Caffeine

Download this page as PDF · Summary

1. What Caffeine Is

Caffeine is a purine alkaloid that acts as a stimulant in the central nervous system. In the studies it appears as an antagonist at adenosine receptors; the receptor variant ADORA2A and the metabolising enzyme CYP1A2 are investigated as potential modifiers of its effects. The substance features prominently in sports nutrition, cognitive research, and safety assessment. Accordingly, most studies stem from performance and tolerability research rather than therapeutic investigation.

Crucial for evaluating all studies is the form of intake. The material distinguishes between pure substance, caffeine-containing coffee, and multi-ingredient preparations in which caffeine is combined with other substances. Mixture studies cannot establish the effect of the single substance; this also applies to research on pre-workout supplements. Habituation also plays a role: several studies explicitly examine habituated individuals, and one study describes the time course of tolerance to performance effects. Measurements are taken across highly diverse levels — performance tests, cognitive tasks, questionnaires, physiological markers such as muscle oxygenation. These levels are not equivalent: a marker is not a disease course, a test score is not everyday performance.

2. How Robust the Research Is

The research is unevenly distributed. The largest block consists of controlled human trials, alongside foundational and observational studies. By application area, safety and tolerability, fatigue and exhaustion, and cognition lead, while musculoskeletal, metabolic, and cardiovascular topics are less represented. Only individual studies exist on gut, inflammation, and skin. Animal and cell culture studies are marginal; statements on performance are almost exclusively based on human research. The most frequently cited work across the entire material is a position statement by the International Society of Sports Nutrition on caffeine and athletic performance.

Methodologically, the field is exceptionally well-structured: randomised, double-blind crossover designs are standard, and there is a systematic review with meta-analysis on cognition in sport. These works underpin the claims on performance. Transferability is weaker. Many studies examine young, trained, and often caffeine-habituated individuals under standardised test conditions. Observational data, such as the NHANES analysis on coffee and cognition in older adults, show associations, not causation. Individual studies also report that measured effects are influenced by expectation and placebo.

Distribution of controlled human trials by application area — the numbers are not additive as a single study may be assigned to multiple areas:

3. What Is Often Claimed — and What the Data Support

4. Profile: Molecule and Charge

Caffeine is not a single molecule but a mixture — there is no molecular formula and no fixed molecular mass. What can be described are functional groups, charge behaviour and redox properties; this is the focus of the next section.

5. Charge, Electrons, Radicals

Direct evidence on the electrochemistry of caffeine itself is limited. Radical-scavenging and cytokine-suppressing activity in the same study is attributed to chlorogenic acids and polyphenols rather than caffeine 26. In humans, lactate and potassium concentrations were measured after caffeine intake during acute resistance exercise, depending on the MCT1-A1470T polymorphism 25.

Direct measurements of charge, cation exchange, electron transfer or redox potential in humans are not reported in the evidence statements; research in this area is still pending. A foundational study analysed muscle biopsies before and after exertion for phosphorylation of β2AR, PKA and MAPK (ERK, JNK, p38) 27; this relates to signal transduction, not electron transfer. The dose-dependent effect on osteoblasts and osteoclasts is noted as a review statement 26. It remains to be clarified whether the measured potassium concentrations 25 are linked to shifts in charge carriers and whether redox markers can be detected in blood under caffeine.

6. Interactions with Other Substances

With which other substances on this platform this substance has been studied or discussed together in human trials and reviews — combinations, comparisons or shared pathways. The number indicates studies in which both are present.

Established Genetic variants of the CYP1A2 polymorphism (rs762551) did not alter the ergogenic effects or side effects of a moderate caffeine dose in controlled human studies 4444. One controlled human study reported greater caffeine effects in CYP1A2 'fast' compared to 'slow' metabolisers: reaction time under stress (−18 ± 9 vs. −1.0 ± 11 ms), fastest 10 % of reaction times at rest (−18 ± 11 vs. −3 ± 15 ms), and lapses at rest (−3.8 ± 2.7 vs. +0.4 ± 0.9; P < 0.05) 7. Serum caffeine and paraxanthine levels did not differ between genotypes (P > 0.05) 7.

Established Four genotype groups were formed for ADORA2A in combination with CYP1A2 29. Acute intake of 6.0 mg·kg⁻¹ caffeine improved several aspects of physical performance, seemingly independent of ADORA2A genotypes alone or in combination with CYP1A2 genotypes 29. Agility (15.8 ± 1.2 s vs. 15.9 ± 1.3 s; p = 0.070) and push-ups (26.6 ± 12.0 vs. 25.0 ± 11.0; p = 0.280) remained unchanged 29. The ergogenic effect of 6 mg/kg may stem from biological actions and expectation, as this dose only outperformed the control condition without differing from placebo 28. It remains unclear how caffeine intake, enzyme activity, and receptor binding interact during drug administration.

7. Overdose, Tolerability, Limits

Established In a controlled human study, 3, 6 and 9 mg·kg⁻¹ body weight were tested in female team sport athletes with moderate caffeine use 22. Side effects occurred most frequently at 9 mg·kg⁻¹; no significant differences were found between 6 and 9 mg·kg⁻¹ across measured parameters 22. The study recommends 6 mg·kg⁻¹ to limit side effects 22. 31.3 % of C-allele carriers reported increased nervousness after caffeine intake, whereas none of the AA homozygotes reported this side effect 4.

