experience in SE Asia
I live below China in Ho Chi Minh City(Saigon), Vietnam. I can't really speak for China,as I've only been there as visitor, but I can tell you a bit about my experience here in VN which is quite similar in culture.
First of all, there is a great variety of fruits(papaya, dragonfruit, mangos, pineapple, coconuts, pumello, jackfruit, starfruit, and on and on) all fresh and very cheap. A whole pineapple goes for 10 cents U.S. Fresh chicken, seafood, and beef is also available very cheaply in the markets, on the street and in shops. Fresh herbs and spice are abundant and virtually free. I'll often grill up a chicken breast with just fresh basil and vegetables. Most of the locals are thin, which gives you motivation and also says something about their diet. My local gym is also reasonably priced, very friendly staff, and rarely crowded.
Now for some downsides: It is very expensive to get 'organic' food. I have to go to special shops to get different things, i.e. one place to get whole wheat pasta, a different place to get raw almonds, and a third place to get natural peanut butter. If I wanted something as exotic as almond butter, I'd be looking at $17 dollars a jar! Anything 'processed' that's made in Vietnam is going to be loaded with salt and oil. I only know of one place to by unsalted cashews. It's also very hard to adhere to a strict regimen when you're living an 'expat' lifestyle. I mean you'll want to try the local cuisine, go out drinking with other expats, etc. The locals don't really understand what you're trying to do either, they see fat as a sign of prosperity. Another thing that makes me laugh are nutrition labels that say: "flour, eggs, and other..."
Beijing is a much bigger city than Saigon, so you may have more specialty health food stores, though I'd guess the goods are imported and still expensive.
Yes, it's a challenge, as are many aspects of your life living overseas, but you are always learning something, and there's always an adventure around the corner. Vietnamese saying: "Den day den do cho biet, o nha voi me biet ngay nao khon?" "Travel and you'll gain wisdom, stay home with your mother - when will you be wise?"