Iron status in athletic females, a shift in perspective on an old paradigm Iron status in athletic females, a shift in perspective on an old paradigm

Abstract

Iron deficiency is a common nutrient deficiency within athletes, with sport scientists and medical professionals recognizing that athletes require regular monitoring of their iron status during intense training periods. Revised considerations for athlete iron screening and monitoring have suggested that males get screened biannually during heavy training periods and females require screening biannually or quarterly, depending on their previous history of iron deficiency. The prevalence of iron deficiency in female athletes is higher than their male counterparts and is often cited as being a result of the presence of a menstrual cycle in the premenopausal years. This review has sought to revise our current understanding of female physiology and the interaction between primary reproductive hormones (oestrogen and progesterone) and iron homoeostasis in females. The review highlights an apparent symbiotic relationship between iron metabolism and the menstrual cycle that requires additional research as well as identifying areas of the menstrual cycle that may be primed for nutritional iron supplementation. ARTICLE HISTORY

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Iron status in athletic females, a shift in
perspective on an old paradigm
Claire E. Badenhorst , Kazushige Goto , Wendy J. O’Brien & Stacy Sims
To cite this article: Claire E. Badenhorst , Kazushige Goto , Wendy J. O’Brien & Stacy Sims
(2021): Iron status in athletic females, a shift in perspective on an old paradigm, Journal of Sports
Sciences
To link to this article: https://doi.org/10.1080/02640414.2021.1885782
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PHYSIOLOGY AND NUTRITION
Iron status in athletic females, a shift in perspective on an old paradigm
Claire E. Badenhorst
a
, Kazushige Goto
b
, Wendy J. O’Brien
a
and Stacy Sims
c
a
School of Sport, Exercise and Nutrition, College of Health, Massey University, Auckland, New Zealand;
b
Graduate School of Sport and Health Science,
Ritsumeikan University, Kusatsu, Japan;
c
Te Huataki Waiora - School of Health, the University of Waikato, Hamilton, New Zealand
ABSTRACT
Iron deciency is a common nutrient deciency within athletes, with sport scientists and medical
professionals recognizing that athletes require regular monitoring of their iron status during intense
training periods. Revised considerations for athlete iron screening and monitoring have suggested that
males get screened biannually during heavy training periods and females require screening biannually or
quarterly, depending on their previous history of iron deciency. The prevalence of iron deciency in
female athletes is higher than their male counterparts and is often cited as being a result of the presence
of a menstrual cycle in the premenopausal years. This review has sought to revise our current under-
standing of female physiology and the interaction between primary reproductive hormones (oestrogen
and progesterone) and iron homoeostasis in females. The review highlights an apparent symbiotic
relationship between iron metabolism and the menstrual cycle that requires additional research as well
as identifying areas of the menstrual cycle that may be primed for nutritional iron supplementation.
ARTICLE HISTORY
Accepted 1 February 2021
KEYWORDS
Hepcidin; oestrogen;
progesterone; iron stores;
iron regulation; iron
homoeostasis
Introduction
Iron is considered an essential mineral for athletic performance,
supporting the processes of oxygen delivery and energy pro-
duction at a cellular level (Beard, 2001). Symptoms of iron
deciency include lethargy, fatigue, negative mood, and in
cases of iron deciency anaemia, a reduced work capacity
(Pasricha et al., 2010; Sim et al., 2019). This cumulative list of
symptoms is likely to impact an athlete’s training and compe-
titive performances (Sim et al., 2019). As such, researchers in
sport physiology and medicine frequently suggest that iron
status in athletes be routinely measured with appropriate
actions taken to correct deciencies if, and when, required
(Sim et al., 2019). Female athletes are encouraged to undergo
quarterly or biannual iron screenings (dependent of history of
iron deciency) due to higher incidence rates of iron deciency
which, in previous and current literature has largely been
attributed to increased iron loss through menses (Bruinvels
et al., 2016; Mayer et al., 2019; Pedlar et al., 2018). Of note,
female athletes presenting with menorrhagia may have an
exacerbated risk of iron deciency as compared to eumenor-
rheic females with regular or normal blood loss (Bruinvels et al.,
2016; Clancy et al., 2006). Research has demonstrated declines
in iron status in female athletes over prolonged training periods
(Auersperger et al., 2013; Mielgo-Ayuso et al., 2018), with
changes being the result of increased exercise-induced iron
loss. Research on the changes in iron status within the men-
strual cycle of female athletes is limited; therefore, researchers
with an interest in female athletes’ health may have to draw
conclusions from studies in non-athletic populations. Within
the non-athletic populations there is variability in the results
for the change in iron status within the menstrual cycle, with
two studies demonstrating a change in iron status (Heath et al.,
2001; Kim et al., 1993), while others report no change at all
(Belza et al., 2005; Puolakka, 1980). However, as will be dis-
cussed throughout this review, there is evidence from previous
research to suggest that in healthy eumenorrheic females,
changes in iron status will likely occur throughout the men-
strual cycle and this should be a consideration for future
research in female athletes. It should be noted that exercise-
associated menstrual disturbances are well documented in
female athlete health research, with regular menstruation con-
sidered a marker of endocrine and metabolic health. Thus, the
question is raised as to how a marker of female athletic health
may negatively impact an essential mineral required to main-
tain athletic performance and physiological processes such as
energy metabolism and immune function. This article aims to
review and critique the current understanding of iron regula-
tion in females and proposes and challenges the paradigm that
menstrual blood loss increases the risk of iron deciency.
