Am J Physiol Endocrinol Metab 301: E1033–E1039, 2011.
First published August 16, 2011; doi:10.1152/ajpendo.00291.2011.
Effects of aerobic vs. resistance training on visceral and liver fat stores,
liver enzymes, and insulin resistance by HOMA in overweight adults
from STRRIDE AT/RT
Cris A. Slentz,1 Lori A. Bateman,1 Leslie H. Willis,1 A. Tamlyn Shields,8 Charles J. Tanner,8
Lucy W. Piner,1 Victoria H. Hawk,3 Michael J. Muehlbauer,6 Greg P. Samsa,5 Rendon C. Nelson,4
Kim M. Huffman,10 Connie W. Bales,7,9 Joseph A. Houmard,8 and William E. Kraus1,2
1
Submitted 13 June 2011; accepted in final form 10 August 2011
Slentz CA, Bateman LA, Willis LH, Shields AT, Tanner CJ, Piner
LW, Hawk VH, Muehlbauer MJ, Samsa GP, Nelson RC, Huffman
KM, Bales CW, Houmard JA, Kraus WE. Effects of aerobic vs. resistance training on visceral and liver fat stores, liver enzymes, and insulin
resistance by HOMA in overweight adults from STRRIDE AT/RT. Am J
Physiol Endocrinol Metab 301: E1033–E1039, 2011. First published August
16, 2011; doi:10.1152/ajpendo.00291.2011.—While the benefits of exercise are clear, many unresolved issues surround the optimal exercise
prescription. Many organizations recommend aerobic training (AT)
and resistance training (RT), yet few studies have compared their
effects alone or in combination. The purpose of this study, part of
Studies Targeting Risk Reduction Interventions Through Defined
Exercise-Aerobic Training and/or Resistance Training (STRRIDE/
AT/RT), was to compare the effects of AT, RT, and the full combination (AT/RT) on central ectopic fat, liver enzymes, and fasting
insulin resistance [homeostatic model assessment (HOMA)]. In a
randomized trial, 249 subjects [18 –70 yr old, overweight, sedentary,
with moderate dyslipidemia (LDL cholesterol 130 –190 mg/dl or HDL
cholesterol ⱕ40 mg/dl for men or ⱕ45 mg/dl for women)] performed
an initial 4-mo run-in period. Of these, 196 finished the run-in and
were randomized into one of the following 8-mo exercise-training
groups: 1) RT, which comprised 3 days/wk, 8 exercises, 3 sets/
exercise, 8 –12 repetitions/set, 2) AT, which was equivalent to ⬃19.2
km/wk (12 miles/wk) at 75% peak O2 uptake, and 3) full AT ⫹ full
RT (AT/RT), with 155 subjects completing the intervention. The
primary outcome variables were as follows: visceral and liver fat via
CT, plasma liver enzymes, and HOMA. AT led to significant reductions in liver fat, visceral fat, alanine aminotransferase, HOMA, and
total and subcutaneous abdominal fat (all P ⬍ 0.05). RT resulted in a
decrease in subcutaneous abdominal fat (P ⬍ 0.05) but did not
significantly improve the other variables. AT was more effective than
RT at improving visceral fat, liver-to-spleen ratio, and total abdominal
fat (all P ⬍ 0.05) and trended toward a greater reduction in liver fat
score (P ⬍ 0.10). The effects of AT/RT were statistically indistinguishable from the effects of AT. These data show that, for overweight and obese individuals who want to reduce measures of visceral
fat and fatty liver infiltration and improve HOMA and alanine aminotransferase, a moderate amount of aerobic exercise is the most
time-efficient and effective exercise mode.
Address for reprint requests and other correspondence: C. A. Slentz, Div. of
Cardiology, Dept. of Medicine, PO Box 3022, Duke Univ. Medical Center,
Durham, NC 27710 (e-mail: [email protected]).
http://www.ajpendo.org
aerobic training; liver fat; resistance training; weight lifting; homeostasis model assessment
physically active are clear, many
unresolved issues surround the optimal exercise prescription
for these benefits. Many organizations recommend both aerobic training (AT) and resistance training (RT) for all adults.
However, these recommendations are mainly based on the
evaluation of each modality separately, as few studies have
investigated the effects of combined AT and RT regimens
compared with each modality individually. Furthermore, adherence to exercise recommendations of physicians is notoriously poor, and many patients cite lack of time as a reason for
noncompliance. Understanding the effects of AT and RT is of
critical importance if we are to apply evidence-based approaches to exercise recommendations to a wide population.
