What Is Caffeine? (Benefits & Side Effects Revealed)

Benedict Ang, CPT, PN1-NC
Published by Benedict Ang, CPT, PN1-NC | Staff Writer & Senior Coach
Last updated: December 28, 2023
FACT CHECKED by Dr. Kristy Dayanan, BS, MD
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Caffeine is a universally used substance, and it’s present in so many different products for a good reason.

While talking with my colleagues, I realized that while almost everyone uses it, many people still don’t know the mechanism of action and effects of caffeine.

So, I spent a full day with my dietitian discussing the scientific literature in order to make this comprehensive article on this substance and make sure nothing important slipped by.

Let’s dive in.

Quick Summary

  • Caffeine, a natural substance in many plants, acts as a stimulant on the central nervous system, improving alertness and concentration.
  • By blocking adenosine receptors in the brain, caffeine prevents drowsiness and enhances mental focus and energy.
  • Studies published in PubMed suggest that moderate caffeine consumption can increase fat burning by up to 13% and boost metabolism by approximately 11%.
  • In my opinion, the right amount of caffeine can significantly enhance physical and mental performance.

What Is Caffeine?

Close up shot of coffee beans with caffeine

Caffeine is a bitter naturally-occurring substance found in more than 60 plants, commonly extracted from coffee beans, tea leaves, and cacao pods [1].

Caffeine, a key stimulant for the nervous system, also acts as a natural insecticide in plants. With a history dating back to ancient times, the term 'caffeine' comes from 'qahwa', an Arabic word, reflecting its journey from Africa to Europe through Arabia. It was first identified as a distinct compound in 1819.

“Unlike most other psychoactive substances, caffeine use is socially acceptable, and the drug is widely used. In fact, caffeine is the most commonly used psychoactive substance in the world.”

- Steven E. Meredith, Postdoctoral Researcher

In addition to natural, there’s also synthetic (man-made) caffeine which is added to some foods (energy bars and energy gums), drinks (soft drinks and energy drinks), and medicines (cough syrup and fat loss supplements).

As a mild stimulant, caffeine prevents the onset of tiredness and helps you stay alert. It increases the activity in your nervous system, and it raises the adrenaline and cortisol in your circulation, according to the Psychosomatic Medicine [2].

How Does it Work?

A person drinking coffee outside

When absorbed in the gut (usually within 45 minutes), caffeine travels to the liver via the bloodstream, where it’s broken down.

Since it’s one of the few substances that can cross the blood-brain barrier, its main effect is on the brain [3]. By affecting the brain, caffeine elevates mood and postpones fatigue [4].

After consuming caffeine before my training sessions, I've noticed it kicks in pretty quickly, helping me focus and reduce fatigue during intense MMA workouts.

The Mechanism Behind It

Caffeine keeps us awake by blocking one of the body’s vital sleep-inducing molecules - adenosine. It’s the neurotransmitter that relaxes the brain and makes you feel tired, based on the research published in PubMed [5].

Adenosine comes from breaking down a high-energy molecule called ATP, which is basically the body’s main “energy currency”. In the process, it releases adenosine, ATP’s chemical backbone.

Neurons in your brain have receptors tailored to lock with this molecule.

Caffeine works by blocking adenosine receptors in the brain. Normally, adenosine binding to these receptors slows brain activity, leading to drowsiness.

Caffeine, an adenosine receptor antagonist, prevents this binding, keeping you alert and focused by reducing the sleep-inducing effects of adenosine [6].

It may also boost mood by increasing brain activity of the “feel-good” neurotransmitters norepinephrine and dopamine and by increasing the blood adrenaline levels as well.

Health Benefits

A woman happily drinking coffee at the park

Contrary to past beliefs, modern research largely supports the benefits of moderate caffeine consumption.

With thousands of studies exploring its effects, the consensus is that caffeine, when used in moderation, can be beneficial to health.

Now, let's delve into these health benefits.

Heart Disease Prevention

Even though caffeine increases heart rate, blood flow, and blood pressure, cohort studies haven’t shown that drinking coffee is associated with a higher risk of hypertension, atrial fibrillation (abnormal heartbeat), heart disease, or stroke, according to the New England Journal of Medicine [7].

As an MMA fighter, I keep an eye on my heart health. Despite my high caffeine intake, regular health checks show no increased risk of heart issues.

Moreover, a study from 2009 showed 16-18% lower risk in women and men who drink 100-400 mg of caffeine daily [8].

Yet another study shows evidence that drinking between 2-4 cups of coffee or green tea lowers the risk of stroke by 14-20% [9].

Fat-Burning and Metabolism Boost

A woman with high metabolism and low fat drinking coffee

Stimulating the sympathetic nervous system, caffeine indirectly enhances satiety, suppresses hunger, and increases the breakdown of fat cells for energy [10].