Brief overview of iron and exercise
The adult human body contains approximately 3000–4000 mg
of iron, obtained through dietary intake and recycling of senes-
cent red blood cells. Majority of iron within the body is found in
red blood cells (haemoglobin) and in muscles (myoglobin),
intrinsically linking iron status with oxygen delivery and energy
metabolism (Williams, 2005). Approximately 25% of iron in the
body is stored as ferritin and 5% is bound to transferrin, which
facilitates iron movement into cells (Beard et al., 1996). Daily
iron losses are estimated to be ~1 mg per day due to small
blood losses from gastrointestinal bleeding (Beard et al., 1996).
As there is no specic physiological method for eliminating
CONTACT Claire E. Badenhorst c.badenhorst@massey.ac.nz School of Sport, Exercise and Nutrition, College of Health, Massey University, Level 3, Sir Neil
Waters Extension Building, Fernhill Road, Auckland, New Zealand
JOURNAL OF SPORTS SCIENCES
https://doi.org/10.1080/02640414.2021.1885782
© 2021 Informa UK Limited, trading as Taylor & Francis Group

excess iron from the human body (Sharp & Srai, 2007), iron
levels are tightly regulated, typically favouring iron conserva-
tion and recycling (Nemeth & Ganz, 2006). The regulation of
iron is dependent on hepcidin, a 25-amino-acid peptide hor-
mone (Nemeth, Tuttle et al., 2004), and iron stores are shown to
be the strongest determinant of hepcidin activity in athletes
(Peeling et al., 2017, 2014). Athletes are at a higher risk for the
development of iron deciency due to the cumulative eects of
exercise-associated causal mechanisms for increased daily iron
loss, including haemolysis with foot strike, eccentric muscle
contraction, gastrointestinal bleeding and sweating (Peeling
et al., 2008). However, over the last decade, research into
exercise-associated iron loss in athletes has primarily been
focused on exercise-induced elevation in hepcidin (~3-6 hours
post-exercise), which likely results in a period of altered iron
metabolism during the post-exercise period (Sim et al., 2019).
In iron sucient athletes (serum ferritin >30 ug·L
−1
(Clénin
et al., 2015)), the post-exercise increase in hepcidin levels
appears to occur in response to inammation from the exercise
session, demonstrating the evolution of iron physiology to
respond appropriately to selective internal pressures of infec-
tion. Bacteria rely on iron from the host for their development
(Ratledge & Dover, 2000); therefore, an increase in hepcidin
promotes “iron withholding” within the body and can limit
bacterial growth (Nemeth, Rivera et al., 2004). The iron with-
holding process instigated by hepcidin eectively binds free
iron to storage and transport proteins including ferritin, trans-
ferrin and lactoferrin, preventing the growth of bacteria that
initiate infections (Jurado, 1997). Inammation is the signal to
commence iron withholding within the body, a result that has
been shown in chronic disease in response to C-reactive pro-
tein (Ganz & Nemeth, 2009; Thurnham et al., 2010) and exercise
(Peeling et al., 2009). With multiple factors, including exercise,
inammation and diet, all likely aecting an athlete’s iron
status, this eld of exercise physiology and sport nutrition is
still investigating appropriate methods to support athlete iron
status (Sim et al., 2019) during regular training and
competition.