Visceral fat and liver fat are associated with type 2 diabetes,
metabolic syndrome, and metabolic abnormalities (7, 10, 18,
27, 33). Visceral and liver fat are also independent risk factors
for all-cause mortality (14). Elevated concentrations of circulating liver enzymes are also associated with type 2 diabetes,
fatty liver, metabolic syndrome, and mortality (9, 12, 15, 16,
22, 32). In fact, alanine aminotransferase (ALT) is considered
to be a marker of fatty liver infiltration (23). Elevated liver
enzymes, even at concentrations considered to be within normal levels, are independent predictors of incident diabetes (9,
22) and nonalcoholic fatty liver disease (NAFLD) (2). The
importance of insulin resistance [homeostatic model assessment (HOMA)] is well established, and aerobic exercise consistently improves insulin sensitivity (28). While the benefits of
aerobic exercise on visceral adiposity and insulin sensitivity
are well established, few studies have compared the effects of
AT, RT, and AT ⫹ RT on these outcomes (5, 26), and, to our
knowledge, none have examined the effect of RT on liver fat or
concentrations of circulating liver enzymes.
The present study, STRRIDE-AT/RT (Studies Targeting
Risk Reduction Interventions Through Defined Exercise-Aerobic Training and/or Resistance Training), was designed to
address three major questions relating to exercise recommendations for overweight, sedentary adults. 1) What are the
specific benefits of RT in this population? 2) How do these
benefits compare with those that accrue when a similar amount
WHILE THE BENEFITS OF BEING
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Division of Cardiology, 2Duke Center for Living, 3Center for Aging, 4Department of Radiology, 5Department of Biostatistics
and Biometrics, 6Sarah W. Stedman Center for Nutrition and Metabolism, and 7Division of Geriatrics, Duke University
Medical Center, 9Geriatric Research Education and Clinical Centers, and 10Physical Medicine and Rehabilitation, Veterans
Affairs Medical Center, Durham; and 8Department of Exercise and Sports Science and Human Performance Laboratory, East
Carolina University, Greenville, North Carolina
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EFFECTS OF AEROBIC VS. RESISTANCE TRAINING ON ECTOPIC FAT
of time is spent in AT? 3) What are the additive, synergistic, or
possibly antagonistic effects of the combination of AT and RT
(AT/RT)? These findings should improve the ability of clinicians, exercise professionals, and the lay public to more accurately understand the benefits of different exercise regimens,
target exercise recommendations to specific outcomes, and
more efficiently utilize precious exercise time. This report
summarizes the effects of AT, RT, and AT/RT on visceral fat
and fatty liver infiltration, as represented by liver density and
the circulating liver-derived enzyme ALT, as well as fasting
insulin resistance as presented by HOMA.
METHODS
AJP-Endocrinol Metab • VOL
Table 1. Baseline demographics, baseline calorie and
alcohol intake, and exercise prescriptions
Variables
RT (n ⫽ 52)
AT (n ⫽ 48)
AT/RT (n ⫽ 44)
Age, yr
Body mass index, kg/m2
Race
Caucasian
African American
Other
Sex
Female
Male
Food intake, kcal/day
Alcohol intake, g
Resistance exercise
Rx frequency,
sessions/wk
Intensity
Rx amount, sets/wk
Rx time, min/wk
Adherence, %
Actual frequency,
sessions/wk
Actual amount,
sets/wk
Aerobic exercise
Intensity, %peak V̇O2
Rx amount,
kcal 䡠 kg⫺1 䡠 wk⫺1
Rx time, min/wk
Adherence, %
Actual frequency,
sessions/wk
Actual time, min/wk
49.7 (11.4)
30.5 (3.4)
49.5 (9.8)
30.4 (3.2)
46.9 (10.0)
30.7 (3.4)
43
8
1
42
6
0
37
6
1
30
22
2,168 (773)
4.7 (8.8)
26
22
1,971 (733)
2.9 (6.5)
25
19
2,104 (559)
5.9 (11.6)
3
Progressive
72
135–180
83.0 (13)
3
Progressive
72
135–180
81.4 (14)
2.5 (0.4)
2.46 (0.4)
59.7 (9)
58.6 (10)
75
75
14
132 (24)
89.8 (10)
14
133 (25)
82.2 (17)
3.0 (0.5)
117 (20)
2.9 (0.6)
109 (27)
Values are means (SD). AT, aerobic training; RT, resistance training;
AT/RT, AT ⫹ RT; V̇O2, O2 uptake; Rx, prescribed. There were no significant
baseline differences between groups. Rx amount (72 sets/wk) ⫽ 3 days/wk, 3
sets of 8 –12 repetitions, on 8 different machines. Actual amount (min/wk) ⫽
approximate range of the number of minutes per week to complete the
prescribed sets/wk. Actual amount (sets/wk) ⫽ Rx amount ⫻ adherence. Rx
amount (14 and 23 kcal 䡠 kg⫺1 䡠 wk⫺1) is approximately calorically equivalent
to 12 and 20 miles of jogging per week, respectively. Actual time (min/wk) ⫽
Rx time ⫻ adherence.
