Iron Status and Physical Performance in Athletes

on is an important mineral in the body, essential for muscle function and oxygen transport. Adequate levels of iron in the blood are necessary for athletes, as iron-deficiency anemia can reduce physical performance. Several studies have investigated iron status and supplementation in iron- deficient athletes, and determined how physical strain can change iron balance and markers related to iron status. The question of how to influence and optimize iron status, as well as other markers that can affect iron metabolism, has been less thoroughly investigated. Therefore, the aim of this review is to take a closer look at the importance of iron values, iron markers, and factors that can change iron metabolism for physical performance and the extent to which physical performance can be influenced in a positive or negative way. A systematic search of the PubMed database was performed, with the use of « iron» or «iron deficiency» or «hemoglobin» AND «athletes» AND «athletic performance» as a strategy of the search. After the search, 11 articles were included in the review after the application of inclusion and exclusion criteria. Major findings include that iron supplementation had the best effect in athletes with the lowest iron status, and effects on physical performance were mostly achieved in those who were originally in a deficit. Iron supplementation could be beneficial for optimal erythropoietic response

After the search, 11 articles were included in the review after the application of inclusion and exclusion criteria. Major findings include that iron supplementation had the best effect in athletes with the lowest iron status, and effects on physical performance were mostly achieved in those who were originally in a deficit. Iron supplementation could be beneficial for optimal erythropoietic response during altitude training, even in athletes with normal iron stores at baseline, but should be performed with caution. Alteration of the hepcidin response can affect the use of existing iron stores for erythropoiesis. Energy intake, and the amount of carbohydrates available, may have an impact on the post-exercise hepcidin response. Optimal vitamin D and B12 levels can possibly contribute to improved iron status and, hence, the avoidance of anemia. Keywords:iron; athletes; sport performance; iron supplementation 1. Introduction Iron is an essential mineral; its main tasks are reversibly carrying oxygen in hemoglobin molecules in red blood cells and in myoglobin in muscle cells. It has other important roles, such as contributing to the electron transport chain, enzymes, DNA synthesis, and energy metabolism [1,2]. Iron is taken up by cells and transported to mitochondria where it can be used to form heme (iron bound to porphyrin), which is the form of iron included in hemoglobin and myoglobin molecules [3]. Iron’s chemistry is important in biological pro- cesses because its ions can give away or accept electrons. It can facilitate redox reactions by working as a cofactor for different enzymes and proteins [2]. Iron can also 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 [4]. To avoid unwanted reactions, iron is transferred in the blood with the transport protein, transferrin [5], and excess iron is stored as ferritin in the liver and the reticuloendothelial system [1,5]. Ferritin is the most used biomarker for iron status [6]. Most of the iron in the body is actively used in hemoglobin, myoglobin, and enzymes. Iron storage is approximately 4 g of iron in men and 2.5 g

blood with the transport protein, transferrin [5], and excess iron is stored as ferritin in the liver and the reticuloendothelial system [1,5]. Ferritin is the most used biomarker for iron status [6]. Most of the iron in the body is actively used in hemoglobin, myoglobin, and enzymes. Iron storage is approximately 4 g of iron in men and 2.5 g of iron in women, but only 1–2 mg is lost per day due to intestinal iron absorption and the efficient recycling system of iron [1,7]. Normal intake of iron is around 10–15 mg each day, but only Life2023,13, 2007.

Life2023,13, 2007 2 of 18 10% is absorbed under normal conditions, when iron losses only occur in small doses due to epithelial desquamation and minor bleeding [7]. Iron balance in the blood is carefully regulated by the peptide hormone, hepcidin, which is produced by the hepatocytes in the liver [8]. Hepcidin is upregulated when serum ferritin is high and is regulated by the need for iron for erythropoiesis. Hepcidin inhibits the absorption of iron by binding to its receptor on ferroportin that transports Fe 2+ from enterocytes to plasma [7,8]. 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 [9–11]. 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 [11,12]. Recent studies have focused on a possible altered hepcidin response with low energy availability (LEA) and a low-carbohydrate or ketogenic diet [13–15]. McKay et al. highlighted, in their review, an increased hepcidin response under both LEA and low-carbohydrate conditions in athletes [15]. Iron deficiency can occur with or without anemia [1,16–18]. Iron deficiency without anemia (IDNA) is diagnosed when ferritin levels are low (<30 mg/L) but hemoglobin levels are normal (>130/120 g/L in men/women). Iron deficiency with anemia (IDA) occurs when low ferritin levels lead to low levels of hemoglobin (<130/120 g/L in men/women), defined by WHO [16,17]. A ferritin cut line of 30 mg/L is normal for adults, but it can vary, and the optimal level of ferritin for athletes is highly debated [19–22]. Because of iron’s role in biological processes, outside of carrying oxygen in hemoglobin molecules, IDNA may have a negative impact on multiple functions. 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 [1,18,23]. Symptoms such as fatigue, reduced concentration, and impaired physical performance can occur with IDNA [17,18]. When IDA occurs, the oxygen-carrying capability

