FORT WORTH, Texas — Stumped over what's "natural" food?

You're not alone.

Jana Morgan considers herself a careful shopper who nonetheless was somewhat stumped when asked whether she considered high-fructose corn syrup and partially hydrogenated vegetable oil — a sweetener and a fat widely used by food manufacturers — to be "natural."

"I am not really sure. I know 'organic,' " said Morgan, who operates a UPS Store in Fort Worth with her husband. "My assumption is that a lot of processed food will not be purely natural."

The federal Agriculture Department clearly defines "natural" when applied to labeling meat and poultry: no artificial or synthetic ingredients, including added hormones, and minimally processed.

But the Food and Drug Administration says it has no plans to do likewise for the galaxy of groceries it regulates.

With no clear definition, confusion and even controversy have been generated. Consumer groups are urging that the FDA restrict the use of the word "natural" and demand that food manufacturers stop being so free with it on labeling until the government acts. This spring, one organization threatened legal action against a popular soft drink, "100 percent natural" 7UP.

" 'Natural' means nothing," said Urvashi Rangan, a toxicologist and a senior scientist at Consumer Reports, which has urged government action. "You have to flip the box over and examine the ingredient list. You've got to do your homework. But there's no requirement for what the ingredients have to be, to be considered 'natural.' "

The resulting vacuum has allowed manufacturers and supermarket chains to divine their own, often conflicting, definitions.

Critics say products labeled "natural" or "all-natural" should contain neither high-fructose corn syrup nor hydrogenated vegetable oil, whose trans-fatty acids are created in an industrial process.

Whole Foods, for example, does not sell "natural"-labeled foods if they contain hydrogenated oil.

Kroger, the nation's biggest food retailer after Wal-Mart, sells a store-brand granola, "100 percent natural cereal," that contains partially hydrogenated cottonseed and soybean oil. But none of its natural-category products include high-fructose corn syrup, Kroger spokesman Gary Huddleston says.

Zone Perfect All-Natural Nutrition Bars, made by a unit of Abbott Laboratories, have no hydrogenated oil but do have high-fructose corn syrup.

And the newly reformulated "100 percent natural" 7UP similarly uses the ubiquitous corn-derived sweetener.

Studies have linked hydrogenated oil, which contains trans-fatty acids, to heart disease. The Harvard University School of Public Health, which has warned about health risks of the highly processed oil since the early 1990s, said in an April study that removing trans fats from the industrial food supply could prevent tens of thousands of heart attacks and cardiac deaths each year.

Although high-fructose corn syrup has detractors, scientists have yet to find a conclusive link to obesity and have not determined whether it's any less healthy than other sweeteners, says Marion Nestle, a New York University nutritionist and author of "What to Eat."

But although consumer groups argue that calling the corn sweetener "natural" misleads the public, manufacturers maintain it's as natural as other approved additives.

With the organic and natural market burgeoning, billions of dollars are at stake. Strict guidelines govern the growing and processing of organic food products, but industry analysts frequently lump them with so-called natural items as a segment category.

Sales for all-natural and organic products will soar to $46.1 billion in 2010 from $28.3 billion in 2005, predicts Packaged Facts, a New York market-research firm. Although the Iowa State University Agricultural Marketing Resource Center says organic foods are outpacing natural ones in sales growth, products labeled all-natural were the most frequent "positive" new product category in North America during 2003, according to Mintel, a research firm. One study has shown that shoppers are willing to pay 30 percent more for food products labeled "natural."

In 1993, the FDA indicated it "would consider establishing a definition" because consumers regard many uses of the term as "noninformative." But late last month, spokesman Mike Herndon said there is no plan to do so.

The agency, he said in an e-mail, does not object to the word "natural" on food labels when it's used in a nonmisleading manner and when the food does not contain "added color, artificial flavors or synthetic substances."