Caffeine is an active ingredient in some of the world's favourite drinks and has a powerful effect on humans. What happens if you take too much?
It is there naturally in your morning coffee and cup of tea or added to your favourite energy drink and many popular soft drinks. For many of us, a slug of caffeine can give our bodies and brains a much needed boost if we are feeling a little sluggish.
But recently, a brand of lemonade on sale in the US was discontinued amid accusations its caffeine content was dangerously high, despite reportedly being within the country's recommended daily intake of caffeine for adults. It has raised questions about just how much caffeine is too much, and does it matter where we get it from?
While there's a vast number of studies showing that some caffeinated drinks have health benefits, there's still some uncertainty around how we should be consuming it.
How caffeine affects the body
There are many crucial functions at play in our bodies all the time, including heart rate, blood flow and sleep-wake cycles. Many of these affected by adenosine – a chemical that occurs naturally in your body, and whose job it is to make us tired at the end of the day.
"Adenosine is one of the naturally produced substances in the body to cause a quieting of activity in various organs that are under stress or in need of lowering energy demand," says Kenneth Jacobson, chief of the molecular recognition section at the National Institute of Diabetes and Digestive and Kidney Diseases in the US.
Adenosine receptors are found on the outer surface of many cells in the body, he says, and they react to variable levels of adenosine near the cell to send a command inside the cell to lower its level of activity. This promotes sleep in the heart, kidneys, immune system and other tissues.
When we consume caffeine, it's quickly absorbed into our bloodstream, where it out-competes adenosine by preventing it from connecting to these receptors and doing its job. This is why consuming caffeine can make us feel more awake and alert.
Caffeine can also boost levels of other neurotransmitters like dopamine and adrenaline, which can make you feel more stimulated, says Damian Bailey, professor of physiology at the University of South Wales in the UK.
"The brain is like a big muscle," Bailey says. "It needs to perform things, and dopamine, adrenaline and caffeine all stimulate it."
Health benefits of caffeine
There's a huge body of evidence around caffeine's effects on our health – particularly coffee, since this is one of the biggest contributors of caffeine in the diets of most populations.
An umbrella review analysing more than 200 meta-analyses in 2017 found that drinking three to four cups of coffee a day was more often linked to health benefits rather than harm, and that studies finding harmful associations could be explained by the higher proportion of coffee drinkers that also smoke.
However, one area where things may get a bit nuanced is heart health. One population study of almost 19,000 people found that, while drinking more than two cups of coffee a day was linked to a higher risk of death from heart disease among people who already had high blood pressure, this wasn't the case among those with healthy blood pressure.
Coffee – but not tea – has been consistently found to protect against depression
And in a review of evidence, scientists say that, while moderate coffee consumption can lower the risk of death, high blood pressure and heart failure, there was no clear effect found on coronary heart disease risk.
Studies show that coffee can also have an effect on how well we exercise. One study of amateur cyclists found it can improve physical performance by up to 1.7%. Caffeine has also been associated with a decreased risk of several forms of cancer, heart disease and type 2 diabetes.
Coffee – but not tea – has consistently been found to protect against depression, and there's also evidence that adenosine's antagonists, including caffeine, that block adenosine receptors, are beneficial for the ageing brain, says Jacobson.
"Caffeine itself, and other more potent, synthetic caffeine-like molecules, have been studied clinically and shown to be beneficial in humans with neurodegenerative diseases, including Alzheimer's dementia," he says.
In fact, research has associated caffeine consumption with up to a 60% reduced risk of developing Alzheimer's.
One explanation for this is that caffeine improves blood flow to the brain, says Bailey. The brain burns a huge amount of fuel, he says – despite only weighing around 2% of our body weight, it accounts for more than a quarter of our body's entire energy requirements.
"But while caffeine's ability to increase blood flow to the brain is a good thing, this can also trigger headaches, so caffeine is a risk factor for migraine suffers," he says.
What does coffee do to your body?
Coffee has also been associated with healthy compositions of gut microbes, partly thanks to the bioactive polyphenol and alkaloid compounds – of which caffeine is one – it contains. It is well established that the right composition of gut microbiota can have widespread effects on our health.
And it's worth noting, some of the health benefits associated with coffee are not down to its caffeine content. Chlorogenic acid, a compound unique to coffee, for example, may be behind some of coffee's beneficial health outcomes, particularly diabetes, says Cornelis.
"Population data strongly shows that coffee-drinking reduces diabetes, but the effects are also found with decaf, so it's probably not related to caffeine," she says.
