I've now lived in Japan for 9 months. I've learned a lot. Seen a lot. Made some major observations. Japanese people are thin. Yes, some of this may be good genes but I don't think it's all of it, or even most of it. Here's why.
People eat real food. Every morning there is a line outside the supermarket before it opens. The people do their chores, hang their laundry out, and go to the market. They buy what they need for the day and come home and cook it. There is a line at the fish counter. Most of the shopping carts are full of fresh fish, fresh produce, and rice. Processed foods are available, yes, but they seem to be seen for what they are, treats, not food. Preservatives, HFCS, genetically engineered crap... not so common. You can't buy soda in 2 liter bottles or in 24 packs. If you want a soda you can buy a soda... but ONE soda at a time.
It's taboo to walk around on the street eating. People don't do it. If you want to eat a meal you sit down at a restaurant and eat a meal or you go home and cook yourself something. OR, better yet, they bring a Bento lunch and find a place to sit down and eat it. Rice, Fish, Veggies.
Fast food is rare. Yes, we have McDonald's but there's no drive through. People don't order HUGE hamburger sets with a giant fries and a giant drink. Most of the people I see in McDonald's grab a small burger and a green tea and sit down to eat it. There aren't really any commercials on TV bragging about things like the "Taco Bell 4th meal" or "Monster bacon whatever Burger." People know where they like to eat and they eat there. Sushi, Ramen, Beef Bowl... Some of it may be a little fatty but nearly all of it is REAL FOOD.
People walk or ride their bikes EVERYWHERE. I spend more time when I'm out driving dodging pedestrians than I do dodging other vehicles. There are 90 year old women who can barely stand hoofing it home from the market with full bags. If they want to get somewhere they GET SOMEWHERE. It's impressive.
So yeah, I've learned a lot. In Japan food is something you eat for fuel. Much of it is delicious but the whole day does not revolve around getting a fix. This doesn't seem to be the truth in America. It'll be a struggle for me to come back. It's really opened my eyes!