Caffeine: Effects, Benefits, and Safety

Which is the most popular psychoactive drug in the world? Caffeine, of course.

Our favorite drug has many different physiological effects, one being potentially enhanced performance during exercise. This effect is mainly exerted by blocking the neurotransmitter adenosine from its receptors.

Caffeine increases both aerobic endurance performance, muscular endurance, and even anaerobic strength and power activities like 1RM or vertical jumping.

In this guide, you’ll learn what caffeine is and how it affects your lifting performance. You will also find all you need to know about the safety of caffeine, proper dosages, and much more.

What is Caffeine?

Caffeine is a naturally occuring central nervous system stimulant. In nature it’s found in plants, with the most famous source being the coffee bean. Which, actually, is a seed found inside a cherry-like fruit, and it is only called “bean” because of it’s resemblance to ordinary beans.

The most popular source of caffeine, coffee, has been consumed since 850 AD, when the coffee bush was discovered in Egypt.¹ Since then, the popularity of coffee has spread across the world, to the point where it is now the most consumed beverage in the world, behind only water.

Caffeine found outside of coffee or tea has usually been synthesized, and thus has not been extracted from plants or seeds.

Pure caffeine is a white, bitter powder with a water solubility of about 2 g/dL, which is about 50 times higher than the concentration in the caffeinated beverages we normally drink. Dry caffeine powder is sometimes called anhydrous caffeine, anhydrous meaning “without water”.

85% of the U.S. population consumes at least one caffeinated beverage per day, and the average daily caffeine intake is 165 mg.²

Below is the average caffeine content of some popular caffeinated beverages in their normal serving size.

Caffeine Uptake and Breakdown

After ingestion, caffeine is rapidly and almost completely absorbed. Within 45 minutes, 99% of ingested caffeine has been absorbed in the gastrointestinal tract.³

Plasma concentrations of caffeine reach peak values after 30 minutes, provided a substantial enough dose, in the range of 5 to 6 mg/kg. Following the intake of a lower dose, plasma caffeine could take up to an hour to reach peak concentrations. Ingesting caffeine in combination with a large regular meal delays absorption enough that it might take two hours or even longer for plasma levels to max out.

The half-life of caffeine is around four hours, meaning that four hours after peak plasma concentrations of caffeine, the concentration is about half that of the peak, and halved again four additional hours later.⁴

The hepatic enzyme CYP1A2 accounts for around 95% of the metabolism of caffeine.⁵ Up to 90% of ingested caffeine metabolizes hepatically into three main metabolites after ingestion. These metabolites are paraxanthine (80%), theobromine (10%), theophylline (trivial amounts).⁶

How Does Caffeine Increase Performance?

Since the early 1900s, research has established that caffeine produces an increase in the capacity for muscular work.⁷

Over the decades, scientists have postulated several mechanisms explaining the ergogenic effects of caffeine. Early ones include glycogen-sparing effects and shifts in fuel use, but the current scientific consensus is that caffeine exerts its performance-enhancing effects by blocking central nervous system adenosine receptors.

Adenosine has negative effects on neurotransmission and pain perception. By binding to the receptors of adenosine, caffeine acts antagonistically on them and produces an increase in forceful muscle contractions which can be kept up for a longer time. Combined with reduced perceived exertion during the exercise and blunted pain reception, adenosine antagonism seems to be the main driver of the ergogenic effects of caffeine.^{8 9}

The Effects of Caffeine on Muscular Strength and Power

Let’s answer the burning question right away: will coffee or caffeine help you lift more and perform better in the gym?

The answer is yes, it probably will. While there are genetic differences in how well we respond to caffeine, and some types of strength training benefit more from caffeine than others, you can expect to lift a bit more weight and grind out a few more reps with caffeine in your system.

A large number of studies, reviews, and meta-analyses confirm the benefits of caffeine on endurance performance. Since there already exists a large body of work documenting the fact that caffeine significantly enhances muscular endurance, and because the focus of StrengthLog lies in the areas of strength, power and hypertrophy, this article will mainly deal with the effects of caffeine on those parameters.

Several meta-analyses investigating the effects of caffeine on muscular strength and power have been published in the last decade.

2010 Meta-Analysis

The first meta-analysis, from 2010, pooled the results of thirty-four studies conducted between 1939 and 2008, and examined the effects of caffeine on maximal voluntary contraction strength and muscular endurance.¹⁰

Although restricted to the knee extensors, caffeine ingestion had a small but significant effect on maximal voluntary contraction strength. Compared to placebo, the increase in knee extensor strength was about 7%. This effect was not apparent in other muscle groups, like the forearm or the knee flexors.

