For people with one genetic variant, eating more processed meat was associated with more colorectal cancer, according to the research, just as the WHO scientists have asserted. But for people with the other genetic variant, eating more processed meat did not appear to raise the risk of getting colorectal cancer.
“What this suggests is that there are some people who should be more careful with processed meat, and that there are some that did not increase their risks by eating processed meat,” Jane C. Figueiredo, a co-author of the study published last year in Plos Genetics and an assistant professor of preventive medicine at the University of Southern California said an interview.
The researchers cautioned that the finding has yet to be replicated and that just because one group appeared to be immune from the risks of processed meat does not exclude the possibility that some other undiscovered genetic combination may raise the cancer risk for some in that group. Processed meats might raise the risks of other chronic diseases, too, they said.
But by highlighting an interaction between diet and genes, the research foreshadows a day when broad dietary guidelines - that is, rules that apply to everyone - may be combined with information tailored to a person's genetic background.
“We are just at the tip of the iceberg in this research,” Ulrike Peters of the Fred Hutchinson Cancer Research Center and senior author of the study said in an interview. “I believe that the public health messages for smoking, physical activity, alcohol and dietary factors will remain the same. But we will also be able to identify some groups who are at a higher risk.”
Exactly how much processed meat people ought to eat continues to be a matter of debate, even in the wake of the announcement. The WHO scientists cited figures suggesting that eating a few strips of bacon per day raises the risk of colorectal cancer from roughly five percent to about six percent, and some researchers have questioned whether the evidence is strong enough to prove that processed meat causes cancer. In coming to their conclusion, the WHO scientists cited 18 cohort studies and found the connection in 12. In the other six, presumably, no such connection was found.
If the genetic findings are replicated, they could help explain why the results connecting processed meat to colorectal cancer have been inconsistent.
The discovery of the genetic variants arose from a massive effort seeking to find genetic reasons for colorectal cancer. Scientists reviewed records of nearly 20,000 people in several countries - the U.S, Canada, Australia and Germany. About half of them had gotten the disease, and about half of them had not. The researchers then tried to find genetic differences between the groups, and combined that with how much processed meat they had been eating.
Their search, which reviewed some 2.7 million genetic variants, turned up one location on 10th chromosome out of the 23 pairs in humans that seemed particularly interesting.
For people with one type of genetic building block (known as a nucleotide) at that location, eating more processed meat seemed not to matter. For people a different type at that location, eating more processed meat was associated with a greater likelihood of having had colorectal cancer.
The problem, of course, is that the vast majority of people have no idea which genetic group they fall in. The odds are somewhat favorable: About 43 percent of people fell into the group for which more processed meat raised the risk of cancer; the remaining 57 percent fell into the other group for which more processed meat did not appear to raise the risk. But the researchers cautioned that the odds may be different in other populations.