Marcus Munafò and his colleagues at the University of Bristol, have looked into the smoking and drinking habits of about 250,000 people. They found that smoking makes you drink more caffeinated drinks, possibly by changing your metabolism so that you break down caffeine quicker, pushing you to drink more to get the same hit.
It’s impossible to do a randomised controlled trial (the most rigorous kind of scientific trial) when it comes to smoking, because it would be unethical to ask a randomly selected group of people to smoke. The next best thing is to study huge biobanks of health data. These biobanks contain information about people’s genes, diets and lifestyles.
To explore the relationship between smoking and caffeine, Munafo and his colleagues analysed data from biobanks in the UK, Norway and Denmark. They were particularly interested in people who had inherited a variant of a gene that has already been shown to increase cigarette smoking.
The team found that people who had this gene variant also consumed more coffee – but only if they smoked. British people with the same variant also drank more tea, although their Danish and Norwegian counterparts didn’t. This is probably due to cultural differences, says Munafò. “People in Norway and Denmark don’t chain drink tea in the same way that people in the UK do,” he says.
The genetic variant seems to influence how much nicotine a person consumes. You can have zero, one or two copies – and each additional copy is linked to an increase in smoking of about one cigarette per day. Each copy also appears to increase coffee consumption by 0.15 cups per day.
“You could extrapolate from that and say that if you smoked 10 cigarettes per day more than the next person, you would be drinking the equivalent of about one and a half extra cups of coffee per day,” says Munafò. He is wary of doing so, though, because the amount of nicotine a person gets from a cigarette will depend on the type of cigarette and the way it is smoked.
The gene variant codes for a nicotine receptor, which is not known to directly interact with caffeine. This suggests that cigarette smoking increases caffeine consumption and not the other way around.
“The team have used a rather clever technique to establish causality, which normally you wouldn’t stand a cat in hell’s chance of doing with an epidemiological study,”
Robert West – University College London.
What’s the link?
There’s a chance that cigarette smoking and caffeine consumption are linked through habit – that smokers tend to pair the two. But Munafò thinks that the nicotine in cigarettes might also influence the way a person metabolises caffeine. “It’s possible that smokers metabolise caffeine more quickly,” he says. If that is the case, smokers might need to consume more caffeine to get the same effects that a non-smoker would experience.
It’s also possible that the apparent link between smoking and coffee drinking could be down to some unknown function of the genetic variant, says West. “It evolved for a purpose, and it wasn’t to smoke,” he says.
A relationship between smoking and coffee might make it harder for smokers to quit, says Munafò. If a smoker stops smoking, but continues to drink plenty of coffee, they might start to experience unpleasant side effects, such as jitteriness. This might be misinterpreted as a symptom of smoking withdrawal, says Munafò. His team plans to investigate this.
Journal reference: bioRxiv, DOI: 10.1101/107037