Iron

Iron is a mineral that the body needs for growth and development. Your body uses iron to make hemoglobin, a protein in red blood cells that carries oxygen from the lungs to all parts of the body, and myoglobin, a protein that provides oxygen to muscles.  Without iron, hemoglobin can't be formed, and fewer red blood cells are produced.

Iron is an important mineral in the body, particularly for athletes. 

On this page, we will review what iron is, how it works in the body for an athlete, how low iron levels impact your performance, and risk factors for an iron deficiency.  Iron is best obtained through the diet, and it is useful to have your levels measured as athletes may have a higher demand for iron.   

Key Roles of Iron

-Oxygen Transport

70% of the iron in our bodies binds to hemoglobin, which is a protein in our red blood cells.  The rest binds to other proteins, such as myoglobin, transferrin, and ferritin, or is stored in the cells.  Hemoglobin and myoglobin are the two main proteins in charge of delivering oxygen to the body (Beard, 2001).

Hemoglobin is responsible for carrying oxygen from the lungs to working muscles and other body’s tissues. Myoglobin resides within muscle cells and is responsible for intra-cellular oxygen transport and temporary oxygen storage (NIH Fact Sheet, 2015). 

-Immune Function
Iron also supports immunity and development as a component of proteins and enzymes that fight oxidative stress, helping athletes ward off illnesses and infections that could disrupt their training and performance (Zhang &B Liu, 2020; Moreira et al, 2020).

-Energy Production
Iron is essential for the proper functioning of mitochondria, the cellular powerhouses responsible for energy production. Iron deficiency can lead to fatigue and decreased exercise capacity.  Without iron, you cannot efficiently produce ATP, the body’s primary energy source. (Davis et al, 1982; Davies et al, 1984; Finch et al, 1976; Clenin et al, 2015; Paul et al, 2017).  This makes iron essential for athletic performance. 

-Cognitive Function:
Iron plays a role in cognitive function, concentration, and memory, which are critical for athletes to make quick decisions during competition (Pivina et al, 2019; Larsen et al, 2020).

Iron Balance in the Body

Ferritin is a protein that stores iron in cells in an inactive form.  Ferritin is essential for maintaining iron balance as it controls the release of iron when levels are too low or high.  If your iron stores deplete, due to low diet intake, the ferritin will release the iron it has stored, acting as a buffer against both iron deficiency and iron overload.  Ferritin is found in many body cells, especially in the liver, meaning that the majority of the body’s iron is stored in the liver (60%), with the remaining found in muscle tissue, and the reticuloendothelial system, which forms part of the immune system (Moustarah & Daley, 2023). 

The blood level of ferritin (referenced as sFer/serumFerritin) is the measure typically used to predict total body iron stores.  When your ferritin levels are adequate, you have enough iron in your body.  If ferritin levels are too low, you are short on iron.

The primary mechanism for maintaining iron homeostasis is a tightly regulated feedback system coordinated by hepcidin, a peptide hormone produced by the liver.  Hepcidin inhibits the absorption of iron. Hepcidin is upregulated when serum ferritin is high, which then limits the iron available (Garza et al, 1997; Ganz, 2011; Goodman et al, 2011).

Hepcidin production has also been shown to be stimulated by inflammatory markers such as interleukin-6 (IL-6), which is upregulated with the inflammation response happening after training (Kong et al, 2014; Dominguez et al, 2018).  This response is also dependent on the baseline levels of ferritin before training; athletes with lower ferritin levels have a lower hepcidin response after training (Peeling et al, 2014).

This control mechanism of iron in the body is important since excess iron can be detrimental to your performance, due to its ability to react with oxygen and generate radical oxygen species, leading to tissue damage (Schmidt, 2015). Accordingly, hepcidin acts as a protective mechanism against excessive iron accumulation. 

Types of Iron

Two types of iron can be found in foods, including heme and nonheme. Heme iron is present only in animal products such as meat, fish, and poultry, whereas nonheme iron is found in fruits, vegetables, dried beans, nuts, grain products, and meat (Hurrell, 1997).  Heme iron is absorbed with better efficiency from the intestine than nonheme iron (Roughead et al, 2002).  The consumption of iron-containing food is one of the main factors determining the body iron status.

Heme iron contributes 10–15% of the total iron intake. However, since heme iron is absorbed better than nonheme iron, with an approximate 15–35% of absorption, it can account for more than 40% of total intestinal iron absorption (Hurrell & Egli, 2010).   Red meat is the most significant source of iron since it is rich in heme iron, which is highly bioavailable (Tamburrano et al, 2019; Czerwonka & Tokarz, 2016).

