Caffeine: Effects on the Body and Safe Doses

Caffeine is a natural stimulant from the methylxanthine group, found in coffee, tea, cocoa, cola, mate, guarana, and many energy drinks and sports supplements. It is rapidly absorbed from the gastrointestinal tract, reaching peak blood concentrations in approximately 30-60 minutes and readily penetrating the brain, heart, and muscles. Its half-life is approximately 5 hours on average, but can vary from approximately 1.5 to 9.5 hours in different individuals, depending on liver enzyme activity and other factors. [1]

Caffeine's primary mechanism of action is associated with the blockade of adenosine receptors in the central nervous system and peripheral tissues. Adenosine normally reduces neuronal excitability and promotes drowsiness, while its blockade by caffeine leads to increased alertness, a decrease in subjective fatigue, and an increased readiness for exercise. Concurrently, caffeine indirectly increases the release of catecholamines, which further activates the cardiovascular system and mobilizes energy substrates. [2]

During physical exercise, caffeine affects not only the brain but also the muscles. Research shows that it can increase motor neuron excitability, enhance central motor signaling, and slightly reduce the perception of effort under the same actual load. For strength training, there is also discussion of its effect on calcium release from the sarcoplasmic reticulum in muscle fibers, which may promote more powerful contractions. [3]

Caffeine also influences energy utilization. Early studies emphasized its ability to increase free fatty acid mobilization and spare glycogen stores, but more recent data indicate that the key role is played by central effects—reducing perceived exertion and improving motor unit recruitment—while changes in fat and carbohydrate oxidation are less consistent and depend on the study protocol. [4]

It's important to understand that reactions to caffeine vary from person to person. They are influenced by genetics (polymorphisms of the cytochrome P450 enzyme and adenosine receptors), habitual consumption levels, body weight, psycho-emotional state, sleep quality, and even the phase of the menstrual cycle. Therefore, the same dose may produce a noticeable boost in performance for one athlete, be barely noticeable for another, and cause tremors, anxiety, and a deterioration in well-being for a third. [5]

Table 1. Caffeine: Key Physiological Effects

Direction of action	What happens in the body	Potential significance for sport
Central nervous system	Adenosine blockade, increased wakefulness	Less subjective fatigue, higher motivation
Cardiovascular system	Moderate increase in heart rate and blood pressure	Maintaining minute blood volume during exercise
Muscular system	Increased central drive and muscle excitability	Potential increase in strength and power
Energy exchange	Mobilization of fatty acids, possible glycogen sparing	Minor improvement in stamina
Psyche and mood	Increased vigor, sometimes anxiety	It can help or hinder, depending on the dose.

Ergogenic effects of caffeine: endurance, strength, and speed

Modern meta-analyses confirm that caffeine provides small but consistent increases in endurance performance. At doses of approximately 3-6 mg/kg body weight taken 30-60 minutes before exercise, time to exhaustion increases and endurance test run times decrease by approximately 2-3%. For running tests, a reduction in run time and an increase in power output relative to placebo have been shown, particularly in protocols lasting approximately 20 to 60 minutes. [6]

For strength and power exercises, evidence has accumulated of small but significant increases in maximal strength and explosive power after caffeine ingestion. Meta-analyses show that the greatest benefits are seen in upper-body exercises and tests assessing maximum one-time effort, as well as short-term muscular endurance in high-repetition work. The effect size is generally assessed as small to moderate, but for a competitive athlete, even a few percent increase can be significant. [7]

Caffeine also demonstrates benefits in speed sports and repeated sprints. Recent studies show improvements in 100-meter sprint times and the ability to maintain average power during repeated sprints, which is particularly relevant for team sports and combat sports. However, the effect may depend on the dose, training status, and caffeine habits, as well as whether caffeine intake is combined with carbohydrates and other supplements. [8]

