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The Physician and Sportsmedicine
ISSN: 0091-3847 (Print) 2326-3660 (Online) Journal homepage: http://www.tandfonline.com/loi/ipsm20
The role of hormones in muscle hypertrophy
Julius Fink, Brad Jon Schoenfeld & Koichi Nakazato
To cite this article: Julius Fink, Brad Jon Schoenfeld & Koichi Nakazato (2017): The
role of hormones in muscle hypertrophy, The Physician and Sportsmedicine, DOI:
10.1080/00913847.2018.1406778
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CLINICAL FEATURE
REVIEW
The role of hormones in muscle hypertrophy
Julius Fink
a
, Brad Jon Schoenfeld
b
and Koichi Nakazato
c
a
Graduate School of Medicine, Department of Metabolism and Endocrinology, Juntendo University, Tokyo, Japan;
b
Department of Health Sciences,
Lehman College, Bronx, NY, USA;
c
Graduate Schools of Health and Sport Science, Nippon Sport Science University, Tokyo, Japan
ABSTRACT
Anabolic-androgenic steroids (AAS) and other hormones such as growth hormone (GH) and insulin-like
growth factor-1 (IGF-1) have been shown to increase muscle mass in patients suffering from various
diseases related to muscle atrophy. Despite known side-effects associated with supraphysiologic doses
of such drugs, their anabolic effects have led to their widespread use and abuse by bodybuilders and
athletes such as strength athletes seeking to improve performance and muscle mass. On the other
hand, resistance training (RT) has also been shown to induce significant endogenous hormonal
(testosterone (T), GH, IGF-1) elevations. Therefore, some bodybuilders employ RT protocols designed
to elevate hormonal levels in order to maximize anabolic responses. In this article, we reviewed current
RT protocol outcomes with and without performance enhancing drug usage. Acute RT-induced hormo-
nal elevations seem not to be directly correlated with muscle growth. On the other hand, supplementa-
tion with AAS and other hormones might lead to supraphysiological muscle hypertrophy, especially
when different compounds are combined.
ARTICLE HISTORY
Received 20 July 2017
Accepted 15 November 2017
KEYWORDS
Resistance training;
endogenous hormonal
elevations; anabolic-
androgenic steroids
Introduction
Although the anabolic effects of anabolic–androgenic steroids
(AAS) are widely recognized, there has been considerable
controversy about the hypertrophic effects of resistance train-
ing (RT)-induced endogenous hormonal elevations. Indeed,
while a recent study found a significant correlation between
RT-induced acute testosterone (T) elevations and long-term
muscle hypertrophy [1], several others failed to observe any
associations between these variables [2–4]. The theory sup-
porting the anabolic effects of RT-induced hormonal eleva-
tions is based on the concept of elevated post-exercise
anabolic hormones binding to hormone receptors and indu-
cing upregulation of several intracellular anabolic pathways
[1]. However, many studies only measured acute hormonal
changes and research about responses several days after train-
ing is lacking. The exact anabolic mechanism of RT-induced
hormonal elevations is not clear, but enhanced protein synth-
esis, decreased protein breakdown [5], satellite cell activation
[6], Wnt signaling [7], and SHBG receptor binding [8] are
speculated to be major factors. In addition, hormones might
act via non-genomic pathways to increase intracellular calcium
[9], thereby allowing for more force production [10] and ulti-
mately leading to improved training intensity and muscular
adaptations [1].
After the discovery and synthesis of T in the 1930s, AAS
were developed for the treatment of various diseases, growth
stimulation, and increase of muscle mass. By the 1970s, the
anabolic and performance-enhancing effects of AAS were dis-
covered by bodybuilders and some high-level athletes who
began to abuse them. Furthermore, the cosmetic effects of
increased muscle mass achieved with AAS led to the wide
spread of AAS abuse among bodybuilders. It has been
shown that supraphysiological levels of T increase muscle
mass not only in elderly and hypogonadal men but also in
healthy young men [11–13]. Previous research on the admin-
istration of T derivatives (AAS) also showed improved body
composition after the treatment [14–16]. Indeed, AAS are
synthetic substances derived from T with several different
anabolic–androgenic ratios, often used by athletes for pur-
poses such as bulking (increase of body mass in the off-
season) or cutting (reduction of body fat while maintaining
muscle mass pre-contest) cycles.
