Fuel metabolism in men and women during
and after long-duration exercise
TRACY J. HORTON, MICHAEL J. PAGLIASSOTTI, KAREN HOBBS, AND JAMES O. HILL
Center for Human Nutrition, Department of Pediatrics,
Universityof Colorado Health Sciences Center, Denver, Colorado 80262
Horton, Tracy J., Michael J. Pagliassotti, Karen
Hobbs, and James O. Hill. Fuel metabolism in men and
women during and after long-duration exercise. J. Appl.
Physiol. 85(5): 1823–1832, 1998.—This study aimed to determine gender-based differences in fuel metabolism in response
to long-duration exercise. Fuel oxidation and the metabolic
response to exercise were compared in men (n 5 14) and
women (n 5 13) during 2 h (40% of maximal O2 uptake) of
cycling and 2 h of postexercise recovery. In addition, subjects
completed a separate control day on which no exercise was
performed. Fuel oxidation was measured using indirect calorimetry, and blood samples were drawn for the determination
of circulating substrate and hormone levels. During exercise,
women derived proportionally more of the total energy expended from fat oxidation (50.9 6 1.8 and 43.7 6 2.1% for
women and men, respectively, P , 0.02), whereas men
derived proportionally more energy from carbohydrate oxidation (53.1 6 2.1 and 45.7 6 1.8% for men and women,
respectively, P , 0.01). These gender-based differences were
not observed before exercise, after exercise, or on the control
day. Epinephrine (P , 0.007) and norepinephrine (P ,
0.0009) levels were significantly greater during exercise in
men than in women (peak epinephrine concentrations: 208 6
36 and 121 6 15 pg/ml in men and women, respectively; peak
norepinephrine concentrations: 924 6 125 and 659 6 68
pg/ml in men and women, respectively). As circulating glycerol levels were not different between the two groups, this
suggests that women may be more sensitive to the lipolytic
action of the catecholamines. In conclusion, these data support the view that different priorities are placed on lipid and
carbohydrate oxidation during exercise in men and women
and that these gender-based differences extend to the catecholamine response to exercise.
gender; exercise fuel oxidation; catecholamines; training
EXERCISE FUEL METABOLISM has been well characterized
in men but not in women. This is relevant, inasmuch as
certain data suggest that men and women may place
different priorities on lipid and carbohydrate utilization during exercise. For example, with the same
relative intensity of submaximal exercise, women have
been reported to oxidize relatively more lipid and less
carbohydrate compared with men (1, 25, 31). Concurrently, a lesser decline in muscle glycogen was observed
in response to endurance exercise in women (31, 32).
Whether these gender-based differences in whole body
fuel oxidation are related to differences in circulating
substrate utilization is unclear (1, 9, 31). With respect
to the hormonal response to exercise, there is some
suggestion that this may differ between men and
women (31) and, in particular, that there is a smaller
increase in epinephrine during exercise in women (23,
31). However, it is still not clear how gender-based dif-
http://www.jap.org
ferences in the pattern of fuel oxidation during exercise
are mediated metabolically.
One problem with the previous studies of genderbased differences in exercise metabolism is the lack of
adequate control measurements. To establish an independent effect of exercise on fuel oxidation, it is important to establish that, under control conditions, i.e.,
resting and fasting for a duration similar to the exercise period, women do not oxidize significantly more
lipid. In addition, fuel oxidation measurements continued into the postexercise period would establish whether
women compensate for their greater exercise fat oxidation by utilizing less lipid than men during exercise
recovery. Such measurements of fuel oxidation need to
be made to appropriately interpret and assess the
implications of the experimental data.
Studies of gender-based differences during exercise
metabolism also require careful attention to issues of
subject selection and study design. This may be the
reason for the lack of gender-based differences in
exercise metabolism observed in certain investigations
(5, 26). For example, it is important to ascertain the
menstrual status of female subjects, as the female
gonadotrophic hormones are known to influence fuel
metabolism (12, 16, 21, 27). Furthermore, when an
attempt is made to match gender groups, it may not be
practical or logical to do so on the basis of body
composition or maximal O2 uptake (V̇O2 max), as these
two parameters naturally differ between men and
women. Thus it may be more appropriate to select and
match subjects on the basis of training status and
training quantity, as this is known to be a major
determinant of exercise fuel utilization (3, 8, 13).
Matching in this way, therefore, has implications for
the exercise conditions selected. Choosing the same
relative work intensity for the exercise test would
represent a similar metabolic challenge for men and
women rather than exercising individuals at the same
absolute workload. Finally, it is important to control
prestudy dietary intake and exercise performance to
ensure that subjects are in a similar state of energy
balance and fuel repletion before testing. Only by
consideration of these factors in the design of studies
may it be possible to clearly establish any gender-based
differences in exercise metabolism.
The aim of the present study, therefore, was to
compare fuel oxidation during and after exercise and on
a control day in women and men. This was done taking
into consideration the above factors in the study design.
Simultaneously, the circulating substrate and hormone
environment was characterized to begin to elucidate
possible mechanisms involved in any observed genderbased differences in exercise fuel oxidation. Finally, we
8750-7587/98 $5.00 Copyright r 1998 the American Physiological Society
1823
1824
EXERCISE FUEL METABOLISM IN MEN AND WOMEN
studied trained and untrained subjects to determine
whether there was any interaction between gender and
training status with respect to exercise fuel oxidation.