functions. 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 [1,18,23]. Symptoms such as fatigue, reduced concentration, and impaired physical performance can occur with IDNA [17,18]. When IDA occurs, the oxygen-carrying capability in the blood is reduced because of lower hemoglobin levels. This reduces physical capabilities because of lack of oxygen to all cells in the body, including those of working muscles during exercise [24]. A reduction in VO2maxand endurance capacity is likely to appear [25,26], whilst supplementation of iron in IDA athletes can contribute to an increase in hemoglobin and, thereby also an increase in VO2maxand endurance capacity [26,27]. Iron deficiency in athletes is normal, and they are prone to it due to several mecha- nisms, including increased losses of iron during training caused by micro-ischemia, hemol- ysis, sweating, etc. Furthermore, women are more prone than men because of menstrual bleeding [1,11,28]. A link between low energy availability (LEA) and poor iron status in athletes is likely, as the dietary intake of iron may not be sufficient [29]. The inflammatory re- sponse that occurs due to training, with increased IL-6 and hepcidin levels, opens a window where less iron is absorbed and recycled [9,24]. In addition, there is an increased utilization of iron for the increased erythropoiesis and rebuilding processes that occur as a result of training. Training and living at altitude, or in hypoxic environments, leads to an increase in hemoglobin following an increase in erythropoietin (EPO) production [30,31]. A rise in EPO increases erythropoiesis: the production of red blood cells in the bone marrow. Living and/or training at altitude is a widely used regime for endurance athletes to increase the oxygen-carrying capacity of their blood and their endurance capabilities [11,30,31]. Hematological adaptations to hypoxia are dependent on adequate iron stores. Ferritin values > 50 ng/mL are recommended prior to altitude training, due to the increased need of iron in these environments [1]. Okazaki et al. showed, both in a

is a widely used regime for endurance athletes to increase the oxygen-carrying capacity of their blood and their endurance capabilities [11,30,31]. Hematological adaptations to hypoxia are dependent on adequate iron stores. Ferritin values > 50 ng/mL are recommended prior to altitude training, due to the increased need of iron in these environments [1]. Okazaki et al. showed, both in a retrospective study and a prospective study, that iron deficiency inhibited erythropoietic responses to altitude train- ing and highlighted the importance of iron supplementation when aiming for an optimal adaptation to altitude training [32]. 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 [24]. All of these factors can affect endurance, as well as power, speed, coordination, concentration, recovery, and consequently, performance

Life2023,13, 2007 3 of 18 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 [33]. Given iron’s crucial biological role and its impact on performance when deficiency occurs, there are still conflicting findings and debates about how to recommend optimizing iron status for athletes. There are no clear recommendations on how to supplement iron in different forms, monitor blood markers, and adjust other factors that affect iron status. Optimizing these factors may have an impact on how athletes, both with and without iron deficiency, can improve their physical performance. The aims of this study were to investigate the importance of iron status, and factors that can alter iron metabolism for physical performance. It also aimed to determine to what extent physical performance can be influenced positively or negatively in relation to iron status. Furthermore, it sought to determine whether only athletes with iron deficiency experience improved effects with better iron status or if there is room for optimization regardless of the athletes’ initial iron status. • Which blood markers should be emphasized to optimize an athlete’s iron status and prevent its reduction? • What is the significance of iron supplementation for physical performance in athletes with iron deficiency compared to those with sufficient iron stores? With iron deficiency vs. sufficient iron stores? • Is it sufficient to aim for normal iron status in athletes, or can we optimize further by using more specific reference ranges or other markers? 2. Materials and Methods This study followed the guidelines of Preferred Reporting Items for Systematic Re- views and Meta-Analyses (PRISMA) [34]. A structured literature search was performed using the PubMed database. The follow- ing terms were included in the search: “Hemoglobins” or “Iron” or “Iron Deficiencies” or “Hepcidins” or “Ferritins” or “Transferrins” or “Hemoglobin”, AND Athletes or “Athletic performance”. The algorithm for the complete advanced search performed in PubMed is presented in the Supplementary Materials. The search was limited to the last 10 years (2013–2023), and

