration
increased fat oxidation and subsequently reduced glucose oxidation
during exercise. When they gave estrogen and progesterone concurrently,
substrate oxidation was "restored" to control values. Recently,
Campbell and Febbraio (9) used exogenous supplementation
with estrogen and/or progesterone to study their independent and
combined actions on key enzymes regulating the transport [carnitine
palmitoyltransferase I (CPT I)] and oxidation [
-hydroxyacyl
dehydrogenase (
-HAD)] of fatty acids in muscle from control and
ovariectomized rats. In that study, Campbell and Febbraio reported that
physiological levels of estrogen increased the maximal activity of CPT
I and
-HAD by ~15% relative to control. Ovariectomy and
progesterone, alone or in combination with estrogen (physiological
levels), reduced the activity of those enzymes by ~20%. However,
when the physiological dose of progesterone was combined with a
pharmacological dose of estrogen to raise circulating estrogen
concentrations to very high levels, the activities of both enzymes were
similar to the estrogen-only condition. Our data are consistent with
this work by Campbell and Febbraio, suggesting that up- or
downregulation of key enzymes in fat and/or carbohydrate oxidation
pathways may play an important role in mediating the changes observed
at the whole body level.
It appears that progesterone and estrogen act in opposition to each
other with respect to setting the mixture of oxidized substrates during
exercise. Estrogen alone reduces carbohydrate oxidation by decreasing
muscle glycogenolysis and blood glucose uptake (ultimately sparing
liver glycogen). The addition of progesterone reverses the
carbohydrate-sparing effect on muscle glycogenolysis but does not seem
to oppose the reduction in blood glucose uptake (and may even
potentiate it). The results suggest that the modulation of exercise
substrate utilization by ovarian hormones is dependent on the relative
concentrations of estrogen and progesterone. These data may help to
explain the discrepant results obtained from studies in which the
natural variations in estrogen and progesterone across the menstrual
cycle have been used to study their regulatory roles in substrate metabolism.
Studies of menstrual cycle phase.
As previously described, data from well controlled studies of women in
different phases of the menstrual cycle suggest that blood glucose
uptake and/or whole body carbohydrate oxidation is similar in both
phases (4, 6, 25) or lower in the luteal phase (8,
20, 47). Methodological issues, such as whether subjects were
exercising below the lactate threshold and whether they were overnight
fasted or received a preexercise meal, might explain some of the
discordance. However, even when exercise is below the lactate threshold
and at comparable intensity, investigators have reported no cycle phase
differences in either fasted (4, 25) or fed (6,
8) subjects.
When the relative changes in estrogen and progesterone between the
follicular and luteal phases are compared across studies, however, a
potentially useful pattern emerges. In four studies, stable isotope
tracers were used to quantify glucose kinetics, exercise intensity was
below the lactate threshold, and timing of the preexercise meal was
controlled for. When no differences across cycle phase were observed
(6, 25), estrogen concentrations were ~1.5-fold greater
in the luteal phase. In the two studies in which carbohydrate oxidation
was lower in the luteal phase, estrogen levels were 2.3- to 2.8-fold
elevated relative to the follicular phase (8, 47).
Campbell and colleagues (8, 10) suggest that antagonistic
actions by estrogen and progesterone could underlie the differences
noted across menstrual cycle phase, and they argue that progesterone
may counter the actions of estrogen to facilitate glucose uptake during
exercise. Hansen et al. (22) reported that glucose uptake
during muscle contractions was impaired in estrogen-deficient rats, and
recent data from the Febbraio laboratory (see Campbell and Febbraio,
Ref. 11) show that estrogen, but not progesterone,
replacement restored glucose uptake to control conditions in
ovariectomized rats. Thus the specific actions of estrogen on blood
glucose kinetics appear to be complex and influenced by the
presence/absence of other factors. In contrast, progesterone, even when
paired with physiological concentrations of estrogen, reduces blood
glucose uptake relative to the baseline condition (11 and present
study) and, at least under the conditions we used, raised total
carbohydrate oxidation by increasing other carbohydrate (i.e.,
glycogen) use. A more accurate representation of the metabolic events
will require understanding what is happening in specific tissues. In
the luteal phase, the presence of progesterone may constrain blood
glucose uptake by peripheral tissues and thereby spare liver glycogen
use, at the expense of increased muscle glycogen use. The net effect
would be no change in whole body substrate oxidation. But if the
concurrent rise in estrogen is high enough to overcome the progesterone
effect on muscle glycogenolysis, the dual constraints on both blood
glucose uptake and muscle glycogen utilization would clearly have the
net effect of reducing whole body carbohydrate use. The magnitude of
the estrogen rise in the luteal phase may therefore determine the
balance between changes in blood glucose uptake, liver glucose output,
and intramuscular glycogenolysis.