Only 2 of the 34 included studies tested the effect of caffeine intake on performance in 1RM strength. This means that the results from the 2010 meta-analysis can only be applied to isokinetic and isometric strength, since the 1RM-studies are too few to draw any statistically sound conclusions from, regarding the effects of caffeine on maximal strength.

2016 Meta-Analysis

The second meta-analysis, published in 2016, aimed to review the performance effects of caffeine on strength and endurance in isotonic resistance exercises.¹¹ This meta-analysis found that caffeine improves muscular endurance, but failed to discover any meaningful effects of caffeine on performance in maximal strength.

This meta-analysis only include five trials related to muscular strength, and out of these, only three examined the effect of caffeine on 1RM strength, with a pooled amount of subjects fewer than 50. In addition, subgroup analysis was not possible because of the low number of included studies. As a result, this limits the use of this meta-analysis to determine the effects of caffeine on strength performance.

2018 Meta-Analysis

A third review, from 2018, performed a meta-analysis of the effects of caffeine ingestion on maximal muscle strength and muscle power as assessed by tests on vertical jump performance.¹²

After combining the data from 10 studies on strength outcomes and 10 studies on power outcomes that met the inclusion criteria, this meta-analysis concluded that caffeine has the ergogenic potential to improve both maximal strength and power in a significant way. 

Subgroup analysis revealed that this ergogenic effect was most pronounced in the strength of the muscles of the upper body, while no pooled effect could be seen for the lower body muscles.

The difference between the effects on upper and lower body strength warrants further research, since individual studies have found that caffeine prior to lower body resistance training can result in a greater amount of weight lifted compared to placebo.¹³

The forest plot below from the 2018 meta-analysis show the differences between the effects of caffeine placebo on measures of maximal muscular strength.

2019 Meta-Analysis

In 2019, yet another meta-analysis reviewed the effects of caffieine on isokinetic muscular strength.¹⁴ In line with earlier meta-analyses, pooling the data from 10 studies resulted in the conclusion that caffeine may significantly enhance isokinetic strength, primarily in the knee extensor muscles.

Umbrella Review

A so-called umbrella review summarized the findings of all meta-analyses examining the effects of caffeine on exercise performance of any kind, be it muscle strength, muscle endurance, power, aerobic endurance, jumping performance and exercise speed in 2019.¹⁵

Analyzing a total of 21 meta-analyses, including the three mentioned above, the umbrella review found that caffeine had performance-enhancing effects on a broad range of exercise tasks, including those related to strength and power. Moderate-to-high quality reviews and a moderate quality of evidence support these findings, which indicates that caffeine can benefit almost any type of exercise performance. In general, that effect is greater for aerobic as compared with anaerobic exercise like resistance training.

In Summary

The collected evidence indicates that caffeine can exert significant ergogenic effects on muscle strength and power. As a result, this could benefit performance in a number of strength-dependent sports and the performance during training sessions in the gym.

Inter-individual variability in the ergogenic response to caffeine makes it difficult to speculate on any exact performance increases from caffeine ingestion. In general, you can expect to be able to lift more and perform better in the gym if you drink coffee or ingest some other source of caffeine before a training session.

The Effects of Caffeine on Muscle Growth

You won’t get bigger muscles from caffeine alone, although you might from the performance benefits it gives you. Rather, there has been some concern that caffeine could reduce anabolic signaling and muscle protein synthesis. This is probably not something you need to worry about.

Cell culture studies have indicated that caffeine might reduce anabolic signaling by inhibiting the AKT/mTOR/S6K, NF-κB and MAPK pathways.^{16 17}

These observations are the result of direct incubation of cells in supraphysiological amounts of caffeine. This makes the them unlikely to be transferable to humans consuming moderate amounts of caffeine orally. R

The results might indicate medical uses for caffeine such as the ability to enhance the effect of anticancer drugs by inhibiting DNA repair pathways. This is a good thing. That normal coffee or caffeine habits could affect muscle protein synthesis and anabolic signaling in humans is less likely.

What Does Animal and Human Research Say?

