An international proverb (that is, one found in many languages and cultures).
It is quite true literally since our bodies are built mostly from the food we eat. I say mostly, because what we drink and breathe is also a factor.
In several languages this proverb/adage is in the one is what one eats form, mostly because in these languages it is a perfect pun, as they use the same word for is and eats.
In some cases the words are spelled differently but sound the same (e.g. German - ist and ißt), in others they are even spelled identically (e.g., Slovak - je).