Caffeine
Evidence Fact Sheet
1,3,7-Trimethylxanthine
Caffeine (1,3,7-trimethylxanthine) is a xanthine alkaloid that acts as a non-selective adenosine A1/A2A receptor antagonist. Human meta-analyses report small ergogenic effects on endurance, muscle strength/power, and exercise fat oxidation, plus a clear dose-timed disruption of night-time sleep. FDA GRAS; EFSA authorizes alertness and endurance claims.
Also known as: 1,3,7-trimethylxanthine · Caffeine anhydrous · Guaranine
Overview
Caffeine (1,3,7-trimethylxanthine) is a naturally occurring xanthine alkaloid and one of the most-studied dietary stimulants. Its primary mechanism is non-selective antagonism of adenosine A1 and A2A receptors, which relieves central adenosine-mediated inhibition; secondary actions include phosphodiesterase inhibition, catecholamine release, and facilitation of sarcoplasmic-reticulum calcium release relevant to muscle contraction and fat oxidation. Research protocols commonly use 75-200 mg as a single acute dose for alertness/attention and 3-6 mg/kg body weight 30-60 min pre-exercise for endurance and strength/power outcomes. Regulatory status: FDA GRAS (21 CFR 182.1180) and a lawful supplement ingredient with a recommended ceiling of 400 mg/day for healthy adults (200 mg/day in pregnancy); EFSA authorizes alertness/attention and endurance-performance claims but rejected fat-oxidation and weight-reduction claims; ANVISA permits it with a mandatory caffeine label warning. This is an educational evidence summary, not a dosing or treatment recommendation.
Mechanism of Action
Non-selective adenosine A1/A2A receptor antagonism (relieves central adenosine-mediated inhibition) · Phosphodiesterase inhibition (raises intracellular cAMP) · Catecholamine release promotion · Enhanced fatty-acid oxidation during exercise · Sarcoplasmic-reticulum Ca2+ release facilitation
Body systems: Neurological & Cognitive · Musculoskeletal · Mitochondrial & Cellular Energy · Body Composition · Mood & Stress Response
Evidence-Based Benefits
Each benefit below is anchored to a specific PubMed-indexed study. Effect sizes, sample sizes, and p-values are reported as published; no values are inferred. Honest negatives and null results are kept alongside the positive findings, and disease-research populations are described as such — Caffeine is not characterized as a treatment for any disease.
Endurance exercise performance
Meta-analysis supported- ES = 0.41 ± 0.2time-trial completion
- 3.03 ± 3.07%mean power output
In a pooled meta-analysis of 46 studies, moderate-dose caffeine (3-6 mg/kg) produced a small but consistent improvement in endurance performance versus placebo, with faster time-trial completion and higher mean power output. The authors noted heterogeneous responses, including some trials showing slower performance.
Reported effect: Improvement vs placebo in mean power output 3.03 ± 3.07% (effect size 0.23 ± 0.15) and time-trial completion time 2.22 ± 2.59% (effect size 0.41 ± 0.2)
“an overall improvement following caffeine compared to placebo in mean power output (3.03 ± 3.07%; effect size = 0.23 ± 0.15) and time-trial completion time (2.22 ± 2.59%; effect size = 0.41 ± 0.2)”
Source: PMID 29876876 · Southward 2018 · Sports Med
Muscle strength and power
Meta-analysis supported- SMD = 0.20strength · p = 0.023
- SMD = 0.17power · p = 0.047
- SMD = 0.15lower-body · p = 0.147 (ns)
Pooling ten strength and ten power trials, caffeine ingestion significantly improved maximal muscle strength and jump power versus placebo. The effect reached significance for upper-body strength but not lower-body strength, illustrating a small and region-dependent benefit.
Reported effect: Strength SMD = 0.20 (95% CI 0.03, 0.36; p = 0.023); power SMD = 0.17 (95% CI 0.00, 0.34; p = 0.047); lower-body strength SMD = 0.15 (95% CI -0.05, 0.34; p = 0.147)
“Caffeine ingestion improved both strength (SMD = 0.20; 95% confidence interval [CI]: 0.03, 0.36; p = 0.023) and power (SMD = 0.17; 95% CI: 0.00, 0.34; p = 0.047). A subgroup analysis indicated that caffeine significantly improves upper (SMD = 0.21; 95% CI: 0.02, 0.39; p = 0.026) but not lower body strength (SMD = 0.15; 95% CI: -0.05, 0.34; p = 0.147).”
