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Fe9.0
Iron Research
Likely helps
814 peer-reviewed studies
What the evidence says
Likely helps
Iron appears to help in 12 of 16 studies with measurable effects — the evidence leans clearly favourable.
Most evidence is from high-quality meta-analyses and randomised trials published 1966–2026 with a typical study size of 449 participants.
Based on 814 studies · 146 meta-analyses · 603 RCTs · 615,092 total participants
Confidence
High confidence
What the studies found
12helped2unclear2didn't help· 798 more without graded effect data
By outcome
Iron deficiency & anemia
Likely helps604 studies
Women's healthMay help reduce menstrual discomfort and PMS symptoms · 1-3 cycles · Provides essential nutrients for healthy pregnancy · Ongoing
Likely helps179 studies
Therapeutic & clinical
Probably helps96 studies
Kidney & renal health
Mostly mechanism / observational46 studies
Heart & blood pressure
Mostly mechanism / observational23 studies
Cognitive function
Mostly mechanism / observational17 studies
Endurance & exercise performanceImproved endurance and recovery if deficient · 4-8 weeks · Restores VO2 max and endurance when deficient · 4-12 weeks
Mostly mechanism / observational16 studies
Safety profile
Mostly mechanism / observational16 studies
Digestive health
Mostly mechanism / observational13 studies
Energy & fatigueSignificant energy improvement if deficient · 2-4 weeks for symptoms; 3-6 months to replenish stores
Likely helps12 studies
By the numbers
Pulled from 39 studies with measurable effects
Likely real effects
100%
across studies
People studied
615k
typical study: 449 people
Strongest designs
749
146 pooled, 603 randomised
Showed benefit
75%
12/16 studies
How long studies ran
1–4 weeks
1
1–3 months
4
Populations Studied
Adults2
General population2
Pregnant women1
Women1
Steady research
168 studies in the last 5 years · Latest meta-analysis: 2026
196619962026
1Meta-Analysisn=7,475 · very large study2026
Iron supplementation may lead to improved blood indices and iron stores, but we are uncertain about the effect due to heterogeneity in the population and different methods and doses of iron supplementation.
Geneen LJ et al. · The Cochrane database of systematic reviews (2026)
Overall, we downgraded the certainty of the evidence primarily for risk of bias, high heterogeneity (between-study inconsistency) that was not explained through subgrouping, and imprecision (wide confidence intervals (CIs)), largely due to very small sample sizes in some studies.
More participants may experience adverse events with oral iron compared to no iron, including abdominal pain, constipation, and diarrhoea (RR 1.69, 95% CI 1.15 to 2.47; 8 studies, 2641 participants), but the evidence is very uncertain.
Serum ferritin may also be higher with IV iron at the first donation since commencement of treatment (MD 78.16, 95% CI 37.20 to 119.12 ng/mL higher; 2 studies, 479 participants), but the evidence is very uncertain.
2Low birthweightMeta-AnalysisCited 22×n=48,971 · very large study2024
Daily oral iron supplementation during pregnancy may reduce maternal anaemia and iron deficiency at term.
Finkelstein JL et al. · The Cochrane database of systematic reviews (2024)
Noticeable benefit
← WorseNo effectBetter →
There is probably little to no difference in maternal death (2 versus 4 events, RR 0.57, 95% CI 0.12 to 2.69; 3 trials, 14,060 women; moderate-certainty evidence).
Infant outcomes: Women taking iron supplements are probably less likely to have infants with low birthweight (5.2% versus 6.1%; RR 0.84, 95% CI 0.72 to 0.99; 12 trials, 18,290 infants; moderate-certainty evidence), compared to placebo or no iron supplementation.
However, the evidence is very uncertain for infant birthweight (MD 24.9 g, 95% CI -125.81 to 175.60; 16 trials, 18,554 infants; very low-certainty evidence).
3Lung cancer riskMeta-AnalysisCited 8×n=21,943 · very large study2024
However, a positive association was observed between dietary heme iron and the risk of lung cancer in women, which may require further investigation.
Mohammadzadeh M et al. · International journal for vitamin and nutrition research. Internationale Zeitschrift fur Vitamin- und Ernahrungsforschung. Journal international de vitaminologie et de nutrition (2024)
Noticeable harm
← WorseNo effectBetter →
Methods: We searched PubMed, Web of Science, Scopus and Google scholar for eligible articles published through May 2023 reporting the Relative Risk (RR), Hazard Ratio (HR) or Odds Ratio (OR) with 95% confidence interval (95% CI).
