Your diet does make a real difference, more than you credit for in your reply. In university I had pizza almost every day for a month, I gained 15 lbs (7 kg) and had a way flabbier stomach. I stopped doing that and tried to eat healthier, incorporating salad with my meals, and in just about a week I started noticing it going away, and I was back to where I was before in 3 months.

It wouldn’t surprise me too much to see how a body would react noticeably to a drastic change from a sedentary, highly processed carbohydrate diet and lifestyle to an active, more balanced one. Everyone’s body is different of course so it won’t always be the case, but to me the OP’s claims seem far from impossible. Japan still has its share of oily foodstuff, but the average portion is tiny compared to the US.