“I used to eat a lot of natural foods until I learned that most people die of natural causes” is a saying that has been written on T-shirts. The saying is recorded in print since at least 1989 and is of unknown authorship.
 
   
Zazzle.com   
7 September 1989, Orlando (FL) Sentinel, pg. D1:
MY NEIGHBOR WOLFGANG sez he avoids natural foods after reading where so many people were dying from natural causes.
 
Google News Archive
23 April 1992, Pittsburgh (PA) Post-Gazette, pg. 22, col. 3:
Berle (Comedian Milton Berle—ed.) even borrowed a joke for the occasion: “I stopped buying natural foods.” he said. ‘“when I found out that 80 percent of people die of natural causes.”
   
I used to eat a lot of natural foods until I learned that most people die of natural causes.
T-shirt
 
Google Books
The New Official Rules:
Maxims for muddling through to the twenty-first century
By Paul Dickson
Reading, MA: Addison-Wesley Publishing Company
1989
Pg. 196:
People who eat natural foods will die of natural causes. — Robert H. Smith, Oceanside, Calif.

Google Books
The Mammoth Book of Zingers, Quips, and One-Liners:
Over 10,000 Gems of Wit and Wisdom, One-Liners and Wisecracks
By Geoff Tibballs
New York, NY: Carroll & Graff
2004
Pg. 254:
I used to eat a lot of natural foods until I learned that most people die of natural causes.
JO BRAND
       
Google Books
Bathroom Reader:
Meditation Gone Wild
By David Arland Allen
Lulu.com
2008
Pg. 70:
I used to eat a lot of natural foods until I learned that most people die from natural causes.
 
Google Books
Life Changing Thoughts:
Thousands of Inspiring, Life-changing, and Humorous Thoughts
By Gary W. Smith
AuthorHouse
2009
Pg. 301:
I used to eat all natural foods until I learned that most people are dying of natural causes.