Kim Kardashian says that she is serious about slimming down since gaining a reported 15-20 pounds during her relationship with Kanye West.

And in a concerted effort to shift the weight, Kim Kardashian slipped on a pair of geeky black-rimmed reading glasses to work-out on the beach on Wednesday.

However, her more conservative and smarter style failed to distract from her sexy curves as she did some strenuous lunges on the sand.

Kim Kardashian, 31, displayed plenty of cleavage in a black sheer tank top, which she paired with tight leggings, that in turn highlighted her famous derriere.

She has admitted that she has put on weight, and tells Life & Style she’s determined to get back into shape after letting herself go since dating beau West.

“I’m hoping to slim down a little bit, and I’m not afraid to say it,” she tells the magazine in their latest issue.

“I think everyone goes up and down in whatever that comfortable love relationship phase is where you like to eat out, but now it’s time to get it together again.”

Friends of Kim Kardashian tell the magazine that she has gained several pounds since the duo started dating earlier this year.

“She’s gained 15 to 20 pounds. All Kim and Kanye ever do is go out to eat,” a friend tells the magazine.

“She feels so comfortable around him – he loves her curves – she hasn’t been vigilant about dieting.”

Meanwhile, Kim Kardashian says that she has also started taking QuickTrim again – the diet supplement for which she and her sisters are the face of.

“I’ve started [diet supplement] QuickTrim again,” she says.

“When I start that, it’s full focus.”

On Wednesday during her work-out, Kim Kardashian was also joined by her sister Kourtney and their Real Housewives Of Miami pal, Larsa Pippen.

The trio was seen going for a jog – the exercise session recorded by a film crew for the sister’s new reality show.

Tying her hair back in a ponytail and toning down her usual full-force of make-up, Kim Kardashian slipped her spectacles on and off during the workout.

The glasses, from the sister’s own eyewear line, boasted dark and almost squared frames, which also gave the star a look reminiscent of the well-known sexy secretary style.

Meanwhile, Kourtney Kardashian displayed her post-pregnancy figure in some black leggings paired with a yellow sports bra worn underneath a sheer blue T-shirt.

The combination revealed her flat tummy, obtained just under three months since welcoming her daughter Penelope Scotland into the world on July 8.

Kourtney Kardashian wore her hair in a similar style to her sister’s and opted to shield her eyes with some sunglasses.