There were no significant associations between the highest dietary total iron (heme and non-heme) (RR: 1.09, 95% CI: 0.78 to 1.51) or heme iron (RR: 1.01, 95% CI: 0.73 to 1.38) intake compared to the lowest intake with lung cancer risk.
However, in the subgroup of women (cases n=5074), heme iron was associated with a 14% increase in the risk of lung cancer (RR: 1.14, 95% CI: 1.01 to 1.29).
4Iron deficiency at termMeta-AnalysisCited 19×n=2,822 · very large study2023
Daily iron supplementation in iron replete non-anemic pregnant women probably reduces the risk of maternal iron deficiency anemia at term and low birthweight.
Hansen R et al. · Acta obstetricia et gynecologica Scandinavica (2023)
Large benefit
← WorseNo effectBetter →
In addition, it may reduce iron deficiency at term (RR: 0.74, 95% CI: 0.60-0.92; 4 RCTs, 1663 women; I2 = 58%; low-certainty evidence) and the incidence of small for gestational age babies (RR: 0.39, 95% CI: 0.17-0.86; 1 RCT, 213 infants; I2 not estimable; low-certainty evidence).
Daily iron supplementation in iron replete non-anemic pregnant women probably reduces the risk of maternal iron deficiency anemia at term and low birthweight.
5Hemoglobin levelsMeta-AnalysisCited 4×n=529 · large study2024
ALA supplementation had no statistically significant effect on iron-related parameters.
Sharifi-Zahabi E et al. · International journal for vitamin and nutrition research. Internationale Zeitschrift fur Vitamin- und Ernahrungsforschung. Journal international de vitaminologie et de nutrition (2024)
Noticeable benefit
← WorseNo effectBetter →
Likely real
In subgroup analysis, ALA significantly increased hemoglobin in patients with hematological disorders (WMD = 1.23 g/dL; 95% CI: 1.00, 1.45 g/dL; I2 = 96.6%, p < 0.001) and in studies with durations longer than 8 weeks (WMD = 1.03 g/dL; 95% CI: 0.82, 1.25 g/dL; I2 = 96.5%, p = 0.02).
ALA supplementation had no statistically significant effect on iron-related parameters.
6Anemia prevalenceMeta-AnalysisCited 19×n=722 · large study2024
It was found that dietary factors were one of the major causes of anemia, and iron-containing supplements and nutrition counseling could be effective interventions to reduce the prevalence of anemia, ID, and IDA among Chinese pregnant women.
Zhou Y et al. · Nutrients (2024)
No clear effect
← WorseNo effectBetter →
The results showed that the prevalence of anemia, ID, and IDA among pregnant women in China were 30.7% (95% CI: 26.6%, 34.7%), 45.6% (95% CI: 37.0%, 54.2%), and 17.3% (95% CI: 13.9%, 20.7%), respectively.
Generally, lower prevalence was observed in the economically more advanced eastern region of the country, while the prevalence of ID was higher in the eastern region than that in the western region.
The prevalence of anemia and IDA in rural areas was higher than that in urban areas, but ID prevalence was higher in urban areas.
7Fatigue reductionMeta-AnalysisCited 5×n=572 · large study2024
Intravenous iron probably reduces fatigue slightly in the early postpartum weeks (8 to 28 days) compared to oral iron tablets, but probably results in little to no difference after four weeks.
Jensen MCH et al. · The Cochrane database of systematic reviews (2024)
Noticeable benefit
← WorseNo effectBetter →
Likely real
Intravenous iron versus oral iron supplementation The evidence is very uncertain about the effect of intravenous iron on mortality (risk ratio (RR) 2.95, 95% confidence interval (CI) 0.12 to 71.96; P = 0.51; I² = not applicable; 3 RCTs; 1 event; 572 women; very low-certainty evidence).
Intravenous iron probably results in a slight reduction in fatigue within 8 to 28 days (standardised mean difference -0.25, 95% CI -0.42 to -0.07; P = 0.006; I² = 47%; 2 RCTs; 515 women; moderate-certainty evidence).
Oral iron probably increases the risk of constipation compared to intravenous iron (RR 0.12, 95% CI 0.06 to 0.21; P < 0.001; I² = 0%; 10 RCTs; 1798 women; moderate-certainty evidence).
8Sports performanceSystematic ReviewCited 12×n=669 · large study2025
However, endurance performance improved by 2%-20% when ID athletes were treated with 100 mg/day of elemental iron for up to 56 days via oral supplementation, or bi-daily via parenteral administration over 8-10 days.
