Metabolism in Men , Laura Gerson , Todd Hagobian , Daniel Grow

Articles in PresS. J Appl Physiol (June 30, 2005). doi:10.1152/japplphysiol.00565.2005
Title: No Effect of Short-Term Testosterone Manipulation on Exercise Substrate
Metabolism in Men
Running Head: Testosterone, Exercise and Substrate Metabolism
Authors: Barry Braun1, Laura Gerson1, Todd Hagobian1, Daniel Grow2,1, Stuart R.
Chipkin1
1
Department of Exercise Science, University of Massachusetts, Amherst, MA 01003,
Department of Obstetrics and Gynecology, Baystate Medical Center, Springfield, MA
2
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Correspondence to:
Dr. Barry Braun
Dept. of Exercise Science
106 Totman Building
University of Massachusetts
Amherst, MA 01003
Phone: (413) 577-0146
Fax: (413) 545-2906
Email: [email protected]
Braun et al., JAP-00565-2005 R1
Copyright © 2005 by the American Physiological Society.
1
ABSTRACT
Compared with women, men use proportionately more carbohydrate and less fat during
exercise at the same relative intensity. Estrogen and progesterone have potent effects
on substrate use during exercise in women but the role of testosterone (T) in mediating
substrate use is unknown. The purpose of this investigation was to assess how large
variations in the concentration of blood T would impact substrate use during exercise in
men. Nine healthy active men were studied in 3 distinct hormonal conditions:
physiological T (no intervention), low T (pharmacological suppression of endogenous T
carbohydrate oxidation (CHOox), blood glucose rate of disappearance (Rd) and
estimated muscle glycogen use (EMGU) were assessed using stable isotope dilution
and indirect calorimetry at rest and while bicycling at ~60% of VO2peak for 90 minutes.
Relative to the physiological condition (T = 5.5±0.5 ng/ml), total plasma T was
considerably suppressed in low T (0.8±0.1) and elevated in high T (10.9±1.1). Despite
the large changes in plasma T, CHOox, glucose Rd, and EMGU were very similar across
the 3 conditions. There were also no differences in plasma concentrations of glucose,
insulin, lactate or free fatty acids. Plasma estradiol (E) concentrations were elevated in
high T but correlations between substrate use and plasma concentrations of T, E or the
T/E ratio were very weak (r2<.20). In conclusion, unlike the effect of acute elevation in
estradiol to constrain carbohydrate use in women, acute changes in circulating
testosterone concentrations do not appear to alter substrate use during exercise in men.
Key words: androgen, carbohydrate oxidation, fat oxidation, glucose uptake, stable
isotope, glycogen
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with a GnRH antagonist) and high T (supplementation with transdermal T). Total
INTRODUCTION
The majority of well-controlled human studies (adequate sample size, men and
women matched for appropriate characteristics, exercise intensity below the lactate
threshold) show that men oxidize more carbohydrate and less fat than women working
at the same relative exercise intensity (9,15,18,19,32). The mechanisms underlying this
male-female differences is inevitably complex (gender-related patterns of body
composition, muscle fiber type, blood flow, etc. are likely to play some role) but the
prime suspect has been the obvious difference in circulating sex hormones
of a typically “female” sex hormone environment (i.e. characterized by high
concentrations of estrogen and sometimes progesterone) at least partly explains the
results (7-9,11,13,14,23,25,33). Acute changes in circulating concentrations of the
ovarian hormones have potent effects on substrate utilization in response to metabolic
stress (7-9,11,13,14,23,25,33). Every review article written has suggested that, in
similar fashion, acute changes in circulating testosterone could mediate substrate use
(3,6,8,10,20,36,39). There are few data to support or refute that proposition however.
