“Death begins in the colon” has been credited to the Greek physician Hippocrates of Cos (ca. 460 BC – ca. 370 BC) and to Nobel prize-winning Russian biologist Elie Metchnikoff (1845-1916), although it’s not clear what writing of theirs is most similar to the saying.
 
“Cancer begins in the colon” has been cited in print since at least 1919 and “death begins in the colon” from at least 1944. “Death begins in the colon” began to be used with great frequency in certain health communities from 1976.
 
Variant sayings include “disease begins/starts in the colon” and “health begins/starts in the colon.”
 
     
Google Books
September 1919, The Medical Summary, pg. 151:
We don’t know whom to credit for the dictum that “Cancer begins in the colon.” Dr. John Bell, the well-known head of the cancer department of Battersea Hospital, London, and investigator of cancer and several of our American specialists on cancer have lately been emphasizing this view; and at a meeting of a national medical society, a few years ago, at which cancer was the special subject of discussion, several hundred physicians who had had large experience in its treatment agreed that they had never seen a case of cancer in which pronounced constipation (often not fully recognized, as when there is an incomplete daily movement) was not a prominent feature.
 
Newspapers.com
24 September 1944, Miami (OK) Daily News-Record, pg. 5, col. 5 ad:
DEATH—
BEGINS IN THE COLON
The pollution of the bloom stream, from poison generated in the lower bowel, does not cause the same kind of malady in all afflicted persons.
(...)
Dr. J. P. VAN HORN
Colon Therapy—Chiropractic X-ray
LADY TECHNICIAN
 
31 January 1976, Chicago (IL) Metro News, “...But By Prayer and Fasting” by Victoria Mucie, pg. 2, cols. 4-5:
(“Death begins in the colon” is one of the good doctor’s main tenets.)
(Dr. Alvenia M. Fulton, Chicago naturopath and nutritionist—ed.)
 
11 August 1977, Robesonian (Lumberton, NC), “Health Care Author Says Clue To Better Health Is In Eyes” by Karen Vela, pg. 9, col. 1:
La Dean Griffin, herbalist, iridologist, author of almost a dozen books and pamphlets on the natural approach to health care, strayed from the beaten Lecture path Wednesday night to tell slightly more than two dozen persons that
—“People today are really, really sick.”
—“A cold is nature’s way to save your life.”
—“Death begins in the colon.”
 
Google News Archive
6 August 1979, Merced (CA) Sun-Star, advertising supplement, pg. 4, col. 1:
Death Begins in the Colon—Our Answer—Colon Therapy
(“Back Talk” ad for Dr. Robert Noli, of Noli Chiropractic—ed.)
 
Google News Archive
23 October 1988, Albany (GA) Herald, “Diet & Health,” Ask Cathy Summerlin, pg. 12A, col. 5 ad:
As a lot of you already know, “Death” begins in the colon.
 
Google News Archive
26 July 1993, Spokane (WA) Spokesman-Review, pg.A7, col. 3 ad:
Many Doctors Believe
DEATH BEGINS IN THE COLON
(Northwest Wellness Center—ed.)
 
OCLC WorldCat record
Death begins in the colon
Author: Bill Giles
Publisher: Canberra : B. Giles, 1996.
Edition/Format:  Book : English
 
Google Books
Dead Man’s Dance
By Robert Ferrigno
New York, NY: Berkley Books
1996
Pg. 245:
Quinn had left his Jeep behind a hippy health-food store, hopefully protecting it from being towed away by plastering the bumper with “Meat Is Murder” and “Death Begins in the Colon” bumper stickers.
 
Salon.com
July 14, 1999
A thing of beauty
Should we flush our colons or leave them alone?
By Jon Bowen
(...)
Naturopathic doctors like to declare that “death begins in the colon.” What they mean is that all the Big Macs, Ding Dongs, corn dogs and Cheez Whiz you’ve ever eaten have left remnants in your colon that loiter there to this day, emitting toxins into your body’s bloodstream that will, eventually, inevitably, poison the bejesus out of you. Autointoxication, the process is called.

Google Books
Dr. Jensen’s Guide to Diet and Detoxification
By Bernard Jensen
Los Angeles, CA: Keats Pub.
2000
Pg. 1:
CHAPTER 1
DEATH BEGINS IN THE COLON
The expression “Death begins in the colon” apparently originated with the Nobel prize-winning Russian biologist Elie Metchnikoff (1845-1916) during his later years of laboratory research at the Pasteur Institute in Paris. While studying the flora of the human intestine, he became convinced that aging and death were gradually caused by the destructive influence on the body by toxic metabolic by-products of bowel bacteria. He advocated the use of fermented milk products rich in beneficial microorganisms, such as Lactobacillus acidophilus, to crowd out the harmful bacteria and reduce their negative influence on health and aging processes.
   
Google Books
Blood Never Lies:
A practical guide to health and nutrition
By Ted Aloisio
Coral Springs, FL: Llumina Press
2004
Pg. 45:
The Colon
When the late Dr. Bernard Jensen, a chiropractor coined the phrase “death begins in the colon,” his medical contemporaries laughed. With the advancement of medical science, drugs and surgery, why be overly concerned about just another organ, they mused.
   
Google Books
Lose Weight, Have More Energy & Be Happier in 10 Days:
Take Charge of Your Health with the Master Cleanse
By Peter Glickman
Clearwater, FL: Peter Glickman
2005
Pg. 27:
(You can read more about this in the excellent books by Dr. Norman W. Walker, such as Colon Health: The Key to a Vibrant Life, or Become Younger. Dr. Walker, by the way, lived well past 100 years of age, was mentally sharp and physically fit throughout his life, and is credited with the statement, “Death begins in the colon.”)
   
OCLC WorldCat record
Health begins in the colon
Author: Edward F Group, III.
Publisher: Houston, TX : Global Healing Center, ©2007.
Edition/Format:  Book : English
 
Acne.org Message Boards
Colonics - Death Starts In The Colon, or so the saying goes
Apple_Blossom
Jan 13 2007, 04:35 PM
For those of you who do kidney flushes, strict diets, etc…for your health, how many of you have done colonics?

Ask Me About Colon Health
Death Starts in the Colon
October 20, 2008 by ardithkay40
It has been said that, “Death Begins In The Colon.” This is so true!
 
Google Books
Transcend: Nine Steps to Living Well Forever
By Ray Kurzweil and Terry Grossman
Emmaus, PA: Rodale
2011
Pg. 71:
Performing a CSA can provide vital and lifesaving information—as Hippocrates observed centuries ago, “Death begins in the colon.”