The Pros and Cons of Caffeine
by Eric Trexler
Articles, Nutrition, Supplements
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Caffeine can give a boost of energy and acutely improve performance, but concerns
about potential downsides (such as sleep disruption or heart issues) are common.
This article discusses both the pros and the cons of caffeine consumption.
Note: This article was the MASS Research Review cover story for February 2024
and is a review of a recent paper by Marcus et al. If you want more content like
this, subscribe to MASS.
Key Points
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The present study examined the effects of acute caffeine
consumption on cardiac arrhythmias, daily step counts, and sleep
outcomes in 100 healthy adults.
Participants took more steps on coffee days (10,646 steps) than
caffeine avoidance days (9665 steps). However, caffeine also led to
less sleep (36 fewer minutes per night) and more daily premature
ventricular contractions (p < 0.05).
Caffeine isn’t inherently bad or unhealthy, but we still don’t fully
understand its long-term impact on training adaptations and there
are plenty of people who are better off without it. If you’re
experiencing issues that could potentially be related to caffeine
(e.g., headaches, poor sleep, anxiousness, etc.), experimenting with
caffeine avoidance might be worthwhile.
Caffeine is one of the few fitness-related topics where the passion of fitness
enthusiasts is matched, if not exceeded, by the general public. The interest
from lifters is understandable, as meta-analyses indicate that caffeine can
acutely enhance a variety of performance outcomes including strength and
power (as covered in this MASS video). Interest from non-lifters is equally
understandable, as everyone could use a morning boost of energy and
caffeinated beverages like tea and coffee play a prominent role in many
cultures. Approximately 85% of American adults consume a caffeinated
beverage daily (2), and some estimates place their average daily caffeine intake
just north of 200 mg/day (3). That might sound like a lot, but average daily
caffeine intakes in Sweden and Finland exceed 400 mg/day (3). To date, one of
the top highlights of my career has been presenting at an incredible exercise
science conference in Finland – having been there during a relatively cold
month, I can vouch for the 400+ mg/day estimate. Coffee was virtually always
within reach, and as a coffee lover, I was in heaven.
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Having said that, many governments advise citizens to limit their daily caffeine
intake to no more than 300-400 mg/day, while some caution that daily intakes
above 200 mg/day might lead to unwanted side effects like nervousness or
anxiety (4). Caffeinated beverages are also in the news after two recent
lawsuits attributed fatal cardiac arrhythmias to the consumption of a particular
product at an American restaurant chain. I wouldn’t dare get between a coffee
drinker and their morning dose of caffeine, but we shouldn’t turn a blind eye to
caffeine’s potential drawbacks just because we enjoy its most desirable
characteristics. Instead, we should consult the evidence to determine if the
pros outweigh the cons, and that’s exactly what this article intends to do.
Marcus et al (1) recently examined the effects of acute caffeine consumption on
cardiac arrhythmias, daily step counts, and sleep outcomes, so let’s dive in and
see what they found.
Purpose and Hypotheses
Purpose
The purpose of the presently reviewed study was “to examine the effects of
caffeinated coffee on cardiac ectopy and arrhythmias, daily step counts, sleep
minutes, and serum glucose levels.”
Hypotheses
The researchers did not explicitly state a hypothesis.
Subjects and Methods
The present study screened 113 potential participants and ended up enrolling
100 eligible adults who consumed caffeinated coffee at least once per year. The
researchers excluded participants who had certain heart conditions, took
certain medications impacting heart rhythms, or had a medical reason to avoid
coffee. The study was a randomized crossover trial; participants were
instructed to consume caffeinated coffee or avoid caffeine in a series of seven 2day sequences (on-off or off-on), with the trial lasting 14 days in total. Each
morning, participants received a text message informing them of their assigned
condition (caffeinated coffee or no caffeine). These assignments were sent in a
randomized order with two-day sequences to ensure that participants would
never go more than two consecutive days with caffeine or more than two
consecutive days without caffeine.