For team sports, comprehensive reviews note the positive impact of acute caffeine intake on specific game skills: improved passing accuracy, increased jumping height, and an increase in the number of high-intensity segments per match. However, the effect on subjective perception of exertion and overall fatigue is not always pronounced, suggesting a complex interplay of central and peripheral mechanisms of action. [9]

Despite the overall positive picture, it's important to remember interindividual variability. There is evidence that caffeine has little or no effect on performance in some athletes, while in others it even slightly impairs results due to increased anxiety, impaired coordination, or stomach upset. Therefore, caffeine recommendations always include individual testing during training, rather than the first experiment on race day. [10]

Table 2. Effect of caffeine on different types of sports activities

Load type	What most studies show	Estimation of effect size
Long lasting endurance work	Decreased time to complete the distance, increased time to exhaustion	Small but stable
High-intensity aerobic work	Moderate improvement in power and average speed	Small to moderate
Maximum strength work	Increased maximal strength, especially in upper body exercises	Small to moderate
Short-term power and repeated sprints	Improving the first sections and average power of the series	Small, significant for elite athletes
Team sports	Improvement of a range of gaming skills, the number of intensive actions	Small, depends on the sport

Doses, forms and strategies for using caffeine in sports

Most recommendations for caffeine use in sports agree on a range of approximately 3-6 mg per kg of body weight, taken 30-60 minutes before exercise. These dosages have been most frequently used in randomized studies and have been shown to provide a significant ergogenic effect with a moderate risk of side effects. For a 70 kg person, this range is approximately 210-420 mg of caffeine, which is roughly equivalent to two to four large cups of strong coffee, depending on the roast and brewing method. [11]

Many athletes are well-suited to the lower end of this range. Doses of 2-3 mg/kg of body weight can improve endurance and specific skills, especially if the individual isn't a heavy, regular caffeine consumer. Moreover, the risk of tachycardia, severe anxiety, and gastrointestinal upset is lower at these doses. Therefore, a reasonable strategy is to start with smaller amounts, gradually increasing the dose under training conditions and assessing the "effect plus tolerance" combination. [12]

Caffeine can be consumed not only through coffee and tea, but also in the form of tablets, capsules, gels, chewables, and energy drinks. Standardized tablets or capsules are more commonly used in research because they make it easier to control the dose. Drinks and gels are convenient for use immediately before and during long races, but require careful consideration of the volume of liquid, sugar, and other components. Rinsing the mouth with a caffeine solution without swallowing is of particular interest: some studies show a short-term increase in power output even with this method, but the effect is less pronounced than with oral administration. [13]

In addition to a single dose before a race, other regimens are being discussed. For example, distributing the dose in small portions throughout a long marathon or multi-day race to maintain alertness and focus. Combining caffeine with other supplements, such as creatine, beta alanine, and sodium bicarbonate, is also being considered to enhance the overall ergogenic effect. However, combination regimens require particularly careful individual testing, as the risk of side effects increases and the contribution of each component is more difficult to understand. [14]

It's also important to consider your daily caffeine intake. For most healthy adults, the upper safe limit is estimated to be around 400 mg per day, taking into account all sources—coffee, tea, cola, energy drinks, chocolate, and medications. If an athlete plans to use caffeine as a supplement, it's wise to calculate their normal intake in advance so that when adding a sports dose, they don't exceed this limit significantly, especially on non-competition days. [15]

Table 3. Practical caffeine dosing options

Application scheme	Approximate dosage	Where appropriate	Pros and cons
One-time dose before the start	3-6 mg per kg of body weight in 30-60 minutes	Classic starts in endurance and strength events	The most studied protocol
Low test dose	2-3 mg per kg of body weight	First meeting, training	Less risk of side effects
Fractional reception during a long race	Small portions every 1-2 hours	Marathons, cycling races, ultras	Supports wakefulness, but is more difficult to dose
Mouthwash format	Low dose in drink	Short, high-intensity workouts	Less pronounced effect, minimal risk to the stomach
Daily moderate consumption	Up to 400 mg per day	Regular days, light workouts	It is important to avoid taking it in the evening due to sleepiness.

