Are there heavy isotopes in food?

Of course. They are natural and therefore they occur everywhere. In every plant or animal tissue, including humans, there are naturally occurring 34S, 15N, 13C, 18O, 2H etc. In total, in the body of an adult man or woman are more than 200 g of heavy isotopes. Since they are omnipresent in every environmental compartment on Earth, we are adapted to them and at natural abundances, they are harmless to plants, animals and the people.