Red meats - beef, pork and lamb - are rich in a sugar called Neu5Gc and provide the primary sources of this sugar in the human diet.
The researchers wanted to understand why people who eat a lot of red meat are at higher risk for certain cancers, while people who eat other types of meat are not.
The team first conducted a systematic survey of common foods and found that red meats - beef, pork and lamb - are rich in a sugar called Neu5Gc and provide the primary sources of this sugar in the human diet.
From previous studies, the researchers had found that Neu5Gc can be absorbed into human tissues.
From these findings, the team hypothesized that eating red meat could, therefore, promote potentially cancer-forming inflammation if the body is constantly generating antibodies against Neu5Gc, which is a foreign molecule.
In a mouse model engineered to have a deficiency of this sugar, the scientists found that feeding the mice Neu5Gc resulted in systemic inflammation, which was associated with a fivefold increase in spontaneous tumor formation.
Principal investigator Dr. Ajit Varki, distinguished professor of Medicine and Cellular and Molecular Medicine, and member of the UC-San Diego Moores Cancer Center, says:
"Until now, all of our evidence linking Neu5Gc to cancer was circumstantial or indirectly predicted from somewhat artificial experimental setups. 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."
As the researchers did not expose the mice in the study to carcinogens or attempt to artificially induce cancers, they believe Neu5Gc is strongly associated with increased cancer risk.
Proving the association in humans will be 'much harder'
"The final proof in humans will be much harder to come by," Dr. Varki says. "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."
However, Dr. Varki also admits the team's findings - which are published in the Proceedings of the National Academy of Sciences - represent a "catch-22," as moderate amounts of red meat can be a source of good nutrition for young people. Dr. Varki hopes that the team's future work will provide practical solutions to this.
The researchers behind that study suggested, therefore, that by adding natural sources of resistant starch - bananas, beans, chickpeas, lentils, whole grains - to a diet high in red meat, the increased risk of colorectal cancer may be diminished.
Another study in 2014 also suggested that higher red meat intake during early adulthood was associated with a 22% increased risk for breast cancer.
That study found that replacing one portion of red meat a day with a portion of another high-protein food - for instance, legumes, poultry, nuts or fish - was associated with a 14% lower risk of breast cancer.