The Science of Caffeine!

Coffee and tea are substances that are enjoyed by many and, for some, is a critical point for beginning the day. But is it really not a good idea to talk to someone who hasn’t had their morning dose of caffeine yet? The purpose of this article is to investigate caffeine including effects, tolerance and sources.

Tea (Camellia) has been a part of Chinese culture for thousands of years with the first physical evidence of its use dating back to the 2nd century BC, and written evidence potentially dating back to the 10th century BC. Originally consumed medicinally, it was later consumed because it ‘makes one think better’ as stated in a medical text Shi Lun by Hua Tuo (220 AD). The use of tea spread to Japan and Korea (6th century AD), and through the rest of the world in the 17th and 18th century.

The first breed of coffee plant, Coffea arabica, was found in Ethiopia in the 11th century. There it was called the ‘magical fruit’ where the leaves would be boiled in water and used medicinally. Through the 14th century, movement and popularity of the plant and resulting beans continued through Yemen, reaching Istanbul (16th century) and then Venice, Marseilles, Paris, Vienna, London and then the rest of Europe in the 17th century. Thanks to it’s popularity in Europe and the British beginning their cultivation of the plant in the 18th century in Jamaica, coffee is now consumed globally.

Currently, there are several species of the Coffea plant where the seeds can be cultivated for the production of coffee. The main species is the one mentioned previously (arabica) with the other popular species, Coffea canephora, accounting for most of the world’s coffee production. The different types of tea however are far more extensive but originated from the Camellia plant. Since their discoveries, species have been crossbred to allow for a greater variance of flavors and allow for plants that can be grown in a variety of environmental conditions for both tea and coffee. The question was asked; what is it that you want to know about caffeine? Let’s have a look at the answers.

Note: Before we venture further into the science, it is to be noted that one cup of coffee (8-oz, 250 ml) contains anywhere from approx. 63 mg (espresso) up to approx. 175 mg (brewed cup eg. french press, etc), whereas the average cup of tea contains approx. 35 mg of caffeine. Check what it is you’re consuming to keep track of your personal intake.


Effects of Caffeine on The Body

A lot of the effects associated with the consumption of caffeine is because of the abundance of adenosine receptors in the body (blocks receptors) as well as its effect on calcium. Caffeine inhibits the uptake of calcium by the sarcoplasmic reticulum (SR). SR is a membrane structure that stores calcium around skeletal muscle fibers, and it’s release is due to an action potential (nerve impulse) that prompts muscles to contract; basically. Inhibited calcium uptake makes the muscles fibers more sensitive to nerve impulses. Blocking of the adenosine receptors however interferes with your sleep-awake cycle as adenosine cause the firing of neurons to slow down making one feel sleepy.

Though the study was conducted in 1981, there has been little variance in the findings since them. The abstract identified that people who hadn’t consumed caffeine previously (naive to the consumption of tea or coffee) experienced an increase in heart rate, blood pressure, plasma epinephrine (adrenaline), norepinephrine (noradrenaline) and renin activity (secreted by the kidneys), and urinary catecholamines (nor- and epinephrine). Once people had been consuming caffeine for 4 straight days, near complete tolerance had been achieved since there was no identifiable change in the previously mentioned measures. After 7 days, discontinuation of the administration of caffeine occurred which resulted in no detectable withdrawal symptoms. Let’s have a look at a more in depth at the study and explanation of the effects.

Most of the effects of caffeine is because it is an antagonist of adenosine receptors and phosphodiesterase (enzyme important for the regulation of signal transduction). In the abstract in mentioned that people were naive to the consumption of caffeine but the subjects had not consumed caffeine products 21 days before the study. The study consisted of 18 subjects (12 female, 6 male) ranging from 21 to 52 years of age. Over the duration of the study, subjects either consumed 300 ml of placebo beverage or 250 mg of caffeine.

After 15 minutes, caffeine was present in their plasma (clear part of blood). Peak caffeine levels varied between the subjects with a mean (average) peak at around 60 mins after consumption. Final day of caffeine administration, plasma was checked prior to consumption and there was still the presence of caffeine in the plasma 15 hours since last consumed. Blood pressure was altered after 2 hours, epinephrine after 15 mins and nor-epinephrine after 30 mins.

