I'd like to say that eating garbage doewnt necessarily make you garbage. Thats like insulting people in poor countries wbo are desperately trying to survive (and yes this occurs a lot more than a lot of us would like to admit).
But it means exactly that. That, for example, you eat garbage you are garbage. It's said by people who has such cozy lifes that they're so self absorbed to understand different people all live under different circumstances.
Also, check out nuttcase's comment. That's also true.
It shows how much you VALUE your self, your body and it is assumed that it somehow reflects the same to who you are and how you value other people as well.
If you eat healthy and treat your body good by eating healthy foods you are assumed to have high regards to yourself and as to how you want to live your life. Thus, reflecting who you are in some respects.