Moreover, as a consequence of high training loads, endur-
ance athletes are considered an at-risk population for develop-
ing iron deciency. Research suggests that the prevalence of
iron deciency is exacerbated in athletic populations, ranging
from 15% to 35% in female and 3–11% in male athletes (Fallon,
2004, 2008; Malczewska-Lenczowska et al., 2009; Nabhan et al.,
2019; Parks et al., 2017; Sinclair & Hinton, 2005), compared to
females (10–14%) and males (1–4%) in industrialized countries
(Marx, 1997; WHO, 2019). Evidence-based practices primarily
promote increased daily iron intake, via food or supplementa-
tion, as a key intervention for athletes to oset the increased
daily loss of iron through exercise-associated mechanisms
(Nielsen & Nachtigall, 1998; Ponorac et al., 2020). The recom-
mended daily iron intakes for males and pre-menopausal
females of 8 mg and 18 mg per day, respectively, allow for
10% absorption rates, reecting the relatively poor absorption
of iron from food (Monsen et al., 1978). A concern for female
athletes is the lack of total daily intake of iron (Kokubo et al.,
2016; Nielsen & Nachtigall, 1998) that may be driven from
a sociocultural context of specic dietary practices such as
ketogenic (McKay et al., 2019) or plant-based diets (Venderley
& Campbell, 2006). However, recent research suggests that the
original concept of low iron bioavailability in plant-based diets
increases the risk of low iron status, is unsupported (Nebl et al.,
2019). Low energy availability, a precursor to Relative Energy
Deciency in Sport (RED-S), is a dietary concern for athletes,
both female and male (Mountjoy et al., 2014), and may be
a contributing factor to the low iron intake in an athlete’s diet
(Kokubo et al., 2016; Mountjoy et al., 2014). Interestingly, ath-
letes participating in regular and high training loads can lower
their iron stores by 25–40% despite meeting the recommended
daily iron intake through food (13–18 mg·day
−1
) (McKay et al.,
2019). Thus, one may conclude that the current recommenda-
tions for iron intake are insucient for athletes, and that elite
level athletes or individuals undertaking regular high volume
training may require higher intakes of iron to support their
training and adaptation (Thomas et al., 2016). Caution should
be applied when interpreting the decline in iron stores within
a training block, especially if, within the training period,
improvements in performance are observed. In these instances,
the decline in iron status may serve as a marker for physiologi-
cal adaptation to the training stimulus which has been applied.
Therefore, regular monitoring of iron status as a biomarker of
training adaptation and physiological health could be bene-
cial (Pedlar et al., 2019). Regardless, during prolonged training
period, regular monitoring of iron status could assist in appro-
priate nutritional interventions for athletes to ensure that
depletion of iron status does not aect physical performance
(Pedlar et al., 2019). For an in-depth and updated review on iron
regulation in athletes please refer to Sim et al. (2019).
Female athletes and iron status
Only a small proportion of placental mammals present with
menstruation, a process of shedding the endometrial lining in
response to declines in reproductive steroid hormones, indicat-
ing the end of the fertility cycle (Maybin & Critchley, 2011). The
history and evolutionary explanation of menstruation of
women is rich in context and theory (Clancy, 2009; Miller,
2016). Briey, in 1993, it was rst suggested that menstruation
existed to prevent the growth of infectious pathogens in the
uterus (Profet, 1993); however, this theory was quickly dispro-
ven from a lack of evidence (Strassmann, 1996). Strassmann
(1996) proposed that maintaining an endometrium was ener-
getically costly; thus, menstruation could be viewed as a cost-
eective mechanism of the female physiology. Both of the
aforementioned hypotheses however failed to account for
why menstruation has evolved. Recent research has proposed
that menstruation is an evolutionary and mechanistic conse-
quence of spontaneous decidualisation (a hormone-induced
dierentiation of the endometrium) (Emera et al., 2012). This
more recent hypothesis appears to be supported by the corre-
lation between this phenomenon of spontaneous decidualisa-
tion and menstruation patterns across species (Emera et al.,
2012), with evidence that dierentiated endometrial stromal
cells are committed to apoptosis upon progesterone withdra-
wal (Alvergne & Högqvist Tabor, 2018). What should be con-
sidered is that the benets of regularly losing the endometrial
lining as a result of spontaneous decidualisation outweigh the
cost for mothers in the presence of invasive foetus or increased
2C. E. BADENHORST ET AL.

incidence of impaired embryos, thus selective pressures may
have promoted the genetic assimilation of regular spontaneous
decidualisation in the human reproductive system. As this arti-
cle does not seek to discuss the evolution of menstruation, we
can direct readers to in-depth reviews on the topic (Emera et al.,
2012).
Given the proposal that the female reproductive system
likely evolved in line with the cost/benet for the mother,
then the regular loss of iron through menstruation may be
considered to be a substantial cost to female physiology.
Therefore, a review on iron regulation throughout the men-
strual cycle is considered to align with the current evolutionary
view on the development of the menstrual cycle and iron
homoeostasis in females.