Side effects, safety and risk groups

For healthy adults, moderate caffeine consumption of up to approximately 400 mg per day is generally considered safe: large reviews have found no association between such doses and a significant increase in the risk of hypertension, arrhythmia, or cancer. However, individual sensitivity is important to consider: for some people, even small doses cause palpitations, tremors, and a deterioration in well-being, while others tolerate higher amounts without problems. [16]

The most common side effects relate to the cardiovascular system, psyche, and sleep. Caffeine can temporarily increase blood pressure and heart rate, increase anxiety, cause tremors, sweating, and difficulty falling asleep. These effects are particularly pronounced at doses of 4-6 mg/kg and above, when taken in the afternoon, and in people with hypersensitivity and anxiety disorders. [17]

Sleep is a critical issue for athletes. It has been shown that consuming approximately 400 mg of caffeine 6-12 hours before bed can reduce the duration of deep sleep, impair sleep onset, and fragment nighttime rest, even if a person subjectively feels they are sleeping normally. Chronically disrupted sleep can negate the potential ergogenic effects of caffeine, as recovery and adaptation to exercise are directly dependent on the quality of sleep. [18]

Sensitive populations require special caution. Pregnant women are advised to limit their total caffeine intake to approximately 200 mg per day, as higher doses are associated with an increased risk of miscarriage and low birth weight infants. For adolescents and children, the safe limits are significantly lower, and energy drinks with high doses of caffeine and sugar are considered a risk factor for behavioral problems, high blood pressure, and sleep problems. [19]

The form of caffeine intake is also important. Energy drinks and pre-workout shakes often contain not only caffeine but also significant amounts of sugar, other stimulants, and excipients. Excessive consumption of these supplements increases the risk of arrhythmia, sudden blood pressure spikes, kidney problems, and metabolic disturbances. For athletes, pure caffeine options (tablets, capsules, standardized gels) are more predictable and safer, provided the dosage and total daily intake are monitored. [20]

Table 4. Key risks and precautions for using caffeine

Group or situation	Potential problems	Recommendations
Healthy adults	Tachycardia, anxiety, insomnia at high doses	Do not exceed approximately 400 mg per day, monitor sleep
People with hypertension	Temporary increase in pressure	Test small doses, consult a doctor
Pregnant women	Risk of pregnancy complications at high doses	Limit to approximately 200 mg per day
Teenagers and children	Sleep and behavioral disorders, pressure	Avoid energy drinks, control your consumption
Athletes at the training camp and before the start	Risk of insomnia and fatigue	Avoid caffeine intake in the afternoon

Caffeine in different sports and individual characteristics

In endurance sports (running, cycling, triathlon, rowing), caffeine is traditionally considered one of the most effective and well-studied performance aids. Its primary purpose is to slightly reduce the perceived effort at a given power output, help maintain a competitive pace for longer, and improve concentration at the end of a race. Caffeine is especially beneficial in conditions of monotonous work, heat, and sleep deprivation, such as multi-day races and ultramarathons. [21]

In strength and strength-speed disciplines (weightlifting, powerlifting, CrossFit, sprinting), caffeine helps increase peak power, improve fast-twitch fiber recruitment, and enhance central motor output. Some studies show increased sensitivity of the upper body to caffeine for strength gains, which is important to consider when planning key competitive attempts. At the same time, doses should be adjusted to avoid tremors and impaired coordination, especially during technically demanding exercises. [22]

In team sports (football, basketball, volleyball, hockey), caffeine in small to moderate doses helps improve attention, reaction time, jumping ability, and the ability to perform repeated sprints. It can increase the number of high-intensity movements and sharp-shooting actions per match. However, excessive doses can impair accuracy, increase the risk of fouls due to overexcitement, and impair sleep after evening games, which is critical during a busy schedule. [23]