At least one animal study supports the unlikeliness of a negative effect of caffeine on muscle protein synthesis.¹⁸ Caffeine administration in mice did not inhibit either anabolic signaling or protein synthesis. On the contrary, enhanced activation of rpS6 following caffeine administration and muscle contractions indicates a possible increase in anabolic signaling. In addition, electrically stimulated contractions in the gastrocnemius and soleus muscles of the rats had no negative effects on muscle hypertrophy affected by caffeine.

There are no published studies examining the effects of caffeine on anabolic signaling or muscle protein synthesis following resistance training in humans. There is, however, a doctoral dissertation suggesting that caffeine has no effect on muscle protein synthesis after a resistance training session and no effect on lean mass during long term resistance training.¹⁹ These results can’t be used as scientific evidence, since they are not from a peer-reviewed study. However, they do show the need for more research to clarify these preliminary findings. 

In Summary

This is an interesting subject of research, but likely not something affecting the caffeine-consuming strength athlete.

The Effects of Caffeine on Anabolic and Catabolic Hormones

Everyone knows the connection between testosterone and muscle growth. Caffeine can raise testosterone levels, but probably not enough for long enough to have any meaningful effects.

Caffeine intake is not associated with serum testosterone levels in men in general,²⁰ but several studies have shown that acute caffeine ingestion can increase the testosterone response to resistance training.

• Professional rugby-league players who ingested 200, 400, or 800 mg of caffeine an hour before a resistance-training session showed an increase in serum testosterone levels of 15–21% in a dose-dependent way.

While caffeine seems to induce an acute increase in serum testosterone, it can also attenuate the growth hormone response to resistance training, possibly caused by elevated free fatty acid concentrations.²¹

Do These Hormonal Effects Actually Have Any Practical Relevance?

Some research indicates that acute hormonal changes following resistance training can improve training adaptations²² while other studies have found, at best, a weak correlation between exercise-induced hormone fluctuations and resistance training-induced hypertrophy.²³

A recent review found that hormonal elevations following resistance training did not seem to be directly correlated with muscle growth.²⁴

On the other hand, a Norwegian study from 2016 found that training the elbow flexors immediately following  a squat session led to greater gains in muscle mass than training arms on a separate day.²⁵ This study concluded that the increase in serum testosterone and growth hormone levels following leg training led to greater training adaptations compared to arm training without the same elevated hormones levels.

In Summary

Regardless if elevated levels of anabolic hormones actually induce superior strength training adaptations or not, it seems as if pre-exercise caffeine ingestion can stimulate testosterone release. However, a simultaneous increase in cortisol release and a slight decline in the testosterone:cortisol ratio might counteract the potential anabolic benefits.

In conclusion, while caffeine has the potential to alter the balance of anabolic and catabolic hormones, the magnitude of these effects are likely too minor to matter.

Caffeine Increases the Rate of Glycogen Synthesis

Coffee can help you recover faster from a workout. Sounds great, doesn’t it? How does it work?

Exhaustive or prolonged exercise depletes muscle glycogen. Consuming enough carbohydrate normally replenishes muscle glycogen within a day. When an athlete desires a faster glycogen compensation after exercise, adding high amounts of caffeine to carbohydrate-rich meals can speed the process up.²⁶

This combination of carbohydrate and caffeine after glycogen depletion also seems to improve subsequent exercise performance in real-life settings.²⁷

Only a few studies with few participants provide evidence to support this practice, but it is an interesting concept to improve the rate of recovery after exercise. Whether more than one component of coffee contribute to the increased rates of glycogen synthesis or if caffeine is the sole driver is unknown.²⁸

Dosage of Caffeine

How much caffeine to take is an important consideration. Too little, and you might not get any effect at all. Too much, and you might get effects you don’t want, like nausea and tremors.

• General recommendations for caffeine ingestion prior to a training session range from 3 to 9 mg/kg.

The golden spot for most people is probably somewhere in that range. For most exercise tasks, you can’t go wrong by starting there.

Muscular Endurance and General Endurance Performance

The majority of studies have focused on endurance training, where caffeine seems to have a dose-dependent effect on performance. For every 1 mg increase in caffeine dose, the relative effect size for muscular endurance increases by 0.1.²⁹ However, caffeine dosage only explains a relatively small part of between-study variance, so you should not interpret this dose-related effect as perfectly linear or set in stone.

Low doses of caffeine, below 3 mg/kg have been shown to enhance endurance performance,³⁰ although recent research has indicated a large individual variance in the ergogenic response to caffeine ingestion. 