According to studies published in PubMed, caffeine may increase fat burning by up to 13% and metabolism by up to 11% [11].

Practically speaking, 300 mg of daily caffeine consumption may allow you to burn an additional 79 calories per day [12]. This might not seem like much, but it sure adds up over the long term.

Cohort studies on large groups of people suggest that higher caffeine consumption is associated with lower rates of weight gain in the long run.

I've experienced the fat-burning benefits of caffeine firsthand; it helps me stay lean and energized, especially during weight cuts.

Exercise Performance Improvement

Caffeine might promote muscle fiber recruitment while decreasing the feeling of fatigue and muscle pain. It also may delay mental fatigue and improve physical performance across many sports.

Before my fights, taking caffeine boosts my endurance, which I've found crucial for those long, grueling rounds in the ring.

One study published in PubMed showed that consuming 2.3 mg of caffeine per pound of body weight enhanced endurance performance by up to 5% when consumed an hour prior to exercise [13].

On top of that, even lower doses (1.4 mg per pound) appeared noticeably beneficial for performance [14].

Besides these benefits above, caffeine may also have positive effects on liver protection, longevity, skin protection, gut health, gout prevention, diabetes prevention, etc.


Close up shot of coffee beans

As mentioned earlier, caffeine is naturally found in certain plants’ leaves, seeds, and nuts like coffee, tea, cacao, kola, and guarana.

These natural sources are then harvested and processed to be added to beverages and supplements.

Most people’s caffeine intake comes from beverages [15].

So, we made a list of expected caffeine per 8-ounces serving in the most popular ones:

  • Brewed coffee - A cup of brewed coffee contains around 95 mg, instant coffee 60 mg, and decaffeinated coffee about 4 mg of caffeine.
  • Espresso - Usually contains around 240 mg.
  • Tea - Green tea contains around 28 mg, black tea about 47 mg, and decaffeinated tea 2 mg.
  • Soda - Soft drinks usually contain between 20 to 40 mg of caffeine.
  • Energy drinks - These contain around 85 mg (keep in mind that the standard energy drink serving is 16 oz which doubles the caffeine content to 170 mg).

Some foods, like chocolate, also contain caffeine. One ounce of dark chocolate contains 24 mg, whereas milk chocolate contains between 1-15 mg [16].

Caffeine can also be found in some over-the-counter drugs (pain relievers) and fat-burning supplements.

And how much caffeine is considered safe?

A woman drinking coffee with caffeine at home

Caffeine recommended dosages go up to 400 mg of caffeine daily, according to both the European Food Safety Authority (EFSA) and U.S. Department of Agriculture (USDA).

In my MMA training, I stick to the recommended limit of around 400 mg of caffeine daily. It's enough to fuel my intense workouts without overdoing it.

It’s the equivalent of 2 - 4 cups of coffee per day (depending on the coffee type) - an amount not generally associated with adverse and dangerous effects [17].

However, it’s recommended not to consume caffeine in numbers higher than 200 mg in one take since fatal overdose has been reported with single doses of 500 mg, according to the study published in PubMed [18].

Also, remember that recommended caffeine intake doses have been calculated with healthy adults in mind, which means some risky categories should take that into account.

According to the American College of Obstetricians and Gynecologists, pregnant women shouldn’t exceed 200 mg daily caffeine intake [19].

Another category that should limit caffeine intake is children. Children under the age of 12 shouldn’t consume caffeine at all, and for older ones, caffeine should be limited to 100 mg, according to the American Academy of Pediatrics [20].

Lastly, you should know that tolerances and responses to caffeine differ from person to person, mostly due to genetic differences, so some adverse effects may occur at even lower doses.

Are There Any Downsides?

A woman suffering from the effects of caffeine

The downsides of caffeine consumption are minor if you are a healthy adult and if you use it in moderation. I've learned to be cautious with caffeine; too much can lead to jitteriness and affect my focus in the ring, so moderation is key for peak performance.

However, chronic or acute excessive intake may still give you a host of different symptoms. The symptoms of too much caffeine can range from mild to dangerous, based on the research published in PubMed [21].

The most common are: 

  • Anxiety
  • Insomnia
  • Headaches
  • Dizziness
  • Dehydration
  • High blood pressure
  • Tremors and restlessness
  • Fast or irregular heartbeats

These caffeine side effects may decrease over time when you develop caffeine tolerance since it is a drug.

“Just like many other receptors that respond to drugs, the adenosine receptors become less sensitive to caffeine with repeated exposure.”