METHODS
Subjects
Trained and untrained men (n 5 14) and women (n 5 14)
were recruited for the study (Table 1). Trained subjects were
mainly competitive cyclists, with one triathlete in each male
and female group. Subjects were matched on training status
(volume and intensity of training as determined from selfreported activity records). Untrained subjects participated in
,90 min of aerobic exercise per week. All subjects were of
normal weight, were nondiabetic and nonsmokers, and were not
suffering from any medical condition or taking any chronic
medication known to interfere with intermediary metabolism. All women were eumenorrheic and not using steroidal
contraceptives. The study protocol was approved by the
University of Colorado Committee Institutional Review Board
for the Protection of Human Subjects. All subjects read and
signed a subject consent form before admission into the study.
Preliminary Assessments
V̇O2 max. V̇O2 max was determined using a graded exercise test
on a cycle ergometer (Monark, Varberg, Sweden). Trained
subjects pedaled at a rate of 90 rpm and untrained subjects at
70 rpm while the resistance was gradually increased (commencing at 1 kp and increasing by 0.5 kp every 1–2 min) until
volitional exhaustion. Heart rate was continuously monitored
via a 12-lead electrocardiogram, with blood pressure and
perceived exertion measured in the final minute of each work
level. To ensure that maximum effort was achieved, two of the
following criteria had to be fulfilled: whole body respiratory
exchange ratio $1.1, maximum heart rate within 5% of the
predicted maximum, and an increase in O2 consumption in
response to the final workload of ,2.0 ml·kg body wt21 ·min21.
Body composition. Body composition was determined by
measuring body density using underwater weighing, as previously described (14). Percent body fat was estimated from
body density using the revised equation of Brozek et al. (4).
Resting metabolic rate. Resting metabolic rate (RMR) was
measured using indirect calorimetry via a metabolic cart
system (model 2900, Sensormedics, Yorba Linda, CA). Sub-
Table 1. Subject characteristics
Trained Subjects
Age, yr
Height, cm
Weight, kg
Body fat, %
V̇O2 max
ml · kg21 ·
min21
ml/min
Fat-free mass,
kg
Fat mass, kg
Men
(n 5 8)
Women
(n 5 7)
25 6 4
176 6 6†
69.1 6 7.0*†
16.1 6 3.4*‡
27 6 5
166 6 6
57.8 6 6.5
19.1 6 3.0*
Untrained Subjects
Men
(n 5 6)
Women
(n 5 6)
27 6 3
25 6 3
180 6 5*†
171 6 6
74.1 6 6.7*† 60.7 6 6.2
19.5 6 2.4* 27.6 6 2.3
64.4 6 3.7*†‡ 55.3 6 6.6*‡ 42.9 6 3.7*
34.3 6 3.8
4,450
57.9 6 5.6*†
3,196
46.7 6 4.4‡
2,082
43.9 6 4.6
11.2 6 3.0*‡
11.1 6 2.9*‡ 14.5 6 2.1
3,179
59.7 6 5.8*
16.8 6 2.4
Values are means 6 SD. Trained women (n 5 7) excludes one
trained woman found to be in luteal phase of her menstrual cycle.
V̇O2 max , maximal O2 uptake. * P , 0.05 vs. untrained women. † P ,
0.05 vs. trained women. ‡ P , 0.05 vs. untrained men.
jects were tested in the morning after a 12-h fast. After 30 min
of rest, metabolic rate was measured for 15–20 min using a
ventilated canopy. Gas concentrations were measured in the
air exiting the hood. O2 consumption and CO2 production
were used to calculate metabolic rate (35). The RMR was used
to determine energy intake of subjects during the period of
prestudy diet control.
Health and physical examination. A health and a physical
examination were performed on subjects to confirm that there
was no medical reason for their exclusion from the study.
Prestudy diet and exercise control. Subjects were fed a
controlled diet for 3 days before each study day. All food was
prepared in the General Clinical Research Center (GCRC)
diet kitchen at the University of Colorado, and subjects were
required to consume breakfast in the GCRC, with other food
prepared to take off site. No other food was permitted, and
subjects were required to consume all the food given. The only
optional part of the diet was two food modules (840 kJ each),
one or both of which the subjects could eat if they were
hungry. The diet composition was 30% fat, 15% protein, and
55% carbohydrate, and initial energy intake was calculated
at 1.6 3 RMR and 1.9 3 RMR for untrained and trained
subjects, respectively. One trained man and one trained
woman requested two additional food modules each. Body
weight was measured daily before the subjects ate breakfast
to ascertain weight stability. Subjects were allowed to follow
their usual activity routine for the first 2 days of the diet, and
on the last day they refrained from planned exercise.
