It's both genetics and diet. They consume almost no protein or fat in their diet, almost all long-chain carbs (rice and rice starch) and vegetables. All their protein is from healthy sources like soy, egg and fish. And their fat sources are also healthy like the fragrant sesame oil and trans fat containing nuts.
If you're a girl let me tell you it's a healthy recipe to replace atkins diet, which in contrast is all unhealthy, hard-to-digest proteins like beef. (Something I would NEVER, ever recommend as a diet by itself).