Whether it’s a flat white, espresso or an energy drink, caffeine is firmly entrenched as the UK’s favourite legal stimulant.
We collectively sip an estimated 98million cups of coffee a day – and research is increasingly showing that apart from the taste, regular coffee drinkers also enjoy a range of health benefits, including a lower risk of heart disease and dementia.
But while overall, drinking three to four cups a day is linked to more health benefits than risks, according to a review of 200 studies in the BMJ in 2017, excessive levels of caffeine have also been linked to migraine, heart palpitations and even death.
Just last month, an inquest heard how a 32-year-old woman from Melbourne, Australia, died from a caffeine overdose after taking caffeine tablets.
These pills, which are often used to stay alert while studying or working nights – or to enhance athletic performance – typically contain up to 200mg of caffeine each.
Healthy adults should consume no more than 300-400mg daily, and no more than 200mg at a time, advises the European Food Safety Authority (the UK has no official guidance on caffeine intake).
That’s about the same as five espressos (75mg per shot), two energy drinks, or a large coffee from a high-street chain – although caffeine amounts can vary wildly.
Research is increasingly showing that apart from the taste, regular coffee drinkers also enjoy a range of health benefits, including a lower risk of heart disease and dementia
For instance, a medium cappuccino at Costa contains a massive 325mg of caffeine, according to research by consumer magazine Which? in 2023 – while a same-size cappuccino at Starbucks has a modest 66mg, and 197mg at Pret.
By comparison, a cup of black tea with milk contains around 47mg; and four squares of dark chocolate, 32mg.
‘Taking more than 200mg of caffeine in one go could be harmful, especially if you don’t consume caffeine regularly, or have underlying heart issues such as high blood pressure or arrythmia [a heart rhythm problem],’ says Alex Ruani, a nutrition researcher at University College London.
The good news is that while individual tolerance varies, ‘it’s virtually impossible to reach fatal levels from drinking coffee alone for most healthy people,’ says Dr Oliver Guttmann, a consultant cardiologist at the Westminster Hospital in London.
A lethal dose of caffeine (estimated at 5-10g) is the equivalent of consuming ‘about 100-130 espresso shots at one time’, he explains.
Yet caffeine tablets are another story, as it’s easier to over-consume them in dangerous quantities – especially if taken alongside caffeine drinks.
The effects of caffeine on the body begin just ten minutes after consumption, with the concentration peaking in the blood after 45 minutes.
Once in the bloodstream, caffeine crosses the blood-brain barrier (the protective barrier around the brain), stimulating the central nervous system and making you more alert and focused – but potentially more irritable and jittery.
‘There is evidence that slow metabolisers should drink two or fewer coffees per day, as drinking three or more cups of coffee per day has been shown to raise their risk of non-fatal heart attack,’ says Alex Ruani, a nutrition researcher at University College London
This is due to caffeine’s effect on the brain’s adenosine receptors – these ‘docking stations’ play a role in regulating your heart rate, blood flow and sleep-wake cycles.
When adenosine, a naturally occurring body chemical, binds to them, it reduces cell activity, prompting drowsiness and sleep.
But caffeine blocks this process, resulting in increased alertness but possibly also feelings of anxiety.
The stimulating effect of caffeine also makes the heart pump slightly faster, temporarily raising blood pressure for some people.
A study by South Dakota State University in the US in 2008 found that caffeine can cause a spike in blood pressure within 30 minutes – and this spike may last for up to four hours for people who are caffeine sensitive, warns Dr Guttmann.
‘Short-term adverse effects of consuming more caffeine than your body can handle include heart palpitations, tremor, sleep disturbance, anxiety, hyperactivity and increased blood pressure in some individuals,’ adds Alex Ruani.
Here we look at some of the benefits – and harms – of caffeine…
Can boost your workout
Caffeine can boost sports performance by helping muscles to contract more powerfully and increasing tolerance to fatigue.
A 2020 study, published in the International Journal of Sport Nutrition and Exercise Metabolism, found that caffeine improved cycling performance by 1.7 per cent.
Caffeine can boost sports performance by helping muscles to contract more powerfully and increasing tolerance to fatigue
‘This may not sound like much, but it’s a big deal for even moderately competitive athletes,’ says Lindsy Kass, a sports and nutrition scientist at the University of Hertfordshire.
Drink around 200mg (a flat white or two espressos) 45-60 minutes before exercise to improve your workout, she suggests.
But could it weaken your bones?
Previous small studies have suggested caffeine consumption could reduce how much calcium is absorbed from food, potentially leading to weaker bones and a higher fracture risk.
However, Lindsy Kass says this research only showed a ‘very weak link’ between caffeine and lower calcium levels – and current evidence from larger studies doesn’t support cutting out coffee for bone health.
She says: ‘According to the Royal Osteoporosis Society, up to four cups of coffee a day is fine’, but adds that people with osteoporosis (which makes bone prone to fracture) should also ensure they eat a diet rich in calcium (found in dairy products, nuts and leafy vegetables).