At the onset of puberty, there is a divergence in male and
female haemoglobin and red cell mass levels, which is sug-
gested to be due to the onset of menstruation in females and
increased in androgen levels in males (Bhasin, 2003; Miller,
2016). However, there is conicting research evidence in
females for menstruation as a causal mechanism of iron de-
ciency, among both athletes (Petkus et al., 2019) and the gen-
eral population (Belza et al., 2005; Kim et al., 1993; Jukka
Puolakka, 1980). Research indicates that ~1 mg of iron is lost
per day during menstruation (Pedlar et al., 2018). The decline in
iron levels in females as a result of menstruation would theore-
tically provide a causative positive feedback mechanism for the
increase in production of erythropoietin. This would suggest
that there may be sex-specic dierences in erythropoietin and
an increased drive for the production of red blood cell mass in
females throughout the menstrual cycle. However, the refer-
ence ranges for erythropoietin do not vary between the sexes
(Beverborg et al., 2015), and as stated previously, males have
higher red blood cell mass compared to females. In the absence
of hormonal dierences to stimulate increases in red blood cell
production, inference from previous research would suggest
that there are physiological modulations of the Fahraeus eect
that enables more ecient delivery of red blood cells to capil-
lary circulation in females (Murphy et al., 2010). The sex-specic
dierences in the Fahraeus appear to be the result of dier-
ences in blood ow speeds, changes in blood viscosity, eNOS
regulation and vasodilation of vessels that are governed by
changes in female reproductive hormones (Hayashi et al.,
1995; Murphy et al., 2010). Within eumenorrheic females, no
dierence was observed for oxygen-carrying components
including haemoglobin concentration, haematocrit and red
blood cell indices within the menstrual cycle (Mullen et al.,
2020). As such, research indicates that despite having lower
levels of haemoglobin and red cell mass compared to males,
a female can maintain haemoglobin levels and eective tissue
oxygen delivery throughout the menstrual cycle.
Of note, limitations are recognized with haemoglobin mea-
surement in athletes, as the concentration of haemoglobin may
be aected by shifts in plasma volume. Hypovolemia resulting
from training or heat adaptation may result in the decline in
haemoglobin without aecting declines in ferritin or transferrin
saturation, a physiological change known as sports anaemia
(pseudo-dilution of haemoglobin due to training) (Sim et al.,
2019). The degree of hypovolemia in response to training
appears to be dependent on training intensity and duration;
however, research has suggested that training over 10 hours
per week is adequate to induce this pseudo-dilution in some
athletes (Clénin et al., 2015). Interestingly, while hypovolemia
and the inuence on haemoglobin concentration from training
and environmental conditions is well recognized in athletes,
there is currently no discussion on the inuence of menstrual
hormones on haemoglobin concentration when determining
iron status in females. Changes in reproductive hormones
throughout the menstrual cycle are known to inuence body
uid and sodium retention and the distribution of body water
in the extracellular space (Stachenfeld, 2008). In young eume-
norrheic females, cyclical changes in oestrogen and progester-
one shift osmoregulation throughout the cycle, often resulting
in higher rates of uid retention in the luteal phase (Sladek &
Somponpun, 2008). Increased uid retention is also reported as
a unpleasant side eect of the oral contraceptive pill use, with
research demonstrating that active components of the pill
encourage the increase in plasma volume and maintenance
of extracellular uid within the vascular space (Stachenfeld,
2008). In research investigating biological parameter uctua-
tions within the menstrual cycle for the athlete biological pass-
port, one female participant did produce an atypical uctuation
in haemoglobin; this result was attributed to the increase in
plasma volume in the luteal phase (Mullen et al., 2020).
Therefore, it should be noted that increases in progesterone
may be associated with increases in plasma volume retention.
Thus, recognition of the menstrual cycle phase females are in
may be required to accurately interpret haemoglobin concen-
tration and subsequently iron status (Miller, 2016; Sims &
Heather, 2018).
Regular clinical assessment of iron status in athletes
encourages the use of multiple biological markers.
A minimum of ferritin, haemoglobin and transferrin saturation
are required to appropriately determine the progressive stages
of iron deciency (Clénin et al., 2015). While current practice
emphasizes the importance of rest and hydration for accurate
haematological results, there is currently no consideration for
the phase of the menstrual cycle of female athletes and how
this will inuence the presentation of results. The current sport
medicine clinical denition for iron deciency has improved
over the last decade and now recognizes higher levels for
athletes, and also attempts to identify iron depletion (stage 1)
prior to the development of iron deciency anaemia (stage 3).
However, while these cut o ranges would appear accurate in
determining the stages of iron depletion, consideration of their
interpretation for eumenorrheic females should be recognized
(Figure 1).
Menstrual cycle and inuence on iron metabolism
Research has demonstrated the inuence of female-specic
gonadal hormones in red blood cell production, haematocrit
and tissue oxygenation for the given cell mass in females
(Crawford et al., 2006; Hayashi et al., 1995; Murphy et al.,
2010). However, the impact of gonadal sex hormones and
their inuence on iron metabolism, and the subsequent
changes throughout the menstrual cycle, have not been thor-
oughly deliberated. Inference from current research may indi-
cate that females have evolved a mechanism to withhold or
JOURNAL OF SPORTS SCIENCES 3

retain iron throughout their premenopausal years, therefore
aligning with the evolution of menstruation.
