How Caffeine Affects Our Body

The way caffeine affects our body has made it one of the most frequently ingested substances in the world. A bitter compound that was originally discovered in the coffee plant, caffeine occurs naturally in the leaves and seeds of many plants like tea and chocolate, where it’s believed to deter pests.

Given its presence in a variety of dietary sources as well as in pharmaceuticals, it’s important to take a deeper look at how we consume caffeine and its effects on our body.


How we consume caffeine

The amount of caffeine in caffeinated beverages varies, based on the amount of time and the method that goes into its preparation.

Hot coffee has 80 – 120 mg of caffeine in a 240 ml cup (the small size at most coffee chains, or a big mug at home). A similar amount of iced coffee has 50 – 70 mg of caffeine, while black tea has around 50 mg, whether hot or iced. Caffeinated beverages such as colas have 30 -60 mg per 350 ml, while energy drinks can contain up to 160 mg per serving. Oral caffeine tables, which are used to stimulate alertness, can contain 100 – 200 mg of caffeine per tablet.

Caffeine has some pretty powerful effects, which we feel almost immediately. This begs the question: how much can we consume in a day?

It’s generally best to limit our caffeine intake to 300 mg a day, which would approximately mean 2 large cups of coffee. The European Food Safety Authorities has set 400 mg as the maximum amount we should consume. A good rule of thumb is to stay within 3 to 4 servings of caffeinated beverages a day.

At higher doses, caffeine can be toxic. Ingesting 2 grams can lead to hospitalisation, with severe symptoms ranging from nausea and irregular heartbeat to elevated blood sugar and seizures. That said, it’s extremely difficult to reach these toxic doses unintentionally, without taking caffeine pills. Given that these numbers may vary as per our own weight and body type, it’s safer to limit our daily dose of caffeinated beverages to 2 servings. In these moderate doses, caffeine can actually be quite helpful.


Caffeine’s effects in the body

When we consume a caffeine-rich drink, its caffeine content is quickly absorbed by our small intestine. Its levels in our body peak within 30 to 120 minutes, and take 5 to 6 hours before beginning to decrease. Drinking a cup of coffee at 4 p.m. could still have an effect at bed time and make it difficult to fall asleep.

Here’s how it goes on to influence various parts of our body:


Our brain

Caffeine & adenosine:
Our brain naturally produces a molecule called ‘adenosine’, with its levels increasing from the moment we wake up until we go to sleep. Adenosine is a chemical messenger molecule, i.e., it transmits messages across our nervous system. It helps us sleep, affects our ability to memorise and learn, and protects the cells in our nervous system.

In order to exert these effects, adenosine binds to certain receptors present on the surface of our nerve cells. Caffeine is structurally similar to adenosine and so binds to these very receptors, in place of adenosine. The result: we feel more alert and awake.

Now that there’s a higher level of adenosine freely circulating in the nervous system, the body attempts to balance it out. To do so, it increases its levels of two other two other chemical messengers called noradrenalin and dopamine also increase. Both of these messenger molecules increase our focus and awareness, in addition to other functions; it’s no wonder that studies have shown that coffee increases brain function.

Eventually, however, nerve cells realise something’s amiss and start making more receptors for adenosine to allow it to exert its effects again. This is why your morning cup of coffee can suddenly turn into two; the more receptors you have, the more caffeine you need to bind to them and exert the same effect.

On that note, having more caffeine may increase the amount of time it takes for us to fall asleep and decrease our total sleeping time, especially in the elderly. Caffeine consumed later in the day may also interfere with our sleep because its effects can take several hours to wear off. So, cut off your caffeine consumption by the early afternoon to avoid sleeping problems.

Caffeine and stress hormones:
Caffeine seems to decrease our levels of GABA, another chemical messenger that helps regulate anxiety. It also amplifies the effects of our stress hormone, cortisol.

These chemical messengers make the heart pound, get us to breathe faster, tense our muscles and make us sweat in excess. This response is called the “fight or flight” behavior, which is a survival mechanism that enables humans and other mammals to react quickly to life-threatening situations. Thus, anyone with an anxiety condition should consume caffeine judiciously and infrequently.



Given that caffeine increases our heart rate and muscle contractions, in high doses, it can lead to a higher blood pressure and affect the rhythm of our heartbeat. If caffeinated drinks increase your heart rate dramatically, it’s advisable to reduce your intake.

Despite its ability to increase heart rate, recent evidence has shown a 16-18% lower risk of heart disease in men and women who drink between 1 and 4 cups of coffee a day.



Although this is poorly understood, caffeine can possibly widen the lung’s airways and act like a decongestant or anti-asthmatic drug. Its mechanism is believed to be similar to a pharmaceutical drug called theophylline, which helps reduce inflammatory molecules associated with lung issues.



Many people find that a morning cup of coffee helps get their bowels moving. Caffeine laxative effect has been attributed to the release of gastrin, a hormone produced by our stomach that speeds up activity in the colon. It also seems to stimulate bowel movement by increasing peristalsis, the contractions that move food through your digestive tract.

Caffeine increases the amount of acid in your stomach and may cause heartburn or an upset stomach. Although small to moderate amounts of coffee can improve gut motility, larger dosages may lead to loose stools or acid reflux. If you experience these symptoms, it might help to reduce your coffee intake or switch to tea.



Consuming high doses of caffeine has been linked to increased urinary frequency and urgency in several studies,

A good rule of thumb is to have an extra glass or two of water for every serving of a caffeinated beverage to compensate for this water loss, and avoid dehydration.



Studies have shown that caffeine can slightly reduce our calcium absorption, and that the content of caffeine and phosphorous in colas can lead to bone loss. While these effects shouldn’t matter to a healthy individual with a balanced, calcium-rich diet, it’s important to be aware of this potential effect of caffeine.


Other effects of caffeine

Supports fat loss

Several studies show that caffeine can increase our body’s ability to burn fat and boost our metabolic rate by 3-11%. It’s possible that these effects will diminish in long-term coffee drinkers.


Boosts physical performance

Caffeine can increase our levels of adrenaline, a hormone that prepares our body for its fight or flight response, and even releases fatty acids from our fat tissues. Given these effects, it’s unsurprising that caffeine can improve our physical performance at the gym, when consumed half an hour before beginning the workout.


Reduces risk for liver cirrhosis

Cirrhosis is a liver condition, where it gets damaged due to excessive alcohol intake as well as fatty foods. Coffee drinkers have a much lower risk of developing cirrhosis.


Boosts Longevity

Several studies show that coffee drinkers live longer and have a lower risk of premature death.


Reduces Depression

Coffee appears to lower the risk of developing depression and may dramatically reduce the risk of suicide.



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