For example, 2 and 3 mg/kg of caffeine prior to a training session provided an ergogenic effect in trained cyclists, while 1 mg/kg did not.³¹ However, looking at individual data shows that some subjects showed a performance increase at all doses. Others showed performance increases at some doses, but actual decreases at other doses. One subject showed no ergogenic response at all regardless of dose.

Resistance Training

When it comes to resistance training, there are only a few studies that have investigated the optimal caffeine dosage for maximum effect. 

Several studies have examined the effect of 3 mg/kg of caffeine prior to a resistance-training session on muscular power, strength and performance in trained subjects.^{32 33 34} 

All studies found that 3 mg/kg of caffeine improved muscular performance, but one showed that while 3 mg/kg provided an ergogenic effect when using low to moderate loads, that amount failed to provide any benefits at 90% of 1RM. To improve performance during high intensity lifts, 9 mg/kg was necessary, despite also introducing adverse side effects.

Recently, a study explored the effects of 3 different doses of caffeine on muscle strength and endurance.³⁵ Twenty-eight strength-trained men completed 2 tests under 5 different conditions. The first condition was a no-placebo control, the second was a placebo control, and during the last 3 conditions, the subjects received caffeine doses of 2, 4, and 6 mg·kg-1. The researchers tested the muscle strength of the participants using 1RM-tests in the squat and the bench press. Muscle endurance was measured in the same exercises by using 60% of 1RM.

The results showed that caffeine increased upper-body strength in a linear dose-dependent fashion. Perhaps surprisingly, only the 2 mg·kg-1 dose enhanced lower-body strength. All 3 doses of caffeine improved lower-body muscle endurance, but had no effects on upper-body endurance.

In other words, the more caffeine you take, the more your 1RM upper-body strength improves, but don’t expect the same when it comes to lower-body strength. The effects of caffeine on lower-body strength seem questionable according to this study.

It doesn’t seem to matter how much caffeine you take for lower-body muscle endurance. The caffeine from one or two cups of coffee is enough for a boost, but more doesn’t seem to be any better. Caffeine likely won’t help you grind out more reps with a moderate weight in the bench press, regardless of dose, however.

In Summary

Since low doses of caffeine have fewer, if any, side effects, it might be a good idea to start with a lower dose, around 3 mg/kg. This dose enhances performance in most individuals and muscular tasks.

From there, you can slowly increase the dose to 6 mg/kg, if needed. There is a potential for some minor benefits with even higher doses, but then you also increase the risk of adverse side effects.

Timing of Caffeine Intake Prior to Exercise

In general, consuming whatever source of caffeine you prefer an hour before exercise is a good way to ensure that you are getting proper benefits from it.

Much like dosage, the timing of caffeine ingestion is important to maximize the effect during exercise. Unfortunately, there are not enough high-quality studies to establish an individualized optimal time for caffeine ingestion prior to a training session to ensure maximal performance benefits.

Most of the published studies use a supplementation protocol with caffeine administration one hour prior to the training session or the testing procedure. Studies utilizing a 45 or 90 minute window between caffeine ingestion and exercise have failed to show any performance enhancing effects.^{36 37}

How can 15 to 30 minutes more or less completely eliminate the benefits of caffeine? That is currently unknown. However, it could be because of other confounding factors rather than differences in waiting time per se.

However, the differences in waiting time between caffeine intake and exercise compared to the usual study protocol or of other confounding factors could be the reason for this lack of effect. Maybe future research will clarify the issue.

In Summary

General guidelines recommend caffeine ingestion 60 minutes prior to exercise to reap the performance enhancing benefits. There are a number of factors that could potentially alter that recommendation on an individual basis. These include genetic variations in how fast caffeine metabolizes, training status, habituation, and potential medication use. All these factors have the potential to modify the ideal time for caffeine ingestion prior to exercise.³⁸

Since there are currently no scientific guidelines on how to optimize caffeine recommendations on an individual level, trial-and-error might be the best option at the moment. Caffeine ingestion one hour prior to exercise has proven to be ergogenic in a number of studies, which makes it a logical recommendation to start with. That time window also coincides with the average time it takes for plasma caffeine to reach peak concentrations.

Form of Administered Caffeine on Performance

Caffeine will help you perform better, regardless of the source, as long as you get enough of it.

If you prefer regular coffee to fuel your workouts, that’s fine, but remember that it will take a number of cups to get enough caffeine for any noticeable effect.