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Subjects: screening and inclusion and exclusion criteria. Subjects
recruited for the STRRIDE-AT/RT study were used in this analysis.
The protocol was approved by the institutional review boards at Duke
University Medical Center and East Carolina University. Subjects
(n ⫽ 3,145) responded to local advertisements and were screened by
phone. Of these, 234 met inclusion criteria and were recruited into the
study. Inclusion criteria were as follows: age 18 –70 yr, sedentary
(physically active ⬍2 times per week), body mass index 26 –35
kg/m2, and mild-to-moderate dyslipidemia (LDL cholesterol 130 –190
mg/dl and/or HDL cholesterol ⱕ40 mg/dl for men or ⱕ45 mg/dl for
women). Subjects were nonsmokers without a history of diabetes,
hypertension, or coronary artery disease. After providing informed
written consent, subjects were asked to maintain their current lifestyle
during a 4-mo run-in period followed by randomization into one of
three exercise-training groups. The purpose of the run-in period was
to discourage individuals who were not serious about the study
commitment and, thus, reduce the dropout rate that occurs after
randomization. Of the 234 subjects recruited, 38 dropped out during
the run-in period, leaving 196 subjects for randomization. Of the
subjects who were randomized, 73.5% (n ⫽ 144) completed the study.
CT, cardiopulmonary exercise testing, and strength evaluations.
Body weight of subjects dressed in light clothing without shoes was
measured to the nearest 0.1 kg on a digital scale. The average of three
weights taken over 2 wk, on different days, was used for each time
point. Height was measured once, to the nearest 0.5 cm. CT scans
were performed by a radiological technologist who was blinded to the
subject’s study status. With subjects in the supine position, a single,
10-mm axial image was taken of the abdomen at the level of the L4
pedicle. A second, single 10-mm axial image was taken at the best
visual location of the liver (determined by a scout image frontal
radiograph taken prior to the liver scan). The spleen was also captured
in 67% of these scans. As a result, the liver-only analyses are based on
a higher total number of subjects, whereas analyses of the spleen only
and the liver-to-spleen ratio are based on smaller numbers of subjects.
The CT images were analyzed using OsiriX imaging software, an
advanced open-source picture archiving-and-communication system
(PACS) workstation DICOM viewer (OsiriX Foundation, Geneva,
Switzerland), to determine the surface area of the visceral, subcutaneous, and total abdominal adipose tissue. With this program, once the
parameters are set (e.g., definition of adipose tissue density range was
set at ⫺30 to ⫺190 Hounsfield units), the program is largely automated. Test-retest reliability correlations for surface areas obtained
are generally nearly perfect (r ⫽ 0.98 or 0.99), as the methodology is
extremely reproducible. To obtain liver density, in the liver image,
three 3.0-cm2 circular regions of interest were manually selected, with
care taken to avoid visible vessels, bile ducts, bordering surfaces, and
motion artifact, and averaged to estimate liver density. In the spleen
image, three 2.0-cm2 circular regions of interest were manually
selected using the same analytic guidelines and averaged to estimate
spleen density. Prior to the beginning of the exercise interventions, CT
tests of 117 subjects were done, and the test-retest correlation for liver
density values was 0.910 (P ⬍ 0.0001), with no significant difference
between test 1 and test 2 means (P ⫽ 0.79).
A maximal cardiopulmonary exercise test with a 12-lead ECG and
expired gas analysis were performed on a treadmill using a TrueMax
2400 Metabolic Cart (ParvoMedics, Sandy, UT) before and after the
exercise interventions.