Non-heme iron is found in meat products as well, and also in some vegetables, fruits, nuts, beans, and grains and iron-enriched or fortified foods such as iron-fortified cereal (Blanco-Rojo & Vaquero, 2019; McDermid & Lonnderdal, 2012).   By contrast, only about 2-20% of non-heme iron is absorbed, due mainly to the fact that it has more inhibitors reducing its bioavailability. Non-heme iron is inhibited by calcium, and additionally bran, cellulose (fiber), pectin (in ripe fruits and vegetables, and jams), phytic acid (in grains and beans), and polyphenols (cereal, beans, tea, and coffee).

Iron Absorption

It is important to be aware of the absorption of iron from your food intake, as the items in your meal can greatly impact how much iron you actually absorb.  For example, consumption of an iron-fortified milk product that supplied 100% of the required daily iron intake did not improve iron status in iron-deficient women over four months in a randomized controlled study. The authors concluded that the presence of calcium and casein in the product is the reason for the lack of absorption (Toxqui et al, 2013).

Research shows that nonheme iron absorption can be improved with the addition of animal tissues, such as beef, chicken, fish, port, and lamb.  Veal muscle, veal liver, and fish were demonstrated to enhance nonheme iron absorption by 150% in human subjects consuming meals of maize and black beans (Cook & Monsen, 1976; Layrisse et al, 1968; Bjorn-Rasmussen & Hallberg, 2004).

Adding a vitamin C food source to your meal also enhances non-heme absorption.  The effect of vitamin C has been proven to be dose dependent (Mao & Yao, 1992; Davidsson et al, 1998) and can increase the absorption of iron only when both nutrients are consumed together (Cook & Reddy, 2001).   It has been reported that iron absorption gradually increases from 0.8% to 7.1% when increasing the amounts of ascorbic acid (ranging from 25 to 1000 mg) added to a liquid formula meal such as a protein shake that contains 4.1 mg of nonheme iron (Teucher et al, 2004).  Ascorbic acid aids iron absorption by creating a chelate or combining with ferric iron Fe³⁺  (Lynch & Cook, 1980).

Phytate and polyphenols are the major iron absorption inhibitors in plant-based foods because they make a complex with dietary iron in the gastrointestinal trac (Schonfeldt et al, 2016).   Phytate is a naturally occurring component found in plants, and it has an inhibitory effect on the bioavailability of most minerals (Me et al, 2009; Harland & Morris, 1995).  Phytate cannot be digested by the human body and cannot be absorbed in the small intestine.  As a result, minerals chelated in phytic acid are not bioavailable (Brouns, 2022).  

Polyphenols are found in the human diet mainly due to their presence in vegetables, cereals, spices, tea, coffee, red wine, and cocoa (Petry et al, 2010).  Polyphenols are known iron bioavailability inhibitors and are assumed to work similarly to phytate by forming a complex with iron.  The inhibitory impact of polyphenols on iron absorption has been reported in numerous studies (Hurrell et al, 1999; Bezwoda et al, 1985; Kim et al, 2008).   

Cast iron pots and cookware can also be a source of significant quantities of dietary iron (Sharma et al, 2021).  The amount of iron doubled in meat and vegetables and increased by 1.5 times in legumes when cooked in iron pots, as compared with other two pots (Xing et al, 2018).

Iron Deficiency

Getting enough iron is one of the requirements for a healthy life.   The average daily nutrient intake level for healthy people to meet the adequate nutrient needs of the body, have been established by the Food and Nutrition Board, Institute of Medicine (IOM) (IM, 2001).  Maintaining optimal iron status is considered fundamental for sport performance, athletic training and health.  However, iron deficiency is common in athletes (Beck et al, 2021).

Iron deficiency can occur with or without anemia (Clenin et al, 2015; Percy, et al, 2017; Balendran & Forsyth, 2021; Soppi, 2018).  

There are three levels of deficiency.

-Storage Iron Depletion (ID), is where the iron stores are depleted but there is sufficient functioning iron to produce red blood cells.  Iron deficiency results from an inadequate supply of iron to cells following depletion of the body’s reserves. Iron depletion has a prevalence of 2-5% in the developed world.  This level is not related to declines in athletic performance or general health.

