"Man is what he eats." It was written by a philosopher, Ludwig Feuerbach, in the middle of the 19th century. Originally in German, the verbs in his saying rhymed like a pun (Der Mensch ist was er isst).
Feuerbach investigated the relation between diet and people's well-being and pointed out that human beings are "equal in their stomachs" in the sense that they are physiologically constructed by the nutrition they have taken in.
What he wrote carries social and political implications. Of course, people get different foods, depending on where and how they live. In this sense, his saying can also be interpreted as emphasizing the differences among regions, countries and social classes as reflected in the difference of diet.