In RT and AT/RT subjects, the total amount of weight lifted during
a single RT session was recorded each week by a supervising personal
trainer at the East Carolina University site or electronically by the
FitLinxx Strength Training Partner system (FitLinxx, Norwalk, CT) at
the Duke University site. The total amount of weight lifted in pounds
from a typical single session during week 5 or 6 was used as the
baseline measure of overall strength, and the total from a typical,
single session at the end of training was used as the end-of-training
measure of overall strength.
Exercise training: protocols, ramp period, duration, modes, verification, and adherence. The exercise groups were as follows: 1) RT
(3 days/wk, 3 sets/day, 8 –12 repetitions/set, 8 exercises), 2) AT
[calorically equivalent to 19.2 km/wk (⬃12 miles/wk) at 75% peak O2
uptake (V̇O2)], and 3) AT ⫹ RT (AT/RT), i.e., the full AT regimen ⫹
the full RT regimen.
A ramp period of 8 –10 wk, designed to gradually increase the
amount of aerobic exercise over time, was prescribed to all
subjects in the AT and AT/RT groups. Details about the prescribed
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the ANOVA was significant, a Fisher’s post hoc analysis was performed to determine differences between groups. Post hoc P ⬍ 0.05
was considered significant.
RESULTS
Baseline characteristics, daily caloric and alcohol intake, and
the exercise programs are described in Table 1. There were no
differences in caloric intake or ethanol intake between groups
at baseline. During the 4-mo run-in period, subjects experienced small, but significant, increases in body weight, abdominal subcutaneous fat area, and total abdominal fat area (data
not shown). Caloric intake changes as a result of exercise
training ranged from ⫹13.3 to ⫹72.2 kcal/day, all nonsignificant (P ⬎ 0.30 for all 3 within-group change comparisons).
Daily alcohol consumption also did not change as a result of
exercise training, as changes in consumption ranged from ⫺1.1
to ⫹1.7 g [all changes were nonsignificant (P ⬎ 0.30 for all 3
groups)].
In Table 2, baseline and change scores are shown for the
three intervention groups. In addition, Table 2 includes P
values for the two-tailed t-tests that indicate which withingroup change scores were significant. AT provided a substantial stimulus, as peak V̇O2 increased by 14% in AT and 16% in
AT/RT. RT resulted in a smaller, but significant, increase of
5.5% in peak V̇O2. The RT stimulus was also successful, as the
amount of weight lifted per session increased by 55% in RT
and 41% in AT/RT. In addition to the highly significant
increases in weight lifted, the RT and AT/RT programs resulted in highly significant increases in lean body mass of 1.1
and 0.8 kg, respectively (both P ⬍ 0.001), whereas the AT
group experienced no change in lean body mass (P ⫽ 0.63).
The RT program resulted in a significant increase in body
mass. However, abdominal subcutaneous fat trended toward a
decrease (P ⫽ 0.10), suggesting that the body mass gain was
not due to fat mass gain. No other changes were observed, with
the exception of an unexpected trend toward an increase in
spleen density (P ⫽ 0.06).
In contrast, AT resulted in significant improvements in
nearly every parameter, including a reduction in body mass,
Table 2. Baseline values and change scores for key variables
RT (n ⫽ 52)
AT (n ⫽ 48)
AT/RT (n ⫽ 44)
Variable
Baseline
Change
P value
Baseline
Change
P value
Baseline
Change
P value
Body weight, kg
V̇O2 peak, ml 䡠 kg⫺1 䡠 min⫺1
Strength, kg/session
Visceral fat,‡ cm2
Subcutaneous fat,‡ cm2
Total abdominal fat,‡ cm2
AST,‡ U/l
ALT,‡ U/l
Liver density,‡ HU
Liver fat score,†‡ HU
Spleen density,‡ HU
Liver-to-spleen ratio‡
HOMA,‡ mg 䡠 dl⫺1 䡠 ␮U⫺1 䡠 ml⫺1
88.6 (16)
26.6 (6)
8780 (495)
156 (82)
321 (124)
471 (141)
27.4 (16.9)
29.3 (13.7)
59.2 (7.6)
40.9 (7.6)
50.2 (5.0)
1.17 (0.15)
2.08 (1.1)
0.7 (2.4)
1.4 (3)
4064 (357)
0.8 (19)
⫺8.2 (30)
⫺7.2 (37)
0.7 (8.8)
⫺2.8 (12)
0.4 (4.9)