-Iron Deficiency Non Anemia, (IDNA) is where hemoglobin levels will test normal, but serum ferritin is low (below 20-30 mg/L) 2). Production of new red blood cells is impaired, meaning that oxygen transport around your body is also impaired, reducing performance (Clenin et al, 2015; Soppi, 2018).  Metabolic systems with iron-containing proteins can be affected by IDNA itself, such as reactions in the respiratory chain where iron works as a cofactor, thereby reducing oxidative capacity, which again reduces the muscles’ ability to use oxygen (Reinke et al, 2012).  Symptoms such as fatigue, reduced concentration, and impaired physical performance can occur with IDNA (Balendran & Forsyth, 2021).   

-Iron Deficiency with Anemia (IDA) is when iron stores in ferritin and transport iron have been sufficiently depleted such that the body can no longer keep up with the demands of hemoglobin synthesis, resulting in low levels of hemoglobin (<130/120 g/L in men/women) (Percy et al, 2017). At this stage, individual hemoglobin concentrations fall below two standard deviations of the distribution average for hemoglobin in a healthy population of the same gender and age and living at the same altitude (below 13g/dL in men and below 12 g/dL in women) (WHO, 2001). In this condition, there is inadequate iron to support normal red blood cell formation. 

When IDA occurs, the oxygen-carrying capability in the blood is reduced because of lower hemoglobin levels. This results in decreased oxygen delivery to active tissues and lower energy production (ATP synthesis).  This reduces physical capabilities because of lack of oxygen to all cells in the body, including those of working muscles during exercise (Beard, 2001; Harper & Conrad, 2015; Myhre et al, 2016; Sim et al, 2019). A reduction in VO_2max and endurance capacity is likely to appear.  Iron deficiency is typically associated with impaired aerobic power, with the magnitude of the expected performance reduction related to the severity of the ID (Greg et al, 1985; Davies et al, 1984).

Iron deficiency can affect several of the abilities that athletes need to perform outside of aerobic abilities, including those related to strength, the immune system, fatigue, and mood status.  All of these factors can affect endurance, as well as power, speed, coordination, concentration, recovery, and consequently, performance in various sports variables. Notably, endurance athletes are a frequently studied group when it comes to iron deficiency, because of iron’s role in aerobic metabolism, and the high prevalence of iron deficiency in endurance athletes (Rubeor et al, 2018; Sim et al, 2019).

Iron Deficiency among Athletes

Iron deficiency (ID) is a widely reported issue in athlete populations, at a rate of approximately 15–36% of female and 3–11% of male athletes (Fallon 2004, 2008; Malczewska et al. 2001; Parks et al. 2017; Roy et al, 2022).   In dancer populations, 15% -28% of female adolescent dancers were found to have IDA (ferritin <12 ug/L, Hemoglobin <125 g/L) Mahlamaki & Mahlamaki, 1988; Beck et al, 2015).  Iron deficiency is the most common overall nutrient deficiency in sports (Mehta et al, 2018)

Smaller cohort studies report higher rates of compromised iron stores across a variety of sports settings, with the prevalence reported as >50% in female, and up to 30% in male athletes (Koehler et al. 2012).

In sports the rate of iron deficiency is distinctly higher up to 52% in female adolescent athletes (Sandstrom et al, 2012; Latunde-Dada, 2012; Dubnov et al, 2006] and occurs more often in endurance sports and in disciplines with a high prevalence of eating disorders. 

Several other studies have shown a performance impairment in nonanaemic iron-depleted endurance athletes (Hinton & Sinclair, 2007; Brownlie et al, 2004; Hinton et al, 2000).  One study found that rowers with IDNA had lower VO2peak and higher lactate concentrations (Dellavalle & Haas, 2012). 

A recent study of female military personnel found that iron stores (serum ferritin) decreased from 57 to 38 μg L 1 over 16 weeks of basic combat training (Martin et al., 2019) in New Zealand. As such, the recommended daily intake of iron sufficient to meet the nutrient requirements of 97–98 per cent the population may need to be increased for athletes when compared to age and sex-matched populations (Commonwealth Department of Health and Ageing Australia Ministry of Health New Zealand National Health and Medical Research Council, 2006))

There are several mechanisms which put athletes at risk for iron deficiency, including increased the inflammatory responses from training, and the losses of iron during training caused by micro-ischemia, hemolysis, sweating, etc. Furthermore, women are more prone to iron loss than men because of menstrual bleeding (Dominguez et al, 2018; Roy et al, 2022). 

