I was read this article (interesting read btw) and came across this nice.. or disgusting little tidbit:
Carnivores show high levels of nitrogen-15, which correlates with their high place in the food chain, since the isotope tends to collect in animal tissue, and therefore increases in meat-lovers. The worms Macko discovered were steeped in sulfur-35, like other creatures that feed on sulfide-eating bacteria. Macko can even distinguish between rice and wheat-eaters from those who eat corn (thanks to the different enzymes with which crops turn sunlight into sugar, corn shows higher percentages of carbon-13). Macko's dietary sleuthing culminated last year in an appearance in "King Corn," a documentary about America's corn dependency. Using hair samples, Macko found that 65% of America's nutrition comes from subsidized corn agriculture.
Corn is pretty pervasive, heh.