Anthelme Brillat-Savarin wrote, in Physiologie du Gout, ou Meditations de Gastronomie Transcendante, 1826:
"Dis-moi ce que tu manges, je te dirai ce que tu es." [Tell me what you eat and I will tell you what you are].
In an essay titled Concerning Spiritualism and Materialism, 1863/4, Ludwig Andreas Feuerbach wrote:
"Der Mensch ist, was er ißt."
That translates into English as 'man is what he eats'.
For an in-depth explanation of the origin and evolution of the term click here.