Chicago ? At first it may seem only right for Dean Foods, the nation’s largest organic dairy producer, to roll out a line of yogurts and milk marketed as “natural.”

But Dean’s recent announcement alarmed advocates of organic food, who say the burgeoning market for less expensive “natural” foods reaps billions from consumers while guaranteeing little or nothing in exchange.

Certified organic food products are regulated by the U.S. Department of Agriculture and produced by farmers and manufacturers under a strict set of rules. But the agency defines the term “natural” only for meat and poultry. In the rest of the food industry, the meaning is largely up to the producer.

Adding to advocates’ concerns, a new study shows wide confusion among American consumers about products aimed at the green market. Many mistakenly believe “natural” is a greener term than “organic.”

“They felt organic was just a fancy way of saying expensive,” said Suzanne Shelton, president and CEO of the Shelton Group, which conducted the survey and specializes in marketing sustainability to mainstream consumers. “They think ‘natural’ is regulated by the government but that organic isn’t, and of course it’s just the opposite.”

The U.S. natural food market grew by 10 percent to $12.9 billion from 2007 to 2008, according to the Nutrition Business Journal. And in this tough economy, some observers suspect companies will be watching Dean’s new venture to see if they can shed cumbersome and expensive organic standards.

“Our fear is that they are going to blur this line” between organic and natural, said Mark Kastel of the Cornucopia Institute, a nonprofit organic industry watchdog group. “The concern is they’ll help destroy organics or least chip away a substantial part of it.”

Dean’s new natural dairy line is being launched by its popular Horizon Organic brand and will be cheaper than organic options. Sara Loveday, the brand’s communications manager, said Horizon has created its own definition for “natural.”

“To us, it means it’s produced without added hormones, artificial sweeteners, artificial colors, flavors, preservatives or high fructose corn syrup,” Loveday said.

The new products will hit shelves this month with Little Blends, 4-ounce natural yogurts flavored with fruits and vegetables and aimed at toddlers. A second product, 6-ounce boxes of vanilla and chocolate milk called Milk Breakers, will be test-marketed next month in Florida.

Loveday said the new products will feature Horizon Organic’s familiar spotted cow, which has advocates worried about consumer confusion.

“The move feels sneaky,” said Dawn Brighid, spokeswoman for Sustainable Table, a nonprofit online resource for sustainable food. “The average mom won’t know about the change, and most people are still unclear about the difference between ‘natural’ and ‘organic.’ With milk prices as high as they are, people will be happy to see a lower price point, but I’m afraid they won’t understand what they are getting.”