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What Is Caffeine? (Benefits & Side Effects Revealed)

Michael Garrico
Published by Michael Garrico | Co-Founder & Marketing Director
Last updated: August 2, 2023
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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 fitness clients, 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 is a naturally-occurring substance found in various plants that works like a stimulant on the central nervous system (CNS).
  • Drinking caffeinated beverages prevents adenosine from binding to its brain receptors, thus postponing fatigue and improving concentration.
  • In addition to brewed coffee, the most common caffeinated drinks today are soft drinks, energy drinks, and tea.

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].

It is considered the world’s most popular central nervous system stimulant.

Caffeine is basically an insecticide that plants produce to defend themselves against pests, which seems to have been consumed since the earliest days of recorded human history.

The word caffeine (and coffee) is derived from the Arabic word “qahwa”, as coffee was brought into Europe via Arabia from Africa.

The active substance itself was first isolated from coffee in 1819 by German chemists.

“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 [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].

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 [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.

When adenosine docks to these receptors, it slows down the release of brain-signaling molecules, and you get sleepy.

Luckily, caffeine (also called adenosine receptor antagonist for a reason) connects to these receptors without activating them, essentially blocking the effects of adenosine.

In other words, it inhibits the inhibitor and therefore reduces tiredness and increases concentration [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

It was once believed that caffeine is unhealthy, but scientific evidence points to the contrary.

If you browse recent literature, you’ll find over 19,000 studies investigating various downsides and benefits of caffeine ingestion, and the main takeaway is that, more often than not, moderate caffeine use is beneficial.

Let’s take a closer look at those purported 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 [7].

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, 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.

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.

One study 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).

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 [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.

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 [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

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 [22]. In pregnant women, too much caffeine can increase the risk of miscarriage or low birth weight [23].


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.).

Get the Most Out of Caffeine

In moderate doses, caffeine may enhance your concentration, motivation, and energy levels.

They often have a good caffeine dose but also contain a combination of other natural and scientifically proven brain-boosting compounds that will help you complete your tasks more efficiently.


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