Coffee

The majority of studies examining he effects of caffeine on exercise performance use the anhydrous, powdered form of caffeine, often administered as capsules. In contrast, the most widely used source of caffeine in general is regular coffee.

Endurance Training

Earlier studies found that caffeine provided in the form of coffee did not have the same ergogenic effects as isolated caffeine. Coffee lacks effect compared to pure caffeine, a 2001 review concluded, possibly because of interactions between the caffeine and some of the many other compounds found in the coffee bean.³⁹

However, most later studies indicate that coffee have similar ergogenic effects as anhydrous caffeine, at least with matched total caffeine doses. For example, a 2016 study demonstrated that pre-exercise coffee ingestion was similarly efficacious as anhydrous caffeine in improving endurance performance.⁴⁰

Strength Training

Studies on strength and sprint training have resulted in similar observations.

In one study, resistance-trained men received caffeinated coffee, decaffeinated coffee, decaffeinated coffee plus anhydrous caffeine, or placebo before a training session. The ingestion of caffeine resulted in a greater total amount of weight lifted compared to placebo, but there were no differences depending on the form of administration.⁴¹

Another study, also from 2016, demonstrated that 300 mg of caffeine provide equal ergogenic benefits when consumed in the form of a beverage containing pure caffeine as when consumed in the form of coffee providing the same amount of caffeine. Resistance-trained men completed sprint testing consisting of 1RM and repetitions to fatigue at 80% of 1RM for leg press and bench press, followed by five ergometer sprints. Results showed that both caffeine anhydrous and coffee elevated sprint performance similarly compared to placebo.⁴²

The Problem with Coffee

There are two potential problems with using coffee (or tea) as the source of caffeine before a training session.

The first is that you never know exactly how much caffeine you are getting. Databases listing the caffeine content of foods and drinks, including coffee, only list an average amount. There are many factors, including the type of coffee bean and the roasting process, that influence the amount of caffeine that ends up in your cup.

Even when using the same coffee product and the same preparation procedure, you might get different amounts of caffeine. One study found that the same coffee beverage obtained from the same outlet on six consecutive days had a day-to-day variation in caffeine content of 259 to 564 mg per dose.⁴³

An average-sized cup of coffee contains around 100 mg of caffeine, but the cup you just made might contain either more or less than that amount. This makes it hard to estimate how much you are actually getting. Since you need a minimum amount of caffeine for a robust ergogenic effect, overestimating the amount of caffeine in a cup of coffee might mean that you don’t get that performance boost.

On the other hand, underestimating it might push you over the edge into a small overdose. This can make you feel jittery and nauseous during the workout instead of energized.

The second problem is the amount of liquid necessary to consume enough caffeine in one intake, if the source of that caffeine is regular coffee or tea. If you want 4–600 mg of caffeine, that can be a lot of coffee to consume an hour before a workout. It could leave you feeling uncomfortable and in the need for frequent urination during the training session.

Other Sources of Caffeine

Recently, interest in the ergogenic potential of caffeine administered in other, non-traditional ways has increased. These ways include chewing gum, gels, bars, energy drinks, and mouth rinses.

Chewing Gum

Chewing caffeinated gum leads to rapid absorption, likely more rapid than from most other ingestion methods. Rapid uptake in the oral cavity combined with the caffeine absorption from swallowing while chewing the gum produces this rapid increase in plasma caffeine concentrations.⁴⁴

A later study came to a slightly different conclusion, and indicated that there is no marked difference in caffeine deliverance between caffeinated gum and instant coffee.⁴⁵

The speedy caffeine uptake from caffeinated chewing gum could make it a suitable alternative when there is only a short time before a training session or an exercise event. A 2011 study demonstrated that caffeine administered in the form of chewing gum is an effective way to increase performance in power athletes. Shot put athletes improved both performance and alertness after chewing caffeinated gum compared to a placebo gum.⁴⁶

Energy Drinks

Numerous studies over several decades have examined the effects of caffeinated energy drinks on exercise performance. A recent meta-analysis concluded that pre-exercise ingestion of energy drinks improve performance in both muscle strength and endurance.⁴⁷ However, these improvements are associated with the taurine content and dosage in those energy drinks, not the caffeine content.

Current scientific evidence does not support any ergogenic effects of caffeine administered in the form of energy drinks.⁴⁸ However, many of the studies used relatively small amounts of Red Bull, providing only 80–160 mg of caffeine. This might be too low a dose to have any significant ergogenic effect, regardless of the source.