The plethora of studies conducted on the effects of caffeine has discovered much. Literature suggests that moderate consumption won’t increase risks of cardiovascular disease (approx. <600 mg/day) and 2-5 cups (approx. 400 mg/d) decreases risk of type 2 diabetes and cause specific mortality due to stroke, heart disease or infection to also decrease. So some can be consumed but too much can definitely have some adverse effects.

During exercise, the consumption of caffeine has been shown to decrease the onset of fatigue and exhaustion with increased endurance. Adversely though, the diuretic nature of caffeine increases one’s risk of dehydration during exercise. This can be combated with the regular consumption of water or sports drink during exercise. The benefits are so significant that the International Olympic Committee will ban an athlete from competing if their plasma levels of caffeine exceeds that equivalent to 4 cups of coffee.

A lethal dose of caffeine would be approximately 170 mg per kg of an average human. I’m 89 kg so I would have to consume 15,130 mg of caffeine in a single moment or more than 183 of instant coffee (approx. 82.5 mg/cup). Doubting I can do that. Thank goodness.


Effects of Caffeine on The Brain

Caffeine in the brain works through 2 processes; one inhibits a particular receptor (A1) which increases the firing rate of neurons, and the other is another receptor (A2a) which causes vasoconstriction (narrowing of blood vessels) reducing cerebral blood flow. Though the effect of A1 receptors is understandable, limited blood flow due to A2a activation is counter-intuitive based on what we all experience when consuming caffeine. One would think that less blood flow to the brain would inhibit a persons ability to function. A possible explanation to this is because caffeine inhibits excitatory and inhibitory neurons in different neural areas, the effectiveness of neural transmission is increased and therefore decreases to total energy expenditure within the brain is experienced. 

The consumption of caffeine can negatively affect someone who is suffering from anxiety. A meta analysis of literature found that consumption of less that 100 mg won’t have an effect on anxiety whereas a single dose between 100 to 400 mg of caffeine can increase the feelings associated with being anxious such as tension, worry, nervousness and jitteriness. Feeling anxious but craving a coffee? Maybe have a half strength instead. Caffeine intake was also shown to have no effect on anger, confusion or depression at doses around 400 mg.

When talking about sleep, the same meta analysis found that consuming 400 mg or less of caffeine affected different aspects of sleep; nocturnal activity, sleep-onset latency (time to fall asleep) and sleep efficiency (time in bed spent asleep). Due to this, though caffeine may alter one’s perception of how fatigued they are, dose and timing of caffeine (1 cup or more) will interrupt different aspects of sleep making you more fatigued the next day. This is because caffeine counteracts the bodies adenosine inhibitory actions which is important in the body’s sleep-wake cycle.

Caffeine has also been shown to have a positive mental health effect later on life following consistent consumption. Four studies published in 2010 found that chronic caffeine consumption might have a neuroprotective effect resulting in better cognition which inversely correlates with the risk of Parkinson’s and Alzheimer’s disease development. There are definitely more effects of caffeine on the body and brain but this is a rundown of the basics.


Caffeine Tolerance

Tolerance of caffeine is characterized by a decrease in the effectiveness of caffeine on a habitual consumers physiology. When someone consumes caffeine for the very first time, they have zero tolerance to the chemical.

A 1985 study found that continual and chronic administration of caffeine increases the presence of adenosine receptors in the brain because of the initial receptors becoming blocked by caffeine and this usually takes up to 4 days before tolerance is achieved. This results in the initial effects of caffeine being essentially lost and more caffeine is needed to be consumed to block the increased number of receptors. The tolerance to caffeine can be removed though but depending on the baseline of consumption before the abstinence, it take anywhere from 2 weeks to 2 months. The healthiest way to consume caffeine is occasionally when needed so that a tolerance isn’t established and you still get the health benefits and effects associated with consumption.

Coffee Withdrawal

Sudden discontinuation of caffeine consumption results in symptoms 12-24 hours after last dose and peaks around the 20-48 hour mark; the window being due to genetic and metabolic variations. Symptoms include fatigue, headaches, depression, anxiety, irritability and drowsiness.