The implication, from the aforementioned studies of human sex differences, is
that testosterone opposes the effect of estrogen and contributes to the “male” pattern of
substrate use by reducing reliance on lipid and increasing use of carbohydrate. Indirect
evidence (e.g. lipoprotein lipase activity) from human studies however, suggests that fat
utilization is actually increased with testosterone supplementation (31,37). In rodents,
testosterone treatment has been reported to increase lipid use (42), reduce exercise
glycogen use (40), and reverse the elevated rates of glycogenolysis induced by
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(3,6,8,10,20,36,39). It is clear from strong animal and human studies that the presence
castration (30). Lipolysis in isolated adipocytes was not increased when
hypophysectomized rats were given testosterone however (43) and testosterone
treatment actually lowered the rate of lipolysis in human pre-adipocytes (12) and in
brown adipocytes from rats (28). These contradictory data are insufficient to draw firm
conclusions about the role of circulating testosterone in regulating human substrate use.
To facilitate the study of metabolic regulation by circulating sex hormones in
humans, we developed a “suppression/replacement” approach to create controlled,
reproducible sex hormone environments in healthy individuals (11). Recently, we used
relevant hormones to test substrate kinetics and oxidation in the presence of very low,
normal physiological, and high physiological concentrations of estrogen and/or
progesterone in young women (11). We used the same model system in the current
study to create 3 distinct hormonal environments in young men. The purpose of the
study was to investigate how low, normal and high concentrations of blood testosterone
affected the balance between blood glucose uptake, estimated glycogen utilization and
lipid oxidation during prolonged submaximal exercise.
Methods
Subjects. Subjects for this study were 9 healthy, physically active men who participated
in regular aerobic activity at least 3 times/wk (Table 1). All of the subjects were in
excellent overall health, had no history of cardiovascular, metabolic, or hormonal
disorders, and used no medications other than occasional over-the-counter aspirin or
ibuprofen. After study procedures were explained verbally, subjects signed a written
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a gonadotrophin releasing hormone antagonist and exogenous administration of
informed consent document approved by Institutional Review Boards at both the
University of Massachusetts and Baystate Medical Center.
Pretesting procedures. Body composition was determined by dual-energy x-ray
absorptiometry (Lunar, MA). Maximal oxygen consumption ( O2 max) was measured on
an electronically braked cycle ergometer (SensorMedics 800S, Yorba Linda, CA) with
an incremental ramp protocol starting at 100 W and increasing by 25 W every 2 min until
volitional exhaustion. Oxygen consumption and carbon dioxide production were
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measured by indirect calorimetry with a TrueMax 2400 metabolic measurement system
(Parvo Medics, Sandy, UT).
Hormonal control. Subjects were tested in three different hormonal conditions:
physiological, low testosterone and high testosterone. All subjects were tested in the
physiological condition first, and then in the 2 altered hormonal conditions: low
testosterone (LT) and high testosterone (HT). The order of the low and high
testosterone condition testing was balanced across subjects.
In order to reduce circulating testosterone to concentrations characteristic of
hypogonadal men (<1 ng/mL) subcutaneous injections of 3 mg Cetrotide (Serrono,
Randolph, MA) were given to suppress endogenous production of gonadotrophinreleasing hormone, or GnRH. The injections were given 70-74, 46-50 and 22-26 hours
before LT testing. In order to raise circulating testosterone into the upper end of the
physiological range for young men (> 8 ng/ml) subjects wore 2 Androderm (5 mg
transdermal testosterone) patches (WatsonPharma, Corona, CA) per day for 3 days
(total of 6 patches) before HT testing. No pharmacological intervention was used before
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physiological testosterone (PT) testing. Individual subjects were always tested at the
same time of day in all 3 conditions. At least 7 days elapsed between testing sessions.
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 entire study, all subjects consumed the same preexercise meal 12 hours (for subjects tested in the morning for all 3 conditions) or 3
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hours (for subjects tested in the afternoon for all 3 conditions) before each test. The
meal comprised 35% of estimated daily energy requirements and was composed of
55% carbohydrate, 15% protein, and 30% fat. Resting metabolic rate was estimated
based on standard equations and this value was multiplied by an activity factor of 1.55
or 1.73 (depending on activity level estimated from training diaries) to calculate daily
energy requirements (26). Subjects were instructed to fast after this meal until testing.