Caffeinated Gels and Bars

Only a few exercise studies have used gels or bars as caffeine sources. One study from 2008 showed that caffeine administered in the form of a bar significantly improved exercise performance, time to exhaustion, and cognitive ability during prolonged cycling.⁴⁹

Two studies demonstrated that both rowing performance and intermittent sprint performance can be enhanced by caffeine ingestion in the form of caffeinated gel.^{50 51}

There are no studies investigating the effects of these forms of caffeine administration on resistance training performance.

In Summary

As long as the total amount of caffeine is the same, it likely does not matter much if you prefer pure caffeine in the anhydrous form, coffee, or any other concentrated form of caffeine before a training session. They will likely give you similar performance benefits. 

Genetic Differences in the Ergogenic Response to Caffeine Ingestion

If you find that you don’t notice perform better with caffeine than without, it might be your genes playing tricks on you. Recent research has showed that while most people get a performance boost from caffeine, some don’t. It all comes down to an enzyme with the name CYP1A2 and the genetically determined speed by which you metabolize caffeine.

Most studies have demonstrated that caffeine provide significant performance-enhancing benefits. However, these benefits are not evident in all individuals. When looking at individual responses to caffeine ingestion, it is obvious that some individuals only gain minor, if any, ergogenic effects. Some even exhibit a performance decrease following caffeine administration.

The hepatic enzyme CYP1A2 is responsible for more than 95% of the metabolism of caffeine.⁵² Recent research indicates that CYP1A2 genetic variations also explain at least part of interindividual differences in the performance benefits gained from caffeine. 

The CYP1A2 gene has two alleles, called A and C. The A allele increases the activity of the CYP1A2 enzyme, while the C allele decreases it. Caffeine metabolizes at different speeds, depending on the individual genotype. The AA genotype metabolizes caffeine rapidly, while the AC and CC genotypes are slow metabolizers. 

The Faster the Better

Being able to metabolize caffeine rapidly might benefit performance. In one study, caffeine ingestion improved 10-km cycling time in trained athletes, but only in those of the AA genotype. Caffeine did not increase the performance of the subjects with the AC genotype, and actually decreased performance in those with the CC genotype.⁵³

Similarly, only resistance-trained men homozygous for the A allele showed any performance benefits during resistance training following the ingestion of 6 mg/kg of caffeine. Carriers of the C allele, however, did not show any ergogenic effects from caffeine compared to placebo.⁵⁴

Since C allele carriers metabolize caffeine slower, it might be possible that ingesting caffeine a longer time prior to exercise could enhance the ergogenic effects of caffeine in that group. To date, this possibility remains unexplored.

Another gene that might modify the ergogenic response to caffeine is the ADORA2A gene, which influence nervous system excitability.⁵⁵ The CYP1A2 and ADORA2A polymorphisms have not been studied together, which means that any interactive mechanisms between them are currently not known.

In Summary

There appears to be a potential genetic variation in how much caffeine affects exercise performance.  The available evidence indicates that this variation could be quite substantial, but how it modifies the individual response to caffeine ingestion is currently unknown. This means that any current recommendations based on genetic differences in caffeine metabolism are also largely based on speculation until further research is available.

Effects of Caffeine on Exercise Performance Based on Sex

The vast majority of studies examining the effects of caffeine on exercise performance have used male subjects. Can we say for sure that caffeine has the same performance-enhancing effects in women? The answer is no, we likely can’t, based on the results of a recent systemat review. ⁵⁶

Men and women come in all shapes and sizes, but in general, men are larger and have a different body composition than women. Our hormonal functioning are vastly different. It only makes sense that something that affects the entire central nervous system and has a dose-dependent effect would affect us in different ways.

After a thorough analysis of the available scientific literature, the researchers were able to conclude that when it comes to aerobic exercise, there doesn’t seem to be any difference in the response to caffeine between men and women. If you run, bike or do any other type of endurance training, you can expect the same benefits from caffeine regardless of sex.

However, when it comes to anaerobic exercise, exercise where power, speed, and strength are required, like sprinting and strength training in the gym, things don’t seem that clear-cut. The majority of the scientific evidence suggests that caffeine has a greater performance-enhancing effect in men than women. It allows men to lift more weight, produce more power, and sprint faster than women, after ingesting the same dose. 

Men also seem to have a greater sensitivity to caffeine. This allows caffeine supplementation to reduce the sense of fatigue in men, but not in women.