Source: PMID 29527137 · Grgic 2018 · J Int Soc Sports Nutr
Fat oxidation during exercise
Meta-analysis supported- SMD = 0.73fat oxidation · p = 0.008
- 19 studiesfasted crossover trials
- SMD = -0.33respiratory exchange ratio
Across 19 crossover trials, a moderate pre-exercise caffeine dose (2-7 mg/kg, fasted) significantly raised the fat-oxidation rate during submaximal aerobic exercise, accompanied by a lower respiratory exchange ratio. Note the regulatory contrast: EFSA reviewed and rejected general fat-oxidation/body-fat-reduction claims, so this is an exercise-physiology finding, not a weight-loss claim.
Reported effect: Fat oxidation rate SMD = 0.73 (95% CI 0.19 to 1.27; p = 0.008); respiratory exchange ratio SMD = -0.33 (95% CI -0.65 to -0.01; p = 0.04)
“The meta-analysis revealed that caffeine significantly (p = 0.008) increased the fat oxidation rate (SMD = 0.73; 95% CI = 0.19 to 1.27). This increment was consistent with a significant (p = 0.04) reduction of the respiratory exchange ratio (SMD = -0.33; 95% CI = -0.65 to -0.01)”
Source: PMID 33255240 · Collado-Mateo 2020 · Nutrients
Sleep (honest negative)
Null / no benefit Meta-analysis supported- 45 mintotal sleep time reduced
- 7%sleep efficiency reduced
Across 24 studies, caffeine consumption measurably degraded night-time sleep: it cut total sleep time and efficiency, lengthened time to fall asleep and time awake after sleep onset, and shifted sleep architecture toward lighter stages. The authors estimated coffee should be avoided roughly 8.8 h before bed and pre-workout caffeine ~13.2 h before bed to avoid reductions in total sleep time.
Reported effect: Caffeine reduced total sleep time by 45 min and sleep efficiency by 7%, increased sleep onset latency by 9 min and wake after sleep onset by 12 min; deep sleep (N3/N4) duration decreased by 11.4 min
“Caffeine consumption reduced total sleep time by 45 min and sleep efficiency by 7%, with an increase in sleep onset latency of 9 min and wake after sleep onset of 12 min. Duration (+6.1 min) and proportion (+1.7%) of light sleep (N1) increased with caffeine intake and the duration (-11.4 min) and proportion (-1.4%) of deep sleep (N3 and N4) decreased with caffeine intake.”
Source: PMID 36870101 · Gardiner 2023 · Sleep Med Rev
Cognitive performance / attention
Meta-analysis supported- 13 studiessystematic review
A systematic review of 13 randomized cross-over studies (5 meta-analyzed) found that pooled effects of caffeine reached significance only on attention, accuracy, and speed; across the broader study set, low-to-moderate doses were associated with improved self-reported energy, mood, and attention. The abstract reports the direction of effect but no pooled numeric effect size, so no number is extracted here.
Effect size: this study reports the direction of the finding but does not state a specific numeric effect size, so none is given here rather than estimated.
“After pooling data in the meta-analysis, the significant impacts of caffeine only emerged on attention, accuracy, and speed. The results of the 13 studies, nevertheless, suggest that the intake of a low/moderate dose of caffeine before and/or during exercise can improve self-reported energy, mood, and cognitive functions, such as attention”
Source: PMID 33800853 · Lorenzo Calvo 2021 · Nutrients
Dosage (research context · not a recommendation)
75-200 mg single acute dose for alertness/attention; 3-6 mg/kg body weight 30-60 min pre-exercise for endurance and strength/power; safety ceiling ≤400 mg/day for healthy adults and ≤200 mg/day in pregnancy (EFSA 2015 / FDA); single doses ≤200 mg raise no safety concern. Educational reference, not a dosing recommendation.
Regulatory Status · 4 Markets
- US · FDA
- GRAS (21 CFR 182.1180; cola-type beverages limited to 0.02% / 200 ppm) and a lawful dietary-supplement ingredient under DSHEA 1994; structure/function claims permitted with 30-day FDA notification. FDA recommends ≤400 mg/day for healthy adults. 2018 guidance restricts pure/highly-concentrated bulk caffeine sold direct to consumers (overdose risk).
