Like most xanthines, caffeine is rapidly absorbed and distributed in all body tissues and fluids, including the CNS, fetal tissues, and breast milk.

Caffeine is cleared through metabolism and excretion in the urine. The plasma half-life is about 3 hours. Hepatic biotransformation prior to excretion results in about equal amounts of 1-methylxanthine and 1-methyluric acid. Of the 70% of the dose that is recovered in the urine, only 3% is unchanged drug.

What is caffeine?

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.

How it works

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.

What foods and drinks contain caffeine?

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 May Improve Mood and Brain Function

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 37.5–450 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.

Caffeine May boost Metabolism and fat Burning

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 .