I believe isotope testing is more useful in historical and ancient remains, from the times when most people ate food grown and drank water sourced from the place they lived.
In today's global market, your food and drink could be from anywhere. Just ate noodles from Indonesia. Never been anywhere near Indonesia in my life. Drinking mineral water with it. Where did that water come from? Who knows, could be anywhere in the country. Will probably have some chocolate before bed. Chocolate doesn't grow in my country at all. Neither does the coffee I had for breakfast. See the problem?
MOO