Testing procedures. Subjects reported to the laboratory and a 21 gauge 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 200mg [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
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and 5-min collections of expired oxygen and carbon dioxide were taken at rest and 75
and 90 minutes after the start of the infusion.
Immediately after the last resting measurement, the subject began submaximal cycling
exercise. To maintain a steady isotopic enrichment of blood glucose during exercise,
the [6,6-2H]glucose infusion rate was increased to 6.0 mg/min (11). During the first 15
minutes of exercise in the PT condition, the intensity was adjusted by manipulating the
pedaling resistance until oxygen consumption reached a steady state at ~60% of the
the other 2 conditions. Blood and breath samples (5 minutes) were collected at 15, 30,
45, 60, 75, and 90 minutes of exercise (Fig. 1).
Biochemical assays. Samples of venous blood for analysis of glucose, lactate, and
glucose isotopic enrichment were collected in heparinized syringes and then transferred
to vacutainers containing sodium fluoride (to inhibit glycolysis). Samples for analysis of
testosterone, estradiol, free fatty acids, and cortisol were collected in nonheparinized
syringes and transferred to vacutainers containing a clotting factor. 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).
Testosterone and estradiol levels were determined using radioimmunoassays
(Diagnostic Systems Laboratories, Webster, TX). Free fatty acid concentrations were
measured using a standard colorimetric assay (Wako Chemicals, Richmond, VA).
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previously measured VO2 max. The protocol was repeated in exactly the same way for
For determination of [6.6-2H] glucose enrichment, 0.7 ml of plasma was
deproteinized by adding 1.2 ml of .3 N Zn(SO)4 and 1.2 ml of .3 N Ba (OH)3. Tubes
were vortexed, incubated in an ice bath for 20 minutes, and centrifuged at 4°C at 2500 x
g for 20 minutes. After the supernatant was extracted, water was removed from the
supernatant via freeze-drying (-50o C, 5mTorr). The dried samples were reconstituted
in 100 ul of a 2:1 acetic anhydride and pyridine mixture, capped, heated in a water bath
at 60°C for 60 minutes and transferred to clean 13 x 100-mm borosilicate tubes.
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Double-distilled water (1.5 ml) and 0.4 ml of dichloro-methane was added in that order
and the tubes were centrifuged at 2000 rpm for 10 minutes. The dichloro-methane
phase was transferred to a 1 ml gas chromatography vial and evaporated under
nitrogen. Tubes were capped and 25 ul ethyl acetate was added using a gas tight
syringe. A 1ul sample of the penta-acetate derivative was injected, separated on a gas
chromatograph, and spectra were recorded on a mass spectrometer (Hewlett-Packard
6890, Palo Alto, CA). Selected ion monitoring was used to compare the abundance of
the unlabelled fragment with that of the enriched isotopomer (Chemstation Software).
After correcting for background enrichment, the abundance of the dideuterated
isotopomer (m/z = 202) was expressed as percentage of total glucose species (m/z =
200+201+202).
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 (41)
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Glucose rate of appearance and disappearance:
Glucose Ra (mg/min) = F - V[(C1 + C2) / 2][(IE2 - IE1) / (t2 - t1)] / [(IE2 + IE1) / 2]
Glucose Rd (mg/min) = Ra - V[(C2 - C1) / (t2 - t1)].
F is the isotope infusion rate, IE1 and IE2 are enrichments of plasma glucose with
isotope label at time t1 and t2, C1 and C2 are plasma glucose concentrations, V is the
estimated volume of distribution for glucose (180 ml/kg).
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Percent energy from carbohydrate (%CHO) = [(RER-0.71)/0.29] × 100; where RER is
the respiratory exchange ratio.
