"Sugar molecule to blame for meat eaters' cancers." If you saw a headline like this in your Facebook feed early this year, let's hope you took it with a grain of salt.
Media coverage of a recent study on a sugar molecule called Neu5Gc, found in lamb, pork and beef, highlighted an alarming link between this sugar and a five-fold increase in tumours in mice. Reporters quoted the study authors as saying their data may explain the well-known association between red-meat consumption and human cancers.
But almost none of the media reports included any critical analysis or comments from scientists with no connection to the research.
In a recent Globe interview, Dr. Neal Barnard, an adjunct associate professor at George Washington University in Washington, noted that the study was published in a reputable journal, the Proceedings of the National Academy of Sciences.
Nevertheless, the findings do not come close to settling the question "as to why meat causes cancer in people," he said.
Humans do not naturally produce Neu5Gc, which enters our bodies when we eat red meat. According to the study hypothesis, our bodies attack this animal sugar as a foreign substance, triggering an immune response resulting in inflammation, which may over time promote tumour growth.
To mimic how humans react to this sugar, researchers from the University of California, San Diego, engineered mice that lacked Neu5Gc. The mice were then exposed to the sugar in feed and given antibodies equivalent to the ones humans would produce to attack the foreign sugar. In the study, the mice developed systemic inflammation and a "much higher incidence" of liver cancers, the authors wrote.
But as Barnard pointed out, mice differ from human physiology both in their susceptibility to cancers and in their protective mechanisms. The biggest problem in the study, he said, is that "the type of cancer that was caused in these animals is not the cancer than humans get when they consume meats."
In people, red-meat consumption has been strongly linked to colorectal and breast cancers, not liver cancer.
A major limitation of the Neu5Gc hypothesis is the absence of large human-based studies "to prove this observed association between Neu5Gc and cancer," said Dr. Peter Wang, an epidemiologist at Memorial University of Newfoundland.
Still, the hypothesis is worth pursuing, said Wang, who co-authored a 2014 study of colorectal cancers in red-meat eaters in Newfoundland and Ontario. Wang found a lower incidence of colorectal cancer among Newfoundlanders, who may be more likely to eat wild meats. He suggested that wild red meat may differ from farmed red meat in Neu5Gc content. "It would be interesting to explore this further," he said.
In the meantime, however, the Neu5Gc hypothesis is in its infancy compared to more established theories that explain the red-meat/cancer link. Researchers have found that grilling or barbecuing meats at high temperatures results in potentially carcinogenic byproducts. Nitrites and nitrates in bacon and other processed meats may react with naturally occurring amines in the large intestine to form compounds that may cause colorectal, esophageal and stomach cancers. Another theory supported by numerous population-based studies is that heme iron in red meat may be responsible for higher incidences of cancer and heart disease in meat eaters.
The evidence suggests that the cancer risk associated with red-meat consumption is due to a variety of factors, not just one, Barnard said.
In the press release for the Neu5Gc study, lead researcher Dr. Ajit Varki was quoted as saying that red meat can be a source of good nutrition for young people: "We hope that our work will eventually lead the way to practical solutions for this catch-22," he said.
Media reports suggested that scientists could develop a Neu5Gc antidote to reduce the cancer risk, or that selective breeding could allow farmers to lower the Neu5Gc content in meat. But they neglected to mention that Varki and co-author Dr. Nissi Varki are co-founders and have equity interest in SiaMab Therapeutics, Inc., a biopharmaceutical company with an interest in Neu5Gc and anti-Neu5Gc antibodies.
Many scientists have financial connections to for-profit enterprises, and their research is not necessarily biased as a result, Barnard said.
Even so, he cautioned against the notion that we can keep eating red meat to our hearts' content without suffering the consequences. The mechanisms behind the effects of red-meat consumption require further investigation in human-based models, such as human cell studies, said Barnard, founder of the Physicians Committee for Responsible Medicine, a U.S. organization that advocates for alternatives to animal research and promotes vegetarianism.
But given the strong evidence that eating red meat increases the risk of cancer as well as heart disease and type-2 diabetes, "we also need to stop recommending that people consume meat," he said.