The proverb promotes the idea that to be fit and healthy you need to eat well and consume good food. If you eat well, you will be healthy; but if you eat badly you will have a bad health.
You can use the saying “you are what you eat” when you want to point out the connection between food and health.
The proverb seems to come to us through several sources. Anthelme Brillat-Savarin wrote in 1826 in Physiologie du Gout, ou Meditations de Gastronomie Transcendante:
“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.]
Ludwig Andreas Feuerbach in 1863/4 in an essay entitled Concerning Spiritualism and Materialism wrote:
“Der Mensch ist, was er ißt.” [Man is what he eats.]
The proverb was introduced into English in the 1920s and 30s by the nutritionist Victor Lindlahr. His views were published in an advert in a 1923 edition of the Bridgeport Telegraph, for ‘United Meet Markets’ in the form of:
“Ninety per cent of the diseases known to man are caused by cheap foodstuffs. You are what you eat.”
In 1942, Lindlahr published You Are What You Eat: how to win and keep health with diet, which seems to have popularized it.