Would it be possible for a female to just increase the amount of caffeine to equal the playing field? Maybe, but that would also introduce the risk of negative side effects. In general, females have lower body weight and less body mass. Taking this into account, increasing the caffeine dose from the standard 3–6 mg per kilogram of bodyweight would also drastically increase the risk of running into side effects.

If you are female and wondering why you don’t get the performance improvements you expect after drinking coffee or taking caffeine pills prior to a workout, this might very well be the reason.

Morning and Evening Effects of Caffeine

In general, muscular performance is greater in the afternoon and evening than in the morning.⁵⁷ Given caffeine’s ergogenic potential, it could be an extra potent way to minimize the decline in muscle contraction velocity observed during resistance training early in the day.

Caffeine increases morning resistance-training performance to that of an evening training session.⁵⁸ It also enhances neuromuscular performance to a greater degree when taken before a morning training session compared to an evening session.⁵⁹

Anyone who has drunk coffee too late knows that it can be hard to get to sleep. Ingesting caffeine in the morning should also help mitigate any potential negative effects on sleep compared to ingesting it later during the day.

Caffeine Habituation

The boost you get from caffeine is great, but what if you take it before every workout? Does the effect wear off over time? Actually, yes, this can probably happen, although it does not seem to happen to everyone.

Regular caffeine use by an athlete might lead to less than expected performance benefits over time. Some recommendations suggest that withdrawal periods, either before an important athletic event or regularly throughout a long-term training period, might restore that diminished potential of caffeine due to habituation.

The relatively few studies examining the potential habituation effects of caffeine on exercise performance have produced conflicting results.

Studies Showing No Evidence of Habituation

In one study, habitual and non-habitual caffeine users received placebo, 3 mg/kg of caffeine, or 5 mg/kg of caffeine prior to a time-to-exhaustion trials.⁶⁰ There were no particular differences between the groups, which suggests that the effect of caffeine on exercise performance is the same in both caffeine naive and caffeine habituated individuals.

In another study, 40 trained cyclists who regularly consumed very low, moderate, or high amounts of caffeine on a daily basis performed three time trials after ingesting 6 mg/kg of caffeine, a placebo supplement, or no supplement at all.⁶¹ Low, moderate and high caffeine consumers all responded with similar absolute and relative improvements as a result of caffeine ingestion. This indicates that habitual caffeine intake did not decrease the acute ergogenic effect of caffeine.

Studies Showing Evidence of Habituation

On the other hand, another study also had users and nonusers of caffeine perform time-trials to exhaustion, after consuming placebo or 5 mg/kg of caffeine.⁶² This time, the results showed significantly greater duration and magnitude of the ergogenic effects of caffeine in the caffeine naive participants than in the participants habituated to caffeine.

Another study gave either placebo or increasing doses of caffeine, up to 3 mg/kg per day, over the course of 28 days, to habitually low caffeine users.⁶³ Both before and after that period, they ingested an acute dose of 3 mg/kg of caffeine prior to an exercise trial. In the exercise trial following the month of daily caffeine use, the now caffeine habituated subjects decreased their performance by 7.3%, while the placebo group did not. This shows that the response to caffeine prior to exercise had diminished due to habituation.

Very recently, a study measuring power output and bar velocity during the bench press showed that 15 caffeine habituated strength athletes did not benefit from caffeine ingestion prior to the testing.⁶⁴

A few months later, the same team of scientists replicated these results, this time using a whopping 9 or 11 mg/kg of caffeine. The bench press performance of trained athletes did not benefit from this massive amount of caffeine. Instead, both 1RM and repetitions to exhaustion using 50% of 1RM were the same as after placebo ingestion. On the other hand, many of the athletes reported side effects the day after the caffeine intakes including tachycardia and heart palpitations. ⁶⁵

Effects of Caffeine Withdrawal

A short caffeine withdrawal period in habituated users seems to offer little to no benefits. One study showed that 3 mg/kg of caffeine provided similar performance-enhancing effects during high-intensity exercise with or without a 4-day withdrawal period before the test in habituated caffeine users.⁶⁶

Longer withdrawal periods might offer benefits and restore the effects of caffeine to habituated users, but this has not been explored in any controlled, performance-related studies. Utilizing withdrawal periods could also increase the risk of adverse side effects when caffeine use is resumed by making the user caffeine naive again.

In Summary

Summarizing the available scientific evidence, it seems as if chronic caffeine consumption can result in habituation, leading to a reduced ergogenic response to acute caffeine ingestion. This does not seem to happen to all caffeine users or in all instances, possibly and at least in part due to genetic differences and the individual response to caffeine.