Established Further controlled human studies report good tolerability: tested stimulants were well tolerated, with no negative effects on other cognitive domains 9; in young, healthy individuals, pre-workout intake was considered well tolerated 13. No serious adverse events or severe haemodynamic disturbances were observed after caffeine intake 30. Symptoms were recorded using standardised questionnaires at predefined time points 1631. Hypothesis: Whether the increased side effects at 9 mg·kg⁻¹ also occur outside team sport athletes and how individual symptoms are weighted remains unclear; a study using a uniform side effect questionnaire across multiple doses could clarify this.

8. The Most Robust Studies

Ordered by evidence strength, not prominence. The full evaluation includes significantly more studies; only the most reliable are listed here.

Controlled Human Studies

• 1 Time course of tolerance to the performance benefits of caffeine (2019)
• 2 Caffeine and Cognitive Functions in Sports: A Systematic Review and Meta-Analysis (2021)
• 3 The Effects of High Doses of Caffeine on Maximal Strength and Muscular Endurance in Athletes Habituated to Caffeine (2019)
• 4 CYP1A2 Genotype Variations Do Not Modify the Benefits and Drawbacks of Caffeine during Exercise: A Pilot Study (2017)
• 5 Women Experience the Same Ergogenic Response to Caffeine as Men (2019)
• 6 Effects of caffeine on neuromuscular fatigue and performance during high-intensity cycling exercise in moderate hypoxia (2017)
• 7 The effect of caffeine on cognitive performance is influenced by CYP1A2 but not ADORA2A genotype, yet neither genotype affects exercise performance … (2020)
• 8 The Effects of 3 Different Doses of Caffeine on Jumping and Throwing Performance: A Randomized, Double-Blind, Crossover Study (2019)
• 9 Cognitive enhancement effects of stimulants: a randomized controlled trial testing methylphenidate, modafinil, and caffeine (2021)
• 10 Acute caffeine intake increases muscle oxygen saturation during a maximal incremental exercise test (2020)
• 11 Effects of Different Doses of Caffeinated Coffee on Muscular Endurance, Cognitive Performance, and Cardiac Autonomic Modulation in Caffeine Naive … (2020)
• 12 Acute caffeine intake increases performance in the 15-s Wingate test during the menstrual cycle (2020)
• 13 Effects of acute ingestion of a pre-workout dietary supplement with and without p-synephrine on resting energy expenditure, cognitive function and … (2017)
• 14 The effects of different doses of caffeine on maximal strength and strength-endurance in women habituated to caffeine (2021)
• 15 The Effect of Caffeine on the Velocity of Half-Squat Exercise during the Menstrual Cycle: A Randomized Controlled Trial (2019)
• 16 Short-Term Effects of a Ready-to-Drink Pre-Workout Beverage on Exercise Performance and Recovery (2017)
• 17 Acute Effects of an "Energy Drink" on Short-Term Maximal Performance, Reaction Times, Psychological and Physiological Parameters: Insights from a … (2019)
• 18 Regular Caffeine Intake Delays REM Sleep Promotion and Attenuates Sleep Quality in Healthy Men (2021)
• 19 The Acute Effect of Various Doses of Caffeine on Power Output and Velocity during the Bench Press Exercise among Athletes Habitually Using Caffeine (2019)
• 20 Combined caffeine and carbohydrate ingestion: effects on nocturnal sleep and exercise performance in athletes (2014)
• 21 Acute Caffeine Intake Enhances Mean Power Output and Bar Velocity during the Bench Press Throw in Athletes Habituated to Caffeine (2020)
• 22 Optimizing Short-Term Maximal Exercise Performance: The Superior Efficacy of a 6 mg/kg Caffeine Dose over 3 or 9 mg/kg in Young Female Team-Sports … (2024)
• 23 Aspirin, paracetamol (acetaminophen) and caffeine for the treatment of acute migraine attacks: A systematic review and meta-analysis of randomized … (2022)
• 24 Early absorption sources of caffeine can be a useful strategy for improving female table tennis players-specific performance (2023)

9. Summary

Proven effects on performance: Numerous controlled studies and systematic reviews, predominantly involving trained and sometimes habituated individuals, support these effects. The magnitude of the effects depends on dosage, type of test, and habituation. For sport-related cognitive tasks, a systematic review with meta-analysis supports the claim. Regarding tolerability, a controlled study in female team-sport athletes with moderate caffeine consumption found that side effects occurred most frequently at 9 mg·kg⁻¹. Between 6 and 9 mg·kg⁻¹, no significant differences were observed in the measured parameters, and the study recommends 6 mg·kg⁻¹ to limit side effects. Further controlled studies report good tolerability without serious adverse events.

Plausible Part of the measured effect may be attributable to expectation: a controlled study on placebo in sports nutrition explicitly used caffeine as a model case and found that some performance enhancement also occurs without caffeine. Hypothesis: This suggests that studies reporting only the verum condition may overestimate the substance-specific contribution. Hypothesis: No effects can be derived from the available studies for skin, gut, and inflammation, or for the relationship between coffee consumption and cognition in older adults.

Outstanding questions include how quickly the performance-enhancing effect diminishes with regular consumption and what duration of abstinence is required. Further questions concern the contribution of caffeine to the effects of multi-ingredient supplements when tested as a whole, and whether CYP1A2 and ADORA2A variants explain the large interindividual differences.

10. Open Questions

• Genotype: Do CYP1A2 and ADORA2A variants explain the large interindividual differences, or are previous findings coincidental?
• Tolerance: How quickly does the performance-enhancing effect diminish with regular consumption, and what duration of abstinence is required?
• Multi-ingredient supplements: What contribution does caffeine make to the effects of multi-ingredient supplements when tested as a whole?

11. Further Paths

Pathways, mechanisms, and networks in which this substance is involved — for those looking beyond the substance itself.

• Interaction network