⫺0.4 (4.9)
1.9 (5.0)
⫺0.02 (0.11)
⫺0.09 (1.3)
0.042*
0.001*
⬍0.0001*
0.8
0.095
0.23
0.57
0.20
0.64
0.64
0.063
0.44
0.63
88.5 (11)
27.7 (6)
NA
190 (106)
307 (83)
497 (132)
26.7 (8.3)
31.7 (17.7)
55.7 (11.0)
44.3 (11.0)
51.8 (4.27)
1.07 (0.24)
2.37 (1.6)
⫺2.0 (3.8)
3.6 (3)
NA
⫺15.9 (34)
⫺25.1 (54)
⫺35.2 (69)
⫺0.0 (9.7)
⫺4.3 (11)
2.5 (5.7)
⫺2.5 (5.7)
⫺1.5 (2.8)
0.08 (0.12)
⫺0.40 (0.8)
0.001*
⬍0.0001*
NA
0.007*
0.010*
0.004*
0.99
0.009*
0.012*
0.012*
0.009*
0.001*
0.004*
90.4 (12)
27.2 (6)
8574 (402)
154 (66)
348 (132)
503 (162)
26.8 (8.7)
31.5 (13.9)
56.6 (12.0)
43.4 (12.0)
50.9 (5.26)
1.13 (0.24)
2.12 (1.2)
⫺2.1 (3.2)
4.0 (3)
3492 (386)
⫺10.9 (33)
⫺28.7 (37)
⫺41.0 (61)
0.7 (6.6)
⫺4.4 (10)
1.8 (5.9)
⫺1.8 (5.9)
0.7 (4.2)
0.04 (0.12)
⫺0.50 (0.9)
⬍0.0001*
⬍0.0001*
⬍0.0001*
0.057
⬍0.0001*
0.0003*
0.48
0.008*
0.079
0.079
0.42
0.19
0.002*
Values are means (SD). NA, not available; AST, aspartate aminotransferase; ALT, alanine aminotransferase; HU, Hounsfield, units; HOMA, homeostatic
model assessment. ‡Number of subjects for RT, AT, and AT/RT, respectively, for visceral, subcutaneous, and total abdominal fat (n ⫽ 39, 36, and 35); liver
enzymes (n ⫽ 47, 46, and 44), liver density and liver fat score (n ⫽ 36, 36, and 35), spleen density and liver-to-spleen ratio (n ⫽ 24, 28, and 22), and HOMA
(n ⫽ 48, 45, and 42). There were no significant baseline differences between groups. Fat and density measures are from CT. †Liver fat score ⫽ 100 ⫺ liver
density. HOMA ⫽ [(fasting glucose (mg/dl) ⴱ fasting insulin (␮U/ml)]/405, which is a measure of fasting insulin sensitivity, with lower numbers being more
sensitive. *Significant change (post vs. pre value) using a paired t-test.
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and actual exercise-training amounts, intensity, and frequency are
provided in Table 1. The aerobic exercise modes included treadmill, elliptical trainers, cycle ergometers, or any combination of
these. As the intensity of the AT program was based on and
maintained using heart rate zones, it was important for the subjects
in the AT/RT group to perform the AT program first, followed by
the RT program.
For subjects randomized to RT, the ramp period began with one set
during weeks 1–2 and two sets during weeks 3– 4, with build-up to the
prescribed three sets on week 5. For subjects in the RT group, three
sessions per week (on nonconsecutive days) of three sets of 8 –12
repetitions on eight Cybex weight-lifting machines, designed to target
all major muscle, groups were prescribed. Throughout the training
intervention, the amount of weight lifted was increased by 5 pounds
each time the participant performed 12 repetitions with proper form
on all three sets on two consecutive workout sessions to ensure a
progressive RT stimulus.
All aerobic exercise sessions were verified by direct supervision
and/or with a heart rate monitor that provided recorded, downloadable
data (Polar Electro, Woodbury, NY). Aerobic compliance percentages
were calculated each week and were equal to the number of minutes
completed within the prescribed heart rate range divided by the
number of total weekly minutes prescribed. All RT sessions were
verified by direct supervision and/or the FitLinxx Strength Training
Partner.
Liver enzymes, insulin, and glucose. Plasma samples were taken at
each time point, and ALT and aspartate aminotransferase (AST) were
measured by conventional spectrophotometric methodology using a
DxC 600 autoanalzyer (instrument and reagents from Beckman
Coulter, Fullerton, CA). These tests conform to International Federation of Clinical Chemistry standardization, utilizing pyroxidal
5-phosphate as a cofactor in the enzymatic determination. Insulin was
determined with immunoassay (Access Immunoassay System, Beckman Coulter), and glucose was determined with an oxidation reaction
(model 2300 Stat Plus, Yellow Springs Instrument, Yellow Springs,
OH). All samples were centrifuged, and plasma was frozen at ⫺80°C.
