While the effect of eating meat on the risk of developing cancer is a contentious topic, the evidence that long-term consumption of red meat is strongly linked with a modest, but significant, increase in the risk of developing bowel cancer is convincing. However, scientists have failed to find a mechanism to explain this apparent association, despite several hypotheses having been put forward in the past. But now, a new study conducted on mice is finally offering us some insight.

They found that a particular sugar, which is present in high quantities in red meat, could be triggering an inflammatory immune response, which is known to encourage the development of cancers. Furthermore, long-term exposure of this sugar to mice was found to significantly promote spontaneous cancers. The work has been published in Proceedings of the National Academy of Sciences.

It is well recognized that the long-term consumption of red meat is associated with an increased risk for certain cancers in humans, in particular colorectal (bowel) cancer. But what has perplexed scientists is why the same does not seem to be true for other carnivorous animals.

One factor that scientists identified as a potential culprit for red meat’s apparent carcinogenic effects is a sugar called Neu5Gc, which is found in most carnivores but not humans. Even though humans can’t synthesize this molecule, it has been found in high levels in some cancerous tissue.

To find out more about whether this sugar could be contributing to the increased cancer risk, scientists from the University of California, San Diego, surveyed common foods to find out which contained the most Neu5Gc. They found that it is highly and selectively enriched in red meat, such as pork, beef and lamb.

Previous work discovered that one form of Neu5Gc is bioavailable, meaning that it can be distributed throughout the body via the bloodstream. Furthermore, despite being a foreign substance, it becomes incorporated into human tissues.

The researchers therefore hypothesized that the presence of this sugar in our bodies could be triggering an immune response, which in turn leads to inflammation and subsequently the promotion of tumor formation.

To test this out, they engineered mice to be deficient in Neu5Gc, mimicking the situation in humans. The team notes that these mice are already prone to develop tumors in the liver, an organ that can incorporate Neu5Gc.

They then fed these mice a diet enriched with Neu5Gc for 12 weeks and gave them regular injections with Neu5Gc antibodies to replicate what happens in our bodies. Sure enough, the mice developed systemic inflammation and experienced a five-fold increase in the incidence of cancers.

While this is the first study to show that mimicking the situation in humans increases the development of spontaneous cancers in mice, the researchers acknowledge that it will be much more difficult to prove in humans. However, the results could provide a possible explanation for the links between red meat consumption and other diseases worsened by chronic inflammation, such as type 2 diabetes.

“Of course, moderate amounts of red meat can be a source of good nutrition in young people,” lead researcher Ajit Varki said in a news-release. “We hope that our work will eventually lead the way to practical solutions for this catch-22.”

[Via PNAS, UCSD, Discover and Medscape]