Jean Anthelme Brillat-Savarin (1755-1826), in the book Physiologie du Gout (1825), wrote “Tell me what you eat, and I shall tell you what you are,” often interpreted as “you are what you eat.” This was figurative and not meant literally; one does not become an apple or a donut by eating such things.
 
“Avoid fruits and nuts: after all you are what you eat” is a poster that features the cartoon cat Garfield (a comic strip created by Jim Davis). It’s not known when “Garfield” first said this. “‘Avoid fruits and nuts. You are what you eat.’ Garfield (Jim Davis)” was cited in print in 1990. “Fruit” is slang for “homosexual” and “nuts” is slang for “crazy,” and some people have taken offense to the slang in this saying.
 
   
Wikiquote: Food
Dis-moi ce que tu manges, je te dirai ce que tu es. Tell me what you eat, and I shall tell you what you are.
. Jean Anthelme Brillat-Savarin, Physiologie du Gout (1825); tr. M. F. K. Fisher, The Physiology of Taste: Or Meditations on Transcendental Gastronomy (1949)
. Variants:
..Der Mensch ist, was er ißt. Man is what he eats.
..Ludwig Andreas Feuerbach, Die Naturwißensschaft und die Revolution [Natural science and the revolution] (1850), repeated in Das Geheimnis des Opfers, ober der Mensch ist was er ißt [The mystery of sacrifice, or man is what he eats] (1862)
..You are what you eat.
..Victor Lindlahr, You Are What You Eat: how to win and keep health with diet (1942)
 
Google Books
The fourth—and by far the most recent—637 best things anybody ever said:
Many given heightened piquancy by nineteenth-century line cuts
By Robert Byrne
New York, NY: Atheneum
1990
Pg. ?:
314
Avoid fruits and nuts. You are what you eat.
Garfield (Jim Davis)
 
16 November 1990, Rocky Mountain News (Denver, CO), Entertainment/Weekend:
Food for thought: “Avoid fruits and nuts. You are what you eat.” Garfield (Jim Davis).
 
Google Groups: bit.listserv.catholic
lou nunez  
12/29/93
(...)
Please keep in mind the immortal words of that sage philosopher Garfield T. Cat who said “You are what you eat, therefore avoid fruits and nuts.”
   
Scientific American
dkincheloe
07:42 PM 7/7/10
I’m rather surprised that the editors of Scientific American would make the following crass, insensitive joke here (at the expense of gay men, and of people with mental illness diagnoses), in this journal, in public; a joke that’s better suited to their own private domains, among other less-than-empathic colleagues: “Last but not least, Arcimboldo’s masterpieces also bring to mind the old adage that you are what you eat. So you should avoid fruits and nuts (at least, according to Jim Davis’ Garfield the cat).” The attribution, of course, allows the editors to make the joke, without taking responsibility for it, right?
   
Google Books
Just Kidding:
Using Humor Effectively
By Louis R. Franzini
Lanham, MD: Rowman & Littlefield Publishers, Inc.
2012
Pg. 180:
You are what you eat, so . . . I’m staying away from fruits and nuts.