In addition to steroid hormones, naturally released peptide
and protein hormones (PH) such as growth hormone (GH) and
insulin-like growth factor 1 (IGF-1) have also been shown to
increase after RT [4,17]. GH became popular as performance-
enhancing drug in the early 1990s after the development of its
improved recombinant form [18]. The difficulty to detect GH
doping made this drug popular especially among body-
builders. However, even though some studies have found a
correlation between acute RT-induced GH increases and long-
term muscle hypertrophy [17], other studies failed to support
these findings [2,3]. It seems that GH improves body composi-
tion without direct effects on performance [19], making this
drug especially notorious among bodybuilders. Indeed, GH
increases serum IGF-1 concentrations [20] and might therefore
have anabolic effects besides its well-recognized lipolytic
effects.
CONTACT Julius Fink j-fink@juntendo.ac.jp 2-1-1, Hongo, Bunkyo-ku, Tokyo 113-8421, Japan
THE PHYSICIAN AND SPORTSMEDICINE, 2018
https://doi.org/10.1080/00913847.2018.1406778
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Since the anabolic effects of GH are mainly expressed via
the conversion to IGF-1, doping with IGF-1 has gained in
popularity; however, pharmaceutical grade IGF-1 has only
been recognized for human treatment recently and is not
widely available, giving rise to an increasing illicit production
and trade market. IGF-1 is sought for its anabolic properties
such as the ability to increase muscle fiber volume via activa-
tion of surrounding satellite cells [21].
Even though direct anabolic properties have not been
recognized, the use of insulin is also widespread among body-
builders and strength athletes seeking to increase muscle
mass [22]. Insulin has preventive properties with regard to
protein degradation [22] and insulin is a powerful transporter
hormone often used after or pre workout, despite the risk of
hypoglycemia, in order to transport as much nutrients as
possible to the damaged muscle cells. Insulin is also used by
some bodybuilders throughout the day in order to increase
nutrient uptake.
In view of the anabolic effects of AAS and PH on muscle
mass and performance, some bodybuilders and strength ath-
letes started to attempt to increase their natural levels of T,
GH, and IGF-1 via RT. However, the degree and duration of RT-
induced systemic hormonal increases might not lead to similar
effects as seen with exogenous administration of AAS or PH,
often used at doses leading to levels way above physiological
levels for long periods of time. The purpose of this review is to
elucidate the degree of RT-induced acute hormonal elevations
and if they can, to a certain degree, induce similar anabolic
pathways as compared to the exogenous administration of
AAS and PH.
Hypertrophic effects of RT-induced endogenous
hormonal elevations
The effects of RT on hormonal responses have been widely
investigated for several decades [23–25]. Even though broad
guidelines exist regarding which type of RT protocols max-
imize the post-exercise hormonal response [26], the effects of
these acute RT-induced hormonal elevations on long-term
muscle hypertrophy are not currently clear.
Typical bodybuilding-type training protocols that include
large muscle groups with moderate intensity, high volume,
and relatively short rest periods are generally effective in
inducing acute T responses [26]. A positive correlation
(r= 0.76) between acute RT-induced T increases and muscle
cross-sectional area (CSA) has been previously observed [1,27].
However, acute elevations of T after RT last only for about
60 min and the average peak generally does not exceed
650 ng/dL [28], whereas an average dose of testosterone
replacement therapy (TRT) (200 mg of bi-weekly T-enanthate
supplementation), which would be considered a very low dose
among AAS abusers, leads to a sustained average of 815 ng/dL
[29]. Indeed, healthy males produce between 2.1 and 11.0 mg
of T per day [30]. TRT generally consists of a weekly adminis-
tration of 75–100 mg of T or 150–200 mg every 2 weeks [31]
to restore T within mid-normal physiological ranges (400–
700 ng/dL) [32]. As a survey in the bodybuilding community
showed, more than 50% of AAS users use more than 1000 mg
of AAS weekly [33], which is 10 times more than natural T
synthesis or TRT doses, and leads to chronic serum T levels
several times higher than acute endogenous RT-induced T
elevations. From these numbers, we can extrapolate that
minor T raises occurring in response to RT cannot mimic the
effects of large amounts of exogenous AAS administration.
Besides, the various anabolic effects of AAS are much more
pronounced as compared to endogenous T due to the numer-
ous chemical structures of different AAS.
