While you might have a strong coffee in the morning to wake up, there are some caffeine-fiends out there who sip an espresso after dinner to help them relax and go to sleep.
So how can one stimulant drink – coffee – make some people so alert that they feel anxious and wide-eyed awake for hours and perform no observable effect on others?
The answer has to do with our level of caffeine sensitivity. Our level of caffeine sensitivity is determined by how efficient our body is in processing and metabolising caffeine.
Gabrielle Maston, Accredited Practising Dietitian and Spokesperson for the Dietitians Association of Australia, explains that our genes determine our individual reaction to caffeine and the impact that a specific amount of coffee has on our body.
“Our sensitivity to caffeine varies person-to-person,” says Maston. “So not everyone can consume the same amount of coffee and share the same effect.”
Around 95 per cent of caffeine is metabolised in the liver via an enzyme called CYP1A2, according to the genetics analysis company Gene Planet online. The activity of this enzyme is highly variable and is regulated by the CYP1A2 gene.
“Our sensitivity to caffeine varies person-to-person. So not everyone can consume the same amount of coffee and share the same effect.”
Differences in our DNA will, therefore, produce differences in the rate of our caffeine metabolism – people with more than one C variant in the gene will be more sensitive to coffee and it will yield a stronger effect on them, while those with two copies of a gene present in the A variant will be more tolerant to caffeine.
Maston adds that caffeine sensitivity is also regulated by the receptors in our brain.
Caffeine passes through the blood-brain barrier and influences our brain's adenosine receptors. People who lack the correct adenosine receptors may not experience the same caffeine-related effects of alertness that others do, because the receptors are less responsive and the caffeine molecules cannot bind to receptors.
“That’s why there are people who drink one cup of coffee and are anxious or shaky while others drink six or seven cups of coffee and are fine,” adds Maston.
Research from The Harvard School of Public Health also shows that our genetic differences change the way that various people process, metabolise and become addicted to coffee. The study published in 2014 looked at around 100,000 people from European and African-American cultures and found six genetic variants determining the way coffee habits are formed and variability in how coffee affects our health.
Can I develop a caffeine tolerance?
Maston explains that separate to caffeine sensitivity (which is hardwired into our genes), we may also develop a habitual caffeine tolerance.
“You can build up a tolerance to caffeine, just like you can build up a tolerance to sweetened foods," Maston says. "You acclimatise your tastebuds to the taste of sugar and fat to the point that you don’t taste the sugar and fat in food as much anymore. It’s the same thing with caffeine.
“Take, for example, some people who are Italian or Greek who drink a lot of coffee, socially. Their bodies will most likely become desensitised to the caffeine over time.
“If culturally, you drink coffee at breakfast, lunch and dinner then you will be able to tolerate coffee more than someone who has it sporadically.”
Is caffeine sensitivity something to worry about?
Maston stresses that caffeine sensitivity is not a health condition.
“There’s nothing wrong with one person who is sensitive to caffeine and another person who isn’t – they will just like to drink different levels of coffee.”
However, she says it is wise to be aware of your individual sensitivity levels so you don’t experience the unwanted side effects of drinking too much caffeine – irritability, nervousness, anxiety, sleeplessness and diarrhoea.
Maston also advises that people who are very sensitive to caffeine to switch to tea, which has “about 10 per cent of the amount of caffeine that a regular coffee has”.
“If, culturally, you drink coffee at breakfast, lunch and dinner then you will be able to tolerate coffee more than someone who has it sporadically.”
So how much coffee should I have?
Your recommended level of caffeine consumption is dependent upon the degree of your caffeine sensitivity.
There are three levels of caffeine sensitivity: hypersensitive (very sensitive to caffeine), hyposensitive (minimally or not responsive to caffeine at all) and a ‘normal’ sensitivity to caffeine (which lies somewhere in the middle of the former two categories).
Maston says the majority of people have a normal sensitivity to caffeine and it’s upon this level of sensitivity that recommendations for coffee consumption have been set.
“It’s recommended that most people can have 300-400 milligrams a day or less of caffeine,” says Maston.
“That’s equivalent to around three-to-four cups of instant coffee a day or less. Or, because espressos have higher levels of caffeine content, it’s equal to one-to-two espresso or percolated coffees a day.”
She says most energy drinks combine high doses of caffeine with sugar: “one of those large cans has around three or four shots of coffee in it”.
“We don’t recommend that people drink energy drinks regularly because it can cause issues for your weight control and has strong caffeine content.
“But drinking coffee can be part of a healthy diet and is no problem at all.
“The main message is to have caffeine in moderation. If you are not too sensitive to caffeine, try to stick to the recommended amounts and be sure that you are not adding things like cream and sugary syrups to your coffee because then the sugar content starts to add up.”