Carbohydrate oxidation rate (mg/min) = [(%CHO/100) × (VO2 in l/min) × (5.05 kcal/l)] /
4.0 g/kcal
Estimate of muscle glycogen utilization (EMGU) was determined by:
(total carbohydrate oxidation rate) – (blood glucose Rd)
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% (15,24) 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: Statistical analysis was accomplished using SAS Inc. software
(Cary, NC). A mixed model two-way analysis of variance (ANOVA) was used to
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determine the treatment x time interaction using a compound symmetric covariate
structure for all variables. By convention, significant differences were defined as
P<0.05. When appropriate, post hoc tests of significance were performed with a Tukey
HSD test. In general, data are presented as the group mean, 95% confidence interval
and exact p-value except where noted. In Tables, data from the high and low
testosterone conditions are presented as the difference from the physiological condition,
the 95% confidence interval of the difference, and the exact P value. Pearson productmoment correlations were used to assess the relationship between circulating
glucose kinetics and estimated muscle glycogen use.
Results
Hormonal environment. The treatments resulted in three very distinct testosterone
environments (Figure 2a). Compared with the physiological condition (PT), the serum
concentration of testosterone before exercise was considerably elevated (2-fold greater)
in the high testosterone condition (HT) and markedly suppressed (reduced by 6-fold) in
the low testosterone condition (LT). During exercise, circulating testosterone
concentrations did not change to any appreciable extent relative to pre-exercise levels.
The direction of changes in plasma concentrations of 17-beta-estradiol generally
tracked the plasma testosterone data (Figure 2b). Relative to PT, there was small
decrease in plasma estradiol in LT and a significant increase (p=0.023) in HT.
Exercise oxygen consumption and heart rate. As expected, given that the absolute work
intensity was matched among conditions, oxygen consumption expressed in absolute
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testosterone, estrogen or the T/E ratio and carbohydrate oxidation, fat oxidation,
terms or as a percentage of VO2 max, and heart rate were very similar in PT, HT, and
LT (Table 2). These results suggest that the relative exercise intensity did not vary
across the three conditions. In addition, subjects’ rating of perceived exertion was
almost identical across conditions (data not shown).
Blood concentrations of substrates. Blood glucose (Fig. 3a) concentrations declined
slightly (approximately 10%) over time during exercise. Blood levels of lactate rose from
resting values at the onset of exercise and plateaued around 1.5 mM for the final hour
of exercise (Fig. 3b). Free fatty acid concentrations rose progressively throughout
circulating compounds either at rest or during exercise.
Gas exchange. At rest, there were no significant differences in resting RER (PT=
0.84±0.01, LT= 0.83±0.01, HT=0.81±0.02, p= 0.231). Respiratory exchange ratios rose
during exercise and remained fairly steady around 0.90 during the last 45 minutes of
exercise (Figure 4). A modest depression in the HT condition ( 0.02 units) was not
significantly different from either of the other 2 conditions either at rest or during
exercise. Based on respiratory exchange ratios, rates of carbohydrate (PT= 2.64±0.48,
LT= 2.36±0.29, HT= 2.18±0.35 mg/kg/min, p= 0.144) and lipid oxidation (PT=
1.44±0.09, LT= 1.35±0.10, HT= 1.54±0.13 mg/kg/min, p= 0.321) at rest were calculated
and were not different among conditions. Steady-state exercise values (averaged over
the last 45 minutes of exercise) are shown in Table 3. Small mean differences in
carbohydrate oxidation (9% lower in HT compared with PT ) between conditions were
not statistically significant. Conversely, the mean rate of lipid oxidation was slightly
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exercise (Fig. 3c). There were no significant differences among conditions in any of the
elevated in the HT condition (16% higher than PT) but there was no statistically
significant difference.
Glucose kinetics. Isotopic enrichment of plasma glucose was approximately 1.4% at
rest and was maintained at approximately the same value (1.3-1.5%) during the last 45
minutes of exercise in all 3 conditions (data not shown). Resting glucose rate of
appearance (Ra) was not different among conditions (PT= 4.09±0.43, LT= 3.78±0.46,
HT= 4.37±0.51 mg/kg/min, p= 0.303). During steady-state exercise, glucose Ra for
PT, LT and HT were very similar, with no significant differences among conditions
rates of disappearance (Rd) were marginally higher than the Ra but followed the same
pattern across conditions (Table 3). Expressed as the difference between total
carbohydrate oxidation and glucose Rd, estimated muscle glycogen utilization (EMGU)
was almost identical in the HT vs. LT condition. The EMGU in both conditions was 910% lower than PT but this minor difference was not statistically significant.
