Put Off Their Food

Alice and Bob are sitting down at a restaurant to eat. Alice orders spaghetti. Then Bob, who is a doctor, starts talking about how he operated on a patient with flatworms today, and describes in graphic detail how he pulled out the long, thin, white, worms from the patient's intestines...

...And on second thought, Alice decides she'll have a salad instead.

This trope is when a character had planned to eat a certain food, but changes their mind based on a conversation or experience that causes them to make a mental association between it and something nasty. The character doesn't lose their appetite entirely, they just can no longer stand the thought of eating that particular thing.

Examples of Put Off Their Food include:

Film

At the end of Spaceballs, when a diner patron who ate the Alien Surprise starts moaning in pain, another character who ordered the same thing quickly changes his order.
In Rear Window, Stella's musing over murder methods put Jeff off his breakfast.

"Now just where do you suppose he cut her up?"
(Jeff stops just before putting some bacon in his mouth)
"Oh -- of course! In the bathtub. That's the only place he could wash away the blood."
(Jeff puts down the bacon)

Literature

Tour of the Merrimack: In The Myriad, Captain Farragut describes over dinner how gorgons melt into caustic brown slime when they die, then tells his chef to skip the French onion soup.
Slayers: In the light novel The Battle of Saillune, Lina and Gourry are attacked by a number of lesser demons that possessed items of food, including a nasty tentacle monster that emerges from their squid stew. After the fight, they order more food, and Gourry requests they skip the squid stew—though Lina disagrees, saying it'll take more than that to make her lose her appetite.
Animorphs: In The Android, Marco's parents try to serve him chicken for dinner after he was nearly killed by a bird while in spider morph. Marco opts out.
In The Hitchhiker's Guide to the Galaxy, during the dinner at the Restaurant at the End of the Universe, Arthur initially orders a steak, changes to a salad when he finds out what the steak comes from, and eventually settles for a glass of water.
Happens to Jessie and Cooper in Shotgun Sorceress when they discover that, as a side-effect of some necromantic magic they performed, they can feel the death of each item of food they taste. They regretfully pass up the rib-eye steak in favor of a salad.
The Canterbury Tales has an example in the description of The Cook in the prologue. While the Cook is a Supreme Chef, Chaucer unfortunately can't enjoy a dish of his because its appearance reminds him too much of a nasty running sore the Cook has on his leg.

Live Action TV

Leverage: Parker's attempt at social skills in "The Juror #6 Job". When told to convince Eliot to eat an orange instead of an apple, she chooses this tactic.
In the Doctor Who serial The Two Doctors, the events of the story make the Doctor and Peri turn vegetarian. This holds until Christopher Eccleston's tenure.
In The Suite Life of Zack and Cody, there was the episode "Twins at the Tipton" where Maddie and London double date with twins and Maddie goes out with the nerdy twin. He starts saying all of this gross trivia at the dinner table, including that it takes "3 hours for meat to move through your intestines so the bile can churn up the gastric acid." Maddie then replies that she'll have a salad.

Newspaper Comics

In a Calvin and Hobbes strip, Mom tells Calvin that the icky stuff on his plate is monkey brains in order to get him to eat it. But now Dad can't eat it because it Squicks him out.
A FoxTrot strip where Jason ties the ends of his spaghetti noodles together so he can eat them all in one long, unending slurp ends with Andy adding spaghetti to the list of things she can no longer make for dinner.
- Another Fox Trot example: Paige just dissected a frog in Bio class and she thought it was really cool. She tells her family about the intestines and they stare in horror at the spaghetti they're eating.

Video Games

One of the skits in Tales of Vesperia has this happen to Rita and Raven, who were enjoying their croquettes... until Raven asks Yuri about the recipe, and Yuri reveals his "secret ingredient". Rita and Raven's reaction say it all.

Web Comics

Drowtales: In chapter 14, when the group of protagonists sits down for dinner, the drow Kyo'nne tells the human Vaelia that the preserved meat they're eating is human meat. She was just joking—but since they don't actually know what kind of meat it is, and since drow do eat human meat, Vaelia decides to play it safe and have bread instead.
"Mameshiba": In all of the episodes on the website, when a certain bean appears on the screen and tells the person who is about to eat it some random fact, they immediately stop eating their food or say "I'm done now."

Web Original

A Running Gag in After Hours is that any time bodily functions are discussed, someone will be just about to take a bite of their food, and stop with it halfway to their mouths.

Western Animation

The Simpsons: In the episode where Homer becomes a food critic, some chefs plan to assassinate him with a lethal eclair. After other attempts to stop Homer from eating it fail, Lisa tells him that it's low-fat, causing him to throw it away in disgust.