The following originally appeared on The Upshot (copyright 2018, The New York Times Company).
Last week a paper was published in The Lancet that claimed to be the definitive study on the benefits and dangers of drinking. The news was apparently not good for those who enjoy alcoholic beverages. It was covered in the news media with headlines like “There’s No Safe Amount of Alcohol.”
The truth is much less newsy and much more measured.
Limitations of Study Design
It’s important to note that this study, like most major studies of alcohol, wasn’t a new trial. It was a meta-analysis, or a merging of data, from many observational studies. It was probably the largest meta-analysis ever done to estimate the risks from drinking for 23 different alcohol-related health problems.
The researchers also combined almost 700 sources to estimate the most accurate levels of alcohol consumption worldwide, even trying to find drinking that might otherwise be missed (from tourism, for instance). They then combined all this data into mathematical models to predict the harm from alcohol worldwide.
They found that, over all, harms increased with each additional drink per day, and that the overall harms were lowest at zero. That’s how you get the headlines.
But, and this is a big but, there are limitations here that warrant consideration. Observational data can be very confounded, meaning that unmeasured factors might be the actual cause of the harm. Perhaps people who drink also smoke tobacco. Perhaps people who drink are also poorer. Perhaps there are genetic differences, health differences or other factors that might be the real cause. There are techniques to analyze observational data in a more causal fashion, but none of them could be used here, because this analysis aggregated past studies — and those studies didn’t use them.
We don’t know if confounders are coming into play because this meta-analysis could only really control, over all, for age, sex and location. That’s not the researchers’ fault. That’s probably all they could do with the data they had, and they could still model population-level effects without them.
But when we compile observational study on top of observational study, we become more likely to achieve statistical significance without improving clinical significance. In other words, very small differences are real, but that doesn’t mean those differences are critical.
Interpreting the Results
The news warns that even one drink per day carries a risk. But how great is that risk?
For each set of 100,000 people who have one drink a day per year, 918 can expect to experience one of the 23 alcohol-related problems in any year. Of those who drink nothing, 914 can expect to experience a problem. This means that 99,082 are unaffected, and 914 will have an issue no matter what. Only 4 in 100,000 people who consume a drink a day may have a problem caused by the drinking, according to this study.
At two drinks per day, the number experiencing a problem increased to 977. Even at five drinks per day, which most agree is too much, the vast majority of people are unaffected.