It is well known that testosterone plays a specic role in
erythropoiesis, particularly in males (Bhasin, 2003), primarily
through increased secretion of erythropoietin or the stimula-
tion of erythroid progenitor cells (Moriyama & Fisher, 1975). As
mentioned previously, haemoglobin levels prior to puberty are
similar between boys and girls, yet the steady rise in testoster-
one levels in boys from age 13 years is reected in the increase
in haemoglobin levels from this age (Krabbe et al., 1978). In
boys who experience delayed puberty (resulting from delayed
testosterone production), haemoglobin levels do not reect
chronological age and have been associated with the develop-
ment of anaemia (Hero et al., 2005; Krabbe et al., 1978). Higher
baseline testosterone concentrations in males has been attrib-
uted to the sex dierence in absolute haemoglobin levels.
(Ferrucci et al., 2006). Male testosterone concentrations sit
within a constant range of 10.41–34.70 nmol·L
−1
, whereas in
premenopausal females, concentrations range from
0.9 nmol·L
−1
in the low hormone phase, peak at 1.34 nmol·L
−1
around ovulation, then fall again to 1.05 nmol·L
−1
in the luteal
phase (Nóbrega et al., 2009). Of importance, the uctuation of
testosterone in women and the eect on hepcidin and iron
regulation has not been examined and may inuence iron
status over the course of the menstrual cycle.
In cell culture and animal studies, and in females receiving
IVF treatment, oestrogen appeared to markedly suppress hep-
cidin, whereas progesterone increases hepcidin (Bachman
et al., 2010; Hou et al., 2012; Li et al., 2015; Yang et al., 2012),
suggesting that eumenorrheic females will also experience
natural uctuations in hepcidin throughout their menstrual
cycle. To date, there has been only two studies that have
investigated hepcidin uctuations over the menstrual cycle
(Angeli et al., 2016; Lainé et al., 2016) with congruent ndings
of a decline in hepcidin during menses, and a rebound in the
later stages of the cycle (Lainé et al., 2016). Therefore, during
the early follicular phase (days 0–5), the decline in sex and iron
regulatory hormonal activity may facilitate increased iron
absorption and recycling. During this phase, the female body
may be primed to increase its uptake of iron. In the late folli-
cular phase (days 6–14), the gradual rise in oestrogen maintains
low hepcidin activity, enabling iron absorption and recycling in
the days following menses. At ovulation, oestrogen and testos-
terone peak, which may align with increased iron uptake and
erythropoiesis. Therefore, the female physiology may be more
receptive and responsive to nutritional and supplemental iron
treatment in the follicular phase (Figure 2).
Following ovulation, increases in progesterone may increase
hepcidin expression, thus limiting iron utilization (Li et al., 2015)
(Figure 2). The increase in inammatory markers prior to
menses (Sims & Heather, 2018) may exacerbate hepcidin activ-
ity and further limit iron utilization. Previous research results
align with this proposal, demonstrating a rebound in hepcidin,
serum iron and transferrin saturation post-ovulation that stabi-
lizes during the luteal phase (Lainé et al., 2016). Therefore, we
could suggest that the female physiology may not be highly
responsive to iron supplementation in the luteal phase, as
compared to the follicular phase. In addition, higher levels of
Figure 1. Considerations on test set-up and serum markers that should be collected for the accurate determination of iron status in female athletes. This framework is
an extension of the considerations outlined by Sim et al., 2019.
4C. E. BADENHORST ET AL.

serum iron, transferrin saturation and inammation, could be
associated with elevated ferritin in the luteal phase and may
aect iron status interpretation from blood tests. These sugges-
tions further emphasize the importance of recognizing the
phase of the menstrual cycle in iron status monitoring (Figure
1). However, the authors recognize that there is high inter- and
intra-variability in menstrual cycles within females, and as such
it is suggested that a minimum of 3 months of menstrual
tracking be conducted to establish the average cycle length
for a female participant (Sims & Heather, 2018). Since only two
research investigations have considered the menstrual cycle
changes on iron status and neither tracked or established
female menstrual cycle length prior to data collection, the
proposal put forward by this review remains an area in dire
need of further research to clarify the physiological observa-
tions made in this review.
Angeli et al. (2016) indicated that serum iron modied the
release of hepcidin throughout the menstrual cycle; however,
the authors were unable to identify the modulating eect on
hepcidin itself. In two-thirds of the female participants serum
iron did not predict the evolution of hepcidin, and it is possible
that variability in menstrual cycle status (i.e. regular length but
either ovulatory or anovulatory or contraceptive type and
regimes) will inevitably determine the concentrations of endo-
genous hormones within regular cycles (Brown & Thomas,
2011) and as such may be a underlying contributor to the
between-subject variability in the hepcidin production and
elimination. Markers of iron status, ferritin and transferrin
saturation were examined in their model, but showed little
variation throughout the menstrual cycle; however, the partici-
pants were considered a homogenous group based on their
iron status. Several covariates including, biological and demo-
graphic factors were found to inuence both hepcidin and iron
variability throughout the menstrual cycle. Contraception was
associated with higher serum iron levels (Angeli et al., 2016);
however, the type of contraception and length of prescription
regimes was not noted by the researchers, and these two
variables are considered to be key factors in determining the
level of ovarian suppression in females utilizing this form of
contraception (Elliott-Sale et al., 2013; Spona et al., 1996).