Pengelly M, Pumpa K, Pyne DB, Etxebarria N · Journal of Sport and Health Science (2025)
9Meta-AnalysisCited 158×n=451,723 · very large study2020
In addition, they further contribute to the ongoing discourse of choosing antenatal MMN over IFA as the standard of care in LMICs.
Oh C et al. · Nutrients (2020)
IFA supplementation showed notable improvement in maternal anemia and the reduction in low birthweight, whereas LNS supplementation had no apparent effect on outcomes; further research that compares LNS and MMN supplementation could help understand differences with these commodities.
For single micronutrient supplementation, improvements were noted in only a few outcomes, mainly pre-eclampsia/eclampsia (calcium), maternal anemia (iron), preterm births (vitamin D), and maternal serum zinc concentration (zinc).
These findings highlight that micronutrient-specific supplementation should be tailored to specific groups or needs for maximum benefit.
10Haemoglobin concentrationMeta-AnalysisCited 125×n=8,506 · very large study2016
Daily iron supplementation effectively reduces the prevalence of anaemia and iron deficiency, raises haemoglobin and iron stores, improves exercise performance and reduces symptomatic fatigue.
Low MS et al. · The Cochrane database of systematic reviews (2016)
Noticeable benefit
← WorseNo effectBetter →
Women receiving iron had a higher haemoglobin concentration at the end of intervention compared to women receiving control (mean difference (MD) 5.30, 95% CI 4.14 to 6.45, 51 studies, 6861 women, high quality evidence).
Women receiving iron had a reduced risk of iron deficiency compared to women receiving control (RR 0.62, 95% CI 0.50 to 0.76, 7 studies, 1088 women, moderate quality evidence).
Seven trials recruiting 901 women reported on 'any side effect' and did not identify an overall increased prevalence of side effects from iron supplements (RR 2.14, 95% CI 0.94 to 4.86, low quality evidence).
11Blood iron parametersMeta-AnalysisCited 9×n=449 · medium study2024
The doses of OIS, that induced a beneficial effect on hematological parameters differed from 16 to 100 mg of elementary iron daily, over the period between 6 and 8 weeks.
Šmid AN, Golja P, Hadžić V, Abazović E, Drole K, Paravlić AH · Sports Medicine (2024)
12Gastrointestinal side-effectsMeta-AnalysisCited 516×n=3,168 · very large study2015
Our meta-analysis confirms that ferrous sulfate is associated with a significant increase in gastrointestinal-specific side-effects but does not find a relationship with dose.
Tolkien Z et al. · PloS one (2015)
Huge harm
← WorseNo effectBetter →
Likely real
Ferrous sulfate supplementation significantly increased risk of GI side-effects versus placebo with an odds ratio (OR) of 2.32 [95% CI 1.74-3.08, p<0.0001, I2 = 53.6%] and versus i.v. iron with an OR of 3.05 [95% CI 2.07-4.48, p<0.0001, I2 = 41.6%].
Subgroup analysis in IBD patients showed a similar effect versus i.v. iron (OR = 3.14, 95% CI 1.34-7.36, p = 0.008, I2 = 0%).
Likewise, subgroup analysis of pooled data from 7 RCTs in pregnant women (n = 1028) showed a statistically significant increased risk of GI side-effects for ferrous sulfate although there was marked heterogeneity in the data (OR = 3.33, 95% CI 1.19-9.28, p = 0.02, I2 = 66.1%).
13Hemoglobin levelsMeta-AnalysisCited 7×n=120 · medium study2024
There is moderate-quality evidence suggesting that probiotics, prebiotics, and synbiotics may improve anemia management, particularly by enhancing Hb levels.
Hu Q et al. · BMC gastroenterology (2024)
Noticeable benefit
← WorseNo effectBetter →
Likely real
The analysis revealed that probiotics, prebiotics, or synbiotics significantly improved Hb levels in patients with anemia (WMD = 10.760, 95% CI: 4.593 to 16.747, p = 0.001), though heterogeneity was high (I² = 96.5%).
Two RCTs (n = 120 participants) reported significant increases in serum iron levels in the probiotic group (WMD = 3.835, 95% CI: 3.271 to 4.400), with moderate heterogeneity (I² = 38.7%).
Two RCTs (n = 192 participants) reported no significant differences were observed between the groups in serum ferritin levels (WMD = 8.048, p = 0.115), and heterogeneity remained high (I² = 62.6%).