English writer Henry Sambrooke Leigh (1837-1883) wrote in “On Corpulence” (1869):
 
If you wish to grow thinner, diminish your dinner,
And take to light claret instead of pale ale;
Look down with an utter contempt upon butter,
And never touch bread till it’s toasted—or stale.
 
“If you wish to grow thinner, diminish your dinner” has been widely quoted.
 
 
Wikipedia: Henry Sambrooke Leigh
Henry Sambrooke Leigh (1837–1883) was a writer and playwright.
 
Google Books
Carols of Cockayne
By Henry S. Leigh
London: John Camden Hotten, Piccadilly
1869
Pg. 186 (“On Corpulence”):
If you wish to grow thinner, diminish your dinner,
And take to light claret instead of pale ale;
Look down with an utter contempt upon butter,
And never touch bread till it’s toasted—or stale.
 
Google Books
Mixer and Server
Volume 34
1925
Pg. 40:
“If you wish to grow thinner, diminish your dinner,
And take to asparagus, lettuce and kale;
Look down with an utter contempt upon butter,
And never touch bread either toasted — or stale.”
 
Google News Archive
21 February 1940, Berkeley (CA) Daily Gazette, pg. 10, col. 2:
“If you wish to grow thinner, Diminish your dinner.”
 
Google News Archive’
2 August 1955, Milwaukee (WI) Journal, “No Butter for the Glory Boys” by C. P. in Indianapolis Star, pg. 12, col. 5:
We’re not supposed to rub it in, but we can’t help thinking that Henry S. Leigh had them in mind when he wrote his famous stanza:
 
If you wish to grow thinner, diminish your dinner,
And take to light claret instead of pale ale;
Look down with an utter contempt upon butter,
And never touch bread till it’s toasted—or stale.
 
10 July 1964, Albert Lea (MN) Tribune, “Woman’s Garden” by Mrs. A. B. Babcock, pg. 7, col. 6:
The motto for dieters is: “If you wish to grow slimmer, diminish your dinner.”
   
Google Books
Fad-Free Nutrition
By Fredrick John Stare and Elizabeth M. Whelan
Alameda, CA: Hunter House Publishers; Emeryville, CA: Distributed to the trade by Publishers Group West
1998
Pg. 178:
If you wish to grow slimmer, diminish your dinner. — H. S. Leigh (A Day for Wishing)
 
Google Books
Calories and Corsets:
A history of dieting over two thousand years
By Louise Foxcroft
London: Profile Books Ltd.
2011
Pg. ?:
As Punch magazine put it in 1869:
 
If you wish to grow thinner diminish your dinner,
And take to light claret instead of pale ale;
Look down with an utter contempt upon butter,
And never touch bread till it’s toasted—not stale.
 
‘I’ve done it,’ said brave Mr. Banting,
And so may each overfed Briton,
If only he’d adopt resolution severe
And avoid—if he would not grow fatter and fatter,
All bread, butter, sugar, milk, ‘taters and beer.
 
Google Books
Mental Hygiene:
Communication and the Health of the Mind
By Lee Thayer
Xlibris Corporation (Xlibris.com)
2014
Pg. ?:
In his Carols of Cockayne (“cockaigne” was a medieval term for the dream of paradise on earth), Henry Sambrooke Leigh wrote (in 1859):
 
“If you wish to grow thinner, diminish your dinner.”
 
But a large percentage of us are still gluttons.