The primary study outcome was premature atrial contractions, but other
outcomes of interest included premature ventricular contractions, arrhythmias,
step count, sleep minutes, and glucose levels. To measure these outcomes,
participants were fitted with an electrocardiogram (ECG/EKG) patch, activity
tracker (Fitbit), and continuous glucose monitor. The researchers took several
steps to confirm protocol adherence; participants pressed a button on their
ECG patch when they consumed caffeinated coffee, in addition to completing
surveys, providing receipts, and downloading a geolocation app to confirm
their presence in coffee shops. Participant characteristics are presented in
Table 1.
Findings
Participants had more premature atrial and ventricular contractions during
coffee days than caffeine avoidance days. This was statistically significant for
premature ventricular contractions (p < 0.05), but not for premature atrial
contractions (p = 0.10). These outcomes are presented in Table 2.
Participants took an average of 1058 additional steps on coffee days in
comparison to caffeine avoidance days. This difference was statistically
significant (p < 0.05), with a 95% confidence interval ranging from 441 to 1675.
These data are presented in Figure 1.
Participants got less sleep on coffee days (for a total of 397 minutes per night)
than on caffeine avoidance days (for a total of 432 minutes per night). This
mean difference of ~36 minutes was statistically significant (p < 0.05), with a
95% confidence interval ranging from 25 to 47 minutes. These data are
presented in Figure 2.
Average blood glucose levels were not meaningfully impacted by coffee or
caffeine avoidance. Average glucose levels were 95 mg/dL on coffee days and
96 mg/dL on caffeine avoidance days (mean difference = -0.41 mg/dL; 95% CI, 5.42 to 4.60 mg/dL). In past MASS articles (one, two), we’ve talked about how
certain genotypes are associated with faster caffeine metabolism, while other
genotypes are associated with slower caffeine metabolism. In the present
study, these genotypes did not have a consistent impact on outcomes.
Participants with faster genotypes tended to have more premature ventricular
contractions on coffee days, whereas participants with slower genotypes
tended to experience larger sleep reductions on coffee days. In other words,
there was a tendency for fast metabolizers to have better sleep-related
outcomes, but worse heart-related outcomes. Genotype had virtually no impact
on step count responses to caffeine.
Interpretation
We’ve presented the case “for” caffeine on many occasions within the pages of
MASS, and it’s an easy case to make. First and foremost, people love caffeine
and the beverages that carry it. Across numerous countries, the vast majority of
adults consume caffeinated beverages on a regular basis because they love the
beverage itself or enjoy a quick energy boost. In addition, meta-analyses
indicate that acute caffeine consumption improves a variety of exercise-related
outcomes including endurance, strength, and power (5). On top of all that, the
presently reviewed study suggests that caffeine consumption can boost the
amount of physical activity you accrue throughout the day, which is generally
favorable for health-related outcomes. Nonetheless, against my better
judgment, my goal in this article is to present the case “against” caffeine.
Many lifters and athletes refuse to entertain the idea of ditching caffeine
because they are hesitant to forego the ergogenic (performance-enhancing)
effect of caffeine consumption. While that perspective is understandable, there
are valid reasons to challenge (or at least question) the premise. First and
foremost, it’s important to recognize that caffeine’s effects on strength and
power outcomes tend to be modest in magnitude, with meta-analyses
reporting Cohen’s d effect sizes in the 0.1 to 0.3 range (5). That’s a respectable
effect size for a dietary supplement (given that so few of them actually work),
but it’s certainly not going to make a game-changing impact in the long run.
Speaking of the long run, I’ve noted in a previous MASS article that crosssectional caffeine supplementation studies report acute performance
improvements in habitual caffeine users (6). However, there are very few
longitudinal studies that actually quantify caffeine’s performance-enhancing
effects over an extended timeline of repeated use. One such study (7)
was previously reviewed in MASS, and the results hinted at reduced effect sizes
over the course of 20 days. There’s also a glaring lack of studies exploring the
effects of caffeine withdrawal on exercise performance. So, despite crosssectional studies reporting acute performance enhancement when habitual
caffeine users consume pre-exercise caffeine (6), it’s often difficult to determine
if caffeine is merely overcoming performance impairments induced by caffeine
withdrawal symptoms. In such a scenario, caffeine would be restoring typical
performance levels rather than elevating performance beyond an individual’s
typical level. Based on the lack of direct evidence related to these
considerations, it’s hard to definitively conclude that routinely supplementing
with caffeine before exercise will meaningfully boost performance in the long
run.