Good for some people’s hearts
Most studies suggest that moderate amounts of caffeine in fact have a heart-protective benefit, says Dr Guttmann.
‘We don’t know exactly why this is, but scientists think it might be that caffeine has an anti-inflammatory effect on the body – and we know that high levels of inflammation are linked to plaque building up on the artery walls.’
Tim Spector, a professor of genetic epidemiology at King’s College London and co-founder of the health app Zoe, agrees: ‘It’s incontrovertible that the studies show coffee drinkers have about a 25 per cent lower overall risk of a heart attack or heart disease than non-drinkers.’
He adds that people with poorly controlled blood pressure may need to be careful with caffeine, however.
‘Coffee drinkers have about a 25 per cent lower overall risk of a heart attack or heart disease than non-drinkers,’ says Tim Spector, a professor of genetic epidemiology at King’s College London
Your genetic make-up can also play a role. Scientists have identified genes that control how quickly the body breaks down caffeine: People who are slow metabolisers experience stimulating effects at lower doses, while fast metabolisers need more caffeine for a kick.
‘There is evidence that slow metabolisers should drink two or fewer coffees per day, as drinking three or more cups of coffee per day has been shown to raise their risk of non-fatal heart attack,’ says Alex Ruani.
‘Meanwhile, the fast metabolisers reduced their heart attack risk by the drinking the same amount.’
Dr Guttmann adds: ‘Research suggests being a slow metaboliser of caffeine could put pressure on your heart.’
He suggests this may be because caffeine blocks hormonal activity that widens arteries – as a result, it causes a spike in adrenaline release, which in turn leads to raised blood pressure.
‘So you might need to be a bit more cautious around caffeine – although one coffee per day should still be fine,’ he says.
How can you tell if you’re a slow metaboliser? If coffee has a strong effect on you and makes you feel jittery, for instance, you’re likely a slow metaboliser and should stick to lower levels, says Alex Ruani.
…But bad for some people’s sleep
Caffeine’s power to disturb sleep is well known. On average it takes half of the caffeine five hours to leave your system – but for some people, that ‘half-life’ can stretch up to nine-and-a-half hours, which is why caffeine can interfere with sleep, ‘even if consumed six hours before bedtime’, explains Alex Ruani.
Again, this effect is thought to be down to genetic differences, with slow metabolisers needing to leave longer between drinking coffee and sleeping.
If you’re sensitive to caffeine, it’s best to stick to a cup or two early morning.
Helps you burn off fat?
Caffeine has long been associated with speeding up your metabolism and burning fat faster – but can it really help keep weight off?
One 2023 study found that the levels of caffeine in your blood could affect the amount of body fat you carry – and help determine your risk of developing type 2 diabetes and cardiovascular disease.
In 2023 researchers at the Karolinska Institute in Sweden found that high levels of caffeine were associated with a lower BMI and a lower risk of type 2 diabetes – they attributed this to the fact caffeine increases thermogenesis (heat production) in the body, which in turn encourages it to burn stored fat off as energy, reducing overall body fat.
The researchers advised that moderate consumption of caffeinated drinks should be explored as a potential means of helping reduce body fat levels.
Feeds ‘good’ gut bugs …but acts like a laxative
Caffeine can also potentially change the makeup of the gut microbiome (the trillions of microbes that live in our digestive tract), by reducing the growth of harmful bacteria and promoting the growth of good bacteria, vital to digestion and also a healthy immune system.
Indeed, regular coffee intake is associated with an increased diversity of gut microbes, a marker of a healthy gut, according to one 2020 study by the University of Oviedo in Spain.
Coffee contains polyphenols, protective plant compounds that help to feed beneficial gut bacteria.
But caffeine can also have a laxative effect, particularly in those with a sensitive gut.
Has a cheering effect
A 2023 study found that consuming at least one small cup of coffee could help ease depression symptoms
Caffeine’s blocking effect on adenosine may also have an uplifting effect on your mood.
Adenosine builds up throughout the day, making you feel sleepy.
By blocking it, caffeine not only helps you feel more alert, but also leads to an increased release in other brain chemicals.
These include the ‘feel-good’ hormones dopamine and serotonin, which are closely involved in mood regulation – boosting their activity may help alleviate symptoms of depression in some people.
A 2016 analysis of all current research, by Qingdao University Medical College in China, found that daily caffeine consumption reduced depression.
Meanwhile, a 2023 study published in the journal Frontiers in Psychiatry found that consuming at least 90mg of caffeine per day (equal to a small cup of coffee or two teas) could help ease depression symptoms.
Could prevent dementia
Growing evidence suggests that caffeine could help ward off neurodegenerative diseases, including dementia.
Research by Toronto University in Canada in 2016 found that consuming caffeine was associated with up to a 60 per cent reduced risk of Alzheimer’s – with one to two cups of coffee enough to enjoy this benefit.
It’s thought the benefits may be due to caffeine’s blocking of adenosine receptors, which has also been linked with a reduction in amyloid plaque formation in the brain (a hallmark of Alzheimer’s).
Another explanation is that it improves blood flow to the brain, keeping the cells and neural pathways healthy.