Typically, the monophasic pill contains consistent concentra-
tions of oestrogen and progestin, which is then ingested daily
until the placebo pills (Burrows & Peters, 2007). However, the
progestogen component is the active component and depend-
ing on the generation of contraceptive pill, has weak andro-
genic and oestrogenic activity or high androgenic and weak
oestrogenic activity. These factors are rarely considered in
investigations that have assessed iron status and regulation in
females (Angeli et al., 2016; Sim et al., 2015). During the sugar or
placebo pill week, a rebound in endogenous hormones occurs,
raising luteinizing hormone, follicle-stimulating hormone and
particularly oestradiol levels to those that are equivalent to the
early follicular phase in naturally menstruating women (Reape
et al., 2008; Rechichi et al., 2009; Schla et al., 2004; Spona et al.,
1996; Sullivan et al., 1999). With previous research demonstrat-
ing that oestrogen is positively correlated with serum iron
levels in females (Bajbouj et al., 2018), the elevations of serum
iron with contraceptive use (Angeli et al., 2016) during the
withdrawal bleed period is likely to be associated with the
increase in endogenous oestrogen levels. Angeli et al. (2016)
noted that female participants taking the contraceptive pill
maintained higher serum iron concentrations throughout the
cycle (or during the active pill phase), a result that may be due
to several undocumented factors including; dose of ethinyles-
tradiol based on the brand of pill ingested and hepatic meta-
bolism of ethinylestradiol by the individual. The current
Figure 2. Changes in reproductive hormones and hepcidin throughout the menstrual cycle. During menses the decline in hepcidin enables the female physiology to be
more receptive to increasing iron intake during the period of blood loss. During the late follicular phase, increase in oestrogen may facilitate lower levels of hepcidin
thus allowing the female body to recover iron from food and supplement sources following menstrual bleeding. The rebound in the luteal phase aligns with increase in
progesterone, allowing for the stabilization of hepcidin and markers of iron stats in females. In the luteal phase the ability to uptake iron from nutritional and
supplement sources may be lower relative to the follicular phase in eumenorrheic females.
JOURNAL OF SPORTS SCIENCES 5

understanding of the daily combined oral contraceptive pill
hormonal environment is that a surge occurs in synthetic oes-
trogen and progestin, peaking within ~1 hour post ingestion
(~2-6 pg·ml
−1
), then falling rapidly for 6 hours before declining
more slowly. Thereafter, baseline oestrogen accumulates to
reach 2–3 pg·ml
−1
over the 21 day period (Elliott-Sale et al.,
2020, 2013; Reape et al., 2008; Rechichi et al., 2009; Spona et al.,
1996). Samples from Angeli et al. (2016) were collected in the
morning (8–9am) and as such may have occurred within
a period of time where there was a peak in synthetic oestrogen
levels after pill ingestion, that may have elevated serum iron
levels during sample withdrawal. However, the timing and dose
of various and commonly used oral contraceptives on serum
iron measurements was not reported and as such is unknown.
The metabolism of the oral contraceptive pill occurs through
the hepatic cytochrome p450 (CYPs) system, with genetic varia-
bility of these enzymes and the presence of induces or inhibi-
tors largely inuencing the rate at which oral contraceptives are
metabolized (Lynch & Price, 2007). Oestrogen is a recognized
inducer of CYPs (Choi et al., 2011; Zhang et al., 2018), thus the
formulation of the contraceptive pill as noted previously, in
addition to the individual genetic expression of CYPs will likely
determine the duration of the exogenous hormones and the
subsequent impact on serum iron levels. However, to date, the
inuence of the genetic polymorphism of CYPs and rates of
disappearance of exogenous hormones from oral contraceptive
pills of dierent formulations on serum iron levels in females
has not yet been investigated and could explain the variations
that we have seen in the current literature.