Endogenous GH release seems also to respond to smaller
muscle groups with low to medium intensity and short rest
periods [3], peaking between the period immediately after and
15 min after RT, coming back to baseline values around 60 min
post-RT [3,23,34]. Depending on the muscle group trained, RT-
induced GH responses can reach around 24 ug/L for large
muscle mass (whole body) [23] and around 12 ug/L for small
muscle mass (biceps and triceps) [3]. Studies supporting an
association between acute hormonal responses and long-term
muscle hypertrophy have either found a strong positive cor-
relation between GH and muscle fiber type I (r= 0.74) and II
(r= 0.71) [17] or a weak positive correlation (fiber type I
(r= 0.36) and II (r= 0.28)) [17,35]. Normal GH levels in healthy
males are <5 ug/L and GH-deficient adults are generally pre-
scribed around 0.25–0.5 U/kg weekly in order to restore
healthy GH levels [36], that is, between 25 and 50 U weekly
for a 100 kg individual. However, many bodybuilders seem to
add 2 to more than 15 U daily on top of their natural produc-
tion spread over the day divided in several injections in order
to constantly maintain elevated GH levels. Similar to T, acute
RT-induced elevations in GH may not be large and long
enough to induce similar effects to exogenous recombinant
GH administration.
Several studies have found the correlation between RT-
induced endogenous hormonal increases and muscle hyper-
trophy as trivial to weak [2,3,34,37]. Differences in measure-
ment methods and statistical analyses might contribute to the
discrepancies observed among studies. Indeed, studies report-
ing a correlation between GH and muscle hypertrophy have
measured muscle fiber CSA [17,35] and not total muscle CSA.
On the other hand, one study reporting anabolic effects of
acute T increases measured the correlation between differ-
ences in T responses before and after a 21-week training
period and changes in muscle CSA [27]. The other study
supporting a correlation between RT-induced acute T eleva-
tions and muscle hypertrophy used partial least squares
regression structural equation modeling in order to analyze
the relationships between endogenous responses to RT and
muscle hypertrophy [1]. From the results above and the large
body of evidence showing a lack of correlation between RT-
induced endocrine responses and muscle hypertrophy, it
appears that even if an association between RT-induced
acute hormonal elevations and long-term muscle hypertrophy
exists, the correlation might be so weak that it is undetectable
depending on the study design.
Attempts have been made to determine causality between
acute hormonal fluctuations and hypertrophy by carrying out
longitudinal research on the topic. West et al. employed a
within-subject design whereby 12 untrained young men
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performed unilateral arm curl exercise under two different
hormonal environments on separate days for 15 weeks [34].
One condition involved performance of 3–4 sets of elbow
flexion alone, thereby minimizing the post-exercise hormonal
response while the other condition involved the contralateral
arm carrying out the same elbow flexion protocol, which was
then followed immediately by performance of 5 sets of 10
repetitions of leg press and 3 sets of 12 repetitions of leg
extension/leg curl supersets to induce a high post-exercise
hormonal response. Results showed similar increases in mus-
cle girth between conditions, indicating that the acute sys-
temic response does not impact muscle growth. Subsequently,
Ronnestad et al. employed a similar design to that of West
et al., the primary difference being that the lower body exer-
cise was carried out prior to elbow flexion in the high hor-
mone condition [38]. Contrary to the findings of West et al.,
muscle CSAs of the elbow flexors were greater in the high
hormone condition in the middle aspect of the muscle, sug-
gesting a beneficial hypertrophic effect of acute systemic
elevations. As we can see from the studies above, it is difficult
to make comparisons between studies due to the differences
in study design and measurement methods, especially for
highly fluctuating data such as endocrine hormones (Table 1).
Hypertrophic effects of hormonal elevations induced
via exogenous administration of AAS
Initially developed for medical purposes, AAS have spread to
strength athletes and bodybuilders seeking supraphysiological
strength, endurance and muscle mass while taking serious
risks due to severe side effects [40]. A survey of 500 AAS
users showed that more than 75% of respondents were non-
competitive bodybuilders and non-athletes [33]. Furthermore,
more than half of the respondents acknowledged the use of
more than 1000 mg of T or similar AAS weekly, 25% reported
the use of GH and insulin, and more than 99% admitted
suffering from side effects related to AAS [33].
The administration of supraphysiological doses of T
(600 mg/week) has been shown to improve muscle mass
and strength with or without concomitant RT in healthy men
compared to individuals without T supplementation [41], leav-
ing no doubt of the anabolic effects of supraphysiological
levels of T. Studies on other AAS such as trenbolone, stanozo-
lol, or nandrolone have also shown improved body composi-
tion and/or performance in human or rodent studies [42–44].
