Which one is true, an apple or a banana a day will keep the doctor away?

For me, it's an idiom. Apple was just used to represent all other fruits and vegetables. This is to encourage people to eat fruits and vegetables in order for them to have a healthier body.

It's an old English proverb that basically means eating nutritious food will make you healthier. As a result, you won't need to go to the doctor very often.