“Diet is a four-letter word”

A “four-letter word” usually means a profanity. “Diet” is a four-letter word (that is, something profane and not to be discussed) for certain people who like to keep their food choices.
 
“Diet is a four-letter word” has been cited in print since at least 1969.
 
 
23 May 1969. Scottsdale (A) Daily Progress, “Take it off with a Bloody Shame!” by Mary Bach, pg. 7, cols. 1-2:
And the ladies (Weight Watchers—ed.) even maintain that “diet” is really NOT a “four-letter word”.
 
5 June 1969, The Oregonian (Portland, OR),  “The Book Shelf,” pg. 12, col. 3:
Most weight-watchers will agree with one terse chapter heading: Diet Is a Four Letter Word.
(100 Delicious Ways to Stay Slim by Shirley Bright Boody—ed.)
 
2 March 1970, Omaha (NE) World-Herald,  “Animalogic” comic strip, pg. 3, col. 3:
“Remember, son—‘diet’ is a four letter word.”
   
Google News Archive
29 March 1983, Kingman (AZ) Daily Miner, pg. B1, col. 5:
Diet is a four-letter word
By the Economical Gourmet
Once I wrote a line in the columns that said “Diet is a four letter word.” I am still of that opinion. So much so, that with Susan Farger,  a nutritionist with a masters degree, and with John O’Donnell as our editor, we are writing a book called “The Diet Is A Four Letter Word Cookbook.”
 
OCLC WorldCat record
Diet is a four letter word : Mary Wright’s success with the Duke diet and fitness programme
Author: Mary Wright; Pamela Westland
Publisher: Colchester : Bruton, 1987.
Edition/Format:  Book : English
 
Google Books
8 June 1987, New York magazine, ‘Underground Gourmet” by Jane Freiman, pg. 80, col. 2:
“DIET” IS A FOUR-LETTER WORD THAT means no more pizza and no more candy. Bye-bye, bagels and cream cheese. Adios, frozen margaritas.
 
OCLC WorldCat record
Diet is a 4 letter word
Author: Suzie Heyman
Publisher: North Miami Beach, FL : Starlight Pub. Co., ©1990.
Edition/Format:  Book : English
 
OCLC WorldCat record
The answer : ‘Diet’ is a four letter word! : your ultimate guide to achieving safe, realistic, permanent weight loss and maintaining good health
Author: Della David
Publisher: Dudley : Limited Lime Publishing, 2008.
Edition/Format:  Book : English