performed using the PubMed database. The follow- ing terms were included in the search: “Hemoglobins” or “Iron” or “Iron Deficiencies” or “Hepcidins” or “Ferritins” or “Transferrins” or “Hemoglobin”, AND Athletes or “Athletic performance”. The algorithm for the complete advanced search performed in PubMed is presented in the Supplementary Materials. The search was limited to the last 10 years (2013–2023), and to younger athletes aged 19–44 years old. Out of the advanced search, 410 articles were reviewed for selection, based on title and abstract. The following inclusion criteria were applied: - - - - - Including studies of iron’s influence on physical performance, which is required beyond ordinary physical activity in the general population. This includes athletes training with the intention of improving physical performance, and those with higher workload demands for performance enhancement, beyond what is necessary for survival. This excludes studies related to the influence on iron status due to illness, old age, etc. Studies dealing with changes in iron metabolism because of exercise or factors surrounding exercise, but not directly related to iron status and its correlation to physical performance, were also excluded. Out of the 410 articles obtained from the initial search, 384 were excluded, not meeting the present inclusion criteria. Duplicates were checked through EndNote (0), and 26 articles were retained for full review. Of these, 2 were excluded for age, 1 for being non-English- language, 1 for being centered around non-athletes, and 1 for not being an original article. Additionally, 10 articles were excluded for not involving iron’s correlation to physical performance. Hence, 11 articles met the inclusion criteria for this systematic review. The

Life2023,13, 2007 4 of 18 flow diagram of the inclusion/exclusion process is illustrated in Figure. A brief overview of the articles included is given in Table.Life 2023, 13, x FOR PEER REVIEW 4 of 21 article. Additionally, 10 articles were excluded for not involving iron’s correlation to physical performance. Hence, 11 articles met the inclusion criteria for this systematic review. The flow diagram of the inclusion/exclusion process is illustrated in Figure 1. A brief overview of the articles included is given in Table 1. Figure 1. Flow chart of the inclusion/exclusion process of the systematic review. 3. Results 3.1. Literature Review From the articles included, the following information was obtained: authors, sample size and type of sport, population characteristics, experimental protocol (including aim, interventions, cut off values), main outcome, and potential effects on physical performance. The key findings from the 11 articles are summarized in Table 1. Identification Articles identified trough PubMed search (n= 410) Screening Articles in primary screening (n= 410) Non-relevant articles excluded (n= 384) Duplicates (n=0) Articles in secondary screening (n= 26) Records excluded: -non athletes (n= 1) -age not 19-44 (n=2 ) -non-English language (n= 1) -no original article (n= 1) Full-text articles assessed for egilibility (n= 21) Records excluded: -iron deficiency without the influence on perfomance (n= 4) -no affection of iron status (n= 2) -changes in iron markers without affecting performance (n= 3) -iron supplementation without influence on performance (n= 1) Included Studies included (n = 11) Figure 1.Flow chart of the inclusion/exclusion process of the systematic review. Table 1.Summary of the articles in this review with methods, results, and which impact the results can have on performance. Author Population Study Experimental Protocol Main Outcomes Potential Conclusions Okazaki et al. [32] 19 + 39 runners Retro- and prospective study 4 week training camp at 2500 m altitude in athletes with low (n= 9) and normal (n= 10) ferritin. Iron-sufficient athletes in identical altitude exposure (n= 26), against control group (n= 13) at sea level. VO2max and RVC measured before/after. 44–306 mg iron given with Vitamin C Only athletes with normal ferritin levels increased RCV

runners Retro- and prospective study 4 week training camp at 2500 m altitude in athletes with low (n= 9) and normal (n= 10) ferritin. Iron-sufficient athletes in identical altitude exposure (n= 26), against control group (n= 13) at sea level. VO2max and RVC measured before/after. 44–306 mg iron given with Vitamin C Only athletes with normal ferritin levels increased RCV and VO2maxafter 4 weeks altitude. Supplementation normalized low ferritin levels, maintained normal levels during altitude. RVC and VO2maxonly increased in altitude group. Daily iron requirement 1.9 mg/day for men and 2.3 mg/day for women at sea level +4.9 mg/day at altitude. VO2maxincrease in iron-sufficient athletes exposed to altitude. No optimal response with low ferritin levels. Normalized iron stores prior to altitude training are important for erythropoietic adaptation to hypoxic stimulation. Recommends Se-Fe 40–90 ng/mL before 4-week altitude. Córdova et al. [35] 18 male cyclists RCT Effect of iron supplementation during a 3-week stage race on hematological status, S-cortisol, and muscle damage. IG (n= 9) given 2×40 mg iron/day, CG (n= 9) cyclists as controls. Blood test 1 week before and at end of race: S-iron, ferritin, Hb, ht, TIBC, TSAT, sTrF, CK, LDH, cortisol. 80 mg/day of iron prevented decrease in iron, ferritin, Hb, and ht compared to CG. More decreased cortisol levels in supplementation group. No difference in muscle damage, but hematological levels associated with muscle damage biomarkers. Iron supplementation (80 mg/day) can be considered in athletes with hard physical load to prevent decline in iron, and then reduced performance. Can cause lower levels of effort-stress on the body.