- EU · EFSA
- Four authorized Reg 432/2012 / Reg (EU) 2017/676 health claims: alertness and attention (Art. 13.1, ≥75 mg/serving) and endurance performance + endurance capacity (Art. 13.5, 3 mg/kg pre-exercise). EFSA 2011 REJECTED fat-oxidation/body-fat-reduction (ID 735, 1484) and energy-expenditure/weight-reduction (ID 1487) claims. EFSA 2015 safety: ≤400 mg/day adults, ≤200 mg/day pregnancy, single dose ≤200 mg of no concern.
- BR · ANVISA
- Permitted in food supplements under the RDC 243/2018 framework (IN 28/2018 authorized-constituent list); energy drinks limited to ≤350 mg/L (RDC 273/2005); mandatory 'CONTÉM CAFEÍNA' label warning plus pregnancy/child cautions. Weight-loss and fatigue-treatment claims are not permitted.
Safety
Common adverse effects at higher intakes: anxiety, insomnia, palpitations, tremor, GI/acid reflux. Withdrawal symptoms (headache, fatigue, irritability) appear 12-24 h after cessation, peaking ~20-51 h. Wide CYP1A2-driven inter-individual variability (~50% slow metabolizers; cardiovascular sensitivity may rise at high doses). Tolerance to alertness/metabolic effects develops over ~7-12 days. Afternoon/evening intake shortens and degrades sleep (Gardiner 2023 meta PMID 36870101). Not for children/adolescents; pregnancy ≤200 mg/day. High-purity bulk caffeine powder carries fatal-overdose risk (FDA 2018). Do not combine with ephedrine/synephrine/DMAA. WADA-monitored (not banned).
References
PubMed-indexed citations anchoring the benefit findings above. Effect sizes are reported as published.
- PMID 29876876 · Southward 2018 · Sports Med — Endurance exercise performance
- PMID 29527137 · Grgic 2018 · J Int Soc Sports Nutr — Muscle strength and power
- PMID 33255240 · Collado-Mateo 2020 · Nutrients — Fat oxidation during exercise
- PMID 36870101 · Gardiner 2023 · Sleep Med Rev — Sleep (honest negative)
- PMID 33800853 · Lorenzo Calvo 2021 · Nutrients — Cognitive performance / attention
Frequently Asked Questions
1. What does the strongest evidence say caffeine actually does?
The best-pooled human evidence is from exercise and sleep research. Meta-analyses report small ergogenic effects: endurance time-trial completion improved with an effect size of 0.41 ± 0.2 across 46 studies (Southward 2018), and muscle strength and power improved with SMDs of 0.20 (p = 0.023) and 0.17 (p = 0.047) across ten trials each (Grgic 2018). A 19-study meta-analysis found caffeine raised exercise fat-oxidation rate (SMD = 0.73, p = 0.008; Collado-Mateo 2020). These are research findings in studied populations, not treatment claims.
2. Are the performance effects large?
No — they are consistently small. The pooled effect sizes are modest (e.g., endurance time-trial effect size 0.41 ± 0.2; strength SMD 0.20; power SMD 0.17), and subgroup analysis showed caffeine significantly improved upper-body but not lower-body strength (SMD 0.15, p = 0.147, non-significant). The endurance meta-analysis also noted heterogeneous responses, with some trials showing slower performance. Caffeine is best characterized as a small, reliable ergogenic aid rather than a dramatic one.
3. Does caffeine affect sleep?
Yes, and this is a well-documented honest negative. A 24-study meta-analysis (Gardiner 2023) found caffeine reduced total sleep time by 45 minutes and sleep efficiency by 7%, increased the time to fall asleep by 9 minutes and time awake after sleep onset by 12 minutes, and shifted sleep toward lighter stages (deep sleep duration fell by 11.4 minutes). The authors estimated coffee should be avoided roughly 8.8 hours before bed and pre-workout caffeine about 13.2 hours before bed to avoid reductions in total sleep time.
4. What doses are used in caffeine research, and is there a regulatory ceiling?
Studies typically use 75-200 mg as a single acute dose for alertness/attention and 3-6 mg/kg body weight 30-60 minutes pre-exercise for endurance and strength/power outcomes. Regulatory note: caffeine is FDA GRAS, with a recommended ceiling of 400 mg/day for healthy adults and 200 mg/day in pregnancy; EFSA authorizes alertness and endurance claims but rejected fat-oxidation/weight-reduction claims. This is educational reference information, not a dosing recommendation.
Last evidence review: 2026-06-27