Interestingly enough, the symptoms of caffeine withdrawal are usually not harmful or long term so, for the American Psychiatric Association, it doesn’t meet the criteria of addiction. Withdrawal symptoms can last, as previously mentioned, from 2 weeks (light caffeine consumption eg. 400 mg/d) up to 2 months (1,000 mg/d). If trying to cut back, exercising, drinking plenty of water, pain relievers and not going completely cold turkey will ensure that the symptoms don’t hit you so aggressively.


Why Does Coffee Make You Poop?

This is something that is very common and, for some, a morning bowel movement can’t be accomplished without them first having their morning coffee. Why though? Caffeine in coffee, and any other source, is absorbed quickly by the stomach where it enters the bloodstream. There, it acts on muscles which causes and increase in peristalsis (directional contractions of the intestinal wall) to speed up the movement of waste. Coffee is special though.

In 1995, scientist used 12 subjects to wear anal probes that measured pressure in different parts of a persons intestines. A fixed amount of coffee, decaffeinated coffee and hot water was given to them along with a 1,000 calorie burger and they were assessed for 10 hours. Coffee drinkers experience intestinal contractions 60% and 23% stronger than decaf coffee and hot water. That’s not all though, compounds in coffee like motilin and gastrin stimulates contractions and increases the release of acid in the stomach. These chemicals combined with the fact that your digestive system is twice as active in the morning than any other part of the day results in you making camp on the toilet in the morning. Recommendations is to give you bowel 2 hours in the morning to settle down before drinking a coffee; unless it is a part of your morning ritual then drink away.

Combination of Hyperthyroidism and Caffeine?

Hyperthyroidism is an autoimmune disorder in which your thyroid (located in the neck) produces too much triiodothyronine (T3) and thyroxine (T4) which causes metabolism to speed up as well as a list of other symptoms. Some of the symptoms like rapid heartbeat, irritability, sleeping difficulties and mood changes are also shared with coffee consumption. Not only that but caffeine can also negatively impact on some of the medication prescribed for hyperthyroidism. So caffeine is not recommended for people with hyperthyroidism because it makes the associated symptoms worse. 


Variations in the Effects of Caffeine from Tea vs Coffee

A study conducted in 2013 polled over 1,100 people and found the 50% of Australians aged 15 – 49 were getting their caffeine from coffee with the rest from varying sources; soft drink (18%), tea (16%) and energy drinks like Redbull (5%). Caffeine has the same effect on the physiology of the body regardless of the source. The different feelings one experiences during different sources is due to other chemicals present; like the extra chemicals in coffee that help you poop. The source however, definitely does have varying amounts.

  • Drip Coffee:                             115 – 175 mg
  • Instant Coffee:                        65 – 100 mg
  • Redbull:                                   80 mg
  • Tea:                                           15 mg (green) – 60 mg (brewed)
  • Soft Drink:                               40 mg (Sunkist) – 55 mg (Mountain Dew
  • Diet Soft Drink:                      34 mg (Cherry Coke) – 45 mg (Diet Coke)
  • Dark Chocolate (1 oz):           20 mg
  • Hot cocoa:                               14 mg

Though coffee has more caffeine than tea, the tea leaves (particularly Camelia sinensis var. Assamica aka Assam tea) actually have more caffeine in them than the coffee beans. The brewing process dilutes the amount of caffeine present in the drink.

Why Does Caffeine Not Affect Me Like It Affects Others?

Currently, there are around 7 genetic polymorphisms (alterations in genes) that alter the way in which caffeine is metabolized by the body or the receptor-mediated effects due to dietary consumption of caffeine. The receptor-mediated effects is because the density and sensitivity of the adenosine receptors (caffeine binding site) are different between individuals. It has been identified however that the polymorphisms don’t affect caffeine pharmacokinetics once 160 mg of caffeine had been ingested with a majority of individuals. I would say, if you’re craving to feel what it is like to consume coffee then go for it but please read over the benefits and consequences of consuming too much caffeine.


Wherever you get your caffeine from (mine constantly changes), there are definitely some pros and cons to it’s consumption. Sure, there is currently no Acceptable Daily Intake in Australia but many studies have concluded that 400 mg/d in adults will result in no adverse effects for most of the population. Like mentioned in previous posts, moderation is key and you just have to determine what is the right amount for you.


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