Study Days
Subjects spent the evening before each study in the GCRC
and were tested the next day after a 10-h fast. All subjects
completed 2 test days: an exercise day and a control day. The
order of testing was randomized within each group, and 3–6
wk separated the 1st from the 2nd study day. Women were
studied in the follicular phase of their menstrual cycle
(exercise study: days 7.3 6 2.8 and 6.8 6 3.5 in trained and
untrained women, respectively, where day 1 is the 1st day of
menses; control study: days 7.5 6 3.9 and 6.5 6 3.3, respectively). Previous menstrual history was used to estimate
desired study day, and menstrual cycle phase and day were
confirmed by report of most recent menses and determination
of serum estradiol and progesterone on the study day. One
trained woman was found to have been in her luteal phase on
both study days but was not able to repeat either day.
Analysis of the data with and without her results did not
affect the substrate oxidation results but did impact some of
the hormone data. Her results were therefore excluded from
the final analysis. On the morning of the study, subjects were
awakened at ,0545. An intravenous flexible catheter (18–20
gauge) was placed in a vein close to the antecubital space of
one arm. A three-way stopcock was attached for sampling and
infusion of normal saline. The subject was then transported
by car (3-min drive) to the laboratory for testing.
Exercise day. Subjects emptied their bladder and completed a urine collection over the subsequent 24 h. They then
lay down and rested for 30 min before a 15- to 20-min
measurement of RMR. Immediately thereafter, baseline blood
samples were taken. Subjects then rode on a stationary
bicycle ergometer (Monark) for 2 h at 40% V̇O2 max after a
5-min warm-up. After the termination of the exercise bout,
subjects rested in a semireclined position for 2 h.
Control day. The control day followed exactly the same
format as the exercise day, except the subjects did not exercise
but rested for the same study duration in a semireclined
position (4 h total).
1825
EXERCISE FUEL METABOLISM IN MEN AND WOMEN
Metabolic Determinations
Respiratory gas exchange measurements. Respiratory gas
exchange during each 4-h test period was measured via
indirect calorimetry using the metabolic cart and Hans
Rudolph mask. Measurements were made continuously for
periods of ,30 min, with recalibration of the metabolic cart
between measurements. Carbohydrate and fat oxidation were
calculated from the volume of O2 consumed and the volume of
CO2 expired after correction for protein oxidation (17). Protein oxidation was estimated by dividing 24-h urinary nitrogen excretion into 2-h portions. This time frame was used
rather than the study period itself, as 1) some subjects could
not urinate immediately after the exercise and 2) we observed
a time delay in the excretion of nitrogen resulting from
exercise protein oxidation. We estimated a difference of less
than 60.5% in the estimation of carbohydrate or fat grams
oxidized when using the study period urine collection vs. 2-h
nitrogen excretion values estimated from the 24-h urine
collection.
Determination of circulating hormone and substrate levels.
Blood was sampled at baseline, every 30 min on the exercise
day, and at baseline and every hour on the control day. Blood
samples were drawn from a forearm vein; therefore, the
circulating measurements represent arterial concentrations
as well as venous drainage from the local muscle bed.
Approximately 1 ml of whole blood was added to a preweighed
tube containing 3 ml of iced perchloric acid (8%). After it was
vortexed, the tube was weighed and spun to separate the
supernatant. Whole blood (2.5 ml) was added to 40 µl of
preservative (3.6 mg EGTA 1 2.4 mg glutathione in distilled
water) for plasma catecholamine determinations. Samples
were immediately placed on ice and spun. Approximately 7 ml
of whole blood were allowed to clot, and the serum was
separated off after spinning. All plasma, serum, and supernatant samples were stored at 270°C until analysis. Catecholamines were determined in duplicate by radioenzymatic
assay [intra-assay coefficients of variation (CV) 5 16 and 8%
for norepinephrine and epinephrine, respectively] (24). RIAs
were used to determine serum insulin (Kabi Pharmacia,
Piscataway, NJ), cortisol, progesterone, estradiol, and testosterone (Diagnostic Products, Los Angeles, CA) and sexhormone binding globulin (SHBG; Wien Laboratories, Succasunna, NJ). Samples were run in duplicate, with intra-assay
CVs of 10, 6.7, 8.8, 7.5, 9.1, and 7.0%, respectively. Serum
glucose was measured enzymatically, one time only, using the
hexokinase method on an automated Rohe COBAS Mira Plus
glucose analyzer (intra-assay CV 5 0.7%). Enzymatic assays
were used to determine serum glycerol (Boehringer Mannheim Diagnostics, Indianapolis, IN) and free-fatty acids (FFA;
Wako Chemical, Richmond, VA) one time only (intra-assay
CVs 5 7.8 and 1.2%, respectively). b-Hydroxybutyric acid
(b-HBA) and blood lactate (Sigma Diagnostics, St. Louis, MO)
were run in duplicate with intra-assay CVs of 10.8 and 4.2%,
respectively. For each subject, samples from both study days
were run simultaneously for all assays.