Interindividual differences in caffeine response are associated with genetic variants of the cytochrome P450 enzyme and adenosine receptors. Rapid metabolizers tend to tolerate caffeine better and experience a more frequent ergogenic effect, while slow metabolizers are more susceptible to tachycardia, anxiety, and an increased risk of cardiovascular complications with high consumption. Even in professional athletes, the proportion of caffeine "non-responders" remains significant, and this should be taken into account when personalizing strategies. [24]

The role of habitual consumption is discussed separately. For people who regularly consume large doses of caffeinated beverages, some of the effects gradually wear off, tolerance develops, and a sudden reduction in dosage can lead to withdrawal symptoms, including headaches, drowsiness, and a depressed mood. For athletes, this means either maintaining a relatively stable "baseline" or planning a gradual reduction in consumption in advance to avoid a period of withdrawal during the peak season. [25]

Table 5. Caffeine in different sports scenarios

Situation	Potential benefits	Special risks and limitations
Marathon, cycling, triathlon	Less fatigue, stable pace	It is important to monitor hydration and stomach
Weightlifting, powerlifting	Slight increase in maximum strength and power	Tremors and excess excitement are unacceptable.
Team sports	Better attention, jump, repeated sprints	Possible impairment of accuracy and sleep
Night starts, multi-day competitions	Supports alertness and concentration	The risk of accumulated sleep deprivation
Athletes with high sensitivity	Effective even with small doses	Frequent side effects, individual regimens required

Anti-doping status of caffeine and practical recommendations

Caffeine has a long history on anti-doping lists. In the past, it was considered a prohibited substance when detected in urine above a certain concentration, but was later removed from the list because it is such a common dietary component that a complete ban was deemed inappropriate. Caffeine is currently not on the World Anti-Doping Agency's list of prohibited substances, but it is included in the monitoring program as a substance that continues to be monitored for potential abuse. [26]

The monitoring program means that laboratories regularly assess caffeine levels in athletes' samples and analyze patterns of use. If systematic abuse of high doses is shown to create an unfair advantage or a health risk, the substance's status may be reconsidered in the future. For now, caffeine is permitted for athletes competing under the World Anti-Doping Agency code, although individual national federations and student leagues may impose their own restrictions. [27]

From a practical standpoint, a rational strategy for managing caffeine is as follows. First, it makes sense to clearly understand your usual daily consumption level—how much coffee, tea, cola, and chocolate a person consumes on a typical, non-competition day. Second, any additional "sports" doses should only be introduced alongside a well-established diet, training, and sleep schedule, rather than using caffeine as a substitute for a healthy lifestyle. [28]

When preparing for important events, it's recommended to test different doses and forms of caffeine during training sessions, recording not only power and time, but also heart rate, blood pressure, well-being, sleep quality, and anxiety levels. Based on these observations, an individual working dose and timing are selected that maximize the benefit-to-risk ratio. It's also wise to avoid caffeine intake around bedtime to avoid impairing recovery. [29]

Finally, it's important to be critical of energy drinks and pre-workout supplements with high doses of caffeine and additional stimulants. They can provide a subjective "energy boost," but they also increase stress on the heart, nervous system, and liver, especially when combined with alcohol, heat, and dehydration. For most athletes, it's more prudent to use moderate, calculated doses of caffeine in predictable forms under the supervision of a physician or sports nutritionist, rather than searching for the "most powerful pre-workout shake." [30]

Table 6. Caffeine: Brief practical guidelines for the athlete

Question	A reasonable guideline
Is caffeine allowed under WADA rules?	Yes, it is allowed, but it is in the monitoring program.
Working dose range	About 3-6 mg per kg of body weight before the start, test individually
Daily safe limit	For most healthy adults, about 400 mg per day
Reception time	Usually 30-60 minutes before exercise, avoid late evening
Who especially needs to be careful	Pregnant women, teenagers, people with hypertension and anxiety disorders