Between the lines, the character is saying that the statement he is divulging is true because it has scientific proof, however he does not show how.
Explanation:
"Talking between the lines" is an expression that refers to something that an interlocutor does not speak clearly, but leaves implied and even hidden in the middle of his message. An example of this can be seen in the text shown in the question above, where the narrator says, subjectively that it is scientifically proven that milk and apples are good for the health of pigs. It is likely that the narrator said this between the lines, so that no one would question him about this scientific evidence that he just cited, but did not present evidence.
