What are these white marks on my nails? How do I prevent them?

I'm a 13 year old girl and I've had these marks on my nails for about 5 years now.The marks go away after about a month then reappear on a different finger! Right now I have about six, each on different fingers. I was once told that these appear because I don't consume enough calcium or meat, which is true, I don't drink much milk or have a lot of meat. Another person told me that this is caused by stress but I don't know which is right! Does anyone know what really causes them? And how do I prevent these annoying little marks? Thanks