Diet Episode

"So I'm on a diet... Big deal. You know what a "diet" is, don't you? It's "die" with a "T," that's what it is!"

—Garfield, August 30, 1978

A character is sneaking out to the kitchen for a midnight snack. He is careful not to wake up his wife or else he'll get caught. The minute he opens the refrigerator, he suddenly hears footsteps, so he turns around just in time to see his wife standing behind him with a stern expression on her face. The wife then promptly puts her husband on a diet for trying to raid the refrigerator at night.

This is a type of episode that shows up in almost every form of media. When a character is caught trying to eat more than he usually does, he will promptly be put on a diet, or the character decides to go on a diet himself.

Becoming a Dead Horse Trope as the obese become Unacceptable Targets.

Has nothing at all to do with Japanese Politics.

Examples of Diet Episode include:

Anime and Manga

There is one in Kamen no Maid Guy.
Azumanga Daioh's Yomi is seemingly always on a diet. Her Day in The Limelight episodes often feature her trying several unsuccessful diets.
Usagi tried to go on a diet but failed in an early Sailor Moon episode that involved the villains taking energy from girls who were obsessed with getting thin.
There is also a Yes! Pretty Cure 5 episode where Coco goes on a diet because he got fat (although you never really see the difference in his human form) from eating too many cream puffs.
Panty and Stocking With Garterbelt has one in which, thanks to a Ghost, Stocking finds herself gaining weight (something that never happened before, considering all the fat she consumes goes to her breasts) and starting a DIETO.
Taken Up to Eleven in a season 2 episode of Gintama where most of the major female characters became obese within the same time period and were enrolled into a fat camp. They all go back to normal at the end of the episode after eating laxitive-laced chips.
In Aikatsu! Ichigo gains a lot of weight over New Year's. After a bit of prodding by her friends she loses the extra weight, but becomes overly obsessed with staying skinny (e.g. getting worried when she gains 100 grams (about 3.5 oz) of weight). At the end of the episode she learns not to overly obsessed, after Mizuki (the school's top idol) tells her that even she let's herself go occasionally
In Magical Idol Pastel Yumi the title character puts herself on a diet.

Newspaper Comics

Numerous Garfield strips had Garfield get put on a diet by Jon, and sometimes by Liz.

Literature

The Dursley portion of Harry Potter and the Goblet of Fire involves Dudley being put on a diet.
Jelly Belly fits the bill for its first half, subverting the trope once Ned gets serious and realizes cutting out junk food isn't as torturous as he thinks it is.

Live-Action TV

One not played for humor was a series of episodes of Roseanne in which Dan is put on a diet following his heart attack. It eventually leads to the largest fight Dan and Roseanne ever had on the show.
A major plot arc in Frasier, as Daphne started gaining weight after she got together with Niles. She was put on a regimen and eventually sent to some sort of fat camp. This storyline was a clever trick used by the writers to mask the fact that Jane Leeves was actually pregnant.
In one episode of The Partridge Family Danny is teased for being fat, goes on a diet, and loses like 10 pounds, all without being noticibly slimmer. According to Danny Bonaduce's biography, it was not a pleasant episode to film.
In an episode of Everybody Loves Raymond, Marie decides that everyone needs to eat healthier. Unfortunately, it was the Thanksgiving Special...
Glee had an episode where Sue made Mercedes go on a diet in order to remain on The Cheerios. Sadly, this so-called "diet" resulted in Mercedes obsessing over her weight and starving herself, prompting Quinn to intervene.
Played With on Babylon 5: Garibaldi is ordered by Dr. Franklin to go on a restricted diet (or rather... a food plan) because of various problems his body is suffering due to the after effects of being shot In the Back by The Mole in an earlier season. Meanwhile, Captain Sheridan is ordered to eat what he considers "Rabbit Food" because he has gained ten pounds since his previous exam. Commander Ivanova is ordered to eat more because she is suffering nutrition deficiencies because she doesn't eat enough healthy food.
- Justified, of course. None of the officers eat healthy due to the stress of their jobs, and two out of three of them suffered injuries recently that their bodies were still recovering from.
- At one point, the three officers, realizing that each of them is being forced to eat something one of the others prefers, trade their lunches. This is, of course, when Doctor Franklin shows up.
An episode of That '70s Show had Red, after his heart attack, trying to sneak around his diet. After he gets busted, he says "This isn't food! This is what food eats!".
In the 3rd Rock from the Sun episode "Moby Dick", Dick goes on a diet after he starts overeating to deal with the stress of his break-up from Mary. Later we find out Mary is also stress eating.
In an early episode of Family Matters, Carl went on a strict diet to look good for his high school reunion. His family pulled out all the stops to keep him from cheating, to the point where he snuck baby food out of desperation.
Played for drama in The Fresh Prince of Bel-Air. Philip was put on a diet and bribed Will into sneaking him junk food, only to end up having a heart attack.
I Love Lucy had Lucy go on a diet to be able to play Sally Sweet in one of Ricky's show numbers.

Western Animation

Tummi Gummi got put on a diet after sneaking a midnight snack in the Gummi Bears episode "The Oracle".
At least one episode of The Flintstones had Fred being shamed into dieting.
The Goofy short "Tomorrow We Diet".
One episode of Tiny Toon Adventures had Hampton try to go on a diet, only to be constantly tempted by a chocolate cake.

Cake: (in a deep voice) Eat meee... Eat meee...

Doug had two:
- The Nickelodeon version had "Doug Tips the Scales", involving the title character
- The Disney version had "Doug's Chubby Buddy", which was a Very Special Episode invloving Patti
The Simpsons had several of these for Homer.
Chris Griffin had one of these in Family Guy, but that plot took a back seat to Peter getting multiple plastic surgeries.