A non-human molecule found in red meat and milk makes its way into the human system when eaten -- and seems to build up especially in tumors, say researchers.
The compound, called sialic acid, is found on the surfaces of animal cells but is not found in people, and may be one reason why animal-to-human organ and tissue transplants do not work well. Animals have a version called Neu5Gc, while humans carry Neu5Ac.
But researchers at have found it does show up in the human body, and showed it can be absorbed from eating red meat and milk.
They also showed that the body produces an immune response against the molecule.
There are already existing recommendations that people should not consume too much food containing saturated fats, such as dairy products and red meats.
The highest amount [of Neu5Gc] was found in lamb, pork, and beef [so-called 'red meat']," said researchers. Levels were very low or undetectable in poultry and fish, vegetables and hen's eggs.
"The small amounts of Neu5Gc in normal tissues also raise the possibility that anti-Neu5Gc antibodies are involved in autoimmunity," the researchers said.
Autoimmune diseases occur when the body mistakenly attacks healthy tissue and include type-1 or juvenile diabetes and some types of arthritis.
But much research has focused on the fat content of animal fat or byproducts of cooking meat as the cause of disease.
Researchers developed an antibody -- an immune system targeting protein -- that would hook onto Neu5Gc. The team found Neu5Gc in human tumor samples and to a much lower degree in healthy tissue.
More tests showed that most people had made their own antibodies that recognized Neu5Gc, and thus could potentially initiate an inflammatory immune response.
"Meat eating has certainly been a feature of human ancestors for many hundreds of thousands of years," they said.
"Thus, it is indeed possible that humans have developed some kind of tolerance or indifference to Neu5Gc. However, most humans are continuing to make antibodies against Neu5Gc."
It could be that the damage only builds up over years -- and that as people live longer, the consequences make themselves felt.
"However, we are now living longer and the question arises whether the gradual accumulation of Neu5Gc and the simultaneous presence of antibodies against could be involved in some diseases of later life," they said.