Data Analysis
On each study day, data were compared by sex and by
training status, with any interaction between the two grouping factors also determined. Inasmuch as the only significant
effects on exercise fuel oxidation were observed between the
genders, all data are presented using the gender-based
separation. For fuel oxidation and energy expenditure (EE),
each study day was divided into two parts comprising the first
2 h of measurement (cycling period for the exercise day) and
the second 2 h of measurement (recovery period for the
exercise day). Fuel oxidation was expressed as absolute
oxidation per 2-h period and as relative oxidation (percent):
(kJ fuel oxidized/total kJ oxidized) * 100. The latter mode of
expression allows for the different EE between individuals
and groups, which would automatically result in different
absolute fuel oxidation rates. Univariate ANOVA was initially used to determine group differences in EE and fuel
oxidation variables for comparable time periods on each study
day. Where significant differences were observed, these were
confirmed by multivariate analysis that included percent
body fat and V̇O2 max as covariates. These two variables were
included in the statistical model, as they may be considered
confounding factors for the univariate results. All data are,
however, presented as the unadjusted means with statistics
from the univariate analysis. For substrate and hormone
measures, a repeated-measures ANOVA was used with gender as the grouping factor and each time point of sampling as
repeated measures. Any interaction between gender and time
was determined to establish whether the pattern of change in
each parameter differed between the genders. Where significant gender-by-time interactions were observed, an unpaired
t-test was used to determine whether there were also specific
time points where gender-based differences occurred. Group
characteristics were compared using ANOVA, and where
appropriate, post hoc analysis was performed using the
Fisher least significant difference test. Within-group differences in parameters (between periods of measurements and
between days) were compared using a paired t-test. Significant differences were established where P , 0.05.
RESULTS
Physical Characteristics
There were no gender differences in age and fat mass.
However, men were significantly heavier and taller and
had a greater fat-free mass than women (P , 0.05).
V̇O2 max was marginally higher in men than in women
(P , 0.06), whereas percent body fat was significantly
greater in women (P , 0.05). Levels of activity were
similar in trained women and men (cycling: 135 6 41
and 132 6 54 miles/wk; other aerobic activity: 3 and
2.5 h/wk; strength training: 2 and 2.5 h/wk, respectively). Dietary intake and body weight change over the
3 days before each study day are shown in Table 2.
Energy and nutrient intakes were similar for the 3 days
preceding the control or exercise day, and weight
Table 2. Dietary intake and body weight change before
each study day
n
Energy
Intake,
kJ/day
Fat
Intake,
g/day
CHO
Intake,
g/day
Protein
Intake,
g/day
Weight
Change,
kg
Cycling day
Men
14 14,98661,604 121614 495653 138617 20.2160.6
Women 13 10,94461,287 88611 366647 99613 20.1260.22
Control day
Men
14 15,15461,667 122615 502654 139617 10.660.6
Women 13 11,29961,775 91616 380666 105617 20.2060.54
Values are means 6 SD. CHO, carbohydrate. Weight change was
estimated from fasting body weight measured on the morning of the
1st day of diet control 2 fasting body weight measured the morning of
the study day.
1826
EXERCISE FUEL METABOLISM IN MEN AND WOMEN
elevated above the value on the control day in men (P ,
0.001) but not in women.
Fuel Oxidation
Fig. 1. Relationship between fat-free mass and resting metabolic
rate in men (s) and women (r). Resting metabolic rate measured on
cycling and control days were averaged. r 5 0.67 for men and 0.52 for
women.
stability was maintained in men and women before
each study day.
Energy Expenditure
RMR was similar in men and women relative to
fat-free mass (Fig. 1). There was no difference between
RMR measured on the cycling day and that measured
on the control day (Table 3: 5.38 6 0.21 and 5.33 6 0.13
kJ/min, respectively, for men; and 4.37 6 0.08 and
4.41 6 0.12 kJ/min, respectively, for women). EE during the 120-min cycle ride was significantly greater in
men than in women (Table 4). This was related to the
greater absolute work rate performed by the men
(338 6 16 and 219 6 14 kpm/min for men and women,
respectively, P , 0.05). However, the relative intensity
of the exercise was similar between the two groups
(average heart rate: 121 6 4 and 125 6 4 beats/min for
men and women, respectively; intensity 5 42.1 6 0.9
and 42.3 6 0.8% V̇O2 max for men and women, respectively). During exercise recovery, EE was significantly
Resting respiratory exchange ratio did not differ
between the two groups on either study day (cycling
day: 0.80 6 0.01 and 0.79 6 0.01 for men and women,
respectively; control day: 0.80 6 0.01 and 0.79 6 0.01
for men and women, respectively). During the cycle
exercise, absolute total carbohydrate oxidation and
total protein oxidation were significantly greater in
men than in women (Table 4; P , 0.001 and P , 0.01,
respectively), but there were no gender-based differences in the absolute total oxidation of fat. When the
oxidation data were expressed as a percentage of the
total EE, the contribution of fat oxidation to exercise
EE was significantly greater in women than in men
(P , 0.02), whereas the contribution of carbohydrate
oxidation to total exercise EE was significantly greater
in men (P , 0.01; Table 4). No gender-based differences
in the relative contribution of fat and carbohydrate
oxidation to total EE were observed during the postexercise recovery period or on the control day.
