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Head-to-head evidence comparison — which supplement is right for you?
Caffeine vs Soy Protein: Caffeine has the stronger overall evidence (9.5 vs 6.5/10); 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
Too few graded studies
Top outcomes
100-200mg
Morning, About 60 minutes before exercise
Pill/capsule for precise dosing
20-40g per serving for muscle (aim ~2-3g leucine); ~25g/day is the level associated with cholesterol lowering (the cited meta-analysis averaged 47g/day)
Any time of day
Soy protein isolate
and 3 more not shown
15-45 minutes
30-60 minutes
30-60 minutes
30-60 minutes
6-12 weeks
4-12 weeks
Acute
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).
No Difference Between the Effects of Supplementing With Soy Protein Versus Animal Protein on Gains in Muscle Mass and Strength in Response to Resistance Exercise.
Int J Sport Nutr Exerc Metab (2018) · Meta analysis · n=266
Nine studies (266 participants); soy vs whey/animal protein with resistance training ≥6 weeks
Meta-analysis of the effects of soy protein intake on serum lipids.
N Engl J Med (1995) · Meta analysis
38 controlled clinical trials; mean soy intake ~47g/day
Effects of high-quality protein supplementation on cardiovascular risk factors in individuals with metabolic diseases: A systematic review and meta-analysis of randomized controlled trials.
Clin Nutr (2024) · Meta analysis
63 RCTs across soy, milk, whey, and casein protein
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 6.5/10) and wins in 2 of 3 categories.
No documented interaction was found in the data searched. This does not establish that this exact combination is safe.
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Caffeine, citrulline, beta-alanine, creatine — the ingredients that work, at clinical doses (vs fairy dusting).
The right pick depends on your goals. Answer a few quick questions for a personalised recommendation — or dig into the full evidence on each.