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 or a “skinny person” by eating such things. 
     
“Vegetable” is slang for “a brain-dead person.” “What do u say to a vegetarian in a coma? You are what you eat!” was posted on Twitter on August 14, 2010. “What do you call a vegetarian in a coma? A classic case of ‘you are what you eat’” was posted on Twitter on May 18, 2013.
       
       
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)
     
Twitter
Connor Stewart
@Conn1106
What do u say to a vegetarian in a coma? You are what you eat!
3:21 PM - 14 Aug 2010
     
Twitter
AngerMGMT Podcast
@angerpod
If you are what you eat, everybody is a cannibal.  Except for brain damaged coma patients….they’re vegans
10:40 AM - 14 Nov 2011
 
Twitter
razmig
@razmig
If you are what you eat and you happen to be a vegetarian in a coma…does that mean it’s okay to call you a vegetable?
2:00 PM - 1 Aug 2012
   
Twitter
Miles P
@PunMStar
What do you call a vegetarian in a coma? A classic case of “you are what you eat”
4:52 PM - 18 May 2013
   
Twitter
billy jean
@jcorvington
A vegetarian trips, hits his head, and lands himself in a coma.
I guess one could say… you are what you eat
2:34 PM - 12 Sep 2013
   
Twitter
Mikey Martin
@stepdad420
If I ever end up in a coma vigil, can someone PLEASE come to the hospital and say “well, I guess you are what you eat.” [I’m a vegetarian].
5:54 PM - 19 Oct 2015
   
Twitter
Ryan
@ryanthemaker
What do you say to a vegan in a coma?
You are what you eat.
7:18 PM - 15 Jun 2016
   
Twitter
RealChildish
@waltercal25
my vegan girlfriend is in a coma
I guess you are what you eat….a vegetable
12:22 PM - 1 Dec 2017
 
Reddit—Jokes
There’s an old saying that goes “You are what you eat”.
submitted April 7, 2018 by SOMANYGEESE
I suppose that would explain why my vegan friend has been in a coma all this time.