Some smokers have genes that predispose them to heavier smoking. Researchers looked at whether those same genes might trigger heavier drinking — and it turns out, they don't.
Several genetic variants have been identified that can contribute to how much a person smokes. These variants are essentially a genetic indicator of how heavily a person is likely to smoke.
"Some people are genetically predisposed to smoke a little more than others," says Professor Johan Håkon Bjørngaard at the Norwegian University of Science and Technology's (NTNU) Department of Public Health and Nursing.
The genetic variants are no more common among smokers than non-smokers. But people who start smoking and who have these genetic variants smoke more on average than people who do not have them.
One reason may be that the genetic variants change the effect of nicotine in the body. Bjørngaard and his colleagues have studied the genetic variants rs1051730 and rs16969968. The genes they reside in code for a receptor that nicotine can bind to.
But what about alcohol use?
55 967 smokers studied
Bjørn Olav Åsvold is a professor at the Nord-Trøndelag Health Study (HUNT) and NTNU's K.G. Jebsen Center for Genetic Epidemiology. He raised the question whether a person who is genetically predisposed to smoking more would also be prone to drinking more alcohol.
A research group from England, Denmark and Norway decided to find out. The group included researchers from NTNU and St. Olavs Hospital.
The researchers obtained statistics from four major health databases, including the HUNT Study.
The study used data on 55 967 smokers from the four databases. Their results have been published in the International Journal of Epidemiology.
The researchers used a special method to measure causal effect, since they weren't looking at the connection between smoking and alcohol use directly.
Instead, they looked at the genes as an indicator of smoking behaviour using a procedure called "Mendelian randomization".
You could say that Mendelian randomization is nature's way of conducting an experiment. Which genes you inherit from your mother or father are random. That way, you avoid some problematic things that could otherwise affect the research results.
Bjørngaard and Åsvold note that NTNU is a leader in Norway when it comes to using this method.
So what did the researchers find this time?
Not the genes' fault
The research group found a correlation between the number of cigarettes people smoke and the number of alcoholic units they consume during a week. Most people didn't find that to be much of a surprise.
However, the researchers found little evidence of a direct link between alcohol use and the genetic variants rs1051730 or rs16969968. In other words, the results do not indicate that more smoking leads to more alcohol use.
The researchers concluded that previously reported connections between smoking and alcohol unlikely have direct causal links.
In other words, you can't blame smoking if you drink a lot. Other factors that cause both smoking and alcohol are probably more important.