Interactions between carbohydrate and lipid metabolism.
Whether the effects of ovarian hormones on carbohydrate utilization are
primary or secondary to their influence on the availability/oxidation of lipid has been the topic of much discussion (5, 7, 10, 14, 39,
43, 45). Estrogen may reduce glucose/glycogen availability such
that maintaining a given rate of energy expenditure requires increased
fat oxidation (a "carbohydrate constraint pull," Ref.
5). Alternatively, enhanced lipid availability and
oxidation could displace carbohydrate utilization (i.e., a "lipolytic
push," Ref. 5). As we have mentioned, data from some
studies in animal models suggest that estrogen increases fat
availability and oxidation by raising levels of intramuscular
triglyceride and/or by elevating the rate of lipolysis (15, 23,
38). Data in humans are consistent with the idea that estrogen
increases lipolysis and, thereby, fatty acid availability. Conversely,
progesterone is antilipolytic in animals (21, 33), and we
found that blood levels of FFA were lower with the addition of
progesterone relative to estrogen alone.
Whether changes in the availability of fatty acids alter the rate of
fat oxidation and hence spare carbohydrate use has not been
conclusively demonstrated. In the present study, we found a reciprocal
relationship between fatty acid availability and glycogen utilization.
Others have shown this same relationship in other contexts (24,
26, 36, 42); for example, when exogenous fatty acids or
lipolytic agents are infused, the rate of fat oxidation goes up and
muscle glycogen/blood glucose use decreases (36). In
addition, the study by Campbell et al. (8), showing that
estrogen and progesterone cause opposing changes in the key enzymes
related to fatty acid transport and oxidation, lends weight to the
hypothesis that the carbohydrate-sparing effect of estrogen is driven
by a lipolytic push and that the opposing effects of
progesterone are consequent to a reduction in lipid availability.
Although these data suggest that alterations in fatty acid utilization
precede changes in carbohydrate oxidation, several investigators have
shown that changes in carbohydrate utilization (e.g., in response to
raising or lowering exercise intensity) can be primary and can drive
subsequent alterations in fat oxidation (42). The design
of the current study does not allow us to definitively answer this
question; our data could be explained by either the lipolytic push or
the carbohydrate constraint pull models (or a combination of both). In
addition, whether the impact of the sex hormones is on adipose tissue
lipolysis only or also impacts other sources of oxidizable lipids
(e.g., intramyocellular or blood triglycerides) cannot be determined on
the basis of data presented here.
Direct and indirect actions of sex hormones.
The ovarian hormones may act directly on metabolic pathways, and/or
their actions could be mediated via other hormones. In the present
study, the plasma insulin concentration fell during exercise in all
three conditions but tended to be lower (especially at rest and early
in exercise) with E or E + P compared with B. Differences in plasma
insulin could potentially impact blood glucose uptake and total
carbohydrate oxidation, but close examination of the data suggests
otherwise. Despite higher insulin concentrations in the baseline
condition [which, in the absence of any change in glucose
concentrations, indicates some fasting insulin resistance, as noted
recently by Campbell and Febbraio (11) after ovariectomy in rats], total carbohydrate oxidation was the same as with E + P,
when insulin concentrations were lowest. Also, the differences in
glucose Rd are greatest near the end of exercise, when
plasma insulin concentrations are very similar among conditions. The actual plasma insulin concentrations in the last 30 min of exercise are
very low in all three conditions, varying from 23 pM (E + P) to 35 pM
(B), which, in clinical terms, is a difference of only 2 µU/ml. There
is likely to be little physiological relevance to such small
differences at the extremely low end of the plasma insulin range.
