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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
Likely helps
29 of 35 studies with measurable effects showed benefit.
Top outcomes
Verdict
Likely helps
11 of 15 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, 30-60 minutes before exercise
Pill/capsule for precise dosing
18-45mg elemental iron for deficiency (varies by severity)
On empty stomach if tolerated, Away from tea, coffee, dairy
Ferrous bisglycinate (gentle, well-absorbed) or iron protein succinylate
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 · n=2020
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=722
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=572
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).
Based on multiple meta-analyses showing 3-6mg/kg optimal dosing. Performance benefits plateau around 200-300mg with increased jitteriness at higher doses.
Based on meta-analyses showing hemoglobin improvements of 2.01-5.30 g/dL in iron deficient populations. Higher doses show diminishing returns with increased GI side effects. Effectiveness varies significantly by baseline iron status and form used.
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.