Satires were meant to be written with knives.

Is it surprising, then — or troublingly unsurprising — that the device of a knife (long, stainless steel, serrated edge) acts as the central support of “January Joiner,” the self-styled “weight loss horror comedy” now playing at the Long Wharf Theatre?

“We’re going to help you wield a knife,” snarls a Barbie-doll fitness trainer to her cadet class of oversize trainees. Her words of wisdom: Visualize yourselves, you fatties. Now, carve away the fat.

As advice for satirists, that ain’t bad. Wield a knife; the humor should be stinging, sharp. Avoid excess. Cut deep. Aim for the jugular.

So the play’s knife metaphor would be cheekily self-regarding if it actually contributed to the satire. But, at most, I’m left with a paper cut.

Set at a fitness resort on the Florida beachfront, “January Joiner” brings together a small cast of heavyset characters eager to slim down. Shaken by a heart attack — “an event,” as she pooh-poohs it — Terry vows to die another day and lose 50 pounds. Decked out in sun hat and bumbling over enthusiasm, Terry is defined by a Heartland naiveté and a simple, wholesome desire to lose weight. It’s the right thing to do. Myrtle, her slightly thinner and classier sister, tags along to the resort, ostensibly to support Terry. But when no one’s looking, Myrtle tightens the belt on her spa robe. No doubt her body image is victim to the expectations of professional life in waistline-obsessed New York.

The fitness formula? Nibble, avoid meat. Or, for the upstart high priests of the body cult, assume the shape of meatheads.

The play’s narrative formula, on the other hand, cleaves to “Trading Places”: expectations are upset, personalities inverted and crisscrossed. In a tour-de-force monologue delivered as poetry —  but tightly, without artifice — Terry (played by the talented Ashlie Atkinson) comes back from a swim in the ocean, stung by a passerby’s barb about “whale-watching.”

So she vows to lose 100 pounds, not 50. And just like that, sneaking in through the back door of a strong monologue, Terry gives in to insecurity. Along with her elephant skin, she sheds her coherent personality as cultivated by dialogue to that point. What happened to the Terry who revels in fat jokes? The one who enjoyed comments like “Your mom is so fat she looks at the scale and it says, ‘To be continued …’”

But personalities in this play aren’t “to be continued.”

They jerk; they swerve. Terry becomes a fitness nut, losing so much weight that a new actress has to replace Atkinson in the second act of the play. But the new Terry, dubbed Not-Terry in the playbill, diverges so much from the old’s character that she’s borderline unrecognizable; actress Maria-Christina Oliveras ’01 is so made up, her face so hard and unforgiving, that we have to side with a suddenly hysterical — jealous? — Myrtle in accusing her of not being Terry. But what’s interesting in a stage drama if we’ve already settled it?

That encapsulates the logic of “January Joiner”: locally punchy, at home in the tumbling consciousness of dialogue, but unconvincing on a larger scale. Playwright Laura Jacqmin’s’04 Russian dolls are not built to size. Sure, Terry wants to wear bras in the first four letters of the alphabet. That’s funny. But why is that what she wants?

Verbal slapstick orders the screenplay. Darnell, the third trainee at the fitness resort, offers yet another caricature of heartland America. A big baby, he pines for his turtle and crushes on Terry, albeit to little avail. “Sometimes I say things and I can’t stop saying them,” he mumbles at one point, in a crystallization of his comic role. As an unrepentant veteran of the weight-loss program, he comes to Florida to chew the proverbial fat: The fitness center is his social center (though, from the looks of it, he chews more than enough animal fat, too.)

The paragon of defiant consistency — a keen foil to the fitness trainers’ insistence on performance and self-betterment — Darnell starkly strays from his role at the climax, as if to conveniently shake up the drama.

It’s a plot twist that’s unpredictable. And yet a narrative technique that is anything but.

To Jacqmin’s credit, tools multiply in her toolbox; her craft is studied, if a bit too eager. An animated vending machine plays totem and confession box to the hungering dieters, but its moments of psychological manipulation punctuate the narrative without complementing it. Along with the occasional mute zombie that creeps up on the characters to elicit a brief, unresolved scream, the vending machine inspires horror and little else.

Our imagined selves, our body image fantasies, can certainly be scary. But if the body image issue is to command our moral attention, why does the play demonize the fitness trainer who suffers from her own insecurities? Shouldn’t the last scene see her released from a cage, rather than punitively trapped in the vending machine?

Un-transcended, the driving fable of “January Joiner” ushers into enlightenment some apostates of fitness, that great American religion. At the same time, it tracks initiated meatheads replacing their emotional intelligence with dead beef.

And yet — forgive the metaphor — “January Joiner” feels much like the experience of slicing away at the rotisserie meat — gyro, shawarma, what have you — only to throw all the Grade-A away. What a waste! “There are starving artists in Brooklyn,” my mother always chided me at the dinner table.

Never play with knives, she also said.

As social commentary, “January Joiner” succeeds only in playing with knives. Sorry, but we came to eat some meat. Isn’t the point of a play, of any art, to taste its argumentation? (So chewy, so juicy!)

We’re on a diet.