STATS ARTICLES 2005
Colorectal Cancer is Red Meat for Media
January 20, 2005
But how much risk is there from red meat?
The Journal of the American Medical Association (JAMA) recently published a new study investigating whether there was any risk of colorectal cancer from eating red meat.
According to the American Cancer Society (ACS), almost 150,000 new cases of colorectal cancer will be diagnosed in the United States this year, with about 56,000 deaths. The lifetime risk for the disease is 2.5 to 5 percent, and can be much higher for people in high-risk categories. According to the Centers for Disease Control (CDC) and ACS, environmental risk factors, as opposed to innate factors such as family history, may include:
- Diet low in vegetables and fruit
- Diet high in animal fat
- Heavy alcohol consumption
- Low physical activity
The authors, from the American Cancer Society, the Rollins School of Public Health at Emory University, and the National Cancer Institute, studied the effects of different types of meat consumption, including long term, high meat consumption versus short term high consumption; consumption of red meat compared to consumption of processed meat; and a high ratio of red meat to poultry and fish compared to a low ratio of red meat to poultry and fish.
Previous studies have explored the association between meat and colorectal cancer, and the results have been varied. Some studies adjusted for possible confounding factors (such as obesity and smoking), while others did not. Some looked at women and men together, and others did not. Several studies had results that conflicted with other studies. Overall, the results were not conclusive.
The authors of the JAMA study performed statistical analyses in several ways, adjusting for many factors that may be confounders in any study on cancer. Their results? “Study bolsters cancer-red meat link” said CNN; “Diet study reaffirms red meat as a culprit in colon cancer” warned USAToday. According to these and other media accounts, adults who consume large quantities of red and processed meat over a ten-year period are 30 percent more likely to get colon cancer and 40 percent more likely to get rectal cancer.
But if you look at the study itself, red meat is not quite the red flag claimed by the media. Even comparing those who consume the most meat, with those who consume the least, the authors found varying results, some of which were conclusive and many of which were not.
Consider first the risk of colon cancer among men who reported their meat consumption in 1992. The risk was 30 percent higher for men who consume a lot of meat compared to those who consume only a little. This result is not statistically significant.
Statistical significance is the gold standard in statistics and epidemiology; it is the accepted breaking point for when authors can conclude a result in the group of people studied is true of the whole population.
In this case, the authors cannot be confident (defined in a careful statistical sense) that this 30 percent rise did not occur by chance (the group that ate more meat, for instance, might have just unluckier than the group that ate less).
By contrast, there is no indication that red meat causes colon cancer in women: The rates for those who eat a lot of red meat are comparable to those who eat very little.
For rectal cancer, the authors combined the results for men and women, and there is some bad news with the good. Heavy red meat consumers are more likely to get rectal cancer than their low-meat-eating counterparts – about 71 percent more likely among those reporting high consumption in 1992.
But the same is not true of those who ate processed meats. Though their cancer rates were 26 percent higher than those who did not eat much processed meat, the result, again, was not statistically significant.
For long term consumption (over 10 years), the results were also less alarming than the media claimed. Red meat did not affect the rate of upper colon cancer at all (although there was a slight, but not statistically significant, increased risk for lower colon cancer)
The authors did confirm an increased risk – about 43 percent more – for rectal cancer among long term carnivores. In contrast, for processed meats, a heavy consumer has an increased risk for lower colon cancer, but does not see statistically significant increases in the risk for rectal or upper colon cancer.
The numbers seem high, but they should be put in perspective. The increased risk for colorectal cancer associated with high meat consumption affects about one percent of these meat-eaters over the course of their lifetime. That may be too high a risk for some people, but it shouldn’t lead you to avoid eating red meat.