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A wee dram cuts obesity risk? It's not that simple

Ben Goldacre
Saturday December 17, 2005
The Guardian


Christmas is a time for pedantic family disputes, and newspaper stories about how alcohol and chocolate are good for you. This week, the Daily Mail reported on a scientific study which reported an observation from survey data: that people who drink alcohol in moderation have lower levels of obesity than people who drink heavily. No surprise there. But the same study also found, more interestingly, that people who drink moderately have lower levels of obesity than people who don't drink at all.
"Scientists said," they began, "drinking a few times a week can cut the risk of obesity by 27% compared to teetotallers ... Researchers are unsure quite why small, regular amounts of alcohol help to keep drinkers' weight down ..." I freely confess, I am possibly one of the pickiest people on the planet. But read those sentences again: this Daily Mail article is suggesting there is a causal relationship between moderate drinking and lower rates of obesity.

And that's not quite what the research showed. It looked at survey data, and found that people who said they were teetotallers had higher rates of obesity than people who said they were moderate drinkers. It's perfectly possible, and you might even say likely, that there is a third, unrelated factor, one that causes people both to be teetotal and to have higher rates of obesity.

I'm sure teetotallers have their reasons for not drinking. Those reasons might even be our missing link, the factor that causes you to be both a teetotaller and obese: they might be moral or cultural; perhaps people from ethnic groups who drink less are more likely to be obese; perhaps people who deny themselves the indulgence of alcohol are more likely to indulge in chocolate and chips; perhaps pre-existing ill health will force you to give up alcohol, and make you more likely to be obese. Perhaps these teetotallers are recovering alcoholics who are more likely to be fat from all those years of heavy alcohol abuse. Perhaps, in the survey, some of the people who said they were teetotallers were just lying about how much they drank. Perhaps, even, people who lie to themselves, and others, are more likely to be obese.

That's why we do scientific experiments, where we can control as many of the variables as possible. This study is a useful bit of preliminary survey data, but to discount these extra possible causal factors we've listed above, we'd have to take a group of people, and randomly assign them to a moderate-alcohol or no-alcohol group, and know that ex-alcoholics, and big chocolate fans, were equally represented in each group.

But that's also why designing scientific experiments can be difficult at times, or even impossible. I can't take a group of people with strong religious or cultural reasons for not drinking, and persuade them to be randomised into a group where they would have to drink alcohol. Outside of the mechanistic, cause and effect universe of the press, finding an association between two things is not enough to say that there is a causal relationship between them.