It's a good thing Japan is not in Europe.
Moving can have a big influence on someone's body/appetite, but moving specifically to Europe has other advantages. There's a book called The French (don't) Diet. I haven't read it, but I know people who have. The basic idea is that French food (and presumably much of Europe's food overall) is naturally healthy. It's not filled with preservatives or infused with high-fructose corn syrup. Those artificial ingredients in most American foods are hard for our bodies to digest, so they move to fat where they are almost never digested. The French food is made of natural ingredients that our bodies can actually use for nutrition and energy, so we don't store the food away - we use it.
The funny thing is that in America, we don't have this artificial food because it tastes better. We have it here because it's cheaper.
On another note, why is this on HN? I know it's tangentially related to the hacker diet, but an Ask YC: How do you lose weight? thread doesn't seem like it would belong here.