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 and VO2max after 4 weeks altitude. Supplementation normalized low ferritin levels, maintained normal levels during altitude. RVC and VO2max only 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.	VO2max increase 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.
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. VO2peak increased 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 (CG n = 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. VO2peak, 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 VO2peak. 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 distancerunners	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 fatigue, can increase quality in training, which can increase performance.
Mielgo-Ayuso et al. [41]	22 female volleyball players	Follow up study	Investigate if the benefits of an 11 week oral iron supplementation protocol lasted after cessation, in the remaining 8 weeks of the season. IG (n = 11) 325 mg/day iron, CG (n = 11). Adequate iron >100 ng/mL, functional iron deficiency 30–99 ng/mL, absolute deficiency < 30 ng/mL, anemia Hb < 12 g/dL. Blood tests: Hb, Se-Fe, TSAT, Ht, S-iron at W0, 11, 21 and 29. Strength tests: press, squat, clean, pullover and total mean strength.	IG maintained iron status after 11 weeks, CG decreased. IG back to baseline 10 weeks after cessation (W21) and did not maintain for the rest of the season (W29). IG increased strength more than CG in the first 11 weeks. Opposite in the following 8 weeks (W21-29): CG had better ferritin and Hb than IG, and increased strength.	Benefits of iron supplementation will not last for multiple months after cessation in female volleyball players. Suggesting sustained supplementation during the competitive season to keep iron stores sufficient. Blood assessment to decide if supplementation is needed and to avoid toxicity.
Badenhorst et al. [42]	10 male runners	Crossover study	Assess the influence of a 3 h hypoxic (2900 m) recovery period post-running on exercise inflammation (IL-6), Se-Fe and iron, hepcidin, and EPO. 8 × 3 min run at 85% Blood tests: pre/post run, after 3 h and 24 h recovery in hypoxia.	Similar increase in IL-6 immediately after training. Hepcidin levels elevated 3 h post running, but lower in hypoxic recovery group. No difference in EPO, S-iron, or ferritin between different recovery environments.	Hypoxic recovery, after intense endurance exercise, can lower the hepcidin response to training. Can be useful to potentially increase acute iron absorption from the diet.
Krzywànsk et al. [43]	243 track and field athletes	Prospective study	Asses vitamin B12 status and its influence on red blood cell parameters in elite athletes. SG = 189 and EG = 54 athletes, 1131 blood samples over 6 years. 34% used B12-injections. Red blood cell parameters: Hb, Ht, MCV, MCH and vitamin B12. Deficiency cut vitamin B12 < 197 pg/mL.	Average vitamin B12 739 pg/mL in SG, 881 pg/mL in EG, no deficiencies. Significant relationship between B12 and Hb: increase in Hb from low B12 levels up to 400 pg/mL, and no change in Hb from 700 pg/mL and onwards.	Vitamin B12 range between 400–700 pg/mL might favor Hb synthesis in athletes. Should be monitored regularly and consider oral supplementation < 400 pg/mL.
Mielgo-Ayuso et al. [44]	36 male rowers	RCT	8 weeks vitamin D supplementation and its effect on hematological status, testosterone, and cortisol values. Vitamin D group (n = 18) 3000 IU/day, control group (CG n = 18) given placebo. Blood tests at start/end: Hb, Ht, iron, ferritin, transferrin, 25(OH)D, testosterone, cortisol. Deficiency cut 25(OH)D < 30 ng/mL	8-week vitamin D supplementation prevented a reduction in Hb, transferrin and Ht, improved 25(OH)D levels. No improvement in muscle recovery based on testosterone and cortisol levels, but 25(OH)D can be a predictor of anabolic and catabolic hormones.	Vitamin D supplementation can prevent a reduction in hematological parameters, and potentially contribute to a better transport of oxygen.

Abbreviations: CG = control group, CK = creatin kinase, EG = endurance group, EPO = erythropoietin, erythroferrone = regulates iron by inhibiting hepcidin, ferritin = primary iron storage protein, hepcidin = main regulator of iron homeostasis(inhibitor), Ht = hematocrit: proportion of red blood cells in blood, Hb = hemoglobin: protein carrying oxygen in red blood cells, IG = intervention group, IL-6 = interleukin 6, IV = intravenous, LDH = lactate dehydrogenase(enzyme), MCH = mean corpuscular hemoglobin: average hemoglobin in red blood cells, MCV = mean corpuscular volume: average volume of red blood cells, RCV = red blood cell volume, SG = strength group, sTfR = transferrin receptor: membrane receptor involved in iron supply to the cell by the binding of transferrin, TIBC = total iron binding capacity: bloods capacity to bind iron with transferrin, transferrin = main carrier protein of iron, TSAT = transferrin saturation: ratio of serum iron to TIBC, VD = vitamin D group, vitamin = vitamin, VO_2max = maximum oxygen consumption that can be achieved during physical exertion, VO_2peak = the highest VO₂ value attained on a test were intensity increases until the individual reaches volitional exhaustion, 25(OH)D = measure on vitamin D stores.3.2. Population characteristics.