“The first syllable of the word ‘diet’ is “die’”

“Diet is ‘die’ with a ‘T’” is a jocular dieting saying that has been printed on many images. The Garfield the cat comic strip, created by Jim Davis, printed this is many newspapers on August 30, 1978:
 
“So I’m on a diet…big deal. - You know what a ‘diet’ is, don’t you? - IT’S ‘DIE’ WITH A ‘T,’ THAT’S WHAT IT IS!”
 
“Garfield hates diets. He describes diets as ‘die with a t at the end’” was printed in the St. Mary & Franklin Banner-Tribune (Franklin, LA) on March 25, 1980. “‘The first syllable of the word diet is die,’ he (fitness instructor Richard Simmons—ed.) says” was printed in the Chicago (IL) Tribune on August 5, 1981.
 
“Tea” is sometimes used for “T.” “‘Diet is Die without Tea’. Fantastic @chrisdenham Garfield quote” was posted on Twitter by Ed Overend on February 21, 2010. “Diet is die without tea” was posted on Twitter by Lucy O’Brien on March 24, 2011. “Diet is DIE with a TEA !” was posted on Twitter by indian Ravishankarಜಾಯ್. on January 20, 2017. “Diet without tea is just die” was posted on Reddit—Oneliners on January 15, 2020.
   
           
Wikipedia: Garfield (character)
Garfield is a fictional cat and the protagonist of the comic strip of the same name, created by Jim Davis. The comic strip centers on Garfield, portrayed as a lazy, fat, and cynical orange cat. He is noted for his love of lasagna and sleeping, and his hatred of Mondays, fellow cat Nermal and exercise.
     
Garfield comic strips
Newspapers.com
Newspapers.com
30th August 1978
So I’m on a diet…big deal. - You know what a “diet” is, don’t you? - IT’S “DIE” WITH A “T,” THAT’S WHAT IT IS!
 
Newspapers.com
25 March 1980, St. Mary & Franklin Banner-Tribune (Franklin, LA), “Book Look: Lasagna loving cat, Garfield is at it again” by Michele Cohen, pg. 8, col. 5:
Besides not liking dogs, Garfield hates diets. He describes diets as “die with a t at the end.”
     
Newspapers.com
26 July 1981, Quad-City Times (Davenport, IA), “Your Loss Is His Gain” by Jane Ardmore, Sunday Woman, pg. 7, cols. 2-3:
YOUR “LIVE-IT” RECIPES
Richard Simmons says that the first syllable of the word “diet” is “die.” So he prefers to call his weight loss regimen the “live-it” program.
 
Newspapers.com
5 August 1981, Chicago (IL) Tribune, “People” by Bill Plunkett, pg. 24, col. 5:
“The first syllable of the word diet is die,” he (fitness instructor Richard Simmons—ed.) says.
 
Newspapers.com
3 March 1982, Payson (UT) Chronicle, “Robin’s Roost” by Robin Fuellenbach, pg. 2, col. 3:
Since we all know, thanks to Garfield the cat, that “diet” means “die” with a “t”, dieting becomes a very unpopular thing to do.
 
Newspapers.com
27 June 1982, Kingsport (TN) Times-News, “TOPS Queen happiest loser in Tennessee” by Becky Jones, pg. 1E, col. 4:
Like Garfield says, diet is just die with a “t.” 
 
Newspapers.com
7 November 1982, Palladium-Item (Richmond, IN), “Life’s a big lasagne for cat and creator” by Dale Parry, pg. E1, col. 1:
“Diet is ‘die’ with a ‘t,’” quips the lasagne-loving Garfield.
 
Twitter     
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@TehKimber
I’m a baaaaaaad girl! I’m supposed to be on a diet. Diet is DIE with a T.
1:58 PM · Mar 5, 2008·Twitter Web Client
 
Twitter
Ed Overend
@EdOverend
“Diet is Die without Tea”. Fantastic @chrisdenham Garfield quote
5:10 AM · Feb 21, 2010·Twitter Web Client
     
Twitter
Lucy O’Brien
@LucyOB
Diet is die without tea.
11:28 AM · Mar 24, 2011 from Newcastle Upon Tyne, England·Echofon
     
Twitter
Ekans アーボ
@CoffeeDruggie
DIET without Tea is DIE
2:02 AM · May 30, 2011·Twitter Web Client
   
Twitter
Godking Wanhope
@stabini
X_X RT @zeGamelord: If you diet without tea, you die.
2:44 PM · Jun 8, 2011·Snaptu
 
Twitter
Sarah Cole
@The_MissPitstop
Replying to @underscoreella
@ellamiharu Diet is Die but without tea! xxx
4:51 PM · Apr 28, 2013·Twitter for iPhone
   
Twitter
indian Ravishankarಜಾಯ್.
@JOYravishankar
Diet is DIE with a TEA !
6:45 AM · Jan 20, 2017·Facebook
 
Parade
Happy 40th Birthday, Garfield! The Candid Cat’s Best Quotes
JUNE 19, 2018 – 5:00 AM
Isabelle Raphael
By ISABELLE RAPHAEL @issyraph
(...)
“Diet is ‘die’ with a ‘t.’”
 
Twitter 
Lesley Townson
@ulverstonchick
Replying to @dooz_er
Pray tell, what is this “dieting” of which you speak? Imho, I agree with Garfield, “diet’ is’die’ with ‘tea’ removed”
2:06 PM · Dec 21, 2019·Twitter Web App
 
Twitter
Melanie Sakowski
@orendawellness
Equation: Diet - die = tea. Tea sounds better. Tea now, die later.
5:01 PM · Jan 3, 2020 from North Vancouver, British Columbia·Twitter for iPhone
 
Reddit—Oneliners
Posted by u/Ansharcastic January 15, 2020
diet without tea is just die.
   
Twitter
sad we
@The_FlawedQueen
Diet without Tea is just “die”
8:23 AM · Jan 15, 2020·Twitter for iPhone