On a related note, there are very few studies exploring chronic training
adaptations in response to habitual pre-exercise caffeine supplementation.
People often assume that caffeine must facilitate better training adaptations
over time because it acutely enhances performance. While it’s intuitive to
assume that acutely enhancing training performance will inevitably lead to
better training adaptations over time, that’s not always the case. Some folks
draw conclusions about chronic caffeine consumption based on multiingredient pre-workout supplement studies, but this is a risky extrapolation to
make. Pre-workout supplements typically contain caffeine in conjunction with
several other ergogenic ingredients, which makes it difficult to discern the
effects attributable to caffeine itself. We previously reviewed a study (8) that
directly investigated the effects of regular caffeine supplementation on chronic
training adaptations. While the results leaned very slightly in favor of the
caffeine group (compared to the placebo group), the findings were not
statistically significant. In summary, we still don’t know if chronic pre-exercise
caffeine supplementation actually improves training adaptations for lifters.
While habituation is an important consideration for chronic caffeine use, sleep
impairment is as well. Back in 2013, a study by Drake and colleagues (9) sought
to determine how the timing of caffeine consumption impacts sleep quality.
Participants either consumed no caffeine or consumed 400mg of caffeine 0, 3,
and 6 hours prior to bed. Results indicated that caffeine impaired sleep quality
at all time points measured. While this is often misinterpreted as suggesting
that you should stop consuming caffeine 6 hours before bed, the results
actually don’t help us determine when the “ideal” cut-off time would be.
Knowing that caffeine impairs sleep 6 hours before bed doesn’t give us any
evidence indicating that sleep is not impaired when caffeine is consumed 7
hours before bed. As reviewed in a previous issue of MASS, a recent metaregression (10) provides much more practical guidance regarding caffeine
timing. As you might expect, Gardiner and colleagues found that caffeine dose
impacted caffeine timing with regards to sleep disruption. They concluded that
you’d want to consume a 217.5mg dose at least 13 hours before bed and a
107mg dose at least 9 hours before bed, whereas smaller doses (<50mg) don’t
seem to meaningfully impact sleep quality. These findings are summarized in
Figure 3. I believe it’s safe to assume that these doses should be viewed as
cumulative in nature; if you have 100mg at 9 a.m. and 100mg at 1 p.m., the
“dose” at 1 p.m. would be greater than 100mg but lower than 200mg, as only a
small portion of the 11 a.m. dose would be metabolized and cleared from the
bloodstream by 1 p.m.
The presently reviewed study found positive effects on step counts, but it
broadly reinforces concerns about caffeine intake and sleep quality
impairment. Caffeine lovers often downplay these concerns by leaning on one
of two justifications: habituation or genotype. If you dig into the supplementary
materials of the present study, you’ll find partial support for these justifications.
For example, some people suggest that they’re so habituated to caffeine’s
wakefulness effects that late-night caffeine intake fails to impact their sleep. In
the present study, people who regularly consumed 1 or more cups of coffee per
day at baseline experienced an average sleep reduction of -28.9 minutes on
caffeine days, whereas people who regularly consumed less than one cup per
day experienced an average sleep reduction of -52.6 minutes. This may
indirectly suggest that daily caffeine consumption attenuates, but doesn’t
eliminate, negative impacts on sleep. In addition, some people suggest that
they’re “immune” to caffeine’s effects on sleep because they’ve got the
genotype associated with faster caffeine metabolism. In the present study,
individuals with intermediate and slow caffeine genotypes lost an average of 34
and 47 minutes of sleep per night (respectively) on caffeine days, whereas fast
metabolizers experienced a mean increase of 33 minutes per night.
Nonetheless, I’m hesitant to conclude that the typical person can assume
(hope) they’re a fast metabolizer and consume late-night caffeine without
sleep-related consequences for two reasons. First, we have insufficient
information about how much caffeine these individuals consumed and what
time of day they consumed it. Second, there were only 13 participants with
“fast” genotypes. This increases the likelihood of a spurious finding, and also
suggests that the vast majority of participants in this study (who are, in theory,
representative of the population they were sampled from) experienced worse
sleep on caffeine days.
