The truth about caffeine: how coffee really affects our bodies
Coffee. Go juice. Liquid gold. The one with all the psychoactive properties. Once used by Sufi mystics as an aid to concentration during religious rituals, it’s now one of the most ubiquitous drinks on the planet: we get through about 2bn cups a day.
It’s also one of the most valued and pored-over drinks. One particularly sought-after blend, Black Ivory, which is produced by encouraging elephants to digest arabica berries, retails at more than £2,000 a kilogram, while coffeemaking championships attract thousands of spectators.
But what does it actually do to you? You might have a vague idea that caffeine wakes you up, wrecks your sleep and can aid sporting performance, but do you know how much you can drink safely? Considering that a typical americano contains more than 100 biologically active ingredients other than caffeine, what do you know about the drug you are glugging two or three times a day? What is happening inside your body when you have a double espresso in the morning?
How quickly does it act?
The effects may start before you even take a sip. Just inhaling the scent of coffee can improve memory and stimulate alertness, according to a 2019 study of 80 18- to 22-year-olds. Another study, from 2018, found that subjects did better in tests of analytical reasoning after a whiff of the good stuff. That said, the researchers in the 2018 study suggested that the effect probably had a placebo element, with the expectation of improved performance proving at least partly responsible.
What about when you actually drink it? “There’s a chance that the use of any supplement will carry a placebo effect,” says Dr Mike T Nelson, a researcher and performance specialist who recently co-wrote the International Society of Sports Nutrition’s position on coffee. “Many researchers use randomised, double-blinded, placebo-controlled trials to try to ferret that out. And if you look at some of the higher-dose caffeine studies and when they have been compared with a placebo, we still see a performanceenhancing effect of caffeine.”
This is why the effects really kick in some time after you start drinking. While a 2008 study found that the effects of a cup of coffee can occur just 10 minutes after ingestion, it said peak caffeine concentration in the blood occurred after 45 minutes.
How does coffee wake you up?
Caffeine acts as a central nervous system stimulant – making you more alert and focused, but potentially also more irritable and anxious. It’s all to do with your body’s adenosine receptors, which help to regulate your heart rate, blood flow and sleep-wake cycles. When adenosine – an organic compound that occurs naturally in your body – binds to these receptors, it triggers physiological responses that lead to a decrease in cellular activity, often promoting drowsiness and sleep.
Caffeine can fool your nerve cells and bind to these instead, preventing adenosine from doing its thing. This promotes increased alertness, while also allowing the brain’s stimulating neurotransmitters (such as dopamine) to run wild. This makes it a moodbooster for many people, but can also lead to anxiety after high doses. While your body adapts to caffeine’s effects after a while, different people can have very different responses to the same amount of it.
Can it really boost athletic performance?
It certainly can. A 2020 study of amateur cyclists found that coffee improved performance by an average of 1.7%. This may not sound like much, but it’s a big deal for even moderately competitive athletes. An older British study reported a dose-related improvement in tests of reaction times, memory and visual-spatial reasoning among coffee drinkers.
Most of these performance benefits come from caffeine, which is why coffee isn’t always the best option. A recent analysis by the consumer group Which? found that a medium cappuccino at Costa contains 325mg of caffeine, while the Starbucks equivalent has just 66mg. Even more confusingly, a 2003 study found a wide range of caffeine concentrations (259mg to 564mg a serving) in the same drink obtained from the same outlet on six consecutive days. (The NHS doesn’t give an upper limit for daily caffeine intake unless you are pregnant, but the US Food and Drug Administration suggests that 400mg is fine.)
“Beans can make a difference – standard robusta beans are higher in caffeine than arabica beans, for example,” says Nelson. “But there are other factors – roasting actually breaks down caffeine, so in general darker roasts will have a lower caffeine content. But even when outlets try to control for beans, brewing method and so on, the variability in caffeine content is still quite high.
“This is why, for athletes, if you’re using it purely for performance-enhancing effects and you want to be very specific with it, I recommend taking caffeine in a pill form, because you can control your intake better.
“I’ve worked with some endurance athletes in the past who would just stop and get a normal coffee at any shop before the race – and a lot of times their performance might be a little bit different, probably because the caffeine amount wasn’t as tightly controlled.”
When should you stop drinking it?
This is hotly contested. Caffeine has a half-life of about six hours, which means that if you have your final espresso at 4pm, half of the caffeine is still in your system at 10pm, when you should be winding down for the night. Plenty of people subscribe to the idea of a caffeine curfew – stopping at 2pm or 3pm, for instance – but that doesn’t mean it’s open season in the morning.
“It should be a nice, balanced thing,” says Nick Littlehales, a sleep coach who has worked with several high-profile football teams. “I see a lot of people who have three coffees more or less back to back in the morning. They’re at 1,000-1,500mg before they get to lunchtime – then they stop their intake. That’s not a sensible way to do things: it’s about keeping it nice and level, with no big ups and downs. Keep track of when you have a little bit of a lowenergy lull, then you can actually use your caffeine intake strategically, to help you out at key times.”
Is it good for you?
This is where those other biologically active compounds come in. It’s also where the science becomes less clear. A few people suggest that excess intake may be linked to increased cancer risk or heart problems, while others say a few cups a day is fine.
It’s helpful to look at meta-analyses of many studies – or, even better, an “umbrella review”. One of the largest of these, which looked at more than 200 meta-analyses in 2017, said: “Coffee consumption seems generally safe within usual levels of intake, with summary estimates indicating the largest risk reduction for various health outcomes at three to four cups a day, and more