And many caffeinated drinks can have different effects due to other things they contain. This is true even for different types of coffee.
For example, roasted coffee contains bioactive compounds that have antioxidant and anti-inflammatory effects.
And while instant coffee contains more healthy polyphenols than filter coffee, it also contains more of the carcinogen acrylamide, says Ruari.
Tea has also been found to have similar anti-inflammatory effects, due to its flavanol content.
Is all caffeine created equally?
While coffee is best known for its caffeinated properties, tea is a strong contender, too.
"Strongly brewed black and green tea can pack quite a punch as far as caffeine is concerned," says Bailey.
There are potential health benefits to replacing some coffee with tea was better than drinking just coffee
One study that followed more than half a million coffee drinkers over two decades found that people who drank filtered coffee – which involves brewing coffee more slowly through a filter – had lower rates of arterial disease and death compared to those who drank no coffee, or unfiltered coffee. The researchers conclude that this might be because of the caffeine.
In the first known study to look at the effects of substituting coffee with tea, researchers found that there are potential health benefits to replacing some coffee with tea compared to drinking just coffee. The researchers conclude that men who get between a third and one half of their daily caffeine intake from coffee had the lowest risk of death, and women who drunk just coffee, or up to 40% tea, had the lowest risk of death – but those who drank more tea than this were at an increased risk of death.
Another more recent trend is the use of caffeine in energy drinks – a vague term for caffeinated soft drinks that contain other ingredients including sugar and other stimulants, such as guarana. These seeds contain around four times more caffeine than coffee beans, and the combination of other natural chemicals in guarana seeds may heighten its stimulant effects over caffeine alone.
Caffeine levels build up in the blood stream after 20 minutes, and it takes about an hour for 'peak caffeine' – Damian Bailey
Studies show that the some of the most popular caffeinated energy drinks in the UK and US contain between 75 and 160mg of caffeine, but some contain as much as 500mg of caffeine.
"Energy drinks often contain other stimulants like B vitamins, L-carnitine, L-theanine, and glucuronolactone," says Alex Ruani, doctoral researcher of nutrition science education at University College London. "When combined, all these things can upset many body systems, including the brain and the heart. Long term this can cause cardiac harm such as hypotension and arrythmia, and neurological disturbances like panic attacks and seizures."
Timing it right
Whether you consume caffeine to stay alert through meetings or in an attempt to improve your stamina in the gym, the time of day seems to matter.
"Caffeine levels build up in the blood stream after 20 minutes, and it takes about an hour for 'peak caffeine'," Bailey says.
"Caffeine helps muscles to contract more powerfully, and increases our tolerance to fatigue, so it can boost performance quite considerably if taken one hour before exercise."
When you first wake up in the morning, some researchers recommend waiting for 90 minutes to two hours before having your first cup of coffee. Their reasoning is that in the first couple of hours after waking up, adenosine bound to the receptors on the outside of your cells will gradually clear, leaving the way clear for caffeine to bind more effectively. However, other researchers dispute the validity of this idea and say there is little scientific evidence to support it.
But while caffeine enters the gut quite quickly, its effects can take hours to wear off. Scientists recommend having your last 'dose' of caffeine eight hours and 48 minutes before you go to bed.
When is caffeine not advised?
National guidelines in the UK, and the US, advise limiting caffeine to 200mg per day when pregnant.
However, in an analysis of 37 studies, 32 of the studies found that caffeine significantly increased risk of adverse pregnancy outcomes, which can include the mother developing gestational diabetes or preeclampsia, or restricted growth of the foetus. This is because caffeine easily crosses the placenta. Researchers have found that, for miscarriage and stillbirth, there's an increased risk for each increment of 100mg, and low birth rate from 10mg per day.
Caffeine also passes through breastmilk to breastfeeding babies, says Jack James, professor at Reykjavík University in Iceland.
"Although the caffeine concentration in infant blood is lower than that of the mother, it's important to note that infants are unable to metabolise caffeine," he says.
Not only can caffeine effect how difficult it is to get to sleep, but it may also affect how much deep sleep you get
Exposing babies to caffeine this way can cause them to feel caffeine withdrawal symptoms, James says.
"It has been firmly established that caffeine interferes with sleep in adults and that regular consumers become physically dependent, evidenced by a wide range of disruptive withdrawal effects," he says.
These symptoms, he adds, can occur as soon as six hours after abstaining from caffeine, especially among daily caffeine consumers, and symptoms can include headaches and irritability.
