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. 
         
No one gains “muscles” by eating the similarly sounding word “mussels,” but there are jokes. “You no how they say you are what you eat? Since I’m having mussels for lunch do you think I’ll grow some muscles?” was posted on Twitter by Jamie Heathcote on February 12, 2011. “Since you are what you eat, want to look better? Eat mussels!” was posted on Twitter by Backyard Boiler on May 4, 2012. “They say we are what we eat… bodybuilders must eat a lot of mussels” was posted on Reddit—Dad Jokes on September 10, 2018.
 
             
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
Jamie Heathcote
@HeathcoteJ20
You no how they say you are what you eat? Since I’m having mussels for lunch do you think I’ll grow some muscles?
7:26 AM - 12 Feb 2011
 
Twitter
Backyard Boiler
@BackyardBoiler
Since you are what you eat, want to look better? Eat mussels! Available only @BackyardBoiler! Just in time for your summertime pics
6:34 PM - 4 May 2012
     
Twitter
Harry Lewis
@HarryLewis24
They say you are what you eat. So I ate mussels #Buff #Puns
4:15 PM - 13 Aug 2012
   
Twitter
🖤
@GMPent
You are what you eat, thus mussels for muscles, it’s only logic. http://instagram.com/p/nE5emmNIng/
11:10 PM - 21 Apr 2014
 
Twitter
Austin McNeill
@austinmcneill4
You are what you eat #mussels #muscles #carrabbas #imeatingalone #lonelylife #imtakingmyselfonadate… http://instagram.com/p/pSNU9nBG_B/
8:16 PM - 15 Jun 2014
   
Twitter
James_fLb
@Jamie_F_LB
“You are what you eat” I ate mussels tonight so im Arnold Schwarzenegger
2:32 PM - 21 Oct 2014
 
Reddit—Shitty Ask Science
If you are what you eat, will I become jacked if I ate mussels? (self.shittyaskscience)
submitted December 12, 2014 by PrinterJam5
 
Reddit—Dad Jokes
They say we are what we eat… bodybuilders must eat a lot of mussels (self.dadjokes)
submitted September 10, 2018 by SnowmanOlaf
COMMENTS
DoctorMTobogganMD
I suppose it’s probably because seafood is really high in protein?