At least two cannibal jokes involve the magazine name Reader’s Digest. This riddle has been cited in print since at least 1984:
 
Q: Why do cannibals eat readers, not writers?
A: Because writers cramp, but readers digest.
 
A bookworm riddle was posted on Twitter in 2013:
   
Q: What do you call a bookworm who gets eaten by a cannibal?
A: Reader’s Digest.
 
             
Google Groups: net.jokes
Shaggy dog story (sort of)
.(JavaScript must be enabled to view this email address)
4/18/84
This ‘joke’ was told to me the other day by a person who has no access to the net.  Since it is at least as funny as most of the garbage that is posted to this newsgroup I decided to contribute to net pollution.
 
An author of fame and his friend who was an avid reader decided to visit Africa to see the sights and get new inspirations.  They wandered all over the contenent, from the mountains to the forests, from the desert to the
ocean.  One day as they were traveling through a particularly deep forest they where attacked by a tribe of cannibals.  After a fierce but brief struggle the two men were captured and carried off to the tribe’s village. Apon reaching the village the cannibals started a large fire and put a large kettle of water on the fire to boil.  Needless to say our two heros thought that the end had come and started to prepare themselves to meet thier fate. After a while the author noticed that the tribe was involved in a loud and heated debate.  In order to find out what has happening he introduced himself and his friend to their guard and asked what the commotion was about.  The guard informed him that it was the tribe’s custom to free half of the victims captured and that the tribe was having trouble deciding which one of the two to eat and which one to let go.  Well this went on for a little while longer but the tribe finally made up its mind and set the author free and threw his friend into the supper pot.  After the shock of being released wore off the author he asked the village chief why he had been chosen to be released.  The chief replied,“It was quite simple to decide once that we found out that you are a writer and your friend was a reader.”  This responce confused the author even more, so he asked for further clarifacation.  “It is simple”, said the chief,
 
“readers digest but writers cramp”.
   
Google Groups: rec.humor
Cannibal joke
Il Oh
10/22/90
(...)
“I found these two white guys in the jungle.  One was reading a book, the other was writing in a notebook.  The one reading a book saw me coming and ran away, so I ate the other one.”
 
“That’s your problem.  Haven’t you heard?  Readers Digest.  Writers cramp.
     
Google Groups: that reminds me of a joke
Cannible Jokes
tex norman
10/11/09
(...)
Why do cannibals prefer eating readers to writers?
Because writers cramp but readers digest.
 
Google Books
The Mammoth Book of Really Silly Jokes:
Humour for the whole family
By Geoff Tibballs
London, UK: Constable & Robinson Ltd
2011
Pg. ?:
Why do cannibals prefer eating readers to writers?
Because writers cramp but readers digest.
 
Twitter 
James Wester
@jameswester
Replying to @JonahNRO
@JonahNRO You’ll never be eaten by a cannibal but your readers are at risk. Writers cramp, readers digest. *rimshot*
10:00 AM - 14 Jan 2011
 
Reddit—Jokes
Gentlemen…BEHOLD! Puns.
submitted July 28, 2012 by Gingersaurus_Rex
Why do cannibals eat readers instead of writers? It’s because readers digest, and writers cramp.
   
Twitter
mikeymobile
@mikeblesic
Q: What do you call a bookworm who gets eaten by a cannibal? A: “Reader’s Digest.”
4:44 AM - 20 Oct 2013
   
Twitter
Sonia Rossi 🇮🇹
@srossi72
Q: What do you call a bookworm who gets eaten by a cannibal?
A: “Reader’s Digest.”
6:56 PM - 27 Jul 2014
 
Reddit—Jokes
What did the cannibal wipe with . . .
submitted October 4, 2014 by Earthsophagus
. . . after he ate some people at the library?
Pages from Reader’s Digest
 
Twitter
Anthony Melpolder
@TheReverseBolt
@SierraAshura
Q: What did the banana say to the Vibrator?
A: Why are you shaking?! She’s gonna eat me!

Q: What’s the difference between Oooohh and Aaaahhhh?
A: About 3 inches.

Q: What do you call a bookworm who gets eaten by a cannibal?
A: Readers Digest
4:12 PM - 6 Jun 2018
 
Twitter
lorraine Jo King
@LahLahBean
#tuesdaytitters
What do you call a bookworm who gets eaten by a cannibal?
Reader’s Digest.
11:09 AM - 16 Oct 2018