Life2023,13, 2007 5 of 18 Table 1.Cont. Author Population Study Experimental Protocol Main Outcomes Potential Conclusions Kasprowicz et al. [36] 20 ultra- marathon runners RCT Vitamin D baseline and its impact on post-exercise S-iron, IL-6 and hepcidin response. VD (n= 10) given 10,000 UI vitamin D/day, CG (n= 10) given placebo, 2 weeks prior to 100 km run. Blood tests prior to supplementation, before the run, after 100 km, and 12 h after finish. Higher vitamin D levels in VD group. No difference in hepcidin and IL-6 response, but VD had less reduction in iron immediately after the run. Correlation between vitamin D and erythropoiesis. Co-occurrence of vitamin D deficiency and anemia. Vitamin D status can affect iron metabolism: high doses can inhibit post-exercise reduction of iron. Suggested 25(OH)D cut > 30 ng/mL. Supplementation doses not justified. McCormick et al. [37] 16 runners Crossover study Iron absorption after training in the morning vs. the afternoon, in runners with Se-Fe < 50 ng/mL. Iron isotopes given in standardized meal after a 90 min 65% run in the morning, or afternoon. Blood tests: IL-6, hepcidin, Se-Fe, Hb and erythrocyte iron-incorporation before, immediately after, 3 h after, and after 14 days. Increased IL-6 after training, hepcidin increased 3 h after training, and showed diurnal tendency; larger increase when training in the afternoon. Iron best absorbed after exercise in the morning compared to the afternoon, and at rest. Endurance athletes can potentially increase iron absorption from the diet by consuming iron shortly after exercise in the morning. Garvican- Lewis et al. [38] 34 endurance athletes (runners, cyclists, triathletes) RCT Hb and erythropoietic response with iron supplementation, iv or orally, during LHTL (3000 m) simulation. Non-anemic endurance athletes were given intavenous or oral iron, or placebo, 2 weeks before, and for 3 weeks LHTL. Blood tests: Hb, ferritin, iron, sTFr, TSAT. Hepcidin, erythroferrone. Hb increased 3.2% in oral, 3.7% in IV supplementation, and no increase in placebo, after 21 days. Ferritin increased more in intravenous than oral. Stable ferritin levels indicate supplemented iron is used for erythropoiesis. Iron supplementation only increased Hb during hypoxic stimulus.

placebo, 2 weeks before, and for 3 weeks LHTL. Blood tests: Hb, ferritin, iron, sTFr, TSAT. Hepcidin, erythroferrone. Hb increased 3.2% in oral, 3.7% in IV supplementation, and no increase in placebo, after 21 days. Ferritin increased more in intravenous than oral. Stable ferritin levels indicate supplemented iron is used for erythropoiesis. Iron supplementation only increased Hb during hypoxic stimulus. VO 2peakincreased with IV supplementation. Iron supplementation is necessary for optimal erythropoietic adaptation to hypoxic exposure. IV iron gave no additional benefit over oral iron in non-anemic athletes. DellaValle et al. [39] 40 female rowers RCT Effects of iron supplementation in non-anemic women. 2×50 mg iron sulfate (IG n= 21) or placebo (CGn= 19) given with citrus juice for 6 weeks. Defined IDNA (Se-Fe < 20 ng/mL) or normal (Se-Fe > 20 ng/mL) prior to intervention. Blood tests: Hb, Ht, Se-Fe and sTfR at baseline/endpoint. VO 2peak, EF and blood lactate after 6 weeks. IG improved Se-Fe, had slower lactate response, and showed better energy expenditure and EF compared to placebo. Both groups improved VO 2peak. Rowers with lower Se-Fe had better improvement in Fe-stores. IDNA affects lactate response without anemia. Improved iron stores, lactate, and EF during endurance training with iron supplementation. Those with lower iron stores at baseline benefit more from supplementation. Supplementation may increase benefits of endurance training when in risk for iron deficiency. Woods et al. [40] 14 dis- tancerun- ners RCT Effect of IV iron in runners with Se-Fe 30–100 ng/mL for 6 weeks training. IG (n= 7) received IV iron, CG (n= 7) received placebo. 3 injections over 4 weeks. Tested: 3000 m TT and 10×400 m monitored session at start and following each injection. Hb in week 0 and 6. TMD (mood) and TFS (fatigue) every second week until week 6. IG increased ferritin to double, no increase in Hb. Increased TFS and TMD in IG compared to placebo. No improvement in physical performance in both groups. 4 weeks intravenous iron did not increase physical performance in athletes with no clinical iron deficiency, training under normal circumstances for 6 weeks. Improved mood and

This systematic review examines the impact of iron status on athletic performance.