You are what you eat
The notion that to be fit and healthy you need to eat good food.
This phrase has come to us via quite a tortuous route. 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'.
Neither Brillat-Savarin or Feuerbach meant their quotations to be taken literally. They were stating that that the food one eats has a bearing on what one's state of mind and health.
The actual phrase didn't emerge in English until some time later. In the 1920s and 30s, the nutritionist Victor Lindlahr, who was a strong believer in the idea that food controls health, developed the Catabolic Diet. That view gained some adherents at the time and the earliest known printed example is from an advert for beef in a 1923 edition of the Bridgeport Telegraph, for 'United Meet [sic] Markets':
"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. That seems to be the vehicle that took the phrase into the public consciousness. Lindlahr is likely to have also used the term in his radio talks in the late 1930s (now lost unfortunately), which would also have reached a large audience.
The phrase got a new lease of life in the 1960s hippy era. The food of choice of the champions of this notion was macrobiotic wholefood and the phrase was adopted by them as a slogan for healthy eating. The belief in the diet in some quarters was so strong that when Adelle Davis, a leading spokesperson for the organic food movement, contracted the cancer that later killed her, she attributed the illness to the junk food she had eaten at college.
Some commentators have suggested that the idea is from much earlier and that it has a religious rather than dietary basis. Roman Catholics believe that the bread and wine of the Eucharist are changed into the body and blood of Jesus (Transubstantiation).
Is the phrase Catholic rather than catabolic?
Archbishop Thomas Cranmer in 1549:
We offer and present unto thee, O Lord, our selves, our souls and bodies, to be a reasonable, holy, and living sacrifice unto thee; humbly beseeching thee that we, and all others who shall be partakers of this Holy Communion, may worthily receive the most precious Body and Blood of thy Son Jesus Christ, be filled with thy grace and heavenly benediction, and made one body with him, that he may dwell in us, and we in him.
Transubstantiation certainly links food and the body, but there doesn't appear to be a clear link between the belief and the phrase. It's safe to assume the origin is more supper than supplication.