A reader, who happens to write one of the best-named blogs on teh tubes, pointed me toward an article I never would have seen. This parallels a news story we had here in the States late last year. So, since the story is getting press overseas (albeit late), it’s time to dust off the old post and update it a bit.
The story repeats the finding that processed meats increase the risk of colon cancer. This news comes from a large report published by the World Cancer Research Fund, which looks at data surrounding diet and cancer. It states that there is no safe level of processed meat consumption when it comes to colorectal cancer risk. It’s going to take a long time to parse through all the data, but since I love my processed meat, I’ll start there, and once again, my scientist colleagues will please forgive me for oversimplifying.
First, this is a huge report, pooling tons of data. One of the most important conclusions is regarding obesity and cancer risk, but that will have to wait until later.
Per USAToday, “every 1.7 ounces of processed meat consumed a day increases the risk of colorectal cancer by 21%.” Per the Daily Mail, “[e]ating just one sausage a day raises your cancer risk by 20 per cent.” What does that mean? “Risk” is a complicated concept in medicine. It is easy to draw overbroad conclusions from bits of data. When risk is measured, it is rarely intuitive–small percentages can indicate large increases in risk, large numbers can refer to small increases in risk–it depends quite a bit on the base line incidence and prevalence of the disease. A 50% increase in a disease sounds big, but in the right situation it can be big or small. For example, if your “usual” risk of disease A is 2/100, then a 50% increase makes your risk 4 in 100, meaning out of 100 people, 2 more get the disease then they would without the extra risk. If the “usual” risk is 10/100, then a 50% increase means 5 more people get the disease.
I hope you haven’t given up on me here. Keep reading…trust me…
Now, for a disease whose risk is 4/10,000, a 50% increase in risk means that 6/10,000 will get the disease–that’s not so many.
Let’s keep doing that math:
There are about 150,000 new cases of colorectal cancer in the U.S. every year, making the yearly “risk” of colon cancer about 1/2000. A 21% increase means that about 20,000 excess cases yearly, if everybody eats that yummy bacon, and if that risk increase can really be interpreted as applying to a yearly statistic. That’s actually quite a few people, so it’s worth looking at the data even more closely.
Referring to pages 122-123 in the report, about 16 studies were compared. (I pulled up the data from the Goldbohm study to look at a sample of what they were working with.) Many of the studies are small, and show minimal effects of processed meat on colon cancer incidence. The confidence intervals of most of the studies cross 1, meaning bacon may confer either risk or protection.
That being said, the data, in aggregate, show a possible trend in the direction of processed meat causing colon cancer.
So what does all this mean?
When the news anchor says that the risk of colon cancer is 21% higher with sausage, it does not mean your personal risk of cancer goes up 21%. These data show a trend toward increasing colorectal cancer risk with increasing consumption of processed meat. That’s it. That’s the only conclusion. There’s no need to induce vomiting to purge today’s breakfast. It might be wise to cut back, however.
If Mark Chu-Carroll or any other mathematically-inclined reader wishes to correct my math, I certainly won’t be offended.