As expected, absolute fuel oxidation was greater
during period 1 of the cycling day (exercise) than during
period 2 (recovery) and period 1 of the control day
(Table 4). During the postexercise period the contribution of fat to total EE significantly increased relative to
the exercise period in men (P , 0.001) and women (P ,
0.01), whereas the contribution from carbohydrate significantly decreased in both groups (P , 0.001). On the
control day, there was also an increase in the contribution of fat to total EE from period 1 to period 2, but this
was significant only for women (P , 0.01). Consequently, the increased relative fat oxidation in men
during recovery on the exercise day was greater than
the relative fat oxidation during the same time period
on the control day (P , 0.01).
Circulating Substrate Levels
Circulating substrate levels on the exercise and
control days are shown in Fig. 2. For both groups on the
cycling day, glucose, FFA, glycerol, and lactate increased with exercise and decreased after exercise (P ,
0.0001), whereas b-HBA increased mainly in the postex-
Table 3. RMR, exercise EE, and nonprotein RER
n
RMR,
kJ/min
RMR RER
Men
Women
14
13
5.38 6 0.21*
4.37 6 0.08
0.80 6 0.01
0.79 6 0.01
Men
Women
14
13
5.33 6 0.13*
4.41 6 0.12
0.80 6 0.01
0.79 6 0.01
EE Period 1,
kJ/min
RER
Period 1
EE Period 2,
kJ/min
RER
Period 2
0.86 6 0.01†‡
0.84 6 0.01‡
6.17 6 0.17*‡
4.43 6 0.20
0.79 6 0.01
0.79 6 0.01
0.82 6 0.01
0.82 6 0.01
5.38 6 0.21*
4.33 6 0.15
0.81 6 0.01
0.80 6 0.01
Cycling day
34.5 6 2.1*‡
23.5 6 1.9‡
Control day
5.32 6 0.20*
4.29 6 0.16
Values are means 6 SE. RMR, resting metabolic rate; RER, respiratory exchange ratio; EE, energy expenditure. Period 1, rate of EE during
first 2 h of measurement (encompasses entire cycling period on cycling day); period 2, rate of EE during second 2 h of measurement
(encompasses postexercise recovery period on cycling day). Significantly different from women: * P , 0.001; † P , 0.05. ‡ Significantly different
from control, P , 0.001.
1827
EXERCISE FUEL METABOLISM IN MEN AND WOMEN
Table 4. EE and fuel oxidation
Absolute Fuel Oxidation, g/2 h
n
EE, kJ/2 h
CHO
Fat
Relative Fuel Oxidation, % of total EE
Protein
CHO
Fat
Protein
45.6 6 3.5
36.3 6 3.3
6.2 6 0.3b
4.7 6 0.4
53.1 6 2.1b
45.7 6 1.8
43.7 6 2.1a
50.9 6 1.8
3.1 6 0.2
3.4 6 0.3
10.8 6 0.3c,f
8.0 6 0.5f
6.2 6 0.3b
4.7 6 0.4
25.1 6 1.3f
22.4 6 1.8f
58.3 6 1.2f
59.6 6 1.7e
16.6 6 0.7f
18.0 6 1.6f
Cycling day
Period 1
Men
Women
Period 2
Men
Women
14
13
4,137 6 252c
2,821 6 224
14
13
739 6 21c,f
532 6 24f
125.4 6 10c
73.4 6 6.3
10.7 6 0.8c,f
6.9 6 0.7f
Control day
Period 1
Men
Women
Period 2
Men
Women
14
13
638 6 25c,i
514 6 19i
12.0 6 0.8i
9.7 6 1.0i
7.7 6 0.6i
6.6 6 0.4i
6.1 6 0.3c
4.2 6 0.3
33.1 6 2.1i
32.8 6 2.7i
47.8 6 2.8
51.1 6 2.6
18.9 6 2.2i
16.1 6 1.2i
14
13
647 6 25c,i
518 6 18
11.0 6 0.9a,e
8.3 6 0.7e
8.3 6 0.5d,i
7.3 6 0.4f
6.1 6 0.3c
4.2 6 0.3
32.0 6 2.6g
27.9 6 2.1e
50.7 6 2.5h
55.3 6 2.5e
17.2 6 1.6
16.0 6 1.2
Values are means 6 SE. See Table 3 legend for explanation of periods 1 and 2. Significantly different from women: a P , 0.02; b P , 0.01;
, 0.001. Significantly different from period 1 same day: d P , 0.05; e P , 0.01; f P , 0.001. Significantly different from same period cycling
day: g P , 0.05; h P , 0.01; i P , 0.001.
cP
ercise period (P , 0.0001). Although lactate levels
increased with exercise, the rise was relatively small
(maximum change of ,0.2 mmol/l for both groups) and
was unlikely to have had any impact on nonmetabolic
CO2 production. During exercise the increase in FFA
was significantly greater in women than in men (P ,
0.004). Conversely, glucose and b-HBA were higher in
men during the postexercise period (P , 0.005 and P ,
0.003, respectively). On the control day the small
increases in FFA, glycerol, lactate, and b-HBA were
also significant with time (P , 0.001). On this day,
substrates increased similarly in both groups. Lactate
level was significantly higher in men than in women on
the control day (P , 0.02), although the differences
were small in magnitude (,0.2 mmol/l).
