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Head-to-head evidence comparison — which supplement is right for you?
Caffeine vs Iron: Caffeine has the stronger overall evidence (9.5 vs 9/10); they're alternatives for boost daily energy — the best pick depends on your goals. Take the 60-second quiz for a pick tailored to your goals.
Caffeine wins 2 of 3 categories. Both are solid choices — the best pick depends on your specific goals.
Verdict
Probably helps
5 of 9 studies with measurable effects showed benefit.
Top outcomes
Verdict
Likely helps
6 of 8 studies with measurable effects showed benefit.
Top outcomes
Shared outcomes (1)
Outcomes where both Caffeine and Iron have evidence — compare verdict strength side-by-side.
100-200mg
Morning, About 60 minutes before exercise
Pill/capsule for precise dosing
18-40mg elemental iron for deficiency (varies by severity) (a typical label dose, not the dose used in the cited trials, which used 60 to 120 mg a day)
On empty stomach if tolerated, Away from tea, coffee, dairy
Ferrous bisglycinate (gentle, well-absorbed) or iron protein succinylate
and 3 more not shown
and 6 more not shown
Coffee can reduce iron absorption by 35-60%
Avoid coffee within 1 hour of iron supplementation.
15-45 minutes
30-60 minutes
30-60 minutes
30-60 minutes
2-4 weeks
4-8 weeks
4-12 weeks
Effect of Acute Caffeine Intake on the Fat Oxidation Rate during Exercise: A Systematic Review and Meta-Analysis
Nutrients (2020) · Meta analysis
A subsequent meta-analysis was performed using the random effects model to calculate the standardized mean difference (SMD).
The effects of caffeine intake on weight loss: a systematic review and dos-response meta-analysis of randomized controlled trials
Critical reviews in food science and nutrition (2019) · Meta analysis · n=606
Thirteen RCTs with 606 participants were included in the meta-analyses.
Effect of Caffeine and Nitrates Combination on Exercise Performance, Heart Rate and Oxygen Uptake: A Systematic Review and Meta-Analysis
Nutrients (2024) · Meta analysis · n=95
The meta-analysis revealed that caffeine and nitrates supplementation (CAF+nitrates) did not enhance performance in time trials (TTs) over the CAF alone (g = -0.06; 95% CI = -0.46 to 0.35; p = 0.78) or nitrates alone (g = 0.29; 95% CI = -0.12 to 0.70; p = 0.17).
Daily oral iron supplementation during pregnancy
The Cochrane database of systematic reviews (2024) · Meta analysis · n=48971
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).
The Prevalence of Anemia among Pregnant Women in China: A Systematic Review and Meta-Analysis
Nutrients (2024) · Meta analysis · n=1376204
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.
Treatment for women with postpartum iron deficiency anaemia
The Cochrane database of systematic reviews (2024) · Meta analysis · n=4558
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).
Exercise trials dose caffeine by body weight, so this curve is in mg per kg, taken about an hour before exercise. A meta-analysis of 46 placebo-controlled trials found a small but clear endurance benefit at moderate doses of 3 to 6 mg/kg (PMID 29876876), and the ISSN position stand reports consistent benefits across that range, a possible minimum as low as 2 mg/kg, and a high rate of side effects at 9 mg/kg with no extra benefit (PMID 33388079), so the curve is flat above 3 mg/kg while side effects rise. 4.5 mg/kg raised time to exhaustion and VO2max in 23 elite endurance athletes (PMID 34033621), and 3 mg/kg raised muscular endurance and strength in 29 resistance-trained adults (PMID 39246027). No single optimal dose is established, and no safe ceiling is plotted: the pregnancy and lactation limits in the safety section apply to total daily intake.
AI-estimated from published studies. Interpret as directional guidance.
Caffeine has a higher evidence score (9.5/10 vs 9/10) and wins in 2 of 3 categories.
For boost daily energy, Caffeine has a higher relevance score (98 vs 90).
Coffee can reduce iron absorption by 35-60% Avoid coffee within 1 hour of iron supplementation. Consult a healthcare provider for personalized advice.
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The right pick depends on your goals. Answer a few quick questions for a personalised recommendation — or dig into the full evidence on each.