Another factor possibly aecting iron availability in females
is the negative correlation between iron absorption and adip-
osity (Angeli et al., 2016); a relationship that may be attributa-
ble to elevated inammatory gene expression with increased
body mass (Engeli et al., 2003). Increased levels of haemoglobin
and ferritin were associated with higher hepcidin levels, a result
that is unsurprising considering the strong inuence of iron
status on hepcidin expression, and may explain the large var-
ibility in baseline iron and hepcidin levels in female participants
(Angeli et al., 2016; Ganz & Nemeth, 2012). Large inter-
individual variability in hepcidin elimination rates, changes in
markers of iron status and the extent of hepcidin rebound at
ovulation were reported, however, the previous studies have
failed to monitor associations to female sex hormone uctua-
tions (Angeli et al., 2016; Lainé et al., 2016). There is large inter-
and intra-individual varibility in female sex hormones between
and within menstrual cycles (Sims & Heather, 2018), as such, the
extent to which variations in sex hormones within females
contribute to the variability in iron, hepcidin and subsequent
iron regulation (absorption and recycling), is still unknown and
is an area that requires investigation.
Further areas that warrant consideration in future research
are variations in testosterone, and whether the ratio of oestro-
gen to progesterone in the luteal phase has any inuence on
hepcidin activity. In addition, a decline in sex hormones with
menopause is likely to aect hepcidin activity. Research has
demonstrated that declines in testosterone with ageing will
reduce iron status in both males and females (Ferrucci et al.,
2006). In animal models, a time-dependent relationship
between oestrogen loss and iron status (Hou et al., 2012)
demonstrated an initial increase in hepcidin followed by
a decrease in hepcidin and an increase in iron levels, which
may be similar to that occurring in menopause (Hou et al.,
2012). Despite a large body of research investigating iron reg-
ulation and supplementation strategies to assist iron balance in
females (Beck et al., 2012; Duport et al., 2003; Februhartanty
et al., 2002; Grondin et al., 2008; Heath et al., 2001; Patterson
et al., 2000) iron deciency is still highly prevalent. Given the
inuence of female sex hormones on hepcidin activity, certain
stages of the menstrual cycle may be primed for absorbing
dietary or supplemental iron. Accordingly, further research is
required to improve the ecacy of existing treatment strate-
gies to ensure that both athletic and general population
females can maintain a healthy iron status.
Changes in hepcidin and spontaneous
decidualisation
For spontaneous decidualisation to occur, researchers have
recognized that there are multiple signalling pathways which
are expressed in the uterus during decidualisation, including
BMP2, WNT, JAK/STAT and cyclic adenosine monophosphate
(cAMP) (Emera et al., 2012). The mechanisms for spontaneous
decidualisation are still under investigation; however, it is
acknowledged that progesterone is required for the process
to occur and be maintain in the luteal phase of the cycle. While
progesterone is needed, research has suggested that cAMP is
essential for inducing the decidualisation process in endome-
trial stroma cells prior to progesterone reaching its peak in
luteal phase. Elevations in cAMP are reported from the secre-
tory phase relative to the proliferation phase of the menstrual
cycle (Tanaka et al., 1993). In response to induced endometrial
stress and gluconeogenic signalling, increases in cAMP and
cyclic AMP response element-binding protein have been
shown to bind to and transactivate the hepcidin promoter,
enabling an increase in serum hepcidin levels that occurs in
the absence of inammatory signals (Vecchi et al., 2014, 2009).
The regulation of hepcidin in response to cAMP response ele-
ment-binding protein throughout the menstrual cycle has not
yet been investigated. Such research may provide insight into
the cellular mechanism associated with iron homoeostasis
throughout the menstrual cycle. This may be of relevance,
especially when considering the associations with the com-
mencement of decidualisation via cAMP, and the uctuations
in hepcidin throughout the cycle that may be initiated by
increases in cAMP in the luteal phase and subsequently sus-
tained by increased progesterone.
Measurement of iron loss in menstrual blood
Traditional methods of determining menstrual blood loss have
relied on largely indirect measurements including pictorial
representation of sanitary napkin saturation (Wyatt et al.,
2001), weight of sanity pads (Heath et al., 1999), self-reported
menstrual disturbances (Bruinvels et al., 2016; Gerlinger et al.,
2007) and questionnaires on menstrual blood loss (Bruinvels
et al., 2016; Heath et al., 1999, 2001). Such data collection is
reliant on accurate reporting on information from participants,
6C. E. BADENHORST ET AL.

and is subject to error, potentially resulting in over or under-
estimation of menstrual blood loss.
Calculations on menstrual iron loss from indirect methods
are based on the ndings from Hallbery and Nilsson (1964),
whereby they utilized blood collected from sanitary napkins
and determined haem iron content from alkaline haematin
methodologies. It is important to note that not all menstrual
blood is lost during menstruation (Strassmann, 1996), and that
the cellular processes for variations in menstrual blood resorp-
tion are still equivocal (Miller, 2016). For example, there is
evidence indicating that menstrual blood loss is sensitive to
energic conditions (Clancy, 2009). In 2010, Miller investigated
the eects of the high physical loads and poor nutritional
intake of Kenyan women, and found higher rates of blood
resorption from the endometrium with higher energy expendi-
ture (Miller, 2010). Moreover, population data on the rates of
menorrhagia and menstrual blood loss support the hypothesis
of a positive association between energetic condition and
blood loss (Miller, 2016). However, there is a lack of controlled
trials to have investigated the variations in menstrual blood
resorption, energetic condition and eects on iron status in the
athletic female population.