Introducing regular withdrawal periods from caffeine might restore caffeine-sensitivity in habituated individuals, but this has not been properly explored. It might also re-introduce potential side effects upon resuming caffeine use.

A moderate, chronic intake of around 3 mg/kg of caffeine per day does not seem to be problematic in a healthy, athletic population. Before an important event or training session, or when energy levels are especially low, a one-time increase of the caffeine dose might mitigate the reduced ergogenic effects caused by habituation.⁶⁷

Caffeine Safety and Side Effects

Recommended safety thresholds for caffeine vary from region to region. For example, The European Food and Safety Authority (EFSA) considers single doses of caffeine up to 200 mg or 3 mg/kg to be safe.⁶⁸

A recent meta-analysis of around 400 studies concluded that up to 400 mg of caffeine per day does not seem to have any adverse effects, like cardiovascular issues or behavioral changes.⁶⁹

The doses athletes use to improve exercise and training performance are a bit higher than the daily limit recommended as safe for healthy adults. They are definitely much higher than the average intake of the population as a whole. 

Potential negative long-term health effects of these large amount of caffeine consumed on a regular long-term basis are currently unknown.

Acute side effects are much more common, and while unpleasant, are usually not dangerous and pass relatively quickly. After the ingestion of 600 mg of caffeine, side effects like tremors, nervousness, and nausea are fairly common. Caffeine consumption later in the day or in the evening might disrupt sleep patterns and lead to trouble falling asleep.

Individual tolerance dictates the limit where caffeine ingestion could result in adverse side effects. Some might feel nauseous and trembling after ingesting a couple of hundred mg, while others can consume a gram in a single intake and feel nothing. Habitual caffeine consumption likely leads to increased tolerance.⁷⁰

Dehydration

One of the most common concerns about caffeine is that is a diuretic. This is something you most likely don’t have to worry about. Here’s why.

The diuretic properties of caffeine have been explored since 1928, with the publication of the first scientific article reporting increased urine volume following caffeine ingestion.⁷¹

Since then, numerous studies have investigated the effects of caffeine on hydration, and while the results of these studies are varied, the trend indicates that higher doses of caffeine do indeed induce a slight diuretic effect, while small to moderate doses do not.⁷²

A 2016 study assessed the effects of 13 different commonly consumed drinks, including coffee and tea, on hydration status.⁷³ It found that coffee scored the lowest on the Beverage Hydration Index, meaning that coffee resulted in the worst hydration response after ingestion compared to the other drinks.

In caffeine habituated individuals, this slight diuretic effect of caffeine and coffee does not seem to induce actual dehydration. In fact, recent evidence indicates that a moderate daily coffee intake provides similar hydrating qualities to water.⁷⁴ The fluid provided by the coffee likely offsets any negative effects of the diuretic properties of caffeine.

In Summary

Ingestion of up to 400 mg of caffeine per day seems to be safe. Individual tolerance dictates how much more than that can be tolerated without significant adverse side effects. While extreme doses of caffeine can be harmful or even lethal, the side effects of minor overdoses are unpleasant but benign.

Regardless of any potential caffeine induced diuresis following intake at rest, exercise seems to completely negate that effect. A meta-analysis pooling the results of 16 studies showed that caffeine exerts a minor diuretic effect at rest, but that this effect does not exist at all with exercise.⁷⁵ It concluded that concerns regarding fluid loss following caffeine ingestion is unwarranted, especially if ingestion precedes exercise.

Conclusions

• Caffeine ingestion in the range of 3 mg/kg to 6 mg/kg provide significant benefits for most types of power and strength training. You might get some minor benefits from even higher doses, especially when training with very high loads, but you also increase the risk of adverse side effects.
• Moderate intakes of caffeine, acute or chronic, seem to be safe for healthy individuals. Concerns about dehydration are unsubstantiated.
• Currently, the optimal timing of caffeine ingestion prior to exercise is unknown, and likely modulated by several factors. According to numerous studies, however, consuming caffeine one hour before a training session is effective.
• There are large interindividual differences in both the ergogenic response and tolerance to caffeine. To minimize the risk of adverse side effects, you should start with a relatively low dose, like 3 mg/kg. This dose provides a good effect in most instances. From there, you can gradually increase it, if needed.

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That’s it! You’ve reached the end of our guide on caffeine.

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