All posttraining samples were collected within 16 –24 h of the last
training session.
Statistical analyses. Data were analyzed using Statview (SAS
Institute, Cary, NC). Two-tailed, paired t-tests were used to determine
if the posttraining-pretraining difference within each group was significant. P ⬍ 0.05 was considered significant. ANOVA was used to
determine if there were significant differences between groups. When
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EFFECTS OF AEROBIC VS. RESISTANCE TRAINING ON ECTOPIC FAT
DISCUSSION
To our knowledge, this is the first randomized trial to
investigate the effects of RT, AT, and RT/AT on visceral fat
area and fatty liver infiltration (indicated by liver density on CT
and substantiated by circulating ALT levels) in overweight and
obese inactive adults. Although RT and AT are vastly different
in terms of the nature of the training stimulus (i.e., intermittent
vs. continuous contractions, time skeletal muscle is under load,
metabolic pathways utilized, and others), the basis for comparison was that the prescriptions utilized were consistent with
national recommendations for the general population (31).
Thus, information gathered in this study should be useful when
considering the optimal exercise prescription for improving
visceral and liver fat, as well as improving fasting insulin
resistance (HOMA).
In this respect, there were several important findings. 1) A
RT program, even a very substantial one, did not significantly
reduce body mass, visceral fat, liver fat, or ALT liver enzyme
levels. RT also did not reduce total abdominal fat, nor did it
improve fasting insulin resistance. 2) In contrast to RT, a
typical vigorous AT program resulted in significant reductions
in visceral fat, liver fat, and abdominal subcutaneous fat and
also led to improvements in circulating ALT and HOMA
(fasting insulin resistance). The decrease in liver fat in AT
subjects is an important finding, and this is the first time, to our
knowledge, such a decrease has been observed. 3) In a direct
comparison of AT with RT, AT more effectively reduced
Fig. 1. Effects of exercise mode(s) [aerobic training
(AT), resistance training (RT), and AT ⫹ RT (AT/RT)]
on changes in liver fat score [A; 100 ⫺ liver density in
Hounsfield units (HU)], visceral fat (B; cm2), subcutaneous abdominal fat (C; cm2), liver-specific enzyme alanine
aminotransferase (D; ALT, mg/dl), fasting insulin resistance (E; HOMA), and liver-to-spleen ratio (F). †Trend
toward difference from RT (P ⬍ 0.10). Significant difference from RT: ††P ⬍ 0.05; †††P ⬍ 0.01. *Trend
toward significant within-group change score (P ⬍
0.10). Significant within-group change score: **P ⬍
0.05; ***P ⬍ 0.01.
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visceral fat area, subcutaneous and total abdominal fat area,
and, presumably, in fatty liver infiltration as represented by the
combination of a reduction in liver fat score (liver attenuation
on CT) and the liver enzyme ALT. AT resulted in an unexpected reduction in spleen density (P ⬍ 0.01), which was in the
opposite direction of and significantly different from the outcome of the RT program (P ⬍ 0.01). Spleen density was
included to serve as a standard compared with liver density, as
it is expected that spleen density would remain constant. While
we report liver-to-spleen ratio changes, the observed enigmatic
changes in spleen density for AT and RT led us to believe that
the liver-to-spleen ratio may not be an accurate indicator of
changes in liver fat.
AT/RT subjects generally experienced the additive effects of
AT and RT for the outcomes in this study. Where RT had little
or no effect, the effect of AT/RT reflected that of AT alone.
This was true for visceral fat area, ALT, and liver density.
Where RT had a positive effect, the effect of AT/RT resembled
the additive effects of RT ⫹ AT, as in subcutaneous and total
abdominal fat area.
Figure 1 details the results of group comparisons for key
variables. AT led to larger improvements in visceral fat and
liver-to-spleen ratio (P ⬍ 0.05) and also trended toward a
greater reduction in liver fat score (P ⬍ 0.10) than did RT.
AT/RT trended toward larger improvements in visceral fat,
abdominal subcutaneous fat, and fasting insulin resistance
(HOMA) (all P ⬍ 0.10) than did RT. However, AT/RT was not
different from AT for any of these key variables.