If female athletes are able to maintain iron status through
adequate dietary intake and an energetic condition supporting
eumenorrhea, then elevated blood loss during menses may not
increase the risk of iron deciency, but instead, may indicate
endometrial health and eective regulation of iron status
through moderation of hepcidin activity during menses. In sup-
port of this concept, research has demonstrated that at high
altitude eumenorrheic female athletes have higher relative hae-
moglobin mass compared to amenorrhoeic females (Heikura
et al., 2018), and that their physiology responded to the altitude
training camp with a 5.4% increase in relative haemoglobin
mass. Results from this study also demonstrated a strong nega-
tive trend between symptoms associated with low energy avail-
ability assessed using the Low Energy Availability in Females
Questionnaire score and percentage change in haemoglobin
mass. While not the primary ndings of the aforementioned
study, the results as discussed in the context of the current
review may provide support for the premise that the presence
of a regular menstrual cycle facilitates healthy adaptation to
training and environmental conditions (Heikura et al., 2018).
Within the athletic population a state of poor energetic con-
dition that would likely aect reproductive and endometrial
health is referred to as low energy availability (Mountjoy et al.,
2014). This low energetic state has been suggested as the aetiol-
ogy of RED-S and the female athlete triad (Mountjoy et al., 2014).
In an acute setting, research that systematically induced
a nutritional energy decit through a 3-day nutritional interven-
tion, resulted in abrupt elevations in serum hepcidin without
inammation (Hayashi et al. unpublished research). In a recent
review, elevations in hepcidin activity in response to low energy
availability has been suggested as a state of altered iron meta-
bolism (Badenhorst et al., 2019). In a state of low energy avail-
ability, elevated hepcidin activity serves to initiate an “iron
withholding” mechanism similar to that seen when inammation
is present, resulting in the shunting of iron to its storage protein,
ferritin. Female athletes presenting with chronic or persistent low
energy availability may also present with elevated ferritin stores.
However, this is likely due to the increased hepcidin activity in
response to the low energetic condition and may reect a state
of functional iron deciency. Such athletes may be limiting the
utilization of iron and increasing their presentation of symptoms
associated with iron deciency (e.g., fatigue, increased eort,
poor mood and concentration) (Badenhorst et al., 2019; Petkus
et al., 2019). Research that focuses on iron status and utilization,
and low energy availability in athletes is still lacking and may
provide further insights into this specic relationship. This
research would be relevant to female athletes who regain men-
strual cycle function yet are reported to be at an increased risk of
iron deciency. In light of the information presented in this
article such an interpretation would be inaccurate with regards
to the expression of hepcidin activity and its impact on iron
regulation (Badenhorst et al., 2019).
Conclusion
This review has challenged the previous theory on menstrua-
tion as a risk factor for female athletes and their development
of iron deciency. By shifting the perspective and viewing iron
as a fundamental resource to female health, it has allowed the
authors to suggest that female reproductive physiology and
iron metabolism have evolved in a symbiotic relationship. The
proposed relationship between menstruation and iron regula-
tion would support the concept that the human body and its
physiology would not evolve to self-sabotage. In line with the
theory that regular menstruation of the endometrium results
from spontaneous decidualisation (Emera et al., 2012), the
changes in hepcidin activity in response to sex hormones and
iron status within females may provide insight into iron resorp-
tion and utilization throughout the menstrual cycle. This article
sought to provide insight into how the body has adapted to
regulate consistent levels of such a fundamental mineral in
females, especially when adequate iron is supplied through
nutritional intake or when cumulative exercise-associated
mechanisms encourage excess iron loss. The review has high-
lighted several areas of future research including association
with sex hormones and hepcidin expression in premenopausal
and menopausal females and inuence on inter- and intra-
variability in females. In addition, validated investigations on
endometrial health and correlations to iron status, hepcidin
activity and menstrual blood loss in female athletes on both
ends of the menstrual cycle function spectrum (eumenorrheic
through to amenorrhoeic) are still required. Results from this
prospective research may oer insight on sex dierences in iron
status and provide inference for nutritional and supplemental
protocols for athletes that would aid in reducing incidence of
iron deciency.
Disclosure statement
The authors report no conict of interest.
Funding
No external funding was required or provided for this project.
JOURNAL OF SPORTS SCIENCES 7

ORCID
Claire E. Badenhorst http://orcid.org/0000-0002-8434-9730
Stacy Sims http://orcid.org/0000-0002-3255-5428
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