The inflammatory response that occurs due to training, with increased IL-6 and hepcidin levels, opens a window where less iron is absorbed and recycled (Peeling et al, 2008; Peeling, 2010; Sims et al, 2019).  The increases in hepcidin activity lead to a decrease in iron absorption and recycling from the gut, respectively.  In addition, there is an increased utilization of iron for the increased red blood cell production and the rebuilding processes that occur as a result of training.  Regular exposure to periods of altered iron homeostasis post-exercise have been shown to reduce iron stores by 25–40% over a 6-week training period (McKay et al., 2019).

Training and living at altitude, or in hypoxic environments, leads to an increase in hemoglobin following an increase in red blood cell production (erythropoietin (EPO) which may positively improve exercise performance (Hahn & Gore, 2012; Bonato et al, 2023).

Inference from previous research has suggested that for every 1 g L 1 decrease in Hb, there is a 1.04 kcal increase in energy expenditure (Hinton et al, 2000) decreasing the aerobic exercise efficiency and increasing the work capacity in iron deficiency anemic athletes (Hinton et al, 2000; Garvican et al, 2011).

Female Athletes

There is a greater prevalence of iron depletion (ID) or iron depletion without anemia (IDNA) in female athletes.  An estimated 11% of male and 35% of female athletes suffer from ID (Beard & Tobin, 2000; Malczewska et al. 2001; Dubnov and Constantini 2004). Across 24 different sports, the prevalence of low ferritin (<35 ug/L) was almost double in females compared to males (57% vs 31%) (Koehler et al, 2012).

Active women are estimated to be twice as likely to present with IDNA compared to sedentary women (Sinclair and Hinton, 2005), with 24-47% of exercising women experiencing IDNA (Rowland, 2012).  In 193 elite young German athletes (Mean age 16.2 years) ~ 50% female, the prevalence of low ferritin (< 35 ug/L) was almost double in females compared to males (57% vs. 31%) (Koehler et al. 2012).

This higher risk for female athletes may reflect the increased iron demand to account for blood loss from menses (Loveless, 2017; Cook & Monsen, 1976; Pedlar et al. 2018). Female athletes are at an even higher risk of iron deficiency due to monthly blood loss from the menstrual cycle. The current recommendation for iron in females is 18 mg/day, however, athletes may need to consume more than this to keep stores adequately full (Sim et al, 2019).

The recommended daily intake for iron consumption in females are more than double that of males (18 versus 8 mg/day). However, this RDI can be quite difficult for females to achieve as their generally smaller body size means a lower absolute energy intake is required when compared with males.  To compensate for the greater iron needs, female athletes may need to eat more nutrient dense foods to achieve their dietary iron target (Beard & Tobin, 2000).

Complicating this issue, female athletes commonly follow diets that are restrictive, including vegetarian or vegan diets low in quality (haem) iron sources (Sim, 2019), or those limiting either carbohydrate or energy intake (Castell et al, 2019; McKay et al, 2020; McKay et al, 2022), which have been shown to contain lower amounts of dietary iron. Accordingly, inadequate energy intake, and therefore lower dietary iron intake, contributes to the higher incidence of iron deficiency seen in female athletic populations (Coates et al, 2017).

Iron deficiency in female athletes may also be the result of intense physical training (Alaunyte et al, 2015; DellaValle, 2013).  Endurance athletes use oxygen at a higher rate which drains their iron stores.  For example, one study confirmed that ferritin levels decreased in female athletes over a cross-country season (Martin et al, 2019).

Performance

Generally, the research indicates that low iron levels can impact performance, and those with low iron levels benefit the most with iron supplementation. 

A study of highly trained distance runners (13 male and 14 female athletes) divided into two groups, those with deficient iron stores (ferritin <15 mcg/l-36 mcg/l) or those with better iron stores (ferritin < 65 mcg/l).  Both groups were given 6 weeks of IV iron or six weeks of oral iron intake.  Both forms of supplementation increased ferritin levels, but only the groups with deficient iron stores showed an increase in VO2max and run time to exhaustion (Garvican et al, 2014). 

The 2015 meta-analysis by Burden (et al 2015) supported this finding and found that iron treatments improve the iron status and aerobic capacity of iron deficient non-anaemic endurance athletes, as measured by VO2max.  On the other hand, a systematic review of 12 studies involving 283 participants concluded that iron supplementation did not improve performance in 50% of the IDNA athletes, using a ferritin cut of <20 µg/L  (Rubeor et al, 2018). They assessed performance using parameters such as run trials and time to fatigue, rather than solely relying on VO_2max, which makes interpretation of the results different compared to the Burden study.