Contribution to total energy expenditure. The proportion of total energy expenditure
attributable to lipid oxidation, blood glucose use and estimated muscle glycogen
utilization are shown in Figure 5. There are slight variations in the exact mixture of
substrates oxidized across conditions but overall, the patterns are markedly similar.
Relationship between hormone concentrations and substrate use: Correlations between
concentrations of circulating testosterone and carbohydrate use, fat use, glucose
kinetics or estimated glycogen use were all very weak (r2 < .20) and non-significant. The
same pattern of correlations was observed between substrate use and circulating
concentrations of estrogen or the testosterone/estrogen ratio.
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(Table 3). Since blood glucose concentrations declined slightly during exercise, glucose
Discussion
The main finding in this study was that suppression and replacement of testosterone to
create distinctly different concentrations of circulating testosterone had very little impact
on the balance between blood glucose, glycogen and fat utilization during exercise in
young men. Subtle differences in the high testosterone condition hinted at a nudge
toward greater use of fat and sparing of carbohydrate both at rest and during exercise
but the changes were not statistically significant and unlikely to be physiologically
Due to the conflicting results obtained from short-term studies, our data are consistent
with some, but certainly not all, published results. In two studies in humans, lipoprotein
lipase activity increased and there was a reduction in visceral and subcutaneous fat in
response to testosterone supplementation; suggesting a rise in fat utilization (31,37). In
vitro studies by Bjorntorp and associates showed that testosterone treatment increased
lipolysis in rats (42). Also in rats, testosterone supplementation reduced exercise
glycogen use and increased resting muscle glycogen content (40) and reversed the
elevated rates of glycogenolysis induced by castration (30). In contrast, catecholaminestimulated lipolysis was not increased in isolated adipocytes when hypophysectomized
rats were given testosterone (43). Testosterone treatment actually lowered the rate of
catecholamine-stimulated lipolysis (but only in subcutaneous fat cells) in human preadipocytes (12) and reduced lipolysis and increased anti-lipolytic activity in brown
adipocytes from rats (28). To add complexity to the story, Giudicelli et al (16) reported
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relevant.
that testosterone may upregulate both lipolytic and antilipolytic responses in rat and
hamster visceral fat. Rebuffe-Scrive et al. reported that testosterone increased basal,
but not catecholamine-stimulated, lipolysis in human visceral adipose tissue whereas in
subcutaneous fat, catecholamine-stimulated lipolysis actually went down in the
presence of testosterone (31). It is unclear why the responses to short-term changes in
circulating testosterone concentrations are so variable. It is probable that the difficulty of
controlling all of the potential confounding variables (e.g. diet, physical activity, circadian
rhythm, other stressors) obscures any impact of acutely changing the testosterone
circulating estrogen and progesterone concentrations however, we did see marked
metabolic effects in women (11). Taken together, the implication from these two studies
is that the metabolic effects of short-term variations in circulating testosterone, if any,
are subtle.
The current findings are not consistent with data from long-term human studies, which
generally show that chronic (weeks to months) exposure to testosterone increases lean
mass and reduces fat mass in men (1,38). It is very likely that this discrepancy is
related to differences in the length of exposure. Over the long-term (months), subtle
changes in nutrient storage and/or utilization could alter adipocyte mass while being
undetectable when viewed at a “snapshot” point in time. Chronic exposures to elevated
levels of testosterone can also alter receptor sensitivity e.g. the upregulation of receptor
sensitivity that occurs during puberty (17). As previously mentioned, short-term
variations in circulating testosterone have been reported to alter specific metabolic
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environment. In a previous study, using a similar method to effect acute changes in
pathways; with increased rates of visceral adipocyte lipolysis as a common, although
not universal, result. Although we observed clear and distinct variations in circulating
testosterone concentrations on the day of the metabolic measurements, we cannot be
certain that the patterns observed were consistently present during all 3 days of the
treatments. Perhaps with longer exposures the individual would adapt via transcription
of new protein and/or upregulation of receptors so that the shift toward greater fat use
would become more evident at the level of whole-body metabolism. In addition, if longer
exposure to testosterone increased lean body mass, a rise in total energy expenditure
lipid utilization.