When it comes to the heart-related outcomes reported in this study, I want to
contextualize those a bit. I know my goal in this article was to present a viable
case against caffeine, but I don’t want to present a hyperbolic or alarmist
argument. If you’ve ever watched a TV show or movie with a hospital scene,
you’ve probably seen someone hooked up to a heart monitor, also known as an
EKG (elektrokardiogramm) or ECG (electrocardiogram). The classic squiggly
pattern reflects the electrical activity of the heart, which dictates the
coordinated rhythm and contraction of each heartbeat. The Q wave represents
depolarization of the atrium, the QRS wave complex represents the
depolarization of the ventricle, and the T wave represents ventricular
repolarization (you can learn more about ECGs and heart rhythms here). When
we talk about heart arrhythmias, we’re talking about a disruption in the typical
rhythm of the heart. In many cases, arrhythmias are transient and benign (this
study observed over 100 premature ventricular contractions per day, on
average), but they can sometimes lead to potentially life-threatening medical
emergencies.
If you follow the news consistently, you sometimes come across surprising
stories indicating that someone died from a high, but not astronomically high,
dose of caffeine (for example, 400-800 mg or so). Mounting evidence suggests
that individuals with a heart condition called “long QT syndrome” are
particularly susceptible to dangerous arrhythmias in response to caffeine
consumption (11). Long QT syndrome is exactly what it sounds like – individuals
with this condition have a longer-than-average QT interval, which is the length
of time between their Q wave and T wave. Caffeine may prolong QT intervals,
which can be dangerous for people who have long QT intervals to start with
(12). All of that is to say, unless we’re talking about an accidental overdose from
a misidentified powder or a completely reckless attempt to intentionally overconsume caffeine, caffeine is unlikely to induce clinically relevant or severe
heart arrhythmias in otherwise healthy individuals. However, people with preexisting heart conditions should consult with a qualified clinician to determine
a safe intake range for them.
To conclude this article, I want to share a brief anecdote. I have been a highdose (≥400-500 mg per day) caffeine consumer for many years, and I recently
stopped consuming caffeine entirely. I was too sick to comfortably drink
beverages for a few days, so I figured I’d do a little experiment if I was going to
experience caffeine withdrawal either way. I’ve been off of caffeine for about a
month now, and my observations have been quite interesting. As someone who
is generally prone to anxiousness, I’ve noticed a dramatic reduction in anxietyrelated symptoms. I used to get headaches if I consumed too little caffeine, but
I also got headaches if I consumed too much caffeine. Needless to say, that’s
not a problem anymore. I also observed that I feel the need to nap more – on
the surface that seems too obvious to be insightful, but I’ve come to realize that
I had been undersleeping to a much greater degree than I realized, and caffeine
was covering that up like a Band-Aid.
In terms of lifting performance, I haven’t noticed anything at all. At first it felt
strange to lift weights without a pre-exercise dose of caffeine, but I’ve come to
realize that this was more related to the ritual and subjective experience of
caffeination than anything else. I consume caffeine before workouts because
it’s what I’ve always done (for many, many years). As a result, I’ve
subconsciously associated the subjective feeling of “peak caffeination” as the
feeling of being prepared to lift. This psychological link has historically led me
to feel unprepared to lift when I’m not caffeinated, but pushing through
uncaffeinated workouts has opened my eyes to the fact that caffeine wasn’t
doing too much for my workouts in the first place.
We’ve reinforced the importance of sleep many times in previous MASS issues,
with research collectively indicating that adequate sleep can be important for
performance, body composition, appetite regulation, subjective wellness
(generally feeling good), and a variety of other outcomes. We’ve also mentioned
the concept of expectancy effects (one, two), like when we expect a placebo to
have positive effects or expect a nocebo to have negative effects. In my time
away from caffeine, I’ve come to find that caffeine’s net impact on my fitness
progress was probably neutral at best – the potential benefits, which may be
susceptible to some degree of habituation, came with the drawback of
inadequate sleep quality. I’ve also observed that many of the benefits I
previously associated with caffeine were most likely influenced by my
expectations; I tended to overestimate caffeine’s positive effects and
exaggerate the hypothetical downsides of not consuming caffeine. Much to the
dismay of the high-powered marketing machine that dictates my every move,
I’m not going to make the flashy claim that ditching caffeine will change your
life, or that caffeine abstention is the one weird trick you’re missing out on.
