11:58 a.m. EST December 26, 2014    Here's one way for a diet behemoth to get its end-of-the-year ad noticed: Make it about a woman's butt. More specifically, make it about how one woman's butt changes in size and shape through the years.

That's what Weight Watchers, the weight-management specialist whose business has been hit in a rocky economy, has done. It will roll out a commercial this week dubbed "My Butt," about the visual evolution of one woman's backside from birth through middle age.

"This is a montage of one personal life," says Jason Kreher, creative director at Wieden+Kennedy, the ad agency behind the campaign. "It just happens to be filmed through the lens of her butt."

What it's not filmed through is the lens of celebrities. Those days appear to be over at Weight Watchers, says Lesya Lysyj, president of the company's North American operations. Good-bye Jessica Simpson. So long Jennifer Hudson. Bye-bye Charles Barkley. No more celebs in Weight Watchers commercials, which will now focus on the real diet problems of real people.

"A celebrity-only strategy is not something we'll do in the future," says Lysyj. Showing before-and-after photos of celebrities makes dieting look way too easy — and too unrealistic, she says. "From this point on, that's not a major part of our story."

January is when consumers who feel guilty about putting on holiday weight turn to commercial diet companies. Weight Watchers, for example, does about 40% of its business in the first quarter of the year, says Lysyj.

The company's latest move not only comes at a tough time for Weight Watchers, but at a time the broader $60 billion domestic diet industry is in decline. Consumers are increasingly turning away from diet soft drinks and cutting back on commercial diet offerings. Only 7% of Americans who want to lose weight try commercial weight-loss programs, Lysyj says.

Lysyj says Weight Watchers isn't nixing celebrities because they're ineffective or two expensive. It's because dieting is hard, and celebs tend to make it look too easy.

"We did not airbrush anything. We want it to look like real life," she says. "That's a very different approach than we've done before."

Of the four new ads — all focused on the difficulty of dieting — perhaps none is more controversial than the butt spot.

"This is my butt," the commercial begins, showing a woman's backside as she walks in her jeans. "My butt and I came into this world together."

The ad shows a montage of butt sequences from the woman working out at the gym, to dancing a slow dance with a young boyfriend who doesn't know where to place his hands — which slide awkwardly towards her backside.

One of the trickiest parts of this commercial: the casting. After all, it shows a woman's backside through the years. "Getting four people who look relatively consistent was a real challenge," says Lysyj. "Every backside does not look the same."

The casting, says Kreher, was exhaustive. Women were cast from jeans size 4 to 12, he says. "I never cast based on butt movement before. It was as unique a casting as we've ever done."

The ad, which will air nationally, ends with this line: "My relationship with my butt had nothing to do with my butt, and everything to do with my brain."

Brand guru Erika Napoletano loves the spot. "Butts get you from place to place, power your steps, let you lift kids up, hold 'em on your hip, and they offer padding when you finally get a moment to sit down," she says "If we're going to focus on something, let's make it something underappreciated but over-criticized. Find a way to celebrate your back 40."

But Weight Watchers has other plans for a possible, future male version of this ad. You guessed it: a guy and his gut.