EFFECTS OF AEROBIC VS. RESISTANCE TRAINING ON ECTOPIC FAT
AJP-Endocrinol Metab • VOL
liver injury than is AST because of its longer half-life (47 h vs.
17 h). Furthermore, Chang et al. (2) suggest that of the liver
enzymes AST, ALT, and ␥-glutamyltransferase (GGT), ALT is
most closely related to liver fat accumulation, and their finding
of a higher correlation between ALT and the development of
NAFLD might be due to the higher specificity of ALT for liver
injury and/or its contribution as a glucogenic enzyme. They
also indicate that their findings “agree with several previous
studies that ALT is more closely associated than either AST or
GTT with both hepatic insulin resistance and later decline in
hepatic insulin sensitivity.”
The importance of visceral fat and insulin resistance
(HOMA) to cardiometabolic health and disease risk is well
established. It is also known that aerobic exercise training can
significantly reduce visceral fat (5, 11, 20, 21, 27) and consistently improves insulin sensitivity (28). Far fewer randomized
controlled studies have investigated the effects of RT on
visceral fat in overweight or obese subjects. Davidson et al. (5)
observed no effect of RT on visceral fat. However, their study
design involved only 20 min of RT, three times per week, and
therefore cannot rule out that a more substantial program might
have had a significant response. Sigal et al. (26) did not find
that RT had a significant effect on visceral fat; however, nor
did AT in their study. The reason for the difference between
their findings and ours is not clear but may be due to differences in study populations. Schmitz et al. (24), in a 2-yr study,
reported that visceral fat increased by 7% with RT, but this was
significantly less than the 21% increase observed in their
inactive controls. This is an important finding and indicates the
need for more research in this area.
Relevant to and supportive of the findings in this report, we
recently published results from the same cohort of the effects
of AT, RT, and AT/RT on metabolic syndrome score and the
individual components (HDL-cholesterol, triglycerides, waist
circumference, fasting glucose, and blood pressure) (1). RT did
not significantly improve metabolic syndrome or any of the
individual components. AT resulted in a trend toward an
improved metabolic syndrome score (P ⫽ 0.067) and a trend
toward reducing waist circumference. Perhaps particularly relevant to the findings of improved visceral and liver fat observed in this report, AT significantly reduced fasting triglyceride levels, whereas RT had no effect. Interestingly, the
AT/RT group, while not statistically better than AT group for
any variable, experienced the most robust and widespread
benefits and significantly improved metabolic syndrome score,
waist circumference, triglycerides, and diastolic blood pressure. Whether this robust response was due to a synergistic
effect of AT/RT or simply the greater total amount of exercise
cannot be determined from this study design.
Important strengths of this study include 1) the randomized
study design, 2) the inclusion of three training programs in the
same study, 3) direct verification of exercise amount, intensity,
and, therefore, exposure, for nearly all AT, RT, and AT/RT
training sessions, 4) the inclusion of a substantial RT program
that reduces the likelihood that negative findings are due to an
inadequate RT stimulus, 5) a significant proportion of women
and minorities in the study population, 6) the additive nature of
the combination program, permitting the assessment of additive or interacting effects of AT and RT, and 7) the inclusion
of two important ectopic fat depots (liver and visceral fat) and
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visceral fat and improved the liver-to-spleen ratio than did RT
(P ⬍ 0.05), and AT trended toward a larger reduction in liver
fat (P ⬍ 0.10). 4) AT/RT was not superior to AT alone, as
there were no statistical differences between these groups for
any of these variables. That is, for these important physiological variables, there was no additional statistically significant
advantage to adding a substantive RT program (and spending
approximately twice as much time exercising) to a moderate
AT program. It is the overall consistency of these findings of a
greater AT than RT effect on visceral fat and fatty liver
infiltration, rather than any one finding, that is so compelling.
We believe that these data convincingly show that, for overweight and obese individuals who want to lose body weight,
visceral fat, and liver fat and improve liver enzymes and
fasting insulin resistance, AT alone would be the most timeefficient and effective exercise modality.
It is important to point out that RT is known to result in
significantly lower caloric expenditure than a similar amount
time spent in vigorous AT. Davidson et al. (5) estimated that
the typical RT program expended ⬃45% of maximal V̇O2. This
compares with 75% of maximal V̇O2 used in the AT program
from the present study. The result is that ⬃67% more calories
were likely expended in the AT than the RT program. We
would hypothesize that much of the difference in the effects on
ectopic fat is due to the differences in caloric expenditure
between the two training programs.