It is conceivable that, although our “suppression/replacement” model clearly altered
circulating concentrations of total testosterone, the impact on free testosterone (not
bound to sex hormone binding globulin) was less affected. In general however,
supplementation with exogenous testosterone or compounds that change endogenous
testosterone production have roughly equivalent effects on both total and free
testosterone (27,29,38). It has been reported that an acute bout of exercise differentially
affects circulating concentrations of free and bound testosterone (2) but we expect that,
if this was indeed the case, all 3 of our conditions would be equally impacted and the
overall pattern of results would not change.
A potential confounding variable we suspected could interfere with interpretation of our
results is the in vivo conversion of testosterone to estrogen via the enzyme aromatase.
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and mitochondrial mass could increase the capacity for fat oxidation and elevate net
There is evidence that supplementing with exogenous testosterone or its precursors,
DHEA and androstenedione, elevate blood concentrations of testosterone and increase
traffic through this pathway, resulting in elevated blood concentrations of estradiol or
estrone (5). Elevated concentrations of estradiol can increase lipolysis, upregulate
enzymes related to beta-oxidation of fatty acids, and downregulate the use of
carbohydrate for energy (9,11,14,25,33). In the current study, we did see the estradiol
concentration in the blood rise significantly in the high testosterone condition. There was
no obvious impact on exercise substrate use however. It is possible that the effect of
based on several pieces of evidence, this scenario seems unlikely. First, the estradiol
concentrations noted in the HT condition are well below values reported to cause
changes to exercise substrate metabolism (4,21). Second, there were no relationships
between circulating testosterone, estrogen or the testosterone/estrogen ratio and any
aspect of substrate use. Third, physiologically relevant changes in the blood
concentration of estradiol would be expected to increase plasma free fatty acid levels
and this did not occur in our study. Finally, if testosterone at physiological
concentrations does have a major regulatory impact on substrate use we would expect
a difference between the physiological and suppressed conditions, which we did not
observe.
To reduce the probability of missing a true effect of varying the testosterone
environment, we tried to minimize sources of variability by exerting experimental control
over the most likely confounding variables. Subjects included in the study were all lean
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estradiol to constrain carbohydrate use negated an opposing effect of testosterone but,
men with high levels of habitual physical activity and considerable cardiorespiratory
fitness. The exercise intensity was chosen to ensure that all subjects would be working
below the lactate threshold so that the contribution from non-oxidative metabolism was
minimal. The long-duration exercise task was chosen because prior studies have shown
that gender differences may not emerge until exercise duration exceeds 60 minutes
(19,35). For each subject, testing took place at the same time of day for all 3 trials with
a minimum of 7 days between trials. Subjects were provided all of the food consumed
as a pre-exercise meal 12 (morning testing) or 3 (afternoon testing) hours before each
conceivable that protein oxidation was altered by testosterone suppression or
replacement. Since protein oxidation was not measured or estimated in the current
study, a change in the contribution of protein oxidation to total energy expenditure could
affect the actual contribution of carbohydrate and lipid in ways that would be missed by
indirect calorimetry. We estimate that, given the observed steady-state RER of
approximately 0.89, even a 50% increase or reduction in the contribution of protein
oxidation to total energy expenditure would alter the “true” RER by less than 0.01 units.
Most importantly, we used a “suppression/replacement” model to create 3 distinct and
reproducible testosterone environments rather than relying on natural variations in
circulating testosterone or testosterone precursors. The lack of change in group means
across conditions was not a result of averaging positive and negative responses; the
failure of testosterone suppression or replacement to alter substrate kinetics or
oxidation was consistent across subjects.
