Filling up my legs

Max has a lot of allergies and so we have to be very careful to monitor his diet. We do this quite well, but the kid can be pretty sneaky and has been known to get into food he should not have. He did this more when he was younger but now that he is getting older he knows that things make him feel ill and he doesn't like to feel that way, but sometimes he doesn't care - like when it comes to eating dog food.
I have absolutely NO idea why my son likes to eat dog food. I have never personally tried it, but really, it CANNOT taste that great.
When he HAS gotten into the dog food we can tell immediately because he gets a rash on his face instantaneously.
It has been a few months since he has had a rash, so last night after dinner the red on his cheeks was a dead giveaway he had been into something.
Here is the conversation as it unfolded in front of me.
Shel says to him, "Max, Did you eat dog food today?"
Max replies, "NO MOM! I ate cat food. I had to fill up my legs."

Note to self: Fill up Maxies legs or he will have to eat cat food.