"While there has been little study of caffeine-induced withdrawal symptoms in infants, we may assume that any such effects parallelling those experienced by adults are likely to be disruptive," he says.
Coffee and other things: Coffee shop rebound
And not only can caffeine effect how difficult it is to get to sleep, but it may also affect how much deep sleep you get, says Ruani.
There is also evidence showing that adenosine has anti-inflammatory effects against immune and inflammatory disorders.
"Those might be conditions where reducing caffeine intake would be appropriate, after consulting a medical professional," Jacobson says.
How much is too much caffeine?
There are no guidelines in the UK specifically, but the European Food Safety Authority EFSA advises healthy people to drink no more than 400mg per day, and no more than 200mg in a single drink.
Generally, the consensus seems to be that drinking between 200 and 300mg of coffee per day is better than not drinking it at all, Bailey says. But recommendations can be rather futile when we don't know how much caffeine is in any given drink, and the content is so variable, he adds.
If we assume that there are 100mg in a cup of coffee, 400mg is four cups of coffee per day – but, Bailey says, four flat whites would be well over the 400mg limit.
This is because different bean varieties, the amounts of coffee grounds used and the preparation of the drink itself can all have an effect.
A moderate intake of around two or three cups of coffee per day hasn't been linked to any serious negative side effects in healthy people, says Jacobson
"Providers aren't aware of how much caffeine they're giving you," he says. "An espresso can contain anywhere between 250mg and 700mg of caffeine, and some energy drinks can be 500 or 600mg."
So, it's easy to overload on caffeine, Bailey adds – and you'll know when this happens, because you may start to feel nauseous, anxious and irritable, and develop a headache. You might also start to feel premature ventricular contractions – which are extra heartbeats, as the heart's pacemakers become irritable, he adds.
But a moderate intake of around two or three cups of coffee per day hasn't been linked to any serious negative side effects in healthy people, says Jacobson.
And if people need to suddenly stop consuming caffeine, he advises doing so gradually over a few days, to avoid any withdrawal symptoms.
And if you consistently drink high amounts of caffeine over time, you're more likely to experience withdrawal symptoms if you suddenly stop consuming it, but research suggests they can occur with daily doses as low as 100mg. The mechanism behind withdrawal symptoms is the same mechanism that causes caffeine's stimulant effects, says Cornelis.
"People who consume a lot of caffeine over a long time will build up a tolerance effect, because adenosine receptors multiply. And if there are more of these receptors, there's more opportunity for adenosine to take effect in the absence of caffeine," she says.
Individual differences
There are at least eight genes we inherit that are associated with caffeine intake, including ones that influence neurological stimulation, the psychoactive effects of coffee on the brain, how quickly we metabolise coffee, how much we can tolerate, and how much we enjoy it, says Ruani.
It can take as little as two hours to metabolise half the caffeine we've consumed, but for some people, it can take longer than 12 hours, Cornelis says.
Caffeine is a unique drug because it's so readily available
This difference is largely because of the genetic variants in the CYP1A2 enzyme in our bodies, which represents more 90% of caffeine metabolism, Cornelis says.
This is often broken down into two categories of people: caffeine responders, and non-responders – and it's likely you know which category you fall into. Most people's coffee-drinking habits tend to match up to their genetic predisposition, Cornelis says, because we're good at figuring out how many coffees we can tolerate, and sticking within our individual tolerance levels.
But this is where the health benefits associated with caffeine become more complicated.
"Those who metabolise coffee quickly may need to consume more to get the health benefits, because it's gone so quickly," Cornelis says.
More like this:
• Why coffee could be good for your health
• How coffee became an unstoppable force
• How do you decaffeinate coffee?
Our genetic differences may also affect the extent to which habitual coffee consumption can promote health benefits.
"Some people may develop high blood pressure from drinking caffeinated drinks, some don't," says Ruani. "Some benefit from the heart protective benefits of caffeine, and some may not."
And this may contribute to some of the contradictory findings associated with coffee's health benefits, she says.
How should you consume caffeine?
Caffeine is a unique drug because it's so readily available. But this makes it all the more important – albeit, perhaps, more difficult – to consume in moderation.
If you're struggling to relax and feeling edgy, you may have hit your caffeine limit, says Bailey. But the real telltale sign, he says, is struggling to sleep at night.
"Always refuse coffee at the end of an evening meal, and ask for decaf instead," he says. "Sleep is so important for the brain."
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