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trial with a standardized energy content and macronutrient composition. It is
In response to acute exercise or training, plasma concentrations of growth hormone
tend to parallel the pattern of change in testosterone (22). Because growth hormone
increases the rate of lipolysis and shifts substrate use toward greater fat utilization,
effects of testosterone suppression or supplementation on substrate use may have
been mediated by a concomitant change in circulating growth hormone concentrations.
It would have been illuminating to assess whether plasma growth hormone
concentrations tracked the obvious differences in circulating testosterone in response to
suppression or replacement but plasma growth hormone concentrations were not
circulating testosterone environment, the role of growth hormone as a mediator of those
responses is moot.
Based on the results from the current study, it is unlikely that circulating concentrations
of testosterone that characterize the typically “male” sex hormone environment explain
the greater reliance on carbohydrate in men compared with women working at the same
relative exercise intensity. Although acute changes in circulating ovarian hormones,
particularly estradiol, do appear to have a physiologically relevant impact on metabolic
regulation during exercise in women, changes of similar magnitude in circulating
testosterone appear to have far less potent effects in men. Longer-term suppression or
elevation of testosterone may induce genomic or proteomic changes that are
manifested in altered regulation of metabolic pathways.
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measured. Since none of our outcome measures was impacted by altering the
From the perspective of clinicians prescribing testosterone to patients and the men and
women using it, results from the current study suggest that blood glucose use and the
balance between carbohydrate and fat oxidation are not impacted by acute changes in
circulating testosterone concentrations. In contrast, acute changes in circulating
estrogen and progesterone (for example, in response to oral contraceptives or hormone
replacement) do appear to cause significant changes in the metabolic regulation of
carbohydrate and fat metabolism. Whether there are metabolic effects of longer-term
manipulation of circulating testosterone concentrations that could impact fat oxidation,
ACKNOWLEDGEMENTS
The authors would like to thank the research subjects for their exceptional commitment
of time and effort. We also acknowledge helpful assistance from Miki Takeda, RN,
Christine Pikul, RN, Elizabeth Mitchell, BS, Carrie Sharoff, M.S. and Steve Black, Ph.D.
Thank you to Serrono, USA for donating the GnRH antagonist Cetrotide. Funding for
this study was provided by a grant from the Baystate/UMASS Biomedical Research
Program.
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fat storage or blood glucose uptake remains to be seen.
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Braun et al., JAP-00565-2005 R1
23
Figure 1. Schedule to show blood and breath sample collections at 15-minute intervals
during exercise.
Figure 2a. Plasma testosterone concentrations before exercise and at 45 and 90
minutes of exercise in each condition. Dotted lines show the lower and upper bounds of
the typical physiological range for men (ref. 16).
Figure 2b. Pre-exercise plasma 17-beta estradiol concentrations in each condition.
Figure 3a, b, c. Plasma glucose (3a), lactate (3b) and non-esterified fatty acid (3c)
concentrations at rest and during exercise in each condition.
Figure 4. Respiratory Exchange Ratio (RER) at rest and during exercise in each
condition.
Braun et al., JAP-00565-2005 R1
24
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Figure 5. Contribution of plasma glucose, estimated muscle glycogen, and lipid to total
energy expenditure during steady-state exercise (45-90 minutes).
Table 1. Subject physical characteristics and demographic data.
Values are mean ± SD (range) for 9 subjects. VO2 max =
maximal oxygen consumption
Age (yr)
Body weight (kg)
Height (cm)
Body fat (%)
Lean mass (kg)
74.2±6.0 (67-83)
176.2±6.9 (168-188)
16.3±3.6 (11-22)
59.4±4.3 (55-66)
48.9±6.8 (38-59)
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VO2 max (ml kg-1 min-1)
24.8±6.8 (18-39)
Table 2. Work intensity, heart rate and oxygen consumption. Values are mean ± SE
during the steady-state exercise period (45 to 90 minutes).
