Gluten free products are seen on a grocery store shelf in Washington on Friday. (Carole Feldman/AP)

The Food and Drug Administration on Friday detailed the requirements manufacturers must meet before labeling their foods “gluten-free,” a move intended to create certainty for millions of Americans who suffer from celiac disease and to set a standard for the burgeoning industry in gluten-free products.

The official government designation came nine years after Congress asked the agency to establish a uniform definition for companies wishing to label their products gluten-free and five years after the deadline set by lawmakers.

In the interim, manufacturers have continued to leap into the lucrative and exploding market for foods without gluten, a protein found in barley, wheat and rye. Although several independent organizations certify products as gluten-free, until Friday there was no government standard for what it takes to earn that label, leaving companies to make their own determinations.

At the same time, the number of diagnoses of celiac disease in Americans has increased. Celiac disease is an autoimmune condition in which consuming gluten can damage the lining of the small intestine and contribute to a range of health issues, from gastrointestinal problems to increased risk for osteoporosis, infertility, miscarriages and certain types of cancer.

About 3 million Americans, or about 1 in 133, suffer from celiac disease, according to the National Foundation for Celiac Awareness. An estimated 18 million are gluten-sensitive, meaning exposure to the protein can cause such symptoms as headaches, fatigue and joint pain.

The rule published Friday by the FDA says that for a product to be marketed as gluten-free, it must contain less than 20 parts per million of gluten. It also requires that any foods labeled “free of gluten,” “without gluten” and “no gluten” meet the same standard. The agency said it settled on that figure partly because it’s difficult to reliably detect gluten at lower levels and because some celiac experts and previous research suggest that most people who have the disease can tolerate trace amounts of gluten without adverse effects.

Although many foods now marketed as gluten-free already meet the new federal guidelines, manufacturers will have a year to comply with the new rule.

“Adherence to a gluten-free diet is the key to treating celiac disease, which can be very disruptive to everyday life,” FDA Commissioner Margaret A. Hamburg said in a statement. “The FDA’s new ‘gluten-free’ definition will help people with this condition make food choices with confidence and allow them to better manage their health.”

Alessio Fasano, director of the Center for Celiac Research at Massachusetts General Hospital, said the FDA took an “excruciating” amount of time to finalize its gluten-free definition in part because it had to consider a massive amount of research on celiac disease as well as varying opinions from activists who wanted even more stringent standards and industry officials who argued for more lenient requirements. In the end, he said, the agency struck the right balance.

“They made a decision that was based on scientific evidence,” Fasano said, adding that the ruling will finally will give celiac sufferers peace of mind in restaurants and grocery stores. “Now they don’t have to stress out,” he said. “Now they know there are rules, that there are parameters. It’s a big deal.”

Congress directed the FDA to set guidelines for the use of the term gluten-free when it passed the Food Allergen Labeling and Consumer Protection Act of 2004. The agency organized a public meeting on the topic the following year and solicited input from a wide range of parties, including medical professionals, food manufacturers and millers.

The FDA published a proposed rule in 2007 but continued to study the matter for years, saying it required careful consideration and technical analysis, such as determining how accurately manufacturers could test gluten levels in their products. The agency asked for another round of public comment in 2011 and sent its proposal to the White House earlier this year for final review.

While the U.S. government tried to settle on a definition of gluten-free, countries including Australia and Canada have set labeling standards of their own. Those standards generally are in line with the definition the FDA set Friday.

In recent years, more and more people have begun buying gluten-free bread, pasta, cereal and other staples, as well as gluten-free beer and wine.

The research firm Packaged Facts reported last year that the market for gluten-free products in the United States has grown more than expected, reaching $4.2 billion. “The conviction that gluten-free products are generally healthier is the top motivation for consumers of these products,” the firm said in its report, which projected that U.S. sales of gluten-free foods and beverages would cross $6.6 billion by 2017.

“It’s great that there’s finally a standard. There was a lot of unknown before today,” said Courtney Mesmer, president of the D.C. Metro Celiac Organization, who has been dealing with the disease herself for years. “It gives us an element of certainty knowing [that companies] are not just slapping on a ‘gluten-free’ label.”