However, I would encourage you to ask yourself a few questions:
Do you feel better when you consume caffeine, or does caffeine help you avoid
bad feelings (e.g., headaches, drowsiness, etc.) that are caused by caffeine in
the first place? Are you actually sleeping well enough, or is caffeine masking the
signs of insufficient sleep? Does caffeine meaningfully improve your
performance, or have you come to expect poor performance when you’re not
caffeinated? Do you have any predispositions (e.g., anxiety, slow caffeine
metabolism, sleep issues, heart arrhythmias, or family history of heart
conditions) that make you question your current level of caffeine consumption?
Don’t get me wrong – most people consume caffeinated beverages regularly
and tolerate them quite well. They feel great, perform well, and experience
absolutely no adverse outcomes related to their caffeine consumption.
However, for some percentage of the human population, caffeine avoidance
might be preferable in comparison to regular caffeine consumption. In other
words, caffeine is totally fine for most people, but it’s not for everyone, and it’s
not without potential drawbacks. I never imagined I would say this, but I much
prefer the decaffeinated life based on how it makes me feel on a day-to-day
basis. I feel less anxious, well-rested, and ready to perform (mentally and
physically) at all hours of the day, and I never find myself feeling unprepared for
a mental, social, or physical task because I haven’t had my coffee yet.
But enough with the anecdotes – let’s wrap this up by summarizing the
evidence. In the presently reviewed study, caffeine was associated with some
pros and some cons. On the bright side, people were more physically active and
accrued more steps per day on caffeine. However, caffeine consumption led to
sleep impairment and increased frequency of heart arrhythmias. Caffeine is not
inherently bad for you, and most people will experience no meaningful
improvements in their health, wellness, or performance if they scale back from
two daily cups of coffee to zero. Nonetheless, if you have been diagnosed with a
heart condition (or suspect that you might have an undiagnosed heart
condition), you should check with a qualified healthcare professional to
thoroughly explore the matter and determine an appropriate caffeine intake
level for your situation. If you’re experiencing issues that could potentially be
related to caffeine (e.g., headaches, poor sleep, anxiousness, etc.), or if you’re
simply getting way out of hand with your daily caffeine dosage, it might be time
to ask yourself the questions listed in this article to determine if a caffeine-free
experiment is a worthwhile pursuit.
Next Steps
Surprisingly enough, we still have a lot to learn about caffeine. I’d like to see
more experimental research pertaining to how different caffeine doses and
timing strategies impact sleep, and how we can explain different subjective
experiences (i.e., some people suggest that caffeine has no impact on their
sleep, while others report being highly sensitive to it). I’m particularly
interested in finding out if these divergent impacts on subjective sleep quality
are related to rate of caffeine metabolism, habituation among regular caffeine
consumers, inaccurate self-assessments of sleep quality, or something else
entirely. On the fitness side of things, I think we need a few longitudinal studies
to examine how caffeine’s effects on strength and power change over time and
how caffeine withdrawal impacts exercise performance. These types of studies
would be perfect for a master’s-level thesis project or as part of a multi-study
doctoral dissertation – they address a straightforward question and could be
completed with very modest budgets and relatively short study timelines.
Finally, it’d be great to see more longitudinal studies that actually quantify
caffeine’s effects on chronic training adaptations rather than acute
performance metrics.
Application and Takeaways
Caffeine acutely improves performance and may passively lead to higher
physical activity levels. However, caffeine can impair sleep, and its effects on
long-term training adaptations are currently uncertain. In addition, caffeine can
worsen anxiety symptoms and lead to clinically relevant arrhythmias in people
with pre-existing heart conditions. Caffeine isn’t inherently bad, but it’s not for
everybody. Caffeine consumption habits should be individualized to fit your
goals, preferences, subjective experiences, and medical considerations.
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This article was the cover story for the February 2024 issue of MASS Research
Review. If you’d like to read the full, 93-page publication (and dive into the
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