Even though many AAS have been developed, only a few are
for human consumption. Steroids like trenbolone are used as
growth promoters in cattle [45]. Several animal studies
[42,46,47] showed the potency of this compound making it
notorious among bodybuilders seeking trenbolone even
though it is not approved for human use. Moreover, the
combined use of AAS and PH has shown the possibility of
even larger improvements in fat-free mass as compared to the
single use of either compound [48], indicating synergistic
effects of AAS and PH. Especially the combination of GH
with insulin and AAS is believed to induce muscle mass
gains going far beyond the use of AAS only. Indeed, muscle
gene IGF-1 expression tends to increase with the combination
of T and GH [48] while insulin reduces proteolysis [49].
Many AAS share the following effects on performance and
muscle mass:
●Increase in satellite cell and myonuclear number leading
to larger mitochondrial areas and lower nuclear-to-cyto-
plasmic ratio [6] which might lead to increased maximal
aerobic capacity (VO
2max
)[50]
●Increased protein synthesis [30]
●Decreased protein breakdown [30]
●Nitrogen retention [30]
●Increase in red blood cells and the following increase of
oxygen delivery to the muscles [51,52]
After the discovery of T in the 1930s, researchers tried to
find new chemical structures minimizing side effects and
maximizing anabolic effects. In many cases this means
increasing anabolic while decreasing androgenic effects.
In order to classify the muscle-building potency of AAS,
an anabolic to androgenic ratio chart is often used by
athletes. However, this ratio is based on the growth rate
of the levator ani muscle versus the prostate in rodents
after treatment with several AAS [53]. Nevertheless, even
though this ratio based on a specific muscle of rodents can
hardly be replicated to humans, it seems that many AAS
abusers still consider it when choosing their performance-
enhancing compound.
The major AAS used by athletes can be divided into three
groups [30]:
(1) Testosterone derivatives (T, Methyltestosterone,
Methandrostenolone,
Chlorodehydromethyltestosterone, Fluoxymesterone,
Boldenone): The compounds in this group are known
to induce fast strength and muscle gains but show a
high rate of aromatization. Due to the high water reten-
tion caused by aromatization, they are mainly used in
bulking cycles for quick mass gains.
(2) Dihydrotestosterone derivatives (Stanozolol,
Oxandrolone, Oxymetholone, Mesterolone,
Methenolone, Drostanolone): Even though most of
these compounds are highly androgenic, they have a
high binding affinity to the androgen receptor and are
potent strength and muscle mass builders. Due to the
DHT structure, these compounds cannot aromatize to
estrogen. Therefore, these compounds are often used
for cutting cycles and pre-contest.
(3) Nandrolone derivatives (Nandrolone, Trenbolone):
Compounds in this group show the highest anabolic
to androgenic ratio and have strong muscle building
effects. However, administration of nandrolone deriva-
tives can result in elevated progestogenic activity. The
use of this group of AAS is versatile and is used for both
bulking and cutting cycles.
Cellular memory can lengthen the effects of AAS even after
discontinuation of use. Indeed, a rodent study showed that
the increase in myonuclei is largely increased by AAS and
persists for several months after discontinuation [54].
Furthermore, these AAS-treated rodents’muscle fiber CSA
THE PHYSICIAN AND SPORTSMEDICINE 3
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grew more than 30% within 6 days in response to overload
after 3 months of discontinuation of AAS treatment [54].
New discoveries in this field do not remain unnoticed by
AAS abusers seeking supraphysiological enhancement.
Indeed, combinations of several performance-enhancing com-
pounds might lead to synergistic effects. The main anabolic
effects of GH are believed to be indirect via conversion of GH
to IGF-1 in the liver triggering the IGF-1-Akt-mTOR pathway
[55–57]. However, GH administration might induce insulin
resistance believed to be the main cause for cardiovascular
risks of increased GH levels [58–62]. The administration of
insulin might restore normal skeletal muscle glucose transport
[63] and revert the effects of elevated GH. Therefore, many
AAS abusers using GH are thought to combine insulin in order
to avoid the negative side effects on insulin sensitivity induced
by GH. Further, besides its anabolic effects, T administration
also has been shown to improve insulin resistance [64,65].
Moreover, since both insulin and IGF-1 receptors are members
of the superfamily of receptor tyrosine kinase (RTKs) [66], some
athletes using GH seem to add insulin in order to increase
serum IGF-1 levels by preventing large amounts of IGF-1
binding to the insulin receptors [67].