Circulating Hormones
Figures 3 and 4 show the hormone changes during
both study days. On the cycling day, all circulating
hormone levels changed significantly with time (P ,
0.0001), generally increasing with exercise and decreasing after exercise. Most notably, there was a significantly greater elevation in epinephrine (P , 0.007) and
norepinephrine (P , 0.0009) during exercise in men
than in women. Postexercise insulin levels were significantly greater in men than in women (P , 0.0009), but
the absolute magnitude of this difference was small
(1 µU/ml). Although cortisol levels were greater in men
than in women on the cycling day, this did not achieve
statistical significance (P 5 0.08).
With respect to the gonadotrophic hormones, on the
control and the exercise day, SHBG levels were significantly greater in women than in men (P , 0.01 and P ,
0.009, respectively), whereas testosterone levels were
significantly greater in men (P , 0.0001). The increase
in estradiol during exercise was greater in women than
in men (P , 0.04). Testosterone increased with exercise, more in men than in women (P , 0.0001), but
levels were not significantly different from the values
on the control day in either group.
DISCUSSION
This study demonstrated significant gender-based
differences in fuel oxidation and the hormonal response
to exercise. With the same relative intensity of mild-tomoderate exercise, fat oxidation contributed more to
the total EE in women than in men. Consequently, the
contribution of carbohydrate oxidation to exercise EE
was lower in women than in men. The most striking
observation, however, was our finding of significantly
greater epinephrine and norepinephrine levels in men
than in women during exercise. Whether these hormones played a role in the observed gender-based
differences in exercise fuel oxidation could not be
directly established.
The present data confirm previous observations of a
greater relative fat oxidation and lower relative and/or
absolute carbohydrate oxidation in women than in men
(1, 25, 31, 32). It also extends previous observations by
establishing that these gender-based differences are
limited to the exercise condition itself. We observed no
difference between men and women in the contribution
of lipid and carbohydrate fuels to EE before exercise,
after exercise, or on a control day. Data were analyzed
by taking into account the differences in V̇O2 max and
percent body fat between men and women. Therefore, it
appears that there is a gender-specific difference in the
prioritization of fat and carbohydrate fuels for oxidation during exercise. Furthermore, this was independent of training status in both men and women.
Although the magnitude of the differences observed
in the present study was small, others have observed
greater gender-based differences between the contribution of fat and the contribution of carbohydrate oxidation to exercise EE (25, 31, 32). These studies used a
1828
EXERCISE FUEL METABOLISM IN MEN AND WOMEN
Fig. 2. Circulating substrate levels in men (open symbols) and women (filled symbols) on exercise (circles) and
control (triangles) days. A: for lactate, there was a significant effect of time (P , 0.0001) on cycling and control days
and a significant effect of gender (P , 0.02) on control day. B: for glucose, there was a significant effect of time (P ,
0.0001) and a significant gender-by-time interaction (P , 0.005); for the cycling day only, 1 P , 0.05 (unpaired
t-test). C: for free fatty acids, there was a significant effect of time (P , 0.0001) on cycling and control days and a
significant gender-by-time interaction (P , 0.004) on cycling day; 1 P , 0.05 (unpaired t-test). D: for glycerol, there
was a significant effect of time on cycling and control days (P , 0.0001 and P , 0.001, respectively). E: for
b-hydroxybutyric acid, there was a significant effect of time (P , 0.0001) on cycling and control days and a
significant gender-by-time interaction (P , 0.003) on cycling day.
somewhat higher exercise intensity (65–85% V̇O2 max)
than the present study (40% V̇O2 max). Therefore, the
magnitude of the gender difference between fat and
carbohydrate oxidation may be related to the intensity
of exercise performed. It is possible that at mild exercise intensities the gender-based differences in fat
oxidation may be smaller, as both groups can readily
utilize lipid as an energy source. Conversely, it seems
probable that at very high exercise intensities, where
carbohydrate utilization predominates and anaerobic
metabolism is increased, gender-based differences in
fuel utilization may not occur.
In the present cross-sectional study, trained subjects
had a nonsignificantly greater lipid oxidation than
untrained subjects. This contrasts with longitudinal
training studies, in which significantly higher rates of
lipid oxidation are observed after training (15). This
discrepancy may be explained by the differences in
study design (cross-sectional vs. longitudinal) and the
use of different exercise regimens (relative vs. absolute
exercise intensity). In addition, the exercise intensity
used in the present investigation was relatively mild to
moderate for all subjects and would favor lipid utilization in untrained as well as trained subjects alike.
The implication of gender differences in fuel oxidation during aerobic exercise relates to the control of fuel
stores, both lipid and carbohydrate (glycogen). With
respect to total body fat, it may be counterintuitive that
women oxidize more lipid than men during exercise,
given the fact that women normally maintain a higher
body fat level than men. However, the magnitude of the
gender-based difference in the relative fat oxidation
observed during exercise was small and could readily
be offset by postexercise differences in EE and/or food
intake. In regard to carbohydrate stores, the lower
carbohydrate oxidation during exercise in women has
been shown to be related to less glycogen depletion (31,
32). Therefore, increasing glycogen stores to enhance
performance may be less effective in women than in
men (32).
