Heather Illg is one of the newest power players in food retailing. But she doesn't oversee ad budgets or balance sheets. She's more interested in calories, fat and fiber.

Ms. Illg is a registered dietitian at a Hy-Vee, a grocery chain in Iowa, one of hundreds of nutrition experts who are emerging as a major force at supermarket titans such as Wegmans and Giant Eagle. Once confined to hospitals and offices, the dietitians are a marketing weapon for the chains bringing them aboard to aid shoppers seeking the best foods to drop weight, battle diseases or avoid allergic reactions. The trend is another sign that consumers are demanding more from their food providers as the nation's health-care system puts a premium on preventive care. And it also represents an increasingly powerful constituency for the nation's food marketers to win over.

Grocery chains have long had dietitians at the corporate level, but a growing number are now positioned in specific stores. (One-third of stores have a registered dietitian at retail, according to a recent survey by the Food Marketing Institute, which represents food retailers; 86% employ them at the corporate level.)

While their mission is to assist shoppers, dietitians are a major brand asset for stores, with many appearing on local radio and TV shows. The publicity serves as a boost for supermarkets as they look to stay relevant while more competition comes from drugstores like Walgreens and CVS, which have increased their food offerings. "Supermarkets have lost 15% share over the past 10 years, so they need to stave off and differentiate their offerings from drug, dollar [and] warehouse [stores]," Phil Lempert, a food-industry analyst who runs supermarketguru.com, said in an email.

Today there are 500 to 600 retail dietitians. Mr. Lempert predicted that number will at least double within two years. The demand is so strong that he recently launched the Retail Dietitians Business Alliance to educate dietitians on supermarket operations.

The trend to put dietitians in-store began in the heartland, where Iowa-based Hy-Vee began experimenting with the idea several years ago. Today the chain has about 190 dietitians on staff, offering services at all 233 stores it operates in eight Midwestern states. In rural towns, the Hy-Vee might be the only place to find a dietitian, while in urban areas consumers like the convenience, said company spokeswoman Ruth Comer. Just head to the store and the dietitian can "walk you over to the product, show you where it is, what it looks like, what you look for on the label," she said.

A lot of the services are free, like the basic store tours, menu suggestions and shopping lists recently offered by dietitians at a store in a Des Moines suburb where Ms. Illg works. The same store charges for personal consultations, starting at $119 for two visits for new clients, according to its website. Some of the demand comes from shoppers suffering everything from heart disease to digestive conditions. Physicians "are actually just telling people when they release them [to] go to Hy-Vee and ask for the dietitians," Ms. Comer said. On shelves, some products are tagged as a "dietitian's pick," touting attributes like high fiber or low sodium.

A Unique position

With that kind of power, dietitians have emerged as a target for brands seeking what could be a powerful endorsement. "They are in the store and they are right there with the consumers when they are making their purchasing decisions," said Nancy Tringali Piho, a longtime food-industry PR professional who in 2011 launched a service aimed at connecting food associations and brands to store dietitians. She keeps a database of dietitians at more than 50 supermarket chains and sends regular newsletters to them.

Ms. Tringali Piho also runs an annual conference called Shopping for Health at which brands and trade groups pitch their products to dietitians at leading chains. One sponsor of this year's meeting, held at the end of the month at an Arizona resort, is Nestle's Lean Cuisine, which is planning to bring a chef to show how frozen entrees can be paired with homemade, fresh foods, Ms. Tringali Piho said. (See P. 6 to learn how frozen-food makers are seeking to repair their image.) Another client is the Wheat Foods Council, which in the face of the gluten-free trend has found that some consumers are giving up wheat unnecessarily for reasons that are not medical, Ms. Tringali Piho said. Supermarket dietitians are in a unique position to "understand where the industry is and talk to the consumer very credibly ... from a medical perspective," she said.

But while they might listen, dietitians interviewed by Ad Age stressed their independence. "I make it pretty clear to brands [that] I don't make decisions about what is on the shelves," said Leah McGrath, a corporate dietitian for Ingles Markets in the Southeast. When fielding consumer questions, "the only time I would mention a brand is if it's something I personally like for some nutritional reason."

Ms. McGrath is somewhat of a media star, appearing regularly on an ABC TV affiliate in Asheville, N.C., while hosting a weekly radio program called "Ingles Information Aisle." When she is not making public-speaking appearances or giving store tours, she is answering consumer questions via Twitter, Facebook and the corporate website. "Organic vs. all-natural?" one Facebook fan asked recently. Ms. McCrath's advice: "There are no standards for "natural' -- there are standards for organic."

Giant Eagle, which operates more than 200 stores in Ohio, Pennsylvania, Maryland and West Virginia, started its program four years ago and has hired 18 dietitians with plans to add more. The in-store program works better than an office approach because consumers can "look at the actual foods, compare the labels side by side [and] open the product and taste it," said Caroline Passerrello, who manages the chain's dietitian initiatives. "It's more of an experiential learning."

Avoiding marketing hype

East Coast chain Wegmans uses its dietitians to train store employees on topics like why organic foods cost more and what benefits they do and don't provide. "These are things that our customers ask our employees and we want our employees to have ways of answering these questions" while avoiding the marketing hype, said Jane Andrews, the chain's first dietitian, who was hired in 1988.

In recent years, Wegmans has added dietitians for each region to keep up with the demand for nutrition information as more people deal with diabetes, high cholesterol, celiac disease, diverticulitis and other issues, Ms. Andrews said. "Whether you are a mother of a small child or you are bodybuilder or you are trying to lose some weight, it just seems as though more people recognize that food has a function beyond enjoyment," she said. "And that function can really affect how they feel.

"Once you figure that out," she added, "you start expecting more of your supermarket. And once you find a supermarket or food store that helps serve you in that way, you are going to go back there."