Conclusion
In conclusion, acute RT-induced hormonal elevations may
have at best minor effects on muscle hypertrophy. Acute
hormonal responses might give us an indication on the inten-
sity and the following mechanical and metabolic stress of a
given RT protocol but should not be used as causative evi-
dence for a hypertrophic response to exercise.
On the other hand, high doses of AAS and PH have pro-
found effects on body composition by sustaining supraphy-
siological levels of anabolic hormones increasing protein
synthesis, satellite cell, and Wnt pathway activation while pre-
venting protein breakdown. The combination of AAS with GH,
IGF-1, and insulin might lead to supraphysiological levels of
muscle fiber growth (hypertrophy) and increase hyperplasia.
However, high doses of AAS and PH might cause severe side
effects. The fact that many of these compounds are only
available with prescription or are not even produced by phar-
maceutical companies nourishes illegal black markets where
products of questionable quality are sold to unaware athletes
seeking supraphysiological muscle gains. Furthermore, studies
of direct effects of AAS and PH combined with RT on muscle
Table 1. Previous research about the correlation between resistance training (RT)-induced hormonal increases and muscle hypertrophy.
Study Sample size and population Acute changes in hormones
Correlation between endogenous hormonal
increases and muscle size
Ahtiainen et al. [25]n=16
Age = 26.8 ± 3.5 yr
Recreational RT experience
FT, GH, T increased during RT Correlation between the difference in acute T
responses pre and post a 21-week training period
and changes in muscle CSA
Fink et al. [3]n=14
Age = 20.2 ± 0.3 yr
Athletes not involved in RT for
at least 2 years
GH
increased immediately after RT and stayed
elevated for 30 min post-RT
IGF-1 increased immediately after RT
T did not significantly increase
No correlation between hormonal increases and
CSA
Fink et al. [4]n=20
Age = 19.9 ± 1.0 and
19.6 ± 1.0 yr
Athletes with experience in RT
GH increased immediately after RT No correlation between hormonal increases and
CSA
McCall et al. [17]n=11
Age = 18–25 yr
Recreational RT
GH and IGF-1 increased mid- and post-RT
T did not significantly increase
No correlation between GH and CSA but a
correlation between GH and type I and II muscle
fibers
Mitchell et al. [39]n=23
Age = 24 ± 3 yr
Men recreationally active but
without RT for at least 1 year
N/A No correlation between FT, GH or IGF-1 and muscle
fiber hypertrophy
Mangine et al. [1]n=33
Men with at least 2 years of RT
experience
N/A Correlation between T and muscle size
Morton et al. [2]n=49
Age = 23 ± 1 yr
RT for at least the past 2 years
N/A No correlation between acute anabolic hormonal
elevations and long-term muscle fiber
hypertrophy
Rønnestad et al. [38]n=11
Age = 20–34 yr
Untrained men
GH and T increased in the leg + arm RT
No changes in arm only RT
Greater CSA increases in a high hormone condition
as compared to low hormone condition
West et al. [37]n=8
Age = 20.0 ± 1.1 yr
Recreationally active men with
no RT activity over the past
year
GH IGF-1 and T increased immediately after RT
and stayed elevated for 30 min post-RT in the
arm + leg RT protocol
No changes in arm only RT
No improvement in anabolic signaling or acute
post-exercise muscle protein synthesis response
in a high hormone condition
West et al. [34]n=12
Age = 21.8 ± 0.4 yr
Recreationally active men with
no RT experience
GH, T, free T, and IGF-1 increased immediately
after RT and peaked at 15 min post-RT in the
arm + leg RT protocol
No changes in arm only RT
No improvements in total CSA or muscle fiber size
in a high hormone condition
West and Phillips [35]n=56
Young men recreationally active
with no RT activity over the
past 8 months
N/A Weak correlation between C and fiber area and
between GH and fiber area
C: Cortisol; CSA: cross-sectional area, FT, free testosterone, gh growth hormone, I: insulin, IGF-1: insulin-like growth factor 1, T: Testosterone.
4J. FINK ET AL.
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mass and performance are lacking. This lack of knowledge
might be one reason for the ongoing abuse of illicit drugs
among bodybuilders and strength athletes. This field needs
more research in order to prevent AAS and PH abuse among
individuals endangering themselves by using drugs with
effects they are not aware of.
Funding
This paper is not funded.
Declaration of interest
The authors have no relevant affiliations or financial involvement with any
organization or entity with a financial interest in or financial conflict with
the subject matter or materials discussed in the manuscript. This includes
employment, consultancies, honoraria, stock ownership or options, expert
testimony, grants or patents received or pending, or royalties.
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