The bacterial toxin, subtilase cytotoxin, is thought to have originally evolved to attack cattle and other animals that produce Neu5Gc naturally. For these animals, the consequences of the toxin are less severe because the Neu5Gc in their blood serum prevent the toxin from accumulating at the cells of the gut. However, humans long ago lost the ability to produce Neu5Gc and are only exposed to it through the food that enters their intestines. To demonstrate that humans could add the sugar to their own cells, Varki consumed some pure [Neu5Gc]. “It showed up in my urine and got incorporated into my facial hair,” he says. “I stopped eating red meat after that” [New Scientist].