Food
The only thing I can think of where a Japanese person would change a major variable by moving out of Japan is something beneficial that they eat and drink only in Japan, or something bad they eat and drink elsewhere.
There are some other chemicals or pesticides that might only be used outside of Japan, but I don't know them off hand.
Nothing immediately occurs to me, but targeting the research in that direction, plus adding a few more populations from different countries, might nail something down.