why my dog eat her poop????neeedd helppp....

I have a puppy who was the same. She's not a puppy any more, she's nearly 2. When she was younger, I would always catching her eating poop! So gross!!! I did some research about it and also asked my vet. From all the sources that I checked, it was simply because she was missing something that she felt like she needed. I was feeding her what I thought was the absolute best dog food available so I didn't think it possible for her to be missing something, but she was. I don't remember exactly what....some vitamin. Anyway, I changed her food and she stopped eating the poop.