The story I'm looking for might have been on fanfiction.net but I couldn't say for sure. Sam and Dean were in a Diner eating and Dean was eating a lot because he was really hungry. Sam commented on it negatively or something. I think Dean started eating a little less and it was noticeable. Then Sam remembered that when they were younger they would run out of food/money while their dad was gone and Dean made sure Sam had food by eating less or not at all. At the end, I think Sam told Dean that he should eat however much he wants or something.

I would love if anyone could find that story or one that is similar, I would really appreciate it. 

Found: http://www.fanfiction.net/s/4987670/1/Not_Hungry