EXERCISE FUEL METABOLISM IN MEN AND WOMEN
1829
Fig. 3. Circulating hormone levels in
men (open symbols) and women (filled
symbols) on exercise (circles) and control (triangles) days. A: for epinephrine, there was a significant effect of
time (P , 0.0001) and gender (P ,
0.003) and a significant gender-by-time
interaction (P , 0.007) on cycling day
only; 1 P , 0.05 (unpaired t-test). B: for
norepinephrine, there was a significant
effect of time (P , 0.0001) and a significant gender-by-time interaction (P ,
0.0009) on cycling day only; 1 P , 0.05,
# P 5 0.06 (unpaired t-test). C: for insulin, there was a significant effect of
time (P , 0.0001 and P , 0.004, respectively) and a significant gender-by-time
interaction (P , 0.0009 and P , 0.01,
respectively) on cycling and control
days; 1 P , 0.05 on cycling day, * P ,
0.05 on control day (unpaired t-test).
D: for cortisol, there was a significant
effect of time (P , 0.0001) on cycling
and control days.
The significantly greater catecholamine response that
was observed during exercise in men than in women
suggests a role for these hormones in the gender-based
differences in fuel oxidation. Previous observations
have suggested a greater epinephrine (23, 31), but not
norepinephrine (9, 31), response to exercise in men
than in women. One of the major effects of the catecholamines is the stimulation of lipolysis (10, 20). However,
in the present study we observed no gender-based
difference in circulating glycerol levels, an estimate of
whole body lipolysis. This has also been observed
previously (9, 31), although reports are inconsistent (1,
31). The authors recognize that changes in circulating
glycerol levels do not solely reflect the release of
glycerol from triglyceride stores; however, these two
parameters do appear to be highly correlated (2, 29).
Thus, on the basis of the assumption that a change in
glycerol level is a reasonable estimate of lipolysis, the
glycerol data from this study suggest a greater sensitivity to the lipolytic action of one or both of the catecholamines in women. This is consistent with previous data
on the response to hypoglycemia in women and men (6).
Under resting conditions, however, no gender-based
differences have been observed in glycerol levels during
epinephrine infusion (34). This suggests that there may
be an interaction between the catecholamines and
stress (i.e., exercise and/or hypoglycemia) that affects
lipolytic sensitivity in women relative to men.
Epinephrine can also stimulate glycogenolysis, particularly in muscle (7). Therefore, the greater catecholamine level during exercise in men could have resulted
in a greater mobilization and, potentially, utilization of
muscle glycogen. Indeed, a greater decline in muscle
glycogen stores after exercise has been previously
observed in men than in women (31). However, in the
present study we had no measure of glycogenolysis. It
could be postulated that a greater sensitivity in women
than in men, to the lipolytic action of the catecholamines, in conjunction with a similar glycogenolytic
effect, would favor lipid over carbohydrate mobilization
at any given catecholamine level.
The main gender-based difference observed in circulating substrate concentrations during exercise was the
greater FFA levels in women than in men. This has also
been reported by other authors using mild exercise (1),
whereas with more moderate exercise no difference in
circulating FFA has been observed (9, 31). However,
circulating substrate levels do not reflect fuel utilization but represent a balance between production and
utilization. This is relevant, as we observed genderbased differences in circulating FFA concentrations but
not circulating glycerol levels. Therefore, if lipolysis
1830
EXERCISE FUEL METABOLISM IN MEN AND WOMEN
Fig. 4. Circulating gonadotrophic hormone levels in men (open symbols) and
women (filled symbols) on exercise
(circles) and control (triangles) days.
A: for estradiol, there was a significant
effect of time (P , 0.0001) and a significant gender-by-time interaction (P ,
0.04) on cycling day only; 1 P , 0.05
(unpaired t-test). B: for progesterone,
there was a significant effect of time
(P , 0.0001 and P , 0.007, respectively) on cycling and control days.
C: for testosterone, there was a significant effect of time (P , 0.0001) and a
significant gender-by-time interaction
(P , 0.0001) on cycling day only and a
significant effect of gender on cycling
and control days (P , 0.0001). D: for
sex hormone-binding globulin (SHBG),
there was a significant effect of time
(P , 0.0001) on cycling day only and a
significant effect of gender (P , 0.009
and P , 0.01, respectively) on cycling
and control days.
were stimulated to the same extent in both groups, the
lower FFA levels in men could be due to a greater
oxidation of adipose tissue-derived FFA than in women.
This could only be confirmed by measurements of
substrate turnover. Adipose tissue-derived FFA may
not, therefore, be contributing to the increased relative
fat oxidation in women. Other sources of lipid that
could contribute to the greater relative fat oxidation in
women during exercise include intramuscular triglyceride-derived FFA (31) and, to a lesser extent, circulating
very-low-density-lipoprotein triglycerides (33).
In the present investigation, circulating glucose levels were consistently lower in women than in men,
either in response to the same duration of fasting on the
control day or during the exercise and the postexercise
recovery. Others have reported that, with more moderate exercise (65% V̇O2 max), exercise blood glucose is
lower in men than in women (31). This again implies
that exercise intensity is an important determinant of
gender-based differences in exercise fuel metabolism.