- Dr. James J. Galligan

Yet, for the same reason, within the caffeine withdrawal period (when you abruptly stop), another group of symptoms may occur:

  • Nausea
  • Irritability
  • Drowsiness
  • Fatigue
  • Headaches
  • Depressed mood

Addressing the often-overlooked aspect of caffeine withdrawal is essential. Symptoms like headaches, irritability, and difficulty concentrating highlight the physical dependence that can develop with regular caffeine use. Caffeine withdrawal symptoms are most pronounced in the first few days of withdrawal, but luckily they tend to subside after about one week.

Nevertheless, caffeine may also interact with some medications like antidepressants and could even increase the effects of these drugs, according to the PubMed [22]. In pregnant women, too much caffeine can increase the risk of miscarriage or low birth weight [23].

Awareness about caffeine overdose is crucial, especially regarding its symptoms like agitation and irregular heartbeat. Equally important is the proper storage of caffeine-containing products, away from children and pets, to prevent accidental ingestion.


How Long Does Caffeine Last For?

Caffeine lasts for four and six hours. It can stay in the blood from 1.5 to 9.5 hours after coffee consumption, depending on various factors. Smoking speeds up the caffeine breakdown, whereas oral contraceptives and pregnancy can slow down the breakdown.

Are Caffeine and Coffee the Same?

No, caffeine and coffee are not the same. Coffee is a drink made with hot water and powdered roasted coffee beans, while caffeine is a stimulative substance found in coffee beans, among other plant sources (tea, cocoa, kola, guarana, etc.).

Is Mixing Creatine and Caffeine a Good Idea?

Mixing creatine and caffeine is generally considered a good idea. However, caffeine may potentially counteract some of the benefits of creatine due to its diuretic effect, which could increase the risk of dehydration. It's essential to stay adequately hydrated when combining these substances.

How Often Should You Consume Caffeine?

You can consume caffeine every day but in moderation, typically 200-400mg (1-2 cups of coffee) per day for most adults, but this can vary based on tolerance and sensitivity.


  1. https://ift.onlinelibrary.wiley.com/doi/full/10.1111/j.1750-3841.2010.01561.x
  2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2257922/
  3. https://www.ncbi.nlm.nih.gov/books/NBK223808/
  4. https://www.ncbi.nlm.nih.gov/books/NBK209050/
  5. https://pubmed.ncbi.nlm.nih.gov/11283304/
  6. https://pubmed.ncbi.nlm.nih.gov/18088379/
  7. https://www.nejm.org/doi/full/10.1056/NEJMra1816604
  8. https://www.sciencedirect.com/science/article/abs/pii/S0167527308008498
  9. https://www.ahajournals.org/doi/10.1161/STROKEAHA.111.677500
  10. https://www.degruyter.com/document/doi/10.1515/jbcpp-2016-0090/html
  11. https://pubmed.ncbi.nlm.nih.gov/7486839/
  12. https://onlinelibrary.wiley.com/doi/full/10.1038/oby.2007.529
  13. https://pubmed.ncbi.nlm.nih.gov/23573201/
  14. https://link.springer.com/article/10.1007/s40279-014-0257-8
  15. https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/caffeine/art-20049372
  16. https://www.caffeineinformer.com/caffeine-content/dark-chocolate
  17. https://health.gov/our-work/nutrition-physical-activity/dietary-guidelines/previous-dietary-guidelines/2015/advisory-report
  18. https://pubmed.ncbi.nlm.nih.gov/14687776/
  19. https://pubmed.ncbi.nlm.nih.gov/20664420/
  20. https://www.aacap.org/AACAP/Families_and_Youth/Facts_for_Families/FFF-Guide/Caffeine_and_Children-131.aspx
  21. https://pubmed.ncbi.nlm.nih.gov/19049813/
  22. https://pubmed.ncbi.nlm.nih.gov/24676319/
  23. https://bmcmedicine.biomedcentral.com/articles/10.1186/s12916-014-0174-6
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About The Author

Benedict Ang, CPT, PN1-NC
Staff Writer & Senior Coach
Benedict Ang, CPT, PN1-NC is an ex-National Soccer player turned MMA and Kickboxing champion, with ACE CPT and PN1-NC certifications. His advice is rooted in education and experience, ensuring that readers receive scientific and battle-tested insights. His mission is to empower his clients and readers to realize their potential and become the best versions of themselves.
Learn more about our editorial policy
Dr. Kristy June Dayanan, BS, MD is an author with a BS degree from University of the Philippines and an MD from University of Perpetual Help System. Her ability to simplify medical science complexities and dietary supplement jargon for the average reader makes her a valued medical fact checker and reviewer.
Learn more about our editorial policy
Dr. Harshi Dhingra, MBBS, MD is a published peer-reviewed author and renowned physician from India with over a decade of experience. With her MBBS from Bharati Vidyapeeth and an MD from Rajiv Gandhi University, she actively ensures the accuracy of online dietary supplement and medical information by reviewing and fact-checking health publications.
Learn more about our editorial policy

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