While people who eat a lot of red meat are known to
be at higher risk for certain cancers, other carnivores are not, prompting
researchers at the University of California, San Diego School of Medicine to
investigate the possible tumor-forming role of a sugar called Neu5Gc, which is
naturally found in most mammals but not in humans.
In a study published in the Dec. 29 online early
edition of the Proceedings of the National Academy of Sciences, the scientists
found that feeding Neu5Gc to mice engineered to be deficient in the sugar (like
humans) significantly promoted spontaneous cancers. The study did not involve
exposure to carcinogens or artificially inducing cancers, further implicating
Neu5Gc as a key link between red meat consumption and cancer.
"Until now, all of our evidence linking Neu5Gc
to cancer was circumstantial or indirectly predicted from somewhat artificial
experimental setups," said principal investigator Ajit Varki, MD, Distinguished
Professor of Medicine and Cellular and Molecular Medicine and member of the UC
San Diego Moores Cancer Center. "This is the first time we have directly
shown that mimicking the exact situation in humans -- feeding non-human Neu5Gc
and inducing anti-Neu5Gc antibodies -- increases spontaneous cancers in
mice."
Varki's team first conducted a systematic survey of
common foods. They found that red meats (beef, pork and lamb) are rich in
Neu5Gc, affirming that foods of mammalian origin such as these are the primary
sources of Neu5Gc in the human diet. The molecule was found to be
bio-available, too, meaning it can be distributed to tissues throughout the
body via the bloodstream.
The researchers had previously discovered that
animal Neu5Gc can be absorbed into human tissues. In this study, they
hypothesized that eating red meat could lead to inflammation if the body's
immune system is constantly generating antibodies against consumed animal
Neu5Gc, a foreign molecule. Chronic inflammation is known to promote tumor
formation.
To test this hypothesis, the team engineered mice to
mimic humans in that they lacked their own Neu5Gc and produced antibodies
against it. When these mice were fed Neu5Gc, they developed systemic
inflammation. Spontaneous tumor formation increased fivefold and Neu5Gc
accumulated in the tumors.
"The final proof in humans will be much harder
to come by," Varki said. "But on a more general note, this work may
also help explain potential connections of red meat consumption to other diseases
exacerbated by chronic inflammation, such as atherosclerosis and type 2
diabetes.
"Of course, moderate amounts of 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."