NAFLD is directly associated with obesity and hepatic
insulin resistance and is an important emerging metabolic risk
factor (4, 30). An elevated liver fat level is an independent
predictor of type 2 diabetes, dyslipidemia, metabolic syndrome, and cardiometabolic abnormalities (4, 18, 33). It is
important to point out that, in this study, it would appear that
visceral and liver fat are not the best markers of fasting insulin
resistance, since visceral and liver fat tend not to decrease as
much in the AT/RT group, whereas HOMA tends to decrease
more. However, liver enzymes, in particular ALT, even within
the normal reference limits, are correlated with and predictive
of incident NAFLD and type 2 diabetes (2, 9, 22). Evidence
from cross-sectional studies suggests that physical activity will
likely reduce liver fat (29). However, we are aware of only
three exercise studies in humans that have examined the effects
of aerobic exercise on liver fat, and none of the studies has
reported a significant effect (3, 8, 25). This is likely explained
by the short duration (6, 10, and 12 wk) and small number of
subjects in all three of these studies, which resulted in a limited
exercise stimulus and a limited statistical ability to detect an
effect. Evidence from the present study demonstrates that a
moderate amount of aerobic exercise training over 8 mo leads
to statistically significant reductions in liver fat that are clinically meaningful as indicated by significant improvements in
ALT. As this is the first study of the effect of RT on liver fat
or liver enzymes, the data would suggest that RT alone or in
addition to AT is not an effective modality for improving these
variables.
While we measured ALT and AST, only one of these
enzymes was affected by training. It is not entirely clear why
only ALT was improved. However, according to a position
stand by the American Association of the Study of Liver
Diseases, ALT is a better indicator of health and disease (10).
While most of this position stand is about ALT, and not AST,
the authors indicate that ALT is a better indicator of chronic
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EFFECTS OF AEROBIC VS. RESISTANCE TRAINING ON ECTOPIC FAT
GRANTS
This study was funded by National Heart, Lung, and Blood Institute Grant
2R01-HL-057354 (clinical trial registration no. NCT00275145).
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the authors.
AJP-Endocrinol Metab • VOL
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two major metabolic plasma risk markers (insulin resistance
and ALT).
An important limitation of this study is that liver density
measures obtained from CT, while highly correlated to liver fat
measures from MRS and liver biopsy studies (13, 17), are not
direct measures of liver fat. However, the changes in liver
density are very similar to the patterns of change across the
groups in visceral fat, ALT, HOMA, subcutaneous fat, and
body weight changes. Another limitation is that iron status was
not measured, and increased or decreased iron levels can affect
liver density. However, numerous studies have shown that
increases in physical activity (the studies are predominantly on
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the case of AT, would be to decrease liver density, which is the
opposite of the effect in this study. There are far fewer studies
of the effect of RT on iron status, but the few studies that do
exist suggest that the outcome is the same, i.e., a similar
reduction, not an increase, in iron status (6). Furthermore, as
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volume, and not an actual decrease in total hemoglobin or iron
status (6).
Finally, while AT and RT are very different training modes
and comparisons between them should be done with this in
mind, it is still important to determine which mode is most
effective for benefiting which specific risk factors.
Future research directions for comparing RT with AT could
include two programs matched for caloric expenditure. This
could be accomplished by comparing a lower-intensity aerobic
exercise-training program than was used in the present study
and a RT program similar to that used in the present study.
These would be similar in total time required. The benefit of
this approach would be that changes in ectopic fat would likely
be very similar for the AT and RT programs (as previous
research has shown that AT effects on ectopic fat follow a
dose-response relationship). This would allow a more direct
comparison of the RT benefits, which are specific to muscle
mass increases, and the AT benefits, which are specific to the
oxidative/mitochondrial changes.
Conclusions. While RT reliably improves strength and increases lean body mass, its effects on central ectopic fat depots
are less clear. The major finding in this study was that, in
sedentary, overweight and obese adults, AT consistently more
effectively improved visceral fat, total abdominal fat, liver fat,
and the liver-derived enzyme ALT than did RT. When RT was
added to AT, there was no additional beneficial effect on these
variables. These data show that, for overweight and obese
individuals who want to reduce body weight and measures of
visceral fat and fatty liver infiltration and also improve fasting
insulin resistance and liver enzymes, a moderate amount of
aerobic exercise is the most time-efficient and effective exercise mode.
EFFECTS OF AEROBIC VS. RESISTANCE TRAINING ON ECTOPIC FAT
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