In the present study it appeared that women were
better able than men to maintain blood glucose levels
(albeit at on overall lower level) during exercise. In the
only report to measure glucose kinetics in men and
women (1 h of cycling, 50% V̇O2 max), exercise glucose
utilization was not different between the genders (23).
However, it is difficult to make inferences from this
tracer study relative to our present investigation, as
the former study observed no gender-based differences
in whole body fuel oxidation, likely because of the small
number of subjects studied (n 5 4/gender). In the
present study, whether the greater catecholamine levels during exercise in men than in women resulted in
differences in glucose production cannot be determined.
Therefore, potential gender-based differences in the
regulation of blood glucose production and utilization
during exercise require further investigation.
It is presently unknown whether differences in fat
and carbohydrate utilization in men and women can be
explained by differences in substrate delivery, hormone
action, or the capacity for substrate utilization. It could
be hypothesized that the capacity for carbohydrate
oxidation is greater in men than in women; therefore,
by default, women must use more lipid. This hypothesis
is supported by data that demonstrate a higher ratio of
phosphofructokinase to 3-hydroxacyl-CoA dehydrogenase in men than in women, suggesting a greater
enzymatic potential for glycolysis in men (11). Future
work needs to establish how circulating factors (hormones and substrates) and muscle metabolic characteristics dictate the gender-based preferences for carbohydrate vs. lipid fuels.
EXERCISE FUEL METABOLISM IN MEN AND WOMEN
Similar to previous observations, plasma b-HBA
levels were significantly increased in men and women
after exercise (18). In addition, b-HBA levels were
significantly greater in men than in women after
exercise. The elevated FFA after exercise provide a
source for the production of ketones (22). However, the
greater b-HBA level in the men was not associated with
a greater FFA concentration in the postexercise period.
The accumulation of ketones may lead to errors in the
estimation of fuel oxidation (30) after exercise. However, the magnitude of these errors is likely small (30).
The contribution of the greater ketone levels in men
than in women to postexercise fuel oxidation cannot be
estimated, inasmuch as it is not known whether the
greater b-HBA accumulation in men was due to more
production or less removal. Nor did we measure acetoacetate, the other major ketone. Nevertheless, it is
unlikely that the magnitude of these postexercise gender-based differences in ketone levels would significantly affect the estimations of fuel oxidation, such that
the conclusion of no gender difference in postexercise
fuel oxidation would be changed.
Other hormones measured in the present study
showed less of a dramatic difference between men and
women than the catecholamines. We observed a nonsignificantly greater cortisol level in men on the exercise
day during cycling and recovery. No such difference was
apparent during the control day. This suggests that
cortisol could have some role, although not primary, in
determining the gender differences in exercise fuel
oxidation. All the gonadotrophic hormones were observed to increase during exercise. This has previously
been reported by others (19, 36). This elevation in
estradiol, progesterone, and testosterone during exercise appeared to be due to a rise in the free hormone,
inasmuch as SHBG levels did not change. It has been
suggested that a decreased hepatic clearance of these
steroid hormones as a result of a decrease in hepatic
blood flow is responsible for the exercise-induced rise in
their circulating levels (28). Estradiol levels were very
variable in women on the cycling day. This was because
subjects were studied at different points of the follicular
phase, and estradiol is known to increase from the early
through the late part of this phase. Nevertheless, it is
possible that the greater increase in estradiol during
exercise in women had an effect on fuel metabolism. For
example, estrogen has been reported to decrease glucose utilization but may increase lipid availability by
stimulating lipolysis (12, 27). Further studies are needed
to confirm a role, direct or indirect, for the gonadotrophic hormones and especially estradiol in exercise fuel
metabolism.
In summary, we observed a significant gender-based
difference in the pattern of exercise fuel oxidation in
both trained and untrained men and women matched
for level of physical activity. Women derived more
energy during exercise from fat oxidation and less from
carbohydrate oxidation than did men. There were no
gender-based differences in relative fuel oxidation under resting conditions before exercise, after exercise, or
on a control day. The exercise differences in fuel oxida-
1831
tion between men and women were associated with a
significant gender-based difference in the catecholamine response to exercise. Men had a greater elevation in both epinephrine and norepinephrine levels
during exercise. It is hoped that future studies will
provide insight into the sources of the different fat and
carbohydrate fuels that contribute to the observed
gender-based differences in exercise fuel oxidation and
also the hormonal and/or metabolic mechanisms involved in determining the gender-specific pattern of
fuel oxidation. Finally, it could be speculated that,
because of their reproductive role, women have a
greater priority for carbohydrate conservation than
men under conditions of increased energy demand.
The authors thank all the subjects who volunteered for the study
and greatly appreciate their time and cooperation. The authors also
thank the GCRC Core Laboratory and especially Teddi Wiest-Kent
for assistance with analytic aspects of the study.
This investigation was supported by National Institute of Diabetes
and Digestive and Kidney Diseases Grants DK-42549 and DK-48520
and by National Institutes of Health Division of Research Resources
Grant 5-01-RR-00051.
Address for reprint requests: T. J. Horton, Center for Human
Nutrition, Box C225, University of Colorado Health Sciences Center,
4200 East 9th Ave., Denver, CO 80262.
Received 23 October 1997; accepted in final form 13 July 1998.
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