The Atkins diet might forever be associated with the early 2000s, when the low-carb diet craze swept America. But years later, the brand is still alive -- and quietly growing, thanks to trends that have consumers taking a second look at low-carb eating.

The diet marketer, formally called Atkins Nutritionals, wants to accelerate that growth with aggressive marketing plans that will be led by a new ad agency, Interpublic's Lowe Campbell Ewald, Detroit, which won the business after a competitive review. The incumbent agency, independent Goodness Mfg., did not participate in the pitch, which was managed by Roth Associates, said Atkins Chief Marketing Officer Scott Parker.

Mr. Parker said he was impressed with Campbell Ewald's direct-response work, citing clients including banking and insurance provider USAA and the U.S. Navy. Atkins, whose model includes offering free online diet tools while selling branded food, will keep Sharon Osborne as its celebrity spokesman, but other campaign elements are still in development, he said.

The marketer, which spent $20.6 million on measured media in 2012, according to Kantar Media, plans to boost spending by 50% this year, Mr. Parker said. The new investment comes as the brand tries to seize on what Mr. Parker said is a resurgence of the dieting principles that thrust the brand into prominence a decade ago. For instance, he cited newer fad diets, such as the "Paleo Diet," which calls for caveman-like consumption of meats and vegetables. Rising health concerns about sugar are also playing in Atkins favor, he said. "We are riding and leading an excellent trend," he said.

More than bacon and eggs
In the meal-replacement category, which includes products like shakes and snack bars, Atkins' share of sales jumped from 7.4% in 2011 to 8.1% last year, putting it fourth behind Herbalife, Kellogg's Special K and Slim Fast, according to Euromonitor International.

The company has had multiple owners since Dr. Robert Atkins died in 2003. Private-equity firm Roark Capital Group has had control for the past three years. Atkins' revenue had been as high as $700 million in 2004, but declined to $200 million by 2006, the Wall Street Journal reported in late 2010 when the Roark purchase was announced.

Mr. Parker, a former head of global marketing for Jenny Craig who has been at Atkins for more than two years, said the Atkins diet went off track several years ago because of misperceptions about what it is. "The diet fundamentally teaches you to eat a balanced menu, it never did tell you to eat nothing but bacon and eggs," he said. "But that is what word-of-mouth became and people literally were doing their own makeshift diet and they didn't have a very good experience because they didn't do it correctly."

Previous owners also over-proliferated the food product line and lost control of quality, Mr. Parker added. North Castle Partners, which owned the brand before Roark, pared back items to include only bars and shakes. Under Roark, Atkins has slowly begun expanding offerings, launching frozen meals in January. The brand's marketing tactics include hosting a blog by Ms. Osbourne in which she dishes diet advice. In one recent post she plugged her appearance on the celebrity gossip TV show "Extra," in which she promoted Atkins.