Parameter
PT
LT
HT
Work (watts)
147±9
147±9
147±9
Heart rate (beats/min)
155±4
151±5
156±4
VO2 (ml kg-1 min-1)
30.5±1.2
30.6±1.1
30.4±1
% VO2 max
63.2±1.3
63.7±2.6
63.3±2.3
Braun et al., JAP-00565-2005 R1
25
Table 3. Summary of metabolic data during steady-state exercise. Values are mean ±
SE. Statistical comparisons for Low and High T conditions are expressed relative to
physiological T. T= testosterone; = group mean difference; CI= confidence interval.
High T
Testosterone, ng/ml
mean
from PT
95% CI
Exact P value
RER
mean
from PT
95% CI
Exact P value
Total CHO use, mg/kg/min
mean
from PT
95% CI
Exact P value
Estimated muscle glycogen
use, mg/kg/min
mean
from PT
95% CI
Exact P value
Blood glucose rate of
appearance, mg/kg/min
mean
from PT
95% CI
Exact P value
Blood glucose rate of
disappearance, mg/kg/min
mean
from PT
95% CI
Exact P value
Total lipid oxidation,
mg/kg/min
mean
from PT
95% CI
Exact P value
0.7 ± 0.04
-4.1
(-6.2,-1.9)
0.0001
9.7 ± 1.0
4.9
(2.8,7.2)
<0.0001
0.914 ± 0.016
0.004
(-0.039,0.046)
0.83
0.894 ± 0.007
0.016
(-0.059,0.027)
0.34
27.1 ± 1.7
25.5 ± 2.0
-1.6
(-8.3,5.0)
0.53
24.9 ± 1.4
-2.2
(-8.8,4.5)
0.40
18.8±1.6
17.2±1.7
-1.6
(-7.0,3.8)
0.46
17.1±1.1
-1.7
(-7.2,3.7)
0.41
8.3±0.7
8.2±0.7
-0.1
(-2.5,2.4)
0.96
7.8±0.7
-0.5
(-2.8,2.0)
0.65
8.4±0.7
8.4±0.7
0.0
(-2.5,2.4)
0.96
8.0±0.7
-0.4
(-2.8,2.0)
0.64
5.1±0.7
5.4±0.6
0.3
(-1.7,2.3)
0.69
5.9±0.4
0.8
(-1.2,2.8)
0.32
Braun et al., JAP-00565-2005 R1
4.8 ± 0.29
0.910 ± 0.012
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Condition
Low T
Physiological T
26
Figure 1
EXERCISE
6.0 mg/min
REST
2.5 mg/min
Rest
0
Rest
75
Rest
90
Ex
15
Ex
30
Ex
45
Ex
60
Ex
75
Ex
90
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Figure 2a
Testosterone (ng/ml)
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
Rest
Ex 45
Physiological
Braun et al., JAP-00565-2005 R1
Low
Ex 90
High
27
Figure 2b
75
50
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Estradiol (pg/ml)
100
25
0
Physiological
Low
High
Figure 3a
Physiological
Low
6.0
Plasma Glucose (mM)
High
5.5
5.0
4.5
4.0
Pre-Ex
Ex 45
Braun et al., JAP-00565-2005 R1
Ex 60
Ex 75
Ex 90
28
3.0
Physiological
Low
2.5
High
2.0
1.5
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Plasma Lactate (mM)
Figure 3b
1.0
0.5
0.0
Pre-Ex
Ex 45
Ex 60
Ex 75
Ex 90
Figure 3c
Physiological
Low
1.4
Plasma FFA (mM)
1.2
High
1.0
0.8
0.6
0.4
0.2
0.0
Pre-Ex
Ex 45
Braun et al., JAP-00565-2005 R1
Ex 60
Ex 75
Ex 90
29
Figure 4
1.10
Physiological
Low
1.05
High
1.00
RER
0.95
0.90
0.85
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0.80
0.75
0.70
Pre-Ex
Ex 15
Ex 30
Ex 45
Ex 60
Ex 75
Ex 90
Estimated Glycogen
Glucose
Lipid
100%
80%
58.4
53.4
53.1
25.8
25.5
24.2
15.8
16.8
18.3
60%
40%
20%
0%
Physiological
Braun et al., JAP-00565-2005 R1
Low
High
30