Several investigators have theorized that the major influence of the
sex hormones on metabolic regulation is mediated via the catecholamines
epinephrine and/or norepinephrine. Epinephrine concentrations were
lower after estradiol supplementation in amenorrheic women in one study
(41), but not significantly different across menstrual
cycle phases (6, 25) or with estrogen supplementation in
men (12). Studies in both humans (27) and
rats (1, 3) suggest that high concentrations of estrogen
alter tissue responses to favor the lipolytic compared with the
glycogenolytic actions of epinephrine. We observed an increased fatty
acid availability and oxidation in the presence of estrogen alone
relative to baseline, despite no difference in the concentrations of
circulating epinephrine or norepinephrine. The addition of progesterone
also had no impact on catecholamine levels but caused a dramatic shift
toward lower fatty acid availability and oxidation. Our data therefore
support the idea that lipolytic sensitivity to catecholamines is
accentuated by estrogen and imply that the shift is reversed with the
addition of high levels of progesterone.
The potential importance of glucoregulatory hormones not measured in
the current study, e.g., glucagon, growth hormone, and cortisol, cannot
be evaluated. The general consensus, based on available data, suggests
that they play relatively subtle roles in mediating substrate
utilization during submaximal exercise (5, 17, 30, 39,
44).
Conclusions.
Results from this study suggest that estrogen alone reduces total
carbohydrate oxidation during exercise by decreasing both blood glucose
uptake and other carbohydrate (i.e., glycogen) use. The addition of
progesterone may further reduce blood glucose uptake but, conversely,
increases glycogen use such that total carbohydrate utilization is
indistinguishable from an estrogen-absent condition. There is
tantalizing evidence that the changes observed are mediated via
opposing effects of the ovarian hormones on fatty acid availability and
oxidation, but further studies at the tissue-specific level will be
required to address that question. The present data suggest that
alterations in substrate use across the menstrual cycle are dependent
on the relative changes in both estrogen and progesterone. From a
practical perspective, the balance between use of carbohydrate and fat
for energy could shift in response to hormone replacement therapy in
postmenopausal or amenorrheic women. The specific formulation (estrogen
only or combined with progesterone) might alter the pattern of
substrate use and could potentially impact exercise performance and
macronutrient requirements in active individuals.
 |
ACKNOWLEDGEMENTS |
We thank the research subjects for their exceptional commitment of
time and effort. We also acknowledge exceptional assistance from Cindy
Baecher and Kathy Dudzinski. Donna Storton was extremely helpful with
the GC-MS work, and Dr. Juli Jones assisted with the insulin
radioimmunoassays. Catecholamine analyses were done at the University
of Colorado Health Sciences Center core laboratory. Dr. George Wade
provided helpful scientific insight and perspective. Thanks also to
Organon USA for donating Ganirelix.
 |
FOOTNOTES |
Funding for this study was provided by the University of
Massachusetts/Baystate Biomedical Research Program.
Current address for T. M. D'Eon: Jean Mayer USDA Human Nutrition
Research Center on Aging, Energy Metabolism Laboratory, Tufts University, 136 Harrison Avenue, Boston, MA 02110.
Address for reprint requests and other correspondence: B. Braun, Dept. of Exercise Science, 106 Totman Bldg., Univ. of
Massachusetts, Amherst, MA 01003 (E-mail:
bbraun{at}excsci.umass.edu).
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
July 30, 2002;10.1152/ajpendo.00271.2002
Received 20 June 2002; accepted in final form 24 July 2002.
 |
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