Dellavalle, et al. (2014) studied the potential link between iron depletion without anemia (IDNA) and performance in a cohort of 165 female rowers from 5 universities. Serum ferritin and training group were positively related to VO2 max. 40 rowers were given the iron supplement, Ferrous Sulfate or a placebo.  Those who received the iron supplement improved their iron stores, had a slower lactate response and showed greater improvements in energy expenditure.  Rowers with normal iron status trained 10 minutes longer per day on average and had a higher VO2 max.

The most recent review made the same conclusions.  Iron supplementation has the most pronounced impact on physical performance in athletes with lower ferritin levels, demonstrating significant improvements in iron-deficient athletes while offering limited benefits to athletes with sufficient iron stores (Solberg & Reikvam, 2023).

Under exceptional circumstances of environmental stress, such as altitude training designed to stimulate red cell production, iron supplements should be considered in athletes with suboptimal ferritin stores, in an effort to meet the additional erythropoietic demands of the hypoxic stimulus (Sim et al, 2019; Stellingwerff et al, 2019).  This would only be done under the supervision of a medical professional.

Synchrony with other vitamins.

-Vitamin D

Research shows that vitamin D supplementation can prevent a decline in iron and hemoglobin levels.   Kasproviz (et al, 2020) and Mielgo-Ayuso (et al, 2018) demonstrated that vitamin D supplementation could prevent a decline in both iron and hemoglobin levels [36,44]. They showed that when levels of vitamin D were <30ng/mL there was a significant relationship with impaired oxygen transport which negatively impacted physical performance.  Vitamin D supplementation could prevent a decline in hemoglobin, hematocrit, and a transferrin, potentially contributing to better oxygen transport and improved performance.  Ensuring that vitamin D levels remain above this threshold would possibly help athletes prevent a decline in iron and hemoglobin levels.

-Vitamin B
Krzywánsk (et al, 2020) discovered a significant relationship between vitamin B12 and hemoglobin. Athletes using B12 injections had higher levels of hemoglobin and hematocrit.  Since hemoglobin is an important marker for red blood cell status, they recommended serum levels of vitamin B12 between 400–700 pg/mL for optimal hemoglobin levels, with regular monitoring to consider supplementation when vitamin B12 levels are <400 pg/mL.

Recommended Daily Intake

Current recommended intakes are listed below.  This does not include the increased demands that athletes may face:

 

Women

14-18 years: 15 mg/ day

19-50 years: 18 mg/ day

51+ years: 8 mg/ day

Men

14-18 years: 11 mg/ day

19-50 years: 8 mg/ day

51+ years: 8 mg/ day

Iron status can be determined through a simple blood test analysis of serum ferritin and hemoglobin.

It is likely that athletes have a higher iron requirement than the general population. For instance, despite consuming iron at the RDI (13–18 mg/ day), a block of intensified training was shown to reduce ferritin concentrations by 25–40% in a group of international level endurance athletes (McKay et al. 2019). This suggests that the current iron recommendations for the ‘general population’ may not be sufficient for athletes, supporting the case for athlete-specific recommendations to be developed (Thomas et al, 2016).

When iron deficiency occurs, correcting the diet is normally the first step, aiming for 14 mg of iron ingested each day (Clenin et al, 2015). For athletes, it is important to include meat, fish, whole grains, and green vegetables in their diet, as those foods contain heme iron which has better absorption and is more bioavailable.  Food rich in vitamin C can increase iron absorption, while polyphenols found in coffee, tea and certain plants, can inhibit iron absorption (Beard & Tobin, 2000).

Taking an oral supplement is not recommended unless an athlete has been diagnosed with IDA, iron depletion with anemia, and is being professionally monitored and supervised.  Taking an excess amount of iron can be dangerous, as iron can be toxic at high levels because its redox capacity can contribute to the formation of reactive oxygen species (ROS), which can cause cell damage and cell death (Swanson, 2003; Ishibashi et al, 2017).

Athletes are advised to regularly monitor their iron status and take immediate action if deficiency occurs (Clenin et al, 2015; Sim et al, 2019).  The only populations other than IDA athletes that may benefit from an iron supplement are those that are intentionally undergoing hypoxic conditions to increase their red blood cell density (Williams, 2005).

In summary, it is important for all athletes to ensure that your diet contains sufficient amounts of foods with iron.  It would be useful to have a test done of your ferritin and hemoglobin levels to ensure that you have adequate stores.  If you test low, it would be important to work with a medical professional to raise your iron intake.

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