Scientists have discovered that a sugar called Neu5Gc - present in red meat, some fish, and dairy products - is related to the appearance of spontaneous tumors in humans.
According to researchers at the University of Nevada, Reno, about two million years ago, humans experienced a genetic change that differentiated us from most primates.
A spokesperson said: "This change protected us from some diseases, but caused current consumer products, such as red meat, to pose a high risk to health.
"[A gene called CMAH] - which allows the synthesis of this sugar - was deactivated."
Therefore, if humans consume products derived from animals that have the gene, the body suffers an immune reaction to sugar, which is a foreign substance in the body.
This can cause inflammation, arthritis, and even cancer.
The scientists - led by Spaniard David Álvarez-Ponce - carried out an analysis of 322 animal genomes to determine whether or not they have active CMAH genes.
Next, they placed the results in the evolutionary tree of the animals, to determine at what moments of their evolution said gene was deactivated.
This allowed them to understand why certain species have an active CMAH gene, while other similar ones do not.
Different animals carry different quantities of Neu5Gc sugar.
David Álvarez-Ponce said: "Our analyzes show that there are fish that have the CMAH gene and others that do not, but for the moment the Neu5Gc sugar has been measured in very few of them.
"In fish that do have this gene, sugar is found in very small proportions in their meat, but in high quantities in caviar. This may be because the gene is expressed specifically in eggs or oviducts."
Sateesh Peri, a master's course student at the Alvarez-Ponce laboratory, added: "It turns out that caviar, one of the most expensive meals in the world, is also one of the products with the highest concentrations of Neu5Gc."
They did note that some fish that do not have the CMAH gene, and their caviar is expected to be free of Neu5Gc.
The researchers also looked at birds.
A spokesperson said: "Like humans, birds also do not have CMAH genes; so consuming chicken, turkey or duck does not have the negative effects of consuming red meat.
"Another group of animals that does not have CMAH genes are reptiles, except for one species of lizard.
"The presence of the gene in this lizard was unexpected, and invalidates the belief (until now accepted) that the gene had been lost in an ancestor of all reptiles and birds."
According to the researchers: "The presence or absence of the CMAH gene in different animals, which this study has characterized, points out which animals we should not eat (or eat only moderately), and which animals may present pathogenic microbes that affect humans, according to the scientists.
"If the animal has the gene, then its meat can have the same negative effects as red meat.
"If the animal does not present the gene, it may contain pathogenic microbes that bind to the Neu5Ac sugar (the precursor of Neu5Gc) and, therefore, may affect humans."