Alcohol-cancer link is not as scary as it seems

The headline had some of my friends in a panic. Citing evidence, the American Society of Clinical Oncology warned that even light drinking could increase the risk of cancer.

Once again, we’ve been told that something we eat or drink is going to kill us. Once again, we’re provided an opportunity: A more nuanced discussion of risk — and how we communicate it — can leave us much happier, and perhaps healthier.

Let’s begin with the fact that it’s easy to use studies to talk about cancer. Nothing illustrates this better than the classic 2013 study that examined research on 40 common ingredients selected from an ordinary cookbook. Researchers found 264 different studies touching on at least one of those ingredients. Their conclusion? Depending where you look, you can find evidence that says that nearly everything we eat is both associated with higher rates of cancer and lower rates of cancer.

The gist of the oncology society announcement is that there is a reasonable amount of evidence finding an association between some cancers (specifically oropharyngeal and larynx cancer, esophageal cancer, hepatocellular carcinoma, breast cancer and colon cancer) and alcohol. It acknowledges that the greatest risks are with those who drink heavily, but it cautions that even modest drinking may increase the risk of cancer. In the United States, the announcement also notes, 3.5 percent of cancer deaths are attributable to alcohol.

Of course, this means that 96.5 percent of cancer deaths are not attributable to alcohol. If we eliminate heavy drinking, which no one endorses as healthy and where the association is surest, that number climbs. If we also eliminate those who smoke — smoking is believed to intensify the relationship between alcohol and cancer — the number of cancer deaths not attributable to alcohol approaches 100 per cent.

Let’s stipulate that there may be a correlation between light or moderate drinking and some cancers. We still don’t know if the relationship is causal, but let’s accept that there’s at least an association. For breast cancer — which is the cancer that seems to be garnering the most headlines — light drinking was associated with a relative risk of 1.04 in the announcement. Relative risk refers to the percentage change in one’s absolute (overall) risk as a result of some change in behaviour. (And 1.04 is a 4 per cent change from 1.0, which represents a baseline of no difference in risk between an experimental group and a control group.)

A 40-year-old woman has an absolute risk of 1.45 percent of developing breast cancer in the next 10 years. This announcement would argue that if she’s a light drinker, that risk would become 1.51 per cent. This is an absolute risk increase of 0.06 per cent. Using what’s known as the Number Needed to Harm, this could be interpreted such that if 1,667 40-year-old women became light drinkers, one additional person might develop breast cancer. The other 1,666 would see no difference.

Of course, moderate or heavy drinking might increase the risks further. The absolute risks for that 40year-old would go to 1.78 per cent from 1.45 per cent for the moderate drinker, and to 2.33 per cent for the heavy drinker. Those numbers are still not that scary. But maybe any increase in risk is too much for you. That’s fine. If you’re one of those people, though, you have to acknowledge that you can’t look at any one cancer in a vacuum.