It’s long been used by athletes as a legal stimulant to boost their performance.
But a new study claims caffeine has the wrong effect on an unlucky 10 per cent of people when they exercise – making them run slower.
A trial on student athletes discovered the majority ran 6km (3.7 miles) quicker after drinking it – but a handful were 14 per cent slower.
Researchers discovered the people who ran slower have a genetic variation that makes them less able to digest caffeine.
That means it stays in their body and continues to act, maximising the cognitive benefits but increasing blood pressure.
And this also boosts their risk of having a heart attack, according to scientists who led the trial at James Madison University, Virginia.
A small percentage of the population is at a greater risk of heart attacks from caffeine
The researchers investigated the effect of caffeine on 101 male student athletes.
They found that, after consuming a strong cup of coffee’s worth of caffeine pills, some improved their cycle time by five per cent compared with a control.
However, a small number of study participants were actually 14 per cent slower after taking the caffeine pills.
The researchers believe the CYPIA2 gene may be the deciding factor in how our bodies process caffeine.
Previous studies have hinted at the effects of the gene of caffeine metabolism.
For instance, athletes who take caffeine have recorded as running 87 per cent longer, and cycling as much as 156 times longer.
HOW MUCH CAFFEINE IS SAFE?
The EU’s food safety watchdog advised a daily limit of 400mg for adults in its first guidelines on caffeine intake in 2015.
European Food Safety Agency officials suggested pregnant women should keep intakes below 200mg.
It also advised children to consume no more than 3mg of caffeine per KG of body weight – the equivalent of two mugs of milky tea for a child of four.
Health officials warned those who break the limits run the risk of a host of health problems, from anxiety to heart failure.
Its warning also showed links between high caffeine intake in pregnancy and having a baby that is underweight.
The NHS says too much caffeine can cause a miscarriage. There are also links to birth defects.
However, with coffee far from the only food or drink to contain caffeine, people may unintentionally be going over the safe limit.
However this was the first study to link the gene, caffeine consumption and athletic performance, and shows how caffeine can actually decrease performance in some people.
Around 50 per cent of the population have a variant of CYPIA2 that allows them to metabolise caffeine quickly.
While ten per cent have the form of the gene that can’t metabolise caffeine at all. The remaining 40 per cent of us, however, are somewhere in between.
The study was published in the journal Medicine & Science in Sport & Exercise.
Caffeine – good or bad?
Caffeine can increased blood pressure by causing our veins to shrink. However, a number of studies suggest they are health benefits to caffeine, as well.
A recent study looked at the rumoured weight loss properties of the drug, and its results suggests it really can.
Scientists compared coffee to cannabis, analysing its effects on 47 people.
People who smoke cannabis are known to get the ‘munchies’, being drawn to snacks like crisps and chocolate, as the drug hits part of the brain linked to appetite.
Coffee, the researchers have now found, has the reverse effect, which may make people less hungry, using the same system in the body.
The findings, from Northwestern University, support previous evidence that people drinking two cups of daily coffee after a diet are better able to keep the weight off.
It suggests that coffee may help people to slim, not just by speeding up their metabolism, but by making them less hungry.