Caffeine is a natural stimulant most commonly found in tea, coffee, and cacao plants. It works by stimulating the brain and central nervous system, helping you stay alert and prevent the onset of tiredness. Historians track the first brewed tea as far back as 2737 B.C.. Coffee was reportedly discovered many years later by an Ethiopian shepherd who noticed the extra energy it gave his goats. Caffeinated soft drinks hit the market in the late 1800s and energy drinks soon followed. Nowadays, 80% of the world’s population consumes a caffeinated product each day, and this number goes up to 90% for adults in North America.

Once consumed, caffeine is quickly absorbed from the gut into the bloodstream.

From there, it travels to the liver and is broken down into compounds that can affect the function of various organs.

That said, caffeine’s main effect is on the brain.

It functions by blocking the effects of adenosine, which is a neurotransmitter that relaxes the brain and makes you feel tired.

Normally, adenosine levels build up over the day, making you increasingly more tired and causing you to want to go to sleep.

Caffeine helps you stay awake by connecting to adenosine receptors in the brain without activating them. This blocks the effects of adenosine, leading to reduced tiredness.

It may also increase blood adrenaline levels and increase brain activity of the neurotransmitters dopamine and norepinephrine.

This combination further stimulates the brain and promotes a state of arousal, alertness, and focus. Because it affects your brain, caffeine is often referred to as a psychoactive drug.

Additionally, caffeine tends to exert its effects quickly.

For instance, the amount found in one cup of coffee can take as little as 20 minutes to reach the bloodstream and about 1 hour to reach full effectiveness .

Caffeine is naturally found in the seeds, nuts, or leaves of certain plants.

These natural sources are then harvested and processed to produce caffeinated foods and beverages.

Here are the amounts of caffeine expected per 8-ounce (240-mL) serving of some popular beverages

• Espresso: 240–720 mg
• Coffee 102–200 mg
• Yerba mate 65–130 mg
• Energy drinks 50–160 mg
• Brewed tea 40–120 mg
• Soft drinks 20–40 mg
• Decaffeinated coffee 3–12 mg
• Cocoa beverage 2–7 mg
• Chocolate milk 2–7 mg

Some foods also contain caffeine. For instance, 1 ounce (28 grams) of milk chocolate contains 1–15 mg, whereas 1 ounce of dark chocolate has 5–35 mg.

You can also find caffeine in some prescription or over-the-counter drugs like cold, allergy, and pain medications. It’s also a common ingredient in weight loss supplements

Caffeine has the ability to block the brain-signaling molecule adenosine.

This causes a relative increase in other signaling molecules, such as dopamine and norepinephrine.

This change in brain messaging is thought to benefit your mood and brain function.

One review reports that after participants ingested.5–450 mg of caffeine, they had improved alertness, short-term recall, and reaction time.

In addition, a study linked drinking 2–3 cups of caffeinated coffee (providing about 200–300 mg caffeine) per day to a 45% lower risk of suicide.

Another study reported a 13% lower risk of depression in caffeine consumers.

When it comes to mood, more caffeine isn’t necessarily better.

A study found that a second cup of coffee produced no further benefits unless it was consumed at least 8 hours after the first cup.

Drinking between 3–5 cups of coffee per day or more than 3 cups of tea per day may also reduce the risk of brain diseases such as Alzheimer’s and Parkinson’s by 28–60% .

It’s important to note that coffee and tea contain other bioactive compounds (besides caffeine) that may also be beneficial.

Because of its ability to stimulate the central nervous system, caffeine may increase metabolism by up to 11% and fat burning by up to 13%.

Practically speaking, consuming 300 mg of caffeine per day may allow you to burn an extra 79 calories daily.

This amount may seem small, but it’s similar to the calorie excess responsible for the average yearly weight gain of 2.2 pounds (1 kg) in Americans.

However, a 12-year study on caffeine and weight gain noted that the participants who drank the most coffee were, on average, only 0.8–1.1 pounds (0.4–0.5 kg) lighter at the end of the study

When it comes to exercise, caffeine may increase the use of fat as fuel.

This is beneficial because it can help the glucose stored in muscles last longer, potentially delaying the time it takes your muscles to reach exhaustion.

Caffeine may also improve muscle contractions and increase tolerance to fatigue.

Researchers observed that doses of 2.3 mg per pound (5 mg per kg) of body weight improved endurance performance by up to 5% when consumed 1 hour before exercise.

Doses as low as 1.4 mg per pound (3 mg per kg) of body weight may be sufficient to reap the benefits.

What’s more, studies report similar benefits in team sports, high intensity workouts, and resistance exercises .

Finally, it may also reduce perceived exertion during exercise by up to 5.6%, which can make workouts feel easier.

Coffee consumption is linked to several other health benefits:

• Liver protection. Coffee may reduce the risk of liver damage (cirrhosis) by as much as 84%. It may slow disease progression, improve treatment response, and lower the risk of premature death.
• Longevity. Drinking coffee may decrease the risk of premature death by as much as 30%, especially for women and people with diabetes
• Decreased cancer risk. Drinking 2–4 cups of coffee per day may reduce liver cancer risk by up to 64% and colorectal cancer risk by up to 38%.
• Skin protection. Consuming 4 or more cups of caffeinated coffee per day may lower the risk of skin cancer by 20%.
• Reduced MS risk. Coffee drinkers may have up to a 30% lower risk of developing multiple sclerosis (MS). However, not all studies agree.
• Gout prevention. Regularly drinking 4 cups of coffee per day may reduce the risk of developing gout by 40% in men and 57% in women .
• Gut health. Consuming 3 cups of coffee a day for as few as 3 weeks may increase the amount and activity of beneficial gut bacteria.

Keep in mind that coffee also contains other substances that improve health. Some benefits listed above may be caused by substances other than caffeine.

Caffeine consumption is generally considered safe, although habit forming.

Some side effects linked to excess intake include anxiety, restlessness, tremors, irregular heartbeat, and trouble sleeping.

Too much caffeine may also promote headaches, migraine, and high blood pressure in some individuals.

In addition, caffeine can easily cross the placenta, which can increase the risk of miscarriage or low birth weight. Pregnant women should limit their intake.

Caffeine can also interact with some medications.

Individuals taking the muscle relaxant Zanaflex or the antidepressant Luvox should avoid caffeine because these drugs can increase their effects.

Both the U.S. Department of Agriculture (USDA) and the European Food Safety Authority (EFSA) consider a daily intake of 400 mg of caffeine to be safe. This amounts to 2–4 cups of coffee per day.

That said, it’s worth noting that fatal overdoses have been reported with single doses of 500 mg of caffeine.

Therefore, it’s recommended to limit the amount of caffeine you consume at one time to 200 mg per dose.

According to the American